Development of a tracer technique for the study of suspended sediment dynamics in aquatic environments
The development and field testing of a particle tracing technique for the prediction and monitoring of cohesive sediment transport is described. Natural, chemical and water soluble dye tracers have been used for many years to determine water circulation in order to predict sediment transport. Radioactive and fluorescent particles have been used widely to predict sediment particle transport, but have been restricted mainly to non-cohesive sand and gravel transport studies due to the difficulties of preparation, handling, disassociation of the label from the particle and labour-intensive analysis. The development of a fine cohesive tracing technique therefore offered a significant advancement for the understanding and prediction of fine cohesive sediment and pollutant dynamics in aquatic environments if a sediment analogue could be developed. The physical properties, including size, surface charge, fiuorescence and settling velocity of natural fine cohesive sediment were analysed in order to passively and actively adsorb organic fluorescent dyes onto the sediment surface; the tests were largely unsuccessful. The physical properties of artificial fluorescent particles as sediment analogues were examined and found to have a close correlation to natural sediment. Analysis of the fluorescent particles in mud suspensions on an Analytical Flow Cytometer offered an automated and accurate method of tracer concentration determination at low dilutions. A preliminary field study was carried out in a small pool with encouraging results. A study in a shallow freshwater lake was carried out to determine the sediment dynamics in the lake. A depth-averaged model of the wind-driven circulation within the lake was used to interpret the distribution of tracer. Secondary transport and deposition clearly led to an accumulation of sediment and internal loading in the lake driven by hydrodynamical forcing. A study of the particle residence time and deposition-resuspension processes in the turbidity maximum of a macro-tidal estuary. Fluorescent particles were released into the turbidity maximum and were advected down-estuary on the ebb tide and up-esiuary on the flood tide. The residual mass budgets indicated a significant deposition of the particles in the upper estuary at slack high water. The particles were detected in estuarine surface waters 1 week after release. The fluorescent particles behaved in a similar way to the suspended sediment in both the lacustrine and estuarine study and were considered as sutiable tracers for cohesive sediment.