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Mr. Gerald Djuth May 22, 2006 Page 2 Investigation Layout and Sampling Depths The proposed investigation consists of an arc-shaped array of 5 primary borings (P-101 through P-105) and 4 optional borings (P-106 through P-109). As shown on Figure 1, the primary borings are approximately 400 to 550 feet southeast of MW-5R. It is proposed to drill P-102 and P-103 first and conduct CPT stratigraphy logs at all 5 primary boring locations. The purpose of the CPT profiling logs is to establish the degree of continuity of soil layers in the offsite area and attempt to correlate with results of previous CPT profiles conducted in 1998 and on-site stratigraphy. Where possible, CPT stratigraphy logging will be advanced to a total depth of 100 to 110 feet to verify continuity of the lower-permeability intermediate groundwater zone. Groundwater sampling will be conducted as close as possible to the stratigraphy boring locations. At each of the 5 borings in the primary arc, samples will be collected at three depths: • in the first permeable groundwater zone which is typically 48 to 60 feet bgs, • in the lower permeable (intermediate) zone at about 80 to 95 ft bgs, and • at an intermediate depth between the above two zones, to be selected in the field based on the CPT log results. Stepout Groundwater Sampling The initial groundwater samples will be collected at three depths at P-102 and P-103 in the center of the array. P-102 and P-103 were selected as pivotal because they are located in the direction of average downgradient groundwater flow from the vicinity of MW-5R and MW-15. All samples from the inner arc of 5 borings will be rushed to an analytical laboratory in Stockton for overnight analysis and, depending on the results, additional stepout borings will be drilled and sampled. If site contaminants are detected at concentrations above the project water quality objectives (WQOs), stepout locations approximately 200 feet farther downgradient of the detection location will be selected for sampling (P-106, P-107, P-108, or P-109). The selected stepout borings will also be sampled at three depths. The sampling depths will match or span the depths of the contaminant detections in the primary borings. Analytical Methods Groundwater samples for fast turnaround analysis will be analyzed by Method SW8260 with a modified (shorter) list of analytes equivalent to Method SW8021B by a local laboratory certified by the California Environmental Laboratory Accreditation Program (ELAP) for the analytical methods used. The target reporting limits will be 0.5 µg/L and preliminary results will be available for decision-making the next morning. Depending on the results of fast turnaround laboratory analyses, the remaining samples may be analyzed on a standard turnaround basis of approximately two weeks.