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Addressing the contribution of indirect potable reuse to inland freshwater salinization


Inland freshwater salinity is rising worldwide, a phenomenon called the freshwater salinization syndrome (FSS). We investigate a potential conflict between managing the FSS and indirect potable reuse, the practice of augmenting water supplies through the addition of highly treated wastewater (reclaimed water) to surface waters and groundwaters. From time-series data collected over 25 years, we quantify the contributions of three salinity sources—a water reclamation facility and two rapidly urbanizing watersheds—to the rising concentration of sodium (a major ion associated with the FSS) in a regionally important drinking-water reservoir in the Mid-Atlantic United States. Sodium mass loading to the reservoir is primarily from watershed runoff during wet weather and reclaimed water during dry weather. Across all timescales evaluated, sodium concentration in the reclaimed water is higher than in outflow from the two watersheds. Sodium in reclaimed water originates from chemicals added during wastewater treatment, industrial and commercial discharges, human excretion and down-drain disposal of drinking water and sodium-rich household products. Thus, numerous opportunities exist to reduce the contribution of indirect potable reuse to sodium pollution at this site, and the FSS more generally. These efforts will require deliberative engagement with a diverse community of watershed stakeholders and careful consideration of the local political, social and environmental context.

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Fig. 1: Both IPR and human activities in the Bull Run and Occoquan River watersheds contribute to salinization of the Occoquan Reservoir in Northern Virginia, USA.
Fig. 2: Annualized sodium load and concentration in outflow from the Occoquan River, Bull Run and UOSA water reclamation facility.
Fig. 3: Daily sodium load and concentration in outflow from the Occoquan River, Bull Run and UOSA water reclamation facility for an illustrative two-year period (2012–2013).
Fig. 4: Probability density functions of the percentage sodium mass load entering the Occoquan Reservoir from the Occoquan River, Bull Run and UOSA conditioned on rate of flow into the reservoir.
Fig. 5: Sources of sodium discharged by the UOSA water reclamation facility.

Data availability

All data used in this study are publicly available (


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Funding was provided by a US National Science Foundation Growing Convergence Research (GCR) Program award to S.B.G., M.A.R., S.K., P.V., M.E., T.A.B. and T.S. (NSF award #2021015) and by a Metropolitan Washington Council of Governments award to S.B.G. and S.K. (contract #21-001). The authors thank the Upper Occoquan Service Authority—in particular, B. Angelotti and M. Noble-Blair—for providing critical data and guidance, H. Zhang at The Water Research Foundation for a national perspective on freshwater salinization, D. Sedlak for valuable comments on the manuscript and participants in the Occoquan Watershed Monitoring Lab’s Freshwater Salinization Workshop held on 14 January 2020.

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S.V.B. and S.B.G. conceived and drafted the article. E.A.P., M.A.R., A.N.G., P.V., A.M.-M., M.E., G.P., N.S. and S.C. contributed text and analysis. All authors contributed edits.

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Correspondence to Stanley B. Grant.

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Bhide, S.V., Grant, S.B., Parker, E.A. et al. Addressing the contribution of indirect potable reuse to inland freshwater salinization. Nat Sustain 4, 699–707 (2021).

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