A key legislative committee this week could advance a mining bill backed by the Natural Resources Council of Maine that would ban large open pit metal mines and impoundments of mine waste.

Members of the state Legislature’s Joint Environment and Natural Resources Committee spent three hours Monday discussing some of the technical issues of metal mining and mine-related legislation.

The committee could vote Wednesday to send those bills to the full Legislature for a final vote. Among the measures is a bill that would enact newly developed regulations to implement a controversial 2012 law that could open the door for the first large-scale metal mining in Maine since 1977. Yet another bill, however, would repeal that 2012 law, reverting back to stringent mining regulations adopted in 1991.

But a large chunk of Monday’s discussion centered on LD 820, a bill introduced by Sen. Everett “Brownie” Carson, D-Harpswell, that would amend the 2012 mining law to add a range of new conditions for permitting metal mining.

LD 820 has a good chance of passing the committee and the Legislature, said Nick Bennett, a staff scientist with the Natural Resources Council of Maine, which helped develop the bill. He said it addresses many of the problems critics have raised with the 2012 law and with subsequent regulations proposed by the state Department of Environmental Protection to implement it.

The bill, which already called for strictly limiting groundwater pollution and banning mining on public and other sensitive lands, was amended over the weekend to include provisions that effectively ban open pit mines and the use of tailings impoundments. Bennett said the group always intended to advocate for banning open pits and waste impoundments, which can suffer structural problems and release toxic waste into waterways.

The bill would ban open pit mines larger than three acres in aggregate or with more than one “bench,” a ledge created in the mine wall that’s used both to stabilize pits and as a pathway for equipment and trucks.

“It puts a limit on the size of the pit,” Bennett said of the size requirements.

The ban on wet mine waste and tailings impoundments is designed to prevent leaching of natural contaminants from often-acidic waste rock that is removed in the course of mining metals. The bill would instead require “dry waste management,” keeping and storing the waste rock in lined structures that prevent runoff of acidic water.

The bill also would prohibit groundwater contamination anywhere outside the immediate mine shaft — unlike the 2012 law that allowed virtually unlimited contamination inside a broadly defined “mining area,” Bennett said.

“We accepted the argument that you cannot have a mine without groundwater contamination,” Bennett said. “Our 1991 rules said no contamination of groundwater period. On the other hand, the 2012 law said unlimited groundwater contamination in the mining area. There has to be something in between.”

Bennett said the water requirements in the bill treat the areas around the immediate mine shaft much like other facilities that can impact groundwater, such as landfills and factories, which must have secure linings and water collection systems and are not allowed to contaminate groundwater.

“What we’re trying to do is set up a whole series of protections so that a Callahan or something like it never happens again,” said Carson, the bill’s sponsor and former executive director of NRCM, referring to the former Callahan mine in Brooksville, which is now a federal superfund site.