Application of dispersants to surface oil slicks by aircraft and by boat in approved areas of the Gulf of Mexico from April 22, 2010 to final application on July 19, 2010

The use of dispersants began shortly after the Deepwater Horizon (DWH) accident occurred in April of 2010, and ended shortly after the Mississippi Canyon lease block 252 (MC-252) well was capped in July 2010. The U.S. Coast Guard Federal On-Scene Coordinator (FOSC) and U.S. Environmental Protection Agency (EPA) authorized their use to disperse oil that could not be removed by other methods, with the goal of reducing the amount of oil in areas where persons were working onsite to cap and contain the well, and reducing the amount of oil that reached nearshore and shoreline habitats. Dispersants were applied to oil slicks located on the water surface at least 3 nautical miles (nmi) or more from the shoreline. EPA, BP, and others collected samples of water and air around the areas where dispersants were used, and along the Gulf shoreline, to determine whether any person or marine life was exposed to potentially significant levels of the dispersants or dispersed oil. This report contains information about surface dispersant application, and provides additional context for the posted dataset. Characteristics of this dataset include the following: • This dataset combines results from aerial and vessel dispersant spraying activities during the DWH Response in a unified data file. • Before posting, extensive work was done to verify some aspects of the posted information (i.e., positional coordinates and field data). Surface dispersant application began on April 22, (field trial), and continued on days when the FOSC approved surface dispersant use until the well was capped on July 15, 2010. The last application of dispersant occurred on July 19, 2010. An additional application of 50 gallons of dispersant by boat on clean water was conducted on July 28, 2010 to obtain baseline fluorometry measurements, with a smaller volume used on one of the vessels at the source during recovery of the capping stack on September 4, 2010. The two dispersants used for surface application during the response, Corexit EC9527A and Corexit EC9500A, were listed on EPA’s National Contingency Plan Product Schedule (U.S. EPA 2010c) and preauthorized for use in the Gulf of Mexico.