Hydrogeology of Delmarva Coastal Bays: Integration of Technologies

WRD PROJECT #: MD154
PROJECT CHIEF: Krantz, David E.
BEGIN DATE: 01-Jan-2001
END DATE: 30-Sept-2002

Customers currently supporting the project:

U.S. Geological Survey

Problem

The coastal bays along the Atlantic coast of Delaware and Maryland receive a significant but poorly quantified flux of nutrients from ground-water discharge as base flow to small tributaries or as a direct discharge to the estuaries. Because of the restricted circulation and long residence times of water within the coastal bays, these estuaries are vulnerable to eutrophication from excess nutrients. The upper reaches of several coastal bays, Indian River in Delaware and Newport Bay and ST Martins River in Maryland, have been identified by the U.S. Environmental Protection Agency as moderately to severely impacted.

Objectives

The proposed project would build upon the results of three ongoing studies of nutrient fluxes to the Delaware and Maryland coastal bays with the primary goals of creating a hydrostratigraphic framework and mapping the distribution of fresh ground water and zones of mixing with saltwater beneath the bays. These data would be used to develop a finite-element model to simulate ground water flow and mixing. An additional geochemical component of the project would use stable isotope tracers to evaluate sources, transport, chemical transformations (such as denitrification), and fluxes of nitrogen from the ground water and estuarine sediments to the water column. This proposed project integrates several standard and new technologies for field data collection, including marine seismics, horizontal resistivity, hydraulic vibracoring, in situ porewater sampling, and thermal infrared imaging to identify ground-water discharge. The results of this investigation will be valuable to the resource managers within the States of Delaware and Maryland and the National Park Service at the Assateague Island National Seashore for developing and evaluating policies to reduce the loads and mitigate the effects of excess nutrients to the coastal bays.

Approach

Collection of New Field Data:

The field data collection scheduled for Year One will provide additional information necessary to accurately interpret the existing resistivity profiles from Rehoboth and Indian River Bays. The specific information needed includes deeper seismic profiles to map the hydrostratigraphy of the coastal bays system down to the base of the surficial aquifer, vibracoring to provide lithology to interpret the sediment packages seen in the seismic profiles and estimate permeabilities, and in situ porewater sampling to obtain vertical profiles of salinity and other geochemical constituents beneath the bays to directly verify the distribution of fresh and brackish ground waters interpreted in the resistivity profiles.

The marine seismic survey would use a Geopulse system from the office of Ground water Branch of Geophysical Applications and Support. The tracklines for the seismic survey would follow those of the resistivity survey, additional seismic lines may be added if time allows. The Geopulse seismic system has been used previously in Delaware Bay to the north, and obtained excellent data with penetration depths to 100 meters.

Eastern Regional Mapping Group in the USGS Geologic Division has three hydraulic vibracore units mounted on the Hoverprobe, a Ford F450 truck, and a trailer. District personnel have worked closely with the GD drillers with this equipment, and are familiar with the capabilities and limitations. The trailer-mounted vibracore rig is planned to be placed on a catamaran drilling barge for use by this project for coring, geophysical logging, and porewater sampling in open coring and porewater sampling to provide the control necessary for interpreting the resistivity profiles. On land, we have successfully used the vibracore rig to recover 90 foot cores, and have run gamma logs through the flushed out core barrel (after recovering the core). We plan to run gamma and electromagnetic induction logs of the vibracore holes in the estuary to measure lithology and porewater conductance in situ, again, for direct comparison with the seismic and resistivity profiles.

The GD vibracore rig includes a screened drive point probe that is used to collect porewater samples in situ. We have successfully used this tool down to 10 meters, and expect to be able to go deeper. This tool can also be used to measure hydraulic heads beneath the estuary which will be valuable for calibrating the ground-water flow model. Also in support of the modeling, we will compile available water level data and geophysical logs from upland wells near the coastal bays.

Ground-Water Flow Modeling:

The Ground Water Specialist for the Maryland District, will develop a SUTRA (finite element) model to simulate ground-water flow and mixing of fresh and saline waters beneath the coastal bays. This may be developed as a set of 2-D transects across the bays from the upland to the ocean, and will lay the groundwork for development of a 3-D model. Additional funding would be needed to support salary for a more concentrated modeling effort, including the development of a 3-D model for this complex system; we will pursue other sources of funding to build on these projects.

Initial modeling efforts by Greene have produced simulations that recreate the general features of the distribution of fresh and brackish ground-waters beneath a hypothetical coastal bay equivalent to the Chincoteague Bay. For a narrow coastal bay, such as Sinepuxent Bay (1 to 2 kilometers wide), the model predicts that the interlace will be seaward of the barrier island, for a wider bay, such as the main body of the Chincoteague Bay (8 kilometers wide), the model predicts the discharge of fresh ground water close to the landward margin of the bay and a subsurface mixing zone that may extend one-quarter to one-half of the way across the bay. A resistivity survey in Chincoteague Bay would be critical to the model results, and is proposed for the second year of this project. Additional vibracoring and porewater sampling will be valuable for the interpretation of this dataset, and we will pursue other sources of funding to support these activities.