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Commingled Plume#35 Chevron 9-4275/ARCO#2133 Site Conceptual Model September 1,2006 Page 11 6.0 PETROLEUM HYDROCARBON DISTRIBUTION AND CONCENTRATION TRENDS 61 Distribution of Petroleum Hydrocarbons in Soil Soil samples have been collected at both sites during the installation of monitoring wells and various site investigation activities The lateral and vertical distribution of hydrocarbons in soil is discussed below 6.1.1 Lateral Distribution of Hydrocarbons in Shallow Soil SECOR reviewed historical soil data and evaluated the new soil data to determine the lateral distribution of hydrocarbons in soil Maximum adsorbed concentrations are located in the vicinity of the former USTs at the ARCO site and surrounding the former dispenser islands on the Chevron site Based on historical soil data, TPI-Ig, benzene, and MtBE are defined to the north (Chevron wells MW-3R and MW-4), south (ARCO wells MW-3 and MW-4), east (Chevron wells MW-6 and MW-8, and ARCO soil boring AS-11), and west (Chevron wells MW-5 and MW-7) Current and historical soil results for Chevron 9-4275 facility and ARCO 2133 facility are summarized on Tables 7 and 8 6 1 2 Vertical Delineation of Hydrocarbons in Soil Vertical delineation of adsorbed hydrocarbons in soil was completed by the advancement of six soil borings (MW-1 R through MW-3R at Chevron 9-4275 facility and AS-8 through AS-11 at ARCO 2133 facility) to depths of 60 to 70 feet bgs Soil samples collected from the borings at depths below 51 feet bgs did not contain concentrations of TPI-Ig, benzene, MtBE, or TBA above the laboratory reporting limits (Tables 7 and 8) The extent of adsorbed hydrocarbons in soil is illustrated on Figures 8 and 9 62 Petroleum Hydrocarbons in Groundwater During advancement of the six deep borings at the both sites, grab groundwater samples were collected to determine vertical distribution of hydrocarbons in groundwater In addition, six wells destroyed on the Chevron site in 2005 were replaced, developed and sampled for the second quarter 2006 monitoring and sampling event A groundwater elevation contour map, hydrocarbon concentration map, and dissolved concentration maps of TPI-Ig, benzene, and MtBE in shallow groundwater for June 19-21, 2006, are included as Figures 5, 6, and 10 through 1, respectively 6 2 1 Lateral Distribution of TPHg in Groundwater The lateral extent of TPI-Ig in groundwater is delineated by the existing monitoring well network Based on the data presented on Figure 10, the TPI-Ig plume to the east and northeast is delineated to below 50 pg/L by wells MW-8, MW-6 and PR-1 The historical maximum dissolved TPI-Ig concentrations at the sites were 400,000 pg/L in E-10 (ARCO) and 38,000 NgIL in MW-5 (Chevron) During the second quarter of 2006, the maximum dissolved TPI-Ig concentration at either site was observed in well MW-2R at 720 NgIL (Chevron) Concentration graphs for well with historically high TPI-Ig concentrations are shown in Appendix I 94275-ARCO 2133 SCM 9-2006 1