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A systematic approach for evaluation of capture zones at pump and treat systems

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This document describes a systematic approach for performing capture zone analysis associated with ground-water pump and treat (P&T) systems. A “Capture Zone” refers to the three-dimensional region that contributes the ground water extracted by one or more wells or drains (see Figure 1). A capture zone in this context is equivalent to the “zone of hydraulic containment”.

If a contaminant plume is hydraulically contained, contaminants moving with the ground water will not spread beyond the capture zone. Failed capture, illustrated schematically on Figure 2, can allow the plume to grow, which may cause harm to receptors and may increase the ultimate cost or duration of the ground-water remedy.

The purpose of this document is to present a systematic approach to evaluating capture zones at P&T sites. The intended audience is technical professionals that actually perform capture zone analyses (i.e., hydrogeologists, engineers) as well as project managers who review those analyses and/or make decisions based on those analyses. The scope of this document is limited to evaluating capture in porous media and not necessarily karst or fractured rock settings. The methods and techniques presented here may be used for such settings, but other more intensive techniques may also be required.

EPA places considerable emphasis on P&T performance and determination of whether or not these systems are operating properly and successfully. As discussed in Elements for Effective Management of Operating Pump and Treat Systems (U.S. EPA, 2002b), protection of human health and the environment often requires hydraulic containment of contaminants. Capture zone analysis is the process of evaluating field observations of hydraulic heads and ground-water chemistry to interpret the actual capture zone, and then comparing the interpreted capture zone to a “Target Capture Zone” to determine if capture is sufficient.

An optimization study (U.S. EPA, 2002a) of 20 “Fund-lead” P&T systems at Superfund sites concluded that capture zones were not being adequately evaluated. At least 14 of the 20 sites did not have a clearly defined Target Capture Zone. About half of the 20 sites had not attempted to interpret actual capture based on water levels. Only eight of the 20 sites had a ground-water flow simulation model, and capture zone analysis was found to be inadequate or incomplete at six of those eight. Overall, a recommendation to improve the capture zone analysis was made for 16 of the 20 sites. The report also concluded there was a need for improved guidance and training with respect to capture zone analysis. This document is intended to partially address those needs.

This document is intended to be used as a companion document to Methods for Monitoring Pump-and-Treat Performance (U.S. EPA, 1994, link provided in “References” section) when evaluating capture zones. This document is intended to provide more detail regarding capture zone analysis, and includes more complex examples, relative to the previous document. This document is not intended to be a comprehensive reference for each topic presented herein nor is it a “how to” guide. However, a table provided at the beginning of the “References” section helps guide the reader to sources of information (cited within this document) according to specific topics.

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