MODELING OF OXYGEN-TRANSPORT AND PYRITE OXIDATION IN ACID SULFATE SOILS

Drainage and aeration of previously submerged acid sulphate soils coul d lead to oxidation and acidification of the soil and to related probl ems such as high Fe and Al concentrations in the soil solution. Agricu ltural production on these acidified soils is low. Furthermore, leachi ng of acid and toxic compounds from the soil to surface waters may cau se environmental damage, such as degradation of ecologically valuable wetlands. A computer simulation model was developed to compute the rat e and magnitude of pyrite oxidation in pyritic deposits such as acid s ulphate soils, and the resulting acidity, soluble Fe and sulphates pro duced. The model includes vertical gaseous O2 diffusion from the atmos phere 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 pyri te oxidation was modeled by combining the equal diameter reduction mod el with an O2 concentration dependent rate constant for pyrite oxidati on. 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 concentrati ons in the macropores and sulphate concentrations in the soil solution corresponded well. The present model, combined with a chemical equili brium model and a solute transport model, can be applied to predict th e (long-term) effects of water management strategy on the quality of s oils, drainage waters and surface waters in areas with pyritic deposit s.