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• ARCADIS GERAGHTY&MILLER <br /> depth below the mid-level zone of impact Concentrations showed an increase between the <br />' 100 and 110 ft bgs depth However, m the underlying 119 ft bgs depth, a water sample could <br /> not be acquired because of the lower yield of the formation at this depth While the vertical <br /> extent of impact at this location has not been precisely delineated, it is reasonable to interpret <br /> that the combined mechanisms of underlying low permeability zones and biodegradation would <br />' combine to reduce concentrations of benzene within a short vertical distance below that <br /> investigated <br /> Similarly, The TPHg and benzene groundwater analytic data collected using the <br />' Simulprobe technique from NP-2, approximately 80 feet downgradient from the source area, <br /> indicate that the depth of maximum impact is in the 60 to 70 foot bgs depth interval <br />' Concentrations of benzene significantly attenuate with depth below the mid-level zone of <br /> impact Concentrations showed an increase between the 90 and 100 ft bgs depth As <br /> discussed above, it is thought that variation of groundwater flow direction at the Site may <br /> contribute to periodic fluctuations in the sampled concentrations, especially at wells such as <br />' this, located easterly of the source area This may account for the observation that the <br /> 1 concentration, as indicated by the samples collected during the quarterly monitoring of <br /> February 5, 1999, of relatively mobile benzene in well NP-2-116 is greater than the benzene <br /> concentration in the upgradnent well NP-1-111, even though TPHg is opposite with the higher <br /> concentration(1,800 ug/L)m NP-1-111 and the lower concentration m NP-2-116 Additional <br />' monitoring will be conducted to assess this comparative water quality in these wells over time <br /> It is also interpreted that, while the vertical extent of impact at this location has not been <br />' precisely delineated, it is reasonable to interpret that the combined mechanisms of underl& <br /> low permeability zones and biodegradation would combine to reduce concentrations of <br />' benzene within a short vertical distance below that investigated <br />' 39 <br />