Drilling for natural gas in the Marcellus Shale uses millions of gallons of water at each well. The water is mixed with sand and chemicals and pumped deep into the ground at very high pressure to fracture the rock and release the natural gas.

Most of it never returns.

While many have focused on the degree to which that "fracking" process threatens to pollute drinking water supplies, Jeff Schmidt, director of the Pennsylvania chapter of the Sierra Club, recently raised a different concern.

“The majority of the water remains underground, permanently removed from the hydrologic cycle,” said Schmidt at a recent rally in the state Capitol. “It can never sustain life above ground again.”

Schmidt is by no means the first to express unease about pumping millions of gallons of water into a hole from which it will never return.

With fresh water shortages predicted to be a greater source of international tension than oil in the 21st Century, is it wise to be pumping fresh water into the ground?

The average amount used per well is around 4 million gallons.

Scott Perry, head of the oil and gas division at Pennsylvania’s Department of Environmental Protection, said he’s not concerned about the water staying underground.

Perry’s reasoning, however, employs a semantic dodge. He noted that a nuclear power plant uses 40 million gallons of water a day and said, “That water is not necessarily returned to the environment from which it came.”

However, a significant portion of the water used for cooling nuclear facilities such as Three Mile Island is released into the air as steam.

Technically, that’s a different environment from which it came, but it remains in the hydrologic cycle — the process of evaporation and condensation through rainfall that cycles fresh water back to its source in headwaters mountains.

Schmidt says, “The process of fracking will permanently remove hundreds of billions of gallons of fresh water in Pennsylvania from the hydrologic cycle. ... I know of no study that has evaluated the permanent removal of this much fresh water from the hydrologic cycle.”

“No prior activity in Pennsylvania has ever approached the magnitude of Marcellus Shale drillings’ permanent water consumption,” Schmidt said.

What is the impact of removing such huge volumes of water permanently from use?

Not much in Pennsylvania, said Michael Arthur, professor of geosciences and co-director of Penn State University’s Marcellus Center for Outreach and Research.

Arthur said 17 trillion gallons of rain falls every year on the 95,000 square miles underlaid by the Marcellus.

About 7 trillion gallons go to recharging ground water supplies, while the remaining 10 trillion run off into streams or evaporate.

“The Marcellus effort will never get to the point they are using 100 billion gallons a year,” he said. The recharge will always outpace the consumption.

But Schmidt says it’s “bigger than Pennsylvania or even Marcellus.”

“As the relatively new process of deep horizontal drilling, combined with fracking becomes more widespread, the math gets pretty staggering,” he said. “Trillions of gallons of water lost to the hydrologic cycle.”

According to the U.S. Energy Information Administration, there are 48 shale gas basins in 32 countries around the world, containing almost 70 shale gas formations.

The Marcellus is among the largest and most productive, but assuming all those shale formations were equally big and lucrative — a very liberal assumption — it would require over 22 trillion gallons of water to be permanently sequestered a mile below ground.

That’s a lot of water, but all of it taken at once wouldn’t come close to draining Lake Erie, the smallest by volume of the Great Lakes.

If all 70 shale gas formations around the world were equivalent to the Marcellus and were fracked in one day with water from Lake Erie, the lake would diminish by about 17 percent.

Arthur said the problem with the argument that drilling removes water from the hydrologic cycle is that it discounts the fact that most of the water in that cycle comes from the oceans.

The removal of water ultimately affects ocean levels, not rivers and streams, he said, because the evaporation and return by rainfall continues.

But Arthur said the issue is “definitely worth looking at” for other reasons.

“Globally, there could be some impacts,” he said. “It’s not going to impact the hydrologic cycle,” but rather be an issue of competing use for water in areas of limited rainfall.

Arthur said the best climate change predictions “suggest the wet (regions) get wetter and the dry get drier.”

He said in dry lands areas with very little water storage, hydraulic fracturing could be competing for water with industries such as agriculture.

Arthur said potable water is limited in many areas of the world — “places like Turkey and Iraq are already fighting over the Tigris and Euphrates system.”

“We’re blessed in Pennsylvania,” he said, with ample rainfall and agencies such as the Susquehanna River Basin Commission that can make decisions about allocation of water.

In a drought, the SRBC has the authority to prohibit drillers from withdrawing water.

But those withdrawals will take place over decades. According to the SRBC, the maximum need for water at the height of drilling will be less than two minutes' worth of the average flow of the Susquehanna.