SITE INFORMATION AND CORRESPONDENCE

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East Bay MUD - 2 - 10 March 1995 Memo #1 As reported in the 17 November 1989 report, ground water samples collected from the three monitoring wells in June and October 1989 showed no contamination in MW2 and MW3. However, up to 882 ppb benzene, 4300 ppb gasoline, and TEX were detected in MWl. A ground water production well was noted 500 yards to the southwest. Groundwater samples collected from the 3 wells in March and June 1990 continued to show no contamination in MW3. However, MW2 contained 60 ppb toluene and MW1 contained 1300 ppb benzene and 4000 ppb gasoline. Ground water samples from September 1990 and February 1991 showed no detectable contamination in MW2 and MW3, however, MWl contained up to 1200 ppb benzene and 5000 ppb gasoline. Depth to water was about 10 feet but the gradient was not provided. In a Problem Assessment Report; Soil and Groundwater Investigation, dated 3 May 1991, Weston consultants reported that they submitted a work plan, in February 1991 (not in our file), for additional soil and ground water investigation. In February 1991, apparently as proposed in the work plan, Weston installed MW4, MWS, MW6, and MW7. Silty clay was present from the surface to approximately 11 feet with a sand layer to approximately 20 feet bgs. Toluene and xylene were detected at low levels from soil samples collected at 10 feet bgs in all 4 wells. Ground wager contamination .,as not detected in MW2 through MW7. MW1 contained 1600 ppb benzene and 16,000 ppb gasoline. Depth to water was about 12 feet and gradient was southwest. Ground water samples collected in November and December 1991 showed up to 180 ppb benzene and 1200 ppb gasoline in MW1 and 0.6 ppb xylene in MW4. Ground water contamination was not detected in MW2, MW3, MW5, MW6, and MW7. Depth to ground water was about 13 feet. Gradient was not provided. Regional gradient is to the northwest. The direction of ground water flow over most of the site has most often been to the southwest. However, the gradient has been to the northeast and east, and on occasion an apparent depression has been measured near MW4, resulting in different gradients t1joughout the site on the same date. A survey of production wells in the vicinity has not been conducted and no explanation for the difference between regional gradient and local gradient or variations in local gradient has been provided. In July 199:2, Weston conducted a study of tidal influence at the site and determined that the tidal influence was minimal. Over a twelve hour period, the tide varied 4 feet but the maximum ground water flux was 0.48 inches in MW4 and 0.25 inches in MW5 and MW7. Depth to ground water was about 12 feet and gradient was generally southwest. Ground water samples continued to show contamination in MW1 and none in the other wells. Ground water results from October 1992 showed similar results but depth to ground water had dropped to 14 feet. By April 1993, ground water had risen to 7 feet bgs and the gradient was generally easterly. Ground water results were similar to previous results. In August and September 1993, Black and Veatch consultants (BV) installed MW8 and MW9 and collected "Hydropunch" samples at 7 locations. MW8 had low levels of BTEX at 16 feet while soil samples from MW9 had no detectable contamination. Soil information was not collected from the "Hydropunc:h" samples. Depth to water was about 12 feet and gradient was southwest. Ground water contamination was detected in MW1 at 1200 ppb gasoline and 48 ppb benzene, and in