A.L. Collins

Source type ascription for fluvial suspended sediment based on a quantitative composite fingerprinting technique

Abstract We report the successful application of a quantitative composite fingerprinting technique, incorporating both a rigorous statistical verification procedure for composite signatures and a multivariate mixing model, to evaluate sediment source type in the Dart catchment, Devon, UK and the Plynlimon experimental catchment, Powys, UK. Quantitative ascription of source types is discussed in terms of the mean contributions and potential seasonal, inter- and intea-storm variations in the relative contributions of surface erosion from a number of distinct land use categories and of channel erosion to the suspended sediment load sampled at various sites within the study areas. Surface erosion of pasture soils is generally important in both study catchments but the impacts of spring and autumn cultivation in the Dart, and of autumn and winter forestry activity at Plynlimon, are also evident. Inter-storm variations in sediment source type reflect antecedent conditions, variable contributing areas and timing of sediment sample collection, and highlight the individuality of catchment response for the sampled flood events. Intra-storm variations in source type contributions emphasize the necessity for detailed sampling programmes for suspended sediment in storm periods, when using the fingerprinting approach to investigate sediment origin.

Selecting fingerprint properties for discriminating potential suspended sediment sources in river basins

Abstract The absence of guidelines for pre-selecting the most effective combination of properties for inclusion in a composite fingerprint represents a key methodological uncertainty hampering the wider adoption of the fingerprinting approach for identifying suspended sediment sources. This contribution reports a preliminary attempt to address this issue by testing the discrimination of potential sediment sources within a number of contrasting river basins in the UK and Africa, afforded by a range of sediment properties. Statistical analysis confirms that there is no single diagnostic property capable of discriminating the range of potential suspended sediment sources in the study basins. The use of composite fingerprints based on several constituents drawn from a single group of properties consistently improves sediment source discrimination. However, the level of discrimination afforded by a particular combination of properties is not consistent between the study catchments. Composite fingerprints incorporating constituents selected from several groups of properties using a stepwise statistical selection procedure consistently provide the most robust discrimination of potential sediment sources. Whilst it is not possible to identify a universally applicable optimum composite fingerprint, the results suggest that at present, the most effective means of optimising sediment source discrimination is to identify a number of groups of properties for subsequent use in the establishment of a composite fingerprint. More specifically, this study suggests that measurements of a combination of acid and pyrophosphate–dithionite extractable metals, base cations and organic constituents should provide an effective basis for establishing composite fingerprints for discriminating individual sediment source types. Radiometric properties can also provide useful information for improving sediment source discrimination. The final choice of the groups of properties will necessarily reflect the laboratory facilities available.

USE OF COMPOSITE FINGERPRINTS TO DETERMINE THE PROVENANCE OF THE CONTEMPORARY SUSPENDED SEDIMENT LOAD TRANSPORTED BY RIVERS

Sediment fingerprinting appears to offer a valuable alternative to direct monitoring for elucidating the provenance of suspended sediment and the relative importance of spatial zones or subcatchments comprising larger ( >500 km 2 ) drainage basins. Against this background, a quantitative composite fingerprinting technique, incorporating both statistically verified multicomponent signatures and a multivariate sediment-mixing model, has been employed to determine the spatial origin of contemporary suspended sediment transported from the upper and middle reaches of the River Exe (601km 2 ) and River Severn (4325 km 2 ) basins, UK. Spatial origin is addressed in terms of the relative contribution from three distinct geological subareas constituting each study basin. The consistency of the composite fingerprinting approach is examined using the estimates for mean and seasonal variations in source area contributions and also a comparison between the results obtained for individual flood events and alternative lines of evidence provided by flood travel times and the spatial distribution of precipitation. It is argued that fingerprinting estimates for sediment provenance are consistent with existing information on suspended sediment yields from different subcatchments within the study basins, although in the Severn, the role of storage and remobilization in producing signature ‘averaging’ may complicate comparison of the fingerprinting data with typical floodwater routing times. Validation represents the greatest problem for the cost‐benefit of fingerprinting and scope still exists for further refinement of the procedures involved.

Using unsupported lead-210 measurements to investigate soil erosion and sediment delivery in a small Zambian catchment

Abstract Traditional techniques used to assemble information on rates of erosion and soil redistribution possess many important limitations. As a result, the use of environmental radionuclides, and more particularly 137 Cs measurements, has attracted increasing attention in recent years as a means of obtaining spatially distributed information on rates of erosion and deposition. The application of the 137 Cs approach is, however, hampered in some areas of the world where 137 Cs inventories are low and the low concentrations of 137 Cs found in soils and sediments cause problems for laboratory analysis. These problems will increase as time progresses due to the radioactive decay of the existing inventory, most of which was deposited as fallout ca. 40 years ago. This contribution explores the potential for using another fallout radionuclide, namely unsupported 210 Pb, as an alternative to 137 Cs, in the small (63 km 2 ) Upper Kaleya catchment in southern Zambia where 137 Cs inventories are already very low. The approach employed with unsupported 210 Pb is similar to that used for 137 Cs, although the essentially constant fallout of unsupported 210 Pb through time means that the resulting estimates of erosion and soil redistribution rates reflect a longer period of time (ca. 100 years rather than ca. 40 years). The estimates of erosion and deposition rates derived from the unsupported 210 Pb measurements are used to construct typical sediment budgets for the three main land-use types in the Upper Kaleya catchment, namely, commercial cultivation, communal cultivation and bush grazing. The results obtained from the unsupported 210 Pb are compared with equivalent results based on 137 Cs measurements provided by a previous investigation undertaken in the study catchment. The two sets of results are highly consistent. The study reported confirms the viability of using unsupported 210 Pb as an alternative to 137 Cs in this environment and demonstrates that conjunctive use of both radionuclides can provide additional information on the erosional history of a study area.

Use of the geochemical record preserved in floodplain deposits to reconstruct recent changes in river basin sediment sources

Abstract The geochemical record preserved in overbank floodplain deposits is used in combination with a composite fingerprinting technique and the chronology provided by 137 Cs dateable horizons, to reconstruct recent changes in sediment sources in the Exe and Severn basins, UK. Composite fingerprints for discriminating potential source materials are identified from a suite of properties including heavy metals, trace metals and cation bases. These fingerprints are then compared with those of historical floodplain deposits using a numerical sediment mixing model. Relative contributions from distinct spatial sources characterised in terms of the Devonian, Carboniferous and Permian geological sub-areas of the Exe and the Ordovician-Silurian, Devonian-Carboniferous-Permian and Triassic-Jurassic sub-areas of the Severn are shown to fluctuate over the recent past in response to intrinsic (e.g. the occurrence of extreme hydrologic events) and extrinsic (e.g. anthropogenic disturbance associated with land use change) controls. The potential for fingerprint property transformation in association with floodplain storage is assessed and the scope for further research using fingerprinting to reconstruct longer-term sediment sources is discussed.

Tracing sediment loss from eroding farm tracks using a geochemical fingerprinting procedure combining local and genetic algorithm optimisation

Eroding farm tracks represent important spatially distributed features in many agricultural landscapes and there is concern over their role in catchment sediment problems. It is, however, important to place eroding farm tracks in the context of catchment sediment sources more generally, especially since the former afford potential for targeted sediment mitigation. A sediment source tracing procedure was therefore used to assess the importance of eroding farm track surfaces as a contemporary primary suspended sediment source relative to inputs from pasture or cultivated topsoils and channel banks/subsurface sources, in the upper River Piddle catchment (~100km(2)), in southern England. The study provided a timely opportunity to assess the performance of both local and global (genetic algorithm; GA) optimisation techniques in the sediment geochemistry mass balance modelling used to apportion sources. Over the duration of the study, average median source contributions for individual time-integrated suspended sediment samples collected from three sub-catchments ranged between 1±1 and 19±3% for farm track surfaces, 31±3 and 55±2% for pasture topsoils, 1±1 and 19±1% for cultivated topsoils and 23±2 and 49±1% for channel banks/subsurface sources. Comparison of the local and GA optimisation techniques demonstrated that GA with random initial values improved the minimisation of the objective functions compared to local searching by 0.01-0.04% of 5000 repeat Monte Carlo iterations. GA informed by the outputs of the local optimisation as initial values improved corresponding performance by 0.05-0.20%. These findings increased confidence in the outputs from the local optimisation mass balance modelling, but fingerprint property datasets should be treated on an individual basis. Future sediment source tracing studies should always endeavour to combine local and global search tools to avoid the risk of using localised solutions for source apportionment estimates.

Using 137Cs measurements to quantify soil erosion and redistribution rates for areas under different land use in the Upper Kaleya River basin, southern Zambia

Although soil erosion is a serious environmental problem in many African countries, its assessment using traditional techniques is hampered by a range of problems. Reliable information on soil erosion rates is, nevertheless, an essential prerequisite for the design of targeted erosion and sediment control strategies. This contribution reports the use of 137Cs measurements to quantify medium-term (∼40 years) soil erosion and redistribution rates in both cultivated and uncultivated areas within the Upper Kaleya River basin in southern Zambia. Typical net soil erosion rates are estimated to be 4.3 t ha−1 year−1 for areas under commercial cultivation, 2.9 t ha−1 year−1 for bush grazing areas and 2.5 t ha−1 year−1 for areas under communal cultivation. Although these erosion rates reflect land use in these broad areas over the past 40 years, rather than present land use, they are nevertheless thought to also be representative of current conditions. The findings indicate that any attempt to develop effective erosion and sediment control strategies in the study area should involve all land use types and should aim to reduce both on-site erosion and sediment delivery from the slopes to the stream channel.

Sediment source tracing in a lowland agricultural catchment in southern England using a modified procedure combining statistical analysis and numerical modelling.

Catchment erosion, soil losses and resulting sediment pressures continue to represent cause for concern with respect to the ecological vitality and amenity value of riverine systems, including those in the agricultural catchments of southern England. Given that the sources of fine-grained sediment are typically diffuse in nature, it is essential to adopt a catchment-wide perspective to corresponding management strategies and sediment source tracing procedures have proved useful in assisting such planning. There remains, however, scope for further refining sediment sourcing procedures and on that basis, a recent study in the upper River Kennet (~214 km(2)) catchment in southern England, provided an opportunity for designing and testing a refined statistical procedure for sediment source discrimination with composite fingerprints using Genetic Algorithm (GA)-driven Discriminant Function Analysis, the Kruskal-Wallis H-test and Principal Components Analysis. The revised statistical verification of composite signatures was combined with numerical mass balance modelling using recent refinements including a range of tracer weightings and both local and GA optimisation. Comparison of the local and global optimisation increased confidence in the outputs of local optimisation and the goodness-of-fit for the predicted source contributions using the optimum composite signatures selected from the revised statistical testing ranged from 0.914 to 0.965. Overall relative frequency-weighted average median source type contributions were estimated to be 4% (agricultural topsoils; predicted deviate median inputs 1-19%), 55% (unmetalled farm track surfaces; predicted deviate median inputs 9-91%), 6% (damaged road verges; predicted deviate median inputs 4-42%), 31% (channel banks/subsurface sources; predicted deviate median inputs 5-41%) and 4% (urban street dust; predicted deviate median inputs 0-20%). The study provides further evidence of the importance of eroding farm tacks as a catchment scale sediment source and confirms the utility of tracing for assembling information on sediment inputs from both the agricultural and urban sectors.

A reconnaissance survey of the source of interstitial fine sediment recovered from salmonid spawning gravels in England and Wales

The declining salmonid populations reported for many rivers in England and Wales have frequently been attributed to spawning gravel siltation and the need to address this problem through sediment control strategies has been widely recognised. The planning of such sediment control strategies is, however, severely hampered by the lack of reliable information on the source of the fine sediment deposited within spawning gravels. Given the need for information on the source of the fine sediment accumulating in salmonid spawning gravels, a reconnaissance survey, based on the source fingerprinting approach, has been undertaken to establish the source of fine interstitial sediment recovered from spawning gravels in 18 important salmonid rivers in England and Wales. The findings confirm the potential of the fingerprinting approach for establishing the provenance of interstitial fines. More specifically, the results provide useful information on the relative importance of surface and channel/subsurface sources as the source of the interstitial fines collected from the individual study rivers. Significant regional contrasts in the source of this sediment are reported. The findings have important implications for the design and implementation of effective sediment management strategies aimed at protecting salmonid spawning gravels, in terms of both the sources to be targeted and the associated need to adopt different approaches in different rivers.

Suspended sediment source fingerprinting in a small tropical catchment and some management implications

Abstract The targeting of sediment management strategies is a key requirement in developing countries because of the limited resources available. Such targeting is, however, hampered by the lack of reliable information on catchment sediment sources. This paper reports the results of using a quantitative composite fingerprinting technique to estimate the relative importance of the primary potential sources within the upper Kaleya catchment (63 km 2 ) in southern Zambia. During the study period (1997–9), the load-weighted mean relative contributions to the sediment load sampled at the catchment outlet were in the order: commercial cultivation (2.0%), bush grazing (17.1%), channel banks/gullies (17.2%) and communal cultivation (63.7%). Although influenced by the timing of sediment sample collection, these results are judged to provide a meaningful assessment of sediment sources in the Kaleya catchment. They indicate that improved environmental management strategies for the study area must be comprehensive and need to reduce surface erosion and hillslope/channel connectivity in areas of bush grazing and both communal and commercial cultivation and to control gully development.