DISTRIBUTION AND DEGRADATION OF DIESEL OIL IN THE UNSATURATED ZONE FOLLOWING AN OIL-SPILL ON A CHALK AQUIFER

In 1976, there occurred a substantial loss of diesel oil from a storag e facility at Royston in eastern England. The site is on the outcrop o f the important Chalk aquifer, which is protected by an unsaturated zo ne 24-30 m thick. In 1986, a cored borehole was drilled through the si te of the spillage to investigate the fate of the contaminants. The co re samples were analysed by physical and chemical methods to determine the physical structure of the rock, and the characteristics and distr ibution of the oil. The chemically analysed samples included pore wate r extracts, scrapings from fracture surfaces, and non-fracture (matrix ) samples. The results indicate that oil accumulated within a few mill imetres of major fissure surfaces, and entry into the rock matrix was limited by the small size of pores and the presence of water. Oil may also have migrated along microscopic channels away from the major fiss ures. There was no evidence of downward migration of oil since the ini tial phase of movement. The adoption of certain assumptions regarding degradation, evaporation and dissolution processes allows the estimati on of oil depletion caused by these processes. Physical weathering and degradation were found to have been extensive, but highly variable. B oth processes occurred on the major fissure surfaces but, in the matri x, degradation appears to have been restricted. The conclusions have i mplications for the investigation and remediation of fissured Chalk aq uifers contaminated by oil.