Glynne Jones, 68, pleaded guilty at Mold Magistrates Court in Denbigh to causing a water discharge activity, namely the discharge of poisonous, noxious or polluting matter to an inland freshwater contrary to the Environmental Permitting (England and Wales) Regulations 2016 Act, on 18 October.
He was fined £1,600 and ordered to pay costs amounting to £2,950 to be paid in 28 days.
The prosecution followed a Natural Resources Wales (NRW) investigation in December 2020 that found insufficient slurry management had caused an overspill into the Afon Concwest.
NRW environment team leader for North East Wales Anthony Randles said the incident “could have had a devastating impact on the watercourse and its biodiversity”.
He said: “Slurry is high in ammonia which is toxic to aquatic life and can cause fish kills, whilst also containing microorganisms such as bacteria, viruses, and fungi, that can cause diseases to animals in the water.
“This incident at Mr Jones’ Hafodty Ddu property was avoidable and should never have happened. It shows the importance of regular and proper maintenance of slurry stores.”