Studies on heavy metals in the sediments of a storm water drain and the Avon-Heathcote Estuary

The concentrations of chromium, copper, iron, manganese, nickel, lead, and zinc have been estimated, primarily by flame AAS, in the sediments of the City Outfall Drain and Avon-Heathcote Estuary, Christchurch, New Zealand. These, and other metals have also been determined by XRF, and the two sets of results are compared. Sediment samples were obtained by the use of a simple coring device where possible, or by sampling the top two centimetres of sediment where this was not possible. The variation in metal concentration with location, depth, and rainfall are discussed, with reference to metal-metal correlations, organic matter content, particle size distribution, and a theory on the formation of the sediment units in the Avon-Heathcote Estuary. These results are then evaluated with regard to enrichment factors calculated from a set of local background levels defined in this study. The strongest metal-metal correlation is lead with zinc, although lead, zinc, copper, and to a lesser extent chromium, are commonly found in association with one another. Metal concentration generally decreases with depth, except in the case of the estuary core RH2, which exhibits a profile of concentration vs. depth consistent with MacPherson's proposals for the formation of the Estuaries sediment units. Lead and zinc are found to be the metals most highly enriched above background levels in the clay, then sand, then silt fractions. No obvious reason for this was found as for all other metals, including those that correlated well with lead and zinc, the order was clay > silt > sand. The distribution of metal concentrations in the silt-clay size fraction wre investigated using a novel technique not requiring the separation of individual size fractions. The percentage of quartz present in each fraction was found to be the primary factor causing variation in metal concentration down to approximately 4 μm. Below this size metal speciation becomes dominant. The role of organic matter was not determined but is felt to be of possible significance. The thesis concludes with recommendations for areas of further research, including sediment dating, application of sequential extraction techniques, and a possible method of treatment for waste waters