INTERPRETATION OF 2-REGION MODEL PARAMETERS

Apart from advection and diffusion/dispersion, other physical and chem ical processes can affect the movement of solute through a porous medi um. The classical advection-dispersion equation does not usually model adequately the breakthrough curves resulting from such effects. The t wo-region model (TRM) is an attempt to model succinctly and easily the effects of physical and/or chemical nonequilibrium. Physical nonequil ibrium is the focus of this paper. The additional parameters appearing in the TRM are the ratio of mobile to total pore fluid, beta, and the apparent transfer rate of the solute between the mobile and the immob ile regions, alpha. Meaning is ascribed to these parameters by identif ying the various ways in which physical nonequilibrium can arise. An e xamination of published data shows that the dominant trend is a linear variation of the transfer rate alpha with mobile fluid velocity V-m. In order to develop an approach incorporating all porous media types, timescales of solute transport are identified and compared to the mass transfer timescale (1/alpha). It was found that the local advection t imescale best characterizes the mass transfer timescale. Two trends we re observed for the mobile water fraction beta. For aggregated/saturat ed porous media, beta was found to be constant or decreased with incre asing pore water velocity, while for partially saturated soils, beta w as constant or increased with increasing moisture content.