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<br /> 7.1 Distribution of Hydrocarbons in Soil
<br /> A total of 45 onsite and one (1) offsite soil borings have been drilled, and provide data to assess
<br /> ' the distribution of hydrocarbons in soil The maximum depth of soil investigation is
<br /> approximately 120 feet bgs Cross-sections and maps depicting the distribution of TPHg,
<br /> benzene, and MTBE in subsurface soils are included as Figures 3A (A-A'), 313 (B-B'), 3C (C-
<br /> M, 5A(TPHg 0-50 ft bgs), 5B (benzene 0-50 ft bgs), 5C (MTBE 0-50 ft bgs), 5D (TPHg 51-
<br /> 100 ft bgs), 5E (benzene 5 1-100 ft bgs), and 5F (MTBE 5 1-100 ft bgs) and historical soil
<br /> analytical data is summarized in Appendix A A total of 218 soil samples (including 12 tank
<br />' bottom samples) were analyzed for TPHg and 63 samples have been analyzed for MTBE (39 by
<br /> EPA Method 8260B)
<br /> Maximum TPHg, benzene, and MTBE concentrations in the vadose zone soils (0-50 foot depth)
<br /> were detected at 46,600 mg/Kg, 220 mg/Kg, and 71 mg/Kg, respectively in boring TDD-14 at
<br /> 15 5 feet bgs MTBE was not confirmed using EPA Method 8260B The highest MTBE in soil
<br />' in the vadose zone confirmed by EPA Method 8260 was 0 0018 mg/Kg in boring CPT-3 at 50
<br /> feet bgs Maximum TPHg, benzene, and MTBE concentrations from 51-100 feet bgs (at and
<br /> below the water table) were detected in boring CPT-4 as follows TPHg at 9,110 mg/Kg at 75
<br />' feet bgs, benzene at 25 3 mg/Kg at 60 feet bgs, and MTBE at 0 072 mg/Kg at 85 feet bgs
<br /> (confirmed using EPA Method 8260B) As shown in Figures 3A, 313, 3C, 5A, 513, 5C, 5D, 5E,
<br /> and 5F, the highest soil hydrocarbon concentrations underlie westernmost dispenser island just
<br /> north of the former USTs (area 1), the former USTs (area 2), and to the north of the middle
<br /> dispenser island (area 3) The maximum depth of significant vadose zone contamination (>10
<br /> mg/Kg) is approximately 80 feet bgs in areas 1 and 2 with concentrations generally decreasing to
<br /> much lower levels at the total depth of investigation (110 feet bgs), whereas, the maximum depth
<br /> of soil contamination in area 3 is approximately 30 feet bgs The extent of hydrocarbon-
<br /> impacted soil in area 1 is approximately 288,000 cubic feet (60'x60'x80'), area 2 - 264,000
<br /> cubic feet (60'x55'x80'), and in area 3 - 48,000 cubic feet (40'x40'x 30') The assumed soil
<br /> density was 120 pounds per cubic feet Based on an TPHg concentration of approximately 971
<br />' mg/Kg (area 1), 987 mg/Kg (area 2), and 8,333 mg/Kg (area 3), from site assessment data, the
<br /> calculated mass of hydrocarbon in soil was approximately 112,824 pounds However, it must be
<br /> recognized that this is only a rough estimate based on several reasonable assumptions, the actual
<br /> hydrocarbon load in the sods could dramatically differ from this estimate It should also be
<br /> noted that the distribution of hydrocarbons in soil depicted in Figures 5A, 513, 5C, 5D, 5E, and
<br /> 5F was constructed from soil analytical data collected from years 1987 through 2004, before and
<br /> during the SVE system operations that have now removed an approximate cumulative 140,299
<br /> pounds of hydrocarbons The amount of hydrocarbon removed by the SVE system is calculated
<br /> using laboratory results on air inlet samples
<br />' 7.2 Distribution of Hydrocarbons in Groundwater
<br />' A total of nine (9) groundwater monitoring wells have been installed onsite, and are sampled
<br /> quarterly to provide data on the distribution of hydrocarbons in groundwater The maximum
<br /> depth of a groundwater well at the site is 90 feet (MW-4, MW-5, and MW-6) Iso concentration
<br /> maps depicting the distribution of TPHg, benzene, and MTBE groundwater are shown as
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