Geochemistry and petrography of phosphorus in urban canal bed sediment
Introduction
Urban canal bed sediment receives predominantly anthropogenic input and has a composition markedly different from most natural sediments (Bromhead and Beckwith, 1994, Bijlsma et al., 1996, Galvez-Cloutier and Dube, 1998, Bijlsma et al., 1996, Dodd et al., 2000). In particular it contains high concentrations of organic matter (19%), Fe (7.5%), P (2%) and heavy metals (Bromhead and Beckwith, 1994, Dodd, 2000, Dodd et al., 2000). Of these elements P probably has the greatest potential to influence water quality, as it is a nutrient that promotes eutrophication. Understanding the processes controlling P fluxes to and from the canal sediment is therefore important in evaluating the influence of sediment composition on water quality. Geochemical processes operating in the canal sediment may also provide a general example of the processes operating in end-member anthropogenic sediment. The term “end-member” is loosely applied in this context as a wide range in the composition of anthropogenic sediment is expected but it can be reasonably concluded that organic matter, P and Fe are often enriched in these systems (National Rivers Authority, 1996).
To enable interpretation of the geochemical processes controlling P geochemistry in urban canal sediment, seasonal pore water composition profiles were integrated with petrographic observations in samples collected from sites in the West Midland conurbation, England. The canal network in this area is particularly suited to studying anthropogenic sediment as it is isolated from the natural drainage system and does not receive significant natural clastic input.
Section snippets
Methods
Samples were collected from two sites in the West Midland conurbation, the Old Birmingham Mainline Canal, an urban canal, near Smethwick in Birmingham (Fig. 1) and the Walsall Canal at Great Bridge between Birmingham and Wolverhampton (Fig. 1). Pore water and petrographic analysis were undertaken on the Smethwick cores whilst only petrographic analysis was undertaken on cores from Great Bridge. For pore water analysis two closely spaced cores of sediment, 7 cm in diameter and approximately 30
pH, Eh and alkalinity
pH (Fig. 2a) decreased from 7.8 in the overlying canal water to 6.9 at the sediment water interface. Below the sediment-water interface the pH was relatively constant at between 6.4 and 7.0. Alkalinity (Fig. 2b) increased sharply across the sediment water interface from 1.14 meq/l in the canal water to 4.94 meq/l at a depth of 2 cm. Below 2 cm the alkalinity increased steadily to 7.81 meq/l at 24 cm. Eh (Fig. 2c) dropped from a positive value of +0.45 V at the sediment water interface to a
Discussion
Before interpreting the data the cause of the seasonal variability must be considered. A possible key to this is influence of boat traffic, which is highly seasonal and causes sediment re-suspension. Evidence of sediment re-suspension by passing boat traffic is the turbid wake observed behind barges. Statistics provided by British Waterways (pers. com.) for 1999 confirm the seasonal influence of boat traffic with averages of 0.1 boats per day in winter (December–February), 2 boats per day in
Conclusions
In this study the geochemical processes operating in canal bed sediment, dominated by anthropogenic and biogenic input, have been examined. One of the most important conclusions is that sediment re-suspension exercises a considerable influence on canal sediment geochemistry. In particular two interfaces occur in the sediment; a transient sediment-water interface at the maximum depth of re-suspension and the sediment-water interface under undisturbed conditions. The main source of PO4 in the
Acknowledgements
We wish to acknowledge the University of Nottingham and the British Geological Survey for supporting this research. Chris Somerfield, Dave Clift, Bill Culley and Tony Gospel are thanked for analytical and technical assistance. British Waterways are thanked for allowing site access. Brian Atkin, Paul Beckwith, Kevin Taylor and two anonymous reviewers are thanked for their comments and advice.
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