SENECA — Casella Waste Services of Ontario and the state Department of Environmental Conservation are investigating an incident that occurred Wednesday when a new cap slid down the Ontario County Landfill.

In a news release, county Administrator Chris DeBolt said the incident happened late Wednesday afternoon. More than three acres of a newly constructed, seven-acre permanent cap slid down the southwest face of the landfill.

“Fortunately, no personnel were injured during this incident,” DeBolt said. “The displaced clean construction material remains within the waste area of the landfill and has not impacted surrounding areas.”

During the summer construction season, Casella — which operates the landfill for the county through an operations agreement — was placing permanent capping material on a section of the landfill. The work was done by third-party contractors and engineers hired by Casella in accordance with DEC-approved plans and regulatory requirements.

“Ontario County officials have been working closely with Casella and the DEC since the incident occurred to understand what happened and how best to mitigate any potential impacts to the surrounding community or the environment,” DeBolt said. “At this time, the cause of the permanent cap failure is still unknown. Extensive independent testing is being conducted by all parties involved to understand the cause of the failure and develop remedial action plans.”

DeBolt said a large amount of soil went down the landfill slope.

“This moving soil damaged some landfill gas collection infrastructure in the affected area,” he said. “Because of this, there is the potential for increased landfill gas odors in the areas surrounding the landfill.”

Damage assessment in ongoing due to safety concerns at the site. DeBolt said DEC regulatory specifications for permanent cap design are developed to provide redundancy, ensure isolation of the waste mass, and protect the surrounding environment.

“At this time, it appears that while the permanent capping soil was displaced and that some damage was sustained by the synthetic waste barrier, it does not appear that any waste material was exposed, compromised, or migrated outside the confines of the landfill,” he said. “This incident occurred in a portion of the landfill that was not actively accepting waste and therefore has no impact on waste acceptance operations.”