SPEIRAN, GARY K., U.S. Geological Survey, Richmond, VA

ABSTRACT: Flow and Quality of Groundwater in Coastal Discharge Areas of the Eastern Shore, Virginia

Variations in shallow geology, topography, and land cover affect the flow and quality of groundwater near coastal discharge areas on the eastern shore of Virginia. Groundwater discharges from the water-table aquifer to streams, estuaries, and saltwater and freshwater wetlands. Sediments in many discharge areas have variable lithology, mineral content, and organic content because previously deposited sediments were eroded and redeposited in differing nearshore environments. Sediment changes coincide with changes in land-surface elevations because original sediments were eroded and redeposited at different sea level elevations. Where land-surface elevations decrease, the water table approaches land surface.

These variations in sediments coincide with changes in land cover from upland fields to lowland forests and wetlands. Evapotranspiration becomes a major pathway for groundwater discharge in lowlands during the growing season. The effects of evapotranspiration result in low lateral hydraulic gradients, upward hydraulic gradients that can equal the magnitude of lateral hydraulic gradients, and diurnal fluctuations in the water table as great as l ft. Where water-table altitudes decrease, groundwater discharges from underlying confined aquifers.

Groundwater quality reflects sources of water and geochemical changes. Distinctly different water-quality types result from the effects of agricultural practices, water discharged from underlying confined aquifers, and brackish water probably from the combined effects of recharge caused by tidal flooding and lateral inflow caused by evapotranspiration. Organic matter in sediments decreases dissolved oxygen concentrations in groundwater and can cause a shift from oxidizing to reducing reactions.

AAPG Search and Discovery Article #90995©1993 AAPG Eastern Section Meeting, Williamsburg, Virginia, September 19-21, 1993.