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�••. Report: Groundwater-quality Monitoring—January 26,2004: 7500 West Eleventh Street, Tracy,CA. Page 3 facility(Dietz Irrigation. 1999x). An initial phase of site characterization work was completed in May 2000. It included the installation of seven groundwater-quality monitoring wells (Nos. MW-1 through MW-7) at the locations shown on Figure 2. An extended phase of site characterization was initiated on March 25, 2002; it included the installation of an additional eight groundwater-quality monitoring wells, numbered MW-3A, MW-3B, MW-8 through MW-12, and MW-12A. It was completed on April 11, 2002, with a round of groundwater sampling and analysis that encompassed all 15 groundwater-quality monitoring wells by r, then extant on the site (The San Joaquin Company, 2002c). When groundwater was first recovered from groundwater-quality monitoring well MW-7 .r.. on May 11, 2000, the sample contained moderate concentrations of Total Petroleum Hydrocarbons quantified as diesel (TPHd) and Total Petroleum Hydrocarbons quantified as gasoline (TPHg), with commensurate concentrations of benzene, toluene, ethyl .. benzene and total xylene isomers (BTEX) and 22 µg/l, of methyl-tertiary butyl ether, a fuel oxygenate. Thereafter, the concentration of components of fuel hydrocarbons followed an increasing trend until the analytes of concern exceeded their maximum solubility in water (see Table 2). However,as early as the sampling round performed on November 28, 2000, no MTBE was detected in the samples recovered from Monitoring Well MW-7. It is believed that this does not indicate that, by that time, the groundwater 46. in the vicinity of the well was free of MTBE. Rather, that the high concentrations of other components of fuel hydrocarbons were such as to raise the method detection limit of the r MTBE analysis to a level greater than the concentration of that analyte actually present in that groundwater sample. i By April 11, 2002, it was clear that floating product was present in the surface of the groundwater in Monitoring Well MW-7; it continued to be present at various apparent thicknesses up to 0.58 ft. until the LNAPL-purging program in that well was initiated on November 8, 2003. The record of floating product thicknesses and the effects of purging that material from Monitoring Well MW-7 are presented in Table 3. 1.4 Geology and Hydrogeology The subject property and the surrounding area are situated on level terrain on the distal, northern slope of an alluvial fan. The shallow underlying alluvial sediments are of Quaternary to Recent age. y The site and the immediately adjacent property along the south side of West Eleventh Street and the west side of Chrisman Road have been extensively excavated and backfilled during prior filling station construction and remodeling, utility installation, and highway expansion. Beneath the paving and fill, the soils are composed of alluvial materials consisting of interbedded clays, silts and sands. These materials have been deposited in a complex lenticular form composed of relatively low permeability clays and silty clays inter- sic