Agricultural pressures on water quality:
a story of two systems

Summary

The Burdekin WQIP region contains two areas of quite different land use; the very large area of Burdekin rangelands which is dominated by cattle grazing on native pastures and the Lower Burdekin delta and floodplain where irrigated cropping is prominent. Each area has its own water quality issues and drainage pathways to the GBR lagoon (Mitchell et al. 2007).

Cattle grazing on the large, dry catchment of the Burdekin rangelands has historically involved localised tree clearing and the over-utilisation of pastures during drought conditions, while intensification of grazing pressure over the last 30 years has further accelerated the rates of soil erosion from grazing lands beyond natural levels. The localised loss of perennial grasses has led to an increased vulnerability of rangelands and riparian areas to overgrazing and soil erosion. The impacts of livestock on water quality, based largely on the science and practice review by Coughlin et al. (2007), are reported in this Chapter.

Discharge from the Burdekin rangelands proceeds, mostly, through the Burdekin Falls Dam and eventually through to the mouth of the Burdekin River and into Upstart Bay. As the Burdekin River flows across the Lower Burdekin floodplain, the high banks along its lower reaches largely exclude drainage from the irrigated cropping area flowing into the Burdekin River itself. Thus, the vast majority of sediment, nutrients and pesticides found in the Burdekin River discharge are sourced from the Burdekin rangelands.

The Burdekin River discharge is dominated by very high levels of fine suspended sediment and particulate nutrients, derived from grazing-related erosion over this very large catchment. While particulate material, as both suspended sediment and particulate nutrients, is the dominant export from the Burdekin River catchment, the total load of dissolved inorganic nitrogen (DIN) is substantial. A large proportion of the sediment and particulate nutrients that are delivered to the Burdekin Falls Dam (BFD) from the four upstream basins (Belyando, Cape Campaspe, Suttor and Upper Burdekin) are trapped by the dam. However, the trapping efficiency of the BFD varies greatly between years.

Data show a consistently greater contribution of sediment from the Upper Burdekin Basin to the total end-of-Burdekin catchment load than from the other above-dam basins. A disproportionately high sediment contribution to the end-of-Burdekin catchment load from the Bowen Broken sub-basin and other below-dam subcatchments is also apparent.

The development and expansion of the Lower Burdekin irrigated cropping area has produced a number of local farming systems that are distinctly different in features such as scale, soil properties, on-farm cultural practices and related environmental issues. Comparison between Delta and Burdekin River Irrigation Area (BRIA) cane farms are particularly noteworthy in this regard; recommended farming practices in one cane growing district may offer minimal or even counterproductive environmental or productivity benefits in another region, or even on other farms within the same district. Release water from the BFD for irrigation is distributed widely throughout the BRIA by a complex system of built and natural channels, while most tail-water discharge occurs through the Haughton River and Barratta Creek system into Bowling Green Bay. In contrast to the BRIA, irrigation water in the Delta area is largely sourced from groundwater, while tail-water discharge flows directly into Upstart Bay via several small creeks and subsurface flow.

The hydrology of the Lower Burdekin irrigated cropping area is characterised by surface waters that largely drain away from the Burdekin River and into a large number of rivers and creeks that drain directly into Bowling Green and Upstart Bays. Deep drainage to the underlying aquifer is another significant characteristic of the Lower Burdekin irrigated cropping area. Nutrient concentrations, mostly as dissolved inorganic nitrogen (nitrate, nitrite and ammonia or DIN), in surface water discharge from the Lower Burdekin irrigated cropping area is greatly affected by fertiliser-additive land use, which is dominated by sugar cane.

A suite of herbicides have also been commonly detected in the waterways of the Lower Burdekin. Most elevated were concentrations of atrazine, diuron, ametryn and 2,4-D in freshwater samples; all four of which exceeded ANZECC Guidelines in many samples. These pesticides were also the most significant with respect to the total load exported from the Lower Burdekin to the GBR lagoon.

Comparative data between the two major land uses (irrigated cropping and grazing) in the Burdekin WQIP region are presented and discussed below; these data illustrate the different contributions and water quality issues associated with different land uses. While dissolved inorganic nitrogen concentrations and, probably, loads leaving Lower Burdekin irrigated agricultural lands from additive fertiliser use are higher than from the Burdekin rangelands, total load contribution from grazing land is comparable with many drainage systems containing intensive agriculture in neighbouring regions.