Southwest Wisconsin groundwater and geology study (SWIGG)

Farmland and tree-covered rolling hills of the Driftless Area in southwest Wisconsin (photo by Eric Carson, WGNHS)

Southwest Wisconsin has fractured bedrock beneath generally thin soils, putting groundwater at risk for contamination.

The purpose of this study was to improve our understanding of groundwater quality in southwest Wisconsin and to better understand how local hydrogeology and well construction characteristics affect groundwater quality.

UPDATE: The SWIGG study is now complete. Its final report was presented to Grant, Iowa, and Lafayette counties in May 2022 and published on the WGNHS website in May 2023. A video of the presentation to the counties is available below.

View final report

Project objectives

  1. Evaluate private well contamination using indicator bacteria (total coliform and E. coli) and nitrate;
  2. Identify the sources of contamination (people, cows, or pigs) in a subset of wells that tested positive for total coliform and/or elevated nitrate;
  3. Analyze the private well samples collected for Objective 2 for genes specific to certain pathogens;
  4. Assess well construction and geological characteristics (e.g., well age, depth to bedrock) that are related to well contamination; and
  5. Identify land use factors and potential contamination sources (e.g., number of nearby septic systems, distance to an agricultural field) related to well contamination.

Results summary for each objective

Rural residents of Grant, Iowa, and Lafayette counties in Wisconsin rely on private wells for their water. Contaminants like nitrate and bacteria from septic systems, fertilizer, and manure can contaminate the groundwater that residents use. Groundwater is vulnerable to contamination where the soil layer is thin and the bedrock is fractured, which is the case for much of the study region. This study includes five objectives that were designed to assess and understand private well water contamination in the three counties.

Objective 1. Extent of private well contamination

The extent of bacteria and nitrate private well contamination was determined by testing randomly selected wells in two water sampling events. A total of 840 water samples were tested for nitrate, total coliform bacteria, and E. coli, which are standard tests of well water quality. Overall, 126 (42%) of 301 wells sampled in November 2018 and 145 (27%) of 539 wells sampled in April 2019 were positive for total coliform bacteria and/or had nitrate greater than the Wisconsin and U.S. Environmental Protection Agency health standard (10 mg nitrate- nitrogen per liter). The percentage of study wells with total coliforms or high nitrate was generally greater than statewide percentages for private wells.

Objective 2. Human wastewater and livestock manure contamination

Tests that distinguish between human wastewater, bovine (cattle) manure, and porcine (pig) manure were used to identify fecal sources of contamination. For these tests, 138 wells were randomly selected from those 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. The tests identify the three specific fecal sources but not other types of contamination, like chemical fertilizers or manure from other animals. These tests cannot determine the source of contamination for nitrate, total coliforms, or E. coli, which can originate from many places.

Objective 3. Pathogens in private wells

Residents may become ill from drinking water that contains pathogens, like viruses and bacteria. The 138 wells tested for Objective 2 were also tested for pathogens. These wells were randomly selected from those positive for total coliforms or with high nitrate. Pathogens were detected in 66 of the 138 wells (48%). Many of the pathogens can be passed between humans and animals, so their source was often unknown.

Objective 4. Factors related to contamination: well characteristics, well siting, geology, rainfall and groundwater levels

Well characteristics, well siting, geology, rainfall, and groundwater levels were examined for relationships to the contaminants measured in Objectives 1 and 2. These factors affect the tendency for contaminants to reach groundwater or enter wells. Nitrate contamination was generally greater where the geology allows rapid flow of water and contaminants. Microbial contamination was generally greater following periods of rainfall and where bedrock is closer to the surface. Both nitrate and microbial contamination were generally greater for older, shallower wells. The factors reflect probabilities, not absolutes. For example, high nitrate is more likely for shallow wells, but this does not mean that all wells that are shallow will have high nitrate.

Objective 5. Factors related to contamination: septic systems, farms, and cultivated land

Septic systems, farms, and cultivated land were examined for their relationships with the contaminants measured in Objectives 1 and 2. Human wastewater contamination was greater for wells closer to septic systems and for wells with more septic systems nearby. Septic systems were not associated with nitrate and total coliforms. Nitrate and total coliforms contamination were greater for wells closer to farms or cultivated land (fields used for crops like corn). Also, nitrate contamination increased when the area of cultivated land nearby was larger. Like Objective 4, these factors reflect probabilities, not absolutes.

Full results

To see the full results of the SWIGG study, including figures, key findings, context and interpretation, and more, download the final report.

View final report

Public presentation of final report

This video is a recording of the public presentation of the final report that SWIGG researchers gave to residents of Grant, Iowa, and Lafayette counties in May 2022.

Resources for private well owners

Private drinking water wells should be tested annually or more frequently if you notice a change in the water or in nearby land use.

More about the SWIGG study

Contact information

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