Modeling of Oxygen Transport and Pyrite Oxidation in Acid Sulphate Soils

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Abstract
Drainage and aeration of previously submerged acid sulphate soils could lead to oxidation and acidification of the soil and to related problems such as high Fe and Al concentrations in the soil solution. Agricultural production on these acidified soils is low. Furthermore, leaching of acid and toxic compounds from the soil to surface waters may cause environmental damage, such as degradation of ecologically valuable wetlands. A computer simulation model was developed to compute the rate and magnitude of pyrite oxidation in pyritic deposits such as acid sulphate soils, and the resulting acidity, soluble Fe and sulphates produced. The model includes vertical gaseous O2 diffusion from the atmosphere into the soil macropores, lateral diffusion of dissolved O2 into the soil matrix, and O2 consumption inside the soil matrix by pyrite oxidation and organic matter decomposition. Oxygen consumption by pyrite oxidation was modeled by combining the equal diameter reduction model with an O2 concentration dependent rate constant for pyrite oxidation. The model was validated by comparing computations and measurements in an acid sulphate soil subjected to drainage for a period of 440 d. Computed and measured pyrite content profiles, gaseous O2 concentrations in the macropores and sulphate concentrations in the soil solution corresponded well. The present model, combined with a chemical equilibrium model and a solute transport model, can be applied to predict the (long‐term) effects of water management strategy on the quality of soils, drainage waters and surface waters in areas with pyritic deposits.

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