People are being warned to keep off a popular lake after a farm vehicle overturned, spilling 400 litres of a chemical into a nearby drainage ditch.
Tests are being carried out on the water at Llangorse Lake near Brecon after the incident involving diluted herbicide.
Public Health Wales said people should stay off the lake as a precaution until the test results were known.
Llangorse Lake is a popular location for water sports and with anglers.