Cryptosporidium incidence and surface water influence of groundwater supplying public water systems in Minnesota, USA

Authors: STOKDYK, JOEL - U.S. GEOLOGICAL SURVEY (USGS); SPENCER, SUSAN; ANDERSON, ANITA - MINNESOTA DEPARTMENT OF HEALTH; WALSH, JAMES - MINNESOTA DEPARTMENT OF HEALTH; FIRNSTAHL, AARON - U.S. GEOLOGICAL SURVEY (USGS); REZANIA, LIH-IN - MINNESOTA DEPARTMENT OF HEALTH; BORCHARDT, MARK

Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2019
Publication Date: 3/21/2019
Citation: Stokdyk, J.P., Spencer, S.K., Anderson, A.C., Walsh, J.F., Firnstahl, A.D., Rezania, L.W., Borchardt, M.A. 2019. Cryptosporidium incidence and surface water influence of groundwater supplying public water systems in Minnesota, USA. Environmental Science and Technology. 53(7):3391-3398. https://doi.org/10.1021/acs.est.8b05446.
DOI: https://doi.org/10.1021/acs.est.8b05446

Interpretive Summary: Cryptosporidium is an intestinal parasite that can cause severe illness and death in people, cattle, and other livestock. Cryptosporidium was responsible for the largest waterborne disease outbreak in United State history, which occurred during 1993 in Milwaukee, Wisconsin. In response to the outbreak, federal rules were enacted to protect the nation’s drinking water, but these only apply to public water systems that draw water from a surface water source, such as a lake or river. Groundwater sources have been ignored because it is assumed the parasite is too large to move vertically through the soil and reach groundwater. We sampled wells supplying public water systems in Minnesota and found 40% were positive for Cryptosporidium. The parasite was present even when there was no evidence of surface water entering the well, suggesting it can move from the land surface to groundwater. Cryptosporidum was measured by two methods and both methods gave the same result. From the types of Cryptosporidium identified in the well water it appears the sources were cattle and people. We showed Cryptosporidium is more common in groundwater than generally believed and it occurs at concentrations similar to those found in rivers and lakes.

Technical Abstract: Regulations for public water systems (PWS) in the USA 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 non-community 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 judgement 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 judgement. We identified C. parvum, C. andersoni, and C. hominis in 41, 2, and 2 samples, respectively and the predominant subtype was C. parvum IIa A17G2R1. Assuming the study wells were regulated as if they were surface-water-supplied PWS, 7 wells exceeded the allowable oocyst concentration and would have been required to add treatment. Cryptosporidium is not uncommon in groundwater, even when surface water influence is absent.