EVALUATION OF FACTORS INFLUENCING NITRATE CONCENTRATIONS IN GROUNDWATER IN EAU CLAIRE COUNTY

In Eau Claire County, about 6.4% of private wells have nitrate concentrations exceeding the Enforcement Standard (ES) of 10 mg/L. This project investigates whether high nitrate concentrations can be correlated to geologic and hydrologic parameters or to land management. If such correlations are found, they can be used to develop a risk assessment map for Eau Claire County that could be used to predict areas that are at an elevated risk for nitrate contamination. To aid in this study, the Eau Claire County Health Department provided nearly 6,000 nitrate concentrations from groundwater samples acquired in private wells over a period from 1999 to 2004 and again from 2005 to 2009. Using GIS, these data sets were joined to maps of depth to bedrock, depth to water table, soil texture, soil taxonomy, geomorphology, and land use. The resulting data sets enabled correlations to be made between these parameters and nitrate concentrations, and to observe how these correlations changed with time between the two data sets. The factors which appeared to have the most impact on nitrate concentrations were land use, depth to the water table, and depth to bedrock. Agricultural land typically had the highest nitrate values, although the nitrate concentrations in urban areas increased significantly with time. Areas with higher bedrock tended to have higher nitrate values, while areas with shallow water tables had lower average nitrate values.

To better investigate the factors controlling nitrate contamination, combinations of factors that independently appeared to influence nitrate concentration were considered. These analyses helped to show which combinations of natural and anthropogenic conditions were most likely to result in high nitrate concentrations. Although analysis of different combinations of factors is ongoing, the areas which seem most likely to have high nitrate concentrations are rural areas with intermediate water tables (5-50 ft beneath the surface), highly developed (more urban) areas with shallow bedrock, and areas with both shallow bedrock and shallow water tables.