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I A S S 0 C 1 A T E S I N C +sfr. 4.1.2 Petroleum Hydrocarbons in Soil A review of historical soil analytical data indicate the highest concentrations of petroleum hydrocarbons are generally present in borings MW4, SB-13, and SB-2 which are either immediately adjacent to the former UST or within 20 feet to the east of the former UST. Petroleum hydrocarbons at these locations were generally found at depths ranging from 10 to 15 feet bgs. Petroleum hydrocarbons were found further from the former UST at decreasing concentrations in borings MW2, MW3, MW6, S13-1 through SB-6, SB-11, and SB-12. At these locations ;- , petroleum hydrocarbons were detected at depths of approximately 15 feet bgs, In general, the soil samples collected at depths of 15 feet bgs were collected from below the groundwater table. 4.1.3 Mass Balance Calculations-Soil Using the analytical results presented in Table 1, ATC estimated the lateral extent of TPHd, which was then used to perform mass balance calculations to estimate the mass of residual soil contamination. The total mass of TPHd remaining in the soil is estimated to be approximately 1,605 pounds. This .......... estimate is conservative in the fact that it does not take into account degradation of TPHd in the soil samples collected in 1989 and 1993. Mass balance calculations are included in Appendix D. Ff k{q ' 4.2 Groundwater Conditions �= 4.2.1 Site H dro eolo Y g gY On October 18, 2000, static fluid levels were measured in monitoring wells MW1 through MW6. The water table elevation decreased an average of 0.77 feet since July 2000. Table 2 summarizes the water level data collected during the reporting period and provides previous data for comparison. �J A groundwater gradient map was created for the October 18, 2000 monitoring event (Figure 3) using groundwater elevation data from all the wells monitored. Figure 3 depicts generally a north groundwater flow direction at a magnitude of approximately 0.008 ft/ft or 42 ft/mile. During the period of investigation from 1989 to 2000 groundwater was measured at depths E ' ranging from 7.71 feet bgs to 17.34 feet bgs. During the period from 1995 to 2000 groundwater flow direction was consistently to the north at a gradient that ranged from 0.0029 to 0.018 ft/ft. r; The average gradient was approximately 0.0077 ft/ft. Prior to this period the gradient appeared to be affected by the presence of free product in well MW4. w:162574.021reporlslclosure-smith.doe 4 �s