Sources and Risk Factors for Nitrate and Microbial Contamination of Private Household Wells in the Fractured Dolomite Aquifer of Northeastern Wisconsin

Abstract

BACKGROUND: Groundwater quality in the Silurian dolomite aquifer in northeastern Wisconsin, USA, has become contentious as dairy farms and exurban development expand. OBJECTIVES: We investigated private household wells in the region, determining the extent, sources, and risk factors of nitrate and microbial contamination. METHODS: Total coliforms, Escherichia coli, and nitrate were evaluated by synoptic sampling during groundwater recharge and no-recharge periods. Additional seasonal sampling measured genetic markers of human and bovine fecal-associated microbes and enteric zoonotic pathogens. We constructed multivariable regression models of detection probability (log-binomial) and concentration (gamma) for each contaminant to identify risk factors related to land use, precipitation, hydrogeology, and well construction. RESULTS: Total coliforms and nitrate were strongly associated with depth-to-bedrock at well sites and nearby agricultural land use, but not septic systems. Both human wastewater and cattle manure contributed to well contamination. Rotavirus group A, Cryptosporidium, and Salmonella were the most frequently detected pathogens. Wells positive for human fecal markers were associated with depth-to-groundwater and number of septic system drainfield within 229 m. Manure-contaminated wells were associated with groundwater recharge and the area size of nearby agricultural land. Wells positive for any fecal-associated microbe, regardless of source, were associated with septic system density and manure storage proximity modified by bedrock depth. Well construction was generally not related to contamination, indicating land use, groundwater recharge, and bedrock depth were the most important risk factors. DISCUSSION: These findings may inform policies to minimize contamination of the Silurian dolomite aquifer, a major water supply for the U.S. and Canadian Great Lakes region.

Quantitative Microbial Risk Assessment for Contaminated Private Wells in the Fractured Dolomite Aquifer of Kewaunee County, Wisconsin

“…In particular, deeper depths to bedrock produce longer transport times and more opportunity for attenuation of microbial contaminants (Erb and Steiglitz 2007;Rasmuson et al 2020). Exacerbating this problem, the county's rural landscape is home to potential fecal sources in the form of thousands of private septic systems and tens of thousands of cattle (Borchardt et al 2021). Septic systems are spread ubiquitously throughout the county (Figure 1), and though each is a relatively small source on its own, all are typically located in close proximity to one or more private wells and thus represent a constant hazard of drinking water contamination.…”

“…Due to its inherent vulnerability and the combination of human and agricultural influences, Kewaunee County's groundwater is in fact regularly contaminated by several different pollutants. Most recently, Borchardt et al (2021) estimated contamination rates of private wells in the county based on the presence of indicator bacteria and/or nitrate and as a function of depth-to-bedrock, which varies from <1 to >45 m across the county (Clayton 2013). More specifically, Borchardt et al (2021) found that 39%-43% of wells constructed in depth-to-bedrock of ≤6:1 m (i.e., ≤20 ft) were contaminated with total coliforms and/or NO 3 − N > 10 mg=L.…”

“…Most recently, Borchardt et al (2021) estimated contamination rates of private wells in the county based on the presence of indicator bacteria and/or nitrate and as a function of depth-to-bedrock, which varies from <1 to >45 m across the county (Clayton 2013). More specifically, Borchardt et al (2021) found that 39%-43% of wells constructed in depth-to-bedrock of ≤6:1 m (i.e., ≤20 ft) were contaminated with total coliforms and/or NO 3 − N > 10 mg=L. This range compared to a 23%-26% contamination rate for wells constructed in depth-to-bedrock of >6:1 m. Furthermore, Borchardt et al (2021) identified both relevant sources of fecal contamination (i.e., human and bovine) in private well samples along with a number of human and zoonotic gastrointestinal pathogens (e.g., Cryptosporidium and Salmonella among others).…”

“…More specifically, Borchardt et al (2021) found that 39%-43% of wells constructed in depth-to-bedrock of ≤6:1 m (i.e., ≤20 ft) were contaminated with total coliforms and/or NO 3 − N > 10 mg=L. This range compared to a 23%-26% contamination rate for wells constructed in depth-to-bedrock of >6:1 m. Furthermore, Borchardt et al (2021) identified both relevant sources of fecal contamination (i.e., human and bovine) in private well samples along with a number of human and zoonotic gastrointestinal pathogens (e.g., Cryptosporidium and Salmonella among others). As in many other states, private wells in Wisconsin are not monitored by government agencies, and the testing and treatment of private well water is at the discretion of well owners.…”

“…However, the health risk associated with pathogen levels observed by Borchardt et al (2021) is currently unknown. Understanding health risk, and not just pathogen occurrence on its own, is crucial in this context because risk (i.e., AGI cases per capita) can be more easily related to the cost-benefit calculus involved with evaluating proposed policy interventions in the county.…”