A Source to Tap Investigation of Minnesota's Groundwater Supplies Used for Drinking Water

Abstract

Groundwater is often a desirable drinking water source because it is generally free of suspended solids and microbial pathogens and thus requires minimal, if any, treatment prior to distribution. Epidemiological studies have shown, however, that consumption of untreated groundwater increases risk of gastrointestinal illness. Previous work in Wisconsin, USA reported the occurrence of pathogenic viruses in groundwater supplies and resulting health impacts but bacterial pathogens were not investigated. In this study, a high-volume (300 – 1500 L) dead-end ultrafiltration sampling method was used to capture and recover microbes from 16 public groundwater systems throughout the State of Minnesota. The systems were sampled at the wellhead or source, after treatment if any (i.e., two systems did not treat or disinfect before distribution), and from one location in the distribution system. DNA was extracted from the microbes recovered in these samples and used as template for quantitative PCR analyses targeting 14 genes corresponding to pathogenic bacteria, one gene for a DNA virus, and the 16S rRNA gene as a marker for total bacteria. All samples were negative for the targeted genes from Campylobacter jejuni, Shigella spp., and Adenovirus; Escherichia coli-specific genes were only detected in water from a non-potable well with a documented history of contamination. Genes markers for two genera, Legionella and Mycobacteria, that include species that are opportunistic pathogens, were detected in four of the 16 public groundwater supplies, with Legionella levels decreasing in disinfected systems while Mycobacteria levels tended to increase. Raw water 16S rRNA gene concentrations ranged from 10^5 – 10^8 gene copies/L, decreased to background levels after disinfection, then rebounded at the tap in the majority of cities. There was no significant difference in 16S rRNA gene concentrations from source-to-tap in the two non-disinfecting cities. Raw water samples contained diverse and previously uncharacterized organisms as revealed by DNA sequencing analyses, and beta diversity analyses suggest that community composition is driven by source water and/or disinfection. The results from this study suggest that groundwaters supplying public water systems in Minnesota are largely free of enteric pathogens but may contain opportunistic pathogens.