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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)

Conclusions and recommendations,   pp. 33-34 PDF (769.8 KB)


Page 33


Conclusions and Recommendations
Conclusions
Human enteric viruses are a common contaminant in water produced by municipal wells
in Madison, Wisconsin. Viruses were found in all wells sampled monthly, though not in
every sample from every well. The percentage of virus-positive samples ranged from
60% in wells know to have multi-aquifer construction or shallow casings to 18 % in well
30, a new, deep well deeply cased across a regional aquitard. The presence of viruses in
wells cased and grouted 200 to 300 feet below a regional aquitard raises disturbing
questions about aquifer vulnerability in confined-aquifer settings usually thought to be
well-protected from surface contaminants.
Although we are unable at this time to elucidate the transport pathway for viruses from
the surface to the wells, several lines of evidence suggest that transport is rapid - on the
order of months or weeks rather than years. Because they require a human host, these
viruses must originate at or just below the land surface. Identical viral serotypes were
found in sewage and groundwater, and the mix of viral species varied with time through
the project. Moreover, virus detections in wells, and virus concentrations in lakes and
sewage varied together through time. This temporal correlation is consistent with
relatively rapid transport.
The Madison Lakes are probably not the main source of the viruses found in the Madison
municipal wells. Lake water contained some but not all of the serotypes found in the
wells, and virus levels in lake water are generally low. Furthermore, the 180/2H signature
of water produced by most Madison wells is not consistent with a significant lake water
component of recharge.
The most likely source of the viruses in the wells is the leakage of untreated sewage from
the Madison sewer system. Untreated sewage sampled at the Madison sewage treatment
plant contains virus concentrations several orders of magnitude higher than
concentrations observed in wells or lakes. Review of sewer construction and location
data, the shear total length of city sewers (hundreds of miles), and the evidence that
sewers are not completely water-tight suggests that leakage of sewage to the subsurface
environment probably occurs in at least some parts of Madison. Given the high
concentrations (millions of genomic copies per liter) of viruses in sewage, it would take
very little sewage to produce the virus concentrations observed in the wells.
Human enteric viruses might be excellent tracers of recent groundwater. They have the
desirable tracer characteristics of detectability over several orders of magnitude, high
mobility, short analytic times and relatively reasonable cost, and are time-specific due to
constantly changing serotypes. Although the presence of detectable tritium in a well is
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