Sample collection map. The sample collection sites are shown here on a map of the northern coast of the Gulf of Mexico. Sample sites 1-3 and 5-8 are pooled into the unexposed reference samples 1 and 2 respectively and were collected during April of 2008. Samples from site 4 were taken on June 28, 2010, when the oil had been present at the location for at least 2 weeks. The blue star indicates the position of the Deepwater Horizon blowout which occurred on April 20, 2010 and was effectively uncontrolled until July 15, 2010, though the well was not officially sealed until September 19, 2010.