Dispersion and advection in unsaturated porous media enhanced by anion exclusion

IT has been observed^1-4 that the average transport velocity of dissolved anions through soils may be larger than that of the accompanying water molecules, owing to electrostatic repulsion by negatively charged solid surfaces, which forces the anions into pore centres where the velocity is faster. This phenomenon, known as anion exclusion, has been explained by diffusive double-layer theory^5-7. Here we present analyses and numerical modelling of concentration/depth profiles of tritium, chloride and sulphate which were collected from irrigated land in the Israeli coastal plain. We found that the anions travelled at about twice the velocity of tritium. The behaviour of tritium is consistent with advective-diffusive transport, but the values of the dispersion coefficients associated with anion transport greatly exceeded the values expec-ted for molecular diffusion in a porous medium, and were ∼30 times those found for tritium transport. Our results indicate that anion exclusion restricts the number of active pore networks avail-able for anion transport. We present two conceptual models that can explain the observed results - in one model some porous regions are completely blocked, whereas in the other they are only partially blocked.