This paper assesses the relative contribution of point sources, mainly the effluent of the Orange Sewerage Treatment Plant (STP), and diffuses sources from rural and urban areas on the overall quality of stormwater from Orange urban catchment. A comprehensive water quality-monitoring program was carried out between November 1998 and March 2000. Results show that the urban creeks are enriched with total phosphorus, total nitrogen and ammonia. Nutrient level in the creeks exceeded the recommended level even before reaching the central business district (CBD). Provided that other factors are conducive, phosphorus and nitrogen enrichment in water can induce higher plant growth in the aquatic system, including toxic cynobacterial blooms. The stormwater quality was flow dependent as the bulk of pollutant loads were delivered during storm events. The diffuse sources contributed 63% of the total annual nutrient load discharged by the Blackman's Swamp Creek catchment at the monitoring site (Site 1) whereas the Sewerage Treatment Plant (STP) contributed less than 40% of the total annual load. However, during the drier months (December-April) when 'rainfall-runoff is low, the contribution of STP makes up to 80% of total catchment load. In the contrary, this point source contribution declines to 20% of the nutrient loads during storm events. The variability of rainfall-runoff and subsequent impact of point and diffuse sources on water quality need to be established in order to predict pollutant export under different conditions. Such information is critical to the development of sustainable and cost-effective management strategies to regulate stormwater pollution.
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