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AECOM Report on Soil Vapor Sampling and Human Health Risk Assessment 35 <br /> 3.21.2 Results <br /> The cumulative ELCR,and the HI (which is defined as the sum of individual HQs)were calculated by <br /> summing the ELCR and HQ for the individual COPCs, respectively. The results for the TICS were <br /> summed separately from the other compounds since some of the TICS may also make up the other <br /> carbon fraction ranges being evaluated. If the sum of individual HQs exceeds 1,the HI can be further <br /> evaluated by summing only those compounds that act on the same target organ. Therefore,the <br /> resulting HI may be an overestimate.The cumulative risk estimates were compared to CalEPA's and <br /> USEPA's target cancer risk range of 1x10 to 1x104 and target HI of 1 (USEPA, 1991). <br /> Table 5 (attached)presents the EPCS used in the modeling,the resulting potential ELCRs, and non- <br /> carcinogenic His associated with the soil vapor to indoor air vapor intrusion inhalation pathway.The <br /> total potential ELCR and HI results for both the detected compounds and TICS are below the CaIEPA <br /> and USEPA's target cancer risk range of 1x10-1 to 1x104 and total HI of 1,for the residential scenario. <br /> Therefore, based on these results, concentrations of detected volatile compounds(including TICs) in <br /> Soil vapor do not pose a human health concern. <br /> For all COPCs, the maximum detected concentration was at SV-1 (former Unocal site).Therefore, <br /> potential ELCR/HI associated with compounds detected in soil vapor at samples SV-2 and SV-3 (the <br /> former Wards site)also fall below target levels. <br /> 3.2.3 Additional Uncertainty <br /> There is some uncertainty associated with estimating potential risks from vapor intrusion to indoor air. <br /> Multiple factors contribute to uncertainty in the evaluations, as described below. <br /> • The JE Model makes several assumptions in estimating indoor air concentrations. For <br /> instance,the model assumes that a constituent is homogeneously distributed within the <br /> source zone (USEPA, 2004). <br /> • The use of the maximum detected soil vapor concentrations(located at SV-1)as the EPCs for <br /> estimation of ELCR/HI is a conservative assumption, intended to represent the worst-case <br /> exposure scenario. The potential risk/HQ associated with the concentrations detected at the <br /> other soil vaporsample locations are lower than those presented in this evaluation. <br /> Analysis of the soil vapor samples for carbon ranges, using APH analysis, results in <br /> detections of constituents within two carbon ranges, C5-C8 aliphatics and C8-C10 aliphatics. <br /> In addition,TICs include predominantly short-chain aliphatic compounds,which are less toxic <br /> than aromatic compounds.The evaluation of the TICS was conservatively performed to <br /> account for all potential components of TPH that may contribute to ELCR/HI to current/future <br /> on-site receptors. <br /> The results of this evaluation are based on the assumption that current/future on-site <br /> buildings will be of slab-on-grade construction, since this is the typical construction in this area <br /> of California. The evaluation of a building with a basement construction is not included in this <br /> evaluation and is likely to result in a higher estimated ELCR/HI. The evaluation of a building <br /> with a crawlspace would be similar to that performed for slab-on-grade construction. <br /> The results of this soil vapor intrusion risk evaluation are based on the assumption that the <br /> soil vapor concentrations remain constant for the assumed exposure duration (i.e., 25 or 30 <br /> December 15,2010 <br /> 60146576-M10 <br />