ARCHIVED REPORTS XR0001322 - Laserfiche WebLink

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1 groundwater flow has vaned frim northeast to southeast, and the groundwater gradient has varied from 0 009 to 0 01 ' Chemical Distribution in Soil and Groundwater Results Relatively low concentrations of residual gasoline hydrocarbons were detected in soil in the vadose zone ' from 0 to 24 feet bgs near the former tanks (see Figure 5) Total petroleum hydrocarbons as gasoline (TPHG) concentrations ranged from below laboratory detection limits to 15 parts per million (ppm), benzene concentrations ranged from below detection limits to ' 0 82 ppm The highest concentrations were detected in soil within the historical capillary fringe from 25 to 41 feet bgs and near the upgradient property lines of the site (see Figure 6) Concentrations of TPHG from soil samples collected from 25 to 41 feet bgs ' ranged from below laboratory detection limits to 1,700 ppm Residual hydrocarbons were detected in soil samples collected above the historical capillary fringe (above 25 feet bgs) ' in the off-site upgradient soil boring (B-20/MW-6) Residual gasoline hydrocarbons were detected in soil near the product line due to a release of approximately 2 to 5 gallons of gasoline during drilling operations TPHG was ' detected at a concentration of 1400 ppm at 6 feet bgs Analytical results of soil samples are presented in Attachment A ' Dissolved gasoline hydrocarbons, 1,2-dichloroethane, and 1,2 dichloropropane have been detected in samples from the five on-site wells and the two off-site upgradient wells . Concentrations of TPHG have ranged from 12,000 parts per billion (ppb) to 110,000 ppb, and benzene concentrations have ranged from 2,100 ppb to 16,000 ppb The highest concentrations were detected in wells near the western and southern perimeter of the site, ' upgradient of the ARCO product tanks and pump islands, and in the off-site wells Eight isoconcentration maps were generated from groundwater data collected at the ARCO and Texaco stations during the first quarter 1994, second quarter 1994, and third quarter 1994 ' and are depicted in Figures 7 through 14 (EMCON, September 12, 1994, Texaco Corporation, June 10 and August 17, 1994) Groundwater data is presented in Attachment A ' The wellhead elevations of the monitoring wells at the site were surveyed by a licensed land surveyor EMCON used the surveyed elevations and the depth-to-groundwater ' measurements from wells at the ARCO and TEXACO stations to evaluate the potentiometric surface The direction of groundwater flow has generally been northeast since our investigation began Figure 15 shows the current groundwater gradient beneath ' the ARCO and Texaco stations 1 SAON\PJ"805ND8051460 2AS-94\pas 1 Rev 0 5/5/95 0805-14602 2-2