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5 GROUNDWATER MODELING <br /> 1L <br /> California maximum contammant levels (MCLS) for benzene, toluene, ethylbenzene "CM3 <br /> and xylenes compounds in groundwater are 1 µg11, 150 µg11, 700 µg11, and 1,750 µg/l, <br /> respectively Based on current groundwater monitoring data, the MCL for benzene is 0 <br /> exceeded for samples collected from wells MW-1, MW-7, SP-1 and SP-2 a"A ft-1. p �. <br /> 7+4rw13 VA , <br /> ' PSC evaluated a groundwater model for benzene as the contaminant of concern using <br /> an ASTM Risk Based Corrective Action (RSCA) approach (ASTM E-1739-95) This ���� <br /> ' model provides an indication of the risk for benzene to jeopardize potential receptors, <br /> particularly through the groundwater ingestion exposure route <br /> ' The RBCA groundwater model calculation predicts a contaminant concentration at a <br /> distance X along the centerline of a groundwater plume emanating from a vertical <br /> planar source in the aquifer This model accounts for three-dimensional dispersion <br /> ' and biodegradation The model allows for typical default values and for site-specific <br /> data, such as source concentration, source width, source thickness, hydraulic <br /> conductivity and groundwater gradient Parameters in the equation include <br /> ' X = Distance from the planar source to the location of concern, along the <br /> centerline of the groundwater plume (in a downgradient direction) <br /> CX = The concentration of the contaminant at a distance X from the source <br /> Cs = The concentration of the contaminant at the source <br /> ' (X"y Z = Dispersivity in the x, y, and z directions <br /> U = Specific discharge (flow velocity) using hydraulic conductivity (K), <br /> total soil porosity (OT) and hydraulic gradient (I) <br /> = First order degradation constant of the contaminant <br /> SW = Width of groundwater source in the y direction <br /> SD = Depth of groundwater source in the z direction <br /> A spreadsheet showing the RBCA calculation and results are included in Appendix E <br /> ' of this RACR Using a Cs benzene concentration of 0 49 mg/1 (measured in the SP-1 <br /> groundwater sample), the model predicts that the associated C,concentrations <br /> decrease rapidly at a distance of 50 feet from the source This calculated rapid <br /> rreduction is primarily influenced by the relatively low hydraulic conductivity of the <br /> subsurface that has been measured at this facility This model indicates that the <br /> ' concentration of benzene will not exceed its associated MCL beyond its property <br /> boundary, and that it will not pose a risk to potential receptors through the <br /> groundwater ingestion exposure route <br /> � la <br /> Remedial Action Completion Report <br /> The Earthgrains Company Facility <br /> 2651 South Airport Way <br /> Stockton California <br /> Prepared for TuE LARTt1GRAIN5 COMPANY Prepared by Philip Services Corp <br /> 11 TNVt624001321121382@GVStmkton\SwcktunlRemcdialCompleuonRepon doc April 14 2003 <br />