SOURCES. D. E. WALLING, J. C. WOODWARD & A. P. NICHOLAS ..... and Terry Bacon for drawing the diagrams and Phil Owens for assistance in the collection.
Tracers in Hydrology (Proceedings of the Yokohama Symposium, July 1993) IAHSPubl.no. 215, 1993.
A MULTI-PARAMETER APPROACH TO FINGERPRINTING SUSPENDED-SEDIMENT SOURCES D. E. WALLING, J. C. WOODWARD & A. P. NICHOLAS Department of Geography, University of Exeter, Exeter, EX4 4RJ, Untied Kingdom
ABSTRACT There is an increasing need for information on the sources of the suspended sediment transported by rivers, but such information is difficult to obtain using traditional monitoring techniques. The fingerprinting technique offers an alternative approach, although it is important to select appropriate fingerprinting properties. The use of several such properties or tracers with contrasting behavior was used to assist in discriminating between several potential sources and in providing more definitive and reliable results. Examples of a multi-parameter approach to fingerprinting suspended sediment source in two small catchments in Devon, United Kingdom, are presented. Cluster analysis was used to distinguish statistically significant source groupings and a mixing model was employed to estimate the relative contributions from these sources to the overall suspended sediment load. INTRODUCTION There is an increasing need for reliable information concerning the source of the suspended sediment transported by rivers. Such information is required both to design effective sediment and non-point pollution control strategies and to provide an improved understanding of erosion and suspended sediment transport within a basin which is an essential precursor to establishing sediment budgets, developing distributed sediment yield models, and interpreting sediment yields in terms of landscape evolution. In some situations it is important to ascertain the precise spatial location of the sediment sources within a basin. More often, however, the requirement is for information relating to the type of source involved and, for example, whether the sediment has originated primarily from sheet and rill erosion or from channel and gully erosion, and in the first case whether the dominant source is cultivated land or other areas. Precise information concerning the types and relative importance of sediment sources within a drainage basin is, however, difficult to assemble using traditional monitoring techniques, since these involve many operational problems and are difficult to apply to anything other than a very small drainage basin due to spatial sampling constraints (cf. Peart & Walling, 1986; 1988). The "fingerprinting" technique offers an alternative approach to sediment source determination, which avoids many of these problems and would therefore appear to possess very considerable potential (cf. Oldfield et al., 1979; Peart & Walling, 1986,1988; Walling & Woodward, 1992). This technique makes use of chemical and physical properties of the sediment to trace its source. It involves, firstly, the selection of a physical or chemical property which clearly differentiates potential source materials, and, secondly, comparison of measurements of the same property obtained from suspended sediment with the equivalent values for the potential sources, to establish the likely source of the sediment. The essential simplicity of the fingerprinting technique is, however, compli329
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cated by a number of potential problems, including the need to select a property which is able to distinguish sources in an unequivocal manner (cf. Walling & Woodward, 1992). Clay mineralogy (cf. Garrad & Hey, 1989), sediment chemistry (cf. Peart & Walling, 1986), mineral magnetic parameters (cf. Oldfield et al., 1979; Stott, 1986; Oldfield & Clark, 1990; Foster et al., 1990), and radionuclide concentrations (cf. Peart & Walling, 1986; Burch et al, 1988; Wallbrink & Murray, 1990; Walling & Woodward, 1992) have all been successfully employed as fingerprint tracers, but it is increasingly apparent that the quest for a single diagnostic property capable of distinguishing all potential sources is likely to prove elusive. Peart & Walling (1988) have indeed strongly advocated the use of a selection of fingerprint tracers to increase die reliability of the results obtained and to permit a substantial number of potential sources to be discriminated by their fingerprints. Thus a single tracer might clearly discount one or more potential source, but be unable to discriminate between the remainder. Additional fingerprints could confirm the relative importance of those remaining. The use of several fingerprint properties has already featured in a number of existing studies (cf. Peart & Walling, 1986; Oldfield & Clark, 1990; Walling & Woodward, 1992), but, with the notable exception of the work reported by Yu & Oldfield (1989) which was, however, concerned with determining the dominant sources of estuarine sediments rather than fluvial sediment, the approach to using the information from a range of tracers for source identification has generally been essentially qualitative. There is clearly a need to develop more rigorous and objective procedures to deal with the complexities introduced by the use of several fingerprint properties, and to make full use of the evidence provided by the individual fingerprints. This contribution describes a multi-parameter approach to sediment fingerprinting which has been successfully tested by the authors in a pilot study of two small drainage basins in Devon, UK. THE STUDY BASINS The location, relief, and drainage networks of the two study basins are shown in Fig. 1 and these have been described in detail by Walling & Woodward (1992). Marked contrasts between the two study basins in terms of land use, relief, channel form, and underlying geology should be reflected in differences in the relative importance of the major sediment source types. These basins, therefore, were judged to afford useful case studies for testing a multi-parameter approach to fingerprinting suspended-sediment source. DATA COLLECTION Collection of material necessary to characterize the potential suspended sediment sources in each of the study basins was based on representative sampling of topsoil (0-2 cm) material from uncultivated (pasture) and cultivated (arable) sites and of eroding channel banks. This aimed to provide a basis for distinguishing sediment sources associated with surface erosion of both cultivated fields and pasture areas and with erosion of channel banks. The surface soils of areas of woodland were not included since these were judged unlikely to represent a significant sediment source within these two basins. All source samples were screened through a 63 (am sieve to facilitate direct comparison with fluvial suspended sediment. Bulk water samples were collected at the outlets of both drainage basins (Fig. 1) during storm events. Suspended sediment was recovered from these samples by continuous flow centrifugation and freeze dried. These samples consisted almost exclusively of
