SITE INFORMATION AND CORRESPONDENCE

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M 4.0 FATE AND TRANSPORT OF RESIDUAL CONTAMINATION During preparation of the FS/RE, PSC performed a comprehensive assessment of soil and groundwater data collected at the site since 1987. This assessment included a trend analysis of concentrations in source area wells to estimate the time for these source area wells to reach MCLS. In addition, site-specific data and literature values were used to model the fate and transport of residual hydrocarbons left on site. The results of the analysis and modeling were used to assess the risk associated with leaving the residual hydrocarbons in subsurface soil and groundwater at the Site. The following sections present a summary of these assessments. 4.1 Trend Analysis PSC evaluated the trends for TPH-g and benzene concentrations in wells PZ -1, PZ -2, MWA, and SP -2. Data collected from 1994 through 2009 were used to evaluate PZ -1 and PZ -2. The concentrations of benzene decreased from 40% and 88% in PZ -1 and PZ -2, respectively. Similar decreases in concentration were observed for the TPH-g data collected from these wells. However, lower concentrations in these wells during the late 1990s resulted in a trend line that indicated increasing concentrations. These results were not considered valid and therefore, the trend analyses for these wells were not used. Data collected from 1987 to 2009 were used in the evaluation of MW -1. The trend line for benzene in this well project that concentrations would be below MCLS in 35 years and TPH-g concentrations would be below ESLs in 61 years. Data from 1999 through 2009 were used in the evaluation of SP -2. The trend line for benzene in this well project concentrations would be below MCLS in 12.5 years and TPH-g concentrations would be below ESLs in 30 years. Spreadsheets and charts used in these trend analyses are presented in Appendix B. 4.2 Fate and Transport Modeling PSC has assessed the fate and transport of residual benzene contamination using the ASTM RBCA R-26 Model and BIOSCREEN, a USEPA natural attenuation groundwater model. Both models are based on the Domenico analytical solute transport model. The ASTM model uses a first order decay rate that predicts the distance to a point of compliance. BIOSCREEN utilizes retardation and site-specific data on natural attenuation parameters to calculate: • Solute transport without decay; • Solute transport with biodegradation using fust order decay; and • Solute transport with biodegradation as an instantaneous reaction. PSC calibrated the BIOSCREEN model using concentrations from the source area wells in 1999 and concentrations from 2009 for source area and down gradient wells near the centerline of the plume. The model results using a 10 -year simulation for no degradation, and biodegradation with first order decay were compared to the observed results along the centerline. The model was calibrated by adjusting the first order decay rate until the observed data fell on the line. The results of calibration using the 10 -year data required decay rates