Lexikon

in sedimentology sediment is any particulate matter (inorganic or organic) that comes from the weathering of rock and can be transported by water (fluvial processes), by wind (aeolian processes) and by glaciers and which eventually is deposited. The sediment can be classified based on its grain size and/or its composition. Sediment size is measured on a log base 2 scale, called the "Phi" scale, which classifies particles by size from "colloid" to "boulder". Composition of sediment can be measured in terms of: parent rock lithology, mineral composition, chemical make-up. The sediment is transported based on the strength of the flow that carries it and its own size, volume, density, and shape. Sediment motion can create self-organized structures such as ripples, dunes, antidunes on the river or stream bed. These bedforms are often preserved in sedimentary rocks and can be used to estimate the direction and magnitude of the flow that deposited the sediment. When the ground surface is stripped of vegetation and then seared of all living organisms, the upper soils are vulnerable to both wind and water erosion. In a number of regions of the earth, entire sectors of a country have become erodible. Loss of soil due to erosion adds to sediment loads in the river systems.

See also surface water sediments

natural sediments are defined as the organic and inorganic materials found at the bottom of a water body. Sediments may include clay, silt, sand, gravel, decaying organic matter, and shells among other things, but exclude anthropogenic debris, such as vehicle tires.

Sediments can become contaminated in a number of ways. Urban runoff that discharges to surface waters often contains polycyclic aromatic hydrocarbons (PAHs), oil and grease, and heavy metals. Agricultural runoff may contain nutrients and pesticides. Industrial spills and releases, especially those that occurred before controls were in place, can put product into the water. Chemicals that are denser than water, such as polychlorinated biphenyls (PCBs) and some pesticides like DDT, will sink to the bottom of water bodies and directly contaminate sediments. Atmospheric deposition of substances such as mercury is another source of sediment contamination as is the discharge of contaminated groundwater through the sediments to the overlying surface water (USEPA 1999 and USEPA 2005).

The classes of contaminants that are most common in sediment contamination are pesticides, PCBs, PAHs, and to a lesser extent dissolved phase chlorinated hydrocarbons. With the right geochemical conditions heavy metals and metalloids can also occur in sediments or precipitate into them. The sediments of many marinas are contaminated with tributyltin, an organo tin compound that was used as a biocide in marine paints (USEPA 1999).

Sediment investigations are generally conducted in two parts. The first uses common sampling and analytical procedures to determine if the total concentrations of contaminants are high enough to warrant concern. The underlying assumption is that all the contaminant is bioavailable. If the data indicate there may be a problem, then the second part of the investigation is done. This part focuses on bioavailability and determining whether there is physical evidence of an impact such as less biodiversity in the impacted sediments and the presence of the chemicals in the tissue of flora and fauna (USEPA 2005).

In addition to evaluating contaminant concentrations, the site investigation needs to develop a very complete conceptual site model. Unlike conventional soil and groundwater investigations, where rapid change in the site conditions is not expected, sediment systems can be very dynamic and it is important for both the risk assessment and remedy selection to have a full understanding of potential changes in site dynamics. A fuller discussion on investigation techniques is found in the Site Characterization section (USEPA 2005).

The risk assessment estimates the potential impacts of the contaminated sediments on human and ecological receptors. Many common organic sediment contaminants are suspected carcinogens and some, such as PCBs and mercury, bioaccumulate in the food chain. Risk assessors have developed a triad, or weight-of-evidence approach, that integrates sediment chemistry, laboratory toxicity testing, and community structure indices to assess risk (Pinkney et al 2005). A more complete discussion of sediment risk assessment and related guidance documents can be found in the Risk Assessment section.

Sources:

USEPA. 1999. Introduction to Contaminated Sediments. EPA 823-F-99-006, Office of Science and Technology, 24 pp.

USEPA. 2005. Adobe PDF LogoContaminated Sediment Remediation Guidance for Hazardous Waste Sites, EPA-540-R-05-012. Office of Superfund Remediation and Technology Innovation, 236 pp.

Pinkney, A.E., B.L. McGee, P.C. McGowan, D.J. Fisher, J. Ashley, and D. Velinsky. 2005. Adobe PDF LogoUsing the Sediment Quality Triad Approach to Characterize Toxic Conditions in the Chesapeake Bay (2002): An Assessment of Tidal River Segments in the Bohemia, Elk, Northeast and Severn Rivers, CBFO-C05-01. USEPA, Chesapeake Bay Program Office, 234 pp.

sedimentary rock is the type of rock that is formed by sedimentation of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution. Particles that form a sedimentary rock by accumulating are called sediment. Before being deposited, sediment was formed by weathering and erosion in a source area, and then transported to the place of deposition by water, wind, mass movement or glaciers. Though sedimentary rocks form just a small part of the Earth's crust, they cover the largest part of the Earth's surface. Sedimentary rocks are deposited in strata that form a structure called bedding. Sedimentary rocks contain important information about the history of Earth. They contain fossils, the preserved remains of ancient plants and animals. The composition of sediments provides us with clues as to the original rock. Differences between successive layers indicate changes to the environment which have occurred over time. Sedimentary rocks can contain fossils because, unlike most igneous and metamorphic rocks, they form at temperatures and pressures that do not destroy fossil remains. (http://en.wikipedia.org/wiki). Sedimentary rocks are classified into three groups. These groups are: clastic, chemical precipitate and biochemical or biogenic. Clastic sedimentary rocks are composed of discrete fragments or clasts of materials derived from other minerals. They are composed largely of quartz with other common minerals including feldspar, amphiboles, clay minerals, and sometimes more exotic igneous and metamorphic minerals. The classification of clastic sedimentary rocks is complex because there are many variables involved. Particle size (both the average size and range of sizes of the particles), composition of the particles, the cement, and the matrix (the name given to the smaller particles present in the spaces between larger grains) must all be taken into consideration. (http://en.wikipedia.org/wiki). Chemical sedimentary rocks form when minerals in solution become oversaturated and precipitate. In marine environments, this is a method for the formation of limestone. Another common environment in which chemical sedimentary rocks form is a body of water that is evaporating. Evaporation decreases the amount of water without decreasing the amount of dissolved material. Therefore, the dissolved material can become oversaturated and precipitate. Sedimentary rocks from this process can include the evaporite minerals halite (rock salt), barite and gypsum (http://en.wikipedia.org/wiki). Organic sedimentary rocks contain materials generated by living organisms, and include carbonate minerals created by organisms, such as corals, mollusks, and foraminifera, which cover the ocean floor with layers of calcite which can later form limestone. Other examples include coal and oil shale (derived from the remains of tropical plants and subjected to heat) (http://en.wikipedia.org/wiki).