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8.i.2 Sidewalk Area In November 1993, three 25,000-gallon diesel USTs were removed from beneath the Lindsay Street sidewalk Analysis of verification soil samples collected November 10, 1993, revealed the presence of 2,900 ppm diesel in the sample collected at the west end of the middle tank approximately 15 feet below grade This area was over excavated between November 11 and November 18, 1993, to an approximate depth of 17 5 feet in the affected areas During this soil remediation effort, approximately 150 tons of diesel impacted soil was removed and transported from the site The over excavation efforts were halted due to safety concerns relating to undermining the support shoring at the edge of the pit 8.2 Groundwater Remediation No groundwater remediation has been performed or required, as the last nine quarterly groundwater sampling events have revealed no petroleum hydrocarbon impact to site groundwater 9.0 Groundwater Risk Analysis A groundwater risk analysis was completed utilizing an industry-standard computer groundwater model The purpose of the effort was to determine the potential health risk due to residual petroleum impacted soils at the site Parameters used in the model were derived from observed site analytical and engineering data and from published engineering tables The results of this modeling effort were then compared to existing California Environmental Protection Agency Maximum Contaminant Levels (CAL-EPA MCL) and Federal EPA Suggested No-Adverse Response Levels (SNARLS) to determine potential future health risks If no significant health risks are apparent, it is hoped that this information will be used to facilitate conditional regulatory site closure. 9.1 Model Description VLEACH is the computer model used to predict the potential impacts to groundwater from adsorbed petroleum hydrocarbons in the vadose zone at the Pac Bell Stockton site VLEACH is a finite difference model available from the US EPA It is widely used throughout the country VLEACH models one chemical at a time, so separate runs were made to address the fate and transport for each chemical present at the project site MZ/04-06-95/UST/95-0089 l 1