MODELING OF UNSATURATED DUAL-POROSITY COLUMN EXPERIMENT

A dual porosity aquifer refers to a media with mobile and immobile domains. Mass transfer between two domains may cause the long-lasting tailing of contaminant transport in many cases. Therefore, a single porosity model may not be sufficient to describe mass transport in these aquifers. The objective of this work is to illustrate the difference of transport with single porosity and dual porosity models and investigate parameters important for mass transfer and transport.

The column experiments were done with lightly compacted crushed sandstone spoil. As there are plenty of macro pores between the aggregates and micro pores within the sandstone aggregates, it is regarded as a dual-porosity media. Sodium Bromide was injected as a pulse into four columns with different flow rate under unsaturated situation. Transport modeling of conservative tracer Bromide was carried out based on dual porosity and single porosity concept. Some conclusions were obtained for 1D unsaturated situation from the modeling: (1) dual porosity model reproduces the breakthrough curve (BTC) better than single porosity, especially the tailing part; (2) mass transfer coefficient (a) increases with flow rate; (3) mobile water portion accounts for about 70% of the water filled porosity despite of flow rate for this particular material; (4) dispersivity is getting larger with decreasing flow rate; (5) change of diffusion coefficient and mass transfer coefficient have no influence on the peak arrival time but on the spreading and peak concentration of a BTC.

The result shows that mass transfer coefficient and dispersivity are not constant under unsaturated situation. Some parameters obtained from conservative transport modeling can be used in reactive transport modeling to account for the physical processes.