IMPACTS OF IMPERVIOUS SURFACE COVER AND ROADWAY DEICING AGENT APPLICATION RATES ON CHLORIDE CONCENTRATIONS IN SOUTHEASTERN PENNSYLVANIA STREAMS

Roadway deicing agents, including rock salt and brines, have had a profound impact on the water quality and aquatic health of rivers in northeastern USA. Yet, a paucity of long-term streamwater-quality data has limited our understanding of how land use change and salt application rates affect chloride accumulation and delivery trends at watershed scales. Here we use long-term (2004-2020), utility-sourced streamwater quality data combined with daily streamflow for seven watersheds in southeastern Pennsylvania to examine the relationship between chloride concentrations and controlling factors such as impervious surface cover (ISC) and available road-salt application rates. Annual flow-normalized chloride concentrations and temporal differences in chloride delivery were calculated using the weighted regressions on time, discharge, and season (WRTDS) model. Flow-normalized chloride concentrations were subsequently compared to ISC within four riparian buffer widths (30 m, 50 m, 100 m, and 150 m) and the entire watershed. Median monthly chloride concentrations were highest in the late winter months (February and March) for six out of the seven watersheds, which is likely driven by runoff following storm events. A small increase in ISC in all seven watersheds (0.47-2.11%) and within all buffer widths was observed. While available road-salt application rates decreased over time, flow-normalized chloride concentrations increased, indicating the long-term accumulation of chloride in groundwater delivered to streams as baseflow. The strongest correlation between flow-normalized chloride concentrations and ISC was observed at the watershed scale, suggesting widespread contamination of shallow groundwater. Although flow-normalized chloride concentrations are below the U.S. Environmental Protection Agency’s chronic threshold value for impacts to aquatic organisms, year-round exceedances may result by the end of this century given current trends in 6 out of the 7 watersheds. The study results suggest that road salt application can have demonstrable impacts on watersheds with limited ISC (i.e., ISC ranges from 4.81-11.98%).