HYDROGEOLOGICAL AND STATISTICAL EVIDENCE FOR WIDE-SPREAD ENTERIC VIRUS CONTAMINATION OF DEEP MUNICIPAL WELLS

Over the past eight years our research group has repeatedly detected human enteric viruses in water produced from deep (over 800 ft) bedrock water-supply wells in Madison, WI. The likely source of the viruses is leakage from urban sewers. These virus detections have been surprising because human enteric viruses have relatively short (1-2yr) viable lifetimes in northern groundwater environments and the Darcy travel times from near-surface sources to the wells were calculated to be tens to hundreds of years.

In evaluating potential virus pathways and transport mechanisms we have become convinced that the viruses originate from a widespread, distributed source, probably sanitary sewers, related to the entire urbanized area rather than to one or more discrete point sources. Several lines of evidence support this concept. First, viruses have been detected in every one of 10 wells sampled repeatedly over an approximately 30 mi² area. It is difficult to devise a single transport mechanism that explains this widespread virus occurrence. Second, analysis of variance shows that the temporal virus detection pattern is statistically similar throughout the study area, again implying a regional rather than localized phenomenon. Third, both visual inspection and regression analysis show that spikes in virus concentrations are related to regional recharge events following large storms or snowmelt. Fourth, virus subtypes (i.e., serotypes) detected in well water correlate with serotypes in sewage, and variations in the virus composition of sewage correlate with variations in the viruses detected in the wells.

These findings are potentially very significant for several reasons. First, the widespread virus occurrence in the deep wells shows that exfiltration from sanitary sewers can have a significant impact on urban groundwater quality. Second, the frequent virus detections show that the deeply-cased municipal wells are not well protected from near-surface contaminants. Finally, the rapid transport times from the surface to the wells imply that discrete features, which might include fractures, cross-connecting wells, improperly abandoned wells, or failing well casings, must be controlling vertical and horizontal groundwater movement.