The effective management of acid sulfate or pyritic soils is a major issue for many coastal regions in Australia. Drainage and subsequent aeration of potential acid sulfate soils often leads to pyrite oxidation and the acidification of the soil and groundwater. A numerical model has been developed to calculate the rate and magnitude of pyrite oxidation in acid sulfate soils, and the distribution of oxidation products such as H+, SO4 2- and Fe3+ within the soil profile. The pyrite oxidation model includes vertical diffusion of oxygen from the atmosphere through soil macropores, lateral diffusion of dissolved oxygen from the macropores into the soil matrix, and the consumption of dissolved oxygen in the acid sulfate soil layers by pyrite oxidation. The model developed by the authors is used in conjunction with a commercially available water flow model which is used to simulate the groundwater and soil moisture regime in a three dimensional space. The model can be used to assess the effectiveness of different acid sulfate soils management strategies. The acidity generated by various drain management strategies is demonstrated.