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Bradbury, K. R.; Borchardt, Mark A.; Gotkowitz, Madeline B.; Hunt, R. J. / Assessment of virus presence and potential virus pathways in deep municipal wells
[DNR-197] (2008)

Introduction,   pp. 5-8 PDF (2.1 MB)


Page 5


Introduction
Virus contamination of groundwater
Among the many waterbome pathogens of humans, enteric viruses have the greatest
potential to move deeply through the subsurface environment, penetrate aquitards, and
reach confined aquifers. Enteric viruses are extremely small (27-75 nm), readily passing
through sediment pores that would trap much larger pathogenic bacteria and protozoa.
Viruses have been found in groundwater at depths of 67 m (Keswick and Gerba 1980;
Robertson and Edberg 1997) and 52 m (Borchardt et al 2003) and lateral transport has
been reported as far as 408 m in glacial till and 1600 m in fractured limestone (Keswick
and Gerba 1980). Several recent studies have demonstrated widespread occurrence of
viruses in domestic and municipal wells in the United States (Abbaszadegan et al 2003;
Borchardt et al 2003; Fout et al 2003; Borchardt et al 2004), and approximately half of
waterborne disease outbreaks attributable to groundwater consumption in the United
States have a viral etiology (National Primary Drinking Water Regulations, 2006). The
US Environmental Protection Agency has listed several viruses on its drinking water
Contaminant Candidate List, emphasizing that waterborne viruses are a research priority
(http://www.epa.gov/safewater/ccl/index.html). Although the vulnerability of
groundwater to virus contamination is now recognized, the occurrence of viruses in
confined aquifers has rarely been explicitly investigated. In the most comprehensive
groundwater-virus study to date, Abbaszadegan et al (2003) sampled 448 groundwater
sites in 35 states and found 141 sites (31.5%) were positive for at least one virus type.
Previous virus sampling in the Madison area
During 2005 and 2006 we undertook initial virus sampling of three deep bedrock wells
serving the city of Madison, Wisconsin (Borchardt et al. 2007a). Each of these high-
capacity wells is over 700 feet deep and cased to at least 220 feet below the surface. The
vertical hydraulic gradient is downward due to a major cone of depression beneath
Madison. Two of the wells (wells 7 and 24) are cased through the Eau Claire shale, a
regional aquitard described by Bradbury and others (1999) and thought to provide
excellent protection to the underlying sandstone aquifer. A third well (well 5, now
abandoned) was open both above and below the shale. Conventional wisdom suggested
that viruses would not be detected in any of the three wells due to the probable long
travel times from the surface to the wells, the depths of the wells, and the assumed short
(six months to two years) lifetime of the viruses. The surprising result of the study was
that viruses were repeatedly detected in the two wells thought to have greatest protection
due to their deep casings (wells 7 and 24). Viruses were detected in 4 of 10 samples
from well 7 and 3 of 10 samples from well 24 (Borchardt et al. 2007a). Moreover, five of
the seven positive samples tested positive for infectivity, suggesting relatively rapid
transport from the virus source to the wells. Replicate sampling and careful laboratory
procedures have ruled out laboratory contamination as a source for the viruses. The
human enteric viruses detected include serogroups coxsackieviruses and echoviruses as
wells as poliovirus vaccine strain Sabin 1. The Madison, Wisconsin wells are typical of


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