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2 2 Extent of Contamination <br /> More than 70 soil samples have been analyzed during the investigation of the extent of <br /> contamination at the site Based on the laboratory results, Clearwater concluded that soil <br /> contamination is limited to the area between the former UST cavity and the former dispenser <br /> island, and ranges from 10 to 30 feet in depth This conclusion is generally in accord with that <br /> reached previously by Geological Audit, although nunor soil contamination was detected by <br /> Geological Audit in monitoring well MW-1, west of the dispenser island <br /> This area of residual soil contamination apparently serves as a continuing source of <br /> groundwater contamination, as gasoline and BTEX compounds have been detected on several <br /> occasions in MW-1, MW-2, MW-3, and MW-5 High concentrations have been consistently <br /> detected in MW-2, which evidently hes within the core of the contaminant plume (Figure 5) <br /> Due to generally northward groundwater flow, the plume is elongated from southeast to <br /> northwest and extends well beyond the limits of soil contamination <br /> I <br /> 3.0 SITE REMEDIATION <br /> 3.1 DriWng of Injection Borings <br /> With approval from PHS/EHD, a direct-push Geoprobe drilling ng was used to drill seven 1- <br /> inch-diameter borings into the core of the groundwater contaminant plume on Apt it 24, 1998 <br /> (Figure 6) In order to vary the depth of injection of the remedial fluid, the borings were drilled <br /> to different depths(Figure 7) No samples were collected and the borings were not logged The <br /> first boring (A2) was drilled to a depth of 20 feet, and then the drill rods were retracted to a <br /> depth of 10 feet, leaving the interval from 10 to 20 feet open for injection of the remedial fluid <br /> However, this depth interval proved to be exceedingly resistive to fluid injection, and would <br /> not accept any fluid, even at the maximum pressure attainable Therefore, the boring was <br /> deepened to 30 feet and injection was repeated from 20 to 30 feet Injection at this depth was <br /> successful, and all but one of the succeeding borings were drilled to either 30 or 35 feet (Figure <br /> 7) <br /> 3.2 ORC Infection <br /> After each boring was drilled, a slurry of Oxygen Releasing Compound (ORC) was injected <br /> using a portable grout pump capable of injecting at a pressure of 1,000 pounds per square inch <br /> (psi) A pressure gauge was connected to the pump so that the injection pressure could be <br /> monitored, providing a quantitative indication of the ease or difficulty of forcing the ORC fluid <br /> into the soil The injection pressures are shown in Figure 7 <br /> The slurry was obtained by mixing powdered ORC compound and water at a ratio of 2 parts <br /> slurry to 3 parts water Slightly different volumes of slurry were injected into each boring, <br /> depending on the injection pressure required (Figure 7) A total of 500 pounds of ORC, <br /> representing approximately 270 gallons of slurry, were injected <br /> 3 <br />