ARS | Publication request: Evaluation of mechanisms that might control transport of wastewater contaminants in bedrock multi-aquifer systems

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: September 9, 2011
Publication Date: N/A

Technical Abstract: Ongoing research has identified infectious human enteric viruses in the Madison, Wisconsin, public supply wells that draw water from a deep, confined sandstone aquifer. These viruses most likely originate from leaking sanitary sewers and are a potential human health risk. Due to a relatively short (1-2 years) period in which viruses can survive in the groundwater environment, rapid transport of sewer effluent is required to explain the presence of infectious viruses in deep, public supply wells. Sewer effluent contains a high concentration of viruses and transport of only a small volume is required to result in detection at a deep well. Numerical modeling of transport to one of the affected wells, using reasonable estimates of porous media flow parameters, results in travel times greater than the two year period of virus infectivity, suggesting that porous media flow cannot explain the virus presence. The preferential flow mechanisms that would allow rapid transport of near surface sewer effluent into the deep aquifer include some combination of a leaky aquitard, fracture flow, multi-aquifer wells, and defective well casings. Evaluation of transport mechanisms is based on water-level, chemistry, and enteric virus data collected from several monitoring wells near a Madison public supply well, which is cased through the regional aquitard. Borehole geophysics, downhole imaging, borehole flow meter data, pumping test results, and straddle packer tests show that several fractures are present in the upper aquifer and apparently contribute flow in the upper aquifer. Chloride, bromide, nitrate, and other wastewater indicators were present in high concentrations at discrete depths, suggesting wastewater is travelling along fractures. Limited sampling demonstrated the presence of viruses in the upper aquifer and a probable link with recharge events. Fracture flow is a likely transport mechanism at this site for viruses to reach the public supply well. If fractures are present, they could allow relatively small volumes of sewer effluent to travel rapidly through the subsurface and reach wells previously thought to be well protected from these contaminants. However, we have not yet been able to document the presence of through-going fractures in the aquitard.