NUMERICAL SIMULATION OF BIODEGRADATION CONTROLLED BY TRANSVERSE MIXING

A key process in bioreactive transport is the mixing between the substrates: without mixing, there can be no reactions. The various modes by which existing transports models can produce mixing in heterogeneous aquifers are investigated and the validity of the results is evaluated. In the context of biodegradation, the relevant scale of mixing is the pore-scale, on which the single organisms act. Therefore, the smallest scale considered is the REV(representative element volume)-scale, implying that the dispersivities obtained on this scale are suitable for the description of mixing in the context of reactive transport. Demonstration is done that macro-dispersion models are not relevant in this context: artificial mixing results in over-prediction of reaction rates, behaviour in transverse direction differs from that in the longitudinal… Demonstration is also done that dispersivity values based on the second spatial moment transverse to the direction of flow leads to an over-estimation of mixing. Because macro-dispersivities do not reflect the relevant mixing process controlling biodegradation, only local-scale dispersivities which are small in magnitude can be used in the numerical model. The mixing processes can be reproduced accurately by using a streamline-oriented grid to avoid grid-orientation effects and by using dispersivities values representatives of local mixing.