Pennsylvania Water Science Center



Fractured Rock Hydrology Research

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Dan Goode
(djgoode@usgs.gov)

 

In cooperation with the U.S. Environmental Protection Agency, Office of Superfund Remediation and Technology Innovation

Application of Existing Guidance, Methods, and Models for Evaluating Capture Zones in Fractured Rock Aquifers

by Allen M Shapiro, Daniel J Goode, and John H Williams

Abstract

In 2008, US EPA published a guidance document on the evaluation of groundwater capture zones for pump-and-treat operations at sites of groundwater contamination. The document outlines a 6-step procedure for evaluating the robustness of capturing groundwater from a targeted volume of the aquifer. The procedure discusses methods of interpreting water level data and trends in concentrations, and specific groundwater modeling approaches and calculations that are used to support the evaluation. The methodology specifically focuses on capture zones in relatively homogeneous unconsolidated porous media; highly heterogeneous subsurface environments are not the intended focus of the document.

Although US EPA [2008] explicitly focuses on capture zones in unconsolidated porous media, the 6-step approach offers a general framework that can potentially be applied in geologically complex settings, such as fractured rock aquifers. There are numerous points in the 6-step procedure where references are made to incorporating heterogeneous geologic conditions. The report explicitly states that inconsistencies between interpretations and measurements should be reevaluated by considering, among other issues, time-varying processes and heterogeneous formation properties.

Extending the EPA guidance to fractured rock aquifers can be accomplished with the recognition that a geologically motivated conceptual model must be the filter through which evaluations of capture zone are made. Simple areal and cross-sectional interpretative methods of performing groundwater flow calculations and interpreting water levels and concentration data are likely to lead to erroneous results. Monitoring at multiple depths in fractured rock aquifers is needed to evaluate the three-dimensional character of groundwater flow. Interpretations of data must be geologically motivated through the identification of permeable features as they occur with depth and spatial extent in the aquifer. Provided that a geologically motivated conceptual model of the aquifer can be established, numerical models of groundwater flow should become a routine part of evaluating capture zones in fractured rock aquifers.

Citation: Shapiro, A.M., Goode, D.J., and Williams, J.H., 2011, Application of Existing Guidance, Methods, and Models for Evaluating Capture Zones in Fractured Rock Aquifers (abstract): National Ground Water Association Focus Conference on Fractured Rock and Eastern Groundwater Regional Issues, September 26-27, 2011, Burlington, Vermont. (/projects/frhr/shapiro2011.php)

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