Key pollutants and sources
4.1 Regional context
Current estimates of the average annual total suspended sediment ( TSS ) load from the Burdekin region range between 3.2 and 4.0 million tonnes ( Dougall et al . 2014 ; Kroon et al . 2012 ; Kuhnert et al . 2012 ), with the most recent estimate of 3.8 million tonnes per year from the Source Catchments modelling ( McCloskey et al . 2017b ). Using this estimate , the Burdekin NRM region contributes the highest sediment loads to the GBR ( 39 % of the total GBR TSS load ). A large proportion ( 87 %) of this load is from the Burdekin River basin itself ( 3,222 kilotonnes annually ). These loads are estimated to be 5 to 10 times above pre- European loads ( Lewis et al . 2014 ; Kroon et al . 2012 ; Dougall et al . 2014 ; McCloskey et al . 2017b ; Waters et al . 2014 ).
The relative contribution of different land uses in the Burdekin region to TSS export is shown in the panels in Figure 13 ( data derived from C . Dougall DNRM ). Grazing is the greatest contributor of TSS loads exported from the Burdekin WQIP sub-catchments ( 72 %). All other land uses contribute less than 1 % of the regional TSS load , except for conservation areas ( 9 %) which contain gully features and may have been impacted from land uses prior to protection . However , some areas , such as urban development sites can be local high impact sources of suspended sediment .
It is now clear from a range of monitoring and modelling that the bulk of fine sediment delivered from the Burdekin basin to the GBR is derived from a small proportion of the basin area , primarily within the Bowen ( 43 %) and Upper Burdekin catchments ( 27 %) ( Figure 13 ), with a large proportion of this load contributed from grazing lands , the dominant land use . This is supported by a variety of studies and illustrated in Figure 14 . The key points are :
• Tracing data suggest that the Upper Burdekin and Bowen catchments have the highest rates of accelerated erosion relative to pre-European rates at 3.6 and 7.5 times , respectively ( Bartley et al . 2015 ).
• The Burdekin Falls Dam traps a large proportion of coarse sediment (> 63 µ m ), about 50 % of the coarse silt fraction ( 16-63 µ m ) and 30 % of the fine clay fraction (< 4 µ m ). The selective trapping efficiency of the Burdekin Falls Dam also highlighted that the < 16 µ m fraction became the dominant sediment particle size transported through the impoundment to end of river export ( Bainbridge et al . 2016 ; Lewis et al . 2013 ).
• The Upper Burdekin and Bowen catchments also contribute the highest loads of the < 4 µ m , < 16 µ m and < 63 µ m sediment fractions to the end-of-catchment . Importantly both the clay mineralogy and geochemical tracing data agree with the sediment budgets constructed for the catchment and highlight these two dominant sources ( Bowen and Upper Burdekin ) to the end‐of‐catchment sediment loads ( Bainbridge et al . 2014 , 2016 ; Furuichi et al . 2016 ).
• The Bowen River and other coastal streams in the region contribute more fine sediment per hectare than catchments upstream of the Burdekin Falls Dam ( Bainbridge et al . 2014 ; Wilkinson et al . 2013 ).
- 24 -