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06/05/06 MON 10:35 FAX 925 363 7275 TERRA VAC NOR CAL Z002 <br /> Proposal for Due Diligence Sampling <br /> Circle K Store'5449 <br /> Stockton, CA <br /> Purpose: <br /> The purpose of the proposed due diligence sampling is to determine if a new release has <br /> occurred at the Site since 2001. It has been suggested that a recent release has resulted in <br /> increased groundwater dissolved please hydrocarbon (including MtBE) concentrations at <br /> th W Site. <br /> Discussion: <br /> if an uncontrolled hydrocarbon release had occurred at the Site since 2003 vadose zone <br /> soil concentrations beneath the impacted areas would show evidence of increased <br /> petroleum hydrocarbon concentration. Therefore collection and analysis of soil samples <br /> from the vadose zone at select locations can be performed to determine if a recent release <br /> has occurred. Analytical results of collected soil sarrrplcs can be compared.to historic. soil <br /> data collected'at The Sitc. .A significant increase in soil concentrations may indicate that <br /> that a recent release has occurred. However, since gasoline containing.MtBE has not <br /> been stored on the site since before 2003 detection of MtBE. would indicate that the <br /> source of the.Mt13E and the release occurred prior to 2003. <br /> Furthermore, soil samples collected below the static water'table will not provide valuable <br /> data in determination of whether a recent release has.occurred. Given the model of fate <br /> and transport of petroleum hydrocarbons in the subsurface soil, sample collection <br /> significantly below the water table is not necessary unless groundwater elevation clianbes <br /> could have.snrea.red or trapped released hydrocarbons below static water level.- Since <br /> 2003 reported groundvua#er elevations at the site have only varied by approximately 4 feet <br /> acid current groundwater elevations (2/2106) are at elevations similar to groundwater <br /> elevations in 2003. Therelbre collection of soil samples below the water table will. not <br /> yield any data that would sho" a recent release has occurred. <br /> Collection and analysis of`'roundwatcr samples from the.proposed borings locations will <br /> not,yield any additional information that could not already be obtained (or is already <br /> available) froin existing 1-nonitoring wells. Each of the proposed boring locations bas <br /> active monitoring wells Within approximately 8 feet. <br /> Installation of an additional tiroundwater monitoring well at the proposed location BWA <br /> will not provided information usable in determining if a recent release his occurred. <br /> Based on the information available.this area has not been previously accessed. Since it is <br /> known that a release has occurred at this site prior to 2003 detection of hydrocarbons at <br /> this location may occur' yet pt•ovide no comparable data for evaluation. <br /> Proposed approach: <br /> The proposed approach is to advance soil borings at the.locations depicted in the attached <br /> figure. The borings will be advanced to a depth of 45 feet below grade, or to <br />