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Mr. Werner Sicvol <br />0 -2- <br />Fourth Quarter Monitoring Report <br />I concur with the report conclusions except for the following: <br />• 11 February 1999 <br />1. The report states that volatile organic compound concentrations in nearly all MWs, including <br />MW -5R, decreased and these concentration changes appear to be within normal range of variation. <br />Since its installation, MW -5R has had these concentrations: <0.5, 1.3, 11, and 26 parts per billion. <br />These changes in these concentrations are not within the normal range of variation. Instead, they <br />show increasing concentrations. Since MW -5R is the most downgradient well in the main plume, <br />BP should provide plans to prevent further migration of the plume. <br />2. The report states that natural attenuation (NA) parameters yielded limited additional evidence to <br />support natural biodegradation of contaminants of concern. The Technical Protocol for Evaluating <br />Natural Attenuation of Chlorinated Solvents in Groundwater [Protocol (Wiedemeier, et. al )] <br />provides a screening process to determine if NA of chlorinated compounds is a viable option. The <br />screening process consists of collecting NA parameters and using a weighting table (Table 2.3) <br />provided in the Protocol to determine if NA is occurring. Application of this screening process to <br />the site in conjunction with results from selected wells gives the following scores: 1 for GW -1, <br />0 for GW -2, 3 for GW -3, 1 for OB -1, and 4 for OB -2 (see attached table). According to the <br />Protocol, these scores suggest that biodegradation is not occurring or is occurring too slowly to <br />contribute to NA. In a case like this, the Protocol recommends critically evaluating other NA <br />mechanisms or implementing engineered remediation systems. Therefore, BP should look at other <br />remedial alternatives besides NA to enhance the GWETS. BP is required to remediate groundwater <br />to background (nondetectable) or beneficial use protective levels unless BP can demonstrate that it <br />is technologically or economically infeasible to do so. <br />By 5 March 1999, please address the above comments. Please inform me at least 72 hours prior to <br />initiation of field work, such as start up operations of the GWETS, so that I may be present to observe or <br />take split samples. If you have any questions, you may call meat (916) 255-3081. <br />PHILIP'S. ISORENA <br />Associate Engineer <br />Attachment <br />cc: San Joaquin County Public Health Services, Stockton <br />Mr. Fred Kintzer. Parsons Engineering Science, Inc., Oakland <br />PSI:Isb.bp <br />