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Austin Road Landfill Section 4 Contaminant Plume Characterization Report Capture Zone Modeling Analysis Table 4-1 GPTRAC Input Parameters Austin Road Landfill Wel! x-coordinate y-coordinate Duration b T K I Q Designation (ft) (ft) (days) (ft) pelday) (f 1day) (unitless) (gpm) EW-1 3500 4000 varies 40 10200 255 2 x 10'' 40 EW-21 1900 4000 varies 1 40 10200 255 2 x 19' 150 Other model input assumptions include: ■ The aquifer is unconfined and is not influenced by adjacent recharge areas. ■ The aquifer is not subject to influences from flow boundaries,recharge sources or other pumping wells,or other conditions that may alter groundwater steady-state flow conditions. ■ Groundwater flows across the site at a bearing of N30°E and uniform hydraulic gradient of 2.3 x 10"3 ft/ft. The groundwater hydraulic gradient represents a typical value for ARL based on review of historic groundwater monitoring reports prepared by EMCON and CH2M Hill. Two sets of modeling scenarios were conducted for the analysis. The first set of model simulations used the E -1 and E -2 pumping rates measured during the aquifer test as listed in Table 4-1. The second set of modeling scenarios used the higher EW-1 pumping rate(150 gpm) at both extraction well locations. All other parameters listed in Table 4-1 remained the same in both set of modeling scenarios. 4.2 Results of Extraction Well Field Simulation Analysis of the one-year,three-year,and five-year extraction periods shows that the existing extraction well system(assuming pumping rates of 40 and 150 gpm)does not effectively capture groundwater emanating from beneath the landfill. The one-year and three-year simulated capture zones are as depicted in Figures 4-1 and 4-2. Based on the input parameters listed in Table 4-1,a large disparity exists between the size of the capture zones at each well. The capture zone for EW-1 is approximately 1/3 that of the capture zone for EW-2. This difference results from the large disparity in measured pumping rates between EW-1 (Q=40 gpm)and EW-2(Q= 150 gpm). The low pumping rate in well EW-1 may be the result of an undersized pump and/or leaky discharge piping. The aquifer grain size distribution in EW-1 indicates slightly finer materials at the EW-1 location compared to the EW-2 location,however,this alone does not appear to explain the lower discharge rate at EW-1. In addition,the aquifer pumping test determined that the K value near each well was approximately the same,based on results from nearby monitoring wells. CDM Camp Dresser&McKee CS 010783 4-2 W:WEPORTSI.STOCKTONIARLPLUME.98\SEC4.WPD City ofStoekton 190855-00%