An oil flood through an Arkansas subdivision on March 29 is just the most recent example of pipeline problems in the U.S. In recent weeks, months and years diesel has leaked from a pipeline into wetlands near Salt Lake City; oil has spilled into the Yellowstone River in Montana; and about 20,000 barrels of oil have spewed into the Kalamazoo River in Michigan. The question: Is the problem the pipelines themselves or what they carry?

The answer may be an unfortunate combination of the two. Certainly, the infrastructure has issues. The U.S. is crisscrossed by more than four million kilometers of such pipelines, many decades old. These pipelines spring hundreds of leaks every year, most small. The pipelines can fail for reasons ranging from a backhoe inadvertently striking one to the slow but steady weakening from corrosion. "It's not a matter of if, but when," says Susan Connolly, a resident of Marshall, Mich., right near where the Kalamazoo River spill occurred in 2010 as a result of external corrosion.

Critics charge that pipelines carrying diluted bitumen, or "dilbit"—a heavy oil extracted from tar sands mined in northern Alberta—pose a special risk because, compared with more conventional crude, they must operate at higher temperatures, which have been linked to increased corrosion. These pipelines also have to flow at higher pressures that may contribute to rupture as well. Environmental group Natural Resources Defense Council (NRDC) notes that pipelines in the upper Midwest that routinely carry oil from tar sands have spilled 3.6 times more oil per pipeline mile than the U.S. average. The Arkansas and Kalamazoo accidents both involved dilbit.

The chemistry of the tar sands oil could contribute to corrosion as well. In processing, the tar sands are boiled to separate the bitumen from the surrounding sand and water, and then mixed with diluent—light hydrocarbons produced along with natural gas—to make the oil less viscous and able to flow. But even so, the resulting dilbit is among the lowest in hydrogen as well as the most viscous, sulfurous and acidic form of oil produced today.

Some think the Arkansas spill could have resulted from just this combination of aged infrastructure and added stress from dilbit, although an exact cause has yet to be determined. The breached Pegasus Pipeline involved in the Arkansas incident can carry nearly 100,000 barrels of oil per day from Illinois to Texas. Originally constructed in the 1940s to bring Texas crude oil up to Illinois, it had been reversed in recent years to stream dilbit. The operator, ExxonMobil, retrofitted the 50-centimeter tube to compensate for the demands of pushing tar sand oil through in the opposite direction, but the higher temperatures and pressures may nonetheless have contributed to the rupture or sped up preexisting corrosion, suggest critics such as NRDC's Anthony Swift.

A study from the Alberta government (pdf), however, casts doubt on the notion that dilbit is worse for pipelines than any other oil is. It found that dilbit is not corrosive at pipeline temperatures of as much as 65 degrees Celsius, although it is highly corrosive at refinery temperatures above 100 degrees C. Nor is the fine sand that remains in some of the dilbit eroding pipelines, though it does form sludges that must be cleaned. The higher temperature operation may even kill off the bacteria that help to corrode pipelines carrying other types of oil. "There is no evidence that dilbit causes more failure than conventional oil," geologist John Zhou of the provincial government research firm Alberta Innovates said during an interview in November on a trip to the tar sands; Zhou helped prepare the Canadian province's analysis of dilbit. The U.S. National Academies is currently studying the issue.

The good news for residents of Arkansas is that a dilbit spill on land may prove easier to clean than one in water. Thanks to its more viscous nature, Zhou says, "it's not going to move very far on a spill"—as long as it does not get into waterways, as occurred in Michigan. Regardless, the sour smell of dilbit is likely to remain in the air of Mayflower, Ark., until all the diluent evaporates. "Before you get into town, you can already smell the oil," says Glen Hooks of the Sierra Club Arkansas, who visited the spill site. "There is no reason to trust oil companies when they say pipelines are safe when there's been spill after spill after spill."

The mishap also highlights some of the concerns around the building of the proposed Keystone XL Pipeline, which could carry 830,000 barrels per day of dilbit or other tar sands products 2,700 kilometers from Alberta to Texas. That pipeline would incorporate the latest technologies, such as epoxy coatings and electrical current to reduce corrosion. Yet, even brand-new pipelines can spring a leak: TransCanada's Keystone I Pipeline, which began carrying dilbit from Alberta to the U.S. Midwest in 2010, has already suffered 14 different leaks (pdf).