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Blockbuster Video/Yellow Submarine November 24, 1992 Monitoring Welllnstalladon Page 4 I 1 � 10 The soil from each sampled interval was logged by a California State registered geologist until the total depth of the well at 70 feet. Reference samples were taken from interval and stored In a compartmented chip case. A PID was on site to screen the samples. However, it malfunctioned and gave erratically high readings, even in ambient air and refused to zero. Consequently, only olfactory observations were ' recorded. ' 5.0 Geology/Hydrology of Site The subsurface geology of the Yellow Submarine site is characterized by an upper unit composed of plastic silty clay which includes local restricted lenses of clayey silt. This unit extends from the surface down to a depth of approximately 53 feet. MW-4 also encountered a bed of dense hardpan within the clay unit at a depth of 45 feet. Below this clay is a bed of silty clayey sand which was penetrated by the other three t wells on the property and appears to be approximately 15 feet thick. This sand was not penetrated by MW-4, although only the auger cuttings were available for logging below the water table and this contact could easily have been missed. Groundwater gradient (determined by MW-1, MW-2 and MW-3� during the 9/91 and . 3/92 monitoring events, was to the northeast at 0.006 to 0.007 feet/foot. However, ' during the 9/92 monitoring event, the gradient hadzaytrsed and was to the southwest at a gradient of 0.009 feet/foot. 6.0 Distribution of Contamination ' The remedial excavation determined that the contamination was in the shape of a thin, elongated lens which plunged to the north. A small patch of contamination, approximately two feet in diameter was left on the northern wall and a portion of the bottom of the excavation. The diameter of the plume was rapidly decreasing toward the bottom of the excavation and.it was thought that the plume would pinch out within a few feet beyond the bottom. However, the contamination which was encountered in MW-4 shows that the level and extent of the con tantially iD&Lease 25 feet north of the bottom of the excavation. The contamination encountered in the boring is ' concentrated within and adjacent to two lenses of silty clay in the upper clay unit. These silt lenses have a slightly greater permeability that the enclosing clay and would be logical pathways for any contamination. Contamination in .the upper silt lens ' extends to a higher elevation than that of the last appearance of the pod of contamination in the re-excavation. 1 1 .