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i J k n ' 16 August'2001 AGE-NC Project No 97-0312 ' Page 7 of 20 UST-5 PH-g;T-P - and benzene-have_be detected at concentrations as high as 3,400 µg/1 ' (UST-1), 3150,000 µg/1 ST-1} and{840^µg/�ST-5), respectively, during the site investigation . Minor coneentra ions of dissolved petroleum hydrocarbons have been detected in ground water samples from monitoring wells UST-2, UST-3, UST-4 and'UST-6 Moderate concentrations have h" been detected sporadically in these wells(Table 6) The intermittent detections in the perimeter wells may be the result of seasonal shifts of the contamination plume The inferred horizontal extent of -" dissolved TPH-d, TPH-g and benzene m'ground water from on historical monitoring events is ' depicted in Figures 19, 20 and 21, respectively 1 In addition to the six on-site monitoring wells,two on-site domestic wells,referred to as the"Iarge" and "small" wells, have been sampled-since December 1997 Low concentrations of'petroleum hydrocarbons have been sporadically detected in both wells but were below the method detection limits in confirmation samples collected shortly after detectable concentrations were reported by the laboratory (Table 6) ' The extent of TPH-g and BTEX compound-impacted ground water appears to be limited to the immediate area surrounding UST-1 and UST-5 Relative to the inferred source area near the former . UST cluster and UST-1 and UST-5, the TPH-d plume appears to extent dust north of UST-6, the ' western extent of the TPH-d plume appears to be located between the former UST location and UST-2, the eastern extent of the TPH-d plume appears to lie just east of UST-3 and the southern extent of the TPH-d plume appears to be located between UST-5 and UST-4 The distribution of ' dissolved petroleum hydrocarbons can be approximated by utilizing an equation for an ellipsoid, the dimensions of the ellipsoid were estimated to have a short radius of 48 feet, a long radius of 84 feet and an average thickness of 19 5 feet Assuming an average saturated porosity of 20%,the calculated 9U� ' volume of hydrocarbon-impacted ground water is approximately 241,300 gallons Based on the _ average concentration of TPH-d and TPH-g in samples collected from on site ground water ' monitoring wells,the impacted ground water plume contains approximately 47 gallons of TPH-d and 0 2 gallons ofTPH-g Volume/mass calculations are'mAppendix C Areas used for thvolume/mass calculations are illustrated in Figures 23 and 24 II {In addition to the petroleum hydrocarbon and BTEX contamination detected in samples collected from the six on-site monitoring wells, MTBE was detected in grab ground water samples collected I from three soil borings advanced in June 1999 (B-14, B-15 and B-17) MTBE was detected in the grab samples from borings B-14, B-15 and B-17 at concentrations of 6 4 µg/l, 8 4 µg/l and 9,700 µg11, respectively However, the MTBE plume does not appear to be extensive and has not migrated to any of the ground water monitoring wells The inferred horizontal extent of MTBE contamination in'ground water is illustrated in Figure 22 r rl , i Advanced GeoEnvironmental,Inc til