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Section 4 Capture Zone Modeling Analysis Following analysis of the aquifer test data the effectiveness of the existing groundwater extraction well field was evaluated. This was accomplished using the hydraulic parameters derived from the aquifer pumping test(Section 3.4) and using the Well Head Protection Area(WHPA)modeling software developed by USEPA(USEPA,1991). This section describes the modeling inputs and results used to evaluate the existing extraction well system in capturing groundwater migrating from beneath the ARL. The WHPA model is a modular,semi-analytical groundwater flow model that consists of four independent computational modules that may be used to delineate well capture zones. The general particle tracking routine(GPTRAC)module of this program was used in the evaluation of the existing ARL extraction well field since it allows for time-dependent simulation of capture zones from multiple pumping wells. The GPTRAC module option offers the capability to perform pathline and time-related capture zone analysis for the existing extraction well field. The assumptions used by the GPTRAC module of the WHPA program in computing the well capture zones are listed below. The aquifer is a constant thickness,is homogeneous and has a constant K value ■ Extraction wells each pump at a constant rate throughout the period of simulation ■ constant ambient groundwater flow conditions exist ■ each pumping well fully penetrates the entire saturated thickness of the aquifer The effects of well interference from multiple pumping wells(EW-1 and EW-2)and a preexisting aquifer hydraulic gradient were incorporated into the model simulations. 4.1 Model Setup and Simulation A model area consisting of a 5000-foot long by 5000-foot wide area encompassing the upgradient and downgradient of ARL was discretized into a 50 by 50 grid to develop the model boundaries and internal calculation points. The grid patterns,extraction well locations(placed within the internal model coordinate system)and input hydraulic parameters used in this evaluation are presented in Table 4-1 below. The hydraulic parameters determined from the aquifer test were entered into the model to analyze the capture zones of the existing extraction wells. Simulations of well pumping were run for time periods of one,three,and five-year periods. Each simulation run assumed constant operation of the current extraction well system. CDM Camp Dresser&McKee CS 010782 4-1 W:IREPORTSISTOCKTOMARLPLUME.98\SEC4.WP0 City of Scockcon 190855tO06