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u 7.0 CONCLUSIONS AND RECOMMENDATIONS Synthesizing all of the data that have been gathered in the past four years of investigation, several conclusions can now be drawn These are listed below • Petroleum hydrocarbon concentrations in the soil beneath this site are very low Gasoline oxygenates (MTBE, etc) are either below detection limits or present at very low concentrations in all of the samples that have been collected, and total hydrocarbons have only been detected above 100 ppm in two samples, both from VM-1 Volatile aromataic hydrocarbons (BTEX), because of their mobility in groundwater, have spread somewhat farther than TPH and have been detected in several borings, but usually at concentrations less than 1 ppm Neither diesel nor ox to are important contaminants at this site We �in mmend that the sampling protocol a amended to eliminate analysis for these compounds ny uture samples • The stratigraphy beneath the site is complicated by the lenticularity of several sandy channel deposits that channel into and grade laterally into overbank silt beds These variations affect the direction of groundwater flow as well as the direction of migration of hydrocarbons due to differences in permeability • In general, the hydraulic conductivity of the near-surface strata is fair No high-permeability beds have been encountered, but some of the sand beds have conductivities in the range of 104 cm/sec Thus, groundwater flow beneath the site is anticipated to be relatively slow, especially below a depth of 40 feet • The vertical and lateral extent�ofa�scon matronin the soil have been reasonably well de ineated The plume is wlliptical in shape, with a long axis following the probable trace of the former pipeline from the dispenser to the USTs Contamination extends under the sidewalk on the Grant Street side of the property, but does not extend under the sidewalk on the Main Street side Contamination does not extend below the base of the channel sand that lies at a depth of 20-35 feet below the northwestern part of the site • Groundwater in the northwestern part of the site has been impacted, and contamination extends off site north of HP-4 and west of VM-1 Accurate delineation of the extent in those directions would be very difficult, because of subsurface obstructions and drilling restrictions in the streets Delineation of the eastern extent should be possible by completing VM-4 in the✓ shallow aquifer, as originally planned We recommend constructing a monitoring is boring, with screen from 35 feet to the present ottom of thehole, which we estimate to be 55 Data on bacterial activity and nutrient availability suggest that natural bioremediation of the contaminants at the site will be slow in the vadose zone above 35 feet, and negligible below that depth The low concentrations of contaminants also means that the food source for hydrocarbon-degrading bacteria is minimal 10