THE POSITION OF THE FRESH WATER-SALTWATER INTERFACE IN AQUIFERS UNDERLYING SIX COUNTIES IN NORTHEASTERN NORTH CAROLINA

Groundwater is a limited natural resource and is relied upon for domestic, commercial, and agricultural use in northeast North Carolina. Over the next several years, groundwater demands in this area are expected to increase due to significant population, industrial, and agricultural growth. Identifying and studying the aquifers underlying these counties, and particularly locating the freshwater-saltwater interface, is an essential step toward completing a groundwater resource analysis for the area.

Nine east-southeast dipping hydrostratigraphic units (labeled A through I) underlie Bertie, Chowan, Hertford, Martin, Pasquotank, and Perquimans Counties. Geophysical and lithologic data indicate that the sand and limestone units dip east-southeast and thicken coastward. The aquifers generally dip 5-10 ft/mi toward the southeast. These strata were deposited as transgressive and regressive marine and nearshore continental facies from Cretaceous to Recent. Silt and clay confining layers that separate the aquifers are of extremely variable thickness. The aerial extent of the aquifers also varies. Four of the nine aquifers (A, B, H, and I) are identified in all wells drilled to the appropriate depths in the study area.

Saltwater (>250 ppm Cl-) is identified in two hydrostratigraphic units west of the Chowan River (Aquifer H and I) and in all screened units (except the surficial aquifer) east of the Chowan River. Chloride concentration increases to the south-southeast in each of the salty hydrostratigraphic units with concentrations in excess of 7,000 ppm identified in wells screened in Aquifer D in southeast Pasquotank County. The gradient of chloride concentrations ranges from 211 ppm/mile (HU-I) to 1,333 ppm/mile (HU-B). Understanding the physical and chemical characteristics of groundwater in the study area is an essential part of analyzing the water resources for this region.