Advective-diffusive and hydraulic trap modeling in two and three layer soil systems

Downward and upward (hydraulic trap) advective-diffusive transport of chloride was modeled in two and three layer soil laboratory systems with Darcy velocities ranging from 3×10^-9 m/s to 5.7×10^-8 m/s. Two layer soil models simulated a compacted clayey layer over a sandy layer, underlying a landfill.  Three layer soil models simulated an unsaturated secondary leachate collection system in a landfill with overlying and underlying saturated compacted silty liners.  The effect of the hydarulic trap in minimizing diffusive downward chloride movement was investigated in both models.  The agreement between the experimental results and theoretical predictions suggests that existing solute transport theory can adequately predict chloride migration through two saturated layers of clay over sand and also three layer soil systems consisting of two saturated silt layers with an unsaturated sand drainage layer in between. The comparison of the downward and upward advective-diffusive transport in two and three layer soil models, having two different Darcy velocities and soil density, showed that the upward flow (hydraulic trap) could reduce the concentrations in the underlying receptor reservoirs in both models. The rate of the Darcy velocity (or soil density) played a controlling role in chloride movement in both systems