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stilllntec December 17,2009 Page 2 of 3 Reference: Results of Hydrogeologic Modeling Former 76 Service Station#6981,4707 Pacific Avenue, Stockton, California Project Number:211402667 material (Spitz and Moreno, 1996)' and model calibration. An effective porosity of 0.20 was selected based on published values (EPA, 1986)'. TPHg was the contaminant selected for modeling because it is the predominant contaminant of concern at the site. Although rates for first-order decay for TPHg can vary widely a conservative value was used (0.000959 1/day). Dissolved-phase TPHg concentrations in the groundwater from May 2008 were used as the initial concentrations in the model. This is a conservative approach using aIghest concentration in the previous years. e maximum TPHg concentration used by the model was 7,900 µg/L in the source area of the plume (well VW .-7 Other input arameters used in the model include: 1) hydraulic conductivity, 1 ft/day; 2) retardation factor, 1.0; 3) longitudinal dispersivity of 10 feet, 4) transverse dispersivity of 1 feet; and 5) diffusion coefficient of 0.00093 W/day. The largest unknown in the modeling effort is the hydraulic conductivity of the substrate underlying the site. The model also assumed homogeneity of hydraulic properties across the site. Model Results Model simulations were performed to simulate the migration and degradation of TPHg concentrations northeast of the Site in the vicinity of wells V11V-7, MW-2, and MW-16 (Figure 1). Prior to running the contaminant transport portion of the model, modeled groundwater elevation contours were simulated and calibrated against existing groundwater contour elevations from July 2009. Results of the modeling agree with the field data that indicates groundwater flows to the northeast. The groundwater flow and transport model simulated the migration of TPHg impacted groundwater downgradient from well VW-7. The model was run fora eriod of 10 ears using the beginning date as May 2008 and ending May 2018. na ytica results for TPHg during 2009 were used to calibrate the model. Results of the simulation indicate that in May 2018 TPHg concentrations have degraded to below 100 µg/L onsite and below 50 trg/L offsite, and will have migrated approximately 100 feet downgradient of well MW-16 to the middle of Pacific Avenue (Attachment A). Concentrations would be expected to continue to decline further beyond 2018. Conclusions The model is considered to be appropriate for predicting future data trends assuming there is no future release of TPHg, and demonstrates that the potential for significant further downgradient migration of the plume is limited. In summary, the processes of natural attenuation/plume stabilization predicted by the model results are expected to reduce the contaminant concentration within the TPHg plume and to limit further downgradient migration of the plume to an area immediately northeast of the current plume location. If there are any questions or concerns, please call Steve Strait. Spitz, K. and J. Moreno. 1996. A Practical Guide to Groundwater and Solute Transport Modeling. John Wiley and Sons, Inc. pp. 461. z EPA. 1986. Criteria for Identifying Areas of Vulnerable Hydrogeology Under RCRA, Guidance Manual for Hazardous Waste Land Treatment, Storage and Disposal Facility, pp 70, 1