Flow rates, fates of Gulf spill contaminants assessed from the air

WASHINGTON—Scientists have found a way to use air chemistry measurements taken hundreds of feet above last year’s BP Deepwater Horizon oil spill to estimate how fast gases and oil were leaking from the reservoir thousands of meters underwater.

The researchers also determined the fate of most of those gas and oil compounds using atmospheric chemistry data collected from aircraft last June. They say their new methods could be applied to future oil spills, whether in shallow or deep water.

The new analysis has been accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union.

“We present a new method for understanding the fate of most of the spilled gases and oil,” says Tom Ryerson, lead author of the report, from NOAA’s Earth System Research Laboratory in Boulder, Colo. “We found that the spilled gases and oil (spilled fluid) obeyed a simple rule: whether a compound can dissolve or evaporate determines where it goes in the marine environment. That simple rule, and the methods we lay out in this paper, could enable airborne evaluation of the magnitude of future spills.”

Knowing where the spilled gas and oil mixture ended up could also help resource managers and others trying to understand environmental exposure levels.

Using the atmospheric measurements and information about the chemical makeup of the leaking reservoir fluid, Ryerson and his colleagues calculate that at least 32,600 to 47,700 barrels of liquid gases and oil poured out of the breached reservoir on June 10, 2010. This range, determined independently of previous estimates, presents a lower limit.

“Although we accounted for gases that dissolved before reaching the surface, our atmospheric data are essentially blind to gases and oil that remain trapped deep underwater,” Ryerson says. Comparison of the new result with official estimates is not possible because this airborne study could not measure that trapped material.

Not including that trapped material, atmospheric measurements combined with reservoir composition information show that about one-third (by mass) of the oil and gas dissolved into the water column on its way to the surface. The team found another 14 percent by mass, or about 258 metric tons per day (570,000 lbs, per day), was lost quickly to the atmosphere within a few hours after surfacing, and an additional 10 percent was lost to the atmosphere over the course of the next 24 to 48 hours.

Among the study’s other key findings:

• Some compounds evaporated essentially completely to the atmosphere, which allowed scientists to make an estimate of flow rate based solely on atmospheric measurements and reservoir composition information.
• Airborne instruments picked up no enhanced levels of methane, the lightest-weight hydrocarbon in the leaking reservoir fluid, showing that it dissolved essentially completely in the water column.
• Benzene — a known human carcinogen — and ethane were found in only slightly elevated concentrations in the air, meaning they dissolved nearly completely in the water.
• A number of slightly heavier carbon compounds ended up in both the air and water, with the precise fraction depending on the compound. Based on these data, the team inferred different exposure risks of mid- and shallow-water marine species to elevated levels of potentially toxic compounds.

A portion of oil and gas was “recovered” by response activities and piped from the leaking wellhead to theDiscoverer Enterprise drill ship on the ocean surface. The research team calculated this recovered fraction by measuring emissions from natural gas flaring aboard the recovery ship. They calculate a recovery rate of 17,400 barrels of reservoir fluid (liquid gas and oil) for June 10, and which accounts for approximately one-third to one-half of the group’s total estimate of 32,600 to 47,700 barrels of fluid per day.

Ryerson and his colleagues conclude that the technique they developed could be applied to future oil spills, in shallow or deep water. The research flights, conducted at a minimum of 60 meters altitude (200 feet) above the Gulf surface, were possible because a NOAA WP-3D research aircraft had already been outfitted with sensitive chemistry equipment for deployment to California for an air quality and climate study and was redeployed to the Gulf.