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Scientific Investigations Report 2014–5224

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

Hydrogeologic Framework, Hydrology, and Refined Conceptual Model of Groundwater Flow for Coastal Plain Aquifers at the Standard Chlorine of Delaware, Inc. Superfund Site, New Castle County, Delaware, 2005–12

By Michael J. Brayton, Roberto M. Cruz, Luke Myers, James R. Degnan, and Jeff P. Raffensperger

Thumbnail of and link to report PDF (4.03 MB)Abstract

From 1966 to 2002, activities at the Standard Chlorine of Delaware chemical facility in New Castle County, Delaware resulted in the contamination of groundwater, soils, and wetland sediment. In 2005, the U.S. Geological Survey (USGS), in partnership with the U.S. Environmental Protection Agency, Region 3, and the Delaware Department of Natural Resources and Environmental Control began a multi-year investigation of the hydrogeologic framework and hydrology of the confined aquifer system. The goals of the ongoing study at the site (the Potomac Aquifer Study) are to determine the hydraulic connection between the Columbia and Potomac aquifers, determine the direction of groundwater flow in the Potomac aquifer, and identify factors affecting the fate of contaminated groundwater. This report describes progress made towards these goals based on available data collected through September 2012.

The regional hydrogeologic framework indicates that the site is underlain by Coastal Plain sediments of the Columbia, Merchantville, and Potomac Formations. Two primary aquifers underlying the site, the Columbia and the upper Potomac, are separated by the Merchantville Formation confining unit. Local groundwater flow in the surficial (Columbia) aquifer is controlled by topography and generally flows northward and discharges to nearby surface water. Regional flow within the Potomac aquifer is towards the southeast, and is strongly influenced by major water withdrawals locally. Previous investigations at the site indicated that contaminants, primarily benzene and chlorinated benzene compounds, were present in the Columbia aquifer in most locations; however, there were only limited detections in the upper Potomac aquifer as of 2004. From 2005 through 2012, the USGS designed a monitoring network, assisted with exploratory drilling, collected data at monitoring wells, conducted geophysical surveys, evaluated water-level responses in wells during pumping of a production well, and evaluated major aquifer withdrawals. Data collected through these efforts were used to refine the local conceptual flow system. The refined conceptual flow system for the site includes: (a) identification of gaps in confining units in the study area, (b) identification and correlation of multiple water-bearing sand intervals within the upper Potomac Formation, (c) connections between groundwater and surface water, (d) connections between shallow and deeper groundwater, (e) new water-level (or potentiometric surface) maps and inferred flow directions, and (f) identification of major local pumping well influences. The implications of the revised conceptual flow system on the occurrence and movement of site contaminants are that the resulting detection of contaminants in the upper Potomac aquifer at specific well locations can be attributed primarily to either advective lateral transport, direct vertical contaminant transport, or a combination of vertical and lateral movement resulting from changes in water withdrawal rates over time.

First posted April 8, 2015

For additional information, contact:
Director, MD-DE-DC Water Science Center
U.S. Geological Survey
5522 Research Park Drive
Baltimore, MD 21228
http://md.water.usgs.gov/

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Suggested citation:

Brayton, M.J., Cruz, R.M., Myers, Luke, Degnan, J.R., and Raffensperger, J.P., 2015, Hydrogeologic framework, hydrology, and refined conceptual model of groundwater flow for Coastal Plain aquifers at the Standard Chlorine of Delaware, Inc. Superfund Site, New Castle County, Delaware, 2005–12: U.S. Geological Survey Scientific Investigations Report 2014–5224, 61 p., http://dx.doi.org/10.3133/sir20145224.

ISSN 2328-031X (print)

ISSN 2328-0328 (online)



Contents

Abstract

Introduction

Study Approach and Data-Collection Methods

Hydrogeologic Framework

Hydrology

Summary and Conclusions

Acknowledgments

References Cited

Appendix 1. Chemicals of concern detected in soils, sediment, and (or) groundwater at the Standard Chlorine of Delaware, Inc. Superfund Site

Appendix 2. Characteristics of U.S. Geological Survey Potomac Aquifer Study monitoring sites measured at and near the Standard Chlorine of Delaware, Inc. Superfund Site, 2005–12

Appendix 3. Well characteristics and water-level drawdown and recovery data for wells monitored as part of U.S. Geological Survey 72-hour pumping at well OR-6A near the Standard Chlorine of Delaware, Inc. Superfund Site, August 28–31, 2010

Appendix 4. Water-level data for non-U.S. Geological Survey network observation wells monitored as part of U.S. Geological Survey 72-hour pumping at well OR-6A near the Standard Chlorine of Delaware, Inc. Superfund Site, August 26–September 1, 2010


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