CONTAMINANT TRANSPORT AND THE RELATIONSHIP BETWEEN HYDROCHEMICAL AND MICROBIOLOGICAL REACTIONS WITH MATRIX DIFFUSION IN CLAYEY TILL

Macropores in soils have been known for a long time to provide preferential flow paths for rapid downward transport of contaminants into groundwater. Less attention has been paid to the coupling between macropore flow and soil geochemical and microbiological heterogeneity and resulting changes in downward contaminant flux.

High resolution water saturated flow and transport data of bromide, nitrate and pesticides from large undisturbed columns (LUC) and field plots were used to study redox-reactions and evaluate the suitability of various numerical modeling approaches to simulate observed nitrate and pesticide fluxes. Simulating the transport data with an EPM effective porosity model was not successful because calibrated effective porosity values for the same LUC 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 dual porosity 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 model FRACTRAN was capable of consistently simulate the observed changes in solute transport behavior during alternating flow rate and time without changing values of initially calibrated fracture spacing and fracture aperture to represent the macropores.

In the same columns nitrate reduction was shown to occure due to microbial degradation of accumulated organic matter coupled with successive consumption of O2 and NO3- in the macropore water followed by reductive dissolution of Fe and Mn from minerals along the macropores. Simulations using the calibrated discrete fracture matrix diffusion (DFMD) model could reasonably reproduce the denitrification and resulting flux of nitrate of pesticides observed during variable flow rate from the columns and field experiments.