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Treadwell&RDII0 A LANGAN COMPANY Request for Low-Threat Case Closure 6 June 2013 RWQCB case#391149 and#391162 Page 5 420 and 423 N Madison Street Stockton,California Project.•731589001 Groundwater at only one location, boring 15-13-8, contained detectable concentrations of petroleum hydrocarbons in the most recent sampling events. TPHd and TPHmo were detected approximately 45 feet down-gradient of UST B-6 at 190 pg/L and 480 pg/L, respectively in sample 15B-8-GW collected on 5 May 2000. TPHg, MTBE and BTEX were not detected in this sample. The groundwater plume extent is delineated by down-gradient boring 1513-5 and well MW-11 (Figure 9). MTBE and BTEX have not been detected in groundwater wells or borings near UST B-6. Potential For Biodegradation Potential for biodegradation of petroleum hydrocarbons in groundwater at the 423 N Madison site was evaluated through microbiological tests on groundwater samples collected at wells MW-21A and MW-21B in February 2000 (Attachment 3). The analysis indicated that biodegradation is likely to occur in MW- 216,the B groundwater zone. Natural attenuation parameters, including dissolved oxygen (1.48 to 2.47 milligrams per liter [mg/L]), indicate that biodegradation is likely occurring in the A and B groundwater zones(Attachment 4). The microbiological test results and natural attenuation parameters indicate that TPH are likely biodegrading and limited in extent laterally and vertically as a result. Regression Analysis for Benzene The time series data available for 423 N Madison from monitoring well MW-21A were used to calculate benzene degradation rates. These degradation rates were extrapolated and used to estimate the time for benzene to reach the water quality objective (WQO) of 1 pg/L at the Carando source area. The regression analysis uses the benzene concentration at Carando(46 pg/L) and applies the slope of the regression line for benzene at MW-21A at 423 N Madison. This approach was used because the Sites likely have similar geochemical conditions because of their close proximity to each other, similar subsurface materials and similar concentration ranges of hydrocarbons in groundwater. The regression analysis indicates that concentrations at 423 N Madison are decreasing and that source area concentrations at Carando will likely decrease to the WQO in two to 28 years from the date of 2004 sample collection, depending on the statistical method used. The first order decay rate is the most conservative approach, resulting in an estimate of 28 years for benzene to reach the WQO in the source area. This regression analysis is presented in Attachment 5. Estimation of Plume Extent 423 N Madison Street The extent of the TPH plume at 423 Madison is defined by down-gradient borings or monitoring wells, as discussed previously(Figure 9). 420 N Madison Street The down-gradient extent of the benzene plume at Carando was estimated by running BioScreen, a two- dimensional analytical model used to simulate natural attenuation of dissolved hydrocarbons at petroleum fuel release sites (EPA, 1997). The model provides estimates of plume extent and time the plume is likely to persist, and allows the user to input site specific hydrogeologic and lithologic parameters,