During high rainfall or river flow events, gully erosion causes sediment and nutrients to wash out to the Great Barrier Reef. Excess sediment and nutrients means poor water quality, impacting the health of the Reef (read our explainer here).

Fixing these eroding gullies and stopping sediment and nutrient run-off at the source is therefore a priority for our community and government.

Gully remediation projects in the Reef catchments – like those Greening Australia undertakes – reshape massive eroding gullies and stabilise the soil with techniques such as revegetation to reduce sediment. But do these projects also reduce nutrient export? The aim of this project was to understand nutrient export from different remediated gullies to identify the best remediation techniques for overall improvements to water quality.

Researchers from Griffith University and TropWATER at James Cook University assessing water quality sampling equipment.

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The project team from Griffith University, Queensland Department of Environment and Science, TropWATER at James Cook University, and Greening Australia collected samples across four wet seasons and found that using certain remediation materials when fixing eroding gullies can reduce nutrient run-off.

Materials that have high Carbon to Nitrogen (C/N) ratios, such as bagasse or rock capping, can reduce nutrient run-off from fixed gullies. By contrast materials with low C/N ratios, such as Rhodes grass and sorghum hay mulch, can increase nutrient run-off as they decompose.

Locally sourced soils (cracking clay) can be used; however, it is critical to maintain groundcover and reduce grazing pressure to prevent the imported soil and associated nutrients from being mobilised during high rainfall.

Lessons learned and next steps:

• Monitoring nutrients in gullies is difficult due to their commonly remote locations, and the need to use refrigerated autosamplers and recover the samples for filtering in less than 48 hours. Nonetheless, it is necessary to monitor remediated gullies for at least 2-3 wet seasons to get an initial understanding of relationships and effects on particulate and dissolved nutrients.
• Collecting water samples from remediated gullies is challenging since they do not have a concentrated outflow of water, so installing a flume at the gully outlet to concentrate the flow can increase the chance of getting a viable sample. However, the flume must be removed and any damaged area repaired at the end of the project.
• The research raised more questions for investigation about the long-term nutrient processes and changes in these systems. For example, monitoring of un-incised (non-gullied) drainage swales should be undertaken as proxies for the pre-incision landforms to establish the range of Nitrogen loads that might be expected under baseline conditions under fully grassed drainage swales.

For more detailed information on findings and next steps, please refer to the technical report.

Researchers from TropWATER accessed the site by helicopter to collect water samples after a rain flow event for real-time nutrient assessments. Photo credit: Steve Lewis, TropWATER.

This project was part of the $10 million Innovation and Systems Change Program funded by the Great Barrier Reef Foundation’s partnership with the Australian Government’s Reef Trust.

The project built on previous water quality research and gully remediation projects including Strathalbyn Phase Three (funded by the Great Barrier Reef Foundation) and the Innovative Gully Remediation Project (funded through the Queensland Government’s Reef Water Quality Program).