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CHAPTER 1. ENVIRONMENTAL ASPECTS OF EUTROPHICATION

1.2. Eutrophication as an Environmental Problem

1.2.4. Role of Sediments in Eutrophication

Sediments play a significant role in the process of eutrophication of lakes and reservoirs, and lakes in which major controls of nutrient inputs have been implemented may be delayed in their recovery due to the levels of nutrients contained in the sediment. Sediment by definition is material that has accumulated by deposition in water. Virtually all sediments are composed of variable quantities of organic matter, mineral grains, rock fragments, and carbonates and other precipitates, such as the oxides of iron, manganese and aluminum. The size of sediment particles is the most important property in understanding of sediment-water interactions leading to eutrophication of lakes and reservoirs. Most commonly used particle size fractions for characterization of sediments are as follows: clay-size fraction, which contains particles smaller than 2 mm, silt-size fraction, which contains particles of size between 2 mm and 63 mm, sand-size fraction, which contains particles of size between 63 mm and 2.00 mm, and gravel-size fraction, which contains particles larger than 2.00 mm.

The most important particle size fraction in eutrophication is the clay-size fraction, which consists mainly of clay minerals and organic matter. Some clay minerals are usually present in the silt-size fraction. Under specific conditions in lakes and reservoirs, iron and manganese precipitate on the surface of clay mineral particles usually as oxyhydroxides. This phenomenon generates a coating on the particles. Coating of fine-grained particles with organic matter is also common. The coatings provide a highly active physicochemical site for both adsorption and desorption of phosphorus and a wide range of trace metals and organic pollutants of low solubility. Iron oxyhydroxide coating is most important for its capacity to adsorb phosphorus.

Sediment Sources and Transport

Generally, sediment entering a lake or reservoir is derived from rivers, shoreline erosion, sub-aqueous erosion and atmospheric deposition. Rivers are normally the most significant source of sediment to a lake. The physical and chemical characteristics of the riverine sediment reflect the geologic and geomorphological composition of the watershed. Modification of the land surface by man due to deforestation, intensive agriculture and animal husbandry has a large impact resulting in the exposure of bare soil susceptible to erosion by both air and water. Bank erosion may be accelerated in non-protected regions and animal access to the waterway may result in increased bank erosion and direct addition of animal wastes and pathogens. Urbanization of watershed results in a reduction of the land surface area available for infiltration of rain and surface water, which results in increased run-off and river flow with rapid increases in river level response to precipitation. In most river systems the largest percentage of the total sediment delivered to a lake or reservoir occurs in a small number of storms.

Particles eroded from land surfaces by wind may be transported large distances. Particles originating from volcanic activity emitted to the upper atmosphere are transported globally. Soil particles from the atmosphere are of fine-grain size and may have high concentrations of organic carbon, phosphorus, nitrogen and organic micro-pollutants derived from herbicides and pesticides used for both fertilizing and pest control in agriculture. These airborne deposits are major sources of nutrients to remote lakes with little urbanization, and also account for the build up of pollutants in remote parts of the globe. Control of this source must be international and is well beyond the local and regional initiatives to control eutrophication.

Nutrients in Sediments

Sediments play an important role in the accumulation and regeneration of nutrients. Organic matter produced by algae in the lake settles to the sediment and decomposes by aerobic or anaerobic processes, during which different carbon, nitrogen and phosphorus compounds are produced. Further, decomposing organic matter affects changes in oxygen concentrations and redox potential and can generate anoxic conditions at the sediment-water interface. This, in turn affects nitrogen and phosphorus release from the sediments to the overlying water. One of the most important processes, which occur under anoxic condition, is the solubilization of iron and manganese oxyhydroxide coatings on fine-grained sediment particles. Under anoxic conditions, non-soluble trivalent iron and tetravalent manganese change to soluble divalent iron and manganese, with substantial release of adsorbed or co-precipitated elements and compounds, particularly phosphorus.

Contaminants in Sediments

Contaminants are different elements and inorganic and organic compounds which are toxic to aquatic plants and animals. They fall under the two broad categories of trace metals and organic micropollutants. The latter are man-made, are xenobiotic and consist of a wide range of compounds including herbicides, pesticides, industrial compounds and their metabolites, which in total represent many thousands of compounds. Individual elements and compounds can function simultaneously, which may amplify or reduce their impact on the environment.

Most of the persistent organic compounds have low solubility in water (i.e., hydrophobic compounds), and they are fat-soluble (i.e., lipophyllic compounds) and bio-accumulate easily in the fatty tissues of animal bodies. This characteristic is further enhanced by bio-magnification resulting in considerable increases in body burden concentrations up the food chain.

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