TRANSPORT AND RESIDENCE OF CHLORIDE FROM ROAD SALT IN SHALLOW GROUNDWATERS, EAST-CENTRAL MISSOURI, USA

Baseline concentrations of Cl are systematically increasing in many surface waters due to winter applications of road salt, suggesting that Cl is being stored in soil and groundwater reservoirs. We monitored a rural and an urban karst spring in Missouri to characterize Cl variations and residence time in shallow groundwater on short (rain response), seasonal, annual, and decadal timescales. For our rural site, a 19-year long record shows a progressive increase in Cl, likely due to increased salt application and incomplete groundwater flushing. Since July 2014, average Cl at the urban site (192 ppm) was much higher than at the rural site (63 ppm) and frequently exceeded the EPA chronic limit for aquatic life (230 ppm). The highest Cl concentrations were observed within 6 days of road salt application at both springs. After salt application, Cl gradually declined by an average of 12% per month at the rural site through the fall, whereas at the urban site Cl decreased 80% in the month following salting, then monthly averages remained elevated but relatively constant. Neither site recovered to levels of unimpacted Missouri springs (<10 ppm) during the study period. Superimposed on the seasonal Cl trends were short-term responses to rainfall. First flushing of salt in soils or on pavement persists through summer at the rural site and through fall at the urban site. Hydrograph separations based on ¹⁸O and ²H data reveal that event water makes up 89% of the urban spring flow following rainfall, but only 21% in the rural spring. High event water contributions at the urban site, caused either by urban land use or greater conduit flow, lead to sudden Cl peaks during winter followed by dilution after short-term salt sources are exhausted. Conversely, high baseflow at the rural site implies slow movement and greater dilution of salt-laden winter runoff, explaining the modest Cl response to flooding and the gradual Cl decrease during summer and fall. Overall Cl dilution during flood pulses is controlled by event water contributions and soil flushing, while longer-term Cl patterns are controlled by diffuse transport in groundwater and slow loss of Cl retained in soils.