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1.0 INTRODUCTION This Phase II Evaluation Monitoring Report has been prepared on behalf of the County of San Joaquin, Public Works Department, Solid Waste Division (County) by Geo-Logic Associates (GLA) to fulfill a reporting requirement contained in Item 3, Page 4 of the Order for Technical Reports issued by the California Regional Water Quality Control Board —Central Valley Region (RWQCB) on July 7, 2011. Work described herein has been conducted in accordance with the Evaluation Monitoring Program (EMP) workplan that was approved by the RWQCB on August 25, 2011, as part of an ongoing EMP at the ' Corral Hollow Sanitary Landfill (CHSL). EMP SUMMARY AND CONCLUSIONS: The following conclusions regarding the nature and extent of groundwater impacts from the CHSL have been developed from observations, data, and analyses completed during the EMP studies at the site: • There is no evidence to suggest that VOC concentrations exceed State or Federal maximum contaminant levels beyond the landfill property. • Volatile organic compounds (VOCs) have migrated approximately 1000 feet to the north and west of the landfill boundary; however, there are no known users of groundwater, residents, or commercial or industrial facilities within the plume area. • The relatively high dissolved oxygen measurements, as well as elevated nitrate as nitrogen and sulfate concentrations suggest that naturally-occurring reductive dechlorination or microbial degradation of the VOCs is not likely to occur. Given the slow groundwater flow velocities described in Section 4.4.7 (Aquifer Testing), the migration of VOCs beyond wells MW-9A, MW-10A, and MW-11A appears to be severely retarded, and dilution-attenuation of VOC concentration via dispersion is a likely means of reducing VOC concentrations to acceptable levels. ' Formal recommendations for corrective action alternatives will be developed in an Engineering Feasibility Study that will be submitted under separate cover. The types of VOCs detected in groundwater samples are the same types of VOCs that are most common in landfill gas (dichlorodifluoromethane, tetrachloroethene, and trichlorofluoromethane). As a result, the VOC impacts in groundwater appear to be related to a large degree to landfill gas movement at the capillary fringe between the vadose zone and the water table. i 1 EMP Report—Phase II Geo-Logic Page ■ 1 Corral Hollow Sanitary Landfill ASSOCIATES August 21,2012