Dispersants as used in response to the MC252-spill lead to higher mobility of polycyclic aromatic hydrocarbons in oil-contaminated Gulf of Mexico sand

Abstract

After the explosion of the Deepwater Horizon oil rig, large volumes of crude oil were washed onto and embedded in the sandy beaches and sublittoral sands of the Northern Gulf of Mexico. Some of this oil was mechanically or chemically dispersed before reaching the shore. With a set of laboratory-column experiments we show that the addition of chemical dispersants (Corexit 9500A) increases the mobility of polycyclic aromatic hydrocarbons (PAHs) in saturated permeable sediments by up to two orders of magnitude. Distribution and concentrations of PAHs, measured in the solid phase and effluent water of the columns using GC/MS, revealed that the mobility of the PAHs depended on their hydrophobicity and was species specific also in the presence of dispersant. Deepest penetration was observed for acenaphthylene and phenanthrene. Flushing of the columns with seawater after percolation of the oiled water resulted in enhanced movement by remobilization of retained PAHs. An in-situ benthic chamber experiment demonstrated that aromatic hydrocarbons are transported into permeable sublittoral sediment, emphasizing the relevance of our laboratory column experiments in natural settings. We conclude that the addition of dispersants permits crude oil components to penetrate faster and deeper into permeable saturated sands, where anaerobic conditions may slow degradation of these compounds, thus extending the persistence of potentially harmful PAHs in the marine environment. Application of dispersants in nearshore oil spills should take into account enhanced penetration depths into saturated sands as this may entail potential threats to the groundwater.

Figure 1. Setup of the Short-Column Experiment.

Clean seawater, crude oil dispersed by sonication, or crude oil dispersed by Corexit and sonication were flushed through the sand columns by gravity. The effluent of the columns was collected as a time series in 4 vials each.

Figure 2. PAH concentrations in seawater after the addition of oil or oil and Corexit.

PAH concentrations were measured in the seawater 24 h after the addition of Deepwater Horizon Crude oil or addition of the same amount of oil with Corexit. Note logarithmic Y-axis scaling.

Figure 3. Distribution of fluorescence and depth distribution of PAHs after the Short-Column Experiment.

Upper left pane: Fluorescence signal vs. sand depth.; Other panes: PAH concentrations vs. sand depth. The white symbols represent the control experiments, the grey symbols the experiments with oil (treatment I), and the orange symbols the experiments with oil and dispersant (treatment II). Note logarithmic concentration scale in PAH plots, not all PAH species are shown.

Figure 4. Cumulative release of acenaphthylene and chrysene as a function of the volume of water released from the columns.

Triangles indicate the first experiment, circles the duplicate. The white symbols represent the control experiments, the grey symbols the experiments with oil (treatment I), and the orange symbols the experiments with oil and dispersant (treatment II).

Figure 5. Fluorescence signal of the Long-Column Experiments.

The white symbols represent the control experiments, the grey symbols the experiments with oil (treatment I), and the orange symbols the experiments with oil and dispersant (treatment II).

Figure 6. Results of the Long-Column Experiment.

Concentrations of different PAHs in the outflowing water from of sand columns with different lengths: upper row) control columns, middle row) columns that received seawater with oil and lower row) columns that received seawater with oil and Corexit. Note the differences in y-axis scaling.

Figure 7. Results from the in-situ chamber incubations.

Left pane: chamber experiment 1. No dispersant was applied. Right pane: Results from chamber experiment 2. The DOC measurements were less sensitive than the fluorescence measurements and included all dissolved organic carbon. Error bars depict standard error.

Figure 8. PAHs retained in the sand during the Short-Column Experiment.

The columns show the percentage of PAH that was retained in the sediment of the total PAH amount added to the Short-Column Experiments.

Figure 9. Calculated penetration depths for oil concentrations and PAH measured in our Short-Column Experiment one in the presence of Corexit.

Depths were extrapolated from the of PAH concentrations in the sediment sections assuming an exponential trend. Anthracene data did not allow calculation of a trend due to large scatter of data.

Figure 10. Relationship between inferred penetration depth and the hydrophobicity (log K_ow) of the PAHs.

K_ow = Concentration in octanol phase/Concentration in aqueous phase. The K_ow values for the PAHs were retrieved from http://www.env.gov.bc.ca/wat/wq/BCguidelines/pahs/pahs-01.htm.