Sources and risk factors for nitrate and microbial contamination of private household wells in the fractured dolomite aquifer of northeastern Wisconsin

Authors: BORCHARDT, MARK; STOKDYK, JOEL - U.S. GEOLOGICAL SURVEY (USGS); KIEKE, BURNEY - MARSHFIELD CLINIC RESEARCH; MULDOON, MAUREEN - U.S. GEOLOGICAL SURVEY (USGS); SPENCER, SUSAN; FIRNSTAHL, AARON - U.S. GEOLOGICAL SURVEY (USGS); BONNESS, DAVINA - KEWAUNEE COUNTY DEPARTMENT OF LAND AND WATER CONSERVATION; HUNT, RANDALL - U.S. GEOLOGICAL SURVEY (USGS); Burch, Tucker

Submitted to: Environmental Health Perspectives
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/21/2021
Publication Date: 6/23/2021
Citation: Borchardt, M.A., Stokdyk, J.P., Kieke, B.A., Muldoon, M.A., Spencer, S.K., Firnstahl, A.D., Bonness, D.A., Hunt, R.J., Burch, T.R. 2021. Sources and risk factors for nitrate and microbial contamination of private household wells in the fractured dolomite aquifer of northeastern Wisconsin. Environmental Health Perspectives. https://doi.org/10.1289/EHP7813.
DOI: https://doi.org/10.1289/EHP7813

Interpretive Summary: In the past few decades northeastern Wisconsin has seen expansions in dairy farming and rural homes. Both dairy farmers and rural homeowners in this region rely on groundwater from private wells to supply their drinking water. Being rural, fecal wastes from both groups (dairy manure and septic system effluent) are distributed to the landscape. However, the groundwater comes from a type of underground aquifer that is highly connected to the land surface, allowing fecal material to contaminate private wells, and creating tensions between farmers and homeowners. Using a sophisticated sampling strategy, we determined the rate of private well contamination with nitrate and coliform bacteria and identified contamination “hot spots” corresponding to locations where the underlying bedrock was closest to the surface. Using genetic techniques, we identified in private wells a variety of pathogens that cause illness in people and cattle and we showed the fecal material in wells came from both possible sources, dairy manure and septic systems. Lastly, we built statistical models that linked private well contamination with risk factors such as how near wells were to manure storage and the number of surrounding septic systems. Rainfall amount and the amount of water able to reach and replenish groundwater were also factors related to contamination. Well construction factors, like well depth, were not as important to contamination as those factors related to land use. Understanding the “what, where, and how” of private well contamination in northeastern Wisconsin presents opportunities for improving groundwater quality to the benefit of farmers and homeowners alike. Moreover, our findings are relevant to the broader region surrounding the Great Lakes of USA and Canada, which is supplied with drinking water from the same aquifer as northeastern Wisconsin.

Technical Abstract: Groundwater quality in the Silurian dolomite aquifer in northeastern Wisconsin, USA has become contentious as dairy farms and exurban development expand. We investigated private household wells in the region, determining the extent, sources, and risk factors of nitrate and microbial contamination. Total coliforms, E. coli, and nitrate were evaluated by synoptic sampling during groundwater recharge and non-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. 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 the number of septic system drainfields 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, suggesting land use, groundwater recharge and bedrock depth are the most important risk factors. These findings may inform policies to minimize contamination of the Silurian dolomite aquifer, a major water supply for the USA and Canada Great Lakes region.