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Sediment

General Information: Sediment is composed of organic and inorganic particles of various sizes. The major classes of sediment, from largest to smallest, are boulders, cobbles, pebbles, sand, silt, and clay (Friedman et al., 1992).
Class           Size (mm)     Approx size
Boulders        > 256         > Volleyball
Cobbles         > 64          > Tennis ball
Pebbles         > 2           > Match Head
Sand
   V. Coarse    1.5
   Medium       0.375
   V.Fine       0.094
Silt
   V. Coarse    0.047
   Medium       0.0117 (no longer visible to the human eye)
   V.Fine       0.0049
Clay            < 0.00195

(Adapted from  Friedman et al., 1992)
Sediments are classified into four broad categories, according to their origin in relation to the basin of water in which they are deposited: extrabasinal, carbonaceous, pyroclastic, and intrabasinal.
  1. Extrabasinal (terrigenous) particles: Terrigenous particles have been eroded from the land outside the body of water experiencing the deposition. Most terrigenous particles originate from bedrock. Particles are detached by weathering, wind and water erosion, catastrophic mass-wasting (landslides), and glacial activity. The particles either retain their chemical make-up, or become chemically altered to clays and iron oxides.
  2. Carbonaceous particles: These particles are organic in nature and are derived from either solid carbonaceous material (coal, amber, wax, and kerogen), reworked from other geologic formations, or from modern plant detritus.
  3. Pyroclastic particles: These particles are derived during the explosive action of a volcano. Particles include rock fragment, single crystals, or bits of volcanic glass.
  4. Intrabasinal particles: These particles grow biochemically or chemically in the waters experiencing deposition. These particles include carbonate biocrystals, silica biocrystals, particles composed of evaporite minerals, and minerals that grow at the water/sediment interface. Carbonate biocrystals are secreted by marine organisms such as foraminifers and mollusks. Sediment forms when these calcium carbonate skeletons are broken down physically or biologically. Silica biocrystals are formed by diatoms, sponges, and dinoflagellates. Diatoms are the primary contributor to siliceous intrabasinal sediment in sea water and fresh water (Friedman et al., 1992).
Sediment introduced into surface water is either deposited on the bed of the stream or lake or suspended in the water column (suspended load). Bedload is large sediment particles that move by bouncing along the bottom. Generally, the suspended loads in lotic (flowing) water consists of grains less than 0.5 mm in diameter (Dunne et al., 1978). Lentic (lake) suspended loads usually consist of the smallest sediment fractions, such as silt and clays (Dunne et al., 1978). A water body's suspended load is a component of the total turbidity. For more specific information on turbidity, please refer to the Turbidity section.

Particles in both the bedload and the suspended load may be transported by the current. Suspended loads are carried in both the gentle currents of lentic waters and the fast currents of lotic water. Because the particles in the bedload move by rolling or bouncing along the bottom, bedload transportation occurs primarily in lotic (flowing) waters. The volume of sediment transported relies solely on the particle size and the flow velocity. A high flow velocity can transport a greater number of larger particles than can a slower current. Any sediment transported by water is subject to deposition as flow velocity decreases (McCabe et al., 1985).

The amount of sediment deposited on a rocky substrate can be quantitatively defined by an estimation of the percent embeddedness (see Analytical Techniques below). The percent embeddedness is the degree to which fine sediments such as sand, silt, and clay fill the interstitial spaces between rocks on a substrate.

The biota of an aquatic system are thought to have evolved to cope with the natural percent embeddedness of a stream. Any increase above the natural levels may decrease the health of a system. Studies have shown that a 67% embedded substrate will cause changes to occur in the structure of macroinvertebrate fauna. In addition, most fry will leave an area or die when embeddedness levels reach 50-60% (Harvey, 1989).

Numerical Categories: Acceptable Ranges to Maintain Designated Use
            
Optimal Ranges               Designated Use
                   Aquatic life  
< 25% embeddedness         Excellent Conditions
25 - 50% embeddedness         Good Conditions
50 - 75% embeddedness         Fair Conditions
> 75% embeddedness         Poor Conditions

(Plafkin et al., 1989)


      Industry (total solids)        
                          Boiler Feedwater (see psi)
500 - 3,000 mg/l            0 - 150 psi
500 - 2,500 mg/l           150 - 250 psi
100 - 1,500 mg/l           250 - 400 psi
50 mg/l                    > 400 psi

200 mg/l                  Photographic Processing
200 mg/l                  Clear Plastic Production
100 mg/l                  Pulp Production

(AWWA, 1990)

Health Effects: Organic sediment particles may harbor harmful bacteria and pathogens. Infection by the microorganisms may occur if water is used for primary contact or as a raw drinking water source. Treated drinking water will not present the same health risks. In a potable water treatment plant all sediment should be effectively removed before distribution.

Industrial Effects: Suspended sediment in the intake water of industries may present operational problems. Silica-rich sediment may scour pipes and machinery, causing leaks. Clay and organic-rich sediment may clog pipes and machinery, which may be costly to repair. Suspended sediment may be deposited while the water is stored in tanks, decreasing the available volume of the holding tank. Sediment may also drive up the cost of water treatment, making the water source less economical for industrial use (Morton, 1986).

Environmental Effects: The series of sediment-induced changes that can occur in a water body may change the composition of an aquatic community (Wilber, 1983). First, a large volume of suspended sediment will reduce light penetration, thereby suppressing photosynthetic activity of phytoplankton, algae, and macrophytes. This leads to fewer photosynthetic organisms available to serve as food sources for many invertebrates. As a result, overall invertebrate numbers may also decline, which may then lead to decreased fish populations.

In addition, sediment may interfere with essential functions of organisms. The numbers of filter-feeding invertebrates will decline if their filter mechanisms are choked by suspended particles (James et al., 1979). Some zooplankton suffer decline due to clogged feeding mechanisms (McCabe et al., 1985). Likewise, fish may suffer clogging and abrasive damage to gills and other respiratory surfaces. Abrasion of gill tissues triggers excess mucous secretion, decreased resistance to disease, and a reduction or complete cessation of feeding (Wilber, 1983; McCabe et al., 1985). Suspended sediment may also affect predator-prey relationships by inhibiting predators' visual abilities.

*Note: In natural waters fishes avoid areas of high suspended solids when possible by hiding in quieter pools or moving away from the source of sediment. Thus, although experimental studies may suggest certain degrees of injury to aquatic fauna in a given level of turbidity, the actual effects observed may be less pronounced because of the avoidance behavior.

Reproductive success may decline with an increase in fine sediment. If spawning habitats are altered by sediment deposition (e.g., filling of pools and riffles or covering of a gravel bed), fish may be unable to lay eggs. If eggs are successfully produced, the incubation period may be in jeopardy because 1) a shifting-sediment environment is unstable, and 2) burial by fine sediment prevents circulation of water around the egg, decreasing oxygenation. The egg will suffocate and may be poisoned by its own metabolic waste. If eggs do hatch into fry, the young may be less likely to survive in less-than-optimum conditions (Morton, 1986; McCabe et al., 1985).

The settling of suspended solids from turbid waters threatens benthic aquatic communities. Deposited particles may obscure sources of food, habitat, hiding places, and nesting sites (Wilber, 1983). Most aquatic insects will simply drift with the current out of the affected area. Benthic invertebrates that prefer a low-silt substrate, such as mayflies, stoneflies, and caddisflies, may be replaced by silt-loving communities of oligochaetae, pulmonate snails, and chironomid larvae (James et al., 1979).

Increased sediment may impact plant communities. Primary production will decline because of a reduction in light penetration. Sediment may damage plants by abrasion, scouring, and burial. Finally, sediment deposition may encourage species shifts because of a change of substrate.

Sediment deposition may also affect the physical characteristics of the stream bed. Sediment accumulation causes stream bed elevation and a decrease in channel capacity. Flooding is more likely after sediment accumulation because the stream can not accommodate the same volume of water (Morton, 1986). Also, a substrate that is closer to the surface receives more light and supports increased numbers of photosynthetic organisms, such as rooted algae (AWWA, 1990). As a result, recreational use may be threatened because moving parts of boats may become tangled in aquatic plants. Sediment, which is generally negatively charged, attracts positively charged molecules. Some of these molecules (phosphorus, heavy metals, and pesticides) are pollutants. These positively charged pollutants are in equilibrium with the water column and are often released slowly into the water resource.

Sources:

  1. Non point sources:


     


     


     


     

  2. Point sources: Not applicable.

Mode of Transport: Sediment may be transported to surface waters via overland flow. Sediment may also be eroded from stream banks during high flows or resuspended from the substrate by changes of speed or direction of the water current.

Analytical Techniques:

1. Total Suspended Solids (TSS) Sampling Technique:

A number of different methods are available for sampling suspended sediment in streams. First, if the stream is small and well-mixed, a sample can be obtained using a cup or a bucket. However, for the most accurate measurements, suspended sediment samplers are recommended (Gordon et al. 1992).

Two types of suspended sediment samplers are available for perennial streams: depth-integrating and point-integrating. Both samplers are usually made from cast aluminum or bronze and have a tail fin to orient the sampler's intake nozzle upstream. The depth-integrating sampler is designed to sample continuously as it is lowered at a constant speed from the water surface to the streambed and back. The point-integrating sampler is equipped with a mechanism at the end of the sampler that can open and collect a sample at a specified depth in the stream. (Gordon et al. 1992).

Ephemeral streams are sampled using rising-stage samplers. These samplers have a number of bottles arranged on top of one another in a frame. Each bottle is equipped with kinked tubing pointed into the flow and will collect a sample as the water rises. This sampler is better suited for sampling silts and clays because the intake flow velocity is slower than the stream velocity and larger particles might settle before entering the sampler (Gordon et al. 1992).

Many automated samplers are also available. They differ in cost, ease of maintenance, run time, and ability to extract a representative sample. The samplers extract samples by pumping water from the stream and retaining some in a sample bottle. The samplers can be set to extract samples at set intervals or can be attached to a float that will automatically trigger the sampling process when a preselected stream stage is reached (Gordon et al. 1992).

Finally, suspended sediment concentration is often monitored with a turbidity meter. A turbidity meter either measures the amount of light that is transmitted through the sample, or measures the light that is scattered by the sample. Although turbidity can be influenced by other factors such as size distribution, shape, and absorptivity of the sediment, and the color of the water, turbidity meters give satisfactory estimates (Gordon et al. 1992).

2. Percent Embeddedness Technique:

The amount of sediment deposited on a rocky substrate can be quantitatively defined by an estimation of the percent embeddedness. The percent embeddedness is the degree to which fine sediments such as sand, silt, and clay fill the interstitial spaces between rocks on a substrate.

This method is only applicable on substrates of coarse pebbles, cobbles, or rubble. Percent embeddedness is measured at transect points of an area of known size (e.g. a square 0.5m X 0.5m). The following table provides a guide to what percentages are normally assigned to various substrate conditions.

A guide to percent embeddedness: (Simonson et al., 1994)



3. Assessment of Capacity Loss Due to Sedimentation:

The rate of sediment-induced storage loss in lakes and reservoirs can be measured with a series of sedimentation or bathymetric surveys. Transects and sampling points are often established perpendicular to the main axis of tributary inflow or the main axis of the lake. Some base strata (original lake or reservoir bottom) should be established to track the rate (cm/yr) of deposition. A long pole may be used in shallow areas to measure sediment depth. For deeper areas, sonar or a SCUBA diver can record sediment depth measurements. Although measurements every year may not be needed, the same transects and stations should be monitored periodically. Major changes in land use, best management practices, or streambank erosion that could increase sedimentation should also be monitored (Coffey et al., 1995).

Sediment accumulation (deposition rates) can also be determined by measuring radionuclides in the sediment (McIntyre et al., 1989). Cesium-137, a fallout product of nuclear testing, binds tightly to soil particles and can be used to estimate the time of sediment deposition. Measurable levels of Cesium-137 were introduced into the atmosphere during the beginning of the nuclear age and can only be used to estimate deposition after 1957.

Lead-210 may also be used to measure sedimentation rates (McIntyre et al., 1989). Lead-210 is a naturally-occurring uranium isotope that is a decay product of radon. When atmospheric radon decays, the lead-210 is deposited on the earth's surface. Lead-210 will bind to soil particles and can be used to measure sedimentation rates for the past 100 years (McIntyre et al., 1989).