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l 2 r 1994) for completing exploratory groundwater sampling,well installation,pump testing, and preparation of a Final Remediation Plan. In April 1995,Del Monte submitted a Problem Assessment Report and Final Remediation Plan(PAR/FRP)to describe the supplemental investigation. The PAR/FRP followed the guidelines of the Tri-Regional Board Staff Recommendations for Preliminary Evaluation and Investigation of Underground Tank Sites,Appendix A-Reports (Central Valley Regional Water Quality Control Board,1991). At the direction of PHS/EHD,Del Monte is conducting quarterly groundwater monitoring (since 1991), operating an active remediation system at the Disco site (since February 1995), and conducting a 1-year ITB demonstration. The remediation system consists of air sparging and soil vapor extraction(AS/SVE). A work plan was submitted to PHS/EHD for the installation of two multi-purpose monitoring wells in the source area,which were installed on December 11 and 15, 1995. Quarterly Groundwater Monitoring Results Quarterly groundwater samples and water level measurements were collected on February 13,April 18,and June 6, 1996. Groundwater Gradient Water levels were measured by Del-Tech of Oakdale,California,while the AS/SVE system was not in operation. Table 1 presents shallow groundwater surface elevations for each well measured since September 1991.Table 1 includes top-of-screen(TOS)elevations to point out that the water level is above the top of well screens in all of the Disco site monitoring wells. As indicated in Table 1,water levels have risen by approximately 5 feet since the previous monitoring event. Figures 2A,2B,and 2C show an interpretation of shallow groundwater elevation contours which indicate that the groundwater is flowing in a southerly direction. BTEX Analytical Results Table 2 presents analyses of groundwater samples from monitoring wells. Laboratory reports and chain of custody forms are attached. MW-DIS-2 was sampled by two different methods and analyzed by different laboratories.The samples collected on February 13,and June 6, 1996 were taken after a rapid well purge typical for quarterly monitoring. The sample collected on April 19,1996 was taken after minimum aeration purging used for intrinsic bioremediation demonstration monitoring. As expected,the samples collected by minimum aeration technique show BTEX concentrations higher than,but in the same order of magnitude as,the concentrations reported for the rapid purge sample. As reported in the fall quarter 1995,the BTEX results for MW-DIS-2(sample collected 10/18/95)do not fit the data trend.The results from this quarter provide further evidence that the 10/18/95 sample or laboratory analysis procedure was flawed. We recommend excluding this data point from further data consideration. A column was added to Table 2 for sample dilution per the request of PHS/EHD. SFoIC.%PROJECTSCELMONTE1109MOOl_DOC