PITTSBURGH (TNS) — Pennsylvania regulators have released a long-awaited final draft of rules to cut releases of smog-forming and climate warming air pollution from the state’s existing oil and natural gas well sites, but they will still not require companies to find and fix leaks at tens of thousands of low-producing wells.

The rules are a last piece of the methane-reduction strategy that Gov. Tom Wolf announced nearly six years ago to cut down on emissions of the potent greenhouse gas from new and old sites across Pennsylvania’s oil and gas production industry.

They are expected to take effect by the middle of next year.

Scientists attribute about a third of the planet’s warming from greenhouse gases today to human-caused emissions of methane, which traps more than 80 times as much heat as carbon dioxide over 20 years. In the U.S., about a third of methane emissions come from the oil and gas industry.

The new state rules will require some well owners to perform leak searches four times a year and upgrade equipment already in the field to cut down on pollution from controllers, pumps, compressors and tanks.

In total, the rules are expected to reduce emissions of a smog-forming group of chemicals called volatile organic compounds by nearly 12,000 tons per year and methane emissions by about 214,000 tons per year — more than doubling the impact that was expected when the first draft of the rules was published two years ago.

Mark Hammond, director of the Department of Environmental Protection’s air quality bureau, said the major driver for that improvement was better data that state regulators gathered by looking at Pennsylvania facilities to assess the rules’ impact, rather than relying on national estimates.

The department made modest changes to the proposal after receiving comments from roughly 36,000 people and groups.

The final draft of the rules was released in advance of a meeting Thursday of DEP’s Air Quality Technical Advisory Committee.

Most notably, DEP added a requirement for annual leak searches at a small subset of low-flowing wells on multi-well pads.

DEP estimates there are just 38 well sites in that category.

While emissions from those sites “are not huge,” Hammond said, “those numbers actually represent a fairly large percentage of the emissions that would not have been regulated at all.”

Environmental groups said they were disappointed in the rules. They estimate that about 63,000 oil and gas wells will be free from having to perform leak detection surveys based on DEP’s production thresholds, including the vast majority of the state’s conventional wells.

Those groups said that low-producing, leak-prone wells contribute about half of the methane emissions from Pennsylvania oil and gas production. They had pushed for the wells’ owners to be required to conduct routine surveys.

”Most low-producing wells in the state are owned by larger companies with profits that dwarf the compliance costs of taking an optical gas imaging camera to inspect the sites once or twice a year,” said Vanessa Lynch, a southwestern Pennsylvania-based organizer for Moms Clean Air Force.

DEP’s proposal is more stringent than the federal guidelines it is based on. The agency said its rules will reduce emissions of volatile organic compounds by about 700 tons per year and methane by about 12,000 tons per year beyond what federal standards require.

Other changes in the rules between DEP’s proposed and final draft include a uniform threshold for when companies will have to begin controlling emissions from tanks, bringing the standards for tanks at conventional well sites in line with newer shale gas sites.

DEP also removed a provision that would have allowed companies to perform less frequent leak searches if they could show that fewer than 2% of their components were leaking. DEP replaced it with a standard that will allow companies to do fewer or no leak surveys at sites only when production drops to a low level.

Pennsylvania has a deadline to finalize and submit the rules to federal regulators by June 2022.

In the meantime, federal regulators are proposing another set of rules to cut the oil and gas industry’s methane emissions further. If they are finalized, Pennsylvaniafacilities will have to comply with those stricter standards.