STRUCTURAL CONTROLS ON CRETACEOUS SEDIMENT DISTRIBUTION, DEPOSITIONAL HISTORY, AND HYDROLOGY OF THE POTOMAC AQUIFER, VIRGINIA

The distribution, depositional history, and present hydrology of lower Cretaceous fluvial sediments of the heavily used Potomac aquifer in Virginia are controlled by major structural features of the Atlantic Coastal Plain. Coarse quartzo-feldspathic sand and gravel overlying the Norfolk arch were deposited as longitudinal bars and channel fills by immature, high-gradient braided streams. Erosion of persistently uplifted granite and gneiss to the west sustained the supply of coarse sediment to a bilaterally symmetric, eastward radiating, regional fluvial depositional complex. Increasing fine-grained beds were deposited northward into the Salisbury embayment and southward into the Albemarle embayment, as overbank deposits by mature, medium- to low-gradient meandering streams. Another depositional complex centered on the Salisbury embayment eroded schist that underwent three cycles of initial uplift and erosion followed by crustal stability and leveling. Resulting aggregations of fine-grained beds collectively as thick as several hundred feet comprise two continuous confining units that hydraulically separate three vertically spaced subaquifers. The subaquifers across the Salisbury embayment merge southward into a single undivided, hydraulically interconnected aquifer across the Norfolk arch. Current regulatory limits on water-level declines relative to aquifer top surfaces have the potential to be exceeded for the undivided part of the Potomac aquifer.

Potomac aquifer sediments in Virginia and adjacent parts of Maryland and North Carolina are characterized by 2,725 vertical sediment intervals interpreted from geophysical logs of 456 boreholes. Hydraulic connectivity is characterized by vertical hydraulic gradients calculated from water levels in 129 vertically spaced pairs of wells. Pumping tests at 197 locations and published groundwater age dates further indicate that sediment hydraulic conductivity and groundwater flow velocity are greatest in coarse sediments across the Norfolk arch but decrease northward in increasingly fine-grained sediments. The borehole sediment-interval data can potentially support future flow-simulation modeling based on refined discretization of the Potomac aquifer and/or a geostatistical distribution of hydraulic properties.