Sediment pollution

The most obvious effect of high sediment content on water quality is turbidity. High turbidity can have detrimental effects on phytoplankton productivity because of attenuation of incoming light. If the suspended load has a high organic carbon content, the biochemical oxygen demand will be raised, and conversely, the dissolved oxygen levels will decrease.

The turbid plume of water in Vänersborgsviken (a bay in the south-western part of the Swedish lake Vänern) was due to the dumping of dredged material from the barge, shown in the upper, central part of the photo.

Sediments are capable of transporting loads of adsorbed nutrients, pesticides, heavy metals, and other toxins. Sediment particles can contain heavy metals as a part of the structure, as exchangeable cations, or as sorbed and precipitated matter. For estimating the effect of suspended sediment on the content of heavy metals in water I have constructed the nomogram shown below.

Nomogram for estimating the effect of suspended sediment on the content of heavy metals in water.

Most of the transported sediment will be deposited in front of river mouths in lakes and in coastal areas, and on wetlands of floodplain type, where the decrease in flow velocity and the presence of vegetation promotes sedimentation. The effect of salt water in eustarine mixing is to further enhance sediment removal by flocculation of clay particles. Transfer of the sediment from the water column to the bottom has important consequences both for the quality of the water and the properties of the bottom deposits.

Areal distribution of mercury in the bottom deposits of two "black-listed" areas in the river Göta älv before the dredging operations in 1972 - 1975. From Axelsson 1974 (UNGI Rapport Nr 34).

The river Göta älv, the most important of Sweden´s internal waterways, is influenced by industrial pollution. However, in most of the areas, that were dredged in 1972 - 1975, the thickness of polluted deposits was rather thin. The average concentration of heavy metals and chlorinated hydrocarbons in the dredge spoil was therefore often rather low in relation to the concentration in the superficial deposits.

In several lakes as well as in several bays along the coast of Sweden the modern bottom deposits are heavily polluted. One of the most important ways, both from environmental and economical points of view, to improve the bottom conditions in these lakes and bays might (besides the cleaning of waste water) often be to create a new active layer on top of the polluted deposits. This is especially the case for several lakes and coastal bays in Sweden which are covered by ice in winter and where there is plenty of clean glaciofluvial sand in the neighbourhood that can be transported to and deposited as a thin layer on the ice above the polluted bottom areas. During break-up of the ice this layer will settle, partly cover, and partly compact the polluted layers below. This procedure should be repeeted until the new active layer is thick enough. With active layer is here meant the part of the top layer that takes part in the exchange processes between sediment, bottom water, and biota. Except for natural recovery, this type of capping by natural, granular material, such as sand, is the least expensive way of remediation.

The relationship between the organic content and the content of some heavy metals in samples of bottom deposits from Stallfjärden, a broader part of the Swedish river Dalälven just upstream of the rapids at Älvkarleby (ds = dry substance).

Often there is a rather good relationship of organic content to the content of heavy metals in bottom deposits.

Mercury content in the superficial sediments (0 - 10 cm) of the Swedish lake Ekoln around 1970, (ds = dry substance). Modified after Axelsson and Håkanson 1972 (UNGI Rapport Nr 14).

The mercury distribution reflects the flow pattern in the lake and the deposition of mercury contaminated matter transported to the lake by the river Fyrisån. The median mercury content in the upper 10 cm of sediment was 330 ppb (dry weight).

In many lakes and coastal areas the content of heavy metals is high and sometimes highest just in front of the mouth of more or less polluted rivers, as exemplified above by the distribution of mercury in lake Ekoln.

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