Siltation

Water pollution caused by particulate terrestrial clastic material

Siltation is water pollution caused by particulate terrestrial clastic material, with a particle size dominated by silt or clay. It refers both to the increased concentration of suspended sediments and to the increased accumulation (temporary or permanent) of fine sediments on bottoms where they are undesirable. Siltation is most often caused by soil erosion or sediment spill. It is sometimes referred to by the ambiguous term "sediment pollution", which can also refer to a chemical contamination of sediments accumulated on the bottom, or to pollutants bound to sediment particles. Although "siltation" is not perfectly stringent, since it also includes particle sizes other than silt, it is preferred for its lack of ambiguity.

The origin of the increased sediment transport into an area may be erosion on land or activities in the water. In rural areas, the erosion source is typically soil degradation by intensive or inadequate agricultural practices, leading to soil erosion, especially in fine grained soils such as loess. The result will be an increased amount of silt and clay in the water bodies that drain the area. In urban areas, the erosion source is typically construction activities, which involve clearing the original land covering vegetation and temporarily creating something akin to an urban desert from which fines are easily washed out during rainstorms. In water, the main pollution source is sediment spill from dredging, the transportation of dredged material on barges, and the deposition of dredged material in or near water. Such deposition may be made to get rid of unwanted material, such as the offshore dumping of material dredged from harbours and navigation channels. The deposition may also be to build up the coastline, for artificial islands, or for beach replenishment. Climate change also affects siltation rates. Another important cause of siltation is the septage and other sewage sludges that are discharged from households or business establishments with no septic tanks or wastewater treatment facilities to bodies of water.

While the sediment in transport is in suspension, it acts as a pollutant for those who require clean water, such as for cooling or in industrial processes, and it includes aquatic life that are sensitive to suspended material in the water. While nekton have been found to avoid spill plumes in the water (e. g. the environmental monitoring project during the building of the Øresund Bridge), filtering benthic organisms have no way of escape. Among the most sensitive organisms are coral polyps. Generally speaking, hard bottom communities and mussel banks (including oysters) are more sensitive to siltation than sand and mud bottoms. Unlike in the sea, in a stream, the plume will cover the entire channel, except possibly for backwaters, and so fish will also be directly affected in most cases. Siltation can also affect navigation channels or irrigation channels. It refers to the undesired accumulation of sediments in channels intended for vessels or for distributing water.

One may distinguish between measurements at the source, during transport, and within the affected area. Source measurements of erosion may be very difficult since the lost material may be a fraction of a millimeter per year. Therefore, the approach taken is typically to measure the sediment in transport in the stream, by measuring the sediment concentration and multiplying that with the discharge; for example, times gives
Also, sediment spill is better measured in transport than at the source. The sediment transport in open water is estimated by measuring the turbidity, correlating turbidity to sediment concentration (using a regression developed from water samples that are filtered, dried, and weighed), multiplying the concentration with the discharge as above, and integrating over the entire plume. To distinguish the spill contribution, the background turbidity is subtracted from the spill plume turbidity. Since the spill plume in open water varies in space and time, an integration over the entire plume is required, and repeated many times to get acceptably low uncertainty in the results. The measurements are made close to the source, in the order of a few hundred meters. Anything beyond a work area buffer zone for sediment spill is considered the potential impact area. In the open sea, the impact of concern is almost exclusively with the sessile bottom communities since empirical data show that fish effectively avoid the impacted area. The siltation affects the bottom community in two main ways. The suspended sediment may interfere with the food gathering of filtering organisms, and the sediment accumulation on the bottom may bury organisms to the point that they starve or even die. It is only if the concentration is extreme that it decreases the light level sufficiently for impacting primary productivity. An accumulation of as little as may kill coral polyps. While the effect of the siltation on the biota (once the harm is already done) can be studied by repeated inspection of selected test plots, the magnitude of the siltation process in the impact area may be measured directly by monitoring in real time. Parameters to measure are sediment accumulation, turbidity at the level of the filtering biota, and optionally incident light. Siltation of the magnitude that it affects shipping can also be monitored by repeated bathymetric surveys.