HUMAN ENTERIC VIRUSES IN GROUNDWATER FROM A CONFINED AQUIFER

There are numerous reports of human pathogenic viruses in groundwater from shallow sandstone, and sand and gravel, aquifers. Many public water systems draw water from confined aquifers that are considered protected from microbial contamination by overlying aquitards. This study determined the presence of human pathogenic viruses in well water from a deep confined sandstone aquifer.

Most municipal groundwater production in south-central Wisconsin is from the Mount Simon aquifer, a deep sandstone aquifer overlain by the regionally extensive Eau Claire aquitard. The aquitard consists of fine-grained siltstone and shale facies ranging in thickness from zero to more than 60 feet. An upper, unconfined aquifer system overlies the Eau Claire. Pumping from wells completed in the Mount Simon induces downward vertical gradients across the Eau Claire aquitard.

Three municipal water-supply wells were each sampled ten times between June 2003 and August 2004. Raw, untreated groundwater samples were collected according to standard virus filtration methods from sample ports. Samples were analyzed for enteroviruses, rotavirus, hepatitis A virus (HAV), and norovirus genogroups 1 and 2 using a reverse-transcription polymerase chain reaction (RT-PCR) followed by sequencing to confirm virus identity.

Seven of the 30 samples (23%) were positive for enteroviruses; other virus groups tested were absent in all samples. The enterovirus-positive samples were collected from two wells that are cased through the Eau Claire aquitard and presumably draw water from the confined Mount Simon aquifer. The third well, which has a shallow casing and is open to the upper and lower aquifers, was virus-negative throughout the study period. Leaking sanitary sewer pipes are a likely fecal source near the wells. The presence of human enteric viruses in the lower aquifer suggests that viruses are able to penetrate the aquitard or that colloidal-size particles can bypass the aquitard. Possible routes of transport include windows in the aquitard due to heterogeneity in the thickness and/or lithology of the siltstone/shale facies. Flaws in well construction, such as fractures or bridges within the grout seal emplaced between the casing and borehole annulus, may also provide a pathway for rapid migration across the aquitard.