Sources of Elevated Chloride in Local Streams: Groundwater and Soils as Potential Reservoirs

Abstract

Several recent reports document increasing concentrations of chloride in surface waters of northeastern and midwestern North America. These patterns, together with high summertime baseflow concentrations, suggest the possibility of short-term retention of winter-applied chloride within catchments. This study examined the potential of groundwater and soils to act as reservoirs of Cl within the watershed of a small rural stream in New York that has shown a doubling of Cl over the past two decades and almost constant concentrations across seasons. Groundwater samples were obtained from 20 private drinking wells distributed around the catchment to determine whether concentrations were at or greater than surface waters and so could act as a source to the stream. In 16 of the 20 wells, chloride concentrations were lower (mean of 16 samples = 4.10 mg Cl/L (standard deviation = 4.8)) than concurrent streamwater concentrations (mean of eight locations sampled on two dates = 28.9 (5.8)). Four wells, however, showed higher concentrations ranging from 35 to 230 mg Cl/L suggesting either point source contamination or very heterogeneous groundwater chloride sources. Soil cores from the catchment were irrigated in the laboratory with a NaCl solution for 2 weeks followed by 2 weeks irrigation with a chloride-free solution. Chloride concentrations in core leachates were lower than in the irrigation solution during the first 2 weeks indicating some retention of Cl within the cores. After cores were irrigated with chloride-free water, leachate concentrations declined but would not reach streamwater concentrations until the equivalent of 15 cm of precipitation was added to the core.