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Report: Groundwater-quality Monitoring—September 11, 2001: 7500 West Eleventh Street, Tracy, CA. Page 3 clayey material. The depth to groundwater beneath the site varies seasonally between 7 and I1 ft. Regionally, the general direction of groundwater flow is to the north toward the Old River anastomosis 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 '1 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 i. 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 primary 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 plumes of diesel and gasoline (The San Joaquin Company 2001c). 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. 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 SEPTEMBER 11, 2001 GROUNDWATER SAMPLING AND ANALYSIS On September 11, 2001, the fourth 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 September 11, 2001 sampling program, the depth to groundwater in each of the seven monitoring wells was measured using a conductivity probe. The water table elevations were computed relative to mean sea Ievel (MSL). Those measurements and the computed groundwater table elevations are recorded in Table 1. .J sic