OBSERVED MIGRATION OF A CONTROLLED DNAPL RELEASE BY GEOPHYSICAL METHODS

Seven hundred seventy liters of a dense nonaqueous phase liquid (DNAPL ), tetrachloroethylene (PCE), were released into an isolated volume of a completely saturated natural sandy aquifer. The release was monitor ed over a period of 984 hours with a variety of geophysical methods in cluding ground penetrating radar, time domain reflectometry, in situ r esistivity, and a neutron soil moisture probe. The PCE formed a pool o n a low permeability layer at approximately 1 m depth and spread over an area exceeding 32 m(2). In its course of downward migration, the PC E subsequently formed eight smaller pools. At the end of the experimen t an estimated 41 percent of the total PCE volume remained trapped in the upper pool. The PCE mass and its spatial moments were calculated f rom radar reflection amplitudes. Between 48 and 100 percent of the PCE mass was accounted for by radar measurements. The center of mass move d a total of 0.5 m south southeast and 1.3 m downward. Spatial varianc es showed that the greatest lateral spreading occurred in the east-wes t direction. The results demonstrate that natural heterogeneities, eve n in a relatively homogeneous aquifer, can cause DNAPLs to spread late rally over large areas in the subsurface. This experiment also demonst rated that while the ability of geophysics to uniquely measure the pre sence of DNAPL is limited, certain techniques are well-suited to monit oring changes in DNAPL saturation.