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Title: Sediment, Contaminant and Biofilm Interactions in the River Tame
Author: Harper, Stephanie
Awarding Body: Coventry University
Current Institution: Coventry University
Date of Award: 2006
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Abstract:
In-channel storage and release of fine sediment and sediment-associated contaminants were investigated in the urbanised River Tame, Birmingham, UK. Few studies on sediment dynamics have been performed on urban rivers, and there is relatively little data on the distribution of contaminants between the fluvial substrate and the actively transported sediments. Furthermore little research has been performed on the role of the biofilm in regulating sediment and contaminant fluxes in rivers. The field work was conducted concurrently with water quality and flow monitoring by the Environment Agency at the furthest upstream and downstream field sites. Channel resurveying found that channel activity was low, with overall net erosion of the channel during the sampling period, potentially contributing ca. 0.61 t lan-I y-l to the actively transported sediments. Storage of fme sediment at the surface of the channel bed in the Tame was estimated at 1.01 t km-l which was low in comparison to otherUK rivers. . Sediment resuspension may have contributed an estimated 30-60% of the sediment load for intermediate flow events and 2-7% for the highest flow even~. However, suspended sediment concentrations were exceptionally high in the Tame. At the upstream reach, measurements ranged between 53-1750 mg rl during high flow events, and maximum values were two to three times higher than in other urbanised UK rivers. No exhaustion of the sediment supply was shown in the headwaters of the Tame during consecutive high flow events, emphasising the dominance ofpoint sources. Channel bed storage of fme sediment within the gravel matrix of the bed was investigated by deploying pairs of different substrate traps, buried flush with the river bed to a depth of 30 cm. The first was a standard design with rigid impermeable sides (open at the top only). The second trap, designed for this research, had collapsible sides, permitting fine sediment ingress from the surface and from within the gravels. The collapsible trap design improved connectivity with the surrounding riverbed, with higher rates of deposition than the standard design. Median accumulation rates at all sites ranged from 0.32 kg m-2 d-l (1 month) to 0.032 kg m-2 d-l (14 months) in the collapsible traps and from 0.294 kg m-2 d-l (1 month) to 0.046 kg m-2 d-l (14 months) in the impermeable traps. No evidence was found supporting the existence oflateral sediment transfer through the river gravels, although the literature has long assumed its important role in sediment fluxes within the river bed. Greater penetration of the water column into the gravels of the permeable substrate trap is suggested to enhance mechanisms of fine sediment accumulation such as filtration within the pore spaces. There was limited' resuspension of fine sediment from within the top 10 cm of the ~.. !'. substrate gravels, rather than a complete cycle of deposition and resuspension during high flow' events. Over time, retention offine sediment maybe enhanced as the availability ~fpore spaces decreases, combined with the formation of sediment seals, reducing the interaction between the water column and the bed A site-specific upper limit to the mass of fine sediment retention was observed, suggesting that the gravels took 8 months to saturate with fine sediment. Sediment Cu concentrations were exceptionally high in the Tame, with median values ranging between 348-1016 mg kg-l at the upstream and downstream reaches. Cu concentrations in the bed were an order of magnitude higher than for other urban rivers. The bed was also enriched in other heavy metals in comparison to the suspended sediment (124% for Ni and Cd to 756% for Pb) which reflected the historic nature of the pollution in the catchment. Particulate P was also high in the Tame, generally ranging between 3000-6800 mg kg-l , although median values of ca. 12800 mg kg-l were found in channel surface sediment at the upstream reach. These values exceeded safe.ty guidelines oetween three and sevenfold. Site specific factors, such as the degree of catchment urbanisation, the hydrodynamics of the cha.nnel bed, and the number and activity of point sources, were found to be the main controls on sediment-associated contkinant distribution in the Tame. The timing of sediment deposition relative to the flow cycle and the activation of point sources may also playa role, particularly for particulate P. Partic~e size and organic matter content of sediment were shown to be less important although the data were limited. No influence of the biofilm on sediment or contaminant fluxes was found in the River Tame in contrast to results from fluvarium studies, which have shown a role for the biofilm in almost every aspect of sediment dynamics. . Finally, conceptual models of sediment fluxes within engineered and natural gravel beds and of the sediment and contaminant accumulation cycle Within the substrate gravels were produced. By clarifying the processes involved in sediment and contaminant transport and storage in the River Tame, the primaIY aim ofthe research was achieved. The results have implications for the long-term storage and subsequent remobilisation of sediment and associated contaminants. In the future, as the important role of fluvial sediment in river habitat quality is recognised,. sediment sampling will play a key role in the development of sustainable management strategies for urban rivers which face similar challenges to the Tame in improving water and sediment quality. In order to assess the ecological risks of fine sediment in urban rivers, this research suggests that a comprehensive programme of surveying and sampling of sediment at different spatio-temporal scales may be necessary.
Supervisor: Not available Sponsor: Not available
Qualification Name: Coventry University, 2006 Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.487372  DOI: Not available
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