Publication Summary

A groundwater quality assessment was conducted to determine the source(s) of nitrates and other contaminants in groundwater in an area near Smith Prairie Road and 161st Way SE about eight miles southwest of Yelm, Washington. In particular, since 1995 the discharge of one private water-supply well showed elevated concentrations of nitrate+nitrite-N and intermittent green discoloration, fecal coliform bacteria, and particulate matter. A groundwater monitoring network, consisting of four monitoring wells and three private water-supply wells, was established and sampled monthly from September 1999 to May 2000. The hydrogeology and groundwater-flow patterns were characterized.

A groundwater quality assessment was conducted to determine the source(s) of nitrates and other contaminants in groundwater in an area near Smith Prairie Road and 161st Way SE about eight miles southwest of Yelm, Washington. In particular, since 1995 the discharge of one private water-supply well (WS1) showed elevated concentrations of nitrate+nitrite-N and intermittent green discoloration, fecal coliform bacteria, and particulate matter. A groundwater monitoring network, consisting of four monitoring wells and three private water-supply wells, was established and sampled monthly from September 1999 to May 2000. The hydrogeology and groundwater-flow patterns were characterized.

The findings suggest that two groundwater contamination issues exist in the study area: 1. Nitrate loading to the uppermost aquifer from a source upgradient of the monitoring network. 2. Hydraulic connection of well WS1 with infiltrating surface runoff during periods of heavy rainfall.

The primary source of nitrate is likely a dairy upgradient of the monitoring network. The dairy became inactive in 1999 and, provided there is no additional loading to groundwater, nitrate concentrations in groundwater should decrease over time. Based on the estimated groundwater flow rate and assuming nitrate loading to groundwater has ceased, it may take up to 20 years for nitrate levels to decline substantially at WS1. Recharge complexity and denitrification processes may reduce this period. Inspection of the status of the dairy waste storage pond is recommended.

Two potential hydrogeologic contaminant pathways to WS1 are identified. Additional investigations are needed to verify subsurface conditions for both pathways. The probable source of contaminants is infiltrated runoff, but the specific locations where infiltration occurred and where runoff originated were not determined. The sensitivity of groundwater to contamination dictates that runoff be minimized and controlled. To improve water quality at affected water-supply wells, one potential option for well owners is to drill wells to a deeper aquifer zone.

Link to EIM data for User Study ID DERI0001