Structure, Stability, and Transformation of Contaminated Lacustrine Surface Fine-Grained Laminae

An in situ annular flume (Sea Carousel) was deployed in Hamilton Harbour, Lake Ontario, to assess the structure, stability, and transformation of lacustrine surface fine-grained laminae (SFGL). Such laminae typify depositional lacustrine environments and occur at the sediment-water interface. The critical erosion thresholds, erosion rates, and internal friction coefficients were determined for natural undisturbed and physically disturbed sites in order to assess changes in bed stability. Sediment cores, taken at each site, were analyzed for bulk density using a CT scanner in order to provide an intercomparison of our results. Flocs and aggregates, pumped from the flume during erosion experiments, were analyzed for morphological characteristics using conventional optical microscopy and transmission electron microscopy to evaluate the mechanism and development of the SFGL and its structure. A general three-layer model is developed which depicts the organic flocs of the SFGL (Layer 1) compressing within a collapse zone (Layer 2) to form a consolidated bed (Layer 3). The SFGL was observed to be porous, of low density, and of high water content which possessed yield resistance due, in part, to binding by extracellular polymeric substances secreted by a colonized microbial population. Transmission electron microscopy observations showed an active biological community in the sediment promoting biostabilization. Time series of erosion thresholds and friction coefficients of the disturbed bed showed that the SFGL reconsolidated rapidly and increased in shear strength. The artificial disturbance of the sediment resulted in incorporation of the SFGL and the collapse zone with the consolidated bed through mixing. This stimulated the process of consolidation chiefly through removal of gas and breakdown of the organic fibril network. Reconsolidation began with the formation of a collapse zone at the surface of the disturbed sediment. High friction coefficients and an increasing density with depth in the new surface layer indicated that rapid consolidation occurred after disturbance.