ARCHIVED REPORTS XR0007843 - Laserfiche WebLink

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Ms Lon Duncan . May 24, 2002 Page 2 of 5 largely defined in previous investigations Soil contamination in the vadose zone appears to be limited to a small area in the immediate vicinity of the former UST, and soil contamination in the capillary fringe zone appears to extend no more than 25 feet north, 20 feet west, 10 feet south, and 5 feet east of the former UST On December 13 and 14, 2000, four groundwater monitoring wells were installed to further investigate the extent of soil and groundwater contamination at the site Analytical results indicated that petroleum hydrocarbons in soil were limited to a small area in the immediate vicinity of the former UST, and that the extent petroleum hydrocarbon contamination in the groundwater does not extend more than approximately 40 feet downgradient from the source Various chlorinated solvents were also detected in all site wells, and free product was noted in well MW3 Subsequent quarterly groundwater sampling at the site confirmed that the lateral extent of petroleum hydrocarbon contamination is within close proximity to the former UST The presence of free product and groundwater analytical results indicate that petroleum hydrocarbon contamination is present in groundwater in the immediate vicinity of the former UST but does not extend to perimeter wells Various chlorinated solvents continue to be detected at low levels, however, only PCE exceeds its Maximum Contaminant Level (MCL) of 5 0 ppb The measurable product thickness in well MW3 has been steadily declining since regular product bailing was initiated Free product measurements are summarized in Table I Tables 2 and 3 summarizes analytical results of petroleum hydrocarbon constituents and VOCs, respectively WORKPLAN To further investigate the lateral extent of chlorinated solvents in groundwater beneath the site Ground Zero proposes the following scope of work Drilling, Well Construction and Soil Sampling Two groundwater monitoring wells will be installed at the site at the approximate locations shown on Figure 2 The proposed locations have been modified slightly from the previous workplan in order to eliminate the need to obtain encroachment permits from the City of Stockton The two additional wells will be placed at the locations shown in the figure to further investigate groundwater quality upgradient (southwest) and downgradient(northeast) of the former waste oil UST A drilling contractor with a C- 57 drilling license will advance soil borings using a limited access drill ng equipped with S" diameter hollow stem augers The wells will be completed to depths of approximately 34 feet below ground surface (bgs) and will be constructed of 2-mch diameter, Schedule 40 PVC casing and 0 010-inch slotted screen The wells will be screened from approximately 19 feet to 34 feet bgs The filter pack will consist of No 3 Monterey sand from the bottom of the wells to approximately 2 feet above the top of the screens The remaining annulus will be completed with 2 feet of hydrated bentonite overlain by neat cement grout Each well will be fitted at the ground surface with a locking cap and watertight, traffic-rated well box G1Data1GROUNDZL1GABBARDIwcrkplans\3_add_MW_WORKPLAN doc