Water Fracking has a big water footprint, but that's not the whole story A new study finds other energy extraction methods have a bigger impact.

Here’s today’s quiz from the energy-water nexus. Pick the correct answer.

Shale oil and gas wells that are hydraulically fractured in the United States:

a. Consume a total of 250 billion gallons of water per year, much of it in water-stressed regions

b. Consume less than 1 percent of the total industrial water use nationwide

c. Produce 210 billion gallons of water each year

The answer, according to a new study published in Environmental Science & Technology Letters by Duke University scientists, is: All of the above. The fracking required to free up the oil and gas trapped in tight shale formations is a water intensive process, yet the overall water footprint for shale drilling is smaller than other industrial uses, including conventional oil and gas production.

The paper sheds new light on the intense scrutiny fracking has received for its water consumption in these dry times. And both sides of the debate have used the study's seemingly contradictory takeaways to bolster their arguments. Industry PR outlets, like Energy In Depth, have emphasized the “less than 1 percent” finding, saying it proves that fracking is water efficient. Environmentalists, meanwhile, have pointed to the guzzling of 250 billion gallons per year.

That’s a lot of water -- about the same amount that is currently in Colorado’s largest reservoir, Blue Mesa. A fracking job on a single well can gulp three million to 12 million gallons of water, and more often than not, the wells are in areas that don’t have water to spare, even when there’s not a drought.

But as the study points out, it’s a relative trickle compared to just about everything else. California farmers, for example, withdraw 20 billion gallons per day to irrigate their crops, according to the US Geological Survey. Power plants in North Dakota withdraw about 300 billion gallons per year for steam and cooling purposes, dwarfing what’s used to frack wells, even in this shale oil hotspot. And in Nevada, miners use about 150 billion gallons per year; a single mine pumps some 68,000 gallons per minute in order to prevent the massive open pit from becoming a giant, toxic lake.

Still, side-by-side contrasts aren’t quite fair. Irrigators, power plants and mines may withdraw gargantuan amounts of water, but they don't necessarily consume it all. That is, some of it is returned to the source in a relatively clean state. Flood irrigation runs off of fields and back into streams, and water used to cool power plants is put back into the ocean, river or lake. Mines return a portion of the pumped water to other aquifers, or send it directly to farmers and ranchers. Water used for fracking, however, is mixed with tons of sand, along with a cornucopia of chemicals, some of them rather nasty, before being shot into the ground. Most of the water does, indeed, come back out of the earth. But this "flowback," consisting mostly of injected water and chemicals that return to the surface after fracking, is not fit to be used for much without serious treatment.

The second part of the study looks at the water that is “produced” by an oil or gas well. That includes the aforementioned flowback along with formation water, which is found alongside the oil or gas deep underground. This is usually very salty, ancient seawater, that can be contaminated with hydrocarbons and other naturally-occurring but toxic substances, including arsenic, barium and selenium. Sometimes it’s even radioactive.

And it's abundant. For every barrel of oil or cubic foot of gas pulled from the well, a similar amount of water also comes to the surface. The authors of the study found that anywhere from a half-barrel to three barrels of water is produced for every barrel of oil from a horizontal shale well, yielding 210 billion gallons of water per year nationwide.

Other forms of extraction produce even more water. A conventional oil well produces as much as three to seven barrels of water for every barrel of oil. Coalbed methane wells, which extract natural gas bonded to coal by water pressure, are especially prolific, each producing as much as tens of thousands of gallons of water per day. All in all, oil and gas wells in the U.S. produce close to 1 trillion gallons of water each year.

That’s significant because that water, if disposed of improperly by dumping it into a stream or onto the ground, can pollute surface or groundwater. Even when disposed of properly, produced water can be problematic. Often, it’s injected into very deep wells, but this can cause earthquakes in some areas. Oklahoma, for example, home to more than 10,000 injection wells, has been shaken by frequent tremors over the last few years. In other areas the water can be put into huge evaporation ponds, which can leak and contaminate groundwater, kill vegetation, and fatally lure birds. Some places even run the water through municipal treatment plants, which may not be equipped to handle such contaminants.

Another Duke study published last year found that both conventional or unconventional wells can also be pathways for contamination. Methane, fracking fluids and other contaminants can escape through faulty well casings and enter drinking water aquifers, for example.

The studies suggest that those concerned about water should focus less on hydraulic fracturing, and more on oil and gas development in general. And they should worry not just about water consumption, but also about the myriad other ways oil and gas production -- and many other forms of energy development -- can endanger our increasingly scarce water supplies.

Jonathan Thompson is a senior editor of High Country News. Follow @jonnypeace