Spatial and temporal variability of nitrate in Wisconsin’s groundwater
University of Wisconsin - Extension & University of Wisconsin - Stevens Point
Nitrate is a widespread groundwater contaminant found in agricultural regions throughout the world. Wisconsin, USA is located in a humid-continental climate. Nearly one-quarter of its land base is classified as farmland; primary cropping systems include corn (32% of farmland), hay and forage (27%) and soybean (13%). Wisconsin is fortunate that groundwater is generally abundant, freshwater with low salinity and easily accessible. Major aquifers include materials from the Quaternary (sand/gravel), Ordovician (dolomite/sandstone) and Cambrian (sandstone) periods. With over 850,000 domestic wells and 97% of communities relying on public water supply wells, groundwater is the primary water supply for nearly 70% of the state’s 5.8 million residents and many of its industries. Because of the heavy reliance on groundwater wells – both domestic and public – Wisconsin has amassed an extensive set of well water data. The extensive spatial distribution of data has helped create a detailed picture of groundwater quality across the state. The information is useful for helping communities to understand the relationship between land-use in groundwater. Here we investigate what is known about the extent of nitrate pollution in Wisconsin’s groundwater as well as trends in nitrate concentrations over time. Post well construction, private wells are not required to test or report results; however some of that data has been captured by UW-Extension, state agencies and county health departments. Data from these sources has been aggregated into the Wisconsin Well Water Quality Viewer. It contains information on nitrate for 133,882 water samples spanning 1985 to present. Statewide 10% of well samples exceed the 10 milligrams per liter nitrate-nitrogen drinking water standard; while 42% contain concentrations above 2 milligrams per liter providing evidence that land-use is impacting well water quality. Wells located in agricultural regions and installed into sand/gravel aquifers or shallow carbonate rock aquifers are particularly impacted with nitrate. Because most private wells aren’t sampled regularly the data isn’t particularly useful at a local scale for understanding how water quality has changed over time. As a way to investigate temporal variability, we turned to data from transient non-community water systems (bars, restaurants, churches, etc.) which are required to regularly test water for nitrate and report results. This dataset provided a unique opportunity to evaluate temporal changes in shallow groundwater nitrate concentrations using linear regression. While the temporal analysis indicates nitrate statewide has increased in more areas than decreased, the shallow groundwater accessed by 7,447 (87%) of TNC wells did not indicate a significant trend either way. Of the 13% wells with a trend, 726 increased (8%) while 421 decreased (5%). This information will help Wisconsin prioritize outreach strategies to private well users and assist communities in those areas most affected by elevated and potentially increasing nitrate concentrations in groundwater. It may also help other regions with less detailed nitrate data communicate the role of land-use, soils and geology on groundwater quality.