Pollution of groundwater, wetlands, rivers, estuaries and near shore waters by phosphorus is now fairly common due to run-off from agricultural areas and wastewater discharges. In the application of fertilisers in agriculture it has been observed that sandy soils result in high phosphorus concentrations in the run-off. On the other hand loamy soils result in less phosphorus run-off. Phosphate-phosphorus sorption by soils has been observed to be time dependent. A model has been developed to describe the movement of phosphorus through soils to take into account the processes of convection, dispersion and time-dependent sorption. The model enables prediction of phosphorus breakthrough in a soil column. A comparison is made of predicted breakthrough curves with results obtained using two types of soil: a sandy soil from Australia and a loamy soil from Indonesia. The model has direct application to field situations where phosphate-phosphorus moves vertically downward through the unsaturated zone to the water table, and horizontally through the groundwater aquifer. Parameters of the model can potentially be derived from simple batch sorption experiments.

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