The subsurface ground water flow in the Black Hills region is very complex, resulting in overlapping source water assessment areas for public water supply systems throughout this region. Therefore, the Black Hills region assessment approach considers the entire Black Hills uplift as a single, hydrologically interconnected area, and assessment efforts are concentrated in aquifer recharge areas for large sensitive aquifers, and along all perennial streams within the Black Hills. These hydrologically sensitive areas are considered "Zone A" assessment areas, and receive a comprehensive contaminant source inventory. The remaining delineated watershed is a "Zone B" assessment area. A limited contaminant source inventory is conducted in this area.