Clarification appended.

Power plants are increasing the stress of lakes, rivers and aquifers across the United States, according to a study released today by the Energy and Water in a Warming World Initiative.

The report, produced in collaboration with the Union of Concerned Scientists (UCS) and more than a dozen other researchers, is the first to examine how power plant cooling affects freshwater resources -- both in terms of water use and water quality issues, specifically rising temperatures.

"Power plants are contributing to the pressures on water systems to a greater degree than we expected," said Kristen Averyt, lead researcher and deputy director of the Western Water Assessment at the University of Colorado, Boulder. "We're very cognizant of how we use our electricity, but we're not very cognizant of when we use electricity, we're using water, as well."

The UCS study is the culmination of three years of work. Researchers identified a number of inaccuracies in the existing data and had to independently confirm and update information collected by the Energy Information Administration in 2008. The study looks at more than 2,100 different watersheds across the country and takes into account all types of power plants, except for hydroelectric.

"Over 75 percent of the electricity generated in the U.S. requires water. So although coal, natural gas and other fuels are important, it is really water that powers the nation," said Averyt.

Water withdrawal is the amount of water that's taken from a source, some of which is returned. The study found that on a given day, an average of 40 percent of fresh water withdrawn in the United States goes toward cooling power plants. In the United States, that amounts to almost three times the amount of water flowing over Niagara Falls on that same day.

The quantity of water consumed by power plants -- or evaporated and lost to the atmosphere -- is about 5 percent of the country's total water use. That's a small fraction of the national water profile, said Averyt, but a large fraction of withdrawals. Basically, "power plants are thirsty," wrote authors of the report.

Putting ecosystems in hot water

In addition to quantity, the quality of water used by power plants is also an issue. "Once through" cooling systems -- where the water is run through the plant and returned to the source -- can produce water that is 17 degrees Fahrenheit warmer than it came in. Increased water temperatures make plants run less efficiently, since it will require more water to generate a kilowatt-hour of electricity than if the water is colder.

The report found that in 2008, some 350 power plants across the United States reported discharging water at temperatures over 90 degrees Fahrenheit. Some temperatures were over 110 degrees. At those levels, the water released by power plants is harmful to fish and other wildlife.

"It's unsafe for people to sit in a Jacuzzi at 105 degrees, let alone live in it," said Robert Jackson, director of the Center on Global Environmental Change at Duke University and a member of the report's scientific advisory committee, in a statement. "Fish and other species can't climb out of the hot tub."

In the coming decades, climate change is likely to make water management increasingly problematic as it increases water temperature and the frequency of drought, said Averyt. This is particularly true west of the Mississippi River, where the effects of climate change are already being felt, she said.

Water shortages caused blackouts in the southeastern United States in 2007, said Averyt. Also, due to a prolonged dry spell this year, power authorities in Texas have warned that without enough water to cool their power plants, thousands of megawatts of electrical capacity may be taken offline if the drought persists into 2012.

"In the future, climate scientists project more severe droughts in most parts of the U.S., as well as more frequent heat waves," she said. "Compound these events with increasing demands for both electricity and water brought on by a growing population, and one could envision a perfect storm."

Climate change complicates adaptation

Another problem addressed in the UCS report is that efforts to mitigate climate change could further complicate adaptive strategies to manage water. For example, concentrated solar power uses almost as much water per kilowatt-hour as a nuclear plant, said Averyt. This is further complicated by the fact that the areas where concentrated solar arrays are being built are often arid regions that experience water shortages.

"This really gets to the crux of what I consider the mitigation and adaptation nexus," said Averyt. "Yes, we're trying to mitigate our greenhouse gas emissions, but we might be compromising our water resources, which may also be affected by climate change."

"Energy and water needs will continue to collide and the consequences will worsen unless the impact on water supplies is given appropriate consideration in decisions on what power plants we build or retire," said Peter Frumhoff, director of science policy at UCS and head of the scientific advisory committee for the report, in a statement.

But power plants can reduce their impact on water supplies by investing in energy efficiency and low-water cooling options, Frumhoff added, "such as wind and solar photovoltaics, which use essentially no water."

Clarification: The 2007 blackouts Averyt was referring to occurred in the Southeastern region, not just in Texas as an earlier version stated.