EVALUATION OF THE PARTITION INTERWELL TRACER TECHNIQUE FOR POSSIBLE CHARACTERIZATION OF DNAPL POOLS

Chlorinated solvents in the form of dense-non-aqueous phase liquids (DNAPL) cause contamination of soil and groundwater at many sites. Characterization of these sites has become a challenge to develop and implement strategies for cleanup. The Partition Interwell Tracer Technique (PITT) developed in the petroleum industry has been adapted by environmental engineers to characterize the amount and location of DNAPLs. PITT uses a suite of chemical tracers to detect the presence of free phase NAPLs and provide an estimate of its effective or average NAPL saturation. Although this technique has been used with some success in a large number of sites there are still some limitations under complex entrapment morphologies resulting from aquifer heterogeneity. In existing methods, PITT data are analyzed assuming local equilibrium exists between phases. This assumption may be true for low saturations however, it may not be applicable in cases of high DNAPL saturation (pools) that could be expected at heterogeneous field sites. The groundwater flows around pools due to the reduced aqueous relative permeability caused by DNAPL entrapment. Bypassing water may reduce the contact between the tracer solution and the NAPL and tracer diffusion in the oil phase. If these conditions are not recognized, NAPL saturation will be underestimated.

The objective of an ongoing research project is to investigate possible causes for underestimation of saturation by PITT for different entrapment scenarios. The definition of a “pool” has to be evaluated to look closely at the processes that occur within the source zone. Based on controlled experiments that involve the placement of the source zone in a test tank and evaluating tracer breakthrough curves under different flow and pool conditions, an analysis and understanding of causes for this estimation error can be performed. This paper presents the preliminary results from experimental investigations conducted in one-dimensional columns and two-dimensional dissolution cells to obtain a fundamental understanding of how tracers interact with DNAPL pools. The experimental data in conjunction with model analysis are used to make a determination of PITT as a viable technique for characterization of DNAPL source zones.