There are studies that look at the effect of a pesticide on ecosystems, but not many studies have been done on the mixtures of pesticides that can pollute ecosystems. This study looks at 2600 samples, up to 47 pesticides, and 15 waterways that all flow into the Great Barrier Reef. 99.8% of samples contained pesticides. 80% of samples contained anywhere from 2 to 20 different pesticides. The use of land affected the types of pesticide in the samples, and land used to grow sugar cane had mixtures of the most pesticides.
This study aims to identify the mixtures of pesticides that are flowing into the Great Barrier Reef.
Studies on pesticides typically individually report the risk of each one that was identified. This method does not truly report the danger of the pesticide to the ecosystem. The Great Barrier Reef has many stressors causing the ecosystem to decline by over 50% in the last 30 years. Identifying the mixtures of pesticides and where they are coming from can help to assess risk and improve water quality of the Great Barrier Reef. The land that drains into the Great Barrier Reef is mostly used for agriculture, and has highly variable weather conditions.
Results and Discussion
Frequency of pesticide and pesticide mixture occurrence
Out of 2618 samples, only 4 did not contain pesticides and only one contained a single pesticide. The rest contained a detectable mixture of pesticides. DEET, imidacloprid metabolites, triclopyr, and glyphosate were the only pesticides found more in the dry season than the wet season, and the rest of the 40 some pesticides tested were more common in the wet season. The frequency of pesticides was overall higher for the wet season, but pesticides were still present even in the dry season. Different studies seem to report various data on pesticide contamination of surface water and groundwater, but not enough studies have been done on groundwater to clarify any of these conflicting results.
Frequency of pesticides individually and in the mixtures
The five most common pesticides were atrazine, diuron, imidacloprid, hexazinone and 2,4-D. These made up over 50% of the pesticides found.
Number of pesticides in mixtures
The number of pesticides in each sample was anywhere for 0 to 20. Other studies had found up to 71 pesticides in a sample. The difference is likely due to different sampling techniques.
Frequency of various modes of action occurring in the pesticide mixtures
There are different ways pesticides work, usually by blocking the action of an important enzyme. This study recorded 12 different types of pesticides when categorized by how they work. Most samples contained 2 to 4 types of pesticides by function. Pesticides can cause detrimental effects, especially when given at small doses for a long period of time. Mixtures can be even worse for the ecosystem. Even though this study takes into the mixes of pesticides at different concentrations, it will likely underestimate the real risk of the Great Barrier Reef. This study only tested for 47 pesticides, and the collection methods may have been less effective than others.
Spatial variation in the number of pesticides per sample
There are high levels of variation in pesticides found in the samples. There is no north to south trend in pesticide concentration, which is unexpected since there tends to be more agriculture in the southern samples.
Land use based classification of the monitored catchments
The types of land use in this study were conservation, cropping, forestry, grazing, horticulture, and sugar cane. Most sampling sites were classified under more than one type of land use.
Land use based variation in the number of pesticides per sample
Grazing had an average of 3 pesticides per sample. Conservation and forestry had less pesticides per sample, which makes sense because pesticides would not be used on a conservation area and would be used less frequently in forestry. When there was also sugar cane as a land use, the pesticides per sample tripled or quadrupled. Sugar cane significantly increased the pesticides found per sample. This is consistent with previous studies.
Water samples were collected multiple times in 2009/2010 and 2011/2020 over the dry and wet seasons, and depending on the water conditions. Six to ten samples were collected during each specific condition in order to increase accuracy. High performance liquid chromatography and mass spectrometry were used to analyze the samples. The land was categorized by its major uses, like conservation, cropping, forestry, grazing, horticulture, and sugar cane.
Previous studies have underestimated the effects of pesticides by not considering the effect of mixing pesticides. This study shows the high occurrence of mixed pesticides in water that flows into the Great Barrier Reef.