Long-term fate and effects of untreated thick oil deposits on salt marshes

The paper compares two case histories in which thick oil layers were deposited on salt marshes. The first concerns a spill in Milford Haven, Wales, in February 1969, which resulted in deposits of heavy fuel oil on parts of the Martinshaven marsh. The second concerns the Metula spill of light Arabian crude in the Strait of Magellan, Chile, in August 1974. Salt marshes near Puerto Espora, Tierra del Fuego, were affected by thick mousse. Following both spills, vegetation was smothered and killed. For a variety of reasons, including remoteness, there was no cleanup treatment in either case. At Martinshaven, vegetation recovery started within the first year after the spill, and was complete within 15 years. The oil layer is still clearly visible in core samples, but is not visible at the marsh surface because of sediment deposition. Samples analyzed in 1990, 22 years after the spill, were still recognizable as heavy fuel oil, albeit highly degraded. The Puerto Espora marshes were revisited in 1990 and 1991, 16 and 17 years after the spill. Mousse deposits were still visible at the surface. Samples analyzed in 1990 showed that oil beneath the surface skin of thick deposits was still quite fresh. There was no vegetation recovery in such areas. Where deposits were thinner, the mousse was well weathered and vegetation was recovering. Factors influencing recovery include oil type and weathering, oil thickness, sedimentation processes, size of area affected, and plant morphology. It is concluded that in extreme circumstances of extensive, thick oil deposits, natural recovery will take decades, so a cleanup program is likely to be justifiable. Cleanup may not be necessary for small areas of thick deposits on some marshes