Of the hundreds of thousands of gallons of water removed from streams, rivers and public and private water systems every day by the gas industry tapping into the Marcellus Shale, at least 90 percent of it is used to hydrofracture gas wells, according to hydrologist David Yoxtheimer, extension associate with the Penn State Marcellus Initiative for Outreach and Research.
"The other 10 percent is used for various things," Yoxtheimer said. "Some will be used for drilling, cementing the steel (well) casing in place, dust control and pressure testing gas lines, but it's a relatively small amount compared to hydrofracturing."
To hydrofracture - or "frack" - a gas well, millions of gallons of pressurized water mixed with sand and chemicals are pumped deep into the ground to pulverize the shale and release the gas trapped within it.
The practice, along with horizontal drilling, is a key technology in developing shale gas, but it also has created controversy in Pennsylvania and other areas of the country where shale gas is being developed.
Environmentalists say fracking could contaminate - and already has contaminated - drinking water sources, while the industry counters that there have been no substantiated claims of ground water being contaminated by the process.
The federal Environmental Protection Agency plans to study the impacts of fracking on the environment and recently held a series of public hearings to receive input on the scope of the study.
New York state has imposed a moratorium on gas drilling until its impacts can be studied further. Some people, including state legislators, have called for a moratorium in Pennsylvania.
Yoxtheimer said transparency by the industry regarding the chemicals used for fracking is one way of taking the mystery out of the process.
The state Department of Environmental Protection recently released on its website a list of chemicals used industry-wide in the process. Range Resources became the first gas exploration company to divulge the chemicals it uses.
"On some level, being that the chemicals haven't previously been disclosed makes them seem more exotic than they are," Yoxtheimer said. "When you look at it, it's not that exotic or mysterious. It's compounds found in common household chemicals."
According to the industry organization Energy Indepth, more than 99.5 percent of the material used for fracking is water and sand. The rest are compounds found in swimming pool cleaners, laundry detergents, food additives, cosmetics, household cleansers and hair coloring.
Brady Russell, Pennsylvania director for Clean Water Action, said some of those compounds are toxic and capable of polluting fresh water supplies.
For example, ethylene glycol, an ingredient used in frack fluid to prevent scale deposits on pipes, commonly is used as an antifreeze, Russell said.
According to the Agency for Toxic Substances and Disease Control of the U.S. Department of Health and Human Services, ingesting ethylene glycol can cause serious illness or even death.
Russell said industry assertions that no groundwater contamination has ever occurred during the fracking process refers only "to just what happens a mile or two under the ground."
Fracking fluids or the saltier-than-seawater flowback fluids that return to the surface following the fracking process can get into groundwater through poorly constructed well casings, leaking retention ponds or spills from trucks, Russell said.
There have been spills reported in Pennsylvania, Russell said.
"No matter where it comes from doesn't matter to us," Russell said. "Fracking fluids are getting into groundwater and there is no way of getting it out."
"If you consider flowback as a part of hydraulic fracturing, then there has been issues," Yoxtheimer said. "If that water isn't totally captured and spills on the ground, it could seep down into the aquifer. That has reportedly happened."
Dan Spadoni, a spokesman for the DEP, said most spills reported to the agency over the last 16 to 18 months "have been small in quantity and confined to the well pad itself."
Those spills have included drilling mud, flowback or brine water, diesel fuel, hydraulic fluid, motor oil and other materials, Spadoni said.
Some larger spills that escaped the well pad were contained before they reached surface water sources and the contaminated soil was excavated to reduce the chances of groundwater contamination, he said.
A few spills have reached creeks or streams and have harmed aquatic life and degraded the water quality, Spadoni said.
Matt Pitzarella, director of public affairs for Range Resources-Appalachia LLC, said the company uses best management practices designed to reduce pollution-causing mishaps. Those practices must be standard operating procedures industry-wide, he said.
The gas industry has to have zero tolerance for mistakes in order to minimize impacts on the environment, Pitzarella said.
"There is no margin for error. We're under an incredible microscope and we want to get it right," he said.
Pitzarella would not comment on companies that have caused pollution, but he did say: "We're only as good as our weakest link. We want to take (drilling practices) to a higher standard of care."
Pitzarella said he understands the concerns people have regarding the gas industry because of the negative impact resource extraction, particularly with the coal industry, has had on the state.
"If you look at the industrial past of the commonwealth, you can understand where they would be skeptical," Pitzarella said. "We have to demonstrate to the people that we are not the second coming of the coal industry from a century ago."
"If (companies) are not doing it the right way, we need to fix it as an industry," he said.
"It's how you manage your operation," Yoxtheimer said. "It's just like any industry. If you're doing a careful job, there shouldn't be a problem. If your work is sloppy, you will (have problems)."
Yoxtheimer said companies are using more practices that reduce the chances of a pollution-causing accident.
Those practices include using steel tanks instead of retention ponds to contain flowback fluids, covering well pads with liners to contain spills, and treating wastewater for reuse at the well site with mobile treatment units to cut down on trucking incidences.
Yoxtheimer said natural gas exploration, if done properly, can have a positive impact on Pennsylvania.
"If natural gas can be produced to provide a cleaner energy while enhancing economic opportunities, that is something to consider," he said. "It's not all awful.
"The reality is - accidents will happen," he said. "I think a few isolated accidents cumulatively won't have a significant impact on the state compared to the potential benefits that natural gas can provide."
Two companies take on task of cleaning frack water.