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TreadweIEMlo A LANGAN COMPANY Request for Low-Threat Case Closure 6 June 2013 RWQCB case#391149 and#391162 Page 8 420 and 423 N Madison Street Stockton,California Project:731589001 Remaining soil volumes were estimated by defining distinct areas of soil based on concentration differences, as shown in Figures 6 and 8.The vertical extent of remaining TPH was estimated for each distinct soil area based on averaging multiple depth sampling data, where available, or an estimation of the likely vertical extent based on sample concentration and soil types(i.e. higher concentrations or higher permeability soil means greater vertical extent of remaining TPH). Benzene mass was not calculated because it was not detected in soil at either site, and toluene, ethylbenzene, and xylene mass was not calculated because detections were orders of magnitude lower than TPH concentrations. Remaining TEX mass is adequately represented by TPH concentrations. The bulk density of soil was assumed to be 1.7 kg/L or 106 Ibs/ft3(Das, 1998). This represents an approximate average value for a mixture of sandy silt, silty sand, and clay, which represents the site Iithology as indicated in cross sections(Figures 4 and 5). 420 N Madison Street Petroleum hydrocarbons were detected in soil samples from five locations or depths that were not removed during remedial excavations at Carando(CSB-2, CSB-3, CSB-5, CPT-EXC-BT, and CPT-EXC-25; Attachment 1 and Figure 6). Four separate areas with TPH in soil were included for estimating volumes of soil and hydrocarbon mass. The total mass of TPH remaining was estimated to be approximately 725 lbs (Table 1). This estimated mass was almost entirely from the soil volume at CSB-2 where the highest concentrations were observed. The hydrocarbon mass remaining currently is likely less than this estimate due to biodegradation over the last nine years since these samples were collected. 423 N Madison Street Petroleum hydrocarbons were detected in 30 confirmation samples at 423 N Madison after the April 2004 remedial excavation (T&R, 2005b). Only areas associated with UST B-6 and with TPH concentrations greater than 10 mg/kg were used for calculations. Seven separate areas with TPH in soil were included for estimating volumes of soil and mass of TPH (Figure 8). Samples representing TPH remaining in soil, and areas included in the TPH mass estimation, are shown on Figure 8. The total remaining mass of TPH in soil at 423 N Madison was estimated to be approximately 192 lbs (Table 2). The largest contributing areas to this mass are those near the center of the excavation around samples BOT-17 and ESW-15. Because this estimate is based on sample results from 2004,the actual hydrocarbon mass currently remaining at the Site is likely less than this amount due to biodegradation. MASS REMAINING IN GROUNDWATER The TPH mass remaining in groundwater was calculated based on an estimated plume size and average concentrations at each site according to the following formula: Hydrocarbon Mass (lbs) = Plume Area(ft') x Saturated Depth (ft) x Porosity X Average Concentration(m'9/L) x (28.32L/ft3)(16/453,592mg)