A landmark Wisconsin survey of private wells and groundwater contamination shows that land use and geology near a water well, in addition to how it is constructed, can affect drinking water quality.
The Southwest Wisconsin Groundwater and Geology (SWIGG) study tested 978 samples from 816 private wells (some wells were tested more than once) in Grant, Iowa, and Lafayette Counties, revealing the factors that can influence the groundwater quality of private wells. Results from water samples taken in 2018 and 2019 show that 32% of the private wells sampled tested positive for total coliform bacteria and/or nitrate greater than the Wisconsin and EPA health standard (which is 10 mg nitrate-nitrogen per liter). Additionally, the percentage of study wells with total coliforms or high nitrate was generally greater than statewide averages for private wells.
The study reveals several key factors related to private well water quality:
● Older, shallower wells showed increased levels of contamination.
● Human wastewater contamination was more likely for wells closer to septic systems and for wells with more septic systems nearby.
● Nitrate and total coliform contamination was more likely for wells closer to livestock farms or cultivated land (fields used for crops like corn).
● Local bedrock type also affects risk; nitrate contamination was generally greater where the geology allows rapid flow of water and contaminants.
“The report is valuable at two levels. It provides local, comprehensive, and actionable information to the residents of southwest Wisconsin, and it is a significant advance in groundwater science and our understanding of microbial contamination,” says Joel Stokdyk, the report’s lead author.
The survey had five aims:
Determining the extent of private well contamination;
Assessing human wastewater and livestock manure contamination;
Testing for pathogens in private wells;
Identifying factors related to how contaminants move to and through the groundwater or enter wells; and
Identifying factors related to potential contamination sources, such as septic systems, livestock, and cultivated land.
Private wells are the primary water source for all rural residents of southwest Wisconsin. There are approximately 16,000 construction records for private wells in the three-county region and there are additional wells for which construction records are unavailable. Federal, state, or local governments do not regularly monitor water quality for private wells, so homeowners are responsible for the maintenance and testing of their own well. Homeowners can contact their local health department or county Extension office for more information about private well water testing options.
Bacteria in groundwater can originate from fecal sources, like manure and wastewater from septic systems, and non-fecal sources. Nitrate can originate from fecal sources, fertilizers, and natural sources. Bacteria and nitrate are carried to groundwater by rain and melting snow. Well defects can contribute to contamination, but even properly constructed wells can be subject to contamination when groundwater is contaminated from sources on the landscape.
Tests that distinguish between human wastewater, cattle manure, and pig manure were used to identify fecal sources of contamination. For these tests, 138 wells were randomly selected from those that already tested positive for total coliforms or with high nitrate. Human wastewater was detected in 64 wells, cattle manure was detected in 33 wells, and pig manure was detected in 13 wells, indicating that both human wastewater and livestock manure contribute to private well contamination. These tests cannot determine the source of contamination for nitrate, total coliforms, and E. coli, which can originate from many places.
This study provides major pieces of data for the understanding of the relation of land use and geology to private wells and drinking water quality. It is one of the largest and most comprehensive private well studies in the nation.
“We are very grateful to the private well owners, research team, partners, and funders of this study, who have all made it possible for us to better understand, and ultimately better protect, rural drinking water quality,” said Iowa County Conservationist Katie Abbott. “We’ll have many more discussions at community, county, regional, and state levels to look for feasible solutions based on this data. In the meantime, we strongly encourage well owners to test their water at least annually and to learn more about well maintenance.”
Funding for the survey was provided by Grant, Iowa, and Lafayette Counties, with additional funding from the Lafayette Ag Stewardship Alliance, USDA-ARS, residents of Lafayette County, Wisconsin Department of Natural Resources, U.S. Geological Survey, and Iowa County Uplands Watershed Group in addition to collaboration with researchers from UW-Madison Extension and the Wisconsin Geological and Natural History Survey.