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.',/ 5.2 Preliminary Capture Zone Analysis A mathematical model was used to assess the effectiveness of a preliminary extraction option for containment and capture of salt in shallow groundwater. The model WhAEM2000 (Wellhead Analytical Element Model) version 3.1.1, developed by Haitjema Software a subdivision of Haitjema Consulting, Inc. under contract with the U.S. Environmental Protection Agency(USEPA), was used for capture zone simulations. WhAEM2000 is an analytic element model, which solves steady-state groundwater flow in a single aquifer (Strack and Haitjema, 1981a and 1981b; Haitjema, 1985; Strack, 1989; Haitjema, 1995). WhAEM2000 is distributed by the USEPA's Center for Exposure Assessment Modeling (httl2://www.eva.gov/cegMpubl�. 5.2.1 Parameter Input In developing a conceptual model of the groundwater system beneath the SMS Briners facility, it was assumed that an extraction well would be completed in the shallow aquifer to accomplish capture. For preliminary modeling purposes, the shallow aquifer was assumed to be homogeneous with a constant thickness, hydraulic conductivity, porosity, and regional hydraulic gradient. Based on the base elevation of the aquifer and the hydraulic head, the model recognizes confined and unconfined conditions and invokes solving schemes accordingly. Base Elevation The base elevation is typically the elevation of the bottom of the aquifer. However, due to representation of multiple hydraulically connected sand beds within the shallow zone as one homogeneous water-bearing layer, the base elevation was lowered to avoid the occurrence of false unconfined conditions in the model domain. ' Aquifer Thickness The summed thicknesses of shallow sands logged during the construction of the six dedicated monitoring wells was used to approximate the average aquifer thickness. The thicknesses of shallow sands at MWl, MW2, MW3A/3B, and MW4A/4B are 31, 44, 34, and 14 feet, respectively; and the average thickness is 31 feet. Hydraulic Conductivity The hydraulic conductivity is the quotient of transmissivity and aquifer thickness. The preliminary capture zone analysis below used a hydraulic conductivity of 26 friday that was based on an estimated transmissivity value of 6,000 gpd/ft and an aquifer thickness of 31 feet. Porosity Porosity in WhAEM2000 is an effective porosity, which is the volume fraction of an aquifer available for groundwater flow. The effective porosity excludes void spaces in isolated pores and can be two to five percent less than the total porosity in unconsolidated sediments coarser than silt size. The porosity of clean, fine, and coarse sands typically ranges from 0.20 - 0.45 to 0.15 - 0.3 5, respectively(Maidment, 1993). Due to the presence LUHDORFF & SCALMANINI CONSULTING ENGINEERS 8