THE USE OF OVERBANK SEDIMENT FOR GEOCHEMICAL MAPPING AND CONTAMINATION ASSESSMENT - RESULTS FROM SELECTED ENGLISH AND WELSH FLOODPLAINS

Overbank sediment profiles from floodplains in England and Wales conta in a record of both natural geochemical patterns and those showing the influence of man's activities. It has been suggested that this charac teristic can be used to allow maps to be compiled which show human imp act on the fluvial geochemical environment. Studies reviewed in this p aper, however, show that a single overbank profile very rarely spans t he period from before anthropogenic disturbance through to the Industr ial Revolution and later. Significant lateral variations in metal conc entrations occur also over a relatively small area in overbank sedimen ts of the same general age. These, and the nature of vertical changes in chemistry, make the choice of sample sites, and sampling interval w ithin a profile, difficult. Even sediments which appear uncontaminated may record anthropogenic influences from activities such as deforesta tion and agriculture. A means of dating the sediment and an appreciati on of river erosion and sedimentation histories are shown to be essent ial in order to ensure that maps intended to depict natural geochemica l variations are based on material deposited before disturbance of the cathment by human activity. These considerations and associated costs may render overbank sediment non-viable as a regional geochemical map ping medium.