Mobility of sulfate in forest soils: kinetic modeling

Understanding sulfate transport and retention dynamics in forest soils is a prerequisite in predicting SO4 concentration in the soil solution and in lake and stream waters. In this study forest soil samples from the Gårdsjön catchments, Sweden, were used to study SO4 transport in soil columns from the upper three soil horizons (E, Bs, and BC). The columns were leached using a sequential leaching technique. The input solutions were CaSO4 equilibrated with forest floor material. Leaching behavior of SO4 and concentration in the effluent were measured from columns from individual horizons. Sulfate was always retained in the Bs and BC horizons, while the pattern for the E horizon varied. Attempts were also made to model SO4 breakthrough results based on miscible displacement approaches and solute convection-dispersion equation (CDE) in porous media. Several retention mechanisms were incorporated into the CDE to account for possible reversible and irreversible SO4 reactions in individual soil layers. Our modeling efforts were inadequate in describing the mobility of SO4 in the top (E) horizon. Moreover, a linear equilibrium approach was generally inadequate for describing SO4 sorption during transport in the Bs and BC horizons. In contrast, we found that the model provided good descriptions of all breakthrough results when SO4 reactivity was accounted for based on nonlinear equilibrium or first-order kinetic processes. Moreover, based on model parameter estimates, the reactivity or retention of SO4 during transport is concentration dependent. We conclude that sulfate retention during transport in this forest soil is most likely controlled by kinetic reactivity of SO4 of the reversible and irreversible mechanisms.