Balancing phosphorus adsorption and consumption processes in experimental treatment ponds for agricultural drainage water

The paper discusses results from a study of phosphorus (P) forms and dynamics in the sediments of experimental treatment ponds before and after inundation. Five experimental ponds were constructed to treat irrigation drainage from pastureland adjacent to the Murray River in South Australia. The experimental ponds differed in the use of macrophytes and soil sources. A sequential extraction scheme was carried out to identify the different P forms in sediments and investigate changes of P pools over time. The sediments were also subjected to P adsorption and release experiments to study the impact of water plants on P adsorption characteristics. Results showed that: (1) the emergent water plant system had enhanced sediments P adsorption ability indicated by a decrease in P saturation and an increase in P sorption index (PSI); (2) the dense plant cover inhibited other important P removal mechanisms such as uptake by periphytes and co-precipitation; and (3) in ponds with submergent water plants, there was no improvement of sediment P adsorption, but P co-precipitation was encouraged. Based on the findings of this study, maintaining a moderate density of emergent plants was recommended to optimise the overall performance of a treatment pond system.