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. ARG4DISGERAGHTY&MILLER occurs downgradient of a release point, the chances of intercepting and sampling a released constituent are increased downgradient of the release area Groundwater samples will be collected from the saturated zone at a minimum of 10-foot intervals using a HydropunchTm or similar sampling device The first water sample will be collected at the water table based on the water elevation measured in ' the nearest groundwater monitoring well Groundwater samples will be collected to a minimum depth of 70 feet bgs unless a competent confining unit, greater than 3 feet thick, is encountered at which point drilling and sampling will cease Water samples will be analyzed by a State-certified on-site mobile laboratory for TPHg, benzene, toluene, ethylbenzene, and xylenes (BTEX), and methyl tertiary butyl ether (MTBE) The vertical extent of dissolved-phase hydrocarbon impact will be determined to be adequately defined when two groundwater samples in succession, collected below the last sample interval reporting dissolved-phase hydrocarbons, contain no detectable concentrations of TPHg,BTEX or MTBE ' Following the collection of the final Hydropunchrm sample, two 1-inch diameter piezometers will be installed m the borehole One piezometer will be screened in the upper impacted zone and the second piezometer will be screened in the lower, nonimpacted zone as identified by the results of the Hydropunch— sampling The exact depth of the screen intervals will be determined by the field geologist based on site specific lithology and constituent distribution information Water level data ' provided by the piezometers will assist in determining whether a vertical groundwater gradient is present In the absence of a vertical gradient, groundwater will only flow laterally Thus, in the absence of a vertical gradient, there is no substantial reason to ' believe that groundwater would be impacted beneath the depth at which nondetect concentrations are observed A schematic piezometer construction diagram is provided as Figure 2 A summary of field methodologies is provided in Attachment A ' Preparation of a Corrective Action Plan(CAP) ' Upon completion of the focused subsurface investigation, UNOCAL will prepare and submit a CAP The CAP will propose a course of action designed to result in cleanup of the impactsand that has been determined to be the most cost- effective The CAP will be designed such that its implementation will restore or ' protect beneficial uses of the aquifer The CAP submitted to SJCPHS will include ' • An assessment of the vertical and lateral extent of impacts based on the previous site investigations and the focused subsurface investigation proposed herein ' * A feasibility study