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V'Y' AT C <br /> A S S O C I A T E S I N C <br /> 0 In the event improvements are needed to the site in the immediate vicinity of the former <br /> UST, construction workers could be exposed to subsurface soil volatilization to outdoor <br /> air should deep soil excavation (>15 feet bgs)be required, <br /> 0 Subsurface soil or groundwater volatilization to indoor or outdoor air via underground <br /> utilities and/or vertical dispersion is possible for structures in the vicinity of the former <br /> USTs <br /> Although the above scenarios may appear unlikely, and perhaps do not warrant the need for <br /> further evaluation, this conservative approach may further assist in determining nsk to human <br />' health and the environment <br /> 5_2 RBCA Model <br />' Using the RBCA Tier I/ Tier 2 software (version 1 01) developed by GSI in cooperation with the <br /> ASTM committee, individual Site-Specific Target Levels (SSTLs) were calculated for each of the <br /> identified constituents of concern (COCs) The SSTLs are risk-based, site-specific corrective <br /> action target levels for constituents in question The SSTLs represent upperbound constituent <br /> concentrations that, if achieved throughout the source zone, will prevent the exceedence of <br /> applicable risk limits at potential points of exposure The software uses simple analytical models <br /> in conjunction with additional site data to calculate Tier 2 SSTLs in a manner consistent with the <br /> ASTM standard <br />' Model Input <br /> As described above, there does not appear to be any complete pathways at the subject site The <br /> only secondary sources are 1) the subsurface soil which may lead to soil leaching to groundwater <br /> or volatilization from subsurface soil to outdoor or indoor air and 2) the dissolved groundwater <br /> plume which may lead to groundwater ingestion or volatilization to outdoor or indoor air <br />' Constituents of Concern <br /> According to the ASTM standard, TPH is not to be modeled when performing a RBCA <br />' evaluation The following CDCs were therefore identified for the site <br /> • benzene, <br />' • ethylbenzene, <br /> • toluene, <br /> • total xylenes, and <br /> • methyl tertiary butyl ether <br /> Representative COC soil concentrations of the source zone were calculated using soil analytical <br /> data from only soil borings advanced within the delineated zone of contamination (borings SBI, <br /> w 1148851rcNrWNFA-RequcA doc 9 <br />