Sources of sediment, nutrients, pesticides and other pollutants in the Great Barrier Reef catchment
The main source of excess nutrients, fine sediments and pesticides from Great Barrier Reef catchments is diffuse source pollution from agriculture.
Estimates of river pollutant loads to the Great Barrier Reef lagoon have greatly improved since the last Consensus Statement. The results confirm that water discharged from the catchments into the lagoon continues to be of poor quality in many locations. Furthermore, enhanced modelling and monitoring of total suspended solids, nitrogen, phosphorus and photosystem II inhibiting herbicides, and provenance tracing of sediment, has significantly enhanced our knowledge of major sources and processes contributing to these river pollutant loads. The main land uses contributing pollutant loads are rangeland grazing for sediment, rangeland grazing and sugarcane for total nitrogen and total phosphorus, and sugarcane for photosystem II inhibiting herbicides. The Wet Tropics, Burdekin and Fitzroy regions contribute most to these river pollutant loads.
Summary of evidence
- Compared to pre-European conditions, modelled mean-annual river loads to the Great Barrier Reef lagoon have increased 3.2 to 5.5-fold for total suspended solids, 2.0 to 5.7-fold for total nitrogen and 2.5 to 8.9-fold for total phosphorus. However large differences in changed loads exist between rivers due to human factors; e.g. there is almost no change in loading for most pollutants in northern Cape York rivers but much greater changes in rivers in the central and southern Great Barrier Reef. Mean-annual modelled loads of photosystem II inhibiting herbicides, namely ametryn, atrazine, diuron, hexazinone, tebuthiuron and simazine, are estimated to range between 16,000 and 17,000 kilograms per year. The total pesticide load to the Great Barrier Reef lagoon is likely to be considerably larger, given that another 28 pesticides have been detected in the rivers.
- The Fitzroy and Burdekin regions contribute at least 70 per cent to the modelled total suspended solids load to the Great Barrier Reef lagoon from human activity. Grazing lands contribute over three quarters of this load. The dominant sediment supply to many rivers is from a combination of gully and streambank erosion, and subsoil erosion from hillslope rilling, rather than broadscale hillslope sheetwash erosion. Fine sediment (less than 16 micrometres) material is the fraction most likely to reach the Great Barrier Reef lagoon, and is present at high proportions in monitored total suspended solids in the Burdekin, Fitzroy, Plane, Burnett, and Normanby catchments.
- The Fitzroy, Burdekin and Wet Tropics regions contribute over 75 per cent to the modelled total nitrogen load to the Great Barrier Reef lagoon from human activity. Particulate nitrogen comprises by far the largest proportion, followed by dissolved inorganic and dissolved organic nitrogen respectively. Sediment erosion processes, particularly in grazing lands, are sources of particulate nitrogen; sugarcane, other cropping and grazing are sources of dissolved inorganic nitrogen; and land use changes in filter and buffer capacity are the main sources of dissolved organic nitrogen.
- The Fitzroy and Burdekin regions contribute approximately 55 per cent to the modelled total phosphorus load to the Great Barrier Reef lagoon from human activity. Particulate phosphorus comprises by far the largest proportion, followed by dissolved inorganic and dissolved organic phosphorus respectively. Sediment erosion processes, particularly in grazing lands, are sources of particulate phosphorus; sources of dissolved inorganic phosphorus and dissolved organic phosphorus are unclear.
- Most particulate nitrogen and phosphorus is lost or mineralised from fine sediment following delivery to the Great Barrier Reef lagoon and could be readily available for uptake in marine ecosystems.
- The Wet Tropics, Burdekin and Mackay Whitsunday regions contribute over 85 per cent of the modelled total photosystem II inhibiting herbicides load to the Great Barrier Reef lagoon from human activity. Sugarcane contributes 94 per cent of this load. Groundwater potentially may be an important source of photosystem II inhibiting herbicides (as well as dissolved nutrients) to critical near-shore ecosystems of the Great Barrier Reef lagoon; however, insufficient information is available to evaluate the risks.
- The role of modified freshwater flow regimes in driving pollutant transport and affecting reef condition, through surface water diversion, dam construction and wetland drainage and deforestation, has not been fully analysed but is important.
- Compared to diffuse sources, most contributions to suspended sediment, nutrient and pesticide loads from point sources such as intensive animal production, manufacturing and industrial processing, mining, rural and urban residences, waste treatment and disposal, ports and shipping are relatively small but could be locally, and over short-time periods, highly significant. Point sources are the major sources of pollutants such as metals, industrial chemicals and pharmaceuticals. Whilst point sources are generally regulated activities, monitoring may not include this broad range of chemicals, and monitoring and permit information is not always available. In contrast to nutrients, sediments and pesticides, there is a lack of knowledge of the risks posed by these chemicals to Great Barrier Reef ecosystems.