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amec Mr. C. Dean Hubbard California Regional Water Quality Control Board February 1, 2012 Page 3 future exposure in this area. U.S. EPA Supplemental risk assessment guidance stipulates that the exposure point concentration estimates should be based on the 95% upper confidence limit (95% UCL) of the arithmetic mean to estimate a Reasonable Maximum Exposure (RME) scenario. In a future SLRA, 95% upper confidence limits on the mean (UCLs) will be used to characterize cumulative health risks posed by TPHc and will likely demonstrate that concentrations over the footprint of a typical residential lot are below acceptable risk levels. • Naphthalene in one sample each, from two locations in the southwest corner of the site where PAH-affected soil was removed. At two locations, HA-11 C (sample at 2.5 feet bgs) and CS-01 (sample at 7 feet bgs), naphthalene concentrations slightly exceed residential risk-based screening levels for the vapor intrusion pathway. A significant number of samples have been collected from this area and no further delineation of the soil near these detections is being proposed. In the Water Board's September 21, 2011 letter regarding the Report and updated SLRA for the site, a work plan was requested to address the screening level exceedances summarized above. This work plan proposes further investigation in two areas of the site. WORK PLAN FOR ADDITIONAL INVESTIGATION AMEC will advance borings at selected locations (Figure 3) to collect data to further investigate chemical concentrations in soil near former borings GMX-13 and GMX-02. Prior to drilling, Underground Service Alert (USA) will be notified, the soil boring locations will be cleared by a private underground utility locator, and appropriate permits will be obtained from the San Joaquin County Environmental Health Department. One boring will be advanced at the former locations of GMX-13 and GMX-02 and three borings will be advanced surrounding both boring locations for the collection of soil samples (Figure 3). All borings will be advanced to 8 feet bgs via a hand auger. The borings around GMX-02 will be advanced below 8 feet bgs using a hydraulically driven core barrel and drive sampler to a total depth of approximately 10 feet bgs. The borings around GMX-13 will be advanced below 8 feet bgs using a hydraulically driven core barrel and drive sampler to a total depth of approximately 12 feet bgs. The soil from each boring (from the hand auger bucket or a continuous core from the core sample barrel) will be collected and examined from each location and the lithology will be logged by an AMEC field geologist under the supervision of a California Professional Geologist. In addition, organic vapor meter (OVM) readings will be taken and recorded at various depths. Soil samples will be collected from the borings surrounding and at the former GMX-02 location at 1.5, 4, 6.5, and 9 feet bgs, to provide data on shallow soil TPHc concentrations in this area to 4 U.S. EPA, 2002, Calculating Upper Confidence Limits for Exposure Point Concentrations at Hazardous Waste Sites, December.