The Environment and Heritage Service began investigating a site in Hillsborough in 2005 after complaints about dumping. Officers saw McAvoy Construction of Banbridge dump sleech - a foul-smelling clay slurry. The firm later admitted to dumping 2,500 tonnes of the material there.
The waste had come from the extension of a shopping centre in Lisburn and would have cost £13,000 to landfill.
The company admitted three offences under the Waste and Contaminated Land Order (Northern Ireland) 1997 before Lisburn magistrates on 27 April. These included depositing waste on land without a licence and causing others to breach their duty of care. It was fined £8,000.
McLaughlin & Harvey, the construction company that ran the shopping centre site, was also fined £3,000 for one charge under the Order.