SITE INFORMATION AND CORRESPONDENCE_1998-2000

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SUN-01-2004 15:40 9362 8100 • 916 362 8100 P.04 CLEANUP AND ABATEMENT Oi.,JER NO. 5-00-707 PORM8R UNOCAL AND MONTGOMERY WARDS 2 5606 AND 5400 PACIFIC AVENUE, STOCKTON SAN JOAQUIN COLN'fY concentration of 680 µg/1. Groundwater samples collected from the Unocal monitoring wells on 15 June 2000 were found to contain up to: • 15=400 µgil of total petroleum hydrocarbons as gasoline(TPHg); • 4,950µyl of benzene; 204 4911 of toluene; • 301 µg(1 of ethyl benzene; and • 1,290µ9J1 of xylenes. 3. In 1986, Wards removed two 10,000-gallon gasoline USTs from their site. Seven monitoring wells (MW-1 through MW-7) screened from 42 to 68 feet were installed between 1993 aald 1996. Quarterly monitoring was conducted from September 1997 to December 1999, and Wards is now on a somi-annual sampling schedule in conjunction with Unocal In October 1999, Wards started Operating an Air Sparging and Soil Vapor Extraction System on their site; As of July 2000, Wards has reportedly removed 7,561 pounds of petroleum hydrocarbons from soil and groundwater, and results of groundwater sampling show a significant reduction in the concentration of dissolved constituents. MtBE was identified at this site at a maximum concentration of 820 µg/1 by EPA Method 8020 in September 1996- Samples collected in June 2000 showed EDB,MSB, and all other oxygenates as eon-detect by EPA Method 82603; however, all but ED)3 exceeded detection limits. Groundwater samples collected from the Wards monitoring wells on 15 June 2000 were found to contain up to: • 3,420 µg/l of TPH& • 216 9911 of benzene; • 47 pall oftoluene; • <10 994 of ethyl benzene; • 848 41 of xylenes; and • 7.8 µfill of 1,2-ACA. 4. The San Joaquin Delta College(Delta) operates the Robinhood Irrigation Well(RIW), within 350 feet Of the Unocal and Wards sites. The RTW is used.from May to October R supply irrigation water for the 160 acres of landscaping at the College, and operates about 13 hours out of every 24-hour period at an average pump rate of 550 gallons per minute. The well is reported to be 325 feet deep,with perforations across multiple water bearing zones from 40 to 322 feat. The pump intake is reported at 140 feet. Delta employees have complained of gas odor from the water when they irrigate, thereby the pollution creates a condition of nuisance, 5- Unocal installed monitoring well pair MW-22S and—22D approximately 70 feet upgradient of R.IW in May 1999. MW-22S is screened from 30 to 50 feet, and MW-22D is screened from 61 to 66 feel. MW-22D has shown high concentrations of TPH- and benzene in all, four of the past sampling events, and the June 2000 groundwater sample contained TPHg at 4,190 µg11, and benzene at 1,250 µg/1. MW-22S was non-detect for all constituents in June 2000; however,