Waste at an illegal landfill site in Co Tyrone is believed to stem from the Republic, according to the Environment Agency.
The site is in the Galbally townland near Dungannon, and is the third to be discovered in the area since last autumn.
In total, there are five ongoing investigations in the east Tyrone area, involving tens of thousands of tonnes of waste.
The agency said: “It would take considerable effort to transport and dump rubbish on such a large scale.
“The potential environmental effect of any unauthorised waste deposit is clear. Aside from this, there is the cost of cleaning it up.
“These incidents and their links give rise to particular concern that south and east Tyrone is being used by unscrupulous criminals to illegally dispose of waste.
“This is why we have now linked these incidents and have appealed for public help in detecting and preventing further deposits.
A man was arrested as part of the operation on Thursday.
He was later released pending further enquiries.
The Environment Agency has appealed for anyone with information on the illegal dumping of waste to contact them on (028)9056 9453, or email email@example.com .