SITE INFORMATION AND CORRESPONDENCE

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system. The following five stepsorecommended for Interpret Actual Are Zone Achieved thorough P&T system evaluation: An actual capture zone is defined as the three- evaluating plume capture dimensional zone in which all ground water flow paths converge to one or more extraction points. The • performing and interpreting treatment process extent of the capture zone depends on many factors: monitoring • pumping rate • performing and interpreting ground water hydraulic gradient(magnitude and direction) monitoring hydraulic conductivity • vertical flow to other aquifers • evaluating extraction well performance spacing of extraction wells • transient influences (recharge, other pumping) • if applicable, evaluating injection well performance Accurate interpretation of actual capture is difficult and is best evaluated with converging lines of Evaluate Plume Capture evidence. Some potential lines of evidence are listed in Exhibit 3 and described below. Generally, capture For ground water remedies,protection of human health is actually achieved if multiple lines of evidence and the environment often requires hydraulic suggest it; however, capture may not be achieved if containment("capture") of a contaminant plume. only one or two of the multiple lines of evidence Evaluation of containment includes defining an suggest it. Figure 1,on page 6, illustrates the role of appropriate target capture zone, interpreting actual multiple lines of evidence in a capture zone analysis. capture, and then demonstrating that the actual capture zone is consistent with the target capture zone. Care Flour Budget and Analytical Modeling. For idealized should be taken to ensure this consistency is present conditions (i.e.,one well,no recharge, uniform through various temporal changes, such as seasonal saturated thickness, and a homogeneous, isotropic variations in recharge and/or nearby pumping. aquifer), the width of the capture zone some distance upgradient of the extraction system can be estimated Although the complex hydrogeology at fractured for specific flow rates with a straightforward bedrock or highly heterogeneous sites may prohibit analytical equation (see Exhibit 4). Using the same conclusive results, capture zone analyses should be attempted at these sites for the following reasons: (1) Exhibit 3 the analysis may actually be conclusive, and (2) Potential Lines of Evidence for valuable insights to site-specific ground water flow and Ground Water Capture contaminant transport may be gained. calculations of capture zone width based on flow Define the"Target Capture Zone' budget and/or analytical models A three-dimensional target capture zone should be interpretation of ground water flow lines from potentiometric surface maps that are based on indicated on maps and/or on cross-sections of the site. measured ground water elevations from the various It should be based on clearly stated criteria(such as a subsurface stratigraphic units specific concentration contour or a site boundary). In some cases capture of the entire plume may be inward flow relative to a compliance boundary,based required,but in other cases capture of a portion of the on measured ground water elevations at two or more plume may be acceptable (e.g., if natural attenuation of locations oriented perpendicular to the boundary the remaining portion is viable and can be demonstrated). If the target capture zone is based on a concentration trends over time at sentinel wells located specific concentration contour, it may need to be downgradient of the capture zone updated over time as plume boundaries change. If a particle tracking in conjunction with a numerical variety of contaminants of concern are present,the ground water flow model calibrated/verified by actual target capture zone should consider each of those ground water elevations under pumping conditions contaminants. If a target capture zone is not defined, then it will be uncertain if actual capture is sufficient. implementation and analysis of data from tracer tests 4