SOURCES OF WATER QUALITY IMPAIRMENTS IN THE LINCOLN CREEK WATERSHED, LEWIS COUNTY WASHINGTON

Drinking water quality is of growing concern in Washington, especially in rural areas, which are often influenced by agricultural runoff. This study focuses on groundwater and surface water in the Lincoln Creek (LC) watershed in Eastern Lewis County, Washington. Here, elevated phosphate and salinity impair water resources in the glacial outwash aquifer. Most wells in the study area are contained within the shallow groundwater aquifer. The goal of this study is to investigate possible sources of water quality impairments in the region and the hydrologic relationship between contaminated sites.

Groundwater and surface water samples were collected during winter and summer seasons between 2008 and 2009. Surface water conductivity was notably higher in summer months (ranges from 0.09 to 0.20 mS/cm) than in winter (0.07 to 0.08 mS/cm). Discharge was elevated during the winter sampling due to precipitation events, which also served to reduce conductivity. Elevated groundwater conductivities were observed during summer (<4.79 mS/cm), compared to winter months (<3.32 mS/cm). Higher (>2.0 mS/cm) conductivity values were typically observed in deeper wells (> 15 meters), which may penetrate below the glacial outwash. Geochemical analysis of sampled groundwater indicates that Na⁺ and Cl^- are the primary ions in the high conductivity groundwaters. Elevated levels of phosphate were observed in some groundwaters, with concentrations approaching 20 ppm. Phosphate is currently unregulated in drinking water by the EPA, although it is recognized as a contaminant. The EPA limit for phosphate in recreational rivers and streams, however, is 0.1mg/L. Geochemical analysis of surface waters indicates the absence of nutrient contamination as evidenced by the lack of nitrate and phosphate in surface water. We conclude that shallow groundwater communication with the aquifer underlying the glacial outwash is the likely source of water quality impairment in the LC watershed.