SEDIMENTARY EVOLUTION OF DEEPWATER HORIZON/MACONDO OIL ON THE GULFSIDE BEACHES OF MISSISSIPPI, ALABAMA, AND WESTERN FLORIDA: OBSERVATIONS OF A SCAT TEAM MEMBER

Observations of the sedimentary evolution of oil along the Gulfside beaches of Mississippi, Alabama, and western Florida were made in conjunction with shoreline cleanup assessment technique (SCAT) surveys for the Deepwater Horizon spill from early June 2010 until present. Daily helicopter over-flights while the oil was coming ashore gave a broad perspective of its distributional patterns along the sand beaches from Mississippi to western Florida. Repeated ground surveys provided a detailed view of the oils’ sedimentary behavior within the beach and surf zones.

The spill, as it affected sand beaches, can be divided into two phases. Phase 1 was from early-mid June to early July 2010 when floating emulsified oil (mousse patties) stranded onshore in appreciable quantities. High wave and tide conditions resulted in overwash and flooding of the backshore, deposition of oil well into the supratidal zone, and burial to depths of as much as one meter beneath beach berms. Once stranded, the adherence of sand to mousse patties increased their density, promoted burial, and began their transformation into oiled sand aggregates. Oiled sand reworked by wave action from the beachface was generally broken down into smaller aggregates in the surf. Under heavy accumulations, they coalesced into subtidal “oil/sediment” mats in surf zone depressions. As supratidal pools were heated by the sun, submerged oil separated from sand, floated to the surface, and was re-deposited in concentric rings around the pools as water levels decreased.

Phase 2, the period from mid July through December 2010, was characterized by lower water levels and minimal overwash of the berm crest. As a result, the major processes influencing oil deposits in the supratidal zone have been eolian and anthropogenic. Repeated strong northerly winds during Fall/Winter have exposed and concentrated previously buried oil as lag deposits on the surface. Eolian processes exposed oil in areas that have been repeatedly cleaned using both manual and mechanical methods, and wave action exposed oil as the beachface eroded.

Intensified wave energy from the southern quadrant in the future may prove effective in removal and breakdown of oil from these beaches left after active cleanup is terminated.