Research suggests added dimension to use of dispersants in Deepwater Horizon oil spill
When the Deepwater Horizon offshore drilling rig exploded, the US government and industry were forced to take emergency measures to stem the flow of oil from the rig’s ruptured sub-sea wellhead, which sent 900,000 tonnes of oil and natural gas into the Gulf of Mexico.
Emergency response measures sanctioned by the US government involved injecting chemical dispersants a mile below the ocean surface, in an attempt to break up spilled oil and prevent pollutants from reaching environmentally sensitive regions.
The practice is not uncommon. Generally, oil spill clean-ups focus on removing or dispersing oil on the ocean surface and shoreline, limiting damage to these habitats which are deemed more ecologically important than the murky depths of the deep ocean.
Some seven years later, and safety and effectiveness of using dispersants is still under question. Opponents to the Deepwater Horizon case claim the dispersants themselves were toxic, and may have actually exacerbated environmental damage, while being on the whole ineffective at breaking up the dispersed oil.
But today it has emerged that there may have been an added dimension to the use of dispersants in the oil spill clean-up, as the chemicals may have enabled responders to work more effectively to tackle the problem at its source.
According to a report published in the Proceedings of the National Academy of Sciences, in the case of Deepwater Horizon, chemical dispersants may have significantly reduced the amount of harmful gases in the air at the sea surface, diminishing health risks for emergency workers and enabling them to keep working to stop the spill and clean it up sooner.
The work was carried out by collaborators from the Woods Hole Oceanographic Institution, New Jersey Institute of Technology, Texas A&M University and the Swiss Federal Institute of Aquatic Science and Technology.
As part of their research, the team developed physical models and computer simulations to determine the course the oil and gas took following the eruption, including the fraction of larger, more buoyant droplets that floated to the surface and the amount of smaller droplets entrapped deep below it due to sea stratification and currents.
Michel Boufadel, director NJ IT’s Center for Natural Resources Development and Protection (NRDP) and Lin Zhao, a post-doctoral fellow at the centre, developed a model to predict the size of droplets and gas bubbles emanating from the wellhead during the sub-surface blowout. They factored water pressure, temperature and oil properties into the model, and then employed it to analyse effects of the injected dispersants on this stream.
Researchers at Texas A&M then created a model to study movement of pollutants away from the wellhead. It determined that use of dispersants had substantial impact on air quality in the region of the spill by reducing toxic compounds such as benzene that reached the surface of the ocean. As a result, emergency workers on the scene were protected from the full brunt of pollution.
“Government and industry responders were faced with an oil spill of unprecedented size and sea depth, pitting them in a high-stakes battle against big unknowns,” Christopher Reddy, a senior scientist at Woods Hole Oceanographic Institution, and Samuel Arey, a senior researcher at the Swiss Federal Institute of Aquatic Science and Technology, wrote in Oceanus magazine.
“Environmental risks posed by deep-sea petroleum releases are difficult to predict and assess due to lack of prior investigations,” added NJIT’s Boufadel. “There is also a larger debate about the impact of chemical dispersants. There is a school of thought that says all of the oil should be removed mechanically.”
Boufadel highlighted the water-soluble and volatile compounds that did not reach the surface were entrapped in a water mass that formed a stable intrusion at 900 to 1,300 metres below the surface.
“These predictions depend on local weather conditions that can vary from day to day. However, we predict that clean-up delays would have been much more frequent if subsurface dispersant injection had not been applied,” Reddy and Arey said, “But this is not the final say on the usage of dispersants.”