GEOLOGY AND HYDROGEOLOGY OF THE DELMARVA COASTAL BAYS

The coastal bays along the Atlantic coast of the Delmarva Peninsula are representative of a common class of small estuaries. Because of restricted circulation and limited exchange with the ocean, these bays tend to trap both nutrients and fine-grained sediments, and are susceptible to eutrophication. Ground water supplies a significant, but poorly quantified, proportion of the total flux of fresh water and nutrients to the coastal bays. For lack of better information, existing hydrologic models typically assume a homogeneous, isotropic medium for the surficial aquifer. Recent field studies in the coastal bays of Delaware and Maryland have employed several standard and new technologies to survey the hydrogeologic setting of the area. Ultimately, this new, detailed information will be used to improve hydrologic models for simulating ground-water flow and the transport of nutrients in the surficial aquifer of the coastal zone.

The integrated program of field data collection has included: Chirp and boomer seismic surveys in the bays to map the Holocene infill and the Pleistocene and upper Tertiary stratigraphic units that comprise the surficial aquifer; horizontal resistivity to map the distribution of fresh and saline ground waters beneath the bays and identify subsurface zones of mixing; and aerial thermal-infrared imagery to detect temperature anomalies in the bays and tidal tributaries that indicate areas of focused ground-water discharge. In addition, hydraulic vibracoring, in situ porewater sampling, and gamma and electromagnetic-induction logging of boreholes down to 25 m in the estuary are being used as ground truth for the seismic and resistivity profiles. These observational data have provided some fundamental insights into modes of ground-water flow to the coastal bays. A recurring theme is that the stratigraphic complexity of the coastal zone controls many hydrologic processes. Among these are preferential pathways of ground-water flow, subsurface zones of mixing between fresh and salt water, and geochemical transformations of constituents transported by ground water, such as denitrification of ground-water nitrate by chemically reduced sediments.