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Agriculture-Related Trends in Groundwater Quality of the Glacial Deposits Aquifer, Central Wisconsin

U.S. Geological Survey, 8505 Research Way, Middleton, WI 53562. The use of trade names in this paper is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey

^* Corresponding author (dasaad{at}usgs.gov).

Received for publication January 30, 2007. Measuring and understanding trends in groundwater quality is necessary for determining whether changes in land-management practices have an effect on groundwater quality. This paper describes an approach that was used to measure and understand trends using data from two groundwater studies conducted in central Wisconsin as part of the USGS NAWQA program. One of the key components of this approach, determining the age of sampled groundwater, gave a temporal component to the snapshots of water quality that were obtained through synoptic-sampling efforts. This approach can be used at other locations where groundwater quality data are collected, groundwater age can be determined, and associated temporal data are available. Results of these studies indicate measured concentrations of nitrate and atrazine plus deethylatrazine were correlated to historical patterns of fertilizer and atrazine use. Concentrations of nitrate in groundwater have increased over time; concentrations of atrazine plus deethylatrazine increased and then decreased. Concentrations of nitrate also were correlated to screen depth below the water level and concentrations of dissolved O₂; concentrations of atrazine plus deethylatrazine were correlated to dissolved O₂ and annual precipitation. To measure trends in concentrations of atrazine plus deethylatrazine, the data, collected over a near-decadal period, were adjusted to account for changes in laboratory-reporting levels and analytical recoveries. Only after accounting for these changes was it apparent that the median concentrations of atrazine plus deethylatrazine decreased over the near-decadal interval between sampling efforts.

Abbreviations: USGS, United States Geological Survey • NAWQA, National Water-Quality Assessment • USEPA, United States Environmental Protection Agency • Nitrate, nitrite plus nitrate as nitrogen (NO₂+NO³–N) • CFC, chlorofluorocarbon