Storage of sediment-associated nutrients and contaminants in river channel and floodplain systems
Introduction
Many studies have shown that suspended sediment plays an important role in the transport of nutrients, heavy metals and other hydrophobic contaminants through river systems (Allan, 1979, Horowitz, 1991, Horowitz, 1995, Foster et al., 1996, Miller, 1997, Russell et al., 1998). For the majority of trace elements, >90% of the flux through a river system occurs in association with suspended sediment (Gibbs, 1977, Meybeck and Helmer, 1989, Horowitz, 1991). The transfer, dispersal and fate of sediment-associated nutrients and contaminants in river systems is, therefore, strongly controlled by the transport, deposition and remobilisation of suspended sediment, and thus by the sediment delivery or conveyance system.
The sediment delivery or conveyance processes operating within the channel system of a river basin are spatially and temporally complex (Walling, 1983, Trimble, 1995), providing many opportunities for short- and longer-term storage of fine-grained sediment, both within the channel and on the floodplains bordering the channel. Existing studies have shown that typically between 10 and 60% of the sediment delivered to the main channel system may be deposited and stored within the channel or on the floodplains and, therefore, fail to reach the catchment outlet (e.g. Trimble, 1983, Phillips, 1991, Walling and Quine, 1993, Campo and Desloges, 1994, Mertes, 1994, Owens et al., 1997, Allison et al., 1998, Goodbred and Kuehl, 1998). Thus, for example, Walling et al., 1998a, Owens et al., 1999a estimated that overbank deposition and channel bed storage were respectively equivalent to between ca. 40 and 50% and ca. 4 and 10% of the annual suspended sediment load delivered to the main channel systems of the Rivers Ouse, Wharfe and Tweed, in the UK. In the case of overbank deposition on the floodplains, the sediment enters longer-term storage and thus effectively represents a conveyance loss. In contrast, channel storage commonly operates over a shorter timescale and serves to attenuate sediment delivery to the catchment outlet.
The deposition and storage of fine-grained sediment within the channels and on the floodplains of river systems has several important implications for the delivery and fate of sediment-associated nutrients and contaminants. Firstly, estimates of sediment-associated nutrient and contaminant fluxes obtained for downstream locations may underestimate the magnitude of upstream fluxes. Potential environmental problems associated with the transport of contaminated sediment through the river system may, in consequence, also be underestimated. Secondly, the deposition and storage of fine-grained sediment may also result in the contamination of floodplain and channel environments. Many studies have documented elevated levels of sediment-associated nutrients and contaminants on floodplains and channel beds due to inputs from agricultural, urban, industrial and mining activities (Bradley and Cox, 1990, Marron, 1992, Macklin and Klimek, 1992, Brunet et al., 1994, Meade, 1995, Hudson-Edwards et al., 1999a, Hudson-Edwards et al., 1999b, Dawson and Macklin, 1998a, Dawson and Macklin, 1998b, Horowitz et al., 1999, Walling et al., 2000). Such contamination can have adverse effects on water quality and aquatic habitats and the agricultural use of floodplain areas receiving contaminated sediment. Thirdly, remobilisation of contaminated overbank sediment deposits stored on floodplains, by bank erosion and channel migration, can reintroduce nutrients and contaminants into the river system long after the activities causing the contamination have ceased (cf. Lecce and Pavlowsky, 1997, Grove and Sedgwick, 1998). Thus, Lecce and Pavlowsky (1997) describe how high rates of bank erosion in the middle reaches of the Blue River, Wisconsin, USA, are currently remobilising overbank deposits contaminated with Zn, which were deposited during a former period when there was extensive mining activity in the headwaters. Future remobilisation of contaminated sediments could represent a significant problem where the response of a river system changes due to climate change and such potential situations have been referred to as ‘chemical time-bombs’.
Although several studies have documented sediment-associated nutrient and contaminant fluxes at the downstream outlets of river basins (e.g. OSPARCOM, 1995, Jarvie et al., 1997, Russell et al., 1998, Pentreath, 1999) and reported elevated levels of contaminants on floodplains and in channel bed sediments (e.g. Bradley and Cox, 1990, Macklin and Klimek, 1992, Horowitz et al., 1999), few studies have attempted to document the storage of sediment-associated nutrients and contaminants in river channels and on floodplains at the basin scale, particularly for a large river system, and to establish its overall significance. Notable exceptions include the work of Lecce and Pavlowsky, 1997, Hudson-Edwards et al., 1999b on various rivers in Yorkshire, UK. Such information is needed to inform the development of catchment management strategies aimed at controlling environmental problems associated with sediment-associated nutrients and contaminants in river systems and downstream receiving water bodies.
This paper reports a study of the deposition and storage of sediment-associated nutrients and contaminants on the channel beds and floodplains of the main channel systems of two contrasting drainage basins in Yorkshire, UK. The River Aire drains a catchment which is dominated by urban and industrial land use in its middle and lower reaches, whereas the River Swale drains a rural catchment which is dominated by moorland and agricultural land, but which has a legacy of metal mining in its headwaters. The main objectives of the study were:
- 1.
To examine downstream trends in the nutrient and contaminant content of floodplain deposits and channel bed sediment within the main channel systems of the Rivers Aire and Swale, and to determine the magnitude and spatial variation of the contemporary deposition and storage of these nutrients and contaminants on the floodplains and channel beds of the study rivers.
- 2.
To estimate the total sediment-associated contaminant storage in the main channel systems of the study rivers and the total contaminant deposition fluxes on the associated floodplains and to compare these values with estimates of the contaminant fluxes at the catchment outlets.
The study represented part of a larger project aimed at examining the role of fine-grained sediment in nutrient and contaminant fluxes within large UK river basins. Other results from this larger project are presented in Carton et al., 2000, Owens et al., 2001, Owens and Walling, in press.
Section snippets
The study area
The Rivers Aire and Swale are both tributaries of the River Ouse, which drains into the North Sea via the Humber Estuary (Fig. 1). The River Aire has a catchment area of 1932 km2 above the UK Environment Agency (EA) gauging station at Beal (site 1). The River Calder is the main tributary of the River Aire and has a catchment area of 930 km2 upstream of the EA gauging station at Methley (site 9). The River Calder contributes about 52% of the flow and about 56% of the suspended sediment load of
Spatial variation in channel bed storage of fine sediment and associated contaminants
Prior to discussing basin-wide spatial variations in the channel bed storage of sediment-associated contaminants, it is important to consider the within-site variability of both sediment storage (g m−2), and sediment properties and sediment-associated contaminant concentrations (μg g−1) at representative sampling points. Table 1 presents values of the coefficient of variation (CV) of, firstly, <63 μm sediment storage and, secondly, a range of sediment properties (particle size, and organic
Conclusion
The magnitude and spatial variability of the deposition and storage of sediment-associated contaminants on floodplains and channel beds reflect both the contaminant content of the sediment and the amounts of fine-grained sediment involved. The contaminant content of the <63 μm fraction of channel bed and floodplain sediment will be controlled by the properties (e.g. grain size distribution and organic matter content) and minerology (e.g. Fe–Mn oxide content) of the sediment and, more
Acknowledgements
The financial support for the work reported in this paper provided by NERC Research Grant GST 1574 within the framework of the NERC Environmental Diagnostics research programme and by a NERC postgraduate studentship held by JC are gratefully acknowledged. Thanks are also extended to Adam Comerford for assistance with fieldwork, to Art Ames for help with laboratory analysis, to Charles Ward (Environment Agency, Leeds) for providing channel width data, and to Helen Jones for producing the
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- 1
Present address: National Soil Resources Institute, Cranfield University, North Wyke, Okehampton, Devon, EX20 2SB, UK.
- 2
Present address: Institute of Water and Environment, Cranfield University, Silsoe, Bedfordshire, MK42 4DT, UK.