DNAPL spill and plume migration in a naturally heterogeneous aquifer analogue

Spills of dense nonaqueous phase liquids (DNAPLs) in highly heterogeneous porous media were simulated using the compositional flow program CompFlow. Detailed realistic data giving the spatial distribution of sedimentary structures was taken from outcrop studies of sand and gravel deposits. For the scenarios studied, pore entry pressure was a major factor controlling the spatial distribution of the DNAPL, and hence its long-term dissolution into the groundwater. Thus the spatial distribution (including connectivity and local contrasts) of low pore entry pressure/high permeability regions is crucial in determining how far the DNAPL migrates the subsurface. The simulations of DNAPL spills based on the outcrop information were compared with similar scenarios simulated using geostatistical realizations of physical parameters. Gaussian realizations showed spreading of the DNAPL at lower saturations due to the inherently smooth parameter contrasts. Indicator geostatistical realizations, through reproducing the structural nature to the parameter heterogeneities and preserving contrasts, appear to lead to a more realistic reproduction of the behaviour of DNAPLs in these sand and gravel deposits. The results highlight the important role of site characterization and representation in the process of predicting the behaviour of NAPLs in heterogeneous porous media.