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1 <br /> TPHD were not found in groundwater samples collected However, during some quarterly <br /> sampling events TPHD and TOG were detected in the monitoring wells, but the analytical <br />' results did not match the typical TOG or TPHD chromatogram pattern During the second <br /> quarter 1996 sampling event, dissolved petroleum hydrocarbons were below laboratory <br /> detection limits um monitoring wells MW-2 and MW-3 TPHG was detected in monitoring <br /> well MW-1 at a concentration of 120 ppb Benzene concentration have been below <br /> laboratory detection limits during the last seven quarterly sampling events <br /> Methyl-tert-butylether (MTBE) has not been detected in ground water samples collected <br /> from the monitoring wells In general, dissolved petroleum hydrocarbons have decreased <br /> with time as shown on Figures 10 through 12 <br /> 5.5 Evaluation of the Soil Vapor Extraction Test <br /> SVE test methods, procedures, and results are summarized in a letter report entitled <br /> "Vapor Extraction Test Results for ARCO Service Station 760," RESNA, <br />' September 1991 <br /> On June 14, 1991, RESNA conducted an SVE pilot test using three on-site vapor <br /> extraction wells (VW-1, VW-2, and VW-3) Vacuums of 4 5 to 6 inches of water column <br /> (in we) were applied to wells VW-1, VW-2, and VW-3 during the SVE pilot test to <br /> remove hydrocarbon-bearing vapor from vadose zone soils beneath the site Based on <br /> wellhead vacuums ranging from 4 5 to 6 in wc, an average air flow rate of 35 actual cubic <br /> feet per minute or 31 standard cubic feet per minute (scfm), and induced vacuum <br /> responses at the monitoring points, the effective radius of influence (ROI) for wells VW-1, <br /> VW-2, and VW-3 was estimated to be approximately 22 feet <br /> Laboratory analysis of extracted vapor samples collected from vapor extraction wells <br />' VW-1, VW-2, and VW-3 during the test indicated that total volatile petroleum <br /> hydrocarbons as gasoline (TVHG) were detected at concentrations of 980, 2,900, and less <br /> than 5 0 milligrams per cubic meter (mg/m), respectively, and benzene was detected at <br />' concentrations of 44, 44, and 0 09 mg/m3, respectively Based on the achieved extraction <br /> flow rate (31 scfm) and the maximum TVHG concentration in vapor extracted from <br /> well VW-2 (2,900 mg/m3), the hydrocarbon mass extraction rate was calculated as <br /> approximately 8 pounds per day <br /> 1 <br /> SA(\N\PJM80M8051420 4[35 97\prz l Rev 0 3/12/97 <br /> 20805 142 004 5-3 <br />