An estimate of 20,000 gallons of oil from the 1989 Exxon Valdez tanker spill that devastated Alaska's Prince William Sound still lies trapped beneath its beaches, continuing to pollute once pristine shores.
Michael Boufadel and Hailong Li of Temple University in Philadelphia, Pennsylvania collected field data and run computer simulations and found the key lay in the fact that affected beaches consisted of two layers, each with different properties; also, the geographically variable impact of rising and falling water tables played a critical role.
Oil was temporarily stored in the porous upper layer, slowing the rate at which it was subject to weathering, according to the study. An environment lacking the kind of nutrients needed by oil-eating micro-organisms to thrive further protected the fossil fuel.
The second layer, while composed largely of the same materials, was far less porous: on average, water moved through the top layer 1,000 times faster.
When the water level from declining tides fell below the interface between the two layers, oil seeped from the upper to the lower stratum, especially where there was little or no freshwater discharge to compensate.
Because of the even lower oxygen content in the sub-stratum, the crude was not degraded and has remained suspended.
As climate change opens the Arctic region to oil exploration and shipping, the findings could prove crucial in devising effective methods for cleaning up future spills, the researchers said.
Source: Yahoo News Singapore
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