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Ms. Julie Raining • -4- Recommendations GP recommends natural attenuation as the preferred remedial alternative because it is effective and would achieve the remedial goal at the lowest cost. GP proposes to submit a natural attenuation work plan for Board approval during the third and/or fourth quarter of 1997. The work plan would include ground water monitoring parameters and calculation of chloroform degradation rates. I do not agree with GP's conclusion that natural attenuation is the preferred remedial alternative. Although it appears to be the most economical alternative from GP's evaluation, it is not the most effective. GP should further evaluate the effectiveness of natural attenuation, ground water pump and treat, and AS/SVE based on the comments in this letter. GP should recalculate the time required for natural attenuation to remediate chloroform to less than the remedial goal in the source area, considering a half-life of 5.5 years. The conditions for maximum natural degradation should be identified as well as those conditions not as favorable to degradation. The evaluation of ground water pump and treat should include an estimate of the time required to meet the remedial goal by this approach. The potential effectiveness of AS/SVE should also be evaluated further by researching its effectiveness at other sites. The cost to implement ground water pump and treat should be estimated and more details on the cost estimate for AS/SVE provided with consideration given to ways to reduce the costs of AS/SVE. GP proposes to locate, redevelop, and sample wells BC9, BC10, and BCl l if appropriate (or properly abandon these wells), measure water levels, collect water samples for chloroform analyses, submit a report describing field activities and sampling results for the second quarter 1997. The wells should also be analyzed for trichlorofluoromethane (TCFM). TCFM was detected in BC-13 and BC-18 at 2.1 and 3.3 µg/1, respectively. Although significantly lower than the primary MCL of 150 µg/1, TCFM concentrations should be monitored. GP does not recommend any additional investigative work to delineate the chloroform plume either laterally or vertically. As noted above, the extent of chloroform has not been determined in the area of BC-20 at the screened interval of BC-13 (75 to 85 feet bgs) where 7.5 µg/l chloroform was detected in February 1997. The extent of chloroform in this zone should be evaluated after three more quarters of data are available from BC-13. By 5 September 1997, please submit an addendum to the evaluation of remedial alternatives. The addendum shall further evaluate the effectiveness of natural attenuation, ground water pump and treat, and AS/SVE as discussed above, the cost to implement ground water pump and treat, and methods to reduce costs associated with AS/SVE. If you have any questions, you may call me at (916) 255-3048. POLLYWRY Associate Engineering Geologist PAL:pal/lsb cc: San Joaquin County Public Health Services, Stockton Mr. Todd Miller, Brown and Caldwell, Pleasant Hill Recycled Paper Our mission is to preserve and enhance the quality of California's water resources,and ensure their proper allocation and efficient use for the benefit of present and future generations.