MICROBIAL CONTAMINATION OF PRIVATE WELLS IN THE DOLOMITE AQUIFER IN NORTHEASTERN WISCONSIN: ASSOCIATIONS WITH LAND USE, HYDROGEOLOGY, AND WELL CONSTRUCTION

Groundwater quality in the fractured dolomite aquifer in northeastern Wisconsin has become a contentious issue as dairy farms and exurban development expand. We investigated the extent and source of microbial contamination of private household wells open to the dolomite aquifer located in Kewaunee County, Wisconsin and evaluated factors related to contamination. Total coliforms, E. coli, and nitrate were evaluated by synoptic sampling during groundwater recharge and non-recharge periods (n = 317 and 400 wells, respectively). Wells positive for total coliforms or with NO₃-N ≥ 10 ppm were further evaluated for fecal contamination source by qPCR for human-specific enteric viruses, bovine-specific viruses, human and bovine Bacteroides, and agricultural zoonotic pathogens. Risk factors related to land use, precipitation, hydrogeology, and well construction were tested for associations with 1) the probability of well contamination; and 2) well concentrations of microbial contaminants by logistic and gamma regression models, respectively. Contamination by coliform bacteria and high nitrate (NO₃-N ≥ 10 ppm) was strongly associated with the depth of soil overlying bedrock at well locations and agricultural land use (e.g. acreage of agricultural fields within 3,000 feet of a well). Among 131 wells analyzed by qPCR for the source of fecal contamination, 79 (60%) were positive for human- or bovine-related microbes, indicating that both human wastewater and manure contributed to well contamination. The most frequently detected pathogens in private wells included rotavirus group A, Cryptosporidium, and Salmonella. Wells contaminated with human wastewater were associated with the number septic systems within 750 feet and precipitation events. Manure-contaminated wells were associated with groundwater recharge and depth-to-bedrock, but only when manure was being spread on the landscape. Well construction factors were generally not related to contamination. These findings are beneficial for informing policies to minimize contamination of private wells in the fractured dolomite aquifer of northeastern Wisconsin.