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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)

Discussion,   pp. 28-32 PDF (2.1 MB)


Page 28


Discussion
Significance of virus detections
Viruses were detected in at least one sample from all but one of the municipal wells
sampled for this project and in at least two samples from each of the six wells chosen for
long-term sampling. These findings are consistent with our previous work (Borchardt
and others, 2007a) and show that even deeply cased municipal wells in confined aquifer
settings can be susceptible to pathogen contamination.
Potential virus pathways to wells
As stated in the introduction to this report, the four conceptual models of virus transport
to the confined aquifer include (1) transport through the aquitard by porous-media flow;
(2) transport by porous-media flow around the edge of the aquitard or through nearby
"windows" or breaches in the aquitard, including local lakes; (3) transport by rapid flow
through fractures in the aquitard or through cross-connecting nearby wells; and (4)
transport by rapid flow along the well annulus through damaged, deteriorated, or poorly
installed grout or breaches in the well casing. This current project has not been able to
confirm or discount any of these potential flow paths. We had hoped to undertake in-well
borehole sampling during this project in order to evaluate pathway 4 above, however
logistical considerations prohibited this work during the past year. We intend to carry out
the in-well sampling as part of a follow-up project during 2008-2009.
Lakes as a source of viruses
Although at first glance infiltrating lake water seems a plausible source for the viruses
found in the municipal wells, several lines of evidence show that the lakes are probably
not the primary virus source. First, the deuterium/oxygen- 18 relationships (figure 9)
suggest that only two wells (7 and 19) receive a significant proportion of lake-derived
water, while all wells contained viruses. Second, with the exception of the July 2008
levels in Lake Mendota, virus concentrations in the lakes are generally as low as or lower
than virus concentrations in the wells. Assuming significant mixing and dilution with
virus-free water in the aquifer, the lake virus contents are likely too low to account for the
virus levels in the wells. Third, the lakes contained only four of the six virus species
detected in the wells.
Sanitary sewers as a source of groundwater contamination
Sanitary sewers are a major part of civic infrastructure in urban settings and represent a
significant potential source of groundwater contamination. Sewer exfiltration, or outward
leakage of sewage wastes, represents a potential source of pathogens, toxic chemicals,
pharmaceutical compounds and other materials to the subsurface environment (Bishop et
al. 1998). There have been two schools of thought on the significance of sewer
exfiltration (Rutsch et al. 2008). Some investigators argue that the overall impact of
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