ARCHIVED REPORTS XR0002727 - Laserfiche WebLink

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Blockbuster Vdeo/Yellow Submarine November 24, 1992 Monitoring Well Installation Page 5 7.0 Conclusions and Recommendations The amount and extent of contamination encountered in MW-4 was completely unforeseen. The contaminant plume in the re-excavation was pinching out rapidly at the bottom and side of the excavation and MW-1, just twenty feet to the west of MW- 4, had no contamination. ' Because the plume of contamination in the re-excavation was so small and was rapidly diminishing, it is not reasonable to assume that the MW-4 contamination originated from that source. In addition, the MW-4 contamination extends to a higher elevation than that last seen in the re-excavation. Therefore, it is probable that another source is responsible for the contamination found in MW-4. ' During the remedial excavations which were conducted on the site, pipes were encountered that extended out beneath the sidewalk on the north boundary of the ' property in the vicinity of MW-4. It was thought at that time that the pipe run may have indicated that an old underground storage tank was present out beneath Ilarding Avenue. The discovery of contamination increasing in intensity and extent toward Harding Avenue lends credence to the presence of a tank beneath the street. The water table is presently at a depth of approximately 53 feet below surface and the ' closest contamination encountered in MW-4 lies at approximately 36 feet below surface, a separation of 17 feet. The groundwater is presently not contaminated, and the threat of contamination at present is low because of the impermeable character of ' the upper clay unit. ' However, at the present time, the water table is depressed because of the six years of drought that California has experienced. If the rainfall levels return to normal, the standing water level (piezometric surface) will rise at the site. Although the historic elevation of groundwater in this area is somewhat in question, it is clear that thef ' standing water level will approach that of the zone of contamination in MW-4. Nevertheless, even in the event of a higher standing water level, it is believed that the ' risk of groundwater contamination is .low. This is because of the fact that the contamination is enclosed within a unit of plastic clay of low permeability and there does not appear to be any secondary permeability induced by root casts above a depth ' of 50 feet. The clay unit is, in effect, an aquatard and does not conduct groundwater flow. ' If a higher level of standing water is achieved in the future, it will be an-artesian system. Water will rise to the piezometric surface only when the clay unit is penetrated by a boring and water in the aquifer at a depth of 53 feet is allowed to rise up into the 1 � e