Sediment, composed of eroded soil particles like silt, sand, and clay, is a primary non-point source pollutant that significantly degrades the quality of water bodies. Non-point source pollution originates from broad areas of land rather than a single discharge point, carried into waterways by rain or snowmelt runoff. Water quality is typically assessed based on chemical, physical, and biological standards, and excessive sediment impacts all three categories, making it the greatest pollutant by volume in many surface waters. Understanding how sediment contaminates water is key to managing its widespread effects on aquatic environments and human infrastructure.
Altering Physical Characteristics
The most immediate and observable effect of sediment is the physical change it causes, primarily through increased turbidity, which is the cloudiness of the water caused by suspended particles. High turbidity reduces the depth to which sunlight can penetrate the water column, directly inhibiting photosynthesis in submerged aquatic vegetation (SAV) and phytoplankton. This reduction in light disrupts the base of the aquatic food web.
Suspended sediment particles also absorb solar radiation, causing the water temperature to rise, particularly near the surface. Warmer water holds less dissolved oxygen, stressing cold-water species like trout that require high oxygen levels. The resulting changes in temperature and light penetration create a hostile environment for organisms adapted to cooler, clearer conditions.
Transporting Pollutants and Affecting Dissolved Oxygen
Fine sediment particles have a large surface area, which allows them to function as carriers for other contaminants, a process known as adsorption. Pollutants such as phosphorus, heavy metals, and pesticides chemically bond to the sediment and are transported into the water body. When the sediment settles, these adsorbed pollutants accumulate in the bottom layer (benthos), where they can be released back into the water column if disturbed.
Deposited sediment causes the depletion of dissolved oxygen (DO) in the water column and the sediments. Sediment often contains organic material; as bacteria and other microbes decompose this material, they consume oxygen through sediment oxygen demand (SOD). This consumption creates anaerobic (oxygen-depleted) conditions in the bottom waters that are lethal to most aquatic life. Warmer temperatures accelerate decomposition, exacerbating the oxygen depletion problem.
Direct Impact on Aquatic Ecosystems
Excessive sediment causes physical, physiological, and behavioral harm to aquatic organisms. As fine sediment settles, it smothers the coarse gravel substrates that serve as spawning grounds for fish like salmon and trout, reducing the survival of eggs and newly hatched fish. The deposition also eliminates the benthic habitats of invertebrates, which are a primary food source for many fish species.
Suspended particles injure fish and invertebrates by clogging or abrading their gills, impairing their ability to breathe and reducing their resistance to disease. Concentrations of suspended sediment greater than 500 mg/L are often associated with severe gill damage and mortality. Reduced water clarity hinders the feeding efficiency of visual predators, causing stress and reduced growth rates.
Costs and Challenges for Human Use
Increased sediment loads complicate the process of treating drinking water, resulting in higher operational costs for municipalities. High turbidity requires water treatment plants to use greater amounts of chemicals for coagulation and filtration to remove suspended particles. This increases the cost of providing clean water to consumers.
Sedimentation also reduces the storage capacity of reservoirs and fills in navigation channels, necessitating expensive and repeated dredging operations. Dredging costs can vary widely, and the disposal of contaminated sediment adds further expense. Beyond infrastructure, turbid water detracts from the aesthetic appeal and recreational value of water bodies, negatively impacting activities like swimming and fishing.