HIGH-RESOLUTION MONITORING OF NAPL SATURATION IN HETEROGENEOUS POROUS MEDIA: APPLICATION OF X-RAY ATTENUATION TECHNIQUES TO CHLORINATED SOLVENTS AND A NONTOXIC SURROGATE

Slow dissolution of nonaqueous-phase liquids (NAPL) trapped within porous media provides long-term sources for aqueous contaminant plumes. In homogeneous media, NAPL is retained at residual saturation and dissolution occurs at pore-scale interfaces. In heterogeneous media, NAPL is entrapped as pools with high NAPL saturation and dissolution occurs predominately at interfaces between pools and bulk aqueous phase. Pool interfaces are not sharp. Instead, transition zones develop with decreasing NAPL saturation away from the pool.

Our research is focused on scaling NAPL mass transfer and related processes (tracer partitioning, NAPL degradation) from laboratory columns to field sites. Controlled experiments with multidimensional flow fields, specified porous-media heterogeneity, and known NAPL pool configuration are central to the approach. Experiments are being conducted at multiple scales (approximately 0.1 to 10 m) using either chlorinated NAPL or a low-toxicity NAPL surrogate. Success in up-scaling experiments depends partly on our ability to make repeated high-resolution measurements of NAPL saturation within NAPL-water transition zones as NAPL dissolution and degradation proceeds.

We have developed an x-ray attenuation system for monitoring NAPL saturation in one- and two-dimensional heterogeneous porous media at scales of 0.1 to 3 m. X-ray attenuation has been used previously in column studies with chlorinated solvents. But our application to low-toxicity surrogates for chlorinated NAPL has expanded opportunities for conducting realistic intermediate-scale experiments of NAPL entrapment, dissolution and tracer partitioning in heterogeneous media. Calibration experiments and column studies with various NAPL have shown x-ray attenuation to provide the high resolution and low detection limits needed to map NAPL saturation within entrapped NAPL pools and NAPL-water transition zones. The method has been used to determine the distribution of NAPL saturation in column studies for estimation of multiphase retention functions and relative permeability functions. Initial results using x-ray attenuation to map NAPL saturation in a two-dimensional experiment of pool dissolution and tracer partitioning also will be presented.