Nutrients and Suspended Solids

During dry weather conditions, concentrations of nitrogen, phosphorous and suspended solids are generally low, indicating high water quality. There are exceptions, however such as nitrate concentrates at Scheu Creek but these are still well below the upper limit of 10 mg/L recommended for drinking water.

Suspended solids (sediment) concentrations increase markedly when stream flows increase following rainfall and runoff events. During cyclone Sadie in 1994, for example, 200,000 tonnes of sediment were discharged from the river system into the Great Barrier Reef lagoon, carrying 85% of the total nitrogen load discharged in this single event

Pesticides

Prior to undertaking a 'snapshot' survey of pesticides in streams and groundwaters in the catchment, pesticide usage by major industries in the catchment was evaluated. The sampling time (December 1995) was selected as representing a time of major pesticide use, when periodic rainfall would increase the potential for pesticide movement in the catchment. Coincidentally, it was also the time of heavy applications of insecticides used in conjunction with the papaya fruit fly outbreak.

Samples from 23 stream sites and 16 bores were tested for a wide range of pesticide residues, with very few pesticide residues detected. Seven samples (3 stream 4 groundwater) contained very low concentrations of atrazine, while 20 samples (12 stream and 8 groundwater) contained low levels of 2,4-D.

Contaminants in Fauna

Between 1990 and 1992, a variety of aquatic fauna from freshwater and estuarine sites were analysed for pesticide residues and heavy metals. Again, results indicated little evidence of significant pesticide contamination, although several samples contained trace levels of organochlorine, phenoxyacid or atrazine residues. Similarly, most heavy metals detected were well below the maximum permitted concentrations set by the Australian Food Standards Code.