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October 28,.2013 Page 2 of 3 Reference: Residual Mass Estimates,7-Eleven Store#14117 9.5 feet bgs,which was shallower than the historically highest TPHg concentrations(15 to 20 feet bgs).No other,samples have been collected from the estimated area of original soil impact.Based on the consistent decreases in dissolved hydrocarbon concentrations in the shallow groundwater,it is unlikely that significant amounts of hydrocarbons remain present in the soil. Residual Mass Estimates in Groundwater Mass estimates of hydrocarbons originally dissolved in groundwater discovered after the original UST removal in 1998 were projected based on the estimated former area and depth of impact.To be conservative,the maximum concentration of.dissolved TPHg,benzene;and methyl tertiary butyl ether (MtBE)from the years 1999 through 2003 were used in the calculations.Figures 3,4,and 5 illustrate the estimated maximum extent of dissolved petroleum hydrocarbons in groundwater;a summaryof . groundwater analytical data is presented in Table 4 and the mass calculations are presented in Table 5. Based on these calculations,a conservative estimate of the mass of dissolved hydrocarbons in groundwater after the initial release is as follows: 10.7 pounds of TPHg,3.4 pounds of benzene,and 21.9 pounds of MtBE. Mass estimates of hydrocarbons currently dissolved in groundwater were also calculated;based on the absence of detectable benzene and low concentrations of MtBE in recent groundwater samples,only TPHg was used in these calculations.The TPHg plume geometry from the second quarter 2013 is depicted with an isoconcentration map on Figure 6(the third quarter isoconcentration map is not included since MW-4 was not sampled due to damage incurred to the well during the-UST removal activities in June 2013),and the mass calculations are presented in Table 5.Based on these calculations,it is estimated that approximately o.o9 pounds of TPHg remain dissolved in.groundwater;the amount of benzene and MtBE remaining in the groundwater is negligible. I Review of historical groundwater data collected from groundwater monitoring wells indicates that the hydrocarbon impacts to groundwater are well defined. Summaries of groundwater flow directions and gradients are presented in Table 6 and Figure 7. )n addition,review of the groundwater data indicates that the plume is historically stable. Outer wells MW-'1,MW-5,MW-6,and MW-7 have not contained TPHg concentrations above laboratory reporting limits since the fourth quarter 2005. MtBE continues to be detected in well MW-6;however,concentrations have not exceeded 2.2 micrograms per liter(µg/L)since the well was installed in 2001. While TPHg continues to be detected in monitoring wells MW-3 and MW-4, decay rate calculations(included in Attachment B)indicate that TPHg concentrations will reach water quality objectives(100 pg/L)in a maximum of approximately 20 years. i i Design with community In mind