A FIELD EXPERIMENT TO STUDY THE BEHAVIOR OF TETRACHLOROETHYLENE BELOWTHE WATER-TABLE - SPATIAL-DISTRIBUTION OF RESIDUAL AND POOLED DNAPL

This paper describes a field experiment involving the release of 230.9 liters of tetrachloroethylene (PCE) below the water table in a natura lly occurring, unconfined sand aquifer. The release was executed in a 3 m X 3 m X 3.4 m deep, sealable-joint steel sheet-pile cell anchored into an underlying clay aquitard. After allowing 28 days for redistrib ution, excavation of the upper approximately 0.9 m of the cell reveale d PCE pools and residual to be present in relatively coarser grained h orizons, with substantial degrees of lateral flow having taken place. This lateral flow was observed in laminations and lenses ranging in th ickness from a few mm to a few cm, with only subtle variations in text ure separating individual migration pathways. Detailed sampling during the excavation procedure and subsampling of three cores extended down to the clay aquitard revealed a spatially variable distribution of PC E with saturations ranging from 1% to 38% of pore space. Laboratory me asurement of a fully hysteretic capillary pressure curve demonstrated that the degree of nonwetting phase residual is a function of the maxi mum saturation attained along main drainage during the initial infiltr ation process. Various models for consolidated petroleum reservoir mat erials did not fit the experimental data well. The theory governing po ol formation in heterogeneous porous media is also presented, and it i s demonstrated that pools can form in homogeneous media exhibiting a d istinct entry pressure.