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SECTION 5.0 RESULTS AND DISCUSSIONS MW-13, the benzene action level was exceeded in the samples collected at each ch depth Based on the quarterly sampling results, the benzene action level was exceeded in wells MW-13 and VM-2 Concentrations of toluene, ethylbenzene, xylenes, and MTBE did not exceed action levels in the three wells 5.2 HISTORICAL TRENDS OF PETROLEUM HYDROCARBONS IN SUBSURFACE 5.2.1 Soil Trends Based on soil sample results summarized in Table 3-1 for previous subsurface investigations, the majority of the petroleum hydrocarbons appear to be in the vadose zone The vadose zone soil plume appears to be bounded by borings ABA, A13-2, MW-13, SB18, and SB-15 (GEOCON, 1996) The volume of petroleum hydrocarbon-impacted soil was estimated by GEOCON to be 1,530 to 2,290 cubic meters (2,000 to 3,000 cubic yards) 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 EA and GEOCON A discussion of the estimated masses of TPH-g and benzene in soil is included in Section 5 4 5.2.2 Groundwater Trends The historical results of petroleum hydrocarbons in groundwater through Third Quarter 1999 are presented in Table 3-2 Historical concentration trends in groundwater for four wells (MW-13, MW-14, VW-1, and VM-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 in conjunction 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 remained relatively constant since 1997 5.3 SLUG TEST RESULTS ' Slug tests were performed in wells MW-11, MW-13, and MW-24 at Shop 10 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 generally slightly higher for the rising head test as compared to the falling head test The estimated ' hydraulic conductivities ranged from 12 x 10"2 to 1 9 x 10"3 centimeters/second (cm/s) The transmissivity, calculated assuming an aquifer thickness equaled to the well screen length, ranged from 1 7 to 6 9 square centimeters/second (CM2 Is) These results indicate a groundwater velocity ' ranging from 13 x 10'5 to 8 4 x 10-5 cm/s, or 4 to 27 meters/year(14 to 87 feet/year), based on the slug test data and the April 1999 groundwater gradient calculated by PSI 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 S IPRa4+10e0100041sHOPIO�S140PIasaooe 5-3 UBS Greiner Woodward Clyde