IMPLICATIONS FOR THE USE OF SHALLOW PRIVATE VERSUS DEEP MUNICIPAL WELLS FOR WATER SUPPLY AT THE URBAN FRINGE

Municipal drinking-water wells in Madison, Wisconsin pump groundwater from an underlying Cambrian sandstone aquifer. As in many urbanized areas, this pumping has produced significant drawdown in the aquifer. The continued expansion of Madison and its surrounding communities has increased the demand for municipal water supply, even as concerns grow over the impacts of increased pumping on area lakes, streams, wetlands, and springs.

Private on-site water-supply wells and wastewater-treatment systems are frequently used in rural areas, and they could help alleviate the growing demand for municipal water supply and treatment if used more frequently by new developments at or beyond the current urban fringe. However, unsewered developments near Madison have been limited by zoning and regulation at the county level, at least in part due to the potential impacts of these developments on groundwater quality.

We have been studying the impacts of a new 30-lot unsewered subdivision on groundwater quantity and quality since 2001. From a water-quantity standpoint, the main advantage of private on-site wells and septic systems is that pumped water is returned locally to the aquifer; however, the shallow bedrock aquifer that supplies water to private wells in our study area contains atrazine and elevated chloride and nitrate concentrations as a result of widespread agricultural land use. There is also concern about the possible direct cycling of contaminants between septic systems and wells, at least in densely developed areas.

In this study, groundwater models are being used to investigate the effects of using a larger community well, which may improve drinking-water quality, but results in water transfer between the deeper and shallower aquifer systems. Other alternatives considered are pumping water locally through on-site wells, but disposing wastewater through the municipal treatment system, and pumping water from municipal wells, but disposing wastewater through septic systems. These options offer tradeoffs in terms of local water quality and quantity, although both involve transferring water from one basin to another.