Social-ecological-technological drivers of freshwater salinization in the Occoquan Reservoir, United States

Freshwater salinization is an emerging and largely unregulated threat to drinking water security. We identify three dominant, seasonally distinct sources of rising sodium in a drinking water supply serving 1 million people: (1) road deicers, which elevate reservoir sodium in winter, with detectable impacts at watershed impervious cover as low as 3%; (2) reclaimed water, which increases sodium during summer low flows when dilution is minimal; and (3) the drinking water treatment plant (DWTP), which adds NaOH to neutralize acidity from coagulation and in-reservoir microbial processes. In this social-ecological-technological system (SETS), salinization is tied to population growth, impervious cover, sodium-rich waste streams, nitrogen management, reservoir biogeochemistry, and DWTP operations. Framing drinking water salinization as a SETS challenge integrates behavioral and biophysical drivers with engineering and governance responses, providing a framework for adaptation in One Water systems. Long-term monitoring reveals that rising sodium concentrations in a large One Water drinking water supply arise from road salts, reclaimed water, and treatment chemicals, highlighting freshwater salinization as a socio-ecological-technological challenge.