Beached Valdez oil fled in floc.

Shortly after the 1989 Exxon Valdez accident dumped 10.8 million gallons of North Slope crude into Alaska's Prince William Sound, storms, tides, and work crews began washing the beached petroleum into the sea (SN: 2/13/93, p. 102). When little of this oil showed up in tidal or sea sediments -- even near beaches that had been heavily polluted -- spill-assessment teams began puzzling over its fate. Several studies now suggest that an emulsified mixture containing oil, clay, and sometimes silt was probably flushed far out to sea in the form of buoyant microscopic particles.

Exxon scientists discovered hints of this while assaying natural rates of Valdez-oil breakdown by Alaskan microbes (SN: 4/17/93, p.253). James R. Bragg of Exxon's research unit in Houston reported findings from follow-up studies last week in Atlanta at a meeting of the American Society for Testing and Materials.

In one study designed to simulate the effects of waves and tides, contact with seawater gave the black, sticky oil "the fluffy appearance of a flocculated emulsion" that no longer strongly adheres well to sediment, Bragg reports. Seawater made up about 80 percent of these cloud-like aggregates. The rest consisted largely of mineral grains stably bound to oil droplets 1 to 10 microns in diameter.

The concept of oil-sediment flocculation is not new. However, Bragg says, some geochemists expected the process to affect oil at sea - and especially to foster its sinking. Quite to the contrary, his new research suggests that in Alaska the process not only affects mainly beached oil but also virtually guarantees that such oil will float to the surface or remain suspended in the water. And the older (more weathered) the oil, the more likely it will emulsify, the studies indicate.

The new studies also have many important implications for understanding the disposition of the spilled Valdez oil.

For instance, the Exxon scientists found that flocculation can increase the area of the oil-water interface -- sometimes by up to 1,000 times. This increases the likelihood that the more toxic, water-soluble aromatic chemicals will leach from the oil, Bragg says. Moreover, it expands the area available for hydrocarbonhungry bacteria to latch on to, thereby facilitating the oil's breakdown. Indeed, water taken from oiled beach sediment revealed that active bacteria usually make up part of any naturally produced oil-clay floc.

Flocculation also helps explain another formerly puzzling observation: the relatively rapid disappearance of oil from even quiet, sheltered bays. Most researchers expected oil to persist in these areas, where abrading waves and sediment movement seldom occur, even during storms. But experiments by Bragg and his co-workers showed that waves too weak to move sediment sands could still drive the flocculation-fostered removal of oil, initially at rates oI 3 percent per hour. Bragg says it now appears that every ebb tide may remove some oil - even after heavy weathering has rendered oil tarry and very insoluble in water.

The unusually high levels of "mineral fines" -- clay "flours" produced by local glaciers -- along the southern Alaskan shoreline contributed to the unusually efficient emulsification of beached Valdez oil, the Exxon studies indicate. They also suggest that adding such flours to shores with low clay contents might augment the natural cleanup of oil spills in the future.