Fluorescence-estimated oil concentration (Foil) in the Deepwater Horizon subsea oil plume

Abstract

Tracking the subsea oil plume during the 2010 Deepwater Horizon Oil Spill (DWH) was conducted using in situ fluorescence via vertical profilers (n = 1157) and discrete sample chemical analyses (n = 7665). During monitoring efforts, discrete samples provided a coarse picture of the oil plume footprint, but the majority of the samples were below standard analytical detection limits for petroleum hydrocarbons. In situ fluorescence data improved the spatial and temporal resolution of the subsea oil plume characterization. Here we synthesized millions of continuous fluorescence data points from hundreds of contemporaneously discrete samples collected to demonstrate how fluorescence could serve as a proxy for Benzene-Toluene-Ethylbenzene-Xylene (BTEX) concentration. Data mined from Gulf Science Data repository were well correlated, and geographically and temporally aligned to provide direct comparisons. Described here are the methods used to calibrate the fluorescence data and to spatially approximate the three-dimensional geographic extent of the oil plume.

Keywords: Deepwater Horizon; Oil spill surveillance; Petroleum hydrocarbons; Submersible fluorescence.

Fig. 1.

Three-dimensional schematic of matching vertical cast CDOM values to discrete bottle samples based on spatial proximity. Gray cylinders represent discrete sample bottles. Blue color is seafloor. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 2.

Spatial and temporal distribution of all vertical profile cast locations during DWH response. (Top panel) Locations are coded by maximum fluorescence intensity in the subsea plume between 800 and 1400 m. The Macondo wellhead near the mouth of the Mississippi River Birdfoot region is marked by a yellow cross. (Bottom panels) Gray symbols represent all cast locations during response efforts. Black symbols represent sampling locations for each month. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3.

CDOM fluorescence depth profile casts from two different response vessels, prior to calibration (a) and after standardization applied (b).

Fig. 4.

Vertical depth profile of CDOM fluorescence (ppb QSE) and estimated BTEX F_oil (μg/L). Each point represents in situ fluorescence value. Blue points represent the estimated submerged oil plume. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 5.

Linear regression and prediction confidence of fluorescence with BTEX and TPAH from bottle samples.

Fig. 6.

Percent difference between BTEX measured in bottle samples and estimated with fluorescence (F_oil) in the subsea plume. Median, upper quartile and lower quartile BTEX values are marked for given CDOM concentrations.

Fig. 7.

Spatial footprint of subsea plume of BTEX F_oil (μg/L) values in 50 m increments from 1050 m to 1350 m. May values shown in red shades, June values shown in gray shades. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 8.

Distribution of BTEX F_oil in the subsea plume. Inset panel expands the portion of the plot extending out to 25 km from the wellhead and represents the maximum distance from the wellhead most appropriate for BTEX F_oil values.