USGS CMG 01087 Metadata

Collect core samples to: (1) verify results of previous
resistivity surveys by targeting paired low-resistivity (brackish) and
high-resistivity (fresh) sites, and (2) determine the influence of
paleotopography on ground water flow by choosing nearshore and offshore
targets in a known incised paleovalley imaged in previous seismic surveys.

Samples and Chemical Analysis
Resistivity

DNREC Barge Cores and gamma logs from four sites provided a vertical
sequence of lithology to correlate with the seismic profiles and infer
permeability structure. EM logs were run to measure pore fluid
conductivities for comparison with the resistivity profiles. The gamma and
EM logs were interpreted to guide in situ sampling of discrete ground
water masses using a screened drivepoint for geochemistry and age dating.
The combined results show a complex and dynamic system of ground water
flow with appreciable control by stratigraphy. The flow of fresh ground
water produces plumes 20 m thick and 400 to 600 m wide that may extend 1
km or more from shore beneath the estuary. The lateral boundaries of a
fresh water plume may be abrupt and nearly vertical, or extend
horizontally as a diffuse layer for hundreds of meters. The lower boundary
of the plume shallows moving offshore and a mixing zone with brackish
water develops. The ground water plumes studied follow incised valleys of
tributary streams, but the dimensions of the plumes are much larger than
the valleys. The fine-grained, low-permeability sediments filling the
valleys appear to act as semiconfining layers that restrict infiltration
of the overlying saline water of the estuary and permit flow of fresh
water far offshore.

01-7210-34001

USGS Office of Ground-Water, USGS CMGP, Delaware Sea Grant

Bratton, John F., Bhlke, J.K., Manheim, Frank T.,
Krantz, David E., 2004, Ground water beneath coastal bays of the Delmarva
Peninsula: Ages and nutrients:  Ground Water, vol. 42, no. 7, p.
1021-1034.
Manheim, Frank T., David E. Krantz, John F. Bratton, 2004,
Studying ground water under Delmarva coastal bays using electrical
resistivity:  Ground Water, vol. 42, no. 7, p. 1052-1068.
Krantz, David E., Frank T. Manheim, John F. Bratton, Daniel J. Phelan, 2004,
Hydrogeologic setting and ground water flow beneath a section of Indian
River Bay, Delaware: Ground Water, vol. 42, no. 7, p. 1035-1051.
Bhlke, John Karl, and David E. Krantz, Isotope Geochemistry and Chronology of
Offshore Ground Water Beneath Indian River Bay, Delaware, Water-Resources
Investigations Report 03-4192, URL:
http://pubs.usgs.gov/wri/wri034192/