NITRATE PATTERNS, TRENDS, AND PATHWAYS IN GROUNDWATER IN SOUTH-CENTRAL WISCONSIN: A MODEL-BASED EVALUATION

Nitrate, a critical agricultural fertilizer, is a common groundwater contaminant across Wisconsin. Analyses conducted over the past decades in Dane County, Wisconsin, indicate that roughly 25 percent of private domestic wells contained nitrate in excess of the state and federal drinking water standard of 10 mg/L as nitrogen ― well above Wisconsin’s statewide exceedance level of 12 percent. While nitrate contamination is widespread, it is often challenging to quantify its spatial and temporal distribution, as well as track the fate and transport of such non-point source groundwater contaminants.

Our research seeks to improve understanding of the spatial and temporal distributions of nitrate in groundwater by combining GIS mapping, groundwater modeling, and statistical analysis. Using historical databases, we compiled approximately 61,000 individual nitrate analyses from wells within Dane County and evaluated them spatially using ArcGIS. The distribution of all groundwater nitrate concentrations was evaluated based on sampling date and location, and, for a subset of 23,000 records, included well and casing depth, well construction date, and aquifer type. A groundwater flow model (MODFLOW), recently developed for the county, was then used to generate particle traces (MODPATH) to estimate the travel time and source of groundwater. These results provided a basis for determining historical dates of nitrate loading to the groundwater system. Statistical analysis of this historical nitrate loading suggests a strong temporal correlation between nitrate concentrations and the application of nitrogen fertilizer for agricultural production.

The improved spatial and temporal understanding of nitrate concentrations across the county gives local and state decision makers an additional tool for managing groundwater resources within Dane County and for ensuring the sustainability of these resources for farmers, citizens, industry, and the environment.