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• Hydraulic conductivity values calculated from the results of the hydraulic testing of each well range from 2.49 x 10-6 to 2.35 x 10' ft/sec,which correspond to a linear groundwater velocities of 0.13 to 12.72 feet per year. 8.0 SUMMARY OF FINDINGS AND CONCLUSIONS The findings and conclusions of this additional site investigation are as follows: • The extent of TPH-G and BTEX constituents in soil and groundwater appears to have been adequately assessed at this time. The petroleum hydrocarbons detected in the soil samples collected from MW-11 and MW-12 at a depth of 4 feet may be attributable to surface water runoff from the adjacent highway offramp. • The extent of MTBE in the groundwater to the north and south has not been adequately defined at this time. • The concentrations of MTBE detected in the groundwater using EPA Method 8020 were verified by EPA Method 8260 analysis. • Potential onsi to conduits for migration of petroleum release include product line trenches, underground the electrical power supply lines to the USTs and dispensers, automatic shut-off control lines to the dispensers, an abandoned remote fill line to the former used oil LJST, and a leachfield that.extends offsite. These subsurface features are typically to -;' depths of about 2 to 4 feet. • The survey of offsite subsurface utilities revealed the presence of a sanitary sewer trench adjacent to the BP Oil site. Based on its location relative to the hydrocarbon plume, the sewer trench may influence the transport of petroleum hydrocarbons and MTBE in the groundwater. Although construction drawings showing invert elevations were not I available for review,sanitary sewers are typically constructed at depths of at least 8 to 10 feet. Since depth to water beneath the site is typically 4 to 8 feet,it is likely that the sanitary sewer near the site is below the water table. • The calculated hydraulic conductivity values are consistent with published data for the soil types encountered during drilling and soil sampling. • The concentrations of dissolved oxygen detected in the groundwater samples are indicative of aerobic conditions throughout the hydrocarbon plume. • As shown on Figure 5, the concentrations of MTBE in groundwater samples collected from onsite Wells MW-2 and MW-3 have increased since the analysis was first performed in January 1994. The highest increase in MTBE concentrations was observed in Wells MW-2,MW-3,MW-4, MW-5, and MW-6 during the October 1997 sampling event. • Increases in MTBE concentrations in groundwater samples do not appear to be influenced .} by groundwater elevations,which fluctuate by as much as four feet seasonally, 11 . 4