The company that is seeking permission to build a new crude oil pipeline between Canada and the U.S. Gulf Coast has wildly underestimated the damage that could occur in a worst case spill scenario, a University of Nebraska researcher warned this week.

Even a small, undetected leak from an underground rupture of the pipeline in the Nebraska Sandhills could pollute almost 5 billion gallons of groundwater with benzene at concentrations exceeding safe drinking water levels," University of Nebraska researcher John Stansbury said.

TransCanada's proposed Keystone XL pipeline would would carry 700,000 gallons of Canadian crude per day from the tar sands area of Western Alberta to refineries on the U.S. Gulf Coast. Along the way, the 36 inch diameter pipe would cross important waterways, including the Missouri and Yellowstone Rivers in Montana, the Platte River in Nebraska and the Sandhills, where highly permeable soil covers the enormous Ogallala aquifer.

Spills in these areas could pose health threats to people who rely on these sources for drinking water, he said, and could damage agriculture.

Stansbury, a professor of environmental engineering at the University of Nebraska, Lincoln. and former instructor for the U.S. Army Corps of Engineers risk assessment program, said he became motivated to calculate the potential impact of spills on the Keystone XL pipeline when he noticed "flawed and inadequate assumptions" in the risk assessment TransCanada had provided to the State Dept., which is charged with deciding whether to permit the international project.

TransCanada estimated that there would be only 11 spills of more than 50 barrels over the 50-year design life of the pipeline, Stansbury said, but this estimate undercounts that historical records of spills, overestimates how quickly the company could shutdown in the event of a leak, and assumes that the pipeline will be so well constructed that it will have only half the spills of other pipelines even though it will be carrying tar sands crude, which is known to be more corrosive than conventional oil.

According to Stansbury, federal data on spills from similar pipelines suggest that the rate of spills would be more in the range of two major spills a year.

"The existing Keystone I pipeline has had one major spill and 11 smaller spills in its first year of operation," he pointed out.

The most likely failure points on the pipeline are welds, valve connections and pumping stations, he said, and river crossings are especially vulnerable because these segments are below ground at shallow depth where they are more susceptible to corrosion and also because they have valves on either side of the river.

"Small initial leaks from corroded pipe, according to estimates made by TransCanada, could go undetected for up to 90 days," he wrote, "a prescription for catastrophic failure during a pressure spike."

In the case of the Yellowstone River, a worst case scenario would involve a failure of the valve close to where the pipeline crossed the river upstream from Glendive in eastern Montana.

A failure in that spot could release almost 6.9 million gallons of crude oil if it took the company two hours to repair the leak.

Contamination from such a spill could travel all the way south to Lake Sakakawea in North Dakota, Stansbury calculated.

Benzene would contaminate the air, heavy submerged oil could smother plants and animals, and hydrogen sulfide, benzene, polycyclic aromatic hydrocarbons and crude oil would dissolve in the water, potentially contaminating drinking water, harming aquatic species and creating a toxic benzene plume that could reach hundreds of miles downstream.

Pipeline companies and regulators need to have accurate risk assessments in order to prepare to respond in the event of a pipeline spill, said Paul Blackburn, an attorney and expert on oil pipeline emergency response planning.

The ExxonMobil pipeline spill in the Yellowstone River this month involved an estimated 42,000 gallons - a tiny fraction of the oil expected in a worst case Keystone scenario - but this amount has fouled at least 15 miles of river bank and impacted recreational fishing and ranching.

"There was a pathetic amount of equipment positioned in Montana," Blackburn said. "One of the reason this situation exists is because ... what DOT does is look at oil company response plans and rubberstamp them."

"This process of having secret oil response plans is not democratic ... I call on the Obama administration to change the spill response planning process so we are not asking people to blindly trust oil companies and the federal government."

TransCanada defended its risk analysis in a statement.

The company expressed confidence in its ability to respond to spills quickly and said that it plans to avoid corrosion at major river crossings by using horizontal drilling techniques to place heavy walled pipe 25 feet below the river bottom.

It also insisted that the diluted bitumen or tar sands oil the the line will carry will not corrode the pipeline.

"The Keystone Pipeline system is subject to comprehensive pipeline safety regulation under the jurisdiction of the U.S. Department of Transportation, Pipeline and Hazardous Materials Safety Administration (PHMSA)," the company said.