Delineation of Groundwater Capture Zones for Municipal Wells in Dane County, Wisconsin

Background: This report describes the use of a three-dimensional computer model to delineate zones of contribution for municipal water-supply wells in Dane County, Wisconsin. The capture zone, or zone of contribution (ZOC) of a well is the land surface area where recharging precipitation enters a groundwater system and eventually flows to the well (Fig 1). Delineating the zones of contribution for municipal wells is a critical step in establishing wellhead protection areas for the wells. A wellhead protection area (WHPA) is defined by the federal Safe Drinking Water Act as the “surface and subsurface area surrounding a water well or well field, supplying a public water system, through which contaminants are reasonably likely to move toward and reach such water or well field .” In practical terms, the ZOC is a technically-defined area based on groundwater hydraulics, while the WHPA is a legally-defined area including all or part of the ZOC and within which zoning practices or other land-use controls can be implemented to help protect groundwater from contamination.