MULTI-AQUIFER WELLS PROVIDE LARGE-VOLUME CONTAMINANT TRANSPORT PATHWAYS IN URBAN AREAS

Public water-supply wells and sanitary sewers are critical components of urban infrastructure, but sewer leakage threatens the quality of groundwater in urban areas. Human enteric viruses are excellent tracers of sanitary sewage because they are mobile in the subsurface and can be quantified over a broad concentration range. Previous work by our group identified such viruses in groundwater from wells completed in a confined aquifer and cased to depths up to 95 meters. Several preferential flow pathways, including natural fractures or defective well casings, could explain the rapid and deep transport of these contaminants. This presentation focuses on the contribution of multi-aquifer wells to virus transport.

We have used a combination of field and modeling studies to evaluate the contribution of multi-aquifer wells to virus transport. Measurements of vertical borehole flow were collected in five wells, each of which is open to a shallow bedrock aquifer, an underlying shale aquitard, and a deep aquifer. One well located in the regional discharge area showed no measureable flow across the aquitard. The other four wells are located in regional recharge areas. Measured velocities ranged from 1 to 13 meters/minute downward in these wells, and volumetric flux, which depends on well bore diameter, ranged from 190 to over 3,790 liters/minute. Modeling shows that these flow rates are reasonable, and that multi-aquifer wells provide rapid transport from the water table to the deep aquifer. However, model-simulated travel times from these wells to other water-supply wells are not sufficiently fast to explain observed virus transport in this setting.

Both field measurements and modeling confirm that multi-aquifer wells are potential conduits for large volumes of groundwater flow. Their significance with respect to transport of sewer leakage and other contaminants should be considered in managing urban well fields.