SITE INFORMATION AND CORRESPONDENCE

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Ms. Julie Raining • -2- Chapter 6 Summary, Conclusions, and Recommendations GP has evaluated natural attenuation, ground water pump-and-treat, soil vapor extraction (SVE), air sparging/soil vapor extraction (AS/SVE), and in situ bioremediation as feasible alternatives for decreasing chloroform concentrations in groundwater beneath the site. GP determined that natural attenuation and AS/SVE are considered to be the most effective. Natural Attenuation - GP estimates, based on a half-life of three years for chloroform, that the remedial goal established for the site should be achieved in approximately 1.5 to 2.5 years north of the irrigation canal and in approximately 15 years in the source area near the basin. The more conservative half-life of 5.5 years should be used to estimate the time for remediation in the source area, as noted above. GP states that literature reviews indicate chloroform can and does naturally degrade over time in subsurface environments. GP should identify the conditions for maximum degradation as well as those conditions not as favorable to degradation. Groundwater Pump and Treat - GP argues that extracted groundwater reached an asymptotic level within two years of operation and that there is no reason to believe future operation of the existing groundwater extraction system would achieve better results or would be capable of reaching the cleanup goal of five µg/l. I do not agree that chloroform had reached asymptotic levels in the recovery well RW2 before pumping was ceased in that well. Chloroform concentrations varied between 6.3 µg/l and 17 µg/l between the time pumping started in March 1990 until pumping was discontinued in September 1991. Overall chloroform appeared to be decreasing in that time period, with the last measured chloroform at 6.3 yg/1. Continued pumping would likely have resulted in achievement of the remedial goal in a reasonably short time. Currently, chloroform exceeds the water quality goal of five µg/1 in eight monitoring wells. The highest chloroform concentrations are present in wells near the source area, wells BC3, BC4, BC5, BC18, and BC19. Wells BC3, BC4, and BC5 are all screened between 19.5 and 39.5 feet bgs. Chloroform in these wells ranged from 92 to 110 pg/l in February 1997. Wells BC-18 and BC-19 are screened between 29 and 55 feet bgs. Chloroform in these wells ranged from 32 to 55 µg/l in February 1997. The remediation time would be reduced considerably if pump and treat was conducted from all or some of these wells. GP should re-evaluate the effectiveness of extracting ground water from selected source area wells. Soil Vapor Extraction - I agree with GP's statement that SVE would be ineffective at the site since no chloroform was detected in the vadose zone soil during the investigation completed earlier this year. Air Sparging/Soil Vapor Extraction - GP concludes that a combination of AS/SVE could successfully remediate chloroform in groundwater to below the cleanup goal established for the site. Although effective for most areas, this approach would not likely be effective at reducing chloroform concentrations at depth, where overlying fine-grained soils are present, such as at BC-13, BC-14, and BC-16. GP should discuss any literature describing where this approach has been used for remediating chloroform in ground water and its effectiveness. In situ Bioremediation - GP does not consider in situ bioremediation economically feasible since there is not sufficient evidence in the literature reviewed to show augmentation of the existing system by adding nutrients and/or oxygen to the subsurface would achieve the established chloroform cleanup goal (�,3y faster than if natural attenuation were allowed to proceed as it is currently. 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.