An investigation of the geochemistry and petrology of canal sediments
Canals are artificial waterways, which are distinct from natural aquatic systems. As a result of their industrial heritage they have experienced high loadings of anthropogenic material, and consequently their sediments tend to have a bulk composition that is distinct from natural sediment. It is therefore expected that the geochemical behaviour of canal sediment may deviate significantly from that of natural sediment. This study investigates and contrasts the geochemistry and petrology of a rural and urban canal sediment, in order to determine the influence of anthropogenic material upon the urban sediment and to gain an understanding of the diagenetic processes operating within the sediments. Sediment cores were collected from an urban canal in Birmingham and a rural canal in Leicestershire. The cores were analysed at 1 cm intervals in order to build up 24cm depth profiles of their bulk chemistry, metal speciation and porewater chemistry. The petrology of both the sediments was analysed by CryoSEM in order to determine their in situ petrology. The results have shown that the introduction of anthropogenic material to the urban canal has produced sediment that is chemically and mineralogically distinct from natural sediments. The bulk urban sediment contains elevated metal and organic matter loadings, and a significant proportion of its particulate matter is of anthropogenic origin (e.g. slag, fly ash, metal turnings) and is therefore not typical of clastic material in natural sediments. Rural sediment has not been subject to inputs of such material and therefore it has a bulk chemistry of natural materials such as clay, sand, silt and organic matter which is similar to that which is typically observed in natural sediments. The petrological investigation of rural and urban canal sediment has shown that they have distinct authigenic mineralogies. The reduced iron phosphate, vivianite (Fe3(P04)2.8H20) is the most abundant authigenic mineral in urban sediment, as a result of its elevated organic matter and iron concentrations, while in the rural sediment, pyrite (F eS2) is predominant. In order to choose an appropriate scheme for the investigation of speciation in contaminated urban canal sediment, two different sequential extraction schemes were investigated by CryoSEM. The results revealed that they do not yield meaningful speciation results for urban canal sediment. In particular, the application of an oxalic acid buffer to extract oxides resulted in the formation of insoluble oxalates, and the exhaustion of the pH buffering capacity of the extraction reagents used to extract carbonates, resulted in the incomplete dissolution of calcite. The abundance of non-typical sediment components in urban canal sediment highlights the importance of investigating sediment mineralogy prior to the application of sequential extraction techniques. The porewater chemistry was in broad agreement with the observed petrology and, in the case of the urban site, the data provides evidence of sediment disturbance. The periodic resuspension of the sediment by boat traffic results in a significant change to the surface porewater chemistry of iron and sulphate in the urban sediment and results in changes to the stability of certain authigenic phases, most notably vivianite. In the rural sediment, although physical disturbance of the sediment was observed, there was no chemical evidence in the porewater results. However, it does perhaps subtly enhance the organic matter degradation processes that are occurring, although this could not be confirmed by the results of this investigation. The differences in the authigenic mineral assemblages of rural and urban canal sediment are the result of differences in their diagenetic paths. In the rural sediment, the relatively low organic matter and iron loadings result in a diagenesis dominated by sulphate and iron reduction; the abundance of pyrite in this sediment is evidence that sulphate reduction is the predominant process. In the urban sediment, the co-existence of vivianite and iron monosulphides implies that iron reduction is dominating the oxidation of organic matter in the sediment and occurring simultaneously with sulphate reduction. The application of conceptual models of diagenesis, based on porewater studies of natural sediments, can be used to adequately describe the processes that are occurring in the rural canal sediment. However, the periodic physical disturbance and influx of material to urban canal sediment from pollution events prevents the development of steady state conditions. As a result of this, and the nature of the solid material, the diagenetic path observed in the urban sediment is quite distinct from that observed in natural sediments, and simple steady state models are inappropriate for its interpretation.