More than 771,000 gallons of chemical dispersant were applied onto the spewing Macondo wellhead, nearly a mile beneath the surface, in the wake of the 2010 Deepwater Horizon oil spill disaster — a controversial and unprecedented attempt to break up the oil as it poured into the Gulf of Mexico.

But a new study of the cleanup effort suggests that sudden swings in temperature and pressure at that depth were enough to disperse the natural gas-soaked oil into tiny droplets, which “may have minimized the effectiveness of the (subsea dispersant injection) response approach.”

In other words, the extreme conditions by themselves achieved what the chemical dispersants were designed to do — possibly making the dispersants unnecessary.

“Given the potential for toxic chemical dispersants to cause environmental damage … unrestricted (subsea dispersant injection) application in response to deep-sea blowouts is highly questionable,” according to the report, which was published Tuesday. “More efforts are required to inform response plans in future oil spills.”

Eight years after the Deepwater Horizon caught fire and exploded about 50 miles off the Louisiana coast, killing 11 men and setting off one of the worst environmental disasters in U.S. history, many questions remain about the potential long-term impacts of exposure to the millions of gallons of sweet crude and large quantities of dispersants.

About 170 gallons of oil were spilled for each gallon of dispersant that was deployed.

The new study was led by researchers at the University of Miami. They analyzed levels of polycyclic aromatic hydrocarbons or PAHs — considered the most toxic components of petroleum — based on water chemistry samples taken within a six-mile radius of the blowout site.

The use of subsea dispersants in the cleanup response has drawn mixed reviews in the scientific community.

Last year, research led by the Woods Hole Oceanographic Institution argued that the subsea application of dispersants reduced the quantity of harmful gases in the air, making it safer for response workers trying to clean up the oil and contain BP’s runaway well.

Meanwhile, the National Academies of Sciences, Engineering and Medicine continues to assess the effects and efficiency of dispersants as an spill-response tool, as well as the trade-offs that come with their use. The research includes lab and field studies.

A month into the massive response effort in May 2010, the Environmental Protection Agency gave the British oil giant BP three days to find a less toxic alternative to Corexit, the dispersant it was using at and below the water’s surface. BP resisted, and the EPA backed off, instead telling the company to “significantly scale back the overall use of dispersants.”

Based on witness interviews, the Government Accountability Project, a whistleblower advocacy group, released a report in 2013 that said coming into contact with Corexit could lead to a host of ailments, including abdominal pain, hypertension, kidney and liver damage, memory loss and respiratory problems. The report urged a federal ban on the chemical.

The new study found that Corexit may have ultimately added to the ecological damage by suppressing the growth of natural oil-degrading bacteria and by increasing the oil’s toxicity.

“Our earlier work using computer modeling and high-pressure experimental approaches suggested that pumping chemical dispersants at the spewing wellhead may have had little effect on the amount of oil that ultimately surfaced. But empirical evidence was lacking until the release of the BP Gulf Science Data. When completely different approaches converge to the same conclusion, it is time to listen,” said Claire Paris, a professor of ocean sciences at the University of Miami, who led the new research.

“There is no upside in using ineffective measures that can worsen environmental disaster," Paris added.

The study suggested that oil companies develop alternative strategies to manage future blowouts, particularly those occurring in deeper waters. 

Follow Richard Thompson on Twitter, @rthompsonMSY.