Relationships Between Private Well Water, Stream Base Flow Water, and Land Use in the Tomorrow-Waupaca River Watershed

Abstract

Agricultural practices that lead to groundwater pollution by nitrate-N and pesticides remain one of the major environmental issues facing Wisconsin and the nation. In an effort to reduce groundwater quality problems there has been an increasing trend to encourage Best Management Practices (BMPs). Unfortunately, there are few areas where adequate monitoring is being done to establish baseline water quality conditions or to document water quality changes due to the use of BMPs. This project is designed to evaluate the potential for using stream base flow water quality as a representative measure of mean groundwater quality in a watershed. The Tomorrow-Waupaca River Watershed encompasses approximately 300 square miles, is . comprised of sixteen named tributaries, has a reasonably well-defined groundwater flow system in coarse, glacial material, and contains a diversity of land use dominated by agriculture. Forty-one stream sites were sampled during summer and winter base flow conditions between 1997 and 1999. Some sites have been sampled during winter base flow since 1994. The land area contributing groundwater to each sampling site was delineated, resulting in sub-basins that correspond to each sampling site. A total of thirty-eight sampling sites and sub-basins were used in this study. In addition, three databases containing over 3,500 test results of private well water have been combined. A geographic information system (GIS) using Arclnfo and ArcView has been developed to analyze the watershed data. The relationship of nitrate-N levels in well water compared to nitrate-N levels in winter stream base flow water has a strong correlation when the sub-basins are second order or larger, indicating that this method can be used as a valid monitoring tool in the sub-basins greater than first order. Analysis of seasonal variation in the stream data shows that winter concentrations of nitrate-N and atrazine are higher than summer concentrations in the second and third order sub-basins. This relationship is true for chloride only in the third order sub-basins. First order sub-basins do not follow this trend. Overall it appears that winter stream sampling may be critical for the most accurate assessment of groundwater quality. Average well values are higher than average stream base flow values for nitrate-N in first, second, and third order sub-basins. This difference is believed to be largely due to lag time in the groundwater flow to the stream channel, leading to the likelihood that higher stream values will occur in the future. Lastly, broad estimates were made for the Tomorrow-Waupaca River watershed that indicate thirty-five percent of nitrogen applied at UW-Extension recommended rates leaches to groundwater. Groundwater travel times in the watershed range from a mean of 3.4 years in the first order sub-basins, to a maximum travel time of 259.9 years in the largest third order sub-basin.