This story is part of a special National Geographic News series on global water issues.

For two weeks in late August and early September, environmental activists staged sit-ins in front of the White House to protest a pipeline that would carry a slurry of tar sands from Alberta, Canada, to Texas.

Their objection? Because the gooey mixture of oil and sand that comprises tar sand must be broken down to form normal crude, extracting it is a messy business that produces far more carbon emissions than does extracting regular crude.

But while emissions worries have seized much of the attention directed at the nearly 2,000-mile (3,218-kilometer) Keystone XL pipeline, experts are also concerned about another environmental problem: the threat to water quality all along the conduit’s route.

The pipeline, which would transport the tar sands material to refineries near Houston, would cross one of America’s largest underground water reserves, the Ogallala Aquifer, which stretches across 174,000 square miles (450,000 square kilometers) and underlies eight Great Plains states.

Last month, the U.S. State Department said in an environmental review that the project would have “no significant impacts to most resources” during “normal operation.” But what many opponents of the project worry about is what happens if those normal operations fail.

TransCanada, the company seeking to build the pipeline, already runs one pipeline from the tar sands region that crosses the eastern edge of the aquifer. The Natural Resources Defense Council and other environmental groups have argued that such pipelines are dangerous because they carry a watered-down version of the sticky tar sands deposits known as diluted bitumen, or “dilbit.”

Dilbit carries hazardous chemicals such as cancer-causing benzene and toxic heavy metals such as arsenic. Because it also contains particles of sand, the environmental groups say, dilbit is much more corrosive than oil alone, thus more likely to cause leaks.

Susan Casey-Lefkowitz, a co-author of a recent report by the Defense Council, said that piping dilbit is “like sandblasting the inside of the pipe,” making pipes 16 times more likely to leak than when they are carrying regular crude oil.

Keystone XL would pass over the heart of the aquifer, cutting through the Sand Hills of Nebraska, a region of grass-covered dunes that contains one of the largest wetlands ecosystems in the United States. The region's porous ground acts as a thick sponge, environmentalists say, allowing oil to soak into the aquifer more easily than it would if the soil were more solid.

Along much of its length, the pipeline would be buried in a trench, a design that would protect it from harsh weather but one that would also make it harder to spot leaks.

“I could imagine a worst-case scenario where a potential dilbit spill might reach the water table in a matter of hours or days,” said Jason Gurdak, a hydrologist at San Francisco State University. A spill would likely immediately migrate downward, he said, possibly reaching the aquifer and creating a plume. Because much of dilbit is denser than water, he said, it could sink deep, making any contamination worse than that caused by more common pollutants.

TransCanada argues that piping is the safest way to transport petroleum over long distances and that the impact of any spill on the aquifer “would be limited to a very small area.”

Dilbit spills have already occurred in other areas. One, in July 2010, dumped about a million gallons of the substance into Michigan’s Kalamazoo River, closing the waterway to fishing and swimming for more than six months.

Meanwhile, near the mining sites in Western Canada’s Athabasca River basin, fishermen have pulled up fish with crooked spines and strange sores, and even one with two mouths.

Both the oil industry and the provincial government of Alberta have denied any link between fish deformities and the mining. But in two studies, researchers at the University of Alberta reported that near and downstream from the mines, they found higher-than-normal levels of toxic compounds that can cause cancer or developmental problems, including heavy metals such as mercury and thallium, as well as polycyclic aromatic hydrocarbons (PAHs).

“We saw the same thing for every toxin we looked at: mercury, arsenic, lead, you name it,” David Schindler, professor of ecology at the University of Alberta, told the Canadian parliament last year.

Research funded by the oil industry has suggested that the pollution around tar sands sites occurs naturally, from the deposits being exposed and washed away. But an independent panel of experts said the research failed to meet basic scientific criteria, used too few monitoring stations, and had no real baseline.

Panel member Monique Dubé, an aquatic toxicologist at the University of Saskatchewan, said, “There’s no question there’s contamination in the air, water, and land from the oil sands.”