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SECTION 5.0 RESULTS AND DISCUSSIONS 5.2 HISTORICAL TRENDS OF PETROLEUM HYDROCARBONS IN SUBSURFACE 5.2.1 Soil Trends Based on soil sample results summarized in Tables 3-1 and 3-2 for previous subsurface investigations, the majority of the petroleum hydrocarbons appear to be in the vadose zone The vadose zone soil plume was observed to be bounded by borings SB-1, SB-2, MW-1, MW-2, MW-3, and VW-1 (GEOCON, 1996b) The volume of petroleum-hydrocarbon impacted soil was estimated by GEOCON to be approximately 1,150 cubic meters (m) (1,500 cubic yards [yd']) GEOCON did not estimate the mass of petroleum hydrocarbons in soil URSGWC has estimated the masses of TPH-g and benzene in soil based on soil analytical data reported by Alton and GEOCON A discussion of the estimated masses of TPH-g and benzene in sobs is included in Section 5 4 5.2.2 Groundwater Trends The historical results of petroleum hydrocarbon in groundwater through Third Quarter 1999 are presented in Table 3-3 Historical concentration trends in groundwater for four wells (MW-1, MW-6, VW-1, and VW-2) are presented on Figure 5-4 Concentrations below the laboratory �* detection limit were plotted as half of the detection limit The figures depict significant decreases in both TPH-g and benzene concentrations with time along with the increase in groundwater elevations with time MTBE concentrations have fluctuated in the four wells depicted on Figure 5-4, however, the magnitude has been similar since 1997 5.3 SLUG TEST RESULTS Slug tests were performed in wells MW-1, MW-6, and MW-11 at Stockton Yard Table 5-3 provides a summary of the transmissivities and hydraulic conductivities that were estimated for the respective wells for both the falling head and rising head slug tests The results were slightly higher for the rising head test as compared to the failing head test The estimated hydraulic conductivities ranged from 1 3x10"2 to 7 6x10"3 centimeters/second (cm/s) The transmissivity, calculated assuming an aquifer thickness equal to the well screen length, ranged from 4 1 to 9 5 square centimeters/second (cm'/s) These results suggest a groundwater velocity ranging from 5 4 x 10' to 9 2 x 10 cm/s, or 17 to 29 meters/year (56 to 95 feet/year), based on the slug test data and the April 1999 groundwater gradient calculated by PSI using the gradient depicted on Figure 5-2, the groundwater velocity ranges from 0 2x10'5 cm/s, or 5 to S meters/year (16 to 27 feet/year) These hydraulic conductivity estimates are indicative of a soil type consistent with sand/silty sand However, the hydraulic conductivity estimated by slug testing may be . overestimated due to the influence of the surrounding sand pack on the rate of rise and fall of the 51PRC UI040100041SYYARDMAINTYARD5A.DOC 5-3 URS Greiner Woodward Clyde