SITE INFORMATION AND CORRESPONDENCE

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Ms. Julie Raming - 2 - 26 April 2000 "v B&C performed a pilot scale test of the proposed system in July 1998 which is documented in the 11 September 1998 Groundwater Extraction/Aeration Pilot Test report. The pilot test data showed that volatization and impoundment of the spray effluent was successful at reducing chloroform to non- detectable concentrations. The 36-hour pilot test also showed that the basin infiltration rate was less than the extraction rate, and B&C did not observe groundwater mounding. However, it is not known whether hydraulic connectivity was established between the basin and groundwater or whether steady state conditions were reached. B&C incorporated the pilot test parameters into the DIRP, which anticipates that approximately 83% of the extracted groundwater will be discharged to the irrigation canal, and approximately 17%will be returned to groundwater through the infiltration basin. Since the groundwater extraction rate will exceed the recharge rate, B&C does not expect groundwater mounding to occur. I agree with the conceptual plan for the remediation system. The monitoring data consistently show that the highest concentrations of halocarbons are beneath the stormwater basin (former evaporation pond), which the DIRP identifies as the objective of the remediation plan The chloroform plume has extended off-site beneath the housing development to the north which is beyond the anticipated radius of influence of the extraction wells. Even so, some benefit may be provided by the hydraulic control exerted by the extraction wells. The DIRP proposes to run the remediation system for three years, evaluate the system and the off-site plume, and address the off-site portion at that time. B&C claims that the off-site plume is stable. Although concentrations declined in residential area monitoring wells BC-12, BC-15, BC-16 and BC-17 since 1985, an increasing trend is evident in BC-13 and BC-14 between 1985 and 1998. Since 1998, chloroform concentrations have been variable in BC-13, and decreasing in BC-14. The last three monitoring events suggest that chloroform concentrations are stable in wells BC-12, BC-15, BC-16, and BC-20 at 48 ug/1, 5 ug/l, 2.7 ug/l, and 2.5 ug/l, respectively. Monitoring wells BC-12 and BC-13 show the highest of the off-site chloroform concentrations and are nearest the source area. Evaluation of the off-site plume should not be delayed for three more years. However, I concur that the data show that the plume size is not changing appreciably, and the remediation system may exert hydraulic control limiting plume movement. Therefore, I believe that the off-site plume should be evaluated after two years of source area remediation. The DIRP proposes to prepare plans and specifications for the remediation system within 90 days of Board staff approval. However, the plan is labeled a draft, and I cannot approve a draft document. The following additional details need to be addressed, as I discussed with Mr. Todd Miller and Mr. Brad Dickson of B&C during our 18 April telephone conversation. 1. The stormwater basin also serves water quality functions for the City of Tracy. B&C needs to provide an analysis of the volume of storage that the City needs to have available in the basin, when it is needed, the required detention time, and a discussion of how the stormwater needs will be met concurrently with the remediation system. Although the DIRP states that the conditions of basin operation will be contained in an Access Agreement between GP and the City, these elements need to be stated explicity in the remediation plan as well.