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1 ' The submerged residual hydrocarbons entrapped in the saturated soil matrix achieve equilibrium by conserving surface area and adhering to the soil The transfer of submerged residual ' hydrocarbons into groundwater is a very slow diffusion limited process that does not generally produce large concentrations in groundwater This may explain why the operational measurements of the WB system reached the asymptotic stage of operation, as extraction of groundwater by the WB system could not remove the residual hydrocarbons, which are slow to transfer to the dissolved phase ' The dissolved-phase concentrations are also well defined as presented in Figure 6, which presents the most recent June 2001 groundwater analytical results The groundwater flow direction has generally been to the northeast with a historical average of N53E and an average ' gradient of 0 006 as presented in the rose diagram on Figure 2 Six wells (V3, RW4, MW10, MW 12, MW 13, and MW 11) have been installed offsite and northeast of the site to allow for ' downgradient definition of the dissolved phase hydrocarbons As presented in Figure 6, the highest detected concentration of benzene in a downgradient offsite well was 3 6 gglL detected in RW4 on 21 June 2001 MW 13, which is downgradient of RW4, did not contain detectable concentrations of TPH-g, BTEX, or MTBE Based on a review of the historical groundwater data from both onsite and offsite wells, and the consistent groundwater gradient over time, the dissolved phase hydrocarbons are stable and defined tAn assessment of other remedial alternatives for groundwater cleanup was conducted to address current site conditions This review included the following technologies air sparging, slurry wall, interception trench, in situ oxidation, and groundwater extraction Based on a review of the hydrogeology at the site, monitoring and natural attenuation is considered as the only viable option, given current site conditions Accordingly, a RBCA analysis was performed in support ' of evaluating the natural attenuation alternative To support the RBCA analysis, shallow soil vapor sampling was performed at eight onsite locations (SBI-SB8) The results of this sampling indicated the presence of hydrocarbons at residual levels in soil vapor beneath the site ' In addition, a sensitive receptor survey was also performed Based on the sensitive receptor search, there are no wells pumping water from a shallow or deeper water bearing zone within one mile of the site in the downgradient direction The only active well identified (Sta 49) is 512 feet deep, is screened from 170 feet, and is located upgradient and approximately 1,900 feet from the site, such that it is not likely to be impacted by hydrocarbons in groundwater beneath the site ' Based on the above data, a RBCA analysis was performed to assess potential health risks associated with residual levels of hydrocarbons present beneath the site and to determine the viability of no further remedial action Available soil, groundwater, and soil vapor data were evaluated and used together with conservative assumptions adopted by ASTM (1995) for a Tier I RBCA analysis This analysis included evaluation of potential risks to both on- and offsite ' receptors such as residents at the Chateau Hospital adjacent to the site The RBCA analysis included an evaluation of indirect exposure via inhalation of volatiles from all potential subsurface sources (e g , soil, groundwater, and LPH) using soil vapor data Specifically, ' maximum onsite soil vapor data were conservatively used to evaluate risks to both on- and G 1Projectst739425MASTER\WP1RP0801tteet2 doc 12 A