||Aptek, Inc., Colorado Springs, CO. ;Texas Transportation Inst., College Station.;John A. Volpe National Transportation Systems Center, Cambridge, MA.;Federal Highway Administration, McLean, VA.
Stream sediment is deposited on the bed of the active river channel, whereas overbank or floodplain sediment is produced when major floods occur in a river system. During such floods, the water discharge exceeds the quantity that can pass through the ordinary river channel (bankful discharge). Even in streams of moderate size, the water level may reach several metres above normal, thereby covering large areas. At these times, many new sediment sources open up, and the origin of the load suspended in the stream is manifold. Throughout the flood, and especially during its last phases, some of the stream load will be deposited on the floodplain at levels well above those of the ordinary stream channel. In this way, nearly horizontal layers of overbank sediment are successively built up over long periods of time. Extensive research studies were carried out in many European countries on the use of active stream and overbank sediments to assess anthropogenic contamination of drainage basins. The results showed that active stream sediment is susceptible to contamination by mine wastes or other products of human origin, and is also recorded in surficial overbank sediment layers. The lowermost layers of overbank sediments, however, do provide information on pre-industrial geogenic dispersion patterns. Even in strongly contaminated areas, it is possible to find natural geochemical distribution patterns by sampling the lowermost overbank sediment layer. Figure 1 shows the distribution of Pb in active stream sediment, which is strongly contaminated by mining and smelter wastes for a considerable distance from the industrial sources. By sampling the different layers of overbank sediment the geochemical history of the area was revealed. The first two layers have been contaminated from materials derived from the mining and smelter wastes, whereas the bottom horizons show pristine conditions. It is concluded, therefore, that the top layers of overbank together with the active
stream sediment, show present day conditions, including anthropogenic contamination, wherever it exists, whilst the lowermost horizons of overbank sediment exhibit pristine or pre-industrial conditions. Therefore, the degree of contamination can be assessed by comparing element concentration values of top and bottom overbank sediment layers.