WATER-SUPPLY CONSTRAINTS IN THE VIRGINIA COASTAL PLAIN RESULTING FROM ELEVATED GROUND-WATER SALINITY IN THE CHESAPEAKE BAY IMPACT STRUCTURE

The Chesapeake Bay impact structure closely coincides with Virginia’s “inland saltwater wedge”—part of the Coastal Plain aquifer system in southeastern Virginia that contains saltwater as far inland as 30 miles. Ground-water withdrawals in the Virginia Coastal Plain have increased during the past century to about 150 million gallons per day. A major part of this withdrawal has occurred at industrial pumping centers located outside of but in proximity to the saltwater wedge, resulting in water-level declines as great as 200 feet and redirection of flow landward from the saltwater wedge. Although these withdrawals have generally stabilized, significant increases are expected for public supplies in metropolitan areas underlain by the saltwater wedge. Withdrawal and desalinization of brackish ground water are being actively developed in these areas.

Hydrochemical data indicate that the saltwater wedge originated primarily from seawater emplaced throughout the Coastal Plain sediments during one or more inundations of the land surface prior to the Pleistocene Epoch. Much of the continental shelf re-emerged during the Pleistocene, leading to extensive emplacement of fresh ground water in places tens of miles east of the present-day shoreline. Residual saltwater persisted around the impact structure, however, despite having been flushed from adjacent areas. Hydrodynamic details indicate that the impact structure is hydraulically isolated from the surrounding flow system.

Although withdrawals from outside the saltwater wedge have imposed the potential for saltwater intrusion for much of the past century, the extent of the saltwater wedge has not increased. The hydraulic sluggishness of the impact structure that led to formation of the saltwater wedge possibly in turn accounts for its apparent lack of response to present-day flow conditions. Increases in salinity have been reported, however, for some wells located within the saltwater wedge. Thus, localized withdrawal-induced redistribution of salinity poses a potential limitation on the long-term feasibility of desalinization and, in turn, on increased withdrawals. Modeling is planned to predict future responses of the saltwater wedge to ground-water withdrawal.