When TransCanada evaluated the risk of spills on the pipeline, it found that over the next 50 years there could be 11 spills, each releasing more than 50 barrels of oil. (A barrel holds 42 gallons.) But a recent research paper by John Stansbury, a professor of environmental and water resources engineering at the University of Nebraska places the risk at 91 such spills over 50 years.

Gates is particularly concerned about the risk along a 92-mile section of the pipeline that is slated to traverse Nebraska's sandhills. The local geology makes the water wells in that region real "gushers," he said, and the wells are important because they can sustain extremely high pumping rates.

What's So Special About the Nebraska Sandhills?

It's the largest sand dune formation in America. The unique combination of grasslands, wetlands, sand dunes and groundwater-fed lakes hosts more than 1,000 plant and animal species and provides a haven for migratory birds.

How water flows inside the aquifer also increases the area's vulnerability because

1) The aquifer lies beneath permeable layers of sand, gravel and rock. The ground soaks up rain like a sponge and water travels very quickly from the surface to the aquifer, said Gates, "so we'd expect oil to [move] similarly fast."

2) The water table is only a few feet below the surface. In some places the water bubbles up above ground to feed lakes and streams, so an oil spill could contaminate surface water as well as the underground aquifer.

3) Water always flows downhill, and aquifers have high and low points just like the hills and valleys we see above ground. The topography beneath the sandhills is "relatively steep," said Gates, and that could speed up the spread of oil through the aquifer.

"All the conditions are right for producing potentially very short lag times between an oil release near the surface of the earth and water contamination in the aquifer below," he said.

What Do We Know About Oil Spills in Aquifers?

It's difficult to find specific examples of crude oil leaking into underground aquifers.

Most of what we know in the U.S. comes from a 1979 pipeline rupture that spilled 450,000 gallons of crude oil near Bemidji, Minnesota. Initial remediation efforts removed 75 percent of the oil, and the pipeline company cleaned up an additional 30,000 gallons from 1999 to 2004. About 80,000 gallons — enough to fill 1,200 bathtubs — remained in the soil and underground aquifer, but some of it has since been degraded by microbes.

Because the spill occurred in a remote location, the contaminated section of the aquifer wasn't being used for drinking water and no municipal or industrial water sources were affected. In 1983 the U.S. Geological Survey (USGS) began using Bemidji as a research site to study crude oil spills and mitigation.

What Have We Learned From Bemidji?

Mainly, that natural processes go a long way toward repairing the damage. "When a spill like this occurs, microbes start feasting on the oil and multiplying," Geoffrey Delin, a USGS hydrologist, told SolveClimate News. The microbes eat crude oil and its dissolved hydrocarbon components, including benzene and toluene. Scientists call this process "natural attenuation" — the decrease in a contaminant's concentration.

The microbes can't solve everything, but they're keeping the oil contained. The USGS has drilled test wells around the Bemidji site to monitor the spread of contaminants. By 1998 the viscous oil had migrated 130 or 160 feet down gradient — or downslope — within the aquifer, and it hasn't moved much since then. The dissolved hydrocarbons in the aquifer have moved farther, about 660 feet downslope.