THE ROLE OF MATRIX DIFFUSION TO CONTAMINANT TRANSPORT IN FRACTURED CLAYEY AQUITARDS IN DENMARK

This presentation summarizes results of the role of matrix diffusion for downward transport of bromide and pesticides through fractured till aquitards at three field sites in Denmark. In one study high resolution preferential water saturated flow and bromide transport data from large (0.5 m in diameter) undisturbed till columns (LUC) were used to evaluate the suitability of equivalent porous medium (EPM), dual porosity (DP) and discrete fracture/matrix diffusion (DFMD) numerical modeling approaches for assessment of flow and non-reactive solute transport in clayey till. Simulating the transport data with the EPM effective porosity model (FRACTRAN in EPM mode) was not successful because calibrated effective porosity for the same column had to be varied up to 1 order of magnitude in order to simulate solute breakthrough for the applied flow rates between 11 mm/day and 49 mm/day. Attempts to simulate the same data with the DP models CXTFIT and MODFLOW/MT3D were also unsuccessful because fitted values for dispersion, mobile zone porosity, and mass transfer coefficient between mobile and immobile zones varied several orders of magnitude for the different flow rates, and because dispersion values were furthermore not physically realistic. Only the DFMD modeling approach (FRACTRAN in DFMD mode) was capable to consistently simulate the observed changes in solute transport behavior during alternating flow rate and time without changing values of calibrated fracture spacing and fracture aperture to represent the macropores. DFMD simulations of the biodegrading pesticide MCPP (mecoprop)indicated that excessive retardation of MCPP occured due to constant removal of MCPP from the matrix, which maintained a constant high concentration gradient and flux of MCPP from the fractures into the matrix. For long-term predictions of pesticides in groundwater underneath fractured aquitards, this indicate that good data for contaminant diffusivity and degradation has great importance for obtaining reliable results.