ARCHIVED REPORTS RESPONSE TO COMMENTS BIOTREATMENT BEDS POST REMEDIATION INVESTIGATION REPORT

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' VLEACH Summary The soil property and calibrated recharge values in the previous VLEACH model completed for the adjacent UPRR Former Tracy Yard VCA were applied in the model for this Site. There is a wide range of soil property values reported in the literature for TPH-d (e.g., DTSC 2009). Representative values for C9- C,6 developed by DTSC were used in this evaluation (DTSC 2009), with the exception of the soil partitioning coefficient(Kd). The soil-water partition coefficient(Kd)value is a key parameter in evaluating the leaching potential of TPH-d. Soil-water Kd value describes the equilibrium ratio of concentration aqueous and solid phases. For organic compounds, the Kd is calculated as the product of a chemical- specific partitioning coefficient between water and soil organic matter(Kos) and site-specific fraction of the ' soil that is solid phase organic matter(fog), as shown in the following equation. Ka = Ko, x f ' Kd was estimated using site specific data and was calculated to be 20.58 L/kg, which corresponds to a Koc value of 4,678 L/kg, assuming the foc for the site is 0.0044. The fa value was estimated based on six total organic carbon (TOC) samples collected at the UPRR Former Tracy Yard VCA property located adjacent to the Site, as documented in the"Pre-excavation Soil Investigation Results for OU-3 and OU-4, Former Tracy Railyard," report prepared by ARCADIS and dated May 2, 2008 ("the Former Tracy Yard VCA ' result summary"; ARCADIS 2008). A unit area was assigned to a single polygon in VLEACH with 120 cells to simulate the 12 ft of vadose ' zone soil column. VLEACH model simulations provide the impact to groundwater from potential vertical leaching rate in the format of gram per year(g/year) for each time step. The vertical leaching rate in g/year will serve as a source to impact the groundwater quality. To translate the potential vertical leaching impact on groundwater, it was assumed complete mixing in the groundwater aquifer beneath the source area once TPH-d vertical leaching mass reaches the groundwater table. In addition, it was assumed that the depth of interval for the groundwater aquifer was 6 ft, based on groundwater elevations measured ' during March 2010. With the effective porosity of 0.4395 (measured during the OU-3 and OU-4 investigation on the off-site UPRR Former Tracy Yard VCA property [ARCADIS 2008]), the volume of potentially impacted groundwater was determined to be 3 cubic feet(ft) per unit source area. ' To calculate groundwater flux flowing underneath the soil column, the site specific hydraulic gradient of 0.003 (ARCADIS 2010) was used. Based on the lithologic description of the aquifer, literature values for hydraulic conductivity are 60 feet per day(ft/d) for sandy loam (VLEACH 1997). Hydraulic conductivity ranging from 8 to 140 ft/d have been estimated through site-specific aquifer tests summarized in RI Report (IC 1994).A site-specific hydraulic conductivity value of 50 ft/d is assumed for analysis of soil leaching and mixing with groundwater from the Former Tracy Yard VCA result summary(ARCADIS 2008). Using the hydraulic conductivity of 50 ft/day, the groundwater flux was estimated to be approximately 328 ft'/year. The impacted groundwater concentrations due to potential TPH-d leaching were then calculated based on mass/flux balance. Results from the site VLEACH model indicate that soil in the top three feet of the soil column with the maximum TPH-d concentration measured during the biotreatment bed investigation (620 mg/kg at HA-6 sampled on 3/17/2010)would leach at a rate that would result in TPH-d groundwater concentrations of less than 12 pg/L.