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October 30, 1997 Page 3 • havev slowly risen to a current level of approximately 35 feet bgs Direction of ground- water flow has been consistently to the northeast (see groundwater contour map in Attachment C) Water-Supply Wells (11.) According to a report by Resna Industries (Resna) dated December 16, 1992, there is only one municipal water-supply well within 2,000 feet of the site "This well is located at the corner of Bidwell Place and Bianci Road, approxi- r mately 1,800 feet southeast of the site, and cross-gradient to the general direction of groundwater flow at the site The well is maintained by the California Water Service, the total depth of the well is 520 feet below surface grade and the screened intervals of the well are 195 to 240 feet, 260 to 345 feet, and 370 to 495 feet below surface grade" I Previous Assessments (Lc., Ld ) In May 1988, soil containing gasoline was discov- ered surrounding product lines in the area of the dispenser islands In May and June I988 Resna collected soil samples from test pits excavated near the dispenser islands and beneath the USTs during their removal (see map in Attachment C and data in Attachment E) Total petroleum hydrocarbons calculated as gasoline (TPH-g) was detected at concentrations ranging from below detection to 1,900 parts per million (Ppm) Between January 1990 and August 1992, I9 soil borings were drilled at or near the site (see map in Attachment C) Thirteen of the borings were converted into groundwater monitoring wells (MW-I through MW-13) In an October 1, 1992 report, Resna concluded that"The lateral and vertical extent of residual hydrocarbons has been deline- ated " The report presented a series of figures delineating the lateral extent of hydrocar- bons in soil at specific depth intervals (Attachment E) In October 1992, groundwater samples were collected from all 13 monitoring wells Resna in their December 16, 1992 report, presented two figures illustrating that the extent of TPH-g and benzene in the groundwater had been defined (Attachment F) The downgradient extent of dissolved hydrocarbons was defined by below method detection concentrations of TPH-g and benzene in perimeter Wells MW-9 through MW-13 During all but one quarterly monitoring event between October 1992 and January 1995, TPH-g and benzene remained below or near the detection limit in all perimeter wells In the single January 1994 sampling event, all wells reported anomalous lugh concentra- tions The wells were resampled the following month and TPH-g and benzene were found to be below the detection hrrut The anamolous data is believed to be the result of a contaminated sampling hose In early 1995, well screens in perimeter wells became submerged (<5 feet) by rising groundwater levels As water levels rose, TPH-g and benzene c6ncentrations in 3111657ANCL6981 DOC