<P> The oil leak was discovered on the afternoon of 22 April 2010 when a large oil slick began to spread at the former rig site . The oil flowed for 87 days . BP originally estimated a flow rate of 1,000 to 5,000 barrels per day (160 to 790 m / d). The Flow Rate Technical Group (FRTG) estimated the initial flow rate was 62,000 barrels per day (9,900 m / d). The total estimated volume of leaked oil approximated 4.9 million barrels (210 million US gal; 780,000 m) with plus or minus 10% uncertainty, including oil that was collected, making it the world's largest accidental spill . BP challenged the higher figure, saying that the government overestimated the volume . Internal emails released in 2013 showed that one BP employee had estimates that matched those of the FRTG, and shared the data with supervisors, but BP continued with their lower number . The company argued that government figures do not reflect over 810,000 barrels (34 million US gal; 129,000 m) of oil that was collected or burned before it could enter the Gulf waters . </P> <P> According to the satellite images, the spill directly impacted 68,000 square miles (180,000 km) of ocean, which is comparable to the size of Oklahoma . By early June 2010, oil had washed up on 125 miles (201 km) of Louisiana's coast and along the Mississippi, Florida, and Alabama coastlines . Oil sludge appeared in the Intracoastal Waterway and on Pensacola Beach and the Gulf Islands National Seashore . In late June, oil reached Gulf Park Estates, its first appearance in Mississippi . In July, tar balls reached Grand Isle and the shores of Lake Pontchartrain . In September a new wave of oil suddenly coated 16 miles (26 km) of Louisiana coastline and marshes west of the Mississippi River in Plaquemines Parish . In October, weathered oil reached Texas . As of July 2011, about 491 miles (790 km) of coastline in Louisiana, Mississippi, Alabama and Florida were contaminated by oil and a total of 1,074 miles (1,728 km) had been oiled since the spill began . As of December 2012, 339 miles (546 km) of coastline remain subject to evaluation and / or cleanup operations . </P> <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% . </P> <P> 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>

When was the deepwater horizon oil spill capped