Stream Water Age Reveals Hydrological and Human Drivers of Inland Freshwater Salinization

Abstract The salinity of inland freshwaters is rising globally, particularly in human dominated landscapes that receive deicers in the winter. Here we show that the salinization of an urban stream in Northern Virginia (USA) can be linked to salt sources using stream water age as a master variable. Younger stream water is associated with either low salinity rain or high salinity snow melt, while older stream water is associated with a geology-specific groundwater salinity. Long-term deicer use increases salinity of the groundwater over time, transforming an acute snow melt problem into a chronic groundwater problem. Framing freshwater salinization relative to stream water age clarifies relevant physical processes, highlights long-term threats, opens the door to monitoring catchment-scale deicer application rates, and informs potential engineering and policy solutions.