SITE INFORMATION AND CORRESPONDENCE

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Received: 4/26/99 2. 19PM; 916 255 3015 -T CUAA/SJdA 6PU; Page 3 Sent-,B ent qRWOC9 SACRAMENTO; 916 255 3015; Apr L~ 99 14:29; Page 313 r " 1665 Pacific Ave., Stockton, San Joaquin County Response to Response to Regional Board Comments RegardingDraft Order for Cl Site. Closure of UNOCAL Gordon, here are my immediate thoughts on the State Board Response to our comments on the Draft Order for Closure of UNOCAL UST Site at 1665 Pacific Ave., Stockton, San Joaquin County. State Board staff say They have reviewed the report of recent work..andfind that it contains no new information that would alter dw conclusions and recommendations for closure... The following are new data from the January and February 1999 activities and in my opinion,shed question on the plume stability and vertical site defir#ition. i I. MTBE and benzene were detected in monitoring well MW-7 (located approximately 290 feet downgradient of the site), at respective concentrations of 8.5,0.85 p&/L. Prior to the 4 February 1999 sampling event, neither contaminant had ever been reported in this well. if the plume is stable, why are contaminants now being detected nearly 300 feet away, f benzene plumes are not supposed to go that far, right?) 2. Based on groundwater elevation data from MSV-2(screened from 20 to SO feet bgs), and NP-1, (screened from 85 to 90 feet bgs) there is a downward vertical gradient of 0.055 feet per foot. The two wells are separated by a distance of approximately 10 feet. (see page 21 of the 26 February 1999 Geraghty and Miller Report). The groundwater flow direction is consistently north OZ at a gradient of approximately 0.001 to 0.002 ft./ft. 3. MTBE was detected in the duplicate water sample collected from the NP-I boritig (at 60 feet bgs) at a concentration of 77 µg/L by EPA Method 8020. The 8260 confirmation analysis was run using EPA Method 8260 and resulted in ND (<130 pg/L). Sample NP-I (also collected at 60 feet bgs was ND with a detection limit of 100'µg/L. 4. State Board staff refer to the"hits"of MTBE in several on and off-site monitoring wells as background conditions. According to Margaret Lagorio of San Joaquin County. there are no known fuel stations upgradient of the former UNOCAL. Furthermore, if these are in fact background concentrations, all wells would contain low concentrations of MTBE, not just a few. MTBE concentrations are consistent with a pre-1992 release (this release is reported to be 1989 or before). 5. Between August 1990 and early 1993, groundwater surface elevation dropped below 50 feet bgs (leaving MW-t, 2, and 3 dry). During this time period, Cal Water also pumped from many of their production wells in the area. Between 1993 and present groundwater surface elevation has risen and is presently about 24 feet bgs. This significant change in water surface and the corresponding lack of pumping from the nearby Cal Water wells suggests the upper and lower aquifer zones are in communication. 'Che rise in the groundwater table is primarily due increased precipitation and lack of pumping in the area. The lateral extent(below 59 feet) and the vertical extent of hydrocarbons and fuel additive impact is still not defined. At a minimum, one downgradient deep monitoring well is necessary to determine the magnitude and extent of hydrocarbon impact. I do not agree that site characterization activities are complete at this site. y