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1 I <br /> Elft® <br /> I • <br /> Mr James L Barton <br />' July 27, 2005 <br /> Page 5 <br /> IIn ENSR's opinion, inhalation of vapors through indoor air vapor intrusion is considered the only <br /> potential exposure pathway to receptors at this site ENSR prepared a vapor intrusion pathway <br /> evaluation as a separate document and has included herein as Attachment G <br /> Background Areas for Contaminants <br /> I <br /> The COCs are not naturally occurring materials within the study limits for this SCM Therefore, <br />' no background areas can be determined However, screening levels for both groundwater and <br /> vapor intrusion based upon groundwater concentrations are defined in this report The <br /> screening levels for groundwater concentrations are defined in Appendix A — Reports, <br />' Th-Regional Board Staff Recommendations for Preliminary Investigation and Evaluation of <br /> Underground Tank Sites, dated April 16, 2004 The screening levels for vapor intrusion were <br />' derived using the California Environmental Protection Agency (CaIEPA) version of the Johnson <br /> and Ettinger Model and are defined in Attachment G <br /> Receptors <br /> ENSR believes that because there are no drinking water wells within the monitoring well <br />' network, human health is not at risk through groundwater as a pathway The nearest surface <br /> water body is more than 4,000 feet away Therefore, beyond human health, no ecological <br /> receptors are suspected to be at risk as a result of residual and dissolved concentrations of <br /> petroleum hydrocarbons beneath the site Human health risk from vapor intrusion is evaluated <br /> in Attachment G A site conceptual model depicting the exposure pathway described herein is <br /> included as Figure 16 <br /> Remediation System Description <br />' Residual and d ssoIved concentrations of petroleum hydrocarbons beneath the site are <br /> volatilized, extracted, and treated through the operation of a two-phase remediation system at <br /> the crta rnmhmprl air cnnminn [%Aql and cnil vannr Pytrartinn lSVFI AS ctatp d nrPviniisly in <br /> this report the AS and SVE systems began operation in October of 2003 The AS system <br /> distributes air through 17 sparge wells that are each equipped with valves for adjusting air <br /> distribution and with ports for measuring pressure and flow rate <br /> The SVE system extracts vapors from 18 vapor extraction points consisting of 14 soil vapor <br /> extraction wells and four modified groundwater monitoring wells The system discharges <br /> treated vapors to the atmosphere in accordance with the San Joaquin Valley Air Pollution <br />' Celebrating 35 Years of Excellence in Environmental Services <br />