Quantitative microbial risk assessment with microbial source tracking for enteric pathogens in southwest Wisconsin private wells

Authors: Burch, Tucker; STOKDYK, JOEL - Us Geological Survey (USGS); Heffron, Joseph; Opelt, Sarah; FIRNSTAHL, AARON - Us Geological Survey (USGS)

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/17/2026
Publication Date: 3/23/2026
Citation: Burch, T.R., Stokdyk, J.P., Heffron, J.A., Opelt, S.A., Firnstahl, A.D. 2026. Quantitative microbial risk assessment with microbial source tracking for enteric pathogens in southwest Wisconsin private wells. Journal of Environmental Quality. https://doi.org/10.1002/jeq2.70162.
DOI: https://doi.org/10.1002/jeq2.70162

Interpretive Summary: Private wells supply drinking water for many households, including 23 million households in the United States. Contamination of private wells by fecal microbes from household septic systems and land-applied livestock manure presents a risk of acute gastrointestinal illness (AGI) to well-users. Risk is believed to vary by fecal source, but prevailing AGI risk and its association with fecal source are unknown for most of the United States. ARS researchers in Marshfield, Wisconsin, worked with collaborators from the U.S. Geological Survey to characterize AGI risk in rural southwestern Wisconsin based on water sampling and quantitative microbial risk assessment. Estimated AGI risk was moderate to high based on comparison to previous studies, all-cause AGI, and notifiable disease data. Most AGI cases were associated with detection of human microbial source tracking (MST) markers alone or when detected with livestock markers. Relatively few AGI cases were associated with detection of only livestock MST markers. This study illustrates human fecal sources predominating risk associated with drinking water from private wells, even in a rural region characterized by substantial livestock activity.

Technical Abstract: Private wells supply drinking water for many households, and their contamination by fecal microbes presents a risk of acute gastrointestinal illness (AGI). Risk is thought to vary by contaminating fecal source, but specific associations with fecal source are unknown for most private wells in the U.S. This study characterizes AGI risk in Grant, Iowa, and Lafayette Counties in southwestern Wisconsin. AGI risk was assessed for 10 viral, bacterial, and protozoan pathogens detected in private wells using quantitative microbial risk assessment. Exposure assessment was based on sampling 138 private wells in the study area, and risk was associated with fecal source by microbial source tracking (MST). Median Monte Carlo estimates indicated 4,450 AGI cases/year in the study area associated with drinking water from contaminated private wells (95% CI: 90–37,990). Most annual cases were associated with detection of human MST markers, including median estimates of 2,550 associated with only human markers and 880 with human and livestock MST markers detected together. Fifty AGI cases/year were associated with detection of only livestock markers, and nearly 1,000 cases were estimated to occur in the absence of detectable MST markers. This study characterizes AGI risk for households served by private wells in southwestern Wisconsin. It illustrates that human fecal sources can predominate risk associated with drinking water from private wells, even in a rural region characterized by substantial agricultural activity. Microbial source tracking can characterize the relationship between risk and fecal source; contributions of fecal sources to risk cannot be assumed from land use.