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Report: Groundwater quality Moniloring—December 13,1001: 7500 West Eleventh Street, Tracy, C4. Page 3 into each other to forin semi-continuous permeable strata within the less permeable }...I clayey material. The depth to groundwater beneath the site varies seasonally between 7 and I ft. Regionally, the general direction of groundwater flow is to the north toward the Old River anastomosic branch of the San Joaquin River, the closest tributary of which, the Tom Paine Slough, is one and one-quarter miles north of the 7500 West Eleventh Street site. However, locally, the shallow groundwater gradient tends to follow the topography, which, at the subject property, slopes gently to the north-northeast. The local direction of groundwater flow is also affected by the local sedimentary geology, particularly where continuous or semi-continuous sand strata provide channels for subsurface flow through less permeable facies. 1.5 Distribution of Hydrocarbons in the Subsurface The site characterization program conducted during April and May 2000 included installation of seven groundwater-quality monitoring wells at the locations shown on Figure 2. Evaluation of the results of analyses of samples of groundwater recovered from those wells on May 11, 2000 and analyses of soil samples from the well borings and an array of push-probe borings, together with information regarding groundwater flow directions, sources of leakage of fuel hydrocarbons into the subsurface and other information gathered by the site characterization program yielded an interpretation that the site was affected by a primacy plume of diesel and gasoline that affected both soil and groundwater, a secondary plume of diesel and gasoline, and dispersed low concentrations of a variety of hydrocarbons in shallow soils having sources different from those of the primary and secondary phunes of diesel and gasoline (The San Joaquin Company 20014). Figure 2 is a plan view of the site showing the extent of the primary and secondary plumes of diesel and gasoline and the areas in which shallow soils are affected by other dispersed hydrocarbons (Note: The commercial buildings located at 7501 and 7474 West Eleventh Street, until recently occupied by the Fisco Farm and Home Store and C&B Equipment respectively, are now vacant.). Figures 3, 4 and 5 illustrate the distribution of petroleum hydrocarbons along sections A-A', B-B' and C-C', the locations of which are also shown on Figure 2. 2.0 DECEMBER 13, 2001 GROUNDWATER SAMPLING AND ANALYSIS On December 13, 2001, the fifth round of groundwater-quality monitoring was conducted using monitoring wells MW-1 through MW-7, which were installed at the site in April 2000. The well locations are shown on Figure 2. 2.1 Groundwater Elevations and Flow Direction ! To initiate the December 13, 2001 sampling program, the depth to groundwater in each of ' the seven monitoring wells was measured using a conductivity probe. The water table ,i i sic