Cleaning up illegal waste at a sprawling site in Derry has cost £800,000 so far.
More than half a million tonnes of material was buried without permission in and around a licensed recycling site at Campsie.
Experts from the Northern Ireland Environment Agency (NIEA) disposed of high risk refuse and monitored water quality over the last seven months ahead of consideration of a longer term solution.
Environment Minister Mark H Durkan said: "We will pursue the polluters vigorously to recover all clean-up costs."
The 1.4 km site on the Mobuoy Road near the River Faughan, where most of the illegal landfill was buried in old sand and gravel excavation pits, was in and around a recycling centre.
Independent expert Chris Mills investigated and said clearance costs could ultimately be "tens of millions of pounds".
The licence for the owner and operator of the recycling site was revoked and a criminal investigation launched, with two arrests made.
Mr Mills was commissioned by former Environment Minister Alex Attwood to examine the case.
He noted a history of non-compliance with regulations at the licensed site, but said it was still not known who deposited the illegal waste.
"However, this was a sophisticated operation which had been carried out over a number of years," he added.
Mr Durkan told Democratic Unionist MLA Gregory Campbell costs incurred to date by the Department of the Environment were around £800,000.
"NIEA is engaging top-class experts to advise on longer term clean-up options at the Campsie site, including cost estimates," he added.
"Clean-up decisions will be based on this expert advice which will include detailed estimates of the likely costs and timescale."