Laserfiche WebLink
'F F <br /> close enough to impact groundwater conditions at the ST&E site. <br /> The Site Conceptual Model suggests that the piping from the UST leaked and/or surface spillage <br /> of product occurred during fueling operations. The petroleum hydrocarbons moved downward <br /> through the soil to a depth of about 20 feet below ground surface where migration of the majority <br /> of the contaminant mass stopped. A small volume of the petroleum hydrocarbon compounds <br /> may have migrated.vertically downward to.the first water bearing zone. This would explain the <br /> minor concentrations of TPH-d in two of the groundwater grab samples. <br /> The ST&E site warrants a low-risk closure based on the following rationale: (1) The source has <br /> been removed (UST removal); (2) Remaining petroleum hydrocarbon mass is relatively small <br /> and located near the ground surface. The maximum identified current soil.petroleum <br /> hydrocarbon contamination is diesel at a concentration of i 10 mg/kg. No volatile organic <br /> compounds are present in the remaining petroleum hydrocarbon contaminant soil mass. The <br /> majority of the remaining petroleum hydrocarbon contaminant mass had a visqueen sheet placed <br /> in the subsurface to impede downward moving groundwater. The remaining petroleum <br /> hydrocarbon in soil will degrade in petroleum hydrocarbon concentration'with time and is <br /> currently greater than 40 feet above the local groundwater table; (3)Local groundwater has not <br /> been significantly impacted by the unauthorized release. Concentrations in the local groundwater <br /> of TPH-d exceed MCL's in only one sample which is assumed to be upgradient of a low- <br /> detection TPH-d concentration of 65 ug/l in Borehole STE-BH8. <br /> All significant data for this site has been loaded into the State of California Water Resources <br /> I <br /> Env ironmentaRSTETA RSCR09'102008.wpd 3 <br />