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� v <br /> 0 <br /> A S S O C I A T E S I N C <br /> 6.0 DISCUSSION <br />' The following paragraphs summarize the results of the subsurface investigations performed at the <br /> project site <br /> 1 The extent of residual soil contamination appears to be limited to the vicinity of well <br /> MW2 and does not appear to be a significant threat to the site Soil analytical results are <br />' contained in Table 4 <br /> 2 The vadose zone, from 0 to approximately 19 feet bgs, consists of poorly graded sand and <br /> well graded sand The capillary fringe, from approximately 19 feet to 22 feet bgs, <br /> consists generally of poorly graded sand and well graded sand The soil beneath the <br /> capillary fringe, from approximately 22 feet to 31 feet bgs, consists generally of poorly <br /> graded sand and well graded sand with elastic silts and lean clay from 31 feet to 45 feet <br /> bgs <br /> 3 Groundwater in the uppermost water bearing zone appears to be unconfined <br /> Groundwater flow beneath the site is generally southwest with a magnitude of 0 0004 <br /> ft/ft, or approximately 2 ft/mile <br /> 4 TPHg and BTEX constituents were detected in groundwater samples collected from well <br /> MW2 Wells MW1, MW3, MW4, and MW5 have never contained detectable <br /> concentrations of TPHg or BTEX The plume of TPHg and benzene appears to be <br /> confined to an area southeast of the former USTs within 30 feet of well MW2 TPHd was <br />' detected in groundwater samples collected from wells MW1, MW2, MW4, and MW5 <br /> Well MW3 has never contained detectable concentrations of TPHd The plume of TPHd <br /> appears to extend from well MW2 south and southwest to wells MW4 and MW5 The <br /> concentrations of TPHd appear to decrease with distance from well MW2 All five wells <br /> have been sampled for oxygenate fuel additives by EPA Method 8260 and do not contain <br /> detectable concentrations of oxygenate fuel additives <br /> 5 The only potential receptor of the petroleum hydrocarbon plume is the on-site irrigation <br /> well located approximately 500 west-south west of well MW5 The well is <br />' approximately 149 feet deep The screen interval is unknown, but the groundwater pump <br /> in the well is located at approximately 42 feet bgs <br /> 7.0 REMEDIAL ALTERNATIVES <br /> Based on the results of previous investigations, ATC has considered three remedial alternatives <br /> for groundwater Groundwater remedial alternatives include natural attenuation with <br /> . groundwater monitoring, groundwater extraction and treatment, and in-situ vapor extraction with <br /> air sparg>ng A summary of each alternative is presented below <br /> w 1130991repor[slsumweh doc 7 <br />