Summary for Field Activity Number 2001-087-FA

Alternate Leg ID: IB B-87-01-DL

Area of Operations: Holts Landing State Park, south shore, Indian River Bay, Delaware, United States, North America, North Atlantic

Dates: October 22, 2001 to October 30, 2001

Objectives: 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.

Type of Activity: Sampling

Information to be derived: Samples and Chemical Analysis
Resistivity

Summary: 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.

USGS Project Number: 7210-34001

Contact: John Bratton (jbratton@usgs.gov)

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