Sediment Loss from Agricultural Land Use
Sediment has been identified as a significant water quality issue in the Cayuga Lake watershed. Both measurement and observation confirm that visible plumes of sediment flow into the lake from its southern tributaries during storms and snowmelt. Sediment can adversely affect the quality of streams and lakes and diminish their suitability as habitat for plants and animals. Suspended sediment can reduce the amount of sunlight available to aquatic plants, cover fish spawning areas and food supplies, interfere with filter feeding organisms, and clog and harm the gills of fish. Turbidity interferes with the feeding habits of fish. These effects combine to reduce fish and plant populations and decrease the overall productivity of the aquatic resource. In addition, recreation is limited because of the decreased fish population and the water's unappealing, turbid appearance. Turbidity also reduces visibility, making swimming less safe.
According to EPA "Guidance Specifying Management Measures for Sources of Nonpoint Pollution in Coastal Waters" chemicals such as some pesticides, phosphorus, and ammonium are transported with sediment in an adsorbed state. Changes in the aquatic environment, such as a lower concentration in the overlying waters or the development of anaerobic conditions in the bottom sediments, can cause these chemicals to be released from the sediment. Adsorbed phosphorus transported by the sediment may not be immediately available for phytoplankton growth but does serve as a long-term contributor to eutrophication. This adsorbed phosphorus may be immediately available to the macrophyte community.
Sediment from agricultural lands is the solid material, both mineral and organic, that is in suspension, is being transported, or has been moved from its site of origin by air, water, gravity, or ice. The types of erosion associated with agriculture that produce sediment are (1) sheet and rill erosion and (2) gully erosion. Soil erosion can be characterized as the transport of particles that are detached by rainfall, flowing water, or wind. Eroded soil is either redeposited on the same field or transported from the field in runoff.
Sediments from different sources vary in the kinds and amounts of pollutants that are adsorbed to the particles. For example, sheet and rill erosion mainly move soil particles from the surface or plow layer of the soil. Sediment that originates from surface soil has a higher pollution potential than that from subsurface soils. The topsoil of a field is usually richer in nutrients and other chemicals because of past fertilizer and pesticide applications, as well as nutrient cycling and biological activity. Topsoil is also more likely to have a greater percentage of organic matter. Sediment from gullies and streambanks usually carries less adsorbed pollutants than sediment from surface soils.
Soil eroded and delivered from cropland as sediment usually contains a higher percentage of finer and less dense particles than the parent soil on the cropland. This change in composition of eroded soil is due to the selective nature of the erosion process. For example, larger particles are more readily detached from the soil surface because they are less cohesive, but they also settle out of suspension more quickly because of their size. Organic matter is not easily detached because of its cohesive properties, but once detached it is easily transported because of its low density. Clay particles and organic residues will remain suspended for longer periods and at slower flow velocities than will larger or denser particles. This selective erosion can increase overall pollutant delivery per ton of sediment delivered because small particles have a much greater adsorption capacity than larger particles. As a result, eroding sediments generally contain higher concentrations of phosphorus, nitrogen, and pesticides than the parent soil (i.e., they are enriched).
Methods to Control Erosion and Sedimentation
The objective of this class of best management practices (BMPs) is to reduce the mass load of sediment reaching a waterbody. Two different strategies can be used. The first, and most desirable, strategy is to implement practices on the field to prevent erosion and sediment transport. Practices that could be used to accomplish this include conservation tillage, contour strip-cropping, terraces, and critical area planting.
The second strategy is to route runoff from fields through BMPs that remove sediment. Practices that could be used to accomplish this include filter strips, field borders, grade stabilization structures, sediment retention ponds, water and sediment control basins, and terraces. Site conditions will dictate the appropriate combination of practices for any given situation.
Site-specific solutions may be developed for farms through participation in whole farm planning in cooperation with technical staff from County Soil and Water Conservation Districts, NRCS, Cornell Cooperative Extension, Farm Services Agency etc. Education and outreach are an important component of this effort.
Measures to reduce sediment loss have the potential to increase movement of water and soluble pollutants through the soil profile to the ground water. Erosion and sediment control systems must be carefully designed to protect against the contamination of ground water. Ground-water protection will also be provided through implementation of the nutrient and pesticide management measures to reduce and control the application of nutrients and pesticides.
To ensure that the selected BMPs continue to function as designed and installed, some operational functions and maintenance will be necessary over the life of the practices. Since BMPs will be designed to control a specific storm frequency, they may suffer damage when larger storms occur. Damage must be repaired after such storms and practices must be inspected periodically.
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CLW IO 2004