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Three-county study to test water quality
water testing

SOUTHWEST WISCONSIN - The Southwest Wisconsin Groundwater and Geology study, or SWIGG, aims to study the quality of the groundwater assets of Grant, Iowa and Lafayette Counties, and to identify the sources of any contamination found. 

The study’s goal is to test 500 wells randomly selected throughout the three counties.  It is estimated that there are 2,199 private wells in Grant County; 5,829 in Iowa County; and 3,000 in Lafayette County. This means that a sample of 500 will represent five percent of the total approximately 11,000 private wells. 

The first samples were taken on Friday, Nov. 9, and the second round of samples will be taken in March of 2019.

The United States Geological Survey (USGS) is the agency that is taking the lead on design of the study, determining how many wells will be sampled, and when. Joel Stokdyk, of the USGS Laboratory for Infectious Disease and the Environment and Upper Midwest Water Science Center is one of two principal investigators for the project.

The other principal investigator is State of Wisconsin Geologist Kenneth Bradbury, Director of the Wisconsin Geological and Natural History Survey, which is part of UW-Extension.

Co-Investigators for the study will be Mark Borchardt with the USGS Laboratory for Infectious Diseases and the Environment, and Burney Kieke of the Marshfield Clinic Research Foundation, a statistician. Borchardt was one of the lead investigators in the Kewaunee County study, where broad scale groundwater contamination has been documented.

Local point people on the project will be Lynda Schweikert, Administrator of the Grant County Conservation, Sanitation and Zoning Department; Iowa County Conservationist Katie Abbott; and Terry Loeffelholtz, Land Conservation/Planning and Zoning Department Manager for Lafayette County.

“We are sending the samples to the UW-Stevens Point Center for Watershed Science and Education (CWSE) laboratory to have the nitrate and bacterial indicator laboratory analysis done,” Stokdyk said. “When we have the results, then our agency will take over the work to map the water test results with the information from the Wisconsin Geological and Natural History Survey (GNHS) office about the well construction and groundwater depth data.

Kenneth Bradbury of GNHS will do the work of compiling the data on the characteristics of the sampled wells. This data will work off of the construction reports for each sampled well, and will tabulate the age and depth of the wells. He will also look at the depth to groundwater for each well location. The study will not include taking bedrock core samples at the site of each well, though this kind of information is available in Iowa County from a study the GNHS previously conducted.


Approximately 44 percent of the population in the three counties relies on private groundwater wells for their drinking water. Together these residents are served by over 18,000 known wells with construction records, in addition to wells without records available. 

Private wells are not monitored or regulated by federal, state or local government. Instead, homeowners are responsible for the maintenance and testing of their private well, including any treatment or corrective action to address contamination. Contamination of private wells is commonly assessed using tests for nitrate and indicator bacteria (total coliform and E. coli).

Existing data from the online CWSE well water quality viewer show that across the three counties, 13 percent of 4,283 samples exceeded the nitrate drinking water standard of 10 mg/L, and 29 percent and three percent respectively of 1,747 samples were positive for total coliform bacteria and E. coli.

Samples collected for these measurements are voluntary and therefore may not reflect the true extent of contamination as certain geographic areas or geologic settings may be over- or under-represented. And even when the tests show up positive, they still don’t identify the source of the pollution.

Private well contamination can be related to the vulnerability of specific wells or general groundwater contamination by sources on the landscape. Elevated nitrate in groundwater can result from both inorganic fertilizers and fecal sources, while indicator bacteria typically originate from fecal sources.

Given the mix of land uses in rural areas, fecal contamination can originate from agricultural operations through land application of manure, and from homes with septic systems that discharge wastewater through drainfields (e.g. mound systems). Therefore, both agricultural and residential fecal sources can contribute to nitrate and bacteria contamination of private wells, and hydrogeological factors influence the vulnerability of groundwater to these contaminants.

Grant, Iowa and Lafayette Counties are in the Driftless Area of southwest Wisconsin. Unlike much of northern and eastern Wisconsin, these counties were not covered by glaciers, so the landscape is much older than the glacially-modified landscapes found in other parts of the state.

The uppermost bedrock in the region is mostly dolomite and limestone of the Ordovician-age Sinnipee and Prairie du Chien groups. These carbonate rocks contain fractures and karst features such as sinkholes, conduits, and small caves. Generally in the three counties, these rocks are within 50 feet of the surface, qualifying as shallow soil-to-bedrock areas.

Much of the uplands in the three counties are covered with a silty-clay material known as the Rountree Formation, named after exposures along Rountree Creek in the city of Platteville. This soil type is a mix of weathered carbonate bedrock and loess or windblown silt deposited during the Pleistocene age. Over the three counties, the thickness of this soil layer ranges from absent to several feet thick.

Groundwater can occur in any of the rock formations in the three counties, depending on the elevation of the water table. All of these sandstone layers form interconnected aquifers. Deep wells in this area receive most of their water from the Cambrian sandstone aquifer, but locally shallower wells are finished in the rocks of the Sinnipe or Prairie du Chien groups or in the St. Peter Sandstone. Along major river valleys sand-and-gravel aquifers supply water to wells.

Water well construction characteristics can govern the susceptibility of wells to contamination. Although not mandated by the current Wisconsin well code, it is recommended that water wells be cased (a steel casing pipe cemented into the well) to below the water table. Otherwise, an uncased hole provides a direct conduit for potentially contaminated water to move from near the land surface into the well and into deep aquifers.

Preliminary work in Grant County supported by GNHS determined that out of 2,199 wells studied, 912, or 41 percent, have static water levels below the casing and would be classified as ‘vulnerable’ based on construction alone. However, at this time they do not have the water quality tests to compare to well construction practices, so the link between the two can’t be established.

Bedrock type, as described above, and the depth to that bedrock are important hydrogeologic characteristics when considering groundwater contamination, and they are both central to recent revisions to the Wisconsin code that regulates manure application, NR 151. 

WDNR’s NR 151 administrative rule establishes runoff pollution performance standards for non−agricultural facilities and transportation facilities and performance standards and prohibitions for agricultural facilities and practices designed to achieve water quality standards as required by Wisconsin statutes.

The language of the ‘Sensitive Areas’ modification to the NR 151 rule, WT 15-16, states “The department [WDNR] has found that, in areas of the state where Silurian [Karst] bedrock is present, groundwater and surface water standards will not be attained by implementing the statewide agricultural performance standards and prohibitions in ch. NR 151, Wis. Adm. Code.

Carbonate bedrock aquifers are vulnerable to contamination because water is transported quickly through bedrock fractures. Given the bedrock’s limited ability to filter contaminants, the soil overlying the bedrock is essential for removing contaminants prior to them reaching groundwater.

The NR 151 revisions aim to address contamination of vulnerable groundwater resources where fractured carbonate bedrock (karst) is overlain by thin soils (shallow depth to bedrock). These are the conditions that prevail in Iowa, Grant and Lafayettte Counties (and Crawford, Vernon, Richland and parts of Monroe).

The revisions to NR 151 pertain exclusively to the Silurian dolomite bedrock common in the eastern part of the state. Other regions of the state, like the southwestern counties of Grant, Iowa and Lafayette, have similar hydrogeological characteristics and may therefore be vulnerable to groundwater contamination from human and agricultural fecal sources.

Because the hydrogeology of the Driftless Region and the extent and sources of groundwater contamination are not well characterized, this leaves important questions including: How many wells are contaminated? What roles do well construction and geology play? What is the source of contamination, agriculture or septic systems? The proposed work is designed to address these questions.

Proposed work

The objectives of the study are as follows:

1. Evaluate private well contamination in three counties (Grant, Iowa, and Lafayette) using indicator bacteria (total coliform and E. coli) and nitrate based on two randomized sampling events.

2. Assess well construction and geological characteristics (e.g. well age, depth to bedrock) that affect total coliform and nitrate contamination.

3. Identify the source of contamination in a subset of total coliform- and nitrate-positive wells once per season using microbial tests that distinguish between human, bovine, and swine fecal sources.

Additional objectives follow the study plan that are designed to map depth to bedrock in the three counties, characterize the Rountree formation (a geological unit potentially protective of groundwater), and determine the influence land use factors have on contamination.

After the initial two samples are analyzed, and the correlation established between well construction characteristics and any contamination observed, then the study will take an additional step. The next phase will be aimed at identifying the specific source of the contamination, which the first round of tests will not demonstrate.

Wells which test positive for nitrate or bacteria from the first round, will be eligible for a second round of testing that will determine whether the source of contamination is human, bovine or swine. Once each season for four seasons, 35 eligible wells will be tested for a total of 140 tests. Quantitative polymerase chain reaction testing will demonstrate the specific origin of any fecal contamination.


Sept. 2018 – solicit participation

Nov. 2018 – first sampling

March 2019 – second sampling

June 2019 – summer seasonal sampling

Oct. 2019 – fall seasonal  sampling

Jan. 2020 – winter seasonal sampling

April 2020 – spring seasonal sampling

Aug. 2020 – present study results

Sept. 2020 – complete written report