Natural levels of inorganic phosphate in soils of Rottnest Island are quite high: about 300 mg/kg as P (or about 4 t of P per ha per meter depth of soil). In comparison, the production of phosphorus in wastewater from sewage, treated on Rottnest Island, amounts to approximately 2 t per year. The phosphate, occurring in the soil naturally, is mainly in mineral form and not very soluble. Solution concentrations in the soils are less than 0.001 mg/L P-PO4 (at pH values of 8.5-8.9). Conditions in the calcareous soils of Rottnest Island are favourable for precipitation of phosphate as calcium phosphates. Theoretically >>99% of phosphate in wastewater from treated sewage can be stored indefinitely as hydroxy-, fluoro- and chloro-apatites. In practice, however, application of wastewater to these soils will result in a (kinetically defined) finite concentration of phosphate to move through the soil slowly as a sharp front. The effective width of the frontal zone was taken to be equal to the longitudinal hydrodynamic dispersivity. Mobilities, relative to water, of fronts resulting from step increases of phosphate in soils were then calculated with an experimentally obtained, time-dependent, adsorption equation and the average residence time of phosphate in the frontal zone. Calculated mobilities were verified experimentally by leaching phosphate through small columns of soil at different concentrations of phosphate and rates of infiltration. For concentrations in wastewater between 10 and 15 mg/L P-PO4, mobilities of phosphate, relative to water, in soils of Rottnest Island are less than 2% for expected infiltration rates of wastewater between 0.5 and 1 cm/day.
