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1 v <br /> a� <br /> A S S O C I A T E S I N C 011 <br /> oar/'°' <br /> 2 Groundwater in the uppermost water bearing zone appears to be confined to semi- <br /> confined Groundwater flow beneath the site is generally southwest with a magnitude (yrt JmAf) <br /> 1 ranging from 0 0003 ft/ft to 0 03 ft/ft Copies of recent groundwater gradient maps are <br /> included in Appendix D <br /> ' 3 Petroleum hydrocarbon-affected soils are generally present in the vicinity of the former <br /> UST from 5 feet to 30 feet bgs Petroleum hydrocarbon-affected soils are present at <br /> locations away from the former USTs primarily in the vicinity of the capillary fringe at <br /> 1 approximately 10 feet bgs The highest concentrations of TPHg, TPHd, and benzene are <br /> present in soil in the vicinity of the former USTs The horizontal extent of TPHg, TPHd, <br /> benzene and the presence of these contaminants in the capillary fringe are indicative of <br /> ' groundwater impact and advection Soil analytical data is included in Table 3 <br /> 1 4 Petroleum hydrocarbons in groundwater were detected in samples collected from borings <br /> SB1 through S134 Groundwater wells MW2, MW3, and MW4 contained TPHd The <br /> plume of TPHd extends east, northeast, and southeast of the former location of the <br /> ' 10,000-gallon UST as shown on Figure 4 The extent of TPHg contamination is limited <br /> to the vicinity of wells MW2 and MW4 <br /> 5 Several potential receptors have been identified southwest of the site Only one of the <br /> receptors is a drinking water well and it is located approximately 1,300 feet south of the <br /> site The remaining wells are identified as monitoring wells, irrigation wells, or industrial <br /> 1 wells and are located downgradient from the site The Stockton Deep Water Channel is <br /> located approximately 1,000 feet north of the site <br /> 1 40 REMEDIAL ALTERNATIVES <br /> Based on the results of previous investigations, ATC has considered three remedial alternatives <br /> ' for impacted soil and groundwater at the site Soil and groundwater remedial alternatives include <br /> performing a human health risk assessment (HHRA) study, excavation of remaining petroleum <br /> ' impacted soil and extracting and remediating groundwater as it accumulates in the open <br /> excavation, and in-situ vapor extraction with groundwater extraction and treatment A summary <br /> of each alternative is presented below <br /> 1 4.1 Human Health Risk Assessment Stud <br /> ' A HHRA study would be performed using the RBCA Tier I software (version 1 01) developed <br /> by Groundwater Services, Inc (GSI) in cooperation with the ASTM committee Individual Risk <br /> Based Screening Levels (RBSLs) would be calculated for each of the identified constituents of <br /> ' concern (COC) The RBSLs are risk-based, site-specific corrective action target levels for <br /> constituents in question The software uses simple analytical models in conjunction with <br /> additional site data to calculate Tier 1 RBSLs in a manner consistent with the ASTM standard If <br /> the RBSLs are not exceeded, ATC would recommend no further action for the site <br /> w 1625001reporLs\CAP doc 4 <br /> 1 <br />