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ARCADIS GERAGHTY&MILLER Pit where tanks were exhumed in 1967 Data from NP-2 suggest minimal impact ' to mid-level soils(60 to 65 ft bgs depth zone)in the offsite downgradient(east- northeasterly)direction Data from NP-2 also indicate that no offsite impacts to soils have occurred in the deep level zone • The lateral extent of shallow groundwater impacts is largely constrained by data ' from the monitoring well array Based on log-linear extrapolation of the isoconcentration contour intervals, it is calculated that the extent of TPHg impacts ' to shallow groundwater extend approximately 200 ft in the downgradient (northeasterly) direction Calculated upon the same basis, the downgradient extent ' of Site benzene impacts to shallow groundwater at concentrations greater than 1 ug/L extend approximately 165 ft • Based on data from NP-1, the vertical extent of soil impacts at the Site have been identified and do not extend below 110 ft bgs near the source area Based on data from NP-2 the vertical extent of impacts to soil is limited to low level detections in ' the 60-65 ft bgs zone • While the vertical extent of groundwater impacts have not been precisely delineated, concentrations of TPHg and benzene are significantly attenuated below 50 to 60 ft bgs Field sampling evidence indicates that the impacted water at ' depths of 60 ft bgs and below is moving in horizontal zones of somewhat permeable sail material interlayered with lower permeability soils ' • Based on the presence of significant thicknesses of low permeability zones as evidenced by the NP-1 boring log, and the significant attenuation of concentrations with depth, it can be reasonably concluded that vertical migration is also being stabilized and remediated by natural bioremedial processes • Concentrations of petroleum hydrocarbons in groundwater have declined over the -� 5-year period of the monitoring record 5 c!} I ' 48