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Abstract: Hydrodynamics of the Shallow Salt-Water Interface in the Unglaciated Appalachian Plateau


Salt water commonly occurs at shallow depths (100 feet) below major stream valleys in the Appalachian Plateau, resulting in contamination of water wells used for domestic water supplies. Several factors have been suggested for the occurrence of the shallow salt-water interface, including overpressurization from Appalachian orogenic events, maturation of organic material, and the lack of flushing by glacial meltwaters. Anthropogenic causes include improper well plugging and brine disposal. However, anecdotal historical data indicate that shallow wells were drilled to extract brines to obtain salt for food preservation before large-scale oil and gas production was prevalent in the region.

Geochemical and pressure-head data from monitoring wells installed at several sites in eastern Kentucky indicate that the saltwater interface is depressed beneath the uplands and rises to shallow depths below major drainage valleys. A hydrodynamic system is created where fresh-water recharge accumulates in the interior of the upland areas. This results in an increase in hydrostatic pressure below the uplands, creating a gradient from the upland interior areas to the adjacent valley.

The location and orientation of drainage valleys hosting streams of third order or higher are often influenced by joints and regional fracture systems. Stress-relief fractures in valley bottoms, superimposed on regional fracture sets, create paths that allow for the upward migration of the salt water. Salt water is removed by dispersion and diffusion in the fractured ground-water zone, and diluted as the shallow ground water discharges into the overlying stream.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky