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Review of OU-1 FS Report -6- 7 August 1992 DDRW, Tracy constituents which result in a cumulative cancer risk of less than a one in a million. Semi-Annual Monitoring Semi-annual monitoring is proposed for each of the alternatives after two years of quarterly ground water monitoring subsequent to implementation of the remedial action. I do not concur with this proposal for a blanket approach to monitoring. Conceptually, semi-annual monitoring is acceptable in those cases where the system is well understood, and may even be appropriate for some of the monitor wells at the DDRW, Tracy site. However, up until this past year, Tracy has had a poor historical ground water monitoring program and because the hydraulic dynamics of the system are not all understood (i .e. , the easterly shift in the leading edge of the contaminant plume and obvious seasonal pumpage due to the surrounding agriculture) , it would be premature to approve of semi-annual monitoring. Tracy should implement the remedial action, collect and evaluated two years of quarterly data prior to any proposal in reduction to the monitoring frequency and submit an annual evaluation of the monitoring program. Pesticide Monitoring The Draft OU-1 FS Report indicates that monitoring for organochlorine pesticides (Method 8080) would be required for 30 years with Alternative 1. Presently the WDRs for the IRM do require monitoring for pesticides. However, continued monitoring may not be necessary if consecutive quarterly data demonstrate the absence of pesticides. Monitoring for pesticides for 30 years may not be necessary. Extraction Well Placement Figures 3.3-4 and 3.3-8 depict the placement of extraction wells in each of the three horizons for Alternatives 3 and 6. In general , the extraction wells appear to have been placed in a ring along the leading edge of the plume, especially in the shallow horizon, so that complete capture of the VOC plume to the aquifer cleanup level will occur. We concur with the need to install extraction wells near the leading edge of the plume, but are concerned that the extraction wells do not appear to have been placed near the "hot spots" of the plume both on- and off-base. Extraction wells need to be placed in those areas of the plume where the VOC contaminant concentrations are the highest in order to expedite the ground water cleanup. In addition, fewer wells near the leading edge of the plume may be needed if some of these wells are installed in the areas of the plume with the highest VOC concentrations. The location of the extraction wells should meet the overall objective of extracting a maximum amount of contaminants while maintaining hydraulic control of the plume. Extraction Well Flows and Design The Draft OU-1 FS Report indicates that for the Alternatives where the extraction rate is estimated to be 930 gpm (Alternatives 3 to 7) , extraction from each of the horizons will occur at a rate varying from 23 to 50 gpm (page 3-20) . These flow rates seem to be relatively low considering the hydraulic conductivity (K) and transmissivity (T) estimated for the water bearing zones (Table 3.2-1) . Depending on the T of the zone and the design of the extraction well , some of the extraction