The BP Blow-out Oil Material, Steady-state, Deepwater, Horizontal Plume: Analysis of this Opportunistic, Mesoscale, Hydrocarbon-tracer Field Data for Isopycnal and Diapycnal Eddy Diffusion Coefficients.

By all counts the 2010 deep water spill in the Gulf was a unique event. After the rig collapsed the oil-material entered the water column at the seabed 1544 meters depth continuously for 89 days. The near-field, vertical plume contained both liquid droplets and gas bubbles. The gas completely dissolved, no bubbles broke the surface. The liquid that formed the surface slick was nearly completely depleted of its soluble and volatile components. Pyrotechnics and “lighter” fluids were required to start it burning. This means a considerable quantity of oil-material made of the water-solubles and tiny oil droplets went elsewhere. The near-field vertical plume produced a “horizontal intrusion”. A distance ~1 to 2 km down current the blow-out point it morphed into a horizontal oil-material plume. This was a far-field plume not subject to the intense buoyancy forces of the near-field plume.

Controlled to a large degree by deep marine energetics and water stratification this plume was located and shown to occupy a layer between 1100 to 1200 meters depth. It was measured moving SW with average speed of 7.8 m/s. Water samples, along 8 vertical transects through the plume obtained from surface vessels during two sorties, defined the thickness based on the hydrocarbons within. The positions of the eight transects placed the plume layer 2 to 28 km down current (z-direction) with constant thickness maintained, and ~7 to 96 hours Lagrangian flow-time (t) from the blow-out point.

Two-dimension (z,t) concentration profile measurements of C1 to C5 alkanes and BETX (benzene, ethyl benzene, toluene and total xylenes) yielded a set of data equivalent to a designed, steady-state, tracer field experiment. Using classical diffusive transport analytical models, reasonable fitting the data allowed estimates of the effective eddy diffusion coefficients. The results will be presented and compared to reported marine tracer coefficients as well as coefficients for terrestrial streams [aka rivers]!