Sediment Sampling

King County monitors sediments in lakes and streams as part of its ambient monitoring program in the freshwater aquatic environment. Sediment quality is an important indicator of environmental health, and along with information on water quality, habitat, and the aquatic food web (i.e. plankton, invertebrates, and fish), it can present a better picture of environmental quality. Understanding what is in sediments is important because chemical contaminants can be washed into streams and lakes from urban areas and attach to sediments, which then settle to the bottom. In this way, sediments can act as record of both historical and recent contaminants discharged into surface waters. Once contaminants are in these bottom sediments they can persist where people can be exposed to them directly or by eating fish that have been caught in our local lakes and streams.

The Stream Sediment Monitoring Program was begun in 1987 in WRIAs 8 and 9 as part of the overall Lakes and Streams Ambient Monitoring Program. An updated 10-year program began in 2004 to monitor the effects of all sources (point sources, stormwater, and other urban discharges) to the streams. Additional parameters were added to the existing sediment monitoring program to better understand the range of contaminants that affect sediment quality. Each year a set of streams is monitored in the study area. The study is designed so that each program stream will be monitored within a ten-year time-frame. Program streams include:

Thornton, McAleer, Lyon, Swamp, North, Little Bear, Big Bear, Evans, Cottage, Juanita, Forbes, Mercer/Kelsey, Coal (Lake Washington), May, Pine Lake, Eden, Ebright, Tibbetts, Issaquah, Taylor (Cedar River), Longfellow, Newaukum, Soos, Springbrook, Mill, Lewis, Covington, Des Moines, Jenkins, Judd, and Crisp Creeks.

On each creek, sediment stations were located every mile along the creek where possible. Individual sampling locations were based on a series of criteria including whether a creek reach could be accessed, whether a creek reach had low enough gradient which would allow for sediment deposition, and whether enough sediment had accumulated in an area to collect a sample. Typically muddy or silty areas with fine-grain sediments are targeted for collection as these are the areas where not only sediments accumulate but contaminant as well. Samples were tested for conventional parameters, metals, and organic chemicals.

Data were interpreted using tools that include sediment quality guidelines, acid volatile sulfides/simultaneously extracted metals (AVS/SEM) ratio, and particle size analysis. Sediment quality guidelines provide a benchmark to better understand the potential for adverse effects to occur in sediment-dwelling animals from sediment-associated chemicals. (See sediment quality guidelines tab)

AVS/SEM ratio is used to better understand if metals that are present in sediments are bioavailable and able to cause adverse effects. AVS in sediments can bind to certain metals so that sediment-dwelling animals are not likely to be exposed to the toxic potential of these metals. If there is more AVS in sediments than metals, then the metals present in the sediments are not likely to cause adverse effects in the aquatic community near these sediments.

Particle size analysis determines the physical makeup of sediments by determining how much fine silt, clay, sand, or gravel is in a sediment sample. If a sediment sampling site is dominated by sands and gravels, or coarse particulates, it means that the sediments in this area are moving downstream and that water is flowing by too fast for fine particles to settle out. If a sediment sample is dominated by silts and clays or fine particulates it indicates this site is a depositional zone and fine particulates can settle out. Most chemical contamination found in sediments will be in the fine sediments so if chemicals are present they will likely be associated with the fine particulates of a depositional zone.

For more information about this program, contact contact Dean Wilson.