Statewide quantitative microbial risk assessment for waterborne viruses, bacteria, and protozoa in public water supply wells in Minnesota

Authors: Burch, Tucker; STOKDYK, JOEL - Us Geological Survey (USGS); RICE, NANCY - Minnesota Department Of Health; ANDERSON, ANITA - Minnesota Department Of Health; WALSH, JAMES - Minnesota Department Of Health; Spencer, Susan; FIRNSTAHL, AARON - Us Geological Survey (USGS); Borchardt, Mark

Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/22/2022
Publication Date: 5/17/2022
Citation: Burch, T.R., Stokdyk, J.P., Rice, N., Anderson, A.C., Walsh, J.F., Spencer, S.K., Firnstahl, A.D., Borchardt, M.A. 2022. Statewide quantitative microbial risk assessment for waterborne viruses, bacteria, and protozoa in public water supply wells in Minnesota. Environmental Science and Technology. 56(10):6315-6324. https://doi.org/10.1021/acs.est.1c06472.
DOI: https://doi.org/10.1021/acs.est.1c06472

Interpretive Summary: Public wells that provide drinking water for many communities can be contaminated by fecal material from humans, agricultural livestock, and/or wildlife. This fecal material contains pathogenic microorganisms that cause gastrointestinal infections in humans, but the risk of becoming infected by drinking water from contaminated public wells is unknown for most locations in the U.S. This study used an approach called quantitative microbial risk assessment (QMRA) to estimate the risk of infection (per 1,000 people per year) for 3 types of public wells in Minnesota. Risk estimates were based on 9 different waterborne pathogens measured in nearly 1,000 water samples collected over 2 years and from 145 public wells statewide. Results indicate that contaminated public wells could be responsible for anywhere between 38 and 401 infections per 1,000 people per year. Risk was highest for non-community wells (wells that serve 25 or more people per year outside of their homes) and lowest for non-disinfecting community wells (wells that serve 25 or more people per year in their homes but do not disinfect their source water). Risk for disinfecting community wells was between these two values (126 infections per 1,000 people per year). Overall, risk was dominated by Cryptosporidium, a type of microscopic parasite that infects both humans and cattle. However, risk could be reduced significantly by adding filtration as a treatment step for many wells. This study provides data that can be used in Minnesota and elsewhere for reducing the risk of infectious disease associated with public wells contaminated by fecal material from humans, livestock, and wildlife.

Technical Abstract: The risk of infection due to waterborne pathogens can be estimated via quantitative microbial risk assessment (QMRA) and is an important consideration in development of management practices for public groundwater systems. However, few groundwater QMRAs estimate risk based on site-specific hazard identification and exposure assessment, so prevailing risks in such systems remain poorly defined. This QMRA estimates infection risk for 9 waterborne pathogens using measurements from a 2-year pathogen occurrence study in which 964 dead-end ultrafiltration water samples were collected from 145 public wells throughout Minnesota. Risk for individual pathogens was generally low, with average daily doses equal to zero for most pathogens in 75-90% of individual wells. However, annual risk accumulated across all 9 pathogens was higher, varying from 3.8×10-2 infections/person-year for non-disinfecting community wells to 4.01×10-1 infections/person-year for non-community wells. These estimates are higher than the U.S. benchmark of 10-4 infections/person-year, though no waterborne outbreaks were known to occur in these systems during the study. Risk was dominated by Cryptosporidium but could be reduced significantly via application of treatment requirements in the surface water treatment rule. This suggests that improvements to identification of groundwater under the direct influence of surface water are particularly promising for risk mitigation.