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GROUNDWATER <br /> ❑ TECHNOLOGY * Groundwater Technology,Inc. <br /> 1401 Halyard Drive,Suite 140,West Sacramento,CA 95691 USA <br /> Tel:(916)3724700 Fax:(916)372-8781 <br /> January 11, 1996 <br /> 4 <br /> Mr.Brett Hunter <br /> Chevron USA Products Company <br /> 5801 Bollinger Carryon Road,Bldg. L <br /> San Ramon,CA 94583 <br /> Ad to Request for Site Closure <br /> F': rMl Station No.221-8 <br /> 5431 West Grant Line Road, Banta,California <br /> GTI Project 02070-0188 <br /> Dear Brett: <br /> Groundwater Technology,Inc.submits this letter as an addendum to the Request for Site Closure, <br /> dated September 22, 1995,for the site referenced above. This letter addresses the issue of estimating <br /> residual mass of hydrocarbons remaining in soil. An estimate of residual hydrocarbon mass was not <br /> provided in the original report for several reasons including: <br /> • The residual mass remaining in soil is small and presents no threat to groundwater. <br /> • The e)osdng data for the site requires the use of several qualifying assumptions for calculation <br /> of the residual mass of hydrocarbons including thickness and width of the Impacted area. <br /> • The use of the qualifying assumptions mentioned above introduces several sources of <br /> compounding error resulting in a number that may not be representative of the actual site <br /> conditions. An accurate statement of the residual mass would range between 5 and 100 <br /> pounds depending on the assumptions used in the calculation. <br /> As a result of the.above conditions,.Groundwater Technology used conservative assumptions in the <br /> calculation of residual mass. The thickness of the Impeded zone was mumed to be 10 feet thick. <br /> from 5 to 15 feet below ground surface(bgs). The width of the impacted zone was arbitrary yet <br /> conservative,considering the available data(Figure 1). Finally the average TPH-G concentration of the <br /> impacted zone was averaged between the sample collected from B-3 at 9-12 feet and the 10 feet bgs <br /> sample from MW-2. <br /> Using the above assumptions,the residual hydrocarbon mass is calculated at 47 pounds(Figure 1). <br /> o1586MAr <br /> Offices lhroughout the 1:S..Canada and o wnwas <br />