Effect of water content on solute transport in a porous medium containing reactive micro-aggregates

Abstract

The water content of porous media may substantially affect the transport behaviour of conservative and sorbing solutes. Physical processes potentially involved include alterations of the flow velocities, flow patterns, or of accessible surface sites. We performed column experiments using a synthetic porous medium, in which a substantial part of the sorption sites was concentrated in regions within small grained aggregates that were accessible only by diffusion, a feature often found in natural soils and sediments. We investigated the transport of solutes exhibiting very different sorption characteristics under steady state conditions at different water contents of the porous medium. The tracers used were either nonreactive, partitioned into organic matter or sorbed specifically and nonlinearly to clay minerals. Hydrodynamic dispersion generally increased with decreasing water content, reflected by the breakthrough curves (BTCs) of conservative and only slightly sorbing tracers, which exhibited stronger spreading and early breakthrough of the fronts at lower water saturation. Nonlinear sorption and nonequilibrium mass transfer between the mobile region and the immobile water present within the aggregates dominated the BTCs of the strongly sorbing tracer at all degrees of water saturation, and, thus, rendered the effects of increased hydrodynamic dispersion negligible. Due to a relative increase in the ratio of sorption sites per water volume, the retardation of this tracer distinctly increased at low water contents of the porous medium. Solute transport of all tracers was successfully simulated with an advective–dispersive transport model that considered the respective sorption behaviour and retarded intra-aggregate diffusion as predominant processes. All parameter values of the model had been determined previously in independent experiments under completely saturated conditions. Our results demonstrate that a well parameterised transport model that was calibrated under completely saturated conditions was able to describe rate-limited advective-dispersive transport of reactive solutes also under unsaturated steady-state conditions. Enhanced relative retardation of strongly sorbing compounds under such conditions is likely to affect biological and chemical transformation processes of these compounds.