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' Lodi Ready Mix Page 7 <br /> Site Closure Report <br /> Project No.93.1 <br /> May 12, 1995 <br /> The laboratory data show that significant concentrations of gasoline range <br />' petroleum hydrocarbons were detected in the tank removal sample GS-4 @ 9'. <br /> The October 1990 re-excavation confirmation soil samples show that, besides <br />' RE-BS-25' which contained elevated concentrations of aromatic & diesel <br /> range petroleum hydrocarbons, only slight concentrations of TPH-Diesel were <br />' detected in samples collected at the edges of the excavation. <br /> 5.0 CONCLUSION AND RECOMMENDATION <br /> The data obtained during this investigation suggest that the release of <br />' petroleum hydrocarbons sourced from a former leaking UST has not <br /> significantly impacted the subsurface environment beyond the limits of the <br /> remedial excavation. <br /> ' Several re-excavation confirmation samples showed low concentrations of <br /> diesel range hydrocarbons. The amount of hydrocarbons remaining in the <br /> native soil have been conservatively estimated using mass balance <br /> calculations and basic assumptions-(highest concentration 180 mg/kg x in-situ <br /> bulk density 50 kg/ft3 x maximum impacted height 12 ft x projected impacted <br /> ' thickness 5 ft x length of front 22 ft x 6 lbs/gallon diesel). We conservatively <br /> estimate that up to 22 pounds of diesel range petroleum hydrocarbons (3.6 <br /> gal.) may remain above 25 feet (bgs) extending out to five feet beyond the <br /> ' south and east horizontal limits of the October 1990 remedial excavation. <br /> Our experience shows that diesel range contaminants are less soluble and will <br /> ' tend to migrate less than gasoline. The chromatographic distribution of the <br /> aromatic compounds detected in RE-BS-25' sample suggest a weathered <br /> pattern in which the relatively lighter constituents are in lesser concentration <br /> ' than the relatively heavier ones. A weathered pattern suggests an older <br /> release. <br /> ' Vadose zone hydrology dictates that flow from a fine grained unit (silt) to a <br /> coarser grained unit (sand) will only occur at those areas that have become <br /> saturated (when the pore pressure in the finer unit has a lesser negative <br /> ' pressure than the pore pressure exerted by the capillary forces of the relatively <br /> larger aperture pore of the coarser unit). <br /> ' The silt layer encountered at 25 ft in SB-1 may have acted as barrier <br /> • attenuating downward migration. Vadose zone hydrology shows that fluids <br /> 1 <br /> 0' <br />