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3 BACKGROUND The following information was obtained from excerpts of previous soil and groundwater assessment reports prepared by Krazan & Associates, Inc , a summary chronology of the site, and correspondence from the San Joaquin County Public Health Services Department Environmental Health Division (PHS/EHD) provided to Klelnfelder by Wilbur-Ellis In 1965, Wilbur-Ellis Company installed a 1,000-gallon underground gasoline storage tank (UST) at the site In 1987, the UST and associated piping were removed Low levels of total petroleum hydrocarbons as gasoline (TPH-G) were detected in soil samples collected at the time of removal Affected soil was excavated and aerated The former UST site was backfilled ' In 1993, Wilbur-Ellis sold the property to H & H Engineering and Construction, Inc In 1997, H & H applied for refinancing of a loan Since no closure letter for the former UST was on file at the San Joaquin County PHS/EHD, the bank required a subsurface assessment be conducted H & H retained Krazan & Associates A soil assessment conducted 1n February 1998 found Iow levels of TPH-G and BTEX compounds in the soil Petroleum hydrocarbons were found in subsequent groundwater sampling conducted in March 1998 Additional soil and groundwater sampling, 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 PHS/EHD, testing for fuel oxygenates and alcohols needs to be conducted again prior to closure 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 plume to the north and northwest was requested by PHS/EHD iKrazan & 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 areal 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 ' Accordingto Krazan's reports, the areal distribution of the groundwater contarnmation 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 For former-sampling locations conducted by Krazan and Associates see Plate 2 20-4439-01 E01/2000R726 Page 4 of 17 ' Copynght 2000 Klemfelder, Inc October 16,2000