A Field Experiment to Study the Behavior of Tetrachloroethylene Below the 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 naturally occurring, unconfined sand aquifer. The release was executed in a 3 m X 3 m X 3.4 m deep, scalable-joint steel sheet-pile cell anchored into an underlying clay aquitard. After allowing 28 days for redistribution, excavation of the upper approximately 0.9 m of the cell revealed PCE pools and residual to be present in relatively coarser grained horizons, with substantial degrees of lateral flow having taken place. This lateral flow was observed in laminations and lenses ranging in thickness from a few mm to a few cm, with only subtle variations in texture 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 PCE with saturations ranging from 1% to 38% of pore space. Laboratory measurement of a fully hysteretic capillary pressure curve demonstrated that the degree of nonwetting phase residual is a function of the maximum saturation attained along main drainage during the initial infiltration process. Various models for consolidated petroleum reservoir materials did not fit the experimental data well. The theory governing pool formation in heterogeneous porous media is also presented, and it is demonstrated that pools can form in homogeneous media exhibiting a distinct entry pressure.