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' 2.9 SITE CONCEPTUAL MODEL ' To gain a better understanding of the site, and assist in evaluating the applicability of remedial alternatives, a site conceptual model (SCM) of chemical occurrence, fate and transport, was developed Specifically, the SCM documents sources of chemicals, affected media and transport ' mechanisms Development of the SCM was based on available information characterizing hydrogeologic conditions, and soil and groundwater quality Site investigations have identified the presence of TPH-g and BTEX in the subsurface, which is consistent with the petroleum hydrocarbons dispensed at the former service station MTBE has been detected using EPA Method 8260 in groundwater samples collected from onsite and offsite wells (MW2, MW4, MW8, MW9, MW11, and P1) These wells are located (1) furthest downgradient from the site (MWI1), (2) offsite and downgradient of the Shell and 7-Eleven sites (wells MW8 and MW9) or (3) onsite, upgradient of the Exxon's former tank field, and near the 7-Eleven site (MW4, P1, ' and MW2) This would suggest that MTBE from offsite sources has impacted groundwater at the ExxonMobil site The lateral distribution of petroleum hydrocarbons detected in soil samples are beneath and immediately downgradient (northeast) of the former USTs and former pump islands (Figure 8) The highest TPH-g and BTEX concentrations in soil and groundwater have been detected at ' depths ranging from 20 to 46 feet bgs Under this scenario, following an initial release, hydrocarbons moved through the vadose zone by gravity to groundwater, and dissolved constituents in groundwater were subject to offsite migration via groundwater transport The lateral extent of this migration of hydrocarbons in the downgradient direction is limited by the 10 natural attenuation processes Natural attenuation refers to a range of physical and biological processes, which reduce the mass and toxicity of petroleum hydrocarbons and limits the extent of their migration in groundwater These natural in-situ processes include biodegradation, dispersion, dilution, sorption, and volatilization Petroleum hydrocarbons identified in the soil and groundwater at former Exxon RS 7-3942 are being biodegraded (converted/destroyed), producing the nontoxic end products of carbon dioxide and water This overall reduction in concentration of hydrocarbons in groundwater and reduction in the length of the hydrocarbon- impacted groundwater in the downgradient direction is presented in Figure 9 The USTs and associated dispenser islands and piping were removed in 1986, thus the release of hydrocarbons from these USTs predates 1986 Based on a review of the groundwater elevations as presented in Section 2 3, groundwater elevations have risen through the 1990s The groundwater elevation at the time of the release is assumed to have been during a time when the groundwater elevation was Iower than current elevations Under this scenario, the dissolved phase hydrocarbons would be brought up along with the rising groundwater table and the LPH would remain as residual hydrocarbons entrapped in the soil Residual saturation is a function of three factors, the soil texture or particle size, soil moisture content, and viscosity of the petroleum hydrocarbons Typically, fine-grained soils have a higher residual saturation level It appears from soil samples collected below the current water table, that residual hydrocarbons were left within the saturated zone This is also demonstrated by the fact that well UVB-MWI, which is screened at least 10 feet below the groundwater table, has contained detectable amounts of LPH (0 02 feet on 21 June 2001) It is the submerged residual hydrocarbons in the fine- ' GVP"ccU\7T 9421MASTERIWP\WP03031WNX&C 9