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S E C 0 R Ms. Lon Duncan ARCO Station 4493 Site Closure Report October 31,2006 Page 4 l and 35 lbs in the soil. An estimated 11 lbs of MtBE and 0.11 lbs of benzene remain in the groundwater on the site. Tetrachloroethene(PCE),and TCE were detected above laboratory reporting limits in groundwater samples collected from site wells this quarter. Low-level concentrations of TCE in MW-1, MW-2, MW-7, MW-10M, MW-10D, MW-11D, MW-13M, and MW-13D were detected. Graphs of chlorinated hydrocarbon trends are included as Attackment E. In 1999, TCE was reported in groundwater samples from three of the four wells(MW-1, MW-2,and ' MW-3)located on site. These wells are located up-gradient and cross-gradient of the former USTs and pump islands. TCE was not reported in the groundwater samples from Well MW-4, located down gradient of the former USTs and pump islands, nor in any of soil samples collected from the borings at the site. This data suggests that the source of TCE in groundwater is offsite (SECOR 1999). Additionally, the highest concentration of TCE was 97 pg/L (MW-10M) in March, 2006 (Table 2) while the wells clustered at the source (MW-4, MW-8WD, and MW-12M/D) have had a maximum concentration of 1.6 pg/L (MW-8M, December, 2004)since monitoring began. PLUME ANALYSIS To better understand the future plume movement, SECOR examined the data for the site with respect to groundwater concentrations of gasoline range organics(GRO),benzene,and MtBE. The following analyses was performed: • Concentration trends for GRO, benzene, and MtBE in groundwater for wells MW-4, MW- 5, MW-8M, MW-8D, and MW-9M were plotted These wells are located at or near the plume centerline and can be considered representative of the maximum plume concentrations for the constituents. The resulting concentration graphs with exponential trend lines are included in Attachment F. • Boring logs and cross-section data were examined along the plume centerline to determine geologic influences on the plume. • Relative solubility and estimated remaining mass of the constituents were compared between the constituents. The concentration trendlines indicate that all three co stituents are decreasing in concentration in each of the wells considered. This indicates that the plume is shrinking,and source concentrations are decreasing. The rate of decrease varies by constituent. Benzene and MtBE are decreasing more quickly, and can be expected to reach groundwater standards for all the wells considered within approximately the next 10 years, provided that Icurrent trends of decreasing concentration continue. In many of the wells, either one or both of these compounds is already below standards. Concentrations of GRO are also on a decreasing trend, but are not decreasing as quickly as benzene and MtBE. There are likely several reasons for the slower rate of decrease. GRO is not a single compound, but is comprised of many compounds mixed together. Each compound has a different solubility and different partitioning coefficients for soil,water,and vapor phases. The more soluble and more volatile compounds will "strip" out of the GRO mixture preferentially into the soil gas and groundwater, leaving less soluble and less volatile compounds behind. Thus, initial decreases in GRO concentration will be more rapid than later concentration changes. So overtime,