Defa said a “large amount of glass mixed in with gardening and building materials” had recently been discovered on a green lane.
A spokesman said fly-tipping was “not only illegal” but posed “serious risks”.
That included to the natural environment, such as protected upland habitats and watercourses, wildlife and livestock, which may “ingest or be injured by dumped materials” and to walkers and their pets.
The department has urged people arranging for waste to be removed from their premises to check how and where their waste will be disposed of.
That would help to “prevent the illegal dumping of unwanted commercial and household materials”, it said.
It also encouraged people who see fly-tipping happening to “report it immediately” to the local authority.