Intellectual Merit: Simulation of the subsurface and surface dispersal of oil in the Gulf of Mexico will be conducted with the objective of producing probabilistic envelopes of the spread of different size classes of oil as they age over time. The proposed model system is ready to respond. The SABGOM hydrodynamic model of the Gulf of Mexico and South Atlantic Bight has been successfully coupled with LTRANS, a fully three-dimensional Lagrangian particle tracking model capable of simulating sub-grid scale turbulent motion as well as time-varying particle attributes like diameter, density, and rise/sinking velocities. At distances greater than a few hundred meters above the deepwater source (depending on ambient current speed and stratification), the dispersal of oil depends mainly on the behavior of oil droplets which are fractionated into different sizes. These oil droplets can have orders of magnitude differences in ascent rates (e.g., 6 mm/s and 0.06 mm/s for 300 micron and 30 micron diameter particles, respectively) and change in diameter as they age. Emulsification, interaction with suspended particulate matter, dissolution and other processes can also affect droplet behavior. Our Lagrangian approach is ideally suited for simulating oil dispersal because differences in initial droplet characteristics and time-varying droplet behavior are readily incorporated. In this project, the coupled SABGOM/LTRANS model system will be run for the time period of the Deepwater Horizon oil spill, maps and animations of model output will be produce. The model results will be compared with available observations and will be made available to the oil spill response community.
In the near-term, a series of LTRANS simulations will be run using the existing flow field from recent SABGOM model simulations. The Lagrangian dispersion runs will be initialized with a continuous source of particles representing the near-field plume above the well. Each run will simulate the far-field dispersion of those particles based on a specific set of assumptions about particle behavior. As more complete information on the size and composition of gas bubbles and oil droplets emerge, the most realistic particle distributions from the LTRANS ensemble of runs will be selected. As part of this effort, an improved hindcast from the SABGOM model for use with LTRANS will be prodiced and the model skill will be quantified against physical oceanographic observations. In addition, Eulerian and Lagrangian predictions of oil dispersal will be quantitatively compared with observations in order to use the strengths of both approaches to provide the most realistic predictions for the oil response community.
Broader Impacts: Mid-term results will be open-source models and model results using existing and likely new, particle-tracking technology for the geosciences and oil-spill response communities. Incorporation of the model into the framework of the Community Surface Dynamics Modeling System (CSDMS) will ensure that the coding structures are suitable for coupling with other models and future distribution for research and educational purposes. In addition, the team members from the USGS will ensure that LTRANS can run with CF-compliant model output, making it functional with over seventeen coastal models, allowing simulations and forecasts to be made throughout the US coastal waters. In addition to providing timely information for oil spill responders, this project will lay the ground work for future efforts that investigate the interaction between oil and larval transport of commercially and ecologically important organisms in the Gulf of Mexico.
Intellectual Merit: We successfully coupled three computer models: 1) a model of a deepwater oil plume, 2) a hydrodynamic model, and 3) a trajectory model to simulate the dispersal of oil droplets from the Deepwater Horizon oil spill. The plume model provides the initial conditions (depth of oil droplets) for the trajectory model which simulates the transport of oil droplets using circulation predictions from the hydrodynamic model. In addition to being transported by circulation patterns, the oil droplets were programmed to rise (float) at a speed based on the density and diameter of the droplet, the density of water, and the rate at which the oil aged (was degraded). We compared model predictions to observations of subsurface hydrocarbons. Our coupled model successfully reproduced observations of the subsurface plume. We applied the model to better understand how oil droplet size and aging rates influence the transport and dispersal of subsurface plumes of oil droplets. Our major findings are: Subsurface plumes of oil droplets from the Deepwater Horizon spill originated due to the nature of the oil-and-gas plume as well as the characteristics of the surrounding waters. Model predictions of plume depths agree with observations that the majority of the hydrocarbons were found between 800 and 1200 m. Trajectory model predictions indicate that oil droplets with diameters less than 80 microns formed subsurface plumes. Subsurface droplet distributions compared well with observations of the subsurface plume in late June 2010. Model predictions suggest that subsurface oil droplets were transported to the northeast and east of the wellhead in addition to those being observed toward the southwest. Oil droplet degradation rates significantly influence subsurface transport of oil droplets. Broader Impacts: Our research has promoted a better understanding and prediction of the formation and dispersal of subsurface plumes of oil droplets. This information has been communicated to oil spill managers, members of the geoscience community, and graduate and undergraduate student via presentations and publications. In addition, our models are open-source, which means that the technologies we developed are freely available. The trajectory model has been updated and incorporated into the Community Surface Dynamics Modeling System which will allow broad application and enable wide distribution for research and educational purposes. In terms of teaching and training, twelve graduate students were trained on oil spill fate and effects through a seminar entitled "Dispersion, degradation, and ecosystem effects of oil in the marine environment".
