Jo Leslie Eimers has performed hydrologic modeling and analysis for the USGS since 1983. Silvia Terziotti has performed geographic information systems analysis for the USGS since 1988. J. Curtis Weaver has performed surface-water analysis for the USGS since 1987. Overlay and index methods were developed by the U.S. Geological Survey and North Carolina Public Water Supply Section to rate the unsaturated zone and watershed characteristics for use in assessing more than 11,000 public water-supply wells and approximately 245 public surface-water intakes statewide for vulnerability to contamination. Factors selected for rating the vulnerability of the unsaturated zone to surface contamination were vertical series hydraulic conductance, land-surface slope, land cover, and land use. Factors selected for rating vulnerability of watersheds to surface contamination were average annual precipitation, land-surface slope, land cover, land use, and groundwater contribution. Selection of factors, development of ratings, and assignment of weights were based on literature and consultations with experts in hydrology, geology, forestry, agriculture, and water management. Rating factors were assigned values from 1 to 10, covering the possible range of values of a given factor in North Carolina. Factors were weighted 1, 2, or 3 to reflect their relative influence on the vulnerability of the water supply. Factor values were put into geographic information system layers, which were grids having 30-meter by 30-meter cells. The weakness of these rating methods is that consensus among experts does not imply veracity. An investigation of the statistical relations between contributing factors and particular water contaminants in North Carolina would, by confirming these methods, contribute to public water-supply protection efforts in North Carolina. Efforts to increase the accuracy of source water assessment area delineations for public water-supply wells also would contribute to public water-supply protection efforts in North Carolina. In North Carolina, the determination of overall susceptibility of each public groundwater supply and surface-water intake is based on two key components-a contaminant rating and an inherent vulnerability rating. The contaminant rating is determined by the State's Public Water Supply Section (PWSS) from an inventory of existing data bases of potential contaminant sources. Additional factors include the density of contaminant sources in the delineated area, proximity to the intake, and the contaminant risk to the public water supply. The inherent vulnerability rating is a measure of the potential for contaminants within a delineated source area to reach the water supply. The inherent vulnerability of a groundwater source is determined by combining an aquifer rating and an unsaturated zone rating (North Carolina Department of Environment and Natural Resources, 1999a). The inherent vulnerability of a surface-water source is determined by combining watershed classification, intake location, raw-water quality, the...
