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SWIGG Study: Proximity to agriculture biggest drinking water quality risk
Solutions offered
1_SWIGG_County Cons
FOUR COUNTY CONSERVATIONISTS, including from left, Erik Heagle, Grant County Conervationist, Katie Abbott, Iowa County Conservationist, Erica Sauer, Lafayette County Conservationist, and Lynda Schweikert, former Grant County Conservationist (now in Chippewa County) were on hand for the unveiling of the final SWIGG Study results.

DRIFTLESS - About 65 citizens attended the unveiling of final results of the Southwest Wisconsin Groundwater and Geology study (SWIGG) in Lancaster on Monday, May 16. The report included findings from the final phase of the study, establishing risk factors for drinking water quality in Grant, Iowa and Lafayette counties.

The study tested 978 samples from 816 private wells (some wells were tested more than once) in the three counties. Well samples were distributed all across the counties, in all the different geologic formations found there.

Results from water samples taken in 2018 and 2019 show that 32 percent 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 researchers included Dr. Mark Borchardt (USDA Agricultural Research Service), Dr. Maureen Muldoon (Wisconsin Geological and Natural History Survey – WGNHS), Joel Stokdyk (U.S.  Geological Survey), and Dr. Ken Bradbury (Wisconsin State Geologist - WGNHS).

The full report is available at

Key findings

The study reveals several key factors related to private well water quality:

• nitrate and total coliform [bacteria] contamination was more likely for wells closer to livestock farms or cultivated land [fields used for row crops like corn and soybeans]

• older, shallower wells showed increased levels of contamination

• local bedrock type affected risk – nitrate contamination was generally greater where the geology allows rapid infiltration [from the surface] of water and contaminants

• human wastewater contamination was more likely for wells closer to septic systems and for wells with more septic systems nearby

4_SWIGG_Mark Borchardt
Dr. Mark Borchardt (USDA Agricultural Research Service)

Proximity to farms

A key finding of the study is that nitrate and total coliform [bacteria] contamination was more likely for wells closer to livestock farms or cultivated land [fields used for row crops like corn and soybeans].

The researchers clarified that an important component of the random selection for wells to be sampled in the survey is that proximity to agricultural fields or other septic systems was not a factor in their selection.

For this reason, the sampling results and their correlations to risk factors, is scientifically unbiased. Homeowners collected the samples in the first round of testing, and there was a question about whether this could have led to sampling errors.

“The samples in the first two rounds in phase one of the study were homeowner collected, and the samples in phase two were technician collected,” Dr. Borchardt explained. “There have been studies done about the likelihood of sampling error with homeowner sample collection, and the finding is that errors are very low.”

Dr. Muldoon added that “nitrate numbers would not be affected by homeowner sampling errors, but humans are covered in bacteria, so it is possible that coliform numbers could have been impacted.” Muldoon said that the E.coli numbers would not have been affected unless the homeowner’s hands had been covered in fecal matter at the time the sample was taken.

Numbers tell the story

Scott Laeser of Clean Wisconsin put the key finding that proximity to row crop and livestock agriculture is the top risk factor in risk for drinking water contamination in Southwest Wisconsin in perspective.

According to Laeser, there are 20,539 septic systems in the three counties. The amount of nitrogen discharged per system annually is 20 pounds.

Therefore, total nitrogen discharged by septic systems annually in the three counties is 407,180 pounds.

Laeser goes on to say that the number of acres of corn in the three counties in 2017 was 370,100. The average nitrogen application rate per acre of corn (manure, fertilizer, or a combination) is 160 pounds.

Therefore, the total nitrogen applied on corn acres annually is 59,216,000 pounds.

The average leaching rate of nitrogen per acre of corn based on research from UW-Stevens Point Center for Watershed Science and Education (accounting for crop uptake, loss to atmosphere, and estimated remaining soil nitrogen) is 32 pounds.

This means that the total estimated nitrogen leaching from cornfields in the three counties annually is 11,843,200 pounds.

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Dr. Ken Bradbury (Wisconsin State Geologist - WGNHS)

Bedrock and wells

Two of the four findings of the study about risks for contamination of drinking water intertwine in the topics of well construction and bedrock geology. Generally, newer wells, and wells tapping deeper aquifers and cased below the water table, deliver cleaner water. Older, shallower wells, tapping water from a shallower aquifer and/or not cased below the water table, had a higher risk of delivering contaminated water.


Unlike Kewaunee County, where the aquifer that wells tap into is in a shallower bedrock formation, wells in Southwest Wisconsin can tap into multiple aquifers. 

The shallower aquifer is located under the topsoil and weathered rock (Rountree Formation), in what is known as the ‘Sinnippee Group,’ a collection of three highly permeable layers of dolomite and shale. Contaminants on the surface can infiltrate quickly into aquifers in this layer.

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Shallower wells located in this layer, especially if they are not cased to below the water table, are more susceptible to contamination with coliform bacteria and nitrates.

The deeper aquifer, located in sandstone layers, is separated from the shallower aquifer by a thin layer of shale known as ‘Glenwood Shale.’ This shale layer acts to slow down infiltration of water from the dolomites above, and is the layer that many springs and seeps can be found in. Infiltration to the sandstones below is much less than infiltration from the surface into the shallower aquifer.

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Deeper wells located in this layer, especially if they are cased below the water table, are less likely to deliver contaminated water. However, if a well is drilled into this layer, but is cased only into the shallower aquifer above it, then contaminants from the surface could penetrate into the deeper layer. This could in turn, contaminate other nearby wells located in the deeper layer, even if they were cased below the water table.

“In Kewaunee County, because there is only one relatively shallow aquifer, I was fond of saying that you couldn’t build your way out of trouble,” Dr. Borchardt remarked. “However, in Southwest Wisconsin, it is more likely that you could build your way out of trouble.”

Dr. Bradbury was quick to emphasize that both the shallower aquifer and the deeper aquifer would be capable of delivering good water in the absence of contaminants on the surface.

“Without contaminants on the surface, there would be no groundwater contamination,” Bradbury explained.

Bradbury shared a slide that showed nitrate results by aquifer. He said that 121 of the wells sampled were located in the shallower aquifer. Of those, 31.4 percent exceeded health standards for nitrate.

He said that 155 of the wells sampled were drilled to the depth of the deeper aquifer, but were cased to allow water from the shallower aquifer to mix in. For these wells, the percentage with nitrate levels above the health standard dropped to 16.8 percent.

Bradbury went on to explain that 368 of the wells sampled were located in the deeper aquifer. Of those wells, only 5.2 percent showed nitrate levels that exceeded health standards.

Rountree Formation

Dr. Muldoon debunked the theory that the Rountree Formation of weathered rock is protective of the shallower aquifer located below it.

“In Grant, Iowa and Lafayette counties, the Rountree Formation layer is actually comparatively thin – only three-to-five feet, and is prone to forming cracks that allow water and nutrients to infiltrate into the aquifer,” Muldoon said. “Also, not all weathered rock is created equal – in the three counties, the nature of the rock being weathered makes it more likely to allow infiltration.’

Muldoon said that in counties in the ‘Driftless Region’ located north of the Wisconsin River, such as Richland, Crawford and Vernon, the Rountree Formation is much thicker – 13-15 feet – and results from weathering of a different bedrock formation that is less permeable. This means that north of the Wisconsin River, the layer is more protective of the shallower aquifers.

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Joel Stokdyk (U.S. Geological Survey)

Human wastewater

The study demonstrated that septic systems were not correlated with nitrate and coliform bacteria contamination of drinking water, but were correlated with contamination with some pathogens from a human source. 

Joel Stokdyk led the process of four rounds of technician-conducted, seasonal sampling of a total 138 wells that already tested positive for total coliforms or with high nitrate in the first phase of the study.

Tests that distinguish be-tween human wastewater, cattle manure, and pig manure were used to identify fecal sources of contamination. 

10_SWIGG_seasonal microbial source testing

Stokdyk explained that 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. 

Of the 138 wells that underwent microbial source testing in phase two of the study (previously found to be contaminated in phase one of the study), wells found to be contaminated with pathogens from only humans was 40; nine from bovine only; and three from porcine only.

Wells combining sources of pathogens included 22 from human and Bovine; eight from human and porcine; eight from bovine and porcine; and six from human and bovine and porcine.

Overall, 78 of the 138 wells had one or more of those three fecal sources (which means 60 did not have any of those sources).

Comparing the pathogen detections with the fecal sources will be part of the forthcoming risk assessment, so we don't have that worked up yet.

The fecal source tracking results do not indicate the source of nitrate. See page 19 of the report--this is an important point to make clear.   Instead of the fecal sources, we use the land use risk factors (Objective 5) and found that agricultural land use, but not septic systems, were associated with high nitrate.

The wells tested for nitrate, total coliforms, and E. coli were randomly selected from across the three counties.  The 138 wells tested for fecal sources and pathogens were randomly selected from wells that were previously positive for total coliforms, E. coli, or high nitrate. Because those 138 wells were selected from those previously contaminated, the percentage of wells positive for pathogens and fecal sources does not necessarily represent the region-wide percentages

“These tests cannot determine the source of contamination for nitrate, total coliforms, and E. coli, which can originate from many places,” Stokdyk said. “This contamination was more likely for wells closer to septic systems, and for wells with more septic systems nearby (such as in a subdivision).”

“So, this is not saying that septic systems could not be sources of nitrate or coliform, but rather that in the wells sampled in the study, this correlation was not demonstrated,” Stokdyk said.

Borchardt clarified how these findings should be interpreted.

“Generally, it is well established that smoking cigarettes makes people more susceptible to developing cancer. Yet, my Dad always pointed to the guy who smoked two-packs-per-day, and lived to be 102,” Dr. Borchardt said. “As with water quality, the general results which show the probabilities of risk don’t mean that there will be no individual cases where risk varies from the statistical probabilities.”

Borchardt explained that all wells that are located near a septic system (which is basically almost all private wells associated with a home) will show coliform bacteria from human waste when tested.

“Most of the coliform bacteria from human sources found in water samples in the study were a kind of beneficial bacteria commonly found in the human gut,” Borchardt said. “However, if waste containing a harmful pathogen, for instance from an individual who is sick, is deposited into a septic system, then a harmful pathogen might show up in a water sample.”

Borchardt emphasized that bacteria, unlike nitrate, are living organisms that live, and die. Nitrate, on the other hand he said, is persistent over time.

Additional information

Borchardt explained to meeting participants that 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

• 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.

This study provides major pieces of data for the under-standing 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 an-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.