Many estuaries around the world are being filled by net landward transport of sediment from the continental shelf. One well-known result of this process is the silting up of harbours. Our study of a part of the New England coast shows that strong flood tidal currents move silt and clay in nearbottom suspensions up the Acushnet estuary to its head in New Bedford Harbour, causing siltation. Weaker ebb currents move some of the clay back out into nearby Buzzards Bay. We have combined sedimentological and geochemical methods to try to understand the pattern of accumulation and dispersal of solid waste within the estuary. We try to use the sediment column as a complex historical record of industrial contamination, and to use industrial wastes as tracers for sediment movement. This approach allows us to model the long-term effects of the oceanographic regime on sediment dynamics and environmental contamination. Previous studies (reviewed in the text) showed that the estuarine sediments contained as much as one percent of combined Cu, Cr, and Zn. We have tried to find out how, and at what rate these metals spread within and beyond the harbour away from their industrial point sources. We have also tried to ascertain how organic rich sediment formed by the discharge of sewage has been distributed throughout the area. We concentrated our studies on sedimentary processes, on the distribution of sedimentary organic matter, and on the distributions of the following metals: Ag, Cd, Cr, Cu, Fe, Hg, Mn, Pb, and Zn.
Aktualisiert: 2022-04-20
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Estuaries are sites of high primary productivity, resulting in substantial fluxes of organic matter. Bacterial decomposition of this material in the underlying sediment gives rise to significant remobilisation of organic decomposition products and of inorganic reactants that undergo redox transformations during diagenesis, such as the oxides of Mn and Fe. These reactions promote fluxes of solutes across the sediment-water interface at rates which are influenced by the nature and rate of supply of reactants, the extent of sediment reworking and the occurrence of diagenetic mineralisation and sorption reactions leading to their removal from the porewaters. The aim of this paper is to document the porewater distributions of organic decomposition products and associated species and to evaluate the significance of benthic exchange to their distributions in the overlying water of the Tamar estuary, southwest England, a shallow estuary of high organic productivity.
Aktualisiert: 2022-05-04
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This article presents a synthesis of sedimentological and geochemical work carried out on the Gironde estuary until 1978. The drainage area of the Gironde estuary is the Aquitaine Basin. It is a Cenozoic sedimentary basin of about 80,000 km2 largely open to the ocean and edged by different massifs (Pyrenees, Massif Central). This basin profits from a temperate oceanic climate. The estuary is formed by the joining of two hydrologic, well individualized river systems, the Garonne and the Dordogne. As the development of local industrial planning is still rather limited, the equilibrium of the estuary does not appear to have suffered serious harm. Various ecological aspects and properties of the present-day environment are described. pH of waters fluctuates between the extreme values of 7.4 to 8.5. The percentage of saturation of dissolved oxygen may vary (1976) between 10 and 130 o/oo, especially seasonally. The quantitative changes in nutrients result from the upstream dilution and oxidation of organic matter. Upstream, high levels of trace-metals associated with Suspended particles have been determined; further downstream, a drastic decrease of these metals is observed and in the mid-estuary heavy metals concentrations agree with world average values found in not significantly polluted systems. This upstream/ downstream decrease in metal concentrations in the suspended particulate matter is attributed to a mobilization of the metals which become solubilized from the suspended material. In order to precise these mechanisms, special methods were used,such as the double tagged method with radioactive isotopes and chemical sequential extraction. The Gironde estuary, situated in a well known environment, appears to be a rich source of in- formation for specific estuarine studies, and for the evaluation of the substance transfer to the continent/ocean interface.
Aktualisiert: 2022-02-16
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