PLACING MINNESOTA’S VIRAL OCCURRENCE DATA IN A GEOLOGIC CONTEXT

As requested by the Minnesota state legislature, the Minnesota Department of Health (MDH) has conducted a monitoring study to examine the occurrence of human enteric pathogens in groundwater sources used for drinking water in Minnesota, as well as an epidemiological study to evaluate the association between source water virus occurrence and incidence of community acute gastrointestinal illness. Final evaluations of the data are expected in 2018.

Phase 1 of the monitoring study began in May 2014 and ended in April 2015, and consisted of bimonthly sample collection by ultrafiltration at 82 randomly selected public water supply wells that supply non-disinfected drinking water. Phase 2 ran from May 2015 to April 2016 and included 28 wells from Phase 1, along with 56 additional wells that were predominantly located in geologic settings vulnerable to contamination. Additionally, source water at six community systems was sampled on a weekly basis as part of the epidemiological study.

Project partners at the U.S. Department of Agriculture and U.S. Geological Survey analyzed samples for a broad array of pathogens and markers of fecal contamination. Quantitative Polymerase Chain Reaction (qPCR) was used to detect microbial DNA/RNA in source water samples, and in some cases additional analyses involved cell culture or microscopic methods. Chemical indicators such as TOC, ammonia, nitrate, boron, bromide, chloride, tritium and stable isotopes of water were also analyzed.

The microbiological occurrence rates were highest during Phase 1, and were transitory, with most wells only positive in 1 out of 6 sample rounds. Preliminary assessments suggest that detections did not correlate well with standard geologic indicators of vulnerability such as well depth, confining unit presence or thickness, etc., but may be associated with precipitation or snowmelt events. This suggests that detections were largely controlled by small volumes of fast-moving recharge that exploited pathways that are difficult to discern. These may range from naturally occurring secondary porosity to well construction shortcomings.