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i . groundwater Minor levels of gasoline, 2,700 ppb, are described in Advanced's report to be the volatile end of diesel 1 The tanks, which were the focus of the Advanced's investigation, were removed in February and October of 1998 The diesel tank located in the southern portion of the property was removed lin February of 1998, and the tanks located in the tank farm situated in the northern portion of the property were removed in October of 1998 Elevated levels 1 of contamination, 240,000 ppb diesel, were encountered in the southern diesel tank area As groundwater is at approximately 9 feet below ground surface (bgs), the groundwater sample was collected directly from the tank pit excavation Elevated levels of hydrocarbons were also found in the soil and groundwater in the northern tank pit area Water samples were collected directly from the tank pit and the results were as follows 100,000 ppb TPH gasoline, 31,000 ppb MTBE, 4,600 ppb Benzene, 16,000 ppb Toluene, 3,000 ppb Ethyl Benzene, and 170,000 ppb Xylenes Additionally, a small hole was discovered in the unleaded tank during the removal In November of 1998 Avalon, drilled fifteen geoprobe borings on the subject property and 1 tested soil and groundwater. Elevated levels of MTBE were found in the groundwater in the area of the former, northern tank area While the southern tank area did contain some contamination it appeared to be minor in nature The detections of MTBE in the northern tank area prompted the initiation of a groundwater investigation on the subject property 1 • In March of 2000 Avalon, installed six groundwater monitoring wells on the subject property This investigation was undertaken in order to determine the groundwater gradient direction and to determine the levels of MTBE and other Oxygenates in the groundwater The results of the groundwater investigation and subsequent four quarters of monitoring indicate a north/northeast gradient direction The groundwater monitoring 1 wells confirm that the primary area of impact is the northwest portion of the subject property in the area of the former gasoline tanks referred to as the northern tank area Due to the elevated levels of MTBE along the northern boundary of the subject property, the San Joaquin County Environmental Health Department requested that offsite sampling be initiated to determine if an offsite impact had occurred 1 2.1 Purpose The purpose of this investigation was to determine the extent of the MTBE plume both on and offsite 1 2.2 Methodology All work was performed in accordance with the "Scope of Work," prepared and submitted to the client and to the San Joaquin County Environmental Health 1 . Avalon Environmental Consultants 9627-001-6 Joes Truck Plaza 15600 5 Harlan Road April 15 2001 2 Lathrop California 1