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ftadwelEftllo Jeffery Wong San Joaquin County Environmental Division 22 December 2003 Page 2 A 10,000 gallon capacity UST used for gasoline was removed on 3 September 1986 and found to have leaked. Soil samples collected beneath the UST in 1986 contained total petroleum hydrocarbons as gasoline(TPH-g) at up to 3,600 mg/kg. The excavation was deepened in October 1986, and January and July 1987. A monitoring well was installed near the former UST location in 1986, and seven more between 1992 and 2001. Nine groundwater monitoring wells are being sampled as part of an ongoing groundwater monitoring program conducted under the oversight of the SJCEHD. Three soil vapor extraction wells were previously installed as part of a vapor extraction pilot study. Additionally, in June 2002 a remediation system consisting of ten ozone sparging wells began operation at the site. SITE GROUNDWATER CONDITIONS Monitoring well MW-1 was installed in October 1986 near the former UST location and contained free product from 1992 to 1994. In 1995-1996,when free product was no longer present, TPH-g concentrations ranged from 6,100 to 1,100 micrograms per liter(ug/1). Over the next 3 years, MW-1 experienced an approximate one magnitude drop in TPH-g concentrations concurrent with a regional 10-foot rise in groundwater elevations (from around 25-feet below ground surface (bgs) in 1995 to approximately 15-feet bgs in 1998). TPH-g concentrations in MW-1 dropped to below detection limits in 2002. Benzene has only been detected in MW-1 three rimes since 1992 at concentrations ranging from 0.59 ug/1 to 180 ug/1, and was last detected in 1997. Monitoring wells MW-4 and MW-5 are currently the only wells with significant TPH-g and benzene concentrations. Monitoring well MW-4 has had fairly consistent TPH-g concentrations (averaging approximately 16,500 ug/1) since monitoring began in 1992, and has not shown a decrease since the remediation system began operation. Benzene concentrations in MW-4 have generally been greater than 1,000 ug/l but have dropped to slightly less than 1,000 ug/l. Monitoring well MW-5 had TPH-g concentrations below detection limits when installed in 1994. TPHg concentrations gradually rose from 190 ug/l in 1996 to generally greater than 10,000 ug/l (concurrent with the groundwater rise). TPHg concentrations have decreased to several hundred ug/1 since the remediation system began operation. Benzene concentrations have had a rise and fall in concentrations similar to TPH-g, generally occurring at one fifth to one tenth of the TPHg concentration.