<P> Concerns were raised about the appearance of underwater, horizontally extended plumes of dissolved oil . Researchers concluded that deep plumes of dissolved oil and gas would likely remain confined to the northern Gulf of Mexico and that the peak impact on dissolved oxygen would be delayed and long lasting . Two weeks after the wellhead was capped on 15 July 2010, the surface oil appeared to have dissipated, while an unknown amount of subsurface oil remained . Estimates of the residual ranged from a 2010 NOAA report that claimed about half of the oil remained below the surface to independent estimates of up to 75% . That means that over 100 million US gallons (380 Ml) (2.4 million barrels) remained in the Gulf . As of January 2011, tar balls, oil sheen trails, fouled wetlands marsh grass and coastal sands were still evident . Subsurface oil remained offshore and in fine silts . In April 2012, oil was still found along as much as 200 miles (320 km) of Louisiana coastline and tar balls continued to wash up on the barrier islands . In 2013, some scientists at the Gulf of Mexico Oil Spill and Ecosystem Science Conference said that as much as one - third of the oil may have mixed with deep ocean sediments, where it risks damage to ecosystems and commercial fisheries . </P> <P> In 2013, more than 4,600,000 pounds (2,100 t) of "oiled material" was removed from the Louisiana coast . Although only "minute" quantities of oil continued to wash up in 2013, patches of tar balls were still being reported almost every day from Alabama and Florida Panhandle beaches . Regular cleanup patrols were no longer considered justified but cleanup was being conducted on an as - needed basis, in response to public reports . </P> <P> It was first thought that oil had not reached as far as Tampa Bay, Florida; however, a study done in 2013 found that one of the plumes of dispersant - treated oil had reached a shelf 80 miles (130 km) off the Tampa Bay region . According to researchers, there is "some evidence it may have caused lesions in fish caught in that area". </P> <P> First BP unsuccessfully attempted to close the blowout preventer valves on the wellhead with remotely operated underwater vehicles . Next it placed a 125 - tonne (280,000 lb) containment dome over the largest leak and piped the oil to a storage vessel . While this technique had worked in shallower water, it failed here when gas combined with cold water to form methane hydrate crystals that blocked the opening at the top of the dome . Pumping heavy drilling fluids into the blowout preventer to restrict the flow of oil before sealing it permanently with cement ("top kill") also failed . </P>

Who paid to clean up the bp oil spill