PS 51-179 - Elevated specific conductivity and chloride concentration in the headwaters of 3 Southeastern Pennsylvania Streams linked to impervious surface cover

Background/Question/Methods

Relationships between impervious surface cover, specific conductance, and salt concentration are well-known, though the dynamics of these relationships vary spatially. This study examines relationships between impervious surfaces and specific conductance and chloride concentration through high-frequency monitoring of headwater streams in sub-basins of the Delaware River Watershed in Southeastern Pennsylvania. Ten sample sites in the headwaters of the Darby, Crum, and Ridley Creeks in Willistown, PA, were sampled every four weeks from January 2018 through December 2021 to elucidate water chemistry dynamics. Specific conductance was measured in-stream and chloride concentration was analyzed in-lab. Impervious surface cover of each watershed was estimated through mapping with GIS and related to specific conductivity and chloride to better understand relationships between land use and water chemistry.

Results/Conclusions

Specific conductance ranged from 226 µS/cm to 1186 µS/cm during the study period, with maximum values for all sites occurring during winter months. Chloride concentration ranged from 27 ppm to 247 ppm with maximum values again occurring during winter months, indicating that chloride concentration and specific conductance are driven by winter road salt applications. Specific conductance remained elevated above natural background levels in all seasons, suggesting persistence of salts in groundwater and soils. Data analysis reveals that specific conductance is strongly related to chloride concentration (p < 0.001). Moreover, both specific conductance and chloride concentration vary significantly between sites (p < 0.001). Variation between sites is best explained by differences in impervious surface cover: specific conductivity and chloride concentration are related to impervious surface cover in the surrounding catchment (p < 0.001). These analyses highlight the importance of limiting the expansion of impervious surfaces and protecting open spaces to prevent impairments to headwater streams.