HAMPDEN – The state Department of Environmental Protection has agreed to process an application to boost capacity at one of the state’s largest landfills, while the company that owns it continues to address groundwater contamination problems.

Casella Waste Systems Inc. is seeking to increase capacity at its Pine Tree Landfill by nearly 50 percent, saying the site’s existing 6 million cubic yards will be maxed out within two years.

The commercial landfill, situated next to Interstate 95 and Cold Brook Road, accepts waste from both in and out of state.

DEP officials at one point considered requiring Casella to resolve the groundwater issues before considering the company’s application, but have since decided the proposed expansion is a separate matter, Cyndi Darling, an environmental specialist with DEP, said Wednesday.

The added capacity would be built on the uppermost layer of the landfill, known as “secure 3,” while the groundwater contamination is originating in another, older layer, Darling said.

“There isn’t a problem with the secure 3 landfill,” she said. “It doesn’t mean that we don’t consider the landfill as a whole when we process the application.”

Local officials had hoped the groundwater issues would be resolved before they had to consider the volumes of other information that go along with an expansion request, Town Manager Susan Lessard said Wednesday.

“Certainly that is a disappointment. I don’t take it as a defeat,” she said of DEP’s decision to process the application. “I think it may confuse the process.”

The town will conduct its own review of a separate application from Casella, though the final decision ultimately lies under DEP’s authority.

The DEP will soon approve Casella’s plan to address the groundwater contamination, through steps such as installing wells to extract some of the gas, she said.

Casella is pleased the application is going forward and agrees that the proposed expansion is separate from the groundwater concerns, Marty Drew, general manager of Pine Tree Landfill, said Wednesday.

“We think it’s the appropriate decision,” Drew said. “There are absolutely no problems at all with all the secure landfills.”

Casella intends to file the application by July 28, he said. If approved, it would be an amendment to Casella’s existing license for Pine Tree, rather than a separate license.

A public hearing on the application would be held by the request and subject to DEP approval, Drew said.

Causing concern is the escape of gases from deep within the oldest, or “conventional,” layer of the landfill. Some of those gases, including methane, are being picked up by rainwater passing through the landfill, resulting in leachate, which then flows into nearby groundwater.

The conventional landfill, constructed in 1975 under different ownership and less stringent regulation, sits over an unlined gravel pit.

DEP officials are investigating whether some of the contamination also is leaking from another portion of the landfill located next to the interstate, Darling said.

If approved, the extra capacity would be added through a system of “mechanically stabilized earth berms,” nearly vertical structures that are similar to retaining walls. The berms would retain waste that essentially would be packed onto the landfill’s sides.

The expansion would increase neither Pine Tree’s highest elevation point nor what environmental specialists call its “footprint.”

While acknowledging that the groundwater contamination results in an area separate from the proposed expansion, Lessard said she views the landfill as a whole.

“You can divide it up into as many pieces as you want to. It’s a landfill,” she said.

The amended license is far from a sure thing, however, and both DEP and the town will conduct thorough reviews of all the application data, Lessard pointed out.

“There’s no predetermined outcome here,” she said.