Contribution of diffuse sources to the sediment and phosphorus budgets in Ben Chifley Catchment, Australia

Relative contribution of different diffuse sources to the sediment and phosphorous (P) budgets in the Ben Chifley Reservoir was estimated using a multivariate mixing model and geochemical indices and tracers. The influence of the underlying geology on land use characteristics and the dynamics of sediment and P in the catchment were discussed. Sediments currently in transport to the reservoir are derived mainly from areas underlain by Quaternary alluvium, accounting for 33% of the reservoir sediment budget. A major portion of the sediments from the Quaternary alluvium originates from pasture land which also contributes about 60% of sediment-bound P in the reservoir, followed by cropland (30%) and forest area (10%). Phosphorus-based fertiliser contributes more than 50% of the soil P in pasture and cropland and more than 60% in forest plantation area. The remaining P reaching the reservoir originates from natural soils (native sources). Phosphate from both fertiliser and native sources are transported to the water courses and ultimately the reservoir as a consequence of gully, stream bank and surface erosion. Erosion control, trapping of fertiliser runoff, establishing riparian vegetation and reducing application of P-based fertiliser by increasing native pasture cultivation in the risk areas should be considered as management options for controlling the impact of P and sediment in the reservoir.