Staged Spill Sheds Light on Beach Clean-Up

Images of a shoreline after a marine oil spill are often of birds and other wildlife struggling to survive with oil-coated bodies. But there is another longer-term image many people never see: a beach still contaminated years later with sticky black residue. Oiled shorelines pose a major challenge for clean-up crews—particularly if beaches are a porous mix of sand and pebbles or cobbles. To learn more about on-site methods of removing oil from such mixed sediments, an international team of scientists staged an experimental spill on a remote Arctic island some 800 kilometres north of Norway.

The experiment was conceived and coordinated by Environment Canada and sponsored by spill response agencies from Canada, the United States, the United Kingdom, Sweden, and Norway. It replicated the effect of an actual spill event by applying 5500 litres of oil to a three-metre-wide swath in the upper intertidal zone at three beach sites on the island of Spitsbergen. Various in-situ treatments were applied a week later—to simulate the response time for a remote location—and the quantity of oil at each plot was monitored over the first 60 days, and again one year later. Changes in the physical character of the beach, oil penetration and movement, toxicity and biodegradation were also noted.

When beaches are oiled, the physical action of the waves washes the oil off the surface of the sediment and into the water column, where it disperses and eventually biodegrades. Oil that is stranded above the normal active intertidal zone, where it only gets wet once in a while, or that penetrates deep below the surface can persist for decades. The latter situation is of particular concern in Canada, because porous coarse and mixed sediment beaches are common on both the east and west coasts, and are difficult to clean up.

The treatments tested were, therefore, aimed at increasing the exposure of oiled sediment to the natural processes of wave action and biodegradation—both in the water column and in the sediment itself. They included moving the oiled sediment closer to the wave action by using a small bulldozer; tilling it with a plow; and applying soluble and slow-release fertilizers to stimulate microbial activity. The three methods were also compared to no treatment—that is, leaving the sediment to natural cleaning processes.

Oil biodegradation occurred in both the oiled sediment and the fine oil-mineral aggregates washed into the water by waves. None of the techniques elevated toxicity in the nearshore environment to unacceptable levels or resulted in the oiling of the nearshore sediment. There were, however, other significant differences.

Sediment relocation significantly accelerated the rate of oil removal, and reduced oil persistence by at least one year in a relatively high wave-energy environment where oil was stranded above the level of normal wave activity. It also sped the short-term rate of oil loss on relatively low wave-energy shorelines where the stranded oil was in the zone of wave action. Following treatment, the quantity of oil bound in the oil-mineral aggregates—an important natural process that helps remove oil from sediment—also increased three-fold as a result of the increased availability of fine mineral particles.

Bioremediation was also proven an effective treatment, doubling the rate of natural biodegradation. Changes in total oil loading from sediment tilling, on the other hand, were small—however, results suggest that mixing the shoreline sediment made it substantially more permeable to seawater and air for at least 10 days after tilling, which could lead to enhanced microbial activity.

In untreated areas, the natural removal rate of oil stranded in the active intertidal zone was relatively rapid in the first 10 days, but slowed dramatically after that. Four to five per cent of the original oil residue remained in the intertidal zone after a year, and a significant quantity of oil stranded above the normal intertidal zone was present after the same period.

These results prove the effectiveness of sediment relocation as a viable tool for removing oil from coarse or mixed sediment shorelines—particularly in remote or sensitive areas, where the removal of sediment for off-site treatment is not feasible. They also improve our understanding of natural oil removal processes and their application as a response option.

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