Deepwater Horizon oil spill: using microcosms to study effects of crude oil in coastal sediments
Approximately 4.9 million gallons of crude oil traveled with ocean currents to reach the Gulf coast after the Deepwater Horizon oil drilling rig explosion. Microcosm experiments were conducted to determine how oil contamination affects the concentration and distribution (between solid and aqueous phases) of trace elements in a salt marsh environment. Sediment and seawater from a salt marsh at Bayou La Batre, Alabama, were measured into jars and spiked with 500 ppm MC-252 oil. The solid phase and aqueous samples were analyzed by ICP-OES, ICP-MS, and IC. A second experiment was conducted using various concentrations (0 ppm, 10 ppm, 100 ppm, 500 ppm, 1000 ppm, 2500 ppm) of MC252 oil. ICP-OES data show variations in aqueous elemental concentrations occurred over the 14 day experiment. The pH for the water in the experiments ranged from 6.93 to 8.06. Significant positive correlations (r>0.75) were found in the solid phase samples between iron and the following elements: aluminum, cobalt, chromium, and nickel. Aqueous iron concentrations were highly correlated (r>0.75) with solution pH. The presence of iron oxide and clays in the salt marsh sediment indicates potential for adsorption of trace elements sourced from the environment and from crude oil contamination. The release of aqueous Fe (II) observed between two and 14 days is likely caused by reductive dissolution of iron-bearing clays or iron oxide. All the samples that contained oil behaved in similar ways with respect to time, but the controls showed almost no changes in the concentrations of the trace elements. Although the levels of some trace elements in the solid phase changed during the experiments, their final concentrations were at the same levels as the control samples. With the exception of nickel, the 14 day samples contained lower trace metal concentrations than the sterile control which contained no oil. The reason for this is likely attributable to the in situ oil-degrading bacteria, which were found to be present in the sediment. The oil-degrading bacterial community increased in the presence of oil and decreased as the oil concentration decreased. Oil-degrading bacteria are capable of inducing reductive dissolution in Fe (III) minerals.