Geochemistry and petrography of phosphorus in urban canal bed sediment

Urban canal sediment provides an opportunity to investigate the processes controlling P geochemistry in a system dominated by anthropogenic input. To achieve this a combined study was undertaken of pore water geochemistry and petrology of the top 25 cm canal bed sediment from two urban canals in the English West Midland conurbation. Processes in the sediment are dominated by the geochemistry of Fe, P and organic matter. Results indicate that sediment re-suspension by boat traffic has a major influence on pore water chemistry. In addition to the sediment water interface an additional interface at 6–7 cm depth was identified and corresponds to the maximum depth of sediment re-suspension by boat traffic. Phosphorus in the sediment occurs predominantly as vivianite. Low PO4 concentrations occur close to the sediment water interface in summer due to frequent re-suspension and Fe oxidation. High PO4 concentrations occur close to the sediment water interface in winter when boat traffic is infrequent. The greatest potential to exchange phosphate between sediment and water column will therefore occur during infrequent boat traffic regimes.