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Coupled chemical–microbial deterioration in stagnant fire hydrant branches threatens drinking water quality

Nature Water volume 4, pages 44–57 (2026)Cite this article

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Abstract

Fire hydrants are widely installed in drinking water distribution systems, where stagnant water forms multiple ‘high-risk zones’. The stagnant water quality at hydrant terminals has been poorly studied. Here we show that stagnant water exhibited an 18-fold increase in manganese, a 40-fold increase in total cell counts, a 13-fold increase in adenosine triphosphate and enrichment of opportunistic pathogens compared with flowing water. Notable changes were also observed in microbial communities and dissolved organic matter composition, including shifts in dominant bacterial taxa, transformation of saturated oxidized compounds and generation of unsaturated reduced compounds. This study also explored the ecological mechanisms underlying the covariation of microorganisms and dissolved organic matter after water stagnation. This finding provides an additional possibility for drinking water quality deterioration in drinking water distribution systems, highlighting the potential threat posed by stagnant water in non-consumer terminals (fire hydrants) to water safety.

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Fig. 1: Illustration of fire hydrant and chemical-microbial water quality comparison between stagnant and flowing water.
Fig. 2: Microbial community analysis.
Fig. 3: Spectroscopic and molecular characterization of DOM.
Fig. 4: Analysis of unique molecules in two water types.
Fig. 5: Analysis of microbial and DOM diversity, community assembly and co-occurrence network.

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Data availability

Sequence data associated with this project have been deposited in the National Center for Biotechnology Information (NCBI) Short Read Archive database (accession number PRJNA1267700). FT-ICR MS raw data are available via Zenodo at https://doi.org/10.5281/zenodo.17422657 (ref. 99). Source data are provided with this paper.

Code availability

All codes used in this study are available in the Article or its Supplementary Information.

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Acknowledgements

The present work has been financially supported by the National Natural Science Foundation of China (grant nos. 52388101, 52525003 and 52370105 to G.L.; 52170105 to Q.X.).

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Author notes

  1. These authors contributed equally: Mengqing Fan, Qiang Xu.

Authors and Affiliations

  1. Key Laboratory of Environmental Aquatic Chemistry, State Key Laboratory of Regional Environment and Sustainability, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China

    Mengqing Fan, Qiang Xu, Xiaoxuan Wang, Zhiwei Fang & Gang Liu

  2. University of Chinese Academy of Sciences, Beijing, China

    Mengqing Fan, Qiang Xu, Xiaoxuan Wang, Zhiwei Fang & Gang Liu

  3. Department of Biotechnology, Delft University of Technology, Delft, the Netherlands

    Mark C. M. van Loosdrecht & Martin Pabst

  4. School of Eco-Environment, Harbin Institute of Technology, Shenzhen, China

    Yu Tao

  5. Department of Fisheries and Wildlife, Michigan State University, East Lansing, MI, USA

    Joan B. Rose

  6. Science and Technology, University of Twente, Enschede, the Netherlands

    Walter van der Meer

Authors
  1. Mengqing Fan
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  2. Qiang Xu
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  4. Zhiwei Fang
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Contributions

M.F. led the sampling strategy development, data acquisition, data analysis and manuscript writing. Q.X. contributed equally to strategy development and data acquisition, assisted in data analysis and provided overall guidance. X.W. and Z.F. assisted with data acquisition. M.C.M.V.L. and M.P. provided guidance on data analysis and writing. Y.T., J.B.R. and W.V.D.M. contributed editorial and writing guidance. G.L. supervised the entire project from scientific design to data analysis and also contributed to manuscript writing.

Corresponding author

Correspondence to Gang Liu.

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Nature Water thanks Hans-Jørgen Albrechtsen, Weiyi Pan and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Statistical source data for Fig. 3.

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Fan, M., Xu, Q., Wang, X. et al. Coupled chemical–microbial deterioration in stagnant fire hydrant branches threatens drinking water quality. Nat Water 4, 44–57 (2026). https://doi.org/10.1038/s44221-025-00542-4

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