HYDROSTRATIGRAPHIC FRAMEWORK AND CONTROLS ON GROUND-WATER DISCHARGE TO MARYLAND AND DELAWARE COASTAL BAYS

The hydrostratigraphic framework appears to impart a first-order control on the distribution and flow of fresh ground water and brackish zones of mixing in the aquifer beneath the Maryland and Delaware coastal bays. Geophysical surveys of horizontal resistivity, Geopulse seismics, and aerial thermal infrared imaging show vertical and horizontal patterns that deviate from, and are more complex than, the distribution of fresh ground water predicted from variable-density flow modeling. Hydraulic vibracoring, gamma logging, and in situ porewater sampling in the estuary to depths of 20-25 m provide lithologic and geochemical ground truth for interpretation of the 2-dimensional geophysical profiles. These field observational data will be used to guide refinements to the numerical modeling. Key parameters that influence the flow of ground water and modes of discharge to the coastal bays include: the distribution and thickness of fine-grained Holocene estuarine sediments; the depth, geometry, and orientation of incised valleys and paleo-drainage networks; the horizontal and vertical distribution of laterally continuous high-permeability sediments (Pleistocene shorelines and nearshore sands) and low-permeability sediments (back-barrier muds and incised-valley fill); and the depth to the underlying confining layer(s). For Chincoteague and Sinepuxent Bays in Maryland, the Geopulse seismic profiles (frequency range of 200 to 2000 Hz) show three primary sequences down to 280-300 ms (about 225-250 m). The upper sequence of Pleistocene back-barrier, coastal, and inner-shelf deposits comprises a complex surficial aquifer. The Pleistocene sequence is underlain by 60 to 70 m of the Pliocene marine Yorktown Formation, which is a regional confining layer. The Yorktown confining layer overlies the confined aquifers of the upper Miocene marine units. On a more local scale, fresh ground water was identified beneath Chincoteague Bay as much as 1 km from shore along the trend of Pleistocene shorelines; examples include off South Point on Sinepuxent Neck and in Purnell Bay near the Virginia border. Similarly, the vertical distribution of fresh and saline interstitial water is controlled on the decimeter scale by permeability contrasts from the depositional bedding of the Pleistocene and Holocene coastal sediments.