ESTIMATED DEPTH TO GROUND WATER IN NORTH CAROLINA

Estimates of depth to ground water in North Carolina are useful in assessing the vulnerability of public drinking-water supplies to contamination at or near land surface. Depths to ground water range from just below land surface in parts of the outer Coastal Plain Province to greater than 100 feet below mountain peaks in the Blue Ridge Province of North Carolina.

Multiple regression analyses were used to estimate depths to ground water in the Coastal Plain and Piedmont physiographic provinces. Data for the statistical analyses were obtained from one Federal data base--the U.S. Geological Survey Ground-Water Site Inventory data base--and three State data bases--the North Carolina Department of Environment and Natural Resources pollution-incident data base, on-site wastewater treatment data base, and underground storage tank data base. Because the ground-water system in the Piedmont and Blue Ridge Provinces consists of fractured crystalline rock overlain by regolith, measurements of depth to water in shallow fractured rock are considered to represent the water table.

A geographic information system was used to compile data about potential predictor variables, including land-surface altitude, average annual precipitation, slope, distance to the nearest stream, soil thickness, and soil permeability. In the Coastal Plain Province, land-surface altitude and average annual precipitation were the most significant factors, accounting for about 39 percent of variation in the depth-to-water data. In the Piedmont Province, land-surface altitude, slope, and average annual precipitation accounted for about 33 percent of variation in the data; distance to the nearest stream, soil thickness, and soil permeability did not account for significant variation in depth to water. Only sparse depth-to-water data are available in the Blue Ridge Province at sites located exclusively in flood plains. Because representative data are lacking, a conceptual rather than multiple regression model is being considered to estimate depths to water in the Blue Ridge Province.