Estimating solute dispersion coefficients in porous media at low pore water velocities

The relationship between solute dispersion coefficient (D_mech) and porewater velocity during chloride transport in porous media with different particle characteristics at low porewater velocities (u< 1 cmmin-1), relevant for groundwater flowconditions,was investigated.Data for 29 porous media with very different particle shapes (ranging from almost spherical to very angular) and particle sizes (0.088-12mm) at different velocities (340 breakthrough curves) were used in the analyses. The analysis showed that D_mech at low u is significantly different from that at higher u. Whereas the u - D_mech relationship generally can be assumed linear at higher u (based on observations in earlier studies), the data presented in this study clearly show that this is not the case at low u. The results further indicated that D_mech at low u is strongly related to porous medium particle shape and to some degree also to medium particle size range and mean particle diameter. As the current knowledge about dispersion at low u and its dependency on porous medium properties is very limited, the results obtained in this study represent a significant addition to this understanding. A set of expressions for predicting D_mech at low u from medium properties was developed. These expressions yield good accuracy across all 29 media, thus providing a means for predicting dispersion at low u across a very wide range of media.