Chloride dynamics in the hyporheic zone of a flashy urban stream in the Chesapeake Bay watershed

Recent evidence from the mid-Atlantic suggests that drinking water supplies and aquatic biota are threatened by chronic chloride inputs from road salts applied to improve highway safety.  Understanding the behavior of chloride in urban watersheds where road salts are applied is critical to predicting subsequent impacts to ecosystem health and drinking water supplies.  Here we report on chloride dynamics in the hyporheic zone of Minebank Run, a highly urbanized stream in the Chesapeake Bay watershed that receives chronic chloride inputs from local highways.  Chloride (Cl), sodium (Na), and specific conductance (SC) were greatly elevated in the both surface water and ground water of Minebank Run, spiking during winter when salts are applied to roads.  Cl and SC were significantly higher downstream of the 695 Beltway, a major highway that bisects Minebank Run.  SC spikes also occurred during precipitation events, revealing significant ground water and surface water interaction corresponding to the flashy hydrology typical of urban streams.  Cl and SC levels remained elevated throughout the year apparently because ground water continued to supply surface water with chloride even after road salt application ceased.  Thus, ground water may represent a chronic source of chloride to surface water, thereby contributing to the upward trend in freshwater salinity in urbanizing areas.  Geomorphic stream restoration practices widely used in the mid-Atlantic are not designed to address salinity effects.  Source control of road salts may be necessary to reduce environmental risk.