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k' ■ KLEINFELDER including the installation of three groundwater-monitonng wells, was conducted in August through November 1998 Quarterly groundwater sampling has continued since that time In March 2000, the PHS/EHD approved a request by Wilbur-Ellis to reduce the groundwater sampling frequency to semi-annual, and to discontinue sampling for fuel oxygenates and alcohols However, according to recent requests by PHS/EHD, testing for fuel oxygenates and alcohols will need to be conducted again prior to closure The extent of groundwater contamination at the site had been evaluated to the south, east and west of the former underground storage tank location Additional Evaluation of the contaminant plume to the north and northwest was subsequently requested by PHS/EHD Krazan & Associates described the ratio of benzene to gasoline in water samples from MW-1 as elevated Both the PHS/EHD and Krazan have suggested that the aerial distribution of benzene in groundwater grab samples and monitoring well samples is unusual Possible explanations suggested by Krazan for the "elevated" benzene concentrations include a source of non-gasoline- derived benzene from on-site spills or an off-site location According to Krazan's reports, the aerial distribution of the groundwater contamination trends northwest/southeast across the site This observation agrees with field data presented by Krazan indicating more permeable soils and sand lenses are present to the north-northwest (B-2) and south-southeast (B-3) of the former underground tank location Refer to Plate 2 for former- sampling locations conducted by Krazan and Associates K.leinfelder conducted additional environmental services detailed in a report titled "Additional Subsurface Assessment, Sensitive Receptor Survey and I" Semi-Annual Groundwater Sampling, Former Underground Storage Tank Site, H&H Engineering and Construction, Inc , 212 Industrial Drive, Stockton, California 95206", dated October 16, 2000 In summary A total of seven "sensitive receptors," including six wells and one canal, were identified, which were potentially located within a 2,000-foot radius of the former underground storage tank (UST) at the subject site It was Kleinfelder's opinion that the contaminant plume at the subject site at that time had not impacted these receptors Groundwater flow direction during the July 7, 2000 semi-annual sampling event was toward the northeast at a gradient of 0 005 ft/ft This flow direction and gradient are generally consistent with previous groundwater elevation data Total petroleum hydrocarbons as referenced to gasoline and/or benzene were detected in groundwater samples collected from the three onsite monitoring wells (MW-1 through MW-3) and one or more push probe groundwater samples (GP-3 through GP-5) located in the vicinity of the former UST None of the five oxygenates (ETBE, DIPE, MTBE, TAME, TBA) ethanol and methanol were detected during the push probe assessment conducted on June 1, 2000 Benzene was detected in groundwater samples in each of the three-onsite monitoring wells MW-1, MW-2 and MW-3 and in a single push probe groundwater sample The benzene concentrations ranged from 1 0 µg/1 from groundwater collected from MW-2 to 1,100 µg/l from a groundwater sample collected from MW-1 These benzene concentrations are at or exceed State and Federal MCLS of 1 fig/1 and 5 µg/l, respectively 20-4439-01 E04/ST02R397 Page 2 of 6 Copyright 2002,Kleinfelder, Inc August 22,2002