HAMPDEN – The state Department of Environmental Protection has approved a plan to address groundwater contamination at one of the state’s largest landfills while the company that owns the waste facility readies an application to make room for even more trash.

The DEP this week approved a corrective action plan submitted by Casella Waste Systems Inc. that outlines the company’s plans to monitor and reduce groundwater contamination at its Pine Tree Landfill, situated next to Interstate 95 and Cold Brook Road.

“It’s a plan that we worked on with DEP,” Joe Fusco, Casella’s vice president of communications, said Wednesday. “We look forward to getting started on it.”

Casella plans to file an application with DEP on July 28 to increase capacity at the landfill by nearly 50 percent, saying the site’s existing 6 million cubic yards will be maxed out within two years.

The commercial landfill accepts waste from both in and out of state.

The corrective action plan includes drilling monitoring wells and installing gas detection equipment on the perimeter of the landfill to track down gases that are escaping from within the oldest, or “conventional,” layer of the landfill.

Some of the 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.

Levels of arsenic and several other organic compounds detected in wells on the site exceed federal standards for safe drinking, according to Richard Heath of the DEP’s Bureau of Remediation and Waste Management.

“It’s a significant level of groundwater contamination,” he said Wednesday.

Some contamination also is making its way beyond the landfill site, Heath said. Carbon dioxide and methane produced by degrading waste are escaping and migrating into the soil, he said.

“The fact that it is moving away means we need to do something about it,” Heath said.

Casella’s plan includes further monitoring that will help the company and DEP determine how to address the gas migration problem, he said.

An additional flare installed last week at the landfill has helped by burning off gases that are collected through a series of pipes, Heath said.

“We’ve seen some improvement in the locations where we’re monitoring gas,” he said.

Missing from the plan is a method to drill into the landfill itself to extract some of the gas. Pine Tree officials proposed opening up the liner that covers the conventional layer and pumping out the leachate – deemed the most effective method by both Casella and DEP – but puncturing the liner was deemed too risky, Cyndi Darling, a DEP environmental specialist, said Wednesday.

DEP staff preferred a method of using a flexible drill to bore into the landfill at angles, but realized through further investigation that poor control over the drill could lead to inadvertent puncturing of the liner, Darling said.

“We’re left with no surefire solution to work within the landfill,” she said.

DEP will continue further investigation, as outlined in the corrective action plan, while the department reviews Casella’s forthcoming application to increase capacity, Darling said.

“They’ll be on parallel tracks,” she said. “They’re totally separate.”