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Mr. Russell Chapin <br /> 1766 W. Monte Diablo Avenue <br /> Page 6 of 7 <br /> • Compare the remediated mass to the residual contaminant mass and the current <br /> mass extraction rate to allow evaluation of the effectiveness of the current <br /> remedial action. <br /> • Sample all wells with sufficient water in their casings; recalculate the dissolved <br /> contaminant mass, including data from the extraction wells; compare the <br /> addressable dissolved contaminant mass to the extraction rate of GWE. <br /> • Redraw the cross sections, EHD recommends avoiding use of interpreted boring <br /> logs. Rather, extract the raw data and make the interpretations on the cross <br /> sections; make sure each monitoring and remediation well is on at least one <br /> cross section. Utilize the cross sections to define significant hydrogeological <br /> units, contaminant migration pathways and to describe the distribution of the <br /> contaminant mass. This may leads to a 2- or 3-layer MODFLOW model. <br /> • Compare the AGE-derived hydraulic conductivity obtained through field-testing <br /> with the ATC-derived hydraulic conductivity obtained through model matching; <br /> present and discuss the test results running the MODFLOW and MT3D models <br /> utilizing the field-derived value. <br /> • Analyze the contaminant concentration trend in each well in the context of <br /> ground water elevation, flow direction, and remedial activities. <br /> • Calibrate and run MODFLOW and MT3D models to match actual field data; if <br /> monitored natural attenuation (MNA) out performs active, engineered <br /> remediation during the recent past, it would appear that the model would not be <br /> appropriate for this site. <br /> • Use the calibrated model to predict plume response under natural conditions, <br /> focusing on the next year or two under natural attenuation; design a monitoring <br /> program (adding additional monitoring wells if necessary) to test and validate the <br /> modeled predicted response. <br /> EHD is aware that you, the responsible party (RP), are anxious to receive site closure <br /> and that you believe that enough time and money has been spent characterizing and <br /> remediating the effects of the UST release on the site. However, this closure request is <br /> not for a fully remediated site with a simple hydrogeological framework and a plume that <br /> is obviously shrinking and approaching background conditions. The contaminant <br /> migration pathway is oblique or counter to the predominant ground water flow direction. <br /> The consultant preparing the current request estimates that 5733 pounds of TPH-g <br /> contaminates over 10,000 cubic yards of soil that remains on site, a significant portion of <br /> which is now in the saturated zone. Intuitively, this is a mass that must be carefully <br /> evaluated for the risk it poses to ground water and to human health. <br /> ATC estimates that 147 pounds of dissolved hydrocarbons have been removed from the <br /> site and that only 2.6 pounds of dissolved contaminant remain. However, the <br /> concentrations of contaminants extracted in ground water today are not significantly <br /> lower than when extraction began. The EHD roughly estimates that 1 pound of <br /> contaminants is removed with each 25,000 gallons of extracted ground water. According <br /> to ATC's model, extraction of 75,006 more gallons should leave the groundwater with <br />