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} RAMAGE Post Office Box 18353 Reno, NV 8951 Tel(775)851-7239 Fax(775)851-7240 April 1, 2011 .0 Akwp 3 3H 2104t Is. Lori Duncan, . µII, , San Joaquin County EHD 00 East blain Street W r Stockton, CA 95202-3129 Re: o l --Low-Risk Case Closure Evaluation Stockton Metropolitan Airport Former Fuel Farm, 5000 S. Airport Way, Stockton EFID Site ode: 1845, Global Ili #T0607700159 Dear Ms. Duncan: On behalf of the Stockton Metropolitan Airport, Chevron Products Company, and A.G. Spanos Companies, Ramage Environmental, Inc. is pleased to submit to the San Joaquin County Environmental health Department JC EIII )this work plan to evaluate the subject case for low-risk regulatory closure. EIID requested this work plan on February 28, 201 L Background The site is an inactive aviation ffielifacility within the Stockton Metropolitan Airp t property in San Joaquin County, California. As shown on the attached site plan,2 underground fuel storage tanks(USTs) have been removed from the site, and a network of eight groundwater monitoring wells have been installed to monitor the plume ol`i paste 1, groundwater, lased on the cumulative groundwater analytical data collected to date, the plume of impacted roundwa e appears to be suable and adequately defined. The highest concentrations of petroleum hydrocarbons in groundwater are present in MWA, which is located on the east side of the fors er fueling system. As shown in the attached trend charts for Total Petroleum hydrocarbons "1PH)-as-gasoline and benzene in la mA, concentrations have generally increased since the well was installed and first sampled in November 2002. These increases in concentration coincided ith a rise of approximately 15 feet in the groundwater table elevation, between August 2004and Larch 2007.. Since 2007, the groundwater table elevation has stabilized between 30.00 and 41.fid feet below ground surface bgs , and the concentrations of`I'PH-as-gasoline and benzene now appear to be in decline, This suggests that by the egad.of 2007,the rate of natural attenuation exceeded the rate of continued leaching to groundwater. Based on the impacted groundwater plume stability,definition and concentration declines, Ramage Environmental recommended that the case be considered for low-risk regulatory closure.