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Treadwell&RDIIO A LANGAN COMPANY Request for Low-Threat Case Closure 6June 2013 RWQC8 case#391149 and#391162 Page 6 420 and 423 N Madison Street Stockton,California Project:731589001 degradation rates, and source concentrations. The model also calculates area of the plume and incorporates concentration gradients to provide a reasonable estimate of contaminant mass. Benzene was used as the simulated compound for Carando because it is the most mobile of the TPH constituents and is the only compound exceeding WQOs historically. The model was run using the benzene concentration detected in the in groundwater sample from the 2004 UST excavation. The concentration input was 75 pg/L, rather than the detected value of 46 pg/L, a conservative estimate to account for potential mixing and dilution within the base of the excavation. The model was run for a period of nine years(2004 to 2013). The modeling assumptions, site specific parameters, and input and output files are presented in Attachment 6. The results using the first-order decay rate model estimates the benzene plume extends to approximately j�Qfeet owl dient(Figurg J.The output file for the first-order decay rate model is shown on page 4 of Attachment 6.The output file shows the plume length for the run time of nine years. It also shows the calculated mass of the benzene plume. This estimate is conservative because the first-order decay rate used in the model represents the source area at 423 N Madison,and the decay rate downgradient of the source area is expected to be greater. The data for well MW-13 at 423 N Madison (approximately 120 feet downgradient of the source area) shows isolated benzene detections of only up to 1.4 pg/L(AGE, 2003) over a period of seven years, and supports the idea that the benzene plume at Carando is likely less than 140 feet long. Using the"instantaneous reaction"degradation model, which incorporates degradation of the source area,the plume does not extend beyond the source area (page 3 of Attachment 6). Sensitive Receptor Survey A well search was conducted by requesting well records from the California Department of Water Resources for a radius of 2,000 feet around the sites. The water supply wells that were reportedly in the area of the sites are shown on Figure 10. No supply wells were found to be located within approximately 2,000 feet of the Sites. In addition to this well search, the RWQCB indicated on a 5-year Review Summary for 423 N Madison, that there are no water supply wells within a half-mile of the site (SW RCB, 2012b). CONCEPTUAL SITE MODEL Residual petroleum constituents remain in soil and groundwater at the Sites. The receptors that could potentially be exposed to residual hydrocarbons at these Sites include visitors, the public, commercial and construction workers, and nearby residents and fresh surface water ecological receptors. Potential exposure pathways are presented in the Conceptual Site Model shown in Figure 11 and discussed below. The exposure pathways for petroleum hydrocarbons in soil include inhalation, direct contact,and incidental ingestion. This pathway is not complete because the sites are paved or covered by building foundations and depth of residual TPH in soil is greater than 15 feet below ground surface, which is deeper than potential future construction trench work.