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Draft Proposed Plan Memorandum -2- 7 August 1992 <br /> DDRW, Tracy <br /> semi-annual monitoring may be inadequate to monitor the plume dynamics. A <br /> longer period of time than two years may be needed prior to a reduction in the <br /> monitoring frequency. We approve however, of the concept of a reduced <br /> monitoring program on the off-quarters between the dry and wet season quarters. <br /> Minor Comments <br /> 5. The Draft OU-1 PP states that Remedial Investigation/Feasibility Study (RI/FS) <br /> included sampling off-site private supply wells in 1987 (paragraph 3) . This <br /> sentence should be modified to state that these wells were also sampled in 1992 <br /> and that selected off-site wells will be incorporated into the quarterly ground <br /> water monitoring program. <br /> 6. Page 5 of the Draft OU-1 PP discusses the IRM (paragraph 4) . We recommend that <br /> a sentence be added regarding the purpose of the IRM: to capture and treat the <br /> VOC "hot spot" contamination in the shallow zone and to prevent it from <br /> migrating off-site. In addition, the sentences regarding the system being <br /> inoperable may need to be deleted if the re-start of the IRM begins in <br /> September 1992 which would be prior to issuance of the Final PP. This <br /> paragraph should also mention that the IRM has been operated under existing <br /> WDRs (No. 90-275) . <br /> 7. Page 6 of the Draft OU-1 PP lists the potential ground water contaminants that <br /> may have off-site sources. Chloroform should be added to this list as three <br /> off-site sources are known to the Board to have contaminated ground water with <br /> chloroform in this general area. <br /> 8. Page 8 (paragraph 3) of the Draft OU-1 PP states that inorganics such as boron, <br /> nitrates and pesticides do not appear to threaten water quality at off-site <br /> wells. The precise meaning of this sentence is unclear because these <br /> contaminants may be present in elevated concentrations regionally. Any <br /> concentration of contaminants above background concentrations are considered to <br /> have degraded water quality. It is unclear what is considered to be a threat <br /> to water quality since this terminology is usually used by the Board in <br /> reference to a source area and/or a contaminant plume. This sentence needs to <br /> be clarified. <br /> In addition, this paragraph indicates that pretreatment for one of these <br /> compounds may need to be performed. If it is determined that DDRW, Tracy <br /> caused the ground water contamination, then pretreatment of these compounds may <br /> be required depending on the method of disposal of the treated ground water and <br /> if these compounds interfere with the proposed treatment. <br /> 9. Disposal of the treated ground water for Alternative 3 includes the <br /> installation of 31 additional injection wells in the upper and middle horizons <br /> (page 10) . It is unclear why the lower horizon was not considered for disposal <br /> because of potential water quality degradation problems or because of a <br /> hydraulic limitation of the horizon. However, this concern may be more <br /> adequately addressed as part of the Remedial Design/Remedial Action (RD/RA) . <br /> 10. Page 10 (last paragraph) indicates that only the Department of Toxic Substances <br /> Control (DTSC) approved the use of the existing air stripper for the IRM. It <br />