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Working To Restore Nature <br />This calculated velocity, based upon only 10 to 15 minutes of pumping, is significantly greater <br />than what we would expect in silts and clays based upon available literature Driscoll (1986) <br />noted typical K values for silty clay/clayey silt to be between 10-2 and 1, which corresponds to <br />a maximum estimated velocity of approximately 3 ft/yr <br />CONCLUSIONS/RECOMMENDATIONS <br />Field observations and analytical data indicate that vertical migration of residual contaminants <br />has not occurred beyond 14 feet below surface grade and that lateral migration was not <br />extensive Previous investigations indicated that the main concern was the soil in the vicinity <br />of the former diesel fuel tank pit Analytical results of excavation confirmation samples indicate <br />that impacted soil has been removed to the maximum possible extent without causing structural <br />damage to the existing building The organic rich, fine grained soils at the site are highly <br />adsorptive of petroleum hydrocarbons Based upon the discussions presented in this addendum, <br />RESNA does not recommend further soil investigation <br />Laboratory analyses of water samples collected from site monitoring wells have not exhibited <br />detectable levels of petroleum constituents since the initial sampling in March 1991 (0 6 ppb <br />xylenes in MW2) This would suggest groundwater underlying the site is not significantly <br />impacted Excavation sampling results indicated that localized groundwater contamination <br />existed immediately after the excavation where the groundwater came in contact with remaining <br />impacted soils However, the relatively low effective porosity (high specific retention) of the <br />silt and clay soils impedes movement of dissolved hydrocarbons No further action is <br />recommended <br />15(X)541R-AD494 rNL 4 <br />