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r Former Beacon Station No. 12474 March 18,2002 Stockton California Problem Assessment Report/Corrective Action Plan 0 63 on Figure 8, suggest that concentrations are attenuating prior to migrating westward and southward past monitoring wells MW-1, MW-9 and MW-10. The lateral extent of benzene concentrations in groundwater, as reported for first quarter 2002, is limited to the dispenser island area as shown on Figure 8. The lateral extent of benzene in groundwater is delineated by monitoring wells MW-1, MW-3, MW-4, MW-5, MW-7, MW-8, MW-9 and MW-10. With the exception of monitoring well MW-7 and air sparge well AS-1, MTBE has not been reported in quarterly groundwater monitoring samples for at least the past four quarters. Concentrations of MTBE in monitoring well MW-7 ranging from 0.50 to 0.87 ppb have been reported only three times since the well was installed in September 1997. Concentrations of MTBE in air sparge well AS-1 ranging from 5.0 to 32 ppb have been reported in eight of ten quarterly groundwater samples since the well was installed in October 1999. 6.0 MASS OF PETROLEUM HYDROCARBONS IN SOIL AND GROUNDWATER The evaluation of the distribution of TPHg and benzene-impacted soil indicate that the source of petroleum hydrocarbons was likely the result of releases in the dispenser island area, and the subsequent vertical and limited lateral migration through the soil as shown on Figures 3, 4 and 6. The distribution of TPHg and benzene-impacted groundwater was evaluated from second quarter 2001 groundwater monitoring data and are shown on Figures 7and 8. 6.1 Mass of TPHg and Benzene in Soil The estimated soil volumes impacted by TPHg and benzene were evaluated on the basis of soil concentration data from soil borings and distribution assumptions. For the purposes of 474PARCAP3-02.doc 8 ProjectNo. 1474.23 HORIZON ENVIRONMENTAL INC.