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• Review Article
• Published: 31 January 2023

Global water resources and the role of groundwater in a resilient water future

• Bridget R. Scanlon   ORCID: orcid.org/0000-0002-1234-4199 1 ,
• Sarah Fakhreddine 1 , 2 ,
• Ashraf Rateb 1 ,
• Inge de Graaf   ORCID: orcid.org/0000-0001-7748-868X 3 ,
• Jay Famiglietti 4 ,
• Tom Gleeson 5 ,
• R. Quentin Grafton 6 ,
• Esteban Jobbagy 7 ,
• Seifu Kebede 8 ,
• Seshagiri Rao Kolusu 9 ,
• Leonard F. Konikow 10 ,
• Di Long   ORCID: orcid.org/0000-0001-9033-5039 11 ,
• Mesfin Mekonnen   ORCID: orcid.org/0000-0002-3573-9759 12 ,
• Hannes Müller Schmied 13 , 14 ,
• Abhijit Mukherjee 15 ,
• Alan MacDonald   ORCID: orcid.org/0000-0001-6636-1499 16 ,
• Robert C. Reedy 1 ,
• Mohammad Shamsudduha 17 ,
• Craig T. Simmons 18 ,
• Alex Sun 1 ,
• Richard G. Taylor 19 ,
• Karen G. Villholth 20 ,
• Charles J. Vörösmarty 21 &
• Chunmiao Zheng   ORCID: orcid.org/0000-0001-5839-1305 22  

Nature Reviews Earth & Environment volume  4 ,  pages 87–101 ( 2023 ) Cite this article

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• Water resources

An Author Correction to this article was published on 29 March 2023

This article has been updated

Water is a critical resource, but ensuring its availability faces challenges from climate extremes and human intervention. In this Review, we evaluate the current and historical evolution of water resources, considering surface water and groundwater as a single, interconnected resource. Total water storage trends have varied across regions over the past century. Satellite data from the Gravity Recovery and Climate Experiment (GRACE) show declining, stable and rising trends in total water storage over the past two decades in various regions globally. Groundwater monitoring provides longer-term context over the past century, showing rising water storage in northwest India, central Pakistan and the northwest United States, and declining water storage in the US High Plains and Central Valley. Climate variability causes some changes in water storage, but human intervention, particularly irrigation, is a major driver. Water-resource resilience can be increased by diversifying management strategies. These approaches include green solutions, such as forest and wetland preservation, and grey solutions, such as increasing supplies (desalination, wastewater reuse), enhancing storage in surface reservoirs and depleted aquifers, and transporting water. A diverse portfolio of these solutions, in tandem with managing groundwater and surface water as a single resource, can address human and ecosystem needs while building a resilient water system.

Net trends in total water storage data from the GRACE satellite mission range from −310 km 3 to 260 km 3 total over a 19-year record in different regions globally, caused by climate and human intervention.

Groundwater and surface water are strongly linked, with 85% of groundwater withdrawals sourced from surface water capture and reduced evapotranspiration, and the remaining 15% derived from aquifer depletion.

Climate and human interventions caused loss of ~90,000 km 2 of surface water area between 1984 and 2015, while 184,000 km 2 of new surface water area developed elsewhere, primarily through filling reservoirs.

Human intervention affects water resources directly through water use, particularly irrigation, and indirectly through land-use change, such as agricultural expansion and urbanization.

Strategies for increasing water-resource resilience include preserving and restoring forests and wetlands, and conjunctive surface water and groundwater management.

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Change history, 29 march 2023.

A Correction to this paper has been published: https://doi.org/10.1038/s43017-023-00418-9

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Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, Austin, TX, USA

Bridget R. Scanlon, Sarah Fakhreddine, Ashraf Rateb, Robert C. Reedy & Alex Sun

Department of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, PA, USA

Sarah Fakhreddine

Water Systems and Global Change, Wageningen University, Wageningen, The Netherlands

Inge de Graaf

Global Institute for Water Security, National Hydrology Research Center, University of Saskatchewan, Saskatoon, Canada

Jay Famiglietti

Department of Civil Engineering, University of Victoria, Victoria, British Columbia, Canada

Tom Gleeson

Crawford School of Public Policy, Australian National University, Canberra, ACT, Australia

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UK Meteorological Office, Exeter, UK

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Leonard Konikow Hydrogeologist, Reston, VA, USA

Leonard F. Konikow

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Mesfin Mekonnen

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Senckenberg Leibniz Biodiversity and Climate Research Centre (SBiK-F), Frankfurt am Main, Frankfurt, Germany

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British Geological Survey, Lyell Centre, Edinburgh, UK

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Craig T. Simmons

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B.R.S. conceptualized the review and coordinated input. S.F. reviewed many of the topics and developed some of the figures. A.R. analysed GRACE satellite data and M.S. reviewed this output. Q.G. provided input on water economics. E.J. reviewed impacts of land-use change. S.R.K. provided data on future precipitation changes. L.F.K. provided detailed information on surface water/groundwater interactions. M.M. provided data on water trade. C.J.V. provided input on green and grey solutions. All authors reviewed the paper and provided edits.

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Correspondence to Bridget R. Scanlon .

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Scanlon, B.R., Fakhreddine, S., Rateb, A. et al. Global water resources and the role of groundwater in a resilient water future. Nat Rev Earth Environ 4 , 87–101 (2023). https://doi.org/10.1038/s43017-022-00378-6

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Environmental Science: Water Research & Technology  seeks to showcase high quality research about fundamental science, innovative technologies, and management practices that promote sustainable water.

The journal aims to provide a comprehensive and relevant forum that unites the diverse communities and disciplines conducting water research relevant to engineered systems and the built environment. This includes fundamental science geared toward understanding physical, chemical, and biological phenomena in these systems as well as applied research focused on the development and optimisation of engineered treatment, management, and supply strategies.

Papers must report a significant advance in the theory, fundamental understanding, practice or application of water research, management, engineering or technology, within the following areas:

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The journal places special focus on issues associated with water sustainability, as well as research that may lead to more secure, resilient and reliable water supplies. And it welcomes inter- and multidisciplinary work contributing to any of the above developments that are likely to be of interest to the broad community that the Journal addresses.

Manuscripts should be written to be accessible to scientists and engineers in all disciplines associated with the Journal.

All manuscripts must highlight their novel features and explain the significance of the work relative to related studies in their field as well as the likely impact on relevant water communities in the industry, government or academia.

*Please see the below expandable section for specific guidance regarding this area of our scope.

Measurement advances and analysis: these papers are encouraged and must clearly focus on the relevance of the work to engineered water systems and clearly explain the implications of the analysis or observations for sustainable water management. Papers dealing only with analysis, analytical method development or that simply report measured concentrations of target analytes (for example, occurrence and effluent concentrations of novel pollutant classes) will not be considered for publication.

Modeling: papers that lack appropriate validation through either experimental data or available and reliable datasets will not be considered for publication.

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Papers in this area are strongly discouraged from focusing solely on technology demonstrations in model systems with model pollutant targets. Rather, they are encouraged to consider performance in complex (that is, environmentally relevant) systems and performance metrics (for example, efficacy across multiple pollutant targets, longevity, regeneration during application, and sustainability assessment) most relevant to real world application. 

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Sustainability assessments: papers that cover, for example, life cycle assessment or life cycle cost analysis, of water-related technologies and systems must emphasize the fundamental insight into the factors governing technology or system performance. Papers are strongly discouraged from solely reporting absolute or comparative assessments of technologies/systems without uncovering novel insight or identifying critical barriers to sustainability.

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Editor-in-chief

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Sebastià Puig Broch , Universitat de Girona, Spain

Wenhai Chu , Tongji University, China

Ning Dai , University at Buffalo, USA

Lauren Stadler , Rice University, USA

Liu Ye , The University of Queensland, Australia

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Branko Kerkez , University of Michigan, USA

Jeonghwan Kim , Inha University, South Korea

Linda Lawton , Robert Gordon University, UK

Luca Vezzaro , Technical University of Denmark, Denmark

Eveline Volcke , Ghent University, Belgium

Federico Aulenta , National Research Council, Italy

Nicholas Ashbolt , University of Alberta, Canada

Tom Bond , University of Surrey, UK

Joby Boxall , The University of Sheffield, UK

Kartik Chandran , Columbia University in the City of New York, USA

Amy Childress , University of Southern California, USA

David Cwiertny , University of Iowa  

Joel Ducoste , North Carolina State University, USA

Marc Edwards , Virginia Tech, USA

Jingyun Fang , Sun Yat-sen University, China

Maria Jose Farre , Catalan Institute for Water Research, Spain

Yujie Feng , Harbin Institute of Technology, China

Kathrin Fenner , Swiss Federal Institute of Aquatic Science and Technology, Eawag, Switzerland 

Ramesh Goel , University of Utah, USA

Ola Gomaa , National Center for Radiation Research and Technology, Egypt

Chris Gordon , University of Ghana, Ghana

April Gu , Cornell University, USA

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Peng Liang , Tsinghua University, China

Irene Lo , Hong Kong University of Science and Technology, Hong Kong

Julie Minton , WateReuse Foundation, USA

Vincenzo Naddeo , University of Salerno, Italy

Indumathi M Nambi , Indian Institute of Technology Madras, India

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Paige Novak , University of Minnesota, USA

Yong Sik Ok , Korea University, South Korea

Ligy Philip , Indian Institute of Technology Madras, India

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Zhiyong "Jason" Ren , Princeton University, USA

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David Weissbrodt , TU Delft, The Netherlands

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Di Wu , Ghent University, South Korea

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Full papers, perspectives, critical reviews, frontier reviews, tutorial reviews, comments and replies.

Reviews & Perspectives are normally invited, however suggestions for timely Reviews are very welcome. Interested authors should contact the Editorial Office at [email protected] with an abstract or brief synopsis of their intended Review.

These must report preliminary research findings that are novel and original, of immediate interest and are likely to have a high impact on the Environmental Science: Water Research & Technology community. Authors must provide a short paragraph explaining why their work justifies rapid publication as a communication.

Original research papers on any of the subjects outlined in the scope section and related areas are encouraged and welcomed. All papers should give due attention to overcoming limitations and to underlying principles. All contributions will be judged on the following four criteria. 1. Novelty and insight 2. Quality of scientific work and content 3. Clarity of objectives and aims of the work 4. Appropriateness of length to content of new science

These may be articles providing a personal view of part of one discipline associated with Environmental Science: Water Research & Technology or a philosophical look at a topic of relevance. Alternatively, Perspectives may be historical articles covering a particular subject area or the development of particular legislation, technologies, methodologies or other subjects within the scope of the journal.

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All Critical reviews undergo a rigorous and full peer review procedure, in the same way as regular research papers. Authors are encouraged to identify areas in the field where further developments are imminent or of urgent need, and any areas that may be of significance to the community in general. Critical reviews should not contain any unpublished original research.

These are shorter, more focused versions of Critical reviews on a well-defined, specific topic area covering approximately the last two-three years. Articles should cover only the most interesting/significant developments in that specific subject area.

The article should be highly critical and selective in referencing published work. One or two paragraphs of speculation about possible future developments may also be appropriate in the conclusion section.

Frontier reviews may also cover techniques/technologies that are too new for a Critical review or may address a subset of technologies available for a given area of research within the journal scope.

Frontier reviews should not contain unpublished original research.

Tutorial reviews should provide an introduction and overview of an important topic of relevance to the journal readership. The topic should be of relevance to both researchers who are new to the field as well as experts and provide a good introduction to the development of a subject, its current state and indications of future directions the field is expected to take. Tutorial reviews should not contain unpublished original research.

Comments and Replies are a medium for the discussion and exchange of scientific opinions between authors and readers concerning material published in Environmental Science: Water Research & Technology.

For publication, a Comment should present an alternative analysis of and/or new insight into the previously published material. Any Reply should further the discussion presented in the original article and the Comment. Comments and Replies that contain any form of personal attack are not suitable for publication. 

Comments that are acceptable for publication will be forwarded to the authors of the work being discussed, and these authors will be given the opportunity to submit a Reply. The Comment and Reply will both be subject to rigorous peer review in consultation with the journal’s Editorial Board where appropriate. The Comment and Reply will be published together.

Journal specific guidelines

See a summary of ESWRT’s journal-specific guidelines . More details are also provided below.

Use of RSC template

There are no submission specifics regarding formatting; use of Royal Society of Chemistry template is not required. Bibliographies should be formatted according to the following Endnote and Zotero style files to include the cited article’s title.

Authors are encouraged to include line numbering in submitted manuscripts. Although there is no page limit for Full papers, appropriateness of length to content of new science will be taken into consideration by reviewers.

Water Impact Statement

All submitted manuscripts must include a 'Water Impact Statement' (60 words maximum; approximately three sentences) that clearly states in plain language the broad-scale implications and real-world relevance of the work. True potential for immediate real-world impact may be subject to further study, but the pathways towards achieving that impact in future should at least be envisioned and explained.

Read Professor Michael Templeton’s Editorial Perspective “ Achieving real-world impact ” for further discussion on expectations for the journal.

Authors should use this statement to show that they have given serious consideration as to how their work addresses current challenges related to water sustainability in a realistic sense. This statement will be carefully considered by the editors and the reviewers and will help ascertain the relevance of the article for a broad audience. Absence of potential for real-world impact is reason for rejection. If the manuscript is accepted this statement will be included in the published article. Please note that manuscripts without this statement will not be peer-reviewed.

Double-anonymised peer review option

Environmental Science: Water Research & Technology is now offering authors the option of double-anonymised peer review. Both single- and double-anonymised peer review are now available to authors.

• Single-anonymised peer review - where reviewers are anonymous but author names and affiliations are known to reviewers. (This is the traditional peer review model used on Environmental Science: Water Research & Technology)
• Double-anonymised peer review - where authors and reviewers' identities are concealed from each other.

Guidelines for authors and reviewers can be found  here

Organisation of material

An article should have a short, straightforward title directed at the general reader. Lengthy systematic names and complicated and numerous chemical formulae should therefore be avoided where possible. The use of non-standard abbreviations and symbols in a title is not encouraged. Please bear in mind that readers increasingly use search engines to find literature; recognisable, key words should be included in the title where possible, to maximise the impact and discoverability of your work. Brevity in a title, though desirable, should be balanced against its accuracy and usefulness.

The use of series titles and part numbers in titles of papers is discouraged. Instead these can be included as a footnote to the first page together with a reference (reference 1) to the preceding part. When the preceding part has been submitted to a Royal Society of Chemistry journal but is not yet published, the paper reference number should be given.

Author names

Full names for all the authors of an article should be given. To give due acknowledgement to all workers contributing to the work, those who have contributed significantly to the research should be listed as co-authors. Authors who contributed equally can be noted with a Footnote and referenced with a symbol.

On submission of the manuscript, the corresponding author attests to the fact that those named as co-authors have agreed to its submission for publication and accepts the responsibility for having properly included all (and only) co- authors. If there are more than 10 co-authors on the manuscript, the corresponding author should provide a statement to specify the contribution of each co-author. The corresponding author signs a copyright licence on behalf of all the authors.

Table of contents entry

This entry should include a colour image (no larger than 8 cm wide x 4 cm high), and 20-30 words of text that highlight the novel aspects of your work. Graphics should be as clear as possible; simple schematic diagrams or reaction schemes are preferred to ORTEP- style crystal structure depictions and complicated graphs, for example. The graphic used in the table of contents entry need not necessarily appear in the article itself. Authors should bear in mind the final size of any lettering on the graphic. For examples, please see the online version of the journal.

Every paper must be accompanied by a summary (50-250 words) setting out briefly and clearly the main objects and results of the work; it should give the reader a clear idea of what has been achieved. The summary should be essentially independent of the main text; however, names, partial names or linear formulae of compounds may be accompanied by the numbers referring to the corresponding displayed formulae in the body of the text.

Please bear in mind that readers increasingly use search engines to find literature; recognisable, searchable terms and key words should be included in the abstract to enable readers to more effectively find your paper. The abstract should aim to address the following questions.

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Water Impact Statement 

Authors must provide a 'Water Impact Statement' (60 words maximum) that clearly highlights the broad-scale implications and real-world relevance of the work. This statement should be different from the abstract and must set the work in broader context with regards to water sustainability. True potential for immediate real-world impact may be subject to further study, but the pathways towards achieving that impact in future should at least be envisioned and explained in this statement.

When composing your Water Impact Statement, please consider the following points:

1.What is the problem? 2.Why is it important? 3.How does this translate to real-world applications/scenarios? 4.How can this be generalised?  5.Why is this work significant for ensuring sustainable water resources?  

This statement will be seen by the reviewers and will help ascertain the relevance of the article for a broad but technical audience. Authors should use it to show that they have given serious consideration to the impact of their presented study. Absence of potential for real-world impact is reason for rejection. If the paper is accepted this statement will also be published. Please note that papers cannot be peer-reviewed without this statement.

Introduction

This should give clearly and briefly, with relevant references, both the nature of the problem under investigation and its background.

Descriptions of methods and/or experiments should be given in detail sufficient to enable experienced experimental workers to repeat them. Standard techniques and methods used throughout the work should be stated at the beginning of the section. Apparatus should be described only if it is non-standard; commercially available instruments are referred to by their stock numbers (for example, Perkin-Elmer 457 or Varian HA-100 spectrometers). The accuracy of primary measurements should be stated. In general there is no need to report unsuccessful experiments. Authors are encouraged to make use of electronic supplementary information (ESI) for lengthy synthetic sections. Any unusual hazards inherent in the use of chemicals, procedures or equipment in the investigation should be clearly identified. In cases where a study involves the use of live animals or human subjects, the author should include a statement that all experiments were performed in compliance with the relevant laws and institutional guidelines, and also state the institutional committee(s) that have approved the experiments. They should also include a statement that informed consent was obtained for any experimentation with human subjects. Referees may be asked to comment specifically on any cases in which concerns arise.

Results and discussion

It is usual for the results to be presented first, followed by a discussion of their significance. Only strictly relevant results should be presented and figures, tables, and equations should be used for purposes of clarity and brevity. The use of flow diagrams and reaction schemes is encouraged. Data must not be reproduced in more than one form - for example, in both figures and tables, without good reason.

This is for interpretation and to highlight the novelty and significance of the work. Authors are encouraged to discuss the real world relevance of the work reported and how it promotes water sustainability. The conclusions should not summarise information already present in the text or abstract.

Acknowledgements

Contributors other than co-authors may be acknowledged in a separate paragraph at the end of the paper; acknowledgements should be as brief as possible. All sources of funding should be declared.

Bibliographic references and notes

These should be listed at the end of the manuscript in numerical order. We encourage the citation of primary research over review articles, where appropriate, in order to give credit to those who first reported a finding. Find out more about our commitments to the principles of  San Francisco Declaration on Research Assessment (DORA).

Bibliographic details should be cited in the order: year, volume , page, and must include the article title. For example: Lukas Mustajärvi, Ann-Kristin Eriksson-Wiklund, Elena Gorokhova, Annika Jahnke and Anna Sobek, Transferring mixtures of chemicals from sediment to a bioassay using silicone-based passive sampling and dosing, Environ. Sci.: Processes Impacts , 2017, 19 , 1404-1413. See  Endnote style files . For Zotero, please use the Royal Society of Chemistry (with titles) template.

Bibliographic reference to the source of statements in the text is made by use of superior numerals at the appropriate place (for example, Wittig3). The reference numbers should be cited in the correct sequence through the text (including those in tables and figure captions, numbered according to where the table or figure is designated to appear).  Please do not use Harvard style for references.

The references themselves are given at the end of the final printed text along with any notes. The names and initials of all authors are always given in the reference; they must not be replaced by the phrase et al . This does not prevent some, or all, of the names being mentioned at their first citation in the cursive text; initials are not necessary in the text. Notes or footnotes may be used to present material that, if included in the body of the text, would disrupt the flow of the argument but which is, nevertheless, of importance in qualifying or amplifying the textual material. Footnotes are referred to with the following symbols: †, ‡, §, ¶, ║etc.

Alternatively the information may be included as Notes (end-notes) to appear in the Notes/references section of the manuscript. Notes should be numbered using the same numbering system as the bibliographic references.

Journals The style of journal abbreviations to be used in RSC publications is that defined in Chemical Abstracts Service Source Index (CASSI) (http://www.cas.org/expertise/cascontent/caplus/corejournals.html).

Bibliographic details should be cited in the order: year, volume , page. Where page numbers are not yet known, articles should be cited by DOI (Digital Object Identifier) - for example, T. J. Hebden, R. R. Schrock, M. K. Takase and P. Müller, Chem. Commun ., 2012, DOI: 10.1039/C2CC17634C.

Books J. Barker, in Catalyst Deactivation , ed. B. Delmon and C. Froment, Elsevier, Amsterdam, 2nd edn., 1987, vol. 1, ch. 4, pp. 253-255.

Patents Br. Pat ., 357 450, 1986. US Pat ., 1 171 230, 1990.

Reports and bulletins, etc R. A. Allen, D. B. Smith and J. E. Hiscott, Radioisotope Data , UKAEA Research Group Report AERE-R 2938, H.M.S.O., London, 1961.

Material presented at meetings H. C. Freeman, Proceedings of the 21st International Conference on Coordination Chemistry, Toulouse, 1980.

Theses A. D. Mount, Ph.D. Thesis, University of London, 1977.

Reference to unpublished material For material presented at a meeting, congress or before a Society, etc., but not published, the following form is used:  A. R. Jones, presented in part at the 28th Congress of the International Union of Pure and Applied Chemistry, Vancouver, August, 1981.

For material accepted for publication, but not yet published, the following forms are used.

• A. R. Jones, Dalton Trans. , 2003, DOI: 10.1039/manuscript number, for RSC journals 
• A. R. Jones, Angew. Chem ., in press, for non-RSC journals

If DOI numbers are known these should be cited in the form recommended by the publisher.

For material submitted for publication but not yet accepted the following form is used.

• A. R. Jones, Angew. Chem ., submitted.

For personal communications the following is used.

• G. B. Ball, personal communication.

If material is to be published but has yet to be submitted the following form is used.

• G. B. Ball, unpublished work.

Reference to unpublished work should not be made without the permission of those by whom the work was performed.

Software F James,  AIM2000, version 1.0, University of Applied Sciences, Bielefeld,  Germany, 2000. T Bellander, M Lewne and B Brunekreef, GAUSSIAN 3 (Revision B.05), Gaussian Inc., Pittsburgh, PA, 2003.

Online resources (including databases) Please note the most important information to include is the URL and the data accessed.

• The Merck Index Online, http://www.rsc.org/Merck-lndex/monograph/mono1500000841, (accessed October 2013).
• ChemSpider, http://www.chemspider.com/Chemicai-Structure.1906.html, (accessed June 2011).

arXiv references V. Krstic and M. Glerup, 2006, arXiv:cond-mat/0601513.

Figures & schemes

Preparation of graphics.

Artwork should be submitted at its final size so that reduction is not required. The appearance of graphics is the responsibility of the author.

• Graphics should fit within either single column (8.3 cm) or double column (17.1 cm) width, and must be no longer than 23.3 cm.
• Graphical abstracts should be no larger than 8 x 4 cm.
• Schemes and structures should be drawn to make best use of single and double column widths.

Colour figures

Colour figure reproduction is provided free of charge both online and in print.

Journal covers

Authors who wish to have their artwork featured on a journal cover should contact the editorial office of the journal to which the article is being submitted. A contribution to the additional production costs will be requested.

Use of such artwork is at the editor's discretion; the editor's decision is final. Examples of previous journal covers can be viewed via the journal homepage.

Electronic supplementary information

The journal's electronic supplementary information (ESI) service is a free facility that enables authors to enhance and increase the impact of their articles. Authors are encouraged to make the most of the benefits of publishing supplementary information in electronic form. Such data can take full advantage of the electronic medium, allowing use of 3D molecular models and movies. Authors can also improve the readability of their articles by placing appropriate material, such as repetitive experimental details and bulky data, as ESI. All information published as ESI is also fully archived. When preparing their ESI data files, authors should keep in mind the following points.

• Supplementary data is peer-reviewed, and should therefore be included with the original submission.
• ESI files are published 'as is'; editorial staff will not usually edit the data for style or content.
• Data is useful only if readers can access it; use common file formats.
• Large files may prove difficult for users to download and access.

Text and graphics

The preferred format for ESI comprising text and graphics is Microsoft Word. Publishing staff will convert Word files to PDF before publication, as this format can be accessed easily and reliably on most computing platforms using the freely available Adobe Acrobat Reader. If other formats are submitted they will also usually be converted to PDF files prior to publication.

Multimedia files

We welcome submission of multimedia files (including videos and animations) alongside articles for publication. Videos are an excellent medium to present elements of your work that can be difficult to communicate only in words. Please note that any videos of general interest are shared with the wider community via the RSC Journals YouTube channel. Please notify the editorial team if you prefer for your video(s) not to be uploaded to YouTube. If you submit a multimedia file alongside your paper, please refer to it within your paper to draw it to the reader’s attention. Also please see the section on submitting multimedia files

Format Acceptable formats for video or animation clips are listed below.

Please minimise file sizes where you can, by considering the following points.

• The recommended maximum frame size is 640 x 480 pixels.
• Our recommended maximum file size is 5 Mb.
• Many packages output 30 frames per second (fps) as standard, but it's possible to specify a lower frame rate; this may not noticeably affect the quality of your video but will reduce the file size.
• Use a 256 colour palette, if that is suitable for the presentation of the material.

Please consider the use of lower specifications for all these points if the material can still be represented clearly.

If your video is very short (that is, several seconds long) then it is recommended that you loop it and repeat a few times to provide a more detailed view.

Submitting multimedia files Upload your video online, together with your manuscript under the category 'electronic supplementary material' and please supply the following.

• A clear file name for your video.
• A short descriptive title for the video, which can be used when uploading the video onto a streaming channel.
• A video legend of approximately 30 words long; this caption must be provided to aid discoverability.
• Five to 10 keywords that can be used to tag the video; the more accurate the tags are the better discoverability videos will have.

Copies of any relevant 'in press' references

Manuscripts should be submitted with copies of any ‘in press’ articles referenced.

Open access publishing options

Environmental Science: Water Research & Technology  is a hybrid (transformative) journal and gives authors the choice of publishing their research either via the traditional subscription-based model or instead by choosing our gold open access option.  Find out more about our Transformative Journals. which are Plan S compliant .

Gold open access

For authors who want to publish their article gold open access , Environmental Science: Water Research & Technology  charges an article processing charge (APC) of £2,750 (+ any applicable tax). Our APC is all-inclusive and makes your article freely available online immediately, permanently, and includes your choice of Creative Commons licence (CC BY or CC BY-NC) at no extra cost. It is not a submission charge, so you only pay if your article is accepted for publication.

Learn more about publishing open access .

Read & Publish

If your institution has a Read & Publish agreement in place with the Royal Society of Chemistry, APCs for gold open access publishing in Environmental Science: Water Research & Technology  may already be covered.

Use our journal finder to check if your institution has an open access agreement with us.

Please use your official institutional email address to submit your manuscript and check you are assigned as the corresponding author; this helps us to identify if you are eligible for Read & Publish or other APC discounts.

Traditional subscription model

Authors can also publish in Environmental Science: Water Research & Technology via the traditional subscription model without needing to pay an APC. Articles published via this route are available to institutions and individuals who subscribe to the journal. Our standard licence allows you to make the accepted manuscript of your article freely available after a 12-month embargo period. This is known as the green route to open access.

Learn more about green open access .

Subscription information

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*2022 Journal Citation Reports (Clarivate Analytics, 2023)

**The median time from submission to first decision including manuscripts rejected without peer review from the previous calendar year

***The median time from submission to first decision for peer-reviewed manuscripts from the previous calendar year

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The set of journals have been ranked according to their SJR and divided into four equal groups, four quartiles. Q1 (green) comprises the quarter of the journals with the highest values, Q2 (yellow) the second highest values, Q3 (orange) the third highest values and Q4 (red) the lowest values.

The SJR is a size-independent prestige indicator that ranks journals by their 'average prestige per article'. It is based on the idea that 'all citations are not created equal'. SJR is a measure of scientific influence of journals that accounts for both the number of citations received by a journal and the importance or prestige of the journals where such citations come from It measures the scientific influence of the average article in a journal, it expresses how central to the global scientific discussion an average article of the journal is.

Evolution of the number of published documents. All types of documents are considered, including citable and non citable documents.

This indicator counts the number of citations received by documents from a journal and divides them by the total number of documents published in that journal. The chart shows the evolution of the average number of times documents published in a journal in the past two, three and four years have been cited in the current year. The two years line is equivalent to journal impact factor ™ (Thomson Reuters) metric.

Evolution of the total number of citations and journal's self-citations received by a journal's published documents during the three previous years. Journal Self-citation is defined as the number of citation from a journal citing article to articles published by the same journal.

Evolution of the number of total citation per document and external citation per document (i.e. journal self-citations removed) received by a journal's published documents during the three previous years. External citations are calculated by subtracting the number of self-citations from the total number of citations received by the journal’s documents.

International Collaboration accounts for the articles that have been produced by researchers from several countries. The chart shows the ratio of a journal's documents signed by researchers from more than one country; that is including more than one country address.

Not every article in a journal is considered primary research and therefore "citable", this chart shows the ratio of a journal's articles including substantial research (research articles, conference papers and reviews) in three year windows vs. those documents other than research articles, reviews and conference papers.

Ratio of a journal's items, grouped in three years windows, that have been cited at least once vs. those not cited during the following year.

Evolution of the percentage of female authors.

Evolution of the number of documents cited by public policy documents according to Overton database.

Evoution of the number of documents related to Sustainable Development Goals defined by United Nations. Available from 2018 onwards.

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The users of Scimago Journal & Country Rank have the possibility to dialogue through comments linked to a specific journal. The purpose is to have a forum in which general doubts about the processes of publication in the journal, experiences and other issues derived from the publication of papers are resolved. For topics on particular articles, maintain the dialogue through the usual channels with your editor.

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Quantifying Energy Optimization

Developing a framework for reporting

Reducing Legionella Levels

Service lines and premise plumbing

Occurrence of PFAS Compounds

In US wastewater treatment plants

• 02 Energy Optimization
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Topics of Focus

In the United States alone, billions of gallons of water are treated each day at water resource recovery facilities. Once the water is clean, a different challenge remains: determining what to do with the solids that are removed during the treatment process. The resulting mixture is often a unique semi-solid blend of organic and inorganic materials, trace elements, chemicals, and even pathogens, so there is no across the board solution for handling and processing the combinations of constituents that may be present.

Because these solids are often rich in nutrients, like nitrogen and phosphorus—which also happen to be the perfect ingredients for promoting healthy soil and plant growth—many facilities have turned to land application. Before these solids can be put to use for things like fertilizing farmland, however, they must undergo rigorous treatment to meet stringent regulations, at which point they become known as biosolids.

For more information, contact Ashwin Dhanasekar .

Characterization and Contamination Testing of Source Separated Organic Feedstocks and Slurries for Co-Digestion at Resource Recovery Facilities

Project highlights.

A key challenge with source separated organic (SSO) feedstock co-substrate is that its composition, quality, and characteristics differ between geographical locations and can change over time. This causes challenges and uncertainties for pre-treaters, substrate brokers, facilities accepting this material, operators...

The Water Research Foundation Funds 26 New Research Projects Totaling $5.9M

(Denver, CO) 05/28/24– The Water Research Foundation (WRF) is seeking volunteer participants for 26 new research projects funded through WRF's Research Priority Program . This strategic research program enables WRF...

Interview with Dr. William Tarpeh

Turning Waste into Gold with Dr. William Tarpeh A rare few people end up in the career they decided for themselves as children. More often, the question “What do you...

WRF Presents $100K Research Award To Advance Wastewater Resource Recovery

(Denver, CO) 10/11/23 – Last week, The Water Research Foundation (WRF) presented William Tarpeh, PhD, with the esteemed 2023 Paul L. Busch Award. With this $100,000 research prize, Dr. Tarpeh...

Climate Change

Climate change is altering our natural hydrologic cycle, creating uncertainty when it comes to the quality and quantity of water sources. WRF’s research on climate change covers the key areas of climate risk assessment, climate adaptation, and mitigation strategies.

Because the first step in preparing for climate change is understanding the potential and variable impacts these changes can have on water sources and treatment systems, WRF research tracks potential outcomes, considering a variety of possibilities, and provides resources and tools to help facilities identify and address risks and vulnerabilities in their operations and infrastructure.

Implementing climate change adaptation strategies will be critical as the water sector moves forward. WRF’s research in this area helps utilities create better long- and short-term adaptation plans, respond more effectively to severe weather, and improve infrastructure and operations to meet changing needs, including the production of onsite energy systems and reliable back-up power to protect critical services.

The water sector must also have a hand in mitigating the root causes of climate change. By pioneering approaches to improve energy efficiency, including process optimization, improved energy management, and the use of renewable energy, WRF is helping the water sector decrease activity that is driving these changes.

For more information, contact Harry Zhang .

Holistic Approaches to Flood Mitigation Planning and Modeling under Extreme Events and Climate Impacts

Municipalities and utilities are facing unprecedented challenges in planning for extreme precipitation and flooding events, which are occurring more frequently and unpredictably. A holistic approach to flood mitigation planning and modeling, including partnerships between stakeholders, is needed to balance competing...

One Water Cities: A Self-Assessment Framework

Municipalities play key roles in implementing One Water approaches and furthering community resilience. Read the full article.

The Water Research Foundation Honors Outstanding Water Leaders

(Denver, CO) 6/21/23 - The Water Research Foundation (WRF) announced last week that it awarded its 2023 Subscriber Impact Award to Denver Water and its 2023 Research Innovation Award to...

Cyanobacteria & Cyanotoxins

Aquatic microscopic algae and cyanobacteria (formerly known as blue-green algae) occur naturally in most surface waters. However certain nutrient and temperature conditions can cause them to multiply rapidly, leading to “blooms.” Under certain conditions, some species of cyanobacteria can produce toxic secondary metabolites or cyanotoxins, which may pose health risks to humans and animals. Even when cyanobacteria are not toxic, they can produce unpleasant tastes and odors.

Cyanobacteria continue to be among the most problematic organisms in fresh water systems. Without clear guidance or consensus regulations in place, many utilities struggle with responding to cyanobacterial harmful algal bloom (cHAB) events. Since 1994, WRF has completed more than 40 research projects on these microscopic organisms and the cyanotoxins they produce, helping facilities detect, monitor, and manage these organisms—as well as communicate with the public.

For more information, contact Sydney Samples .

Refinement and Standardization of Cyanotoxin Analytical Techniques for Drinking Water

There is uncertainty relating to the screening and confirmation of cyanotoxin samples. Water utilities need robust and dependable methods to monitor cyanotoxins in source water, through the treatment process, and at the tap, as well as to make appropriate decisions...

The Water Research Foundation Funds Utility Research Projects Worth $5M in Research Value

(Denver, CO) 12/19/2023 – The Water Research Foundation (WRF) has selected twelve new projects for funding through its Tailored Collaboration Program, totaling over $5 million in research value. The projects...

PFAS Communication Guidance

Water sector professionals need to be able to communicate with their customers clearly, concisely, and consistently about per- and polyfluoroalkyl substances (PFAS). This may include information on what PFAS are...

Per- and Polyfluoroalkyl Substances

Per- and polyfluoroalkyl substances (PFAS) are man-made compounds with fluorinated carbon chains. They are resistant to heat, oil, and water, making them useful in a wide variety of products, including...

Disinfection Byproducts (DBPs)

The use of strong oxidants to disinfect water has virtually eliminated waterborne diseases like typhoid, cholera, and dysentery in developed countries. However, research has shown that chlorine interacts with natural organic matter present in water supplies to form regulated and emerging disinfection byproducts (DBPs).

To minimize the formation of regulated DBPs and comply with existing regulations, water utilities have increasingly been moving away from chlorine to use alternative disinfectants like chloramine, or installing more advanced and costly treatment processes, such as ozone or granular activated carbon to remove DBP precursors. However, while reducing the formation of halogenated DBPs, alternative oxidants have been shown to favor the formation of other DBPs (e.g., ozone producing bromate and halonitromethanes, and chloramines producing N-nitrosodimethylamine and iodinated DBPs). 

For more information, contact Kenan Ozekin .

Impact of Haloacetic Acid MCL Revisions on DBP Exposure and Health Risk Reduction

The U.S. Environmental Protection Agency (EPA) is considering changes to the disinfectant and disinfection byproducts (D/DBP) rule. Specifically, there may be a shift from the currently regulated five haloacetic acids (HAA5) to nine (HAA9), which would include four additional brominated...

WRF Seeks Pre-proposals for High-Priority Utility Research

(Denver, CO) 02/15/24 – The Water Research Foundation (WRF) is now accepting pre-proposals for its matching research program, the Tailored Collaboration Program. The Tailored Collaboration Program provides an opportunity for...

The Water Research Foundation Seeks Nominations for Paul L. Busch Award

(Denver, CO) 02/08/24 – The Water Research Foundation (WRF) is now accepting nominations for the 2024 Paul L. Busch Award. The $100,000 award recognizes one outstanding individual for innovative research...

Energy Optimization

For most water facilities, energy is one of the highest costs in their operating budget. Stricter regulations are pushing facilities to use even more advanced—and energy-intensive—treatment technologies. Optimizing energy use can provide huge cost savings and numerous additional benefits, including improving air quality, protecting the environment, and bolstering energy security. WRF has published more than 100 projects that explore ways to not only optimize current energy use, but to generate power as well—setting the course for a self-sufficient water sector.

Developing a Framework for Quantifying Energy Optimization Reporting

Energy projects within the water sector are often discretionary and initiated based on projected annual energy savings metrics. The water sector lacks standard energy savings estimation procedures, as well as measurement and verification approaches and procedures that adhere to the...

Intelligent Water Systems

As with other industries, newly developed technologies drive water utilities to adapt their day-to-day operations. Water networks have been a special focus, with new instrumentation options for water production, transmission, distribution, wastewater collection, and consumer end-points coming to market. Implementing these technologies can improve the efficiency and reliability of water networks, but with myriad options, utilities need guidance on which technologies are most worthwhile and how they should be implemented. 

Quantifying the Impact of Artificial Intelligence/Machine Learning-Based Approaches to Utility Performance

The purpose of this project is to survey the water industry and identify the use cases for artificial intelligence (AI) and machine learning (ML), quantify their benefits, and provide a framework for others to be able to make the same...

2024 Intelligent Water Systems Challenge

The Leaders Innovation Forum for Technology (LIFT) program, a joint effort of The Water Research Foundation (WRF) and the Water Environment Federation (WEF), is holding the sixth Intelligent Water Systems...

WRF Seeks Proposals for 22 New Research Projects Totaling $4.9M

(Denver, CO) 9/12/23 - The Water Research Foundation (WRF) is now accepting proposals for 22 research projects totaling $4.9M that will advance the science of water for communities around the...

The Water Research Foundation and Water Environment Federation Launch the Fifth Intelligent Water Systems Challenge

(Denver, CO) 02/6/23 – The Water Research Foundation and Water Environment Federation are pleased to invite teams to participate in the fifth annual Intelligent Water Systems (IWS) Challenge. As technology...

Microbes & Pathogens

Control of microbes in water systems is critical to achieving water quality and public health goals. While most microbes are not considered human pathogens, certain microbes can pose health risks or contribute undesirable tastes and odors. 

Since the early 20th century, modern drinking water treatment has made great advancements in the detection, removal, and inactivation of bacteria, viruses, and protozoa. As technologies in the drinking water space continue to progress, new challenges have arisen in the form of opportunistic premise plumbing pathogens. 

Wastewater and stormwater utilities also play an essential role in reducing the pathogen load to receiving waters used for recreation.  Additionally, more recent advancements in water reuse, especially direct potable reuse, demand more understanding of pathogen detection, removal, and inactivation in wastewater. 

For more information, contact Grace Jang (drinking water & reuse) or Lola Olabode (wastewater).

Demonstrating the Effectiveness of Flushing for Reducing the Levels of Legionella in Service Lines and Premise Plumbing

Legionella are pervasive environmental bacteria that can incidentally cause severe and sometimes fatal infections upon inhalation. Because Legionella inhabit engineered environments and proliferate in warm, stagnant premise water systems, the majority of outbreaks are associated with preventable water system maintenance...

Interview with Cheryl Norton

Cheryl Norton’s Lasting Journey with WRF and the Water Sector From leading a Water Research Foundation (WRF)- funded project right out of college, to becoming an integral member of the...

Resource Recovery

In recent decades, the wastewater sector has moved away from the idea of wastewater treatment plants as waste disposal facilities, instead envisioning these plants as water resource recovery facilities (WRRFs). WRRFs can produce clean water, recover nutrients (such as phosphorus and nitrogen), and potentially reduce fossil fuel consumption through the production and use of renewable energy.

For more information, contact Jeff Moeller .

Recent Updates

Reporting Period: November 2023 – April 2024

Reporting Period: January 1 – April 15, 2024

Reporting Period: November 2023 – April 30, 2024

Reporting Period: August 1, 2023 – March 15, 2024

Reporting Period: September 15, 2023 – March 20, 2024

Reporting Period: October 1, 2023 – December 31, 2023

Reporting Period: December 7, 2023 – March 6, 2024

Reporting Period: October 1, 2023 – April 1, 2024

Reporting Period: January 1 – March 31, 2024

Reporting Period: December 1, 2023 – March 1, 2024

Throughout the year, WRF hosts and participates in events that focus on critical water quality issues. From web seminars to research workshops, these events provide opportunities for you to learn about new research from water quality experts and to share ideas and connect with other industry professionals.

Guidance for Complying with the Lead and Copper Rule Revisions for Water Systems with No- to Low Prevalence of Lead Service Lines (LSL, LSLs)

Advances in water research.

This issue highlights the essential research The Water Research Foundation delivered in 2023 thanks to the valuable contributions of our researchers, participating utilities, and countless volunteers.

Join our mailing list and receive news and updates in your inbox!

Research to uncover the impact of water use in the Colorado River Basin

The Colorado River is a lifeline for many cities and farms in the Southwest United States. It flows for about 1,448 miles before reaching the Gulf of California in Mexico and supplies water to numerous cities and farms along the way.

However, over the past 60 years, the amount of water in the Colorado River has been shrinking. In fact, in some years, the river's water has been used up completely before it reaches the gulf.

Landon Marston, assistant professor in civil and environmental engineering, teamed up with researchers from multiple universities and nongovernmental organizations to find out where the river's water goes and who uses it. This effort is informing state and federal decision-makers as they prepare to introduce new measures to bring the basin's water demands in balance with its dwindling supplies.

The research group, which includes two of Marston's graduate students, started by quantifying how much water people and businesses used from the Colorado River from 2000-19 from the records of the Bureau of Reclamation. These records serve as the basis for decision-making by local, state, and federal stakeholders concerned about the allocation of the Colorado River.

The group filled in gaps in federal records using a collection of models and data detailing crop-specific water consumption, water exported out of the basin by canals and pipelines, and evaporation from reservoirs and wetlands.

The study found that the agricultural demand for water is significantly higher than the water used by cities. The crops that need the most water are ones used for feeding cattle, such as alfalfa and hay, which are abundant in the area. The states that line the Colorado River raise roughly 14 million cattle per year.

"With water becoming scarcer and reservoir levels dropping, there is a growing need to find ways to use water more efficiently," said Marston. The reduction of water is crucial and will likely need to increase in the coming years due to competing demands and climate changes that reduce runoff into the Colorado River.

In 16 of the years analyzed, more water was taken from the river than what naturally flowed into it. To make up for the shortage, water was removed from reservoirs along the river, including Lake Mead and Lake Powell, which are among the largest reservoirs in the United States but have now become three-quarters empty.

The researchers found that the amount of water used actually varies a lot from year to year. In recent years, there has been less water used in the Lower Basin because of new rules that require more efficient use of water and cuts to the overall supply.

Another use of a significant amount of water is to support the natural environment along the river, including plants and wetlands.

"This is important because it wasn't accounted for in previous studies," said Marston. "This is not just about farms and cities. It is about protecting the river's ecosystem and ensuring a sustainable water supply for everyone in the southwestern United States."

Understanding where the water goes is crucial for making informed decisions about water use in the Colorado River Basin.

"The challenges are there," noted Marston. "But this data provides valuable information that can guide the seven states overlaying the Colorado River Basin and the federal government as they continue to negotiate how to share the dwindling water in the river."

• Drought Research
• Resource Shortage
• Environmental Policies
• World Development
• Funding Policy
• Water resources
• Water scarcity
• Anger management
• Sustainable land management

Story Source:

Materials provided by Virginia Tech . Original written by Courtney Sakry. Note: Content may be edited for style and length.

Journal Reference :

• Brian D. Richter, Gambhir Lamsal, Landon Marston, Sameer Dhakal, Laljeet Singh Sangha, Richard R. Rushforth, Dongyang Wei, Benjamin L. Ruddell, Kyle Frankel Davis, Astrid Hernandez-Cruz, Samuel Sandoval-Solis, John C. Schmidt. New water accounting reveals why the Colorado River no longer reaches the sea . Communications Earth & Environment , 2024; 5 (1) DOI: 10.1038/s43247-024-01291-0

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WERF and WateReuse Merge To Advance Concept of One Water

June 20, 2016

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Photo courtesy of the Water Environment & Reuse Foundation (Alexandria, Va.).

The Water Environment Research Foundation and the WateReuse Research Foundation have merged to form the Water Environment & Reuse Foundation (WE&RF; Alexandria, Va.). The merger, announced May 9, brings together the organizations’ expansive portfolio of research on water, wastewater, and stormwater topics. It also reflects on the sector’s movement toward the concept of “one water,” according to a WE&RF news release.

“Bringing these two organizations together allows us to leverage our expertise to pursue an integrated approach to critical research in water,” said WE&RF Board Co-Chair Kevin Shafer, executive director of the  Milwaukee Metropolitan Sewerage District, in the release. “We are now the largest one-stop-shop for research in resource recovery and reuse.”

The Water Environment Research Foundation was established in 1989 to research wastewater and stormwater. The WateReuse Research Foundation was established in 1993 to research water reuse. WE&RF will continue focusing research on resource recovery and reuse to help meet the growing demand for clean water. The nonprofit plans to identify, support, and disseminate research that enhances the quality and reliability of water for natural systems and communities with an integrated approach to resource recovery and reuse. More than 200 utilities; business, industrial, and commercial enterprises; educational institutions; and government agencies support it, the news release says.

“Our organizations shared a common goal, which is to produce research that solved real-world problems with water quality and supply,” said WE&RF Board Co-Chair Doug Owner, executive vice president and chief technical officer at Arcadis (Highlands Ranch, Colo.). “By merging, we will be able to help more communities produce a safe, reliable, locally controlled supply of water that protects the environment and supports economic growth.”

Read WE&RF Chief Executive Officer Melissa Meeker’s thoughts on the future of the water sector and merger in the WEF Highlights article, “ WERF Executive Director Melissa Meeker Discusses Water Sector Future and WERF–WRRF Merger .”

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Advances in the integration of microalgal communities for biomonitoring of metal pollution in aquatic ecosystems of sub-Saharan Africa

• Review Article
• Open access
• Published: 01 June 2024

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• Mary Mulenga   ORCID: orcid.org/0009-0008-9198-280X 1 , 4 ,
• Concillia Monde   ORCID: orcid.org/0000-0001-9856-9120 2 , 4 ,
• Todd Johnson   ORCID: orcid.org/0000-0001-6346-5604 1 ,
• Kennedy O. Ouma   ORCID: orcid.org/0000-0003-2902-9723 2 &
• Stephen Syampungani   ORCID: orcid.org/0000-0003-2629-5807 3 , 4 , 5  

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This review elucidated the recent advances in integrating microalgal communities in monitoring metal pollution in aquatic ecosystems of sub-Saharan Africa (SSA). It also highlighted the potential of incorporating microalgae as bioindicators in emerging technologies, identified research gaps, and suggested directions for further research in biomonitoring of metal pollution. Reputable online scholarly databases were used to identify research articles published between January 2000 and June 2023 for synthesis. Results indicated that microalgae were integrated either individually or combined with other bioindicators, mainly macroinvertebrates, macrophytes, and fish, alongside physicochemical monitoring. There was a significantly low level of integration (< 1%) of microalgae for biomonitoring aquatic metal pollution in SSA compared to other geographical regions. Microalgal communities were employed to assess compliance (76%), in diagnosis (38%), and as early-warning systems (38%) of aquatic ecological health status. About 14% of biomonitoring studies integrated microalgal eDNA, while other technologies, such as remote sensing, artificial intelligence, and biosensors, are yet to be significantly incorporated. Nevertheless, there is potential for the aforementioned emerging technologies for monitoring aquatic metal pollution in SSA. Future monitoring in the region should also consider the standardisation and synchronisation of integrative biomonitoring and embrace the “Citizen Science” concept at national and regional scales.

Graphical abstract

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Introduction

Stream ecosystems play an ecologically significant role in supporting aquatic biodiversity and providing beneficial ecosystem services that sustain the environment and promote human well-being (Limburg 2009 ; Maes et al. 2020 ). Stream ecosystem services include fresh water and food provisioning, sediment retention and transport, pollution control, recreation and ecotourism, flood regulation, disease prevention, nutrient cycling, and cultural heritage preservation (MEA 2005 ). In addition, both stream and riparian systems serve as biodiversity banks for aquatic and semi-aquatic biota as well as developmental stages of several terrestrial fauna such as arthropods, amphibians, and reptiles, among others (Mccabe 2010 ; Steward et al. 2022 ). Streams are also critical in transferring nutrients, matter, and energy, thus acting as sources and sinks of pollutants and disease vectors (Limburg et al. 2013 ; Wohl 2018 ; Bashir et al. 2020 ).

However, the negative impact of both natural and anthropogenic pressures has reduced the ability of stream ecosystems to supply aquatic ecosystem services (Khatri and Tyagi 2015 ). Natural factors such as climate change, droughts, floods, and other natural disasters impact aquatic ecosystems by altering water availability, water quality, and aquatic biodiversity (Costanza et al. 2014 ; Talbot et al. 2018 ; Culhane et al. 2019 ). Anthropogenic factors such as waste disposal, urbanisation, demand for agricultural land and expansion of industrial activities such as metal mining and fossil fuel combustion, and habitat destruction are also significant drivers of deterioration in aquatic ecosystems (Borgwardt et al. 2019 ; Cormier et al. 2019 ; Kimirei et al. 2021 ; Ferreira et al. 2023 ). Metal pollution of aquatic ecosystems from natural and anthropogenic sources is particularly an environmental and health concern in metal mining regions due to metals being persistent, non-biodegradable, and toxic (Yahya et al. 2018 ; Ali and Khan 2019 ; Amoatey and Baawain 2019 ; Zhou et al. 2020 ). In the stream ecosystems of Sub-Saharan Africa (SSA), aquatic metal pollution is an ever-growing environmental concern (Biney et al. 1994 ; Fayiga et al. 2018 ). There has been a steady accumulation of metals in water, sediment, and aquatic biota in rivers and lakes of SSA, mainly from natural and anthropogenic sources (Zhou et al. 2020 ; Yabe et al. 2010 ; Fayiga et al. 2018 ; Ochieng et al. 2009 ). In southern Africa, aquatic metal pollution above permissible limits has been reported for stream water and sediments from mining, coal use, and other industrial activities (Ouma et al. 2022 ; Addo-Bediako et al. 2021 ). Gerber et al. ( 2015 ) and Moyo et al. ( 2015 ) noted high Cu, Co, Pb, and Mn in the Olifants River associated with anthropogenic activities and posed a high risk to aquatic biota. Furthermore, Chetty and Pillay ( 2019 ) linked the influence of anthropogenic activities to elevated Cr, Cu, Pb, and Zn in Palmiet and Sezela rivers in South Africa’s Kwa-Zulu Natal coupled with high mobility and bioavailability. In the Zambian Copperbelt, Cu, Co, Pb, and Zn mining has impacted the water and sediments of Lake Kariba (Chalumba et al. 2021 ) and Kafue River with increased ecological risks to aquatic life. In the Katangese Copperbelt of the Democratic Republic of Congo, extreme sediment enrichment with Cu (190.2 mg/kg) and Zn (1117 mg/kg) in the Bumbu River draining Kinsasha has been reported (Kayembe et al. 2018 ). Banze wa Mutombo ( 2022 ) also associated the high pollution of the Mura and Kimpulande tributaries of the Congo River with Cu-Co-As-Cd-loaded mining effluents that increased the vulnerability of aquatic communities.

Metal pollution in West Africa’s aquatic systems has similarly reached alarming levels. Gbogbo and Otoo ( 2015 ) reported the detrimental impacts of Cd, As, Hg, and Cu pollution on water, macrophytes, algae, and fish in Ghana’s Sakumo II wetland in the Tema Metropolitan area. According to Ngueyep et al. ( 2021 ), Cameroon’s Kadey River tributaries had excess Ni, Fe, Cr, Se, As, and Hg in sediments from artisanal and small-scale gold mining. Tyovenda et al. ( 2019 ) reported contamination of water and algae and sediment enrichment with Pb, Hg, Ni, and Fe in River Benue, Nigeria. Despite the relatively low mineral deposit in Eastern Africa, alarming aquatic metal pollution has been reported in Kenya’s gold mining belt (Ngure et al. 2014 ), Tanzania’s Mara River (Nkinda et al. 2020 ), Awetu watershed in Ethiopia (Astatkie et al. 2021 ), and Namukombe stream in Uganda (Omara et al. 2019 ). For instance, Ngure et al. ( 2014 ) noted high Hg (355 mg/kg) in fish, while Astatkie et al. ( 2021 ) recorded stream sediment contamination with Pb (2,000 mg/kg), As (623 mg/kg), and Cr (375 mg/kg). Based on the representative studies above, there is sufficient evidence of aquatic metal pollution thus, raising the need for monitoring of aquatic ecosystems across SSA.

One of the approaches that can be employed to effectively monitor and assess the magnitude of anthropogenic and natural impacts on stream ecosystems is the bioindicator concept, which utilises sentinel aquatic biota (Lazorchak et al. 2003 ; Schwacke et al. 2013 ; Parmar et al. 2016 ). Bioindicators reflect the bioavailable fractions of pollutants and hence are of potential ecotoxicological significance (Hamza-Chaffai 2014 ; Lamare 2019 ; Kumari and Paul 2020 ) Based on the targeted outcome, three categories of bioindicators commonly used for monitoring environmental health include early warning, compliance, and diagnostic indicators (Hamza-Chaffai 2014 ). “Early-warning bioindicators” signify the impending deterioration of ecosystem health. Deviations from the acceptable aquatic environmental conditions are detected by “compliance indicators” while the “diagnostic bioindicators” reflect the causes for the deviations from the expected ecological conditions of the aquatic ecosystem (Sumudumali and Jayawardana 2021 ). Figure  1 illustrates the bioindicator concept of aquatic metal pollution biomonitoring that utilises the compliance, diagnostic, or early-warning aspects of indicator species or communities.

Conceptualising integrative monitoring of aquatic metal pollution in stream ecosystems

Aquatic microalgal communities have been utilised to monitor the ecosystem health in lotic environments (Yilmaz et al. 2021 ; Feisal et al. 2023 ). The vital ecological role of microalgal communities has been recognised through continuous surveillance to establish their status in the aquatic environment (Li et al. 2021 ; Thompson and Carstensen 2023 ). The ability of microalgae to accumulate high levels of pollutants, their relative sessile nature, ease of sampling, ease of culture in the lab, and their trophic importance as primary producers in the aquatic food web have positioned them as suitable bioindicators of metal pollution (Zhou et al. 2008 ; Parmar et al. 2016 ; Kumari and Paul 2020 ). Freshwater microalgae occur either as planktonic, which dominate the pelagic zone, or the benthic forms associated with substrates such as sediments, rocks, macrophytes, mud, and organic debris (Bellinger and Sigee 2015 ).

Globally, the use of microalgae for biomonitoring as early-warning signals has been widely documented. For instance, in Europe, Dokulil et al. ( 2016 ) documented long-term historical biomonitoring using microalgae responses and community composition in the extensive Danube River hydro system. Furthermore, biomonitoring of metal pollution in the transboundary Danube River delta aquatic complex reported high concentrations of bioavailable Ni, Cd, and other potentially toxic elements (Burada et al. 2015 ; Simionov et al. 2021 ; Calmuc et al. 2021 ). Metal pollution trends were also observed in the southeastern Brazil river basin impacted by metal contamination from the Mariana dam failure, with increased Hg bio uptake by microalgae (Marques et al. 2022 ). Silva et al. ( 2022 ) further reiterated the significance of using morphological and taxonomic responses of microalgae as bioindicators to environmental changes in river basins of southern Brazil. In India, microalgal communities in the tropical freshwater Godavari River, Cu, and Zn exhibited lethal effects at high concentrations for the dominant cyanobacteria and chlorophytes (Chakraborty et al. 2010 ). Feng et al. ( 2021 ) also noted the detrimental impacts of metal pollution on the microalgal community structure, with certain microalgal species being more sensitive to the bioavailable metals in the Yangtze River in China. In the Sefid Rud River, Iran, changes in microalgal assemblages were suitable bioindicators of environmental variability and corresponded to physical and chemical changes in the south Caspian Sea catchment (Ramezanpour et al. 2014 ).

Microalgal communities have also been used to monitor metal pollution in Africa’s stream ecosystems. In West Africa’s Niger River system, Ezewudo et al. ( 2021 ) noted weak to high potential ecological risks to aquatic communities, including microalgae, from As, Cd and Hg contamination. In the Cameroon Centre Region, the spatial-seasonal changes in algal densities in the streams of the Sanaga lotic system network draining urban and industrial settlements corresponded to changes in the aquatic physicochemical environment (Pascale 2023 ). According to Mangadze et al. ( 2019a ), several ecological health studies on southern Africa’s rivers have applied benthic diatoms for biomonitoring. Dalu et al. ( 2014 ) noted a direct response of microalgal communities to changes in the physicochemical environment of the Kowie system riverine-estuarine continuum in South Africa’s Eastern Cape. Recent studies on South Africa’s urban Molopo River depicted anthropogenic Cu, Cr, Zn, and Pb sediment contamination with potentially deleterious ecological impacts on the benthic algal and macrofauna communities (Mohajane and Manjoro 2022 ). Additionally, the diatom-based biomonitoring tools (e.g., the “South African Diatom Index (SADI)” and the “Benthic Diatom Index (BDI)”) have been used to detect and quantify the magnitude of natural and anthropogenic impacts on stream ecosystems (Lang et al. 2013 ; Harding and Taylor 2014 ; Sirunda et al. 2021 ).

Microalgae have the potential to be integrated into conventional monitoring programs as complementary tools to increase the resolution in detecting sub-lethal contamination and thus serve as early-warning bio-systems (Cid et al. 2012 ; Bae and Park 2014 ). Despite the potential of integrating microalgal communities in the biomonitoring of aquatic ecosystems, this approach remains one of the least explored alternatives to sustainable management of freshwater ecosystems in SSA (Lemley et al. 2016 ). Therefore, this review seeks to (1) provide insights into recent advances in the integration of microalgae in biomonitoring metal pollution in the SSA lotic systems, (2) highlight the potential of integrating microalgal as bioindicators in the emerging technologies for monitoring aquatic metal pollution of lotic systems, and (3) identify research gaps and suggest directions for further research in the integrating microalgae as bioindicators of metal pollution in lotic systems of SSA.

Methodological approach

Scope of literature search.

To ensure that high-quality and relevant articles were selected, our review defined explicit inclusion criteria outlined in Cornelissen et al. ( 2009 ). The literature search included articles addressing advances in integrating microalgae for biomonitoring metal pollution in stream ecosystems draining metal-mining landscapes of SSA. The search was restricted to original research, written in English, from articles published between January 2000 and June 2023 to identify ‘gold-standard’ literature on stream biomonitoring of metal pollution with a focus on microalgae as bioindicators.

The article selection process aims to identify the original research papers that present clear evidence of the study objectives (Syeed et al. 2023 ). Page et al. ( 2021 ) preferred reporting items for systematic review and meta-analysis (PRISMA) protocol was followed to ensure a comprehensive and well-defined strategy for the identification, screening, and inclusion of articles for review (Fig.  2 ). Reputable academic databases, SCOPUS, Taylor and Francis, and Semantic Scholar were searched for authentic articles (Kitchenham and Charters 2007 ). Furthermore, snowballing or citation-searching from “gold-standard” literature was used to identify more articles for preliminary screening (Wright et al. 2014 ).

The PRISMA protocol for identifying, screening, and including literature for the review

The literature search was conducted based on the article title, abstract, and keywords using key terms such as “bioindicator”, “aquatic biomonitoring”, “metal mining”, “aquatic pollution”, “algae”, “aquatic ecosystems”, “sub-Saharan Africa”, “Africa”, “e-DNA”, “environmental DNA”, “metagenomics”, “microalgae”, “Biosensors”, “ Remote sensing ”, and “ Citizen science ”. From each article included, the following information was extracted: (i) title, (ii) authors, (iii) publication year, (iv) regional distribution (v) main objective, (vi) methods (physical, chemical, biological) (vii) indicator organism(s), (viii) environment (sediment, water, biota), (ix) pollutant(s) (x) microalgal metrics used, and (xi) key findings, gaps, or recommendations.

Streamlining article evaluation and selection

Quality assurance and bias reduction.

To remove bias in the first stage of the search, the authors searched independently in the digital databases using search terms with slightly varying synonyms to maximise the extraction of articles from the global search. This initial search was followed by the within-results search, where the authors used the same filter criteria specifying the period, the document type, the region of study, and the field of study. In the second stage, the authors verified the extracted articles’ metadata for completeness and originality. Articles that fulfilled the quality assurance process were included for further synthesis.

Article processing

The results from the databases were downloaded and imported into the Mendeley reference software (Mendeley Ltd). The following metadata was checked and updated where necessary for each article: author(s), title, year of publication (and month), volume, page numbers or article number, abstract, keywords, and DOI, if available. However, articles for which pertinent metadata items such as author, title, or publication year that were missing were further excluded from the list.

Exclusion process

An automated keyword-based search was used to explore the database and extract relevant research articles (Beecham et al. 2008 ). The terms were searched in the article titles, abstracts, and keywords. The exclusion criteria for out-of-scope articles were principally based on the following aspects: (1) studies outside freshwater systems, e.g., oceans and seas; (2) other bioindicator categories used, e.g., non-photosynthetic bacteria, marine plankton, freshwater zooplankton, macrophytes, macroalgae, macroinvertebrates, and vertebrates; (3) clinical and laboratory biomonitoring studies, e.g. humans and wild and domestic animals using water resources; and (4) studies involving aquatic ecosystem pollutants other than metals. Furthermore, manual removal was conducted to ensure that only relevant and complete articles were included in the final review process (Petticrew and Roberts 2008 ).

Bibliometric analysis

A bibliometric analysis of the extracted information was conducted to classify articles based on the year of publication, authors, region, main objective(s), bioindicator type, environmental matrix, pollutant(s), methods, and the microalgal metric(s). Following the PRISMA filtering protocol, the review included 21 articles (15 from the digital scholarly databases and six from snowballing/citation search) relevant to the research area, geographical location, and study period. From Fig.  3 a, between January 2000 and June 2023, there was a notable general cumulative 95% increase in the studies incorporating different microalgal taxa in monitoring metal pollution in aquatic ecosystems in the SSA. This indicates a growing interest in incorporating microalgal taxa in aquatic biomonitoring.

a Distribution of publications by year and research focus and b regional proportions of microalgae and environmental DNA (eDNA) integration in the monitoring of metal pollution in aquatic ecosystems of sub-Saharan Africa

Generally, between January 2000 and June 2023, studies indicate that only South Africa, Namibia, and the Democratic Republic of Congo (DRC) integrated microalgae and microalgal-based eDNA, respectively, in aquatic biomonitoring for metal pollution. Sub-regionally, only 24% of the countries in West Africa, 10% in Eastern Africa, 25% in Central Africa, and 75% in Southern Africa conducted microalgal-based biomonitoring of metal pollution in streams. However, microalgal-eDNA integration in aquatic metal pollution biomonitoring is still in its infancy in SSA, with only Central and Southern Africa accounting for 25% and 75% of aquatic-based research to monitor metal pollution (Fig.  3 b). The integration of microalgal-eDNA method in aquatic metal pollution biomonitoring in SSA was first documented by Jordaan et al. ( 2019 ), who noted a 6% variability in bacterial community composition and diversity from the anthropogenic Co, As, Cr, Ni, and U pollution in the rivers within the lower Wonderfonteinspruit catchment of South Africa. Since then, the eDNA approach has been seen as a potential approach to accelerate aquatic biomonitoring by supplementing traditional taxomorphological monitoring in the SSA landscape (Perry et al. 2022 ).

Bioindicator taxa and environmental assessment

The diatoms are the single most preferred microalgal bioindicator taxon (36%) and are also used with benthic macroinvertebrates (18%) to monitor aquatic metal pollution. Considering their specificity and sensitivity to ecological changes in aquatic ecosystems, diatoms have been widely employed to detect perturbations in stream water quality (Lobo et al. 2016 ; Mangadze et al. 2017 ). The preference for both taxa could be attributed to their stationary and benthic nature, which makes them suitable for recording long-term pollutant trends compared to the instantaneous physicochemical methods that only consider a “snap-shot” of the environmental water quality (Beyene et al. 2009 ; Hattikudur et al. 2014 ). Other single-use taxa of microalgae, including cyanobacteria, had equal preferences (18%), while “algae” and macroinvertebrates comprised 9% each as the bioindicators used to assess metal pollution (Fig.  4 a).

a Bioindicator integration in aquatic metal pollution biomonitoring in SSA; b environmental matrices investigated for metal pollution. Macroin macroinvertebrates, Phytopl phytoplankton, Sed sediment

Regional studies elsewhere, for example, in North America (e.g., Smucker et al. 2018 ) and Asia (e.g., Chon et al. 2013 ), have similarly integrated algae and macroinvertebrates as well as microbial communities to monitor metal pollution of freshwaters while leveraging on the producer–consumer trophic changes as indicators of disturbances at catchment scale. Furthermore, Respondek et al. ( 2022 ) integrated mosses and microalgae in monitoring metal pollution in surface water in the smelter area of Ozimek, Poland. They observed diatom taxa as the dominant algal group that indicated responses to metal stress, e.g., the metal-tolerant Achnanthidium sp and Mayamaea sp dominated up to 99% of the algal communities, and served as excellent bioindicators of metal contamination. In addition, Pandey ( 2020 ) compared green algae , cyanobacteria, and diatom species and noted an increased relative taxa abundance, indicating increased tolerance to metal pollution. Moreover, increased lipid production and cell-wall teratologies in diatoms, also indicated by Lavoie et al. ( 2012 ), were observed under Cu, Cd, Zn, and Pb stress. Pandey and Bergey ( 2018 ) also found that diatoms-dominated periphyton biofilms were excellent indicators of metal pollution, thus showing the utility of periphytic diatom communities as an effective tool for biomonitoring of aquatic metal pollution. Gbogbo and Otoo ( 2015 ) used the biomonitoring potential of algae, among other bioindicator biota of an urban wetland system in Ghana, to determine the magnitude of metal pollution algae accumulated up to 12 mg/g Cd. Similarly, Leguay et al. ( 2016 ) and Solak et al. ( 2020 ) reiterated the importance of complementing physicochemical assessment techniques with diatom-dominated biofilm-based proxies, diatom indices (e.g., the Pampean Diatom Index and Specific Pollution Index) to monitor metal contamination in aquatic systems.

In Fig.  4 b, water is the most frequently assessed abiotic matrix (28.6%) and in combination with different bioindicators in the same proportion for assessing metal contamination. Studies by Dalu et al. ( 2017 , 2022a ) and Tyovenda et al. ( 2019 ) included stream sediment plus water, diatoms, algae, and benthic macroinvertebrates in the evaluation of metal pollution to obtain a three-way health status of the aquatic ecosystem. In addition, Mangadze et al. ( 2017 ) incorporated a fourth dimension of atmospheric contribution to stream ecosystem metal pollution to assess the potential of using diatoms as suitable bioindicators of ionic metal pollution along a South African temperate river system.

Application of microalgae for biomonitoring tropical stream ecosystems of SSA

In Fig.  5 and Table  1 , three studies (by Jordaan et al. 2019 Laffite et al. 2020 Perry et al. 2022 ) incorporated the microalgal-eDNA to check for environmental compliance with the established national or international guidelines for metal contaminant levels in freshwater aquatic environments. Jordaan et al. ( 2019 ) and Pereira‐da‐Conceicoa et al. ( 2021 ) used eDNA as a diagnostic tool to determine the causes of deteriorating water quality and changes in microbial communities in South African river catchments. However, no study used microalgal-eDNA for early warning of aquatic ecosystem change, making this a potential area for future research. Most studies (76%) employed various microalgal taxa responses for compliance monitoring, followed by diagnostic and early-warning functions, each at 38% (Fig.  5 ).

Application of the microalgal taxa in the integrated monitoring of the environmental conditions of aquatic ecosystems exposed to metal pollution in SSA

Several studies combined more than one environmental application of algal communities to test for compliance, diagnosis, or early warning to evaluate the overall integrity and potential ecological risks for the respective aquatic ecosystems investigated. Despite its environmental importance, only 30% of the studies used biomonitoring as an early-warning tool, while 40% employed biomonitoring for diagnostic purposes. Nevertheless, 91% of the studies were targeted to determine environmental compliance of anthropogenic activities that introduce metal contaminants to the aquatic environment against the set effluent discharge standards in the pro-active management of aquatic ecosystems in SSA. Table 1 highlights the environmental applications (compliance, diagnostic, and early warning) of microalgal communities based on the main objectives of the reviewed studies. Algal communities are helpful for compliance, diagnostic, or early-warning biomonitoring since they reflect long-term changes in stream water quality (Mangadze et al. 2016 ) (Table  1 ). Ugbeyide and Ugwumba ( 2021 ) assessed the physicochemical and biological status of the Ibuya River in Nigeria, which was impacted by anthropogenic pollution. Cd (0.003 mg/L) and Pb (3.5 mg/L) levels exceeded permissible limits for surface water quality, while the lower species richness and composition, dominated by Bacillariophyceae, reflected a lotic system impacted by allochthonous pollution. Oberholster et al. ( 2016 ) observed increased algal species diversity caused by improved downstream water quality during the rehabilitation of the Grootspruit wetland, South Africa, impacted by acid-mine drainage. The trends concur with a previous study by Ali and Abd el-Salam ( 1999 ) that noted changes in the dominance of microalgal species Cyclotella and Nitzschia (Bacillariophyta), Actinastrum and Scenedesmus (Chlorophyta), and Oscillatoria sp (Cyanophyta).

Furthermore, in the Macedonian Maidanska River, the bioconcentration and biomineralisation of Cu, As, Cr, Se, and Cs were observed in Audouinella sp, while the high bioaccumulation of Ba (3 mg/g) and intracellular biomineralisation were evidenced in Spirogyra sp. thereby positioning these algal species as a biological pathfinder for acid-mine drainage deposits (Bermanec et al. 2018 ). Water and sediment chemistry, including nutrient and metal pollutants, largely influence the stream algal community composition. Dalu et al. ( 2017 ) explored the influence of anthropogenic impacts on diatom communities and noted the dominance of pollution-tolerant taxa in an austral temperate stream in South Africa. The tolerance and morphological changes (teratologies) on epilithic diatom communities have also been employed to monitor and quantify the biological effects of metal stress from an abandoned Coval da Mo mine drainage (Ferreira da Silva et al. 2009 ). The findings agree with Pandey and Bergey ( 2018 ), who correlated non-taxonomical parameters, including teratologies and lipid bodies, to indicate metal toxicity and recovery in fluvial systems. Diatom indices, including the GDI (Generic Diatom Index), BDI (Biological Diatom Index), and TDI (Trophic Diatom Index), were successfully employed to monitor the Dongjiang River in China with BDI and GDI showing an apparent response to water quality changes (Deng et al. 2012 ). Diatoms have also been incorporated in multispecies biomonitoring of the temporal variability of metal pollution in Nigeria’s Calabar River (Hena et al. 2022 ) and Kebena-Akaki Rivers, Ethiopia (Beyene et al. 2009 ). In both studies, a significant response was observed between the algal community structure and metal concentrations. Mangadze et al. ( 2017 ) similarly reflected the role of diatom assemblages as bioindicators of metal pollutants (e.g., As, Zn, Cu, and Cr), particularly on low pollution tolerant species such as Fragilaria , Cyclostephanos , and Gyrosigma transition to high pollution tolerant forms (e.g., Nitzschia and Gomphonema ). This observation is also supported by findings in Dalu et al. ( 2022b ), where changes strongly influenced the structure of diatom communities in water and sediment quality due to the presence of metal contaminants such as B, Cu, and Fe in the Krom River system of the western cape, South Africa. Microalgal communities have also been used to indicate metal pollution in lacustrine systems. For instance, Ogoyi et al ( 2011 ) determined metal concentrations (Zn, Pb, Cd, Cr, and Hg) in algal communities alongside water and sediment as an integrative aquatic ecosystem assessment approach.

Integrating microalgal communities into molecular tools for compliance, diagnostic, or early-warning monitoring of streams in mining regions of SSA is also ongoing. Jordaan et al. ( 2019 ) studied the influence of anthropogenic pollution on the structure and function of aquatic bacterial communities, using 16S rRNA as a proxy indicator, in South Africa’s Wonderfonteinspruit river catchment. Pereira-da-Conceicoa et al. ( 2021 ) demonstrated the merits of incorporating eDNA into existing aquatic biomonitoring metrics with the potential of recovering more diversity and a higher resolution. The ecological advantages of integrating eDNA studies in aquatic biomonitoring above are also evident in other global investigations. Li et al. ( 2018 ) noted that the operational taxonomic units of molecular e-DNA data can predict up to 79% of aquatic pollution. Ancion et al. ( 2010 ) used 16S rRNA gene libraries to examine the impact of Cu, Zn, and Pb on bacterial communities embedded in freshwater biofilms and recorded higher sensitivities, thereby confirming their potential role as compliance indicators of stream health.

Method integration and environmental and biological metrics used for assessment of metal pollution in aquatic ecosystems of SSA

All the reviewed microalgal-based works combined physicochemical and biological techniques to investigate metal pollution, possibly to enhance the detection of contaminants and their impact on biota (Torrisi et al. 2010 ). Several environmental and biological indices were used to quantify the magnitude of the impact of metal pollution, including enrichment and contamination factors, pollution indices, species richness, and diversity indices (Bere et al. 2016 ; Dalu et al. 2022a ; Mangadze et al. 2019b ; Ugbeyide and Ugwumba 2021 ) (Table  2 ). According to Lobo et al. ( 2016 ), biotic indices such as Beck’s index and Renberg’s “Index B” developed from the relative abundances of bioindicator species have been employed for biomonitoring of streams and other aquatic ecosystems. The determination of the physicochemical water quality coupled with the estimation of aquatic biodiversity based on biotic indices has been used to infer the ecological health status and as “early-warning” indicators of aquatic ecosystem health changes (Bellinger and Sigee 2015 ; Forio and Goethals 2020 ). Geochemical indices such as contamination factor (CF), enrichment factor (EF), geo-accumulation index (I geo ), and pollution load index (PLI) were used to evaluate the occurrence and magnitude of pollution in SSA streams receiving metal(loid) contaminants (Tyovenda et al. 2019 ; Hena et al. 2022 ). Changes in algal community composition, abundance, PLI, and metal pollution index (MPI) have also been used to assess aquatic metal pollution stress in aquatic communities of Egypt’s Alexandria coast (Ismail and El Zokm 2023 ).

Integrating molecular techniques in biomonitoring is a potential approach to revolutionise aquatic pollution assessment (Li et al. 2010 ; Lobo et al. 2016 ). In this review, Laffite et al. ( 2020 ) observed a significant correlation between metals and 16 s rRNA, suggesting a close link between metal pollution and human-mediated pressures on an urban river in the Democratic Republic of Congo. Pereira-da-Conceicoa et al. ( 2021 ) demonstrated the relevance of integrating environmental DNA (eDNA) into existing monitoring metrics to provide additional taxonomic resolution for aquatic biodiversity management in South African streams. The application of molecular methods has also been observed to substantially improve the biomonitoring of streams in France, China, and Switzerland compared to the traditional morphotaxonomic methods (Apothéloz-Perret-Gentil et al. 2021 ; Keck et al. 2018 ; Li et al. 2018 ). However, Perry et al. ( 2022 ) noted a significant drawback in the integration of eDNA principally inhibited by inadequate reference data for SSA in the gene banks. The lack of reference eDNA databases, downstream transport, dilution of DNA fragments, and introduction of terrestrial DNA, among other challenges, has also been observed in other regions, e.g., Finland (Norros et al. 2022 ), Switzerland (Deiner et al. 2016 ), Canada (Laporte et al. 2022 ), and globally (Beng and Corlett 2020 ).

Integrating microalgae into emerging technologies for monitoring metal pollution in stream ecosystems

Several cutting-edge emerging technologies are gaining popularity as complementary approaches to support conventional monitoring and assessments of stream ecosystems, as described below.

Microalgal-eDNA metabarcoding

Based on the current review, recently, limited studies have incorporated microalgal eDNA in biomonitoring aquatic metal pollution in the SSA. For instance, Laffite et al. ( 2020 ) investigated the co-contamination and seasonal variability of metal in bed sediments of urban rivers in DRC using bacterial eDNA. Significant correlations were observed between metal concentrations and 165 s rRNA bacterial densities, linking pollution to anthropogenic inputs. In South Africa, Jordaan et al. ( 2019 ), using the 16 s rRNA gene profiles, noted a substantial impact of pH and metal contamination from mining on bacterial diversity and community structure in the lower Wonderfonteinspruit catchment rivers. Furthermore, Perry et al. ( 2022 ) demonstrated the cost–benefit of using bulk samples and eDNA for multispecies biodiversity monitoring of Namibia’s freshwater systems. However, in most SSA countries, few studies, if any, have integrated microalgal eDNA in aquatic metal pollution biomonitoring. Given the sparsity of eDNA biomonitoring research data in SSA, more effort is needed to develop methods adapted to regional and local conditions and to generate eDNA gene-bank reference data to increase our understanding of SSA aquatic ecosystems (Perry et al. 2022 ). In addition, the performance of eDNA tools in biomonitoring aquatic metal pollution in SSA lotic ecosystems compared with the conventional monitoring approaches is not adequately investigated. Therefore, further research is needed to address this methodological gap by integrating microalgae-based eDNA biomonitoring of aquatic metal(loid) pollution at the community, species, and molecular level in stream ecosystems of SSA (Stat et al. 2017 ).

In the recent past, most of the eDNA biomonitoring has been conducted in the global North (Resh 2007 ). For instance, Cilleros et al. ( 2019 ) compared the effectiveness of eDNA metabarcoding and conventional morphotaxonomic techniques while assessing the diversity of fish assemblages in 38 streams of the French Guiana. Their findings revealed that while traditional taxonomic methods offered a more comprehensive inventory of fish taxa, they were spatially limited. In contrast, eDNA metabarcoding, when complemented with classical methods, was a more comprehensive and efficient approach for rapidly assessing and monitoring fish diversity on a larger spatial scale. Similarly, Gleason et al. ( 2021 ) conducted a study in southern Ontario, Canada, comparing eDNA metabarcoding techniques with traditional kick-net sampling to monitor lotic macroinvertebrate communities. Their findings demonstrated that eDNA techniques, especially metabarcoding of bulk tissues, provided a better representation of the diversity of macroinvertebrate taxa at a finer spatial resolution than traditional methods. However, in SSA, few studies have integrated eDNA in aquatic biomonitoring of metal pollution (e.g., Laffite et al. 2020 ; Jordaan et al. 2019 ; Perry et al. 2022 ). Therefore, progressive regional research must be strengthened to overcome the current limitations of aquatic eDNA biomonitoring, such as inadequate e-DNA reference data (Perry et al. 2022 ).

While acknowledging that eDNA is more appropriate for short-term monitoring, eDNA data can be used integratively with long-term monitoring approaches, such as remote-sensing, biosensor, and citizen science (Hansen et al. 2020 ). For instance, eDNA data can be used to validate or ground-truth remotely sensed data to ensure the reliability of long-term monitoring systems. Additionally, the integration of eDNA can increase the resolution of pollutant detection at sub-lethal and ensure the validity and consistency of sensed data.

Biosensor systems for aquatic biomonitoring

Recently, a variety of biosensors gained high attention and have been employed in in-situ for real-time monitoring and detection of environmental contaminants (Huang et al. 2023 ). A biosensor typically comprises a biosensing probe and a transducer that detects a contaminant by producing a quantifiable signal (Mishra et al. 2019 ; Rovira and Domingo 2019 ). The biosensor probe material can be antibody-, DNA-, whole-cell-, or enzyme-based (Singh et al. 2020 ). The transducer translates the biological signals to optical or electrical signals via optical, physicochemical, or piezoelectric material (Nguyen et al. 2019 ). Electrochemical and optical biosensors have been employed to detect and quantify metals, including Hg + , Pb 2+ , Zn 2+ , Cu 2+ , and Cd 2+ in water (Wu et al. 2023 ). Advances in nanotechnology have further improved the performance of biosensors due to the numerous benefits of larger sensing equipment. Nanosensor materials improve biosensor efficiency for colour sensing, target sensitivity, and carrier capacity. Additionally, nanomaterials have high thermal and electrical conductivity (Huang et al. 2021 ; Abdel-Karim 2024 ).

Whole-cell microbial biosensors detect metal ions based on the genetic element that responds to target metals (Huang et al. 2023 ). In aquatic environments, whole-cell bacterial biosensors have been used to detect bioavailable metals with high sensitivity (Cerminati et al. 2015 ). Alfadaly et al. ( 2021 ) applied a complementary target resistive Rhizobium bacteria-based and Rhodotorula fungi-based bioelectrochemical sensor to detect and remove Cr 6+ and Cd 2+ ions from polluted water. The bacterial component exhibited superior performance for metal resistivity and removal. In another study, Cerminati et al. ( 2015 ) confirmed the efficacy of a broad-spectrum whole-cell-based metal biosensor as a screening tool for the presence of bioavailable Au, Hg, Pb, and Cd in water.

Genetically engineered DNA-based microbial biosensors combined with electrochemical transducers broaden the applicability of cell-based biosensors for early monitoring and detection of metal ions in water Jeon et al. ( 2022 ). According to Jeon et al. ( 2022 ), the mutation of a regulatory protein ZntR in Escherichia coli enhanced the selectivity of Pb 2+ ions after metal ion-exporting genes were deleted in the host cells. Furthermore, Nourmohammadi et al. ( 2020 ) observed high specificity for Pb 2+ bacterial biosensor expressing a luciferase reporter gene controlled by pbr / cadA promoters in Cupriavidus metallidurans in a genetically engineered bacterial system.

According to Huang et al. ( 2023 ), biosensors are low-cost, easy to use, and energy-saving and require minimal pre-sample treatment. In addition, biosensor technology uses non-hazardous materials and has considerably low carbon footprints compared to physicochemical methods. Furthermore, the integration of bacteria into biosensor technologies offers numerous benefits in the detection and monitoring of aquatic metal pollution. Biosensors and microalgae serve as complementary tools, offering different perspectives and capabilities. Biosensors enhance the detection and quantification of metal contaminants in real-time or near real-time, thereby allowing for rapid detection and tracking of metal pollution (Wu et al. 2023 ).

In contrast, microalgae are reliable bioindicators of long-term exposure to metal pollution, reflecting the historical trend. A combination of biosensor data with microalgae assessments reflects a comprehensive understanding of short and long-term metal pollution dynamics in impacted streams. Additionally, biosensors often exhibit high sensitivity and specificity for detecting target metal ions, enabling the detection of sub-lethal metal concentrations (Huang et al. 2023 ). Microalgae, while sensitive to metal pollution, may not always provide precise measurements of metal concentrations at low levels or in complex environmental matrices. The integration of microalgal DNA into biosensors has the advantage of sensitivity and specificity, especially in natural environments with multi-elemental metal contaminants. Furthermore, biosensors are robust and can simultaneously be deployed at multiple locations within stream ecosystems. This spatial advantage complements the localised application of microalgae per time to monitor metal contamination.

Additionally, biosensors are often portable and easy to deploy, making them accessible for field-based monitoring in remote or challenging environments. However, the high cost and technological requirements of nanomaterials production could impede the production and application of nanobiosensors, particularly in developing countries.

Remote sensing

Satellite-based remote sensing (RS) and hyperspectral imaging is a cost-effective monitoring approach that enhances extensive and rapid spatial coverage of the Earth’s surface with repeatability capabilities for investigating environmental systems (Reddy 2018 ; Pettorelli et al. 2018 ). By detecting unique spectral signatures of various substances, including metals, RS can pinpoint the presence and concentration of specific metals in waters via hyperspectral electromagnetic radiation. For instance, Lin et al. ( 2024 ) determined the concentration of metals in China’s Dalian Lake using hyperspectral analysis and genetics algorithms. The integration of RS techniques with biosensor data capture probes also improved the spatial mapping of metals and sediments along Egypt’s Red Sea Coast (Mohammed et al. 2024 ). Bresciani et al. ( 2016 ) mapped patterns of cyanobacterial blooms in five Italian lakes using a suite of aerial and space-borne hyperspectral sensors with increased accuracy. Guo et al. ( 2022 ) and Cao et al. ( 2018 ) integrated RS to model metals and chlorophyll- a concentrations in water. The models provided high retrieval accuracy and realistic information.

Data generated from RS enables the creation of detailed spatiotemporal maps of aquatic metal pollution and hotspots mapping. Integrating RS data with water quality measurements and GIS data provides a comprehensive understanding of metal pollution dynamics in aquatic ecosystems (Yu et al. 2020 ; Zhu et al. 2022 ). Furthermore, RS is cost-effective and efficient compared to traditional field-based methods, enabling the rapid collection of extensive data over large areas with lower monitoring costs (Avtar et al. 2020 ). Overall, RS serves as a valuable tool in monitoring metal pollution in aquatic environments, providing timely and spatially explicit information crucial for informed decision-making and effective environmental management strategies.

The complementarity between RS and microalgae in monitoring metal pollution in aquatic systems is multifaceted. RS provides wide spatial coverage, allowing for the monitoring of large water bodies and the identification of metal pollution hotspots (Chi et al. 2016 ). However, RS may lack the spatial resolution needed to detect localised pollution events or variations. Microalgae, on the other hand, can be highly sensitive to rapid changes in metal concentrations and hence reflect localised pollution impacts. Furthermore, RS allows for the monitoring of changes in metal pollution over time by capturing images at different intervals. In contrast, the rapid response of microalgae to changes in metal concentrations makes useful indicators of short-term pollution events.

RS data can be validated and calibrated using ground-truthed data, including responses from bioindicators, including microalgae (Cook et al. 2023 ). This process enables researchers can assess the accuracy of remote-sensing-derived metal pollution estimates and refine remote-sensing algorithms to improve their reliability. Microalgae responses to metal pollution can serve as early warning indicators of environmental degradation. With a combination of RS data with real-time monitoring of microalgae populations, researchers can develop early warning systems to alert authorities to potential pollution events or ecosystem stressors, enabling timely intervention and mitigation efforts.

• Citizen science

Citizen science monitoring involves volunteers (i.e., mainly non-professionals), often the riparian communities, and is fundamentally public participation by stakeholders in environmental stewardship (Moharana 2021 ; Fraisl et al. 2022 ). Citizen science has the potential of upscaling field studies to a regional or global extent coupled with centralised monitoring efforts that enhance extensive and well-coordinated environmental monitoring, which can produce large datasets rapidly. Miguel-Chinchilla et al. ( 2019 ) analysed citizen-sensed catchment data on stream turbidity which contributed nearly 12% value to the study. Babiso et al. ( 2023 ) analysed water quality data collected from the Meki River, Ethiopia, by citizen scientists. The study results indicated a good agreement with selected parameters, which implied the accuracy of citizen-collected data. Additionally, Thornhill et al. ( 2017 ) used citizen science stream data from the metropolis of China to model and classify predictors of water quality using random forest models with reliable results.

The incorporation of smartphone technology to measure and record environmental data under the citizen science programs has greatly improved the speed, volume, and quality of data. Malthus et al. ( 2020 ) examined the impact of citizen science smartphone applications (Apps) on remotely sensed surface reflectance, stream sediment, and algal concentrations in 32 stream sites in eastern Australia. Smartphone Apps provide a friendly interface for citizen scientists to engage with and use sophisticated modern water quality monitoring technology. Smartphones are widely accessible, and the Apps are customised for objective, comprehensive, and accurate data capture (Pattinson et al. 2023 ).

Citizen science can complement microalgal biomonitoring of aquatic metal pollution in stream ecosystems through increased data collection. Citizen science projects engage a broader range of participants, allowing for more extensive data collection across various locations and times (Njue et al. 2019 ; Babiso et al. 2023 ). This can provide a more comprehensive understanding of the spatial and temporal dynamics of metal pollution in stream ecosystems. Furthermore, citizen science involves community engagement and creates social accountability and awareness towards environmental stewardship (Ruppen and Brugger 2022 ). Involving citizens in scientific monitoring fosters a sense of ownership and stewardship over local environments. This leads to increased awareness of environmental issues such as metal pollution and promotes sustainable behaviours to mitigate them. Citizen science, being a cost-effective metal pollution monitoring technique, can leverage the manpower and resources of volunteers, reducing the costs associated with monitoring efforts (Njue et al. 2019 ; Ruppen and Brugger 2022 ). This enables more frequent sampling and monitoring, which is essential for detecting changes in metal pollution levels over time. However, there is a need to identify and address the potential for errors and biases in integrating this approach in the biomonitoring of stream ecosystems (Follett and Strezov 2015 ).

Challenges and opportunities in integrating microalgae in aquatic biomonitoring of metal pollution

Despite the bottlenecks in the integrative monitoring of aquatic metal pollution in lotic systems of SSA using algal communities as bioindicators, several opportunities also present further room for developing a microalgae-based assessment of stream health status in the region. We highlight the challenges and opportunities for developing higher resolution, site-specific, and species-targeted microalgal-based bioassessment in SSA.

From our literature search, while other bioindicator taxa, particularly macroorganisms, are popular options, the use of algal communities to assess aquatic metal pollution, where this has been attempted, has been limited to the morphotaxonomic level. Besides not offering the benefit of higher resolution in detecting sub-lethal metal contamination, the absence of region-specific baseline data in several SSA sub-regions further limits the comprehensiveness of their use as bioindicators. Additionally, accurate identification of microalgal species requires trained morphotaxonomists and special equipment, such as the high-resolution scanning electron microscope, which may be limited in SSA. The uptake and integration of microalgae-based bioindicators into cutting-edge biomonitoring tools such as molecular (eRNA and eDNA), artificial intelligence systems (e.g., biosensors), and geospatial systems are yet to take off significantly. Accurate identification is essential for proper assessment and quantification of the magnitude of metal pollution and potential ecological risks to the provision of stream ecosystem services in SSA.

Microalgal communities and populations exhibit significant seasonal fluctuations, which must be understood to allow the partitioning of metal-pollution-induced impacts. Climate change, seasonality, and natural and anthropogenic factors influence microalgal community composition and species abundance. Seasonal fluctuations, such as variations in rainfall and temperature, significantly influence the hydrology of aquatic ecosystems, affecting the transport and deposition of metals (Maphanga et al. 2024 ). For instance, intense rainfall during the wet season can remobilise metals from soil and sediment into water bodies, causing elevated metal concentrations (Conrad et al. 2020 ). Conversely, dry seasons may concentrate metals due to decreased dilution and increased evaporation rates (Edokpayi et al. 2017 ). Extreme climatic events, such as floods and drought, further exacerbate metal pollution by altering metal transport and sedimentation patterns, which may introduce metals from repositories into water bodies (Xia et al. 2015 ; Wijngaard et al. 2017 ). Anthropogenic activities such as mining, industrial effluents, and agricultural runoff are significant sources of metal pollution in sub-Saharan Africa (Laffite et al. 2020 ). These activities introduce high concentrations of metals such as Pb, Hg, Cu, and Cd into aquatic ecosystems, which are highly toxic to aquatic biota (Hama Aziz et al. 2023 ; Fatmi et al. 2023 ). The interaction between seasonal dynamics, extreme climatic events, and anthropogenic activities underscores the complexity of integrating microalgae in monitoring aquatic metal pollution in SSA.

Furthermore, in the aquatic environment, metal species and mixtures interact differently with microalgae community species and vary in toxicity. Understanding these interactions and the ecological impact on microbial bioindicators, including microalgae, in the SSA is poorly understood due to the complexity and, hence, the need for cutting-edge research using advanced methods and sophisticated analytical equivalents. Unfortunately, many regions in SSA suffer from limited monitoring infrastructure, which hampers effective biomonitoring and data availability. Also, SSA, as a low to middle-income subregion, is marked by limited financial capacity to fully support advanced environmental stewardship programs in light of other “critical” financial obligations.

Opportunities

Similar to other tropical regions, SSA has a high diversity of microalgal communities and species richness due to its varied and extensively interconnected aquatic habitats. The biodiversity and richness can be leveraged to select the highly sensitive and most indicative species that respond specifically to individual and mixtures of aquatic metal pollution at sub-lethal concentrations. Since the most suitable microalgal communities and species have high sensitivity to changes in water quality, they can serve as potential early warning indicators of aquatic metal pollution episodes in river systems. Therefore, integrating microalgae in stream health assessment will improve early detection and further inform proactive interventions in managing stream ecosystems in the SSA mining regions.

Collaborative research within the SSA and with international research institutions and partners will address the limitations of access to advanced analytical tools, expertise, and limited funding. In particular, adopting modern research and monitoring tools such as eDNA and molecular biosensors will improve the resolution of detection of aquatic metal pollution. Establishing and expanding baseline data collection and accessible online databases will further accelerate the integration of microalgae for monitoring aquatic metal pollution in aquatic ecosystems across SSA.

Conclusion and future perspectives

Integrating microalgal communities as bioindicators of aquatic metal pollution in rivers of sub-Saharan Africa holds great promise for enhancing water quality monitoring and environmental conservation efforts. However, in the past decade and a half, the inclusion of microalgal taxa for integrative monitoring into aquatic metal pollution monitoring programs in the SSA has been low but gradually improving past 2020. The region is still lagging in the integration of emerging tools, such as environmental DNA, and technological advances, such as artificial intelligence models, remote sensing, and citizen science, that offer potential benefits of high precision, speed, reduced costs, and eco-friendly green technologies in monitoring and assessment of stream ecosystem health across its mining landscapes.

Despite the lack of a standardised, synchronised, and adequately documented microalgal database, inadequate microalgal taxonomic and molecular assessment expertise, limited monitoring and processing infrastructure, and economic constraints, the integrative microalgae-based approach offers significant opportunities for addressing aquatic metal pollution in the SSA lotic ecosystems. The high biodiversity of microalgae in SSA presents a vast pool for selecting suitable site-specific (sensitive and indicative) taxa that respond specifically to individual and mixtures of metal pollutants. Different microalgal taxa have been combined with other bioindicator groups to increase the sensitivity to pollutant detection source tracking and quantification. This uniqueness emphasises the central role of microalgae in aquatic ecosystem biomonitoring initiatives. Moreover, the rapid response to changes in water quality during integrative monitoring positions microalgae as potential early warning indicators for aquatic metal pollution events that impact aquatic ecosystems.

Fundamentally, there is an urgent need to prioritise efforts to institutionalism and strengthen and standardise national and regional baseline data collection on microalgae dynamics in response to metal pollution in aquatic ecosystems across SSA. Such data will serve as a foundation for accuracy and a reference point for improving the assessments of metal pollution and ecological impacts on the region’s aquatic ecosystems.

Collaboration among regional and international research institutions and organisations can lead to the development of integrated monitoring networks. These networks can leverage advanced analytical tools, technologies, and expertise to enhance data collection and analysis. Furthermore, involving riparian communities in data collection and monitoring efforts fosters a sense of collective responsibility and ownership of the stream ecosystem resources. In fact, “citizen science” initiatives are crucial to empowering riparian communities to participate actively in the conservation of stream ecosystems.

Continued research into the interactions between metal pollutants and microalgal species is essential for better understanding the ecological consequences of aquatic metal pollution. Complementing conventional monitoring with innovative techniques, such as artificial intelligence, molecular tools, and remote sensing, must be prioritised to improve the overall efficiency and maximise the productivity of environmental stewardship in metal mining regions.

Data availability

All the data generated are available in this review. Additional data and information can be sourced from the cited references and online databases or sources.

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Acknowledgements

This research was supported by the Chair-Environment and Development, Oliver R. Tambo of the Africa Research Initiative (ORTARChI) Project at the Copperbelt University, Zambia. ORTARChI is an initiative of South Africa’s National Research Foundation (NRF) and the Department of Science and Innovation (DSI) in partnership with the Oliver and Adelaide Tambo Foundation (OATF), Canada’s International Development Research Centre (IDRC), and the National Science and Technology Council (NSTC) of Zambia. The findings and conclusions in this publication are those of the authors and should not be construed to represent any official position of the organisations that funded the study.

Open access funding provided by University of Pretoria. This review was funded by the Chair-Environment and Development, Oliver R. Tambo of the Africa Research Chair Initiative (ORTARChI), Copperbelt University, Zambia.

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Conceptualization, M.M., C.M., and K.O.; methodology, M.M., K.O., and S.S; validation, M.M., C.M., K.O., T.J., and S.S.; formal analysis, M.M., C.M., and K.O.; investigation, M.M and K.O.; literature and data curation, M.M. and K.O.; writing—original draft, M.M., C.M., K.O., T.J., and S.S.; writing—review and editing, M.M., C.M., K.O, T.J., and S.S.; supervision, C.M., T.J., and S.S. All authors have read and agreed to the published version of the manuscript.

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Mulenga, M., Monde, C., Johnson, T. et al. Advances in the integration of microalgal communities for biomonitoring of metal pollution in aquatic ecosystems of sub-Saharan Africa. Environ Sci Pollut Res (2024). https://doi.org/10.1007/s11356-024-33781-1

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DOI : https://doi.org/10.1007/s11356-024-33781-1

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Environmental Science: Water Research & Technology

Urea and ammonium fluoride di-nitrogen and cu & fe bi-metal co-doped carbon felt as cathode for electro-fenton degradation of norfloxacin: 1 o 2 -dominated oxidation pathway †.

* Corresponding authors

a Department of Environmental Engineering, Taiyuan University of Technology, Jinzhong 030600, Shanxi Province, China E-mail: [email protected].

b Shanxi Key Laboratory of Compound Air Pollutions Identification and Control, Taiyuan University of Technology, Jinzhong 030600, Shanxi Province, China

c Department of Biological and Pharmaceutical Engineering, College of Biomedical Engineering, Taiyuan University of Technology, Jinzhong 030600, Shanxi Province, China

Carbon materials co-doped with both metals and non-metallic heteroatoms have become an important research focus as catalysts for heterogeneous electro-Fenton technology and the removal of refractory organics. However, there is still a lack of in-depth studies on the doping of carbon with multiple nitrogen sources and their possible synergistic mechanism. In this study, two types of nitrogens (urea and ammonium fluoride) and Cu&Fe bimetal co-doped carbon felt electrodes (C–CuFe/N) were designed to explore the effect of co-doping on carbon. C–CuFe/N exhibited a high degradation efficiency towards norfloxacin (97.2%), low ion leaching and high cycling stability (90.3% after ten cycles), better than those shown by C–CuFe, C–CuFe/UN and C–CuFe/FN prepared with none or a single nitrogen dopant. Various characterizations indicated that C–CuFe/N has the largest specific surface area, highest content of Fe( II ), most surface oxygen and active sites, and importantly, stable M–N x bond, indicating that UN and FN exhibited an excellent synergistic effect. According to the quenching experiments, the dominant reactive oxygen species for C–CuFe, C–CuFe/UN and C–CuFe/FN were radical species (·OH and O 2 ˙ − ), while they changed to non-radical species ( 1 O 2 ) for C–CuFe/N under acidic condition. Alternatively, C–CuFe/N showed a good catalytic performance (97.2–92.3%) over a wide initial pH range (1.2–11.3), but during the degradation process, all the pH values changed toward neutral, and the oxidation pathway varied from 1 O 2 -dominated under acidic condition to radical-dominated under neutral or alkaline condition. Generally, a good synergistic effect was found to exist between the dual nitrogens, which promoted the catalytic activity as well as stability of the catalyst, thus providing a good strategy to design catalysts.

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Urea and ammonium fluoride di-nitrogen and Cu & Fe bi-metal co-doped carbon felt as cathode for electro-Fenton degradation of norfloxacin: 1 O 2 -dominated oxidation pathway

J. Li, L. Gao, Y. Chen, X. Meng, X. Li, K. Qi and J. Zhang, Environ. Sci.: Water Res. Technol. , 2024, Advance Article , DOI: 10.1039/D4EW00210E

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Delivering hydrogen to EU’s industry: which are the greenest options?

Using hydrogen produced from abundantly available renewables on-site is the most sustainable option. Delivering compressed renewable hydrogen via pipelines or shipping liquid renewable hydrogen could still be environmentally friendly, research says.

Renewable hydrogen is expected to play a crucial role in reducing carbon emissions in Europe. Previous JRC  research revealed that sourcing it from regions with cheaper renewable energy can prove to be more cost-effective than local production. 

However, environmental concerns arise from transporting large quantities of hydrogen over long distances, as the environmental impact varies significantly according to the production technology and the method of delivery. 

To address these concerns, a  new study compares the life cycle environmental impacts of on-site production through steam methane reforming (SMR) or electrolysis with three different delivery methods, including compression, liquefaction, and chemical bonding to other molecules.  Transportation by both ship and pipeline was considered.

The distance used to compare the different methods of delivery is 2,500 km, compatible with the extent of EU territory and equivalent to the distance between Portugal and the Netherlands. The two countries were considered based on a proposal in an EU funded project which examined the feasibility of sustainable hydrogen transportation.

The results show that the environmental performance of hydrogen supplied to large industries can vary significantly based on the production technology and delivery pathway. 

The study was carried out by the JRC for the  Clean Hydrogen Partnership , a public-private partnership supporting research and innovation (R&I) activities in hydrogen technologies in Europe. The findings result in key recommendations for policymakers and stakeholders to help countries and industries to accelerate the transition towards a more sustainable hydrogen economy.

On-site production versus long-distance delivery

The most environmentally sustainable approach is on-site production using efficient renewable sources, such as wind power in the Netherlands. If on-site production is not viable using local abundant renewable sources, importing renewable hydrogen can still lead to a significant reduction in greenhouse gas (GHG) emissions compared to on-site production with fossil fuels. However, focusing solely on GHG emissions may lead to other, unintended environmental impacts. 

Shipping liquid hydrogen and transporting compressed hydrogen through pipelines appear to have the least environmental impact when delivering hydrogen over long distances. 

Meanwhile, the process of packing and unpacking hydrogen into chemical carriers such as ammonia, liquid organic compounds, methanol, and synthetic natural gas demands larger amounts of energy and resources. It makes these options less desirable to minimise environmental impact. But no significant difference was noticed in comparative environmental impact of delivery methods when comparing chemical carriers one with another. 

Role of renewable energy infrastructure 

The report emphasises the close relationship between the environmental impact of delivered hydrogen and renewable energy infrastructure. 

For imported solar-generated hydrogen to have an environmental advantage over conventional hydrogen production from fossil fuels, the environmental impact of generating electricity through photovoltaic panels must be significantly reduced. 

This can be achieved by improving the efficiency of photovoltaic panels in terms of materials use and utilising renewable energy for their production.

Impact of water use

Water use is another crucial factor to consider. The availability of freshwater affects the impact of hydrogen production. On-site hydrogen generation in water-rich countries proves to be a more sustainable option in terms of water use compared to importing hydrogen from water-scarce nations. 

Hydrogen loss

Hydrogen losses during the delivery chain can significantly increase the environmental impact of delivered hydrogen. However, options that are more susceptible to losses, such as liquid and compressed hydrogen, still have lower environmental impacts than using hydrogen carriers.

When on-site production of hydrogen using local renewable sources is not feasible, importing renewable hydrogen from closer regions becomes the more environmentally sustainable choice. When transporting hydrogen over long distances within Europe, delivering compressed hydrogen through pipelines or liquid hydrogen via ships stands out as the preferred option in terms of environmental impact.

Related links

Environmental life cycle assessment (LCA) comparison of hydrogen delivery options within Europe

Clean Hydrogen Partnership

EU Hydrogen strategy

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