Sediment geochemistry - a case study approach.

Abstract

The geochemistry of sediments is a very wide field and several important aspects must be taken into account, including, but not limited to, various methodological questions, the analysis of distribution patterns, determination of origins and the assessment of risks. Therefore, this research project adapted a case study approach and analysed several important aspects of contamination in sediments at a time. In case study 1, the distribution of metals in the sediments was analysed in Bigge and Olpe, two small and fast running watercourses in Germany. The metal/metalloid concentrations showed very different distribution patterns. Mobile elements like zinc showed a very homogenous and predictable pattern, while elements with low mobility stick to the sediment and do not migrate much, leading to areas with different concentrations. In addition, it was found that the local monitoring tools in force, which are largely based on analyses of the water, are not sufficient for a reliable assessment of the environmental quality. Case study 2 aimed both to investigate the contamination profile caused by a closed landfill within the Christchurch Harbour nature reserve and the strengths/weaknesses of a partial extraction scheme based on the industrial standardised process DIN 19730. It was found that this procedure can predict the actual migration in the homogenous marshland structure rather well. In the vicinity of a linear channel, however, no correlation between the mobility and dispersion could be detected; the channel acts as an effective drainage system for both the landfill itself and the intertidal marshland in its sphere of influence. Partial extractions are only limited in their ability to predict the migration of contaminants in the ground directly affected by the channel. The main objective of case study 3 was the determination of metal distribution within the Poole Harbour estuary, both in regard to total and mobile metal concentrations. In addition, it was tested if the chosen methodology is an efficient protocol (fast, yet scientifically defendable) for the assessment of the environmental quality of an area of that size. The concentrations and mobilities of all analysed contaminants in Poole Harbour were greatest in the heavily industrialised secondary embayment Holes Bay. Although Wareham Channel typically showed higher concentrations in the total content analyses compared to Southern Bights, the potential risk associated with metals, calculated based on both total concentrations and mobile fractions, was comparable in both areas. In case study 4 a simplified grain-size based normalisation scheme was tested. The efficiency of this approach, together several other normalisation schemes was evaluated in Wareham Channel, located in the west of Poole Harbour. In such fine-grained environments, neither geochemical analyses based on aluminium, nor granulometric normalisation schemes yielded a substantial improvement. Normalisations based on the much simpler iron-ratio reduced the variance considerably. This approach was then applied to the sediments close to a former weapons facility. Case study 5 investigated the interactions between the die-back of the cord grass Spartina anglica in Poole Harbour and the metal contamination in the sediments. Despite having several evolutionary advantages over other plants in this environment, S. anglica is dying back in the estuary since 1925 and the reasons for this process are insufficiently understood. No obvious impact of the metal contamination on S. anglica growth/ die-back could be detected, although the die-back has influenced, in turn, the metal concentrations in the estuary. The overall cadmium concentrations and potential risk of adverse effects have risen since 1925, but in the zones affected by die-back, cadmium stored in the sediment by S. anglica appears to have been washed out rapidly. Existing patches still retain elevated concentrations and are potentially at risk of further cadmium release, especially if sea level rise, caused by climate change, would accelerate the die-back.