Pesticide dynamics in the Great Barrier Reef and its catchments

RRRD037 cover

Pesticide runoff from agricultural lands has been recognised as a serious threat to the health and productivity of the Great Barrier Reef (GBR) and is considered a priority pollutant for management in the GBR catchment area. Pesticide residues have been detected in marine waters, sediments, seagrass meadows and in freshwater plumes in the GBR lagoon. Water quality targets have been introduced to reduce the runoff of pesticides with the aim of improving ecosystem resilience in the GBR, although it is not known if the adoption of best management practices in agriculture will meet these immediate targets. This project fostered collaborations between research scientists, growers and industry stakeholders to address the major knowledge gaps in pesticide dynamics in the GBR catchment area. The project package incorporated a range of key science providers with expertise in the management, transport and fate of pesticides and the projects were conducted across several regions of the GBR catchment area.

The projects examined sites within the main agricultural lands in the Great Barrier Reef catchment area including sugar cane, bananas, grazing and broadacre crops. The collected work from this research package on pesticides in the Great Barrier Reef has delivered key outcomes relating to our understanding of their properties (i.e. half-lives, partitioning between dissolved and particulate phases); their potential to be lost in surface runoff and deep drainage; their ability to be managed on farm to reduce losses and their overall risk in the Great Barrier Reef catchment and lagoon. Collectively, the outcomes allow improved modelling of several pesticides used in the Great Barrier Reef under the Paddock to Reef Integrated Monitoring, Modelling and Reporting Program. The overall summary of these projects is provided here.

The project was led by Dr Stephen Lewis and Jon Brodie from TropWater, James Cook University and was a collaborative effort between the James Cook University, Queensland Department of Natural Resources and Mines, CSIRO, Queensland Department of Agriculture, Fisheries and Forestry, the Queensland Department of Science, Information Technology, Innovation and the Arts, Australian Institute of Marine Science, Entox, University of Queensland, Sugar Research Australia and University of Southern Queensland.

Summary of project outcomes

A trial conducted on an irrigated sugar paddocks in the lower Burdekin showed that diuron and atrazine losses in drainage water could be reduced by 90% when applied to the raised beds via the banded spraying technique. Previous studies using rainfall simulation methods suggested that losses of PSII herbicides in drainage water could be reduced by 50-60% when banded spraying was applied in rain-fed sugar cane systems. Further, spot spray trials showed that the losses of herbicides from a paddock reduced in proportion to the area sprayed. The analysis suggested that if banded spraying technique was applied across the Great Barrier Reef catchment then ecological protection guidelines for PSII herbicides in the Great Barrier Reef lagoon would unlikely be exceeded at any time. An assessment of relative risk between the current PSII herbicides, knockdown herbicides and the alternative herbicides was also performed. Provided that the alternative herbicides (used for residual control in place of diuron) are applied via a banded method, preliminary data suggest that these alternatives pose no more of a risk than diuron. A report led by Steven Lewis has been published and can be downloaded here.

Examination of the movement of tebuthiuron in runoff and in soil from Brigalow lands showed that while no runoff event lost > 0.5% of the total applied tebuthiuron, mean concentrations in runoff up to 472 days after application always exceeded the current ecological protection guidelines (95% guideline = 2.2 µg.L-1). These data also indicate that tebuthiuron was largely transported in the dissolved phase. The research at the plot scale indicated how tebuthiuron moved through the soil profile driven by rainfall, with consistent decrease in soil concentrations from ~98 days after application. A report by Craig Thornton and Amanda Elledge can be found here.

The partitioning of dissolved and particle pesticide residues in runoff were examined at a range of spatial scales. At the plot scale, pesticides with high mobility properties were largely transported in the dissolved phase while those with relatively low mobility were transported predominately attached to particulate matter. The findings at the larger sub-catchment and catchment scale sites showed that the pesticides detected were mainly transported in the dissolved phase. These results will allow improved modelling of pesticides throughout the Great Barrier Reef catchment and have provided valuable information to predict the fate of pesticides in the Great Barrier Reef lagoon. A report by Bob Packett can be downloaded here.

The surface runoff and deep drainage loss potential of three pesticides commonly used in the banana industry were examined at a banana plot at the South Johnstone Research Station over three years. The findings showed that the two herbicides analysed (glyphosate and glufosinate) could be detected in surface runoff, and glyposhate was also occasionally detected in deep drainage. Glyphosate could also be detected in both surface runoff and deep drainage following applications from 6 months earlier. The fungicide mancozeb was below detection limits throughout the monitoring program in both surface runoff and deep drainage samples. This research provided valuable data on the runoff potential of pesticides used in the banana industry and an indication of their offsite risk to receiving waters.

A number of studies were also conducted to improve modelling of several pesticides used in the GBR catchments under the Paddock to Reef Monitoring, Modelling and Reporting Program. Investigation of half-lives of herbicides in soils showed considerable variation between soil type, and showed that some herbicides had half-lives much longer or much shorter than values reported in the literature. Half-lives were found to range from 12 days for atrazine to >744 days for paraquat. A report led by Melanie Shaw can be downloaded here.

Measurements of the degradation half-lives of 8 common herbicides (the 6 common PSIIs, as well as metolachlor and 2,4-D) in seawater indicated that these herbicides have half-lives ranging from months to years, which explains the findings from monitoring programs that have detected herbicides in the Great Barrier Reef throughout the year. Furthermore the results show there is little potential for herbicide degradation in flood plumes that typically occur over a few weeks of the year. This finding supports the application of a simple linear relationship in risk modelling where herbicide concentrations measured at the end of rivers can be 'projected' into the GBR lagoon assuming dilution with seawater mixing. A report led by Andrew Negri can be downloaded here.

Continuous monitoring of pesticide residues in the Barratta Creek drainage system provided valuable data on the spatial and temporal trends in pesticide usage, exposure of pesticides in the creek over the long term and responses to the regulation of diuron that occurred during the monitoring program. While concentrations of diuron and atrazine residues decreased as they moved towards and into the estuary, they remained above ecological guideline values for approximately 6 months of the year even at the Ramsar wetland/estuarine site. The findings suggest that the second regulation of diuron (i.e. a no spray window) was much more effective than the first regulation (i.e. appeared that diuron could have been banned altogether). A report led by Dominique O'Brien can be found here.

Read more about these projects on the pages for RRRD037 and RRRD038.