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Open-File Report 2010–1058

Prepared in cooperation with the U.S. Environmental Protection Agency

Preliminary Investigation of Paleochannels and Groundwater Specific Conductance using Direct-Current Resistivity and Surface-Wave Seismic Geophysical Surveys at the Standard Chlorine of Delaware, Inc., Superfund Site, Delaware City, Delaware, 2008

By James R. Degnan and Michael J. Brayton

ABSTRACT

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The U.S. Geological Survey (USGS), in cooperation with Region III of the U.S. Environmental Protection Agency (USEPA) and the State of Delaware, is conducting an ongoing study of the water-quality and hydrogeologic properties of the Columbia and Potomac aquifers and the extent of cross-aquifer contamination with benzene; chlorobenzene; 1,2-dichlorobenzene; 1,4-dichlorobenzene; and hydrogen chloride (hydrochloric acid when dissolved in water) in the vicinity of the Standard Chlorine of Delaware, Inc. (SCD), Superfund Site, Delaware City, Delaware. Surface geophysical surveys and well data were used to identify and correlate low-permeability units (clays) across the site and to search for sand and gravel filled paleochannels that are potential conduits and receptors of contaminated groundwater and (or) Dense Non-Aqueous Phase Liquid (DNAPL) contaminants. The combined surveys and well data were also used to characterize areas of the site that have groundwater with elevated (greater than 1,000 microsiemens per centimeter) specific conductance (SC) as a result of contamination.

The most electrically conductive features measured with direct-current (DC) resistivity at the SCD site are relatively impermeable clays and permeable sediment that are associated with elevated SC in groundwater. Many of the resistive features include paleochannel deposits consisting of coarse-grained sediments that are unsaturated, have low (less than 200 microsiemens per centimeter) SC pore water, or are cemented. Groundwater in uncontaminated parts of the Columbia aquifer and of the Potomac aquifer has a low SC. Specific-conductance data from monitoring wells at the site were used to corroborate the DC-resistivity survey results. For comparison with DC-resistivity surveys, multi-channel analysis of surface wave (MASW) surveys were used and were able to penetrate deep enough to measure the Columbia aquifer, which is known to have elevated SC in some places. MASW survey results respond to solid material stiffness; clays and cemented sediments will have a higher velocity than silts, sands, and gravels (in order of increasing hydraulic conductivity).

Geophysical surveys detected elevated SC associated with contamination of the surficial Columbia aquifer. Groundwater with elevated SC over ambient (by an order of magnitude) produced a decrease in measured resistivity at the SCD site. Where SC data are not available from wells, it is not known if a low resistivity value measured with DC resistivity alone results from the geologic material (clay) or elevated SC in groundwater (in sand or gravel). Seismic surface waves used as part of the MASW technique are not affected by water content or quality and are used herein to distinguish between sand and clay when SC is high. Through concurrent interpretation of MASW and DC-resistivity surveys, information was gained about water quality and lithology over large areas at the SCD site.

First posted May 12, 2010

For additional information contact:
Director
U.S. Geological Survey
New Hampshire-Vermont Water Science Center
361 Commerce Way
Pembroke, NH 03275
(603)226-7800

http://nh.water.usgs.gov

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Suggested citation:

Degnan, J.R., and Brayton, M.J., 2010, Preliminary investigation of paleochannels and groundwater specific conductance using direct-current resistivity and surface-wave seismic geophysical surveys at the Standard Chlorine of Delaware, Inc., Superfund Site, Delaware City, Delaware, 2008: U.S. Geological Survey Open-File Report 2010–1058, 27 p., at http://pubs.usgs.gov/of/2010/1058/.



Contents

Acknowledgements

Abstract

Introduction

Site Description

Purpose and Scope

Hydrogeologic Setting

Methods of Surface Geophysical Data Collection and Analysis

Direct-Current Resistivity

Multi-Channel Analysis of Surface Waves

Paleochannels and Groundwater Specific Conductance

Results of Geophysical Surveys, Paleochannels and Relation to Groundwater Specific Conductance

Direct-Current Resistivity

Multi-Channel Analysis of Surface Waves

Paleochannels and Groundwater Specific Conductance

Summary

References Cited

Appendix 1. Direct-Current Resistivity Cross Sections with nearby Well Water Levels, Screened Intervals, and Specific Conductance Values



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