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TECHNICAL REPORT

Ground Water Quality

Mobility of Sulfate in Forest Soils

Kinetic Modeling

^a Agronomy and Environmental Management Dep., Sturgis Hall, Louisiana State Univ., Baton Rouge, LA 70803
^b Dep. of Ecology and Environmental Research, Swedish Univ. of Agric. Sciences, Uppsala, Sweden
^c Norwegian Forest Research Inst., Høgskoleveien, Norway

^* Corresponding author (mselim{at}agctr.lsu.edu).

Received for publication March 31, 2003. 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.

Abbreviations: CDE, convection–dispersion equation • MRM, multireaction model • PV, pore volume

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This Issue in Journal of Environmental Quality

JEQ 2004 33: 413-418. [Full Text]