The longer it takes to clean up a freshwater oil spill, the more persistent compounds it produces. That’s according to a team led by environmental engineer Dena McMartin, who studies rural water management at the University of Lethbridge and the University of Saskatchewan.

Most research into the fate of oil spills is conducted in marine environments, which is why McMartin’s team decided to focus on freshwater. The researchers examined the chemical changes that could happen to crude oil as it lingers in rivers and lakes by simulating a spill using water and sediment from the North Saskatchewan River.

In a tank in a lab, they combined the river’s water and sediment with conventional crude oil from an Alberta pipeline operator. The lab was kept at 24C and water samples were tested at multiple points over the course of 56 days using Fourier-transform ion cyclotron resonance (FT-ICR) and Orbitrap mass spectrometry.

The team found that the more days that passed, the more oxygen-rich compounds were present – a by-product of microbes in the water digesting the crude oil. Or as the paper’s authors write, “The O2-containing species gradually decreased in relative abundance, while O3 and O4 species increased in relative abundance throughout the simulated spill, which could correspond to a relative degree of oxygen incorporation.”

It turns out those extra oxygen molecules give compounds major staying power in aquatic environments.

“We believe it’s the oxidization process that is making the hydrocarbons more soluble in water, more polar,” says McMartin. Instead of settling out on rocks or sediment, “they become more soluble and thus more mobile in the water,” she says.

McMartin and her team suspect that solubility is a problem for aquatic wildlife, such as fish, which could be more likely to ingest the compounds from the water column. Non-soluble compounds coating rocks and sediment are not out of harm’s way exactly, but are less accessible to fish.

“We’re only confident with the persistence outcomes and are still at the hypothesis and testing phase for toxicity,” McMartin cautions.

Still, other research has shown that oxygen-loaded compounds can damage bacterial cells, inhibit photosynthesis in algae, and cause cancer and digestive problems in waterfowl. “It’s part of why we try to keep ducks off tailing ponds,” says McMartin.

University of Ottawa environmental toxicologist Jules Blais who was not involved in the research says the study highlights the importance of biodegradation of oil following a spill, as opposed to photodegradation from sunlight.

Unlike oceans where natural hydrocarbon seeps support a community of microbes that break down oil, freshwater environments have fewer naturally occurring oil-eating microbes. But that doesn’t mean we can rule out their impact after a spill.

“It would be instructive to know which microbial communities were most active during the oil weathering process,” says Blais.

The bottom line is that you don’t want crude oil to weather, says McMartin. “You want to identify it and contain it quickly. If it weathers, it’s increasingly difficult to remediate.”