According to Dwr Cymru/Welsh Water's Wastewater Services and the Environment Agency, up to 1000 tonnes of partially treated wastewater may have been discharged into the River Dee. Welsh Water will not speculate to the exact amount, but has said there was a burst pipe at the Chester water treatment plant on Tuesday. Combined with heavy rainfall, the facility was dealing with an increased amount of waste water.
Wirral Council has advised residents to avoid contact with River Dee water. This extends to pets. CWaC have not issued an advisory, but it may be worth finding an alternate walking path for your 4-legged friends.