Current best management practice in agriculture often involves reducing mechanical soil disturbance and increased use of chemicals to control weed growth. However, offsite transport of herbicides to the marine environment can impact on non-target species and ecosystems (Haynes et al. 2000; Jones et al 2003). In response to declining water quality in the Great Barrier Reef (GBR) lagoon the Reef Water Quality Protection Plan has been implemented (Reef Water Quality Protection Plan Secretariat, 2009). Major components of the plan are the GBR catchment event monitoring and modelling programs (Joo et al., 2012). The monitoring program does not currently differentiate between dissolved and particulate transport of herbicides and the modelling program is constrained by a lack of data for simulating exports of particulate bound fractions of herbicides to the reef lagoon.
A range of pesticides are used on agricultural lands in the Central Queensland region which includes the Fitzroy, Burdekin and Mackay Whitsunday, Great Barrier Reef catchments (Bainbridge et al., 2009a). The objectives of this project were to collect and analyse water runoff samples from agricultural lands at various scales to determine the percentage of herbicides transported in the dissolved and particulate phase. Sampling was undertaken at a range of scales from rainfall simulator plots (<3m2), farm, sub catchment and catchment (14,700,000 ha) scale. Water samples were collected during 2012 and 2013 specifically for partitioning analysis. Thirty six samples were collected from rain simulation trials on sugar cane plots in the Burdekin and Mackay Whitsunday catchments, and from rainfall simulation trials on grazing plots in the Fitzroy Catchment. Fifty eight runoff event samples were collected at farm, sub catchment and whole of catchment scales from cropping and grazing lands in the Fitzroy basin. Samples were analysed to determine the types of chemicals transported and the percentage of herbicide concentration being transported in the dissolved and particulate phase.
Results from rain simulation trials were in general agreement with data from the literature regarding mobility, a function of solubility and partitioning properties, for common herbicides. Those with low movement ratings tended to move off site with substantial percentages in the particulate phase, while those with high movement ratings were transported mainly in the dissolved phase. Catchment scale runoff events produced by natural rainfall resulted in minor particulate phase transport and major dissolved phase transport. The information gained will assist land managers to make informed decisions about the potential for offsite transport of different herbicide selections. The data produced by this study should also assist catchment modellers and model developers to more accurately represent the transport of herbicides from paddock scale through to the GBR lagoon. Future studies could investigate methods for collecting larger amounts of suspended sediments from flood waters for analysis. Filtering of larger (known) volumes of flood water would provide sufficient amounts of suspended sediments to help increase confidence in results from laboratory analysis. Further studies on half-lives and partitioning of chemicals in a range of soil types under controlled conditions are also suggested. This would allow for increased knowledge of in-situ partitioning of chemicals over time and the potential risk for runoff transport.