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Former Beacon Station No 12 574 June 7,2005 Stockton California Results of Additional Groundwater Assessment • Quarterly groundwater monitoring data from the first quarter 2005 indicate the highest concentrations of TPHg in monitoring well MW-12 Seasonal fluctuation of TPHg and benzene and the relationship to the groundwater level for monitoring wells MW-3 through MW-17, air sparge well AS-1, and vapor wells VW-1 and VW-4 are shown on concentration versus time graphs I through 34 The distribution of TPHg in groundwater is delineated to the north and east by monitoring wells MW-4 and MW-8 Delineation to the west and south of the dispenser island area is not indicated by current data However, concentrations of TPHg in groundwater decrease by 2 to 3 magnitudes from the dispenser island area to monitoring wells MW-10 to the west and MW-9 to the south Concentrations of TPHg in groundwater samples collected in the first quarter 2005 from the monitoring wells MW-7 and MW-9 indicate that the impacted groundwater extends to approximately the south property line The distribution of benzene in groundwater, as shown on Figure 8, is delineated to the north by MW-4, to the east by MW-8, to the south by MW-11 and to the west by MW-I0 Fate and Transport of Petroleum Hydrocarbons in Soil and Groundwater Petroleum hydrocarbons possibly released from UST systems enter vadose zone soils Petroleum hydrocarbons released to vadose zone soils migrate generally downward, under the force of gravity A portion of the hydrocarbons released in the vadose zone adsorb onto soil particles and contribute to relatively stationary, secondary soil sources Fine-grained soils, which are predominant at the site (see Figures 3 and 4), tend to decrease migration rates and, when laterally discontinuous and interbedded, may introduce a lateral component to the downward migration Petroleum hydrocarbons that migrate downward to the capillary fringe and water table tend to dissolve in the groundwater and migrate by advection generally south-southeastward with the groundwater although generally at a lower rate Physical dispersion and molecular diffusion act on the dissolved hydrocarbon constituents to dilute concentrations and tend to Addtn1GWassmnt Project No 1474 15 9 HORIZON ENVIRONMENTAL INC.