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ST Services Stockton Terminal - 3 - 13 August 2007 <br /> i <br /> 2. The Addendum states that based on an evaluation documented in the 2006 Cleanup Plan, <br /> NuStar had determined that dual phase extraction (DPE) is an effective option for site <br /> cleanup but eliminated it from consideration because the bulk of the remaining <br /> contamination underlies the tanks. It is unclear how this was determined. Regional Water <br /> Board staff are not aware of any sampling data collected from beneath the tanks. Federal <br /> AGT regulations in 40 CFR Part 112 require NuStar to ensure that the tanks are not <br /> leaking. Assuming that the Site is in compliance with this regulation, and that the only <br /> source of contamination is the documented unauthorized releases, the monitor wells likely <br /> provide a reasonably accurate indication of the maximum pollutant concentrations at the <br /> Site. Based on this rationale following completion of the MNA analysis in February 2008, <br /> NuStar should re-evaluate use of DPE or another remedial technology to remediate the <br /> elevated hydrocarbons at the Site in less than 4,500 years. <br /> 3. The MNA parameters summarized in Table 3 of the QMRs appear to indicate that the <br /> natural attenuation in groundwater at the Site is most likely occurring under anaerobic <br /> sulfate reducing conditions. This means that biodegradation will occur at relatively slow <br /> rates since this mode of metabolism provides less energy to microorganisms than other <br /> pathways. In addition, a sufficient mass of sulfate has to be available to facilitate the <br /> metabolism of the fuel hydrocarbon molecules by microbes. As part of the ongoing MNA <br /> parameter collection, NuStar should provide preliminary analysis of the observed <br /> parameters in the third and fourth quarters 2007 to (1) identify the metabolic pathway along <br /> which biodegradation will proceed, and (2) verify that there is sufficient electron acceptor <br /> mass to facilitate biodegradation of the entire hydrocarbon mass. <br /> If you have any questions regarding this letter, you may contact me at (916) 464-4811 or <br /> betaylor(aDwaterboards.ca.gov. <br /> BRIAN TAYLOR, P.G. <br /> Engineering Geologist <br /> Enclosure: ST/MW-1 Linear Regression Graphs. <br /> cc: Ms. Margaret Lagorio, San Joaquin County Environmental Health Department, Stockton <br /> Ms. Rita Koehnen, Port of Stockton, Stockton <br /> Mr. Jerry Stauffer, Valero, L.P., San Antonio, TX <br /> Ms. Amanda Spencer, Ash Creek Associates, Inc., Portland, OR <br /> Mr. Rusty Benkosky, SECOR International, Inc., Rancho Cordova <br />