SLIPPERY ROCK — The owners of two apartment complexes near Slippery Rock University say unexplained changes to their garbage collection contract have left refuse piling up at one building, and could send their costs skyrocketing.

The off-campus student housing — Towers and Campus Heights apartments — on Cornish Drive is owned by Brenda and Allen Martin. On Thursday, they said billing changes dating back to August of 2016 could leave them with an annual garbage pickup bill of $22,000.

The Martins said that they normally pay about $3,600 in garbage collection fees annually, for service through Tri-County Industries.

At-issue, the Martins say, is how the apartment complexes’ collection service is being handled. Brenda Martin said that the properties used to fall under a commercial contract with Tri-County, but since August of last year they’ve been billed as residential customers for each individual apartment unit — whether the apartments are vacant or occupied.

The Martins said they have about 155 apartments in total. About 60 percent are occupied, and until earlier this week both complexes were serviced by Tri-County Dumpsters.

But on Monday, when trash at Campus Heights was supposed to have been picked up, it wasn’t according to the Martins. Now trash is piling up around the buildings’ Dumpsters. On Tuesday, the Martins said, Tri-County removed Dumpsters from Towers apartments, leaving tenants no place to dispose of their trash.

Brenda Martin said that on Thursday her husband was told by a borough official that Tri-County had been told to pull the Dumpsters until the issue could be resolved. On Friday borough administrator Lucinda Lipko confirmed that, but said the municipality wouldn’t comment further on the situation.

“We’re investigating it,” Lipko said.

The changes to the Martins’ billing line up with alterations made early last year to the borough’s garbage billing procedures. Amid a long-running dispute over the a new contract with Tri-County that was approved in late 2015, borough solicitor Neva Stotler said her investigation of critics’ claims about the contract turned up a “tradition” of allowing seasonal residents and property owners to cancel garbage collection services when their residences are vacant. Stotler said that was forbidden by borough codes, and the practice would be stopped. She also said that the changes would apply to rental properties, which would be billed for garbage collection whether they were occupied or not.

Brenda Martin said that way of doing things doesn’t make sense to her.

“You’re paying for a service which you’re not getting, which should be totally illegal,” Martin said.

Allen Martin said he’s still waiting for an explanation of where the borough gets authority to change a contract between his business and Tri-County.

“This is a commercial contract with nothing to do with the borough,” Allen said. “Where’s the authority for the borough to come in between a business and a private contractor?”