ARCHIVED REPORTS XR0006135 - Laserfiche WebLink

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The combined data from the Chevron and Kwikee sites have been used to generate a conceptual model of the site, which is presented in schematic form in section 2.0 of this report. In support of the model, section 2.0 also includes: a Three geological cross sections through the site ® Two maps of the water-bearing and contaminated sand body ® Hydraulic conductivity data from soil samples A map of groundwater elevations in the Stockton area m A plot of water depth over time at the site, and o Gaphs showing the time-variant concentrations of gasoline in groundwater samples from selected wells. After presenting the conceptual model, section 3.0 of the report examines the hydrogeology of the site through a series of site-specific groundwater elevation maps for the years 1995-2002. These maps depict the hydraulic gradient and groundwater flow direction during that time, and lay a basis for examining the distribution and migration of gasoline compounds in the groundwater in section 4.0. Implicit in this analysis is that the petroleum hydrocarbons migrate in the direction(s) of groundwater flow, although at a slower rate than the groundwater due to various retardation factors. Maps depicting the distribution of Total Petroleum Hydrocarbons (reported as gasoline by the laboratory) and the gasoline oxygenate methyl-tertiary-butyl ether (MTBE) are presented for the same dates as those depicted in the groundwater elevation maps. The discussion of these maps focuses on the extent to which the groundwater plume has evolved as predicted by the site conceptual model, given the changes in groundwater flow direction and gradient depicted by the maps in section 3.0. Section 5.0 summarizes the latest monitoring event, which was performed on November 14, 2002. The material in this section is normally reported in the regular quarter monitoring report, but because the monitoring results are presented in map form in sections 3.0 and 4.0, section 5.0 is limited to the monitoring procedures.. Any additional specific questions or observations raised by EHD in the September 2002 correspondence that were not covered in preceding sections of this report are answered in section 6.0. The last section (7.0) presents our conclusions and recommendations regarding the future of this case. 1.5 Site Location and Description The site is located in a residential/commercial area, and the local topography is flat. A convenience store occupies the rear (north) third of the site. The remainder of the front of the site is an asphalt- paved parking area. Two 10,000-gallon underground storage tanks, used to store unleaded gasoline, were located in the southwestern portion of this area, and were partially covered by the dispenser • 4