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Stantec February 9,2010 Kristi Shelton Page 5 of 6 Reference: Work Plan for Additional Site Characterization Task 2-VLEACH Modeling Stantec proposes using VLEACH to model the fate of TPH constituents in the vadose zone beneath the site. VLEACH is a one-dimensional finite difference analytical model for estimating the impact due to the mobilization and migration of adsorbed compounds in the vadose zone. Initially, VLEACH calculates the equilibrium distribution of the impacted mass between the liquid, gas, and adsorbed phases, and simulates the transport processes. Liquid advective transport is calculated based on values defined by the user for-infiltration and soil water content. The impact in the vapor phase migrates into or out of adjacent cells. After the mass is exchanged between the cells, the total mass in each cell is recalculated and re-equilibrated between the different phases. These steps are conducted for each time step. At the end of the model simulation, the.results are compiled to determine an overall area-weighted groundwater impact for the entire modeled area. The first stage will consist of calibrating model parameters to known conditions at the site from surface to the first-encountered groundwater-bearing zone. Using known maximum concentrations of TPH in soil and groundwater, soil column parameters will be varied until a close approximation of observed groundwater concentrations is achieved... Soil column parameters will only be varied within known or accepted literature'values for soils similar to the composition of subsurface soils beneath the .site. Once the model has been calibrated to the extent possible by varying soil column parameters, soil concentrations will be modified as necessary to further calibrate the model. The model output will determine if TPH will migrate to groundwater, and if so, what the maximum equilibrium concentration will be. Task 3 -Reporting Stantec will submit a report documenting the findings of the soil investigation and the modeling effort. The report will present aggregate soil data for the site, provide a discussion of the vertical distribution of TPH in soil, and present an estimate of the chemical mass that will be left in place following excavation. The results of the VLEACH modeling will be presented, including raw data and key assumptions. Stantec will also address the RWQCB'.s.request to perform a risk screening calculation using bulk TPH values at the site. I:\PGBE\French Camp\2010 Remediation\Work Plans\PGE French Camp_2010 WP_fnl.doc