The Results of Monitoring the Department of Conservation and Land Management Program of Applying Granulated Herbicides by Helicopter. A Report to the Environment Protection Authority. EPA Bulletin 435 June 1990.

In 1989 an investigation of the environmental impacts of two herbicides proposed for helicopter application by the Department of Conservation and Land Management (CALM), was carried out by the Environment Protection Authority (EPA)...

The environmental impacts of these chemicals on the aquatic environment was also monitored at five sites. The chemicals were leached into adjacent streams and intermittently detected at relatively low concentrations generally, but ranging from 0.8 to 38.0ug/L atrazine and 1.5 to 18ug/L hexazinone.

There was no observed effect on aquatic growth in the Blackwood River, however in the streams at two application sites there did appear to be some impact on aquatic invertebrate drift...

Water samples were also collected from the southern branch of the Collie River and from a stream site called Darrell. The samples were analysed for both atrazine and hexazinone... The treatment site area drained directly into the Collie River...
The concentrations of atrazine observed in the river on 24 July were very high and exceeded what would normally be considered as a safe limit for the maintenance and preservation of aquatic ecosystems..., but were unlikely to be sufficiently high to kill aquatic fauna.
The samples were taken in a wet period during a suddent heavy fall of rain (18mm) from the side of the river and directly downstream of the application area...

Darrell Site: Hexazinone 18ug/L highest level, Atrazine 38ug/L highest level.

https://epa.wa.gov.au/EPADocLib/398_B435.pdf