Water Quality - Trapping Sediment
Trapping Sediments and Retaining Particulates
Wetlands trap sediment and retain particulates from both onsite and offsite sources. Trapping sediments contrasts with the removal of nutrients because sedimentation and particulate removal emphasize physical processes rather than elements and compounds, many of which are in the dissolved state.
There are two primary benefits of trapping sediment. First, the removal of particulates reduces the load of particle bound nutrients, heavy metals, pesticides, and other pollutants into groundwater, and nearby rivers and streams. Second, at natural sustainable levels, these inputs are necessary for the overall maintenance of the nutrient budget and associated characteristic plant and animal communities of wetlands.
Accelerated sedimentation may be the most detrimental impact on wetlands. Accumulation of sediment in wetlands decreases wetland volume, decreases the duration wetlands retain water, and changes plant community structure by burial of seed banks.
Characteristics and Processes that Influence Trapping Sediments
The characteristics and processes that influence a depressional wetland’s ability to perform this function can be divided into two groups. The first deals with the sources and mechanisms by which particulates are transported to, or prevented from entering into, the wetland, or both. The second group of characteristics and processes relate to the immobilization of the particulates that are transported into the wetland.
Sources and Mechanisms of Sediment Transport
The primary characteristic that causes sediment accumulation in wetlands is landscape position. Sediment inputs into most wetlands are derived primarily from wind and water erosion of soils in the immediate catchment and adjacent upwind landscapes. Because most depressional wetlands occur in surficially closed basins they become “targets” for the retention of water-borne sediment.
Sediment and particulates are transported into depression wetlands from several sources. They include dry deposition and precipitation from the atmosphere, overland flow from adjacent uplands, and occasional overflows connecting wetlands during wet periods of high storage. Atmospheric sources are assumed to account for a relatively small amount of the total particulates that typically impact pothole wetlands. However, in areas of intense agriculture, atmospheric deposition of sediments may be significant.
The dominant mechanisms for the input and output of particulates among depression wetlands are surface sources such as overland flow, surface connections between wetlands during wet periods, and man-made ditches. These sources are a function of wetland basin morphology (e.g., catchment size, slope gradient, and natural or man-made surface connections).
Dense vegetation cover reduces surface water velocities and allows for greater infiltration and filtration of particulates, and soils are less likely to erode. Therefore, uplands with dense vegetation cover and wetlands with buffers of perennial vegetation around them will supply fewer particulate inputs to the wetland than uplands with sparse vegetation; prairie wetlands with cultivated (sparse vegetation cover) catchments accumulated sediments at a rate about two times that of basins with dense grassland cover.
A wetland with primarily sheet flow through the wetland and dense emergent vegetation density will allow sediment to drop out more effectively than a wetland with channel flow and no vegetation.
Vegetation structure also influences the ability of the wetland to trap sediment, both from water and wind erosion. Sediment retains elements and compounds through burial and chemical precipitation (e.g., removal of phosphorus by iron III). Dissolved forms may be transported with the particles through sorption and chelation (i.e., heavy metals mobilized with humic and fulvic compounds). Imported sediment can undergo renewed pedogenesis on site, which potentially involves weathering and release of elements that were previously inaccessible to mineral cycling.
Source: A Regional Guidebook for Applying the Hydrogeomorphic Approach to Assessing Wetland Functions of Prairie Potholes (HGM), Army Corps of Engineers