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The first phase field investigation_consisted of the following tasks: (1) a utility clearance; (2) <br /> k a soil gas survey; (3) a soil sampling program; (4) a laboratory analytical program; and (5) a <br /> f site visit to view the entire length of the pipeline to determine any obstructions to assessing <br />{ �? and closing the entire line. Priorto performing any subsurface investigations, Underground <br />{ Service Alert (USA) was contacted'to have utilities located near the proposed work area by <br />'k member organizations such as PG&E, Pacifc.Bell, and the City of Stockton Utilities <br /> Department. In addition, California Utility Surveys, a private utility locator, was contracted <br /> to confirm the locations-of utilities in the area and to locate the diesel pipeline: <br /> 3.0 SOIL GAS SURVEY <br /> 3.1 Methodology <br /> } A total of twenty nine.active soil gas sample points� (Plate 2) were selected as follows: (1)' initial samples were collected from points located at approximately 100 foot intervals <br /> ' (locations SG70 to SG-17), and (2) secondary sampling points were located based on the <br /> r . detection. of petroleum.hydrocarbons from"the initial sample locations (SG-A to SG-K). The <br /> locations of suspected pipeline leaks at 125.and 455,Sixth Street were provided by UPRR <br /> (Plate 2). The dates of the pipeline Jeaks are unknown. The soil gas survey did not <br /> completely extend to Harrison Street, as originally intended, because the City of.Stockton <br /> was in the process of installing a water main line in -this area'. The P <br /> City's work precluded <br /> y <br /> ..our ability to evaluate the suspected pipeline leak at 455 Sixth Street. <br /> 3.2 Sampling Protocol <br /> Tracer Research Corporation (TRC) performed the soil gas survey under the supervision of <br /> the field engineer. TRC cut through.the asphalt and concrete road subgrade prior to <br /> .installing each sample probe. The target.probe depth was 5 to 6 feet below grade surface <br /> ' (bgs) to correlate with the estimated bottom of the backfill of.the pipeline trench. ,Each 314- <br /> ' diameter steelrobe i <br /> p , equipped with a'detachable drive tip, was hydraulically driven to a <br /> - <br /> depth of 6 feet b s The robe was-then then slowl retracted to disconnect the drive ti A <br /> � P g • P Y P•. <br /> vacuum pump was attached to:the probe and two to five probe volumes of air were pumped <br /> from the probe. The evacuation time in minutes versus the vacuum in inches of mercury <br /> (Hg) was used to calculate the necessary, evacuation time. The actual sample depths of each <br /> k <br /> probe are contained in Table L x Details regarding the soil gas sampling protocol and <br /> sampling 1 data such as sample volume, applied vacuum, and sample time are contained in the <br /> TRC report contained in.Appendix.A. <br /> 4 <br /> 3.3 Analytical Prom <br /> Each soil gas sample was analyzed onsite by a gas chromatograph (GC) employing a flame <br /> ionization detector and a packed chromosorb column. The soil gas samples were analyzed <br /> by direct injection for benzene, toluene, ethyl benzene, and total xylenes'(BTEX), in addition <br /> to total volatile hydrocarbons quantified from carbon 4 to carbon 9 (TVH, C4-C9) and from <br /> carbon 10 and greater (TVH, C10-CX). Further details concerning the GC process, detection <br /> h <br /> "f.\calsToacroN.so 2 <br /> 1 <br />