OILED SEDIMENTS IN THE HUMBER ESTUARY FOLLOWING THE SIVAND INCIDENT

Abstract

ABSTRACT The Sivand oil spill of September 28, 1983, was unusual in that 6,000 t of crude oil were spilled in a highly turbid estuary. This study aimed to contribute to knowledge of spatial and temporal oil distribution and to examine its degradation. The role of suspended particulate matter (SPM) in the fate of oil is discussed, but the study concentrates on stranded oil in two sedimentologically distinct shorelines where ecological and conservation interests are high. The sites were Humberston Fitties, a sand flat, and Blacktoft Sand, a muddy shore in the inner estuary. Methods used were replicate sediment core sampling followed by gas liquid chromatography (GLC) and included ancillary sediment analyses. Soon after the spill, the sand total hydrocarbon (THC) concentration was greater than 50,000 ppm. Over 12 months this concentration decreased to about 3,000 ppm, as expected in coarser sediments. The muds were initially less contaminated (about 2,000 ppm), with only slight reductions in concentration over 12 months. However, there was strong evidence of sediment mobility at this site, and natural cleanup was in fact nearly complete after 9 months; the oil found at 12 months was due to transport of Sivand oil-contaminated muds from nearby reed beds. Good drainage at both sites permitted initial oil penetration. Root macropores at the muddy site encouraged penetration, weathering, and release of oil more easily than if it had been found in the sediment matrix. Sediment mobility at the muddy site contrasted with the firmness of the oiled sandflat, and contrary to vulnerability indices, may imply more rapid oil removal under some circumstances. Water washing was implicated as a major process in oil degradation at the sandy site, but no dramatic sediment turnover or erosion occurred that might have removed the oil. The steep gradients in oil concentration and composition with depth contrasted with the muddy site, where oil penetrated as a coherent mass via root macropores. The study illustrates the importance of sedimentological factors in the fate, effects, and mass balance of spilled oil. Published work on the Sivand has emphasized the effectiveness of cleanup and the lack of biological effects, but may have underestimated the amount of oil trapped in sediments.