Potomac Aquifer Study at the Standard Chlorine of Delaware, Inc. Superfund Site, Delaware City, Delaware

Objectives

Based on the contamination found in well PW-1 and concerns by the State of Delaware, the USEPA has requested assistance to define the flow and transport properties that would allow contaminants present in the Columbia aquifer into and through the underlying Potomac aquifer. Based on this broad objective, 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). Since sampling of a newly installed monitoring well (PW-1) in the Potomac aquifer began in November 2003 by the USEPA, detections of benzene (including consistent exceedances of the MCL for benzene), chlorobenzene, 1,2-dichlorobenzene and 1,4- dichlorobenzene have increased through the most recent sampling in December 2004. No other monitoring wells screened in the Potomac aquifer have had consistent detections.

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 geophysical logging to refine the geologic framework, water-level monitoring to define ground-water flow conditions, re-analysis 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.