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    Clay-Oil Flocculation and its Role in Natural Cleansing in Prince William Sound Following the Exxon Valdez Oil Spill

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    Natural interactions of fine mineral particles with residue oil and seawater, in a process called clay-oil flocculation, were found to create solids-stabilized oil-in-water emulsions on shoreline sediments at numerous locations in Prince William Sound following the Exxon Valdez spill. In laboratory tests using oiled sediment samples from Prince William Sound, these emulsions were shown to facilitate natural cleansing and dispersion of oil from sediments by moving water.

    Microscopic examination showed that the emulsions consisted of complex and varied floe aggregates of about 1–100 microns in diameter. Fluorescence under ultraviolet light revealed that the interior of these particles consisted of numerous spheres of oil surrounded by mineral fines and seawater. X-ray diffraction and scanning electron microscope studies showed the minerals to consist of clays, quartz, and feldspar-typical components of glacial flour. Chemical analyses of the oil revealed a correlation between the fraction of polar hydrocarbons present and the amount of mineral fines bound with the oil. These factors were also related to the ease by which the oil could be removed from substrate sediments by hydrodynamic forces.

    To investigate the effect of flocculation on natural cleansing, studies were conducted to determine the hydrodynamic energy needed for seawater to remove flocculated oil residues from sediments sampled from shorelines. Water was pumped at different velocities through a column packed with oiled sediment, and the amount and composition of oil removed from the sediment were measured as functions of water velocity and sediment movement. In separate tests, oil removal was observed in a wave tank that generated wave heights less than and greater than needed to move sediments. These tests demonstrated that, with clay-oil flocculation, substantial amounts of the oil could be removed at wave energies less than those needed to cause sediment movement and abrasion, which helps to explain observed removal of oil from the subsurface and from low energy shorelines where waves were not large enough to move sediments. Further, the large hydraulic cross section and nearly neutral buoyancy of floc particles removed from the sediment help to explain their efficient dispersal. The flocculation process also significantly increases the oil-water interfacial area, enhancing access to the oil by bacteria for biodegradation.

    Clay-oil flocculation was found to be a beneficial process that we believe accelerated the removal of oil from shoreline sediments following the Exxon Valdez spill. The physical properties of the floc should have facilitated dispersion of the oil over large distances, thus helping to reduce buildup of hydrocarbons in subtidal sediments. Clay-oil flocculation, therefore, is another natural cleansing process that must be considered when studying the fate of oil spilled on shorelines.


    oil, flocculation, natural cleansing, shoreline, Exxon Valdez, clay-oil interactions, oil-mineral interactions

    Author Information:

    Bragg, JR
    Exxon Production Research Company, Houston, TX

    Yang, SH
    Exxon Production Research Company, Houston, TX

    Committee/Subcommittee: E47.04

    DOI: 10.1520/STP19864S