Inquiries opened into 'slurry pollution' in south Wales
Several incidents of pollution across south Wales are being investigated by Natural Resources Wales.
One has caused a "significant fish kill", and been traced to a business park near Llantrisant.
Three incidents in south west Wales are believed to be from agricultural slurry spreading.
Natural Resources Wales (NRW) said it was "disappointed" farmers were not following best practice.
Spreading slurry in the days before forecasted heavy rain or in wet ground conditions can lead to water pollution, said Huwel Manley, NRW operations manager.
He added: "We understand that farmers are keen to empty slurry stores at this time of year, but we are disappointed that farmers and contractors are not following best practice.
"Our officers will continue to assess the full impact of these latest pollution incidents and will be out on site again in the morning assessing the environmental impact."
In December, more than 1,000 fish died as a result of what is believed to have been a slurry leak in the Tregaron area.
NRW have urged people to report any pollution, dead fish or fish in distress.