1990’s Mirrool Creek (NSW). Pesticide: Chlorpyrifos

“Environmental monitoring generally does not find chlorpyrifos, even in irrigation
drainage waters with a detection limit of 0.01μg/L. Samples taken from drains in the
Murrumbidgee Irrigation Area found occasional occurrences, generally at concentrations below 0.1 μg/L, with a few samples taken near the exit point from rice bays approaching
10μg/L. Persistent contamination occurs in certain areas during spring, notably Mirrool Creek (used as a drain) where concentrations between 0.01 and 0.1μg/L prevail
through September and October with occasional high detections (approaching 20
μg/L) in grab samples.”
p278 https://apvma.gov.au/sites/default/files/publication/14756-chlorpyrifos-irr-environment.pdf
1991: Mirrooll Creek: 14ug/L, 17ug/L