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

Drinking water quality is of growing concern in Washington. Nitrate is a common drinking water impairment in the state, especially in rural areas. Nitrate and phosphate contamination commonly originate from urban and agricultural runoff. This study focuses on groundwater and surface water in the Lincoln Creek watershed in Eastern Lewis County Washington, where elevated nitrate and salinity impair water resources. The goal of this study is to investigate possible sources water quality impairments in this region.

Groundwater and surface water samples were collected during July/August 2008 and December/January 2008-2009. Surface water conductivity ranges from 0.09 to 0.20 mS/cm in the summer and 0.07 to 0.08 mS/cm in the winter. Discharge was elevated during the winter sampling due to recent precipitation events. Conductivity of sampled groundwaters ranged from 0.17 to 9.40 and 0.06 to 6.40 mS/cm in the summer and winter, respectively. Wells located within 100 meters of the stream have conductivities comparable to that of surface waters. Higher (>2.0 mS/cm) conductivity values were typically observed in the deeper wells, which are may penetrate the lower aquifer. Geochemical analysis of sampled groundwaters indicates that sodium and chloride are the primary ions in high conductivity groundwaters, consistent with communication with the deeper aquifer. Nitrate and phosphate concentrations in sampled surface waters and groundwaters are low, < 0.3 ppm phosphate and < 1.2 ppm nitrate. Preliminary results suggest that communication with the deep aquifer is the primary source of water quality impairment in the Lincoln Creek watershed.