HYDROCHEMICAL EVIDENCE FOR THE ORIGIN OF ELEVATED GROUND-WATER SALINITY IN THE CHESAPEAKE BAY IMPACT STRUCTURE, SOUTHEASTERN VIRGINIA

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 ratios of bromide to chloride, chlorine-36 to total chloride, and among stable hydrogen and oxygen isotopes indicate that the saltwater originated primarily from mixing of freshwater and seawater. Detailed vertical profiles of specific conductance and chloride concentration indicate a convoluted mixing zone along the margin of the impact structure between freshwater to the west and seawater to the east. Toward the interior of the structure, specific conductances and chloride concentrations exceeding that of modern seawater were produced by evaporation, as indicated by stable hydrogen and oxygen isotopic ratios. Distinct mixing and evaporation trends among the isotopic ratios indicate the original seawater to be isotopically lighter than modern seawater and suggest that the seawater pre-dates the Pleistocene Epoch.

Seawater likely was emplaced throughout the Coastal Plain sediments during one or more inundations of the land surface prior to the Pleistocene Epoch. Evaporation possibly occurred either from arid climatic conditions and (or) from heat associated with the impact event. As a result of re-emergence since the Pleistocene, freshwater recharge has displaced most of the seawater, particularly to the north as a result of large volumes of glacial-melt water and a large hydraulic gradient associated with high water-table elevations. By contrast, residual saltwater has persisted around the impact structure. The thick and clayey Chickahominy Formation that overlies the structure impedes upward flow and discharge needed for flushing to advance across the structure, resulting in subdued lateral flushing along the structure’s outer margin.

Although present-day ground-water withdrawals impose the potential for saltwater intrusion, 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.