During the coming two decades, Pennsylvania could lose enough forest land to build a couple of large cities. The forest won't be lost in a single large chunk, but as thousands of small sites that are cleared to drill natural gas wells and connected with hundreds of miles of new pipelines.

While those impacts will be scattered across the landscape, their cumulative impact on forest habitats could be severe, and it could also complicate the state's efforts to meet its nutrient and sediment reduction obligations under the Chesapeake Bay Total Maximum Daily Load, or pollution diet.

"It's not so much that people know it would keep the TMDL from being met," said Nels Johnson, director of conservation programs with The Nature Conservancy in Pennsylvania. "It's that no one knows whether or not this really threatens the state's efforts to meet the TMDL."

Much of the concern about environmental impacts related to the Marcellus Shale natural gas boom has been related to the water quality impacts of hydraulic fracking, the process of injecting huge amounts of water and chemicals under high pressure deep into the ground to break apart rock and access gas.

Johnson led a team that tackled a different question - how the drilling in the Marcellus Shale formation could affect land use and, ultimately, wildlife habitats in Pennsylvania.

By using information about the depth and thickness of the Marcellus formation in different areas and a variety of other variables, they developed a model to project where the 60,000 wells expected to be drilled in the next two decades will go.

The analysis projects that about 60 percent of the wells will be drilled on forest land - the dominant land cover over much of the Marcellus Shale in the state.

A key factor that affects how much forest will be directly affected by drilling is the number of wells drilled on each drilling pad. A typical pad is about 3 acres but requires about six additional acres for roads and other related infrastructure. Right now, the average is less than two wells per pad, Johnson said, but he expects that to increase to between 4 and 10 wells per pad over time.

While scattered pads may not seem to have great impact, the analysis estimates that, across Pennsylvania, 38,000-90,000 acres of forest may ultimately be cleared for wells seeking to tap the Marcellus Shale formation, which underlies the western and northern portions of the state. Another 60,000-150,000 acres of forest could be lost for new pipelines.

"It's a cumulative impact," Johnson said. "Ultimately, that's why we did this - because we wanted to have a better understanding of the cumulative impact, and how worried we should be about this."

Pennsylvania's large tracts of intact forests are important for an array of wildlife, from brook trout to forest interior birds. Forest birds such as the scarlet tanager, which have declined in many areas, have generally held their own in Pennsylvania's large forests.

That could change as forests are chopped up for wells and pipelines. Many predators, from blue jays to raccoons, thrive along forest edges, from which they forage into the woods, picking off birds or the eggs of wood thrush, ovenbirds and other species that normally rely on large forests for refuge. Not only will forests be directly lost to drill pads and pipelines, but forests near those opening will be rendered uninhabitable for many species.

But the analysis also raises a concern for Chesapeake cleanup efforts. The conservancy estimates that about 46 percent of the drilling would take place within the Bay watershed. That suggests the forest loss within the watershed portion of Pennsylvania could be between 45,000-110,000 acres.

For comparison, that's enough land to build between 1 to 2.5 District of Columbias.

Because forests absorb more nutrients and retain more sediment than other land uses, their loss could result in more of those pollutants reaching local streams.

Assuming those forests are converted to meadow, and applying loading rates derived from the Bay Program model, rough estimates suggest it could increase the amount of nitrogen runoff reaching local streams between 30,000-80,000 pounds a year; while phosphorus could increase between 15,000-40,000 pounds; and sediment could increase between 18 million to 45 million pounds. The variation depends on whether the amount of forest lost was at the low, or high end of the conservancy's estimates.

Right now, the land use changes are not included in the state's watershed implementation plan, which shows how it plans to meet nutrient and sediment limits set in the TMDL.

Kevin Sunday, a spokesman for the state Department of Environmental Protection, said sediment and erosion control guidelines would require best management practices to control runoff and well sites would need to be re-vegetated.

Johnson said that, as a practical matter, it is difficult to reforest areas disturbed for drilling as companies need to maintain access to wells and pipelines. Further, a recent study showed that reforestation generally wasn't taking place at drilling sites, he said.

Katherine Antos, water quality team leader with the EPA's Bay Program Office in Annapolis, said state pollution limits set in the TMDL were based on land uses in place in 2010. "If there are any changes to that, any increased loads or new sources, states have to be able to offset those increases," she said.

Antos said the EPA is currently reviewing offset programs for all states in the watershed.

Harry Campbell, a scientist with the Chesapeake Bay Foundation, said concerns about the impacts related to drilling activities on the Bay TMDL were among the reasons that it and several other organizations petitioned the federal government last year seeking the development of an Environmental Impact Statement to examine the full range of Marcellus drilling impacts in the state.

"We just don't know enough about all this to get a handle on what the potential impacts are," he said. "If we don't have that, then we are flying blind."

That petition is still pending.

Meanwhile, Johnson said the conservancy has been using its analyses to work with drilling companies to encourage drilling more wells at existing pads to reduce forest loss. It's also integrating more habitat data into its model to help steer drilling away from sensitive areas. Companies have been "pretty interested," he said. "We're pretty confident it is going to help, but we know it is not going to eliminate impacts."