Evidence that the Deepwater Horizon Oil Spill Caused a Change in the Nickel, Chromium, and Lead Average Seasonal Concentrations Occurring in Sea Bottom Sediment Collected from the Eastern Gulf of Mexico Continental Shelf Between the Years 2009 and 2011

Abstract

Preliminary analysis indicates a change with time in nickel (Ni), chromium (Cr), and lead (Pb) concentrations in sea bottom sediment samples which were collected during the falls of 2009, 2010, and 2011 along the continental shelf of the Eastern Gulf of Mexico. These samples bracket the Deepwater Horizon Oil Spill which occurred during the spring and summer of 2010. The sea bottom samples were collected as part of the recurring NOAA small pelagics survey of the continental shelf from Texas to Florida. A Shipek grab sampler was used to retrieve 37 sediment samples from the upper 15 cm of the sea bottom for analysis from the carbonate platform off of the west coast of Florida and relict sand off of the coast of Alabama. The samples were analyzed for six trace metals, Ni, Cr, Pb, vanadium (V), thallium (Tl), and mercury (Hg). The silt/clay fraction of the sediment had the highest adsorption affinity for Ni, Pb, and Tl metals relative to V, Cr, and Hg. A comparative analysis of the trace metal concentrations indicates that there is a statistically significant (at p < 0.05 or p < 0.01) increase in Ni, Cr, and Pb concentrations in sea bottom sediment 3 months after the oil spill. One year later, however, the 2011 samples indicate that Ni, Cr, and Pb significantly decreased (at p < 0.05 or p < 0.01) in comparison to 2010. Vanadium did not statistically change from 2009 to 2011. Crude oil is known to contain V and Ni, and the V–Ni ratio (V/V+Ni) has been used as a fingerprint for the Macondo crude oil source. The V–Ni ratios for the sediment sampling years 2009, 2010, and 2011 were 0.52, 0.46, and 0.57, respectively. In this study, approximately 1 year and 3 months after the oil spill, there is evidence, such as the V–Ni ratio, that trace metal concentrations in sea bottom sediment are approaching pre-spill levels. We conclude these differences in Ni concentrations, and the V–Ni ratios are an effective means of monitoring the impact of an oil spill and the use of dispersants to alleviate the spill impact.