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C♦ ER1205503 R02 Tosco 76 ev • Based upon ERI's evaluation of 1) The continuous core samples collected from borings 138 through B10 (Attachment F), 2) a review of the boring logs of wells MW1 through MW7S & D (Attachment H), and, 3) a review of the quarterly groundwater monitoring data with respect to the groundwater elevations in single-casing wells MW 1 through 3,and nested wells MW4S & D through MW7S & D, it appears to consist of a single unconfined aquifer of variable permeability Soil CQn;dit;ons Results of laboratory analyses of soil samples are summarized in Table 1 A total of five soil samples were selected for laboratory analysis based on field observations These samples were collected from depths ranging from approximately 5 to 25 5 feet bgs Laboratory analyses of these soil samples did not detect dissolved hydrocarbons or oxygenated compounds at or above laboratory method detection limits Copies of laboratory reports are included in Attachment G Groundwater Conditions Results of laboratory analyses of the groundwater samples are summarized in Table 2 Dissolved hydrocarbons and oxygenated compounds were not detected at or above laboratory method detection limits in groundwater samples collected from borings B8 through B10 except for the following constituents MTBE results reported below were analyzed using Environmental Protection Agency [EPA] method 8260 Laboratory analyses of groundwater samples collected from boring B8 detected total xylenes(0 57 parts . per billion [ppb]) and MTBE (4 4 ppb) at 7 feet bgs, MTBE (2 6 ppb) at 17 feet bgs, and toluene (0 56 ppb), total xylenes(1 6 ppb), and MTBE (1 2 ppb) at 31 feet bgs Laboratory analyses of groundwater samples collected from boring B9 detected toluene(0 52 ppb)at 7 feet bgs, and methanol (91 ppb) and ethanol (21 ppb) at 30 feet bgs Laboratory analyses of groundwater samples collected from boring B 10 detected MTBE (3 3 ppb) at 11 feet bgs, MTBE (35 ppb) and diesel (58 ppb)at 16 feet bgs, and MTBE (90 ppb) and diesel (490 ppb)at 30 feet bgs SUMMARY The data obtained during this investigation indicate the following • Sediments encountered beneath the site typically consisted of heterogeneous mixtures of silt, clay, and fine-to medium-grained sand Groundwater was initially encountered in borings B8 through B 10 at depths ranging from approximately 8 to 10 feet bgs with saturated zones encountered at depths ranging from approximately 13 5 to 18 feet bgs, and approximately 20 to 36 feet bgs Based upon data collected during this investigation and previous investigations, the groundwater beneath and downgradient of the site appears to consist of a single unconfined aquifer of variable permeability