SITE INFORMATION AND CORRESPONDENCE_2

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Del Monte Stockton Property Assessment and Remediation RFP Section III, Page 2 B. Groundwater Monitoring Since 1991, Del Monte has conducted quarterly groundwater monitoring and reported the results to SJCPHS. While groundwater was flowing in the northwest direction, gasoline contamination had not moved far from the location of the gas tanks and pump dispensing areas. The highest concentrations occur at 80 feet bgs, and in October 2001, groundwater in the one 80 foot well contained 28,000 ppb benzene, 53,000 ppb toluene, 3,800 ppb ethyl benzene, 20,000 ppb xylene, and 220,000 ppb TPH as gasoline. MTBE and other oxygenates have not been confirmed on site except for tert butanol, which has been found in random testing but not found consistently in any monitoring well. Within the last two years, groundwater analyses included testing for 1,2-dichloroethane (1,2-DCA) and ethylene dibromide (EDB). In October 2001, the highest concentration of 1,2-DCA found was 1,900 ppb in the 80 foot deep monitoring well (NP-1-80), while the highest concentration of EDB found was 1,000 ppb in the same 80-foot deep monitoring well. More detailed information, including a site map, can be found in the Quarterly Monitoring Results Report for July and October 2001 (Attachment 4). A Cross-Sectional Analysis was requested by SJCPHS on October 4, 1997 and was completed on February 3, 1998 (Attachment 5). The lack of soil boring information in the new down gradient direction made the analysis less useful than originally thought. C. Monitoring Well Sampling Decision Tree A decision tree was developed several years ago because we and the regulators saw no reason to keep sampling and testing all monitoring wells on a quarterly basis. When a monitoring well comes up nondetect for four quarters, testing stops. Currently, water samples are drawn and tested only in the plume area in the first and third quarter of each year and now each quarter in the new down gradient wells. Groundwater levels are checked each quarter to verify direction of groundwater flow. The Decision Tree is presented in the Quarterly Monitoring Results Report for July and October 2001 (Attachment 4). D. Air Sparging and Soil Vapor Extraction System In February 1995, Del Monte initiated remediation at the site by installing an air sparging and soil vapor extraction(AS/SVE) system that utilized an internal combustion engine to burn the VOCs. The engine was connected to two sets of AS/SVE wells and used natural gas as a backup fuel source. Two sets of nested piezometers measured soil vacuum from the SVE system. Two additional SVE wells were installed in November 1995. The engine was shut down in November 1995 when the intake VOC level tested less than 500 ppmv, as originally planned. The