Virus occurrence in private and public wells in a fractured dolostone aquifer in Canada

Groundwater samples from 22 wells completed in a regional fractured dolostone aquifer in the Guelph region of southern Ontario, Canada, were collected over an 8-month period and analyzed for viruses and Campylobacter jejuni. Only 8% of the 118 samples exhibited viruses at extremely low concentrations, but of the 22 wells sampled, 10 (45%) were positive for human enteric viruses (polyomavirus, adenovirus A, and GII norovirus) including 5 of the 8 public supply wells (62.5%) and 5 of the 11 private wells (45%). Each virus-positive well had only one virus occurrence with six sampling events during the 8-month sampling campaign and only one virus type was detected in each well. The probability of virus detection was positively associated with well open-interval length. Virus concentration (in the wells that were virus-positive) was negatively associated with well depth and open-interval length and positively associated with overburden thickness (i.e., the thickness of unconsolidated materials overlying bedrock facies) and the amount of precipitation 8-14 and 15-21 days prior to the sampling date. The ephemeral nature of the virus detections and the low detection rate on a per sample basis were consistent with previous studies. The percentage of virus-positive wells, however, was much higher than previous studies, but consistent with the fact that the hydrogeologic conditions of fractured bedrock aquifers create wide capture zones and short groundwater travel times to wells making them more vulnerable to contamination occurrence but at very low concentrations.