FIELD DOCUMENTS_1985-1988 - Laserfiche WebLink

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� • a ' Harding Lawson Associates ' III SITE HYDROGEOLOGY 1 The wells at the Shell Oil site are screened in an unconfined aquifer zone composed primarily of silty clays and clayey silts with some silty sands and minor ' sand lenses. The uppermost unit, consisting predominantly of course-grained sediments, is unsaturated at nearly all locations. The water table occurs within the underlying fine-grained sediments at depths of about 12 to 14 feet below ground surface. The wells on the Shell Oil site are screened from a depth of 8 feet to the bottom of the hole, which is 28 feet for Wells 1 through 19, and 26 feet for ' Wells 20 through 22. The saturated portions of the screened intervals of the wells are predominantly clays and silts, with occasional clayey sand lenses. The aquifer zone likely extends beyond this depth since no change in geology is noted at the ' bottom of the borings; however, the extent of the aquifer zone is unknown. In the absence of aquifer testing data to provide estimations of aquifer ' parameters for the site such as transmissivity, hydraulic conductivity or storativity, estimates of hydraulic conductivity for the two aquifer materials encountered on site have been based on literature values. A general range of hydraulic conductivity for silty clays is I x 10-2 to I x 10-4 feet/day, and 1 x 10-1 to 1 x 10-2 feet/day for the clayey to silty sands observed on the site. ' Water levels have been measured on the site six times since November 8, 1985. Potentiometric surface maps generated from these data are shown on Plates 6-11. Corrections have been made to the water levels for wells in which free ' product was found to account for the density difference between the ground water and free product. The ratio between the density of free product and the density ' of fresh water was assumed to be 0.7, based on average densities for similar media. November 24, 1987 ' 4 of 9 B2203-R