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Treadwell&Rollo A LANSAN COMPANY Request for Low-Threat Case Closure 6 June 2013 RWQCB case#391149 and#391162 Page 9 420 and 423 N Madison Street Stockton,California Project:731589001 420 N Madison Street The BioScreen model results(discussed above) provided estimates of the plume mass in groundwater (page 3 of Attachment 6). The estimated benzene mass with first-order degredation is 0.0103 kg. A conversion factor of 1.8% (typical fraction of benzene in TPHg) benzene in TPHg was used to extrapolate the mass of TPH in the plume based on the estimated mass of benzene (Heath et al., 1993). Estimating TPH mass based on a typical fraction of benzene results in a conservative value of TPH mass because the modeled benzene would extend farther than TPH in groundwater. The total TPH mass remaining in groundwater was estimated to be 1.3 lbs. This estimate is based on the benzene concentration in the excavation in June 2004,which is nine years ago and prior to the addition of ORCO to the groundwater at the site in April 2005; therefore,the TPH remaining in the groundwater is very likely less than this estimate. 423 N Madison Street The extent of TPH in groundwater at the 423 N Madison site was estimated based on groundwater monitoring and sampling results from 1999 through 2003 (Figure 9). The plume area was calculated to be approximately 2,670 ft2, the saturated thickness was assumed to be 20 feet, and effective porosity 0.25. The average total TPH concentration was taken as the sum of the detected TPHd and TPHmo concentrations at boring 15-B-8 in May 2000, 670 pg/L. The resulting total TPH mass was estimated to be 0.45 lbs. Because it has been 13 years since the sample that contained TPH was collected,the amount currently in groundwater at the Site is likely less than this estimation due to biodegradaton. CLOSURE REQUIREMENTS UNDER LOW-THREAT UST CASE CLOSURE POLICY On 6 November 2012 the California State Water Resources Control Board implemented the Low-Threat Underground Storage Tank Case Closure Policy(LTCP). The LTCP guidelines indicate that closure is appropriate for a site if the following can be demonstrated: • The unauthorized release is within the service area of a public water system (i.e.; untreated groundwater is not a municipal resource or the community relies on surface water imports); • The unauthorized release consists only of petroleum chemicals(including oxygenates); • The unauthorized release has been stopped; • Free product has been removed to the maximum extent practicable; • A conceptual site model has been developed; • Secondary source has been removed to the extent practicable; • Soil and groundwater have been tested for Methyl tert-butyl ether(MTBE), and results have been reported in accordance with Health and Safety Code Section 25296.15