Potomac Aquifer Study at the Standard Chlorine of Delaware Superfund Site

PROJECT OBJECTIVE

The objective of this project being so extensive, a series of questions can be used to guide the work to address more specific objectives.

• What are the directions and rate (vertically and horizontally) of ground-water flow in the Potomac aquifer?
• What are the rates of contaminant migration (flux) into the Potomac aquifer?
• Is it possible to identify and correlate low-permeability units across the site?
• What potential receptors of contaminated ground water may exist in the Potomac aquifer and how might they impact contaminant migration?

STATEMENT OF PROBLEM

The U.S. Geological Survey (USGS), at the request of the U.S. Environmental Protection Agency (USEPA), Region III has been asked to provide hydrogeologic assistance in the definition of the hydrogeologic properties of the Potomac aquifer around the Standard Chlorine of Delaware, Inc. (SCD) Superfund Site in Delaware City, Delaware. The SCD site is located in an industrial area on the north side of Delaware City, just south of Red Lion Creek (fig. 1). The SCD site was listed on the USEPA National Priorities List in 1985, and has had a number of investigations completed by various contractors to investigate the nature and extent of contamination. These studies have identified extensive ground-water contamination of the shallow Columbia aquifer (Roy F. Weston, 1992; Black and Veatch, 2005). Detection of benzene, chlorobenzene, 1,2-dichlorobenzene and 1,4-dichlorobenzene at monitoring well PW-1 (now abandoned), and newer monitoring wells PW-4D (installed 2007), and PW-9 and PW-12 (both installed 2009) have demonstrated localized contamination of the Potomac aquifer that is persistant based on several rounds of sampling over a multi-year period.

The full extent of aquifer contamination is still not known.

STRATEGY AND APPROACH

To address the project objectives, more than one method is recommended since each method provides unique information that help develop a better understanding of the Potomac aquifer system. The work can proceed either simultaneously, or as separate work elements as funding is available. The USGS recommends the use of borehole geophysical logging and surface geophysical surveys to refine the geologic framework, water-level monitoring to define ground-water flow conditions, execution of a multiple-well aquifer test for the determination of hydraulic properties, and a compilation of historical data and data available from other nearby projects to meet the overall objectives of this study.

Additional work has included major ion analyses to characterize both the Columbia and Potomac aquifers, use of the groundwater age tracer SF6, and technical assistance for new well installation.

PROJECT PRODUCTS DELIVERED

Presentations, Delivered: Michael J. Brayton, 2009, Semi-annual project review meeting with cooperators and stakeholders, including USEPA and Delaware Department of Natural Resources and Environmental Control (DNREC)., USGS

^{USGS PROJECT NUMBER: LF00C37}