Contaminated ground and contaminated estuary sediment illustrated by two case histories

Contaminated ground forms a problem in all of the industrialized countries of the world. Contaminated ground may give rise to hazards and that implies a degree of risk which also involves a problem of definition. The investigation of a site which is suspected of being contaminated differs somewhat from a routine site investigation. Sampling of soil, groundwater and gas-producing material may be required. Various precautions may be necessary to do this and personnel may have to wear protective clothing. The first case history considered involves a site investigation for a relief sewer in Glasgow. As the site investigation progressed it ran into made-ground which contained chemical waste. The presence of this waste meant that the nature of the investigation changed and much more stringent safety precautions had to be taken. It also meant that the initial location of the sewer tunnel had to be repositioned at greater depth in uncontaminated sandstone rather than in the superficial deposits above. The other case history considers the contamination of sediments in the Forth Estuary. When trace metals are released into the water column they can be transferred rapidly to the sediment phase by adsorption onto suspended particulate matter, followed by sedimentation. Intertidal flats may be considered as important trace metal sinks since they accumulate large amounts of suspended matter. Hence, in polluted estuaries the deposition of suspended particles on intertidal flats may thus cause severe contamination. The Forth Estuary has unique contamination for British estuaries; it is experiencing significant Hg pollution. In addition, due to the presence of a nuclear submarine base in the Forth Estuary, Co-60 is detectable in the intertidal sediments. Temporal and spatial contamination patterns were analysed in relation to historical and present pollution point sources. The effect of fluvial and marine sediment mixing on trace metals and other processes controlling contaminant levels is reviewed. Preliminary results on quantifying sediment accretion rates using Caesium levels are discussed.