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Geo(gicufTechnics Znc Page 6 Former Moresco Property Site Characterization Report Project No. 251.2 16.. October 11, 1996 3.3 Hydrocarbon Distribution in the Soil & Mass Balance Hvdrocarbon Distribution Laboratory analyses of soil samples suggest that the vertical extent of soil impacted with gasoline range petroleum hydrocarbons is presently above 70 feet bgs. The lateral extent has been defined to the south, west, east and almost to the north. The geologic cross section (Figures 3 & 4) drawn through the site from north to south illustrate the geology of the site +� and the contaminant distribution. The fuel oxygenate compound Methyl Tertiary Butyl Ether (MTBE) is present at this site and -47 may account for the apparent continued mobility of these chemicals. Because testing for MTBE did not occur until the most recent drilling and sampling event we do not have a complete understanding of the concentration or distribution of this chemical. 9"' The heart of the contamination appears to be between 25 to 35 feet below grade in a clean sand containing discontinuous silty sand lenses. Curiously, the contamination appears to have preferentially migrated toward the north as evidenced by field observations and L. laboratory data. We suspect that the upper fine grained zone between 10-20 feet may have directed contaminant flow in that direction. The concentration of contamination attenuates with depth and does not appear to have penetrated much deeper than the thin gravel layer at 63 feet. Because flow characteristics in unsaturated zones is highly dependent on the aperture of interstitial capillaries (pore space, size and permeability) we suspect that the gravel unit will temporary retard vertical movement until the overlying silty sand material becomes saturated with soil moisture/ gasoline and the capillary pressure equilibrates with L. the larger pore spaces of the gravel unit. At that time we would expect that the fine grain unit underlying the gravel will wick the contamination downward from the gravel unit. This data is summarized and shown as Tables I & 2 in Appendix A. Copies of the laboratory analytical reports are attached as Appendix C. Mass Balance Calculations Mass balance calculations were performed in an attempt to evaluate the amount of gasoline remaining in the soil. The calculations divided the impacted area into 4 layers with layers 2 through 4 having an inner and outer zone. Lo The mass balance calculations shown in Table 3 indicate that approximately 1,675 gallons of gasoline is adsorbed onto soil grains and present in a gaseous phase. The distribution of the highest mass concentration is centered around 25 feet which suggests the mass is migrating downward from the point of release at about 5 feet bgs. Relatively high levels of gasoline (-6 ppt - part per thousand) and benzene (-1 ppt) are present in the soil at 25 feet. It is this mass of chemicals that we believe may pose a threat to public health or the environment if it '- is not partially mitigated. Beneath this, detectable concentrations are present down to 60 feet. It is recommended that this information, along with the other data generated during this r.. investigation be used to evaluate the risk of leaving the chemicals in the soil and to what degree the chemicals would have to be mitigated to protect public health and the environment. This approach is discussed further below. L. Lo