SITE INFORMATION AND CORRESPONDENCE

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(CWS), the local water purveyors, to ascertain the location and pertinent well construction information of nearby wells; and a 500 -foot field reconnaissance to locate and/or confirm the presence of wells or other sensitive receptors. Soil Borings URS will drill four (4) soil borings using a hollow -stem auger drilling rig to a depth of approximately 45 feet below ground surface (bgs) to further characterize the lateral extent of petroleum -impacted soil and groundwater, which was reported during the May 2007 activities. See the attached figure for proposed boring locations. Soil samples will be collected at 5 -foot intervals beginning at 5 feet bgs, and will continue until groundwater is encountered. Groundwater is expected at approximately 40 feet bgs. Soil samples will also be collected from areas where hydrocarbon odors, soil discoloration, or elevated photoionization detector (PID) readings are encountered. Once groundwater is encountered, the boring will be extended an additional approximately 5 feet and a grab sample will be collected from the top of the water column. Groundwater samples will be collected using one of three possible methods, depending on field conditions: through the open augers, by installing a temporary casing/screen, or by using HydroPunch or similar sampling technology. A minimum of two soil samples and one groundwater grab sample from each of the four borings will be submitted to the laboratory for TPHd, TPHg, VOCs, and Title 22 metals analysis. Soil samples reporting the highest petroleum impact, based on odors, visual observations, or PID readings, will be selected for analysis. Soil Gas Sampling To assess whether soil vapors impacted with petroleum hydrocarbons are creating a potential risk to CA ARNG staff working in nearby buildings, URS will drill four (4) borings and collect shallow soil gas samples. URS has prepared this addendum to comply with the DTSC/Los Angeles RWQCB "Advisory — Active Soil Gas Investigation" and DTSC's "Guidance for the Evaluation and Mitigation of Subsurface Vapor Intrusion to Indoor Air" (2004). Samples will be collected at 8 feet bgs from locations adjacent to each of four buildings (see the attached figure for proposed soil gas sampling locations). Soil gas sampling will be conducted using a direct -push drilling rig. Sampling tools and rods between 1.0" and 2.0" outer diameter will be advanced into the ground by the application of hydraulic pressure, using the rig's a hydraulic system. Using this method, no soil cuttings (investigation -derived waste) will be generated. A disposable point is pushed to the desired sampling depth, and the rod is then retracted, leaving a cavity between the point and the rod. Hydrated bentonite will be used to seal around the drive rod to prevent ambient air intrusion. Soil gas sampling will not be conducted for at least 20 minutes following probe installation to allow for soil vapor equilibration. A vacuum pump is used to purge the sample tubing prior to sample collection. The soil gas sample will be collected through 1/4 -inch Teflon tubing connected to the sample point push rod adapter and Swagelok sample train, and transferred into a 1 -liter stainless steel canister. A minimum of three (3) purge volumes will be extracted prior to sampling. Purging and sampling rates will be between 100 to 200 milliliters per minute (mt/min) to limit stripping, prevent ambient air from diluting the soil gas samples, and to reduce the variability of purging rates. Leak tests will be conducted at each sampling location.