EFFECT OF WATER-CONTENT ON SOLUTE TRANSPORT IN A POROUS-MEDIUM CONTAINING REACTIVE MICROAGGREGATES

The water content of porous media may substantially affect the transpo rt behaviour of conservative and sorbing solutes. Physical processes p otentially involved include alterations of the flow velocities, flow p atterns, or of accessible surface sites. We performed column experimen ts using a synthetic porous medium, in which a substantial part of the sorption sites was concentrated in regions within small grained aggre gates 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 trac ers used were either nonreactive, partitioned into organic matter or s orbed specifically and nonlinearly to clay minerals. Hydrodynamic disp ersion generally increased with decreasing water content, reflected by the breakthrough curves (BTCs) of conservative and only slightly sorb ing tracers, which exhibited stronger spreading and early breakthrough of the fronts at lower water saturation. Nonlinear sorption and noneq uilibrium mass transfer between the mobile region and the immobile wat er present within the aggregates dominated the BTCs of the strongly so rbing tracer at all degrees of water saturation, and, thus, rendered t he effects of increased hydrodynamic dispersion negligible. Due to a r elative 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 considere d the respective sorption behaviour and retarded intra-aggregate diffu sion as predominant processes. All parameter values of the model had b een determined previously in independent experiments under completely saturated conditions. Our results demonstrate that a well parameterise d transport model that was calibrated under completely saturated condi tions was able to describe rate-limited advective-dispersive transport of reactive solutes also under unsaturated steady-state conditions. E nhanced relative retardation of strongly sorbing compounds under such conditions is likely to affect biological and chemical transformation processes of these compounds. (C) 1998 Elsevier Science B.V. All right s reserved.