Is Metolachlor the Biggest Pesticide Risk to Victorian Waterways?
Friends of the Earth Melbourne
Key Facts:Metolachlor is a herbicide used to control a variety of weeds.
Metolachlor's ecological guideline is set at 36,000 times lower than the Drinking Water Guideline.
Metolachlor was detected 110 times, in 30 water supplies by water authorities between 2017-2025.
Metolachlor hotspots include the Yarra River at Melbourne Water's offtake to Sugarloaf Reservoir, Talbot Reservoir, Evansford Reservoir, Tullaroop Reservoir and Centenary Reservoir all managed by Central Highlands Water, Taylors Lake managed by Grampians Wimmera Mallee Water and Cohuna, Boort and Laanecoorie all managed by Coliban Water.
Metolachlor is problematic to green algae and diatoms which are the foundations of the aquatic food chain.
Is Metolachlor the Biggest Pesticide Risk to Victorian Waterways?
Recent information obtained by Friends of the Earth via Freedom of Information (FoI) requests to Victorian water authorities reveal that the herbicide Metolachlor was detected over 110 times in 30 water supplies between 2017 and 2025 with an average detection level of 0.05µg/L (mean 0.02µg/L). µg/L=parts per billion.
In Australia the drinking water guideline for Metolachlor is 300µg/L which in terms of pesticide health guidelines, is on the higher end of the guidelines.
Metolachlor's lowest ecological guideline however, known as the ANZECC 99% Default Guideline Value (DGV) is almost 36,000 times lower and set at 0.0084µg/L and in terms of herbicides is the second lowest 99% DGV guideline in Australia. Clearly then, the ecological impact of this herbicide warrant much closer attention.
Friends of the Earth is concerned that all 110 detections breached the 99% Default Guideline Value (DGV). The average Metolachlor detection was 600% higher than the 99% DGV.
The FoI's also helped identify several Metolachlor 'hotspots' where urgent action is required to stop this chemical polluting water supplies.
Metolachlor was detected at very low levels in treated water from Winneke Treatment plant in Melbourne in 2019, 2020, 2021 and 2024 and drinking water in Manangatang in August 2023.
Metolachlor was Victoria's sixth most frequently detected pesticide with a rise of 1000% in detections in comparison to detections in Victorian water supplies 2007-2016.
Metolachlor is primarily used as a pre-emergent herbicide for controlling grasses and some broadleaf weeds in a variety of crops. The EU have classified Metolachlor as being a suspected human carcinogen.
The areas of most concern in regards to the ecological impacts of Metalochlor were Talbot Reservoir, Evansford Reservoir, Tullaroop Reservoir and Centenary Reservoir (Central Highlands Water), Taylors Lake (Grampians Wimmera Mallee Water), Cohuna, Boort and Laanecoorie (Coliban Water), and the Yarra River at Yering Gorge (Melbourne Water). These locations indicate possible chronic Metolachlor pollution.
The towns of Ouyen and Quambatook also had 4 and 5 detections at their raw water storages. Average levels at Ouyen were 0.0125µg/L and Quambatook 0.018µg/L, well under the Australian Drinking Water Guidelines, but indicating source water from the Loddon River and Murray River contains Metolachlor.
If it reaches water, Metolachlor is particularly problematic to Chlorophyta (green algae) and Bacillariophyta (diatoms), the foundations of the aquatic food chain. Green Algae and Diatoms produce their own food through photosynthesis using sunlight and carbon dioxide. Both green algae and diatoms create microhabitats within waterways, providing surfaces for bacteria, fungi and invertebrates. They are also important for oxygen production and carbon sequestration.
Water authorities have one primary interest and that is to safeguard the drinking water supply in terms of potential health impacts to their customers. The ecological impact of the pesticides that are detected are usually not a priority. Surely drinking water authorities need to take a more proactive stance in reducing pesticide loads into waterways.
Water authorities also do not have to be informed what pesticides are being used in the water supplies that they are responsible for meaning that they don't necessarily test for all pesticides that may be used within the water supply.
Contact details:
Anthony Amis 0425841564