Risk factors for Cryptosporidium contamination in Minnesota public supply wells.

In a recent monitoring study of Minnesota's public supply wells, Cryptosporidium was commonly detected with 40% of the wells having at least one detection. Risk factors for Cryptosporidium occurrence in drinking water supply wells, beyond surface water influence, remain poorly understood. To address this gap, physical and chemical factors were assessed as potential predictors of Cryptosporidium occurrence in 135 public supply wells in Minnesota. Univariable analysis, regression techniques, and classification trees were used to analyze the data. Many variables were identified as significant risk factors in univariable analysis and several remained significant throughout the succeeding analysis techniques. These factors fell into general categories of well use and construction, aquifer characteristics, and connectedness to the land surface, well capture zones, and land use therein, existence of potential contaminant sources within 200-feet of the well, and variability in the chemical and isotopic parameters measured during the study. These risk categories, and the specific variables and threshold values we have identified, can help guide future research on factors influencing Cryptosporidium contamination of wells and can be used by environmental health programs to develop risk-based sampling plans and design interventions that reduce associated health risks.

Cryptosporidium Incidence and Surface Water Influence of Groundwater Supplying Public Water Systems in Minnesota, USA.

Regulations for public water systems (PWS) in the U.S. consider Cryptosporidium a microbial contaminant of surface water supplies. Groundwater is assumed free of Cryptosporidium unless surface water is entering supply wells. We determined the incidence of Cryptosporidium in PWS wells varying in surface water influence. Community and noncommunity PWS wells ( n = 145) were sampled ( n = 964) and analyzed for Cryptosporidium by qPCR and immunofluorescence assay (IFA). Surface water influence was assessed by stable isotopes and the expert judgment of hydrogeologists using site-specific data. Fifty-eight wells (40%) and 107 samples (11%) were Cryptosporidium-positive by qPCR, and of these samples 67 were positive by IFA. Cryptosporidium concentrations measured by qPCR and IFA were significantly correlated ( p < 0.001). Cryptosporidium incidence was not associated with surface water influence as assessed by stable isotopes or expert judgment. We successfully sequenced 45 of the 107 positive samples to identify species, including C. parvum (41), C. andersoni (2), and C. hominis (2), and the predominant subtype was C. parvum IIa A17G2R1. Assuming USA regulations for surface water-supplied PWS were applicable to the study wells, wells positive for Cryptosporidium by IFA would likely be required to add treatment. Cryptosporidium is not uncommon in groundwater, even when surface water influence is absent.

Microbial pathogens and contaminants of emerging concern in groundwater at an urban subsurface stormwater infiltration site.

Urban stormwater may contain a variety of pollutants, including viruses and other pathogens, and contaminants of emerging concern (pharmaceuticals, artificial sweeteners, and personal care products). In vulnerable geologic settings, the potential exists for these contaminants to reach underlying aquifers and contaminate drinking water wells. Viruses and other pathogens, as well as other contaminants of emerging concern, were measured in stormwater and groundwater at an urban site containing a stormwater cistern and related subsurface infiltration gallery, three shallow lysimeter wells, and a monitoring well. Five of 12 microbial targets were detected more than once across the eight rounds of sampling and at multiple sampling points, with human-specific Bacteroides detected most frequently. The microbial and chemical contaminants present in urban stormwater were much lower in the water table monitoring well than the vadose zone lysimeters. There may be numerous causes for these reductions, but they are most likely related to transit across fine-grained sediments that separate the water table from the vadose zone at this location. Precipitation amount prior to sample collection was significantly associated with microbial load. A significant relation between microbial load and chloride-bromide ratio was also observed. The reduction in number and concentrations of contaminants found in the monitoring well indicates that although geologically sensitive aquifers receiving urban stormwater effluent in the subsurface may be prone to contamination, those with a protective cap of fine-grained sediments are less vulnerable. These results can inform stormwater infiltration guidance relative to drinking water wells, with an emphasis on restricting infiltration near water supply wells finished in geologically sensitive aquifers to reduce public health risks.

Viral, bacterial, and protozoan pathogens and fecal markers in wells supplying groundwater to public water systems in Minnesota, USA.

Drinking water supply wells can be contaminated by a broad range of waterborne pathogens. However, groundwater assessments frequently measure microbial indicators or a single pathogen type, which provides a limited characterization of potential health risk. This study assessed contamination of wells by testing for viral, bacterial, and protozoan pathogens and fecal markers. Wells supplying groundwater to community and noncommunity public water systems in Minnesota, USA (n = 145) were sampled every other month over one or two years and tested using 23 qPCR assays. Eighteen genetic targets were detected at least once, and microbiological contamination was widespread (96% of 145 wells, 58% of 964 samples). The sewage-associated microbial indicators HF183 and pepper mild mottle virus were detected frequently. Human or zoonotic pathogens were detected in 70% of wells and 21% of samples by qPCR, with Salmonella and Cryptosporidium detected more often than viruses. Samples positive by qPCR for adenovirus (HAdV), enterovirus, or Salmonella were analyzed by culture and for genotype or serotype. qPCR-positive Giardia and Cryptosporidium samples were analyzed by immunofluorescent assay (IFA), and IFA and qPCR concentrations were correlated. Comparisons of indicator and pathogen occurrence at the time of sampling showed that total coliforms, HF183, and Bacteroidales-like HumM2 had high specificity and negative predictive values but generally low sensitivity and positive predictive values. Pathogen-HF183 ratios in sewage have been used to estimate health risks from HF183 concentrations in surface water, but in our groundwater samples Cryptosporidium oocyst:HF183 and HAdV:HF183 ratios were approximately 10,000 times higher than ratios reported for sewage. qPCR measurements provided a robust characterization of microbiological water quality, but interpretation of qPCR data in a regulatory context is challenging because few studies link qPCR measurements to health risk.