Overbank sediments : a natural bed blending sampling medium for large—scale geochemical mapping

Abstract

Overbank sediments occur along rivers and streams with variable water discharge. They are deposited on floodplains and levees from water suspension during floods, when the discharge exceeds the amounts that can be contained within the normal channel. Overbank sediments were introduced as a sampling medium in geochemical mapping in 1989, and a number of studies have later been published on this subject. These papers indicate: 1. Depth integrated samples of overbank sediments reflect the composition of many current and past sediment sources upstream of the sampling point, contrary to active stream sediments, which originate in a more restricted number of presently active sediment sources from which they move regularly along the stream channel. In many regions overbank sediments are more representative of drainage basins than active stream sediments and can, therefore, be used to determine main regional to continental geochemical distribution patterns with widely scattered sample sites at low cost per unit area. 2. Samples of overbank sediments can be collected in floodplains or old terraces along laterally stable or slowly migrating channels. In some locations the surface sediments may be polluted, however, natural, pre-industrial sediments may, nevertheless, occur at depth. Mapping of the composition of recent and pre-industrial overbank sediments can, therefore, be used (i) in a characterization of the present state of pollution, and (ii) as a regional prospecting tool in natural as well as polluted environments. 3. Vertical movements of elements in strata of overbank sediments may occur, especially in cases where the distribution of relatively mobile elements in non-calcareous areas are heavily influenced by acid rain. However, the overall impression is that vertical migration of chemical elements is not a major problem in the use of overbank sediments in geochemical mapping. 4. The composition of overbank sediment is of great interest to society in general, since flood plains are very important for agriculture, urbanisation, and as sources for drinking water. Several of the above points indicate that overbank sediments represent a natural analogue to the products of bed-blending. This aspect is mentioned here in light of the Theory of Sampling (TOS).