It has emerged it took eight months for education board members to be told about illegal waste dumped just yards from a Coleraine grammar school.
Up to 90 tonnes of contaminated waste - understood to be the by-product of a process used to launder red diesel - was discovered at the NEELB owned site on the Lodge Road in March.
But it is understood dumping could have begun at the site up to three years ago.
Board members were only told on Monday about the dumping - which cost around £50,000 to remove.
Local Sinn Fein councillor Billy Leonard has demanded answers.
"This material that was dumped is used in the treatment of diesel for illegal trading, so there are a whole lot of issues around this," he said.
"What did the Customs and Excise people do about it? Why did the Education and Library Board, on whose land this was dumped, not inform the public? How did they operate so close to a police station? How did they get access to this land which is fenced off?
"There is a whole plethora of questions to be asked."
© UTV News