Sediment-Associated Contaminants

Many contaminants adhere, or “sorb”, to sediment rather than readily dissolving in water.  Once sorbed, these chemicals can persist in the sediment for years, long after they are no longer detectable in water.  These types of chemicals, sometimes referred to as “hydrophobic” ("water fearing”), include many legacy contaminants, like DDT, PCBs, and chlordane, that were banned decades ago but are still found in lake and stream bed sediment, sometimes at concentrations high enough to be a risk to aquatic organisms.  Other hydrophobic contaminants are being released into the environment, like the pesticide bifenthrin, flame retardant chemicals, and polycyclic aromatic hydrocarbons (PAHs). Most metals, such as lead, zinc, and cadmium, also concentrate in sediment rather than water.

Sediment-associated contaminants are of particular concern for aquatic organisms that live in the bed sediment of streams, rivers, and lakes—referred to as benthic organisms.  Dragonfly larvae, for example, live in sediment, as do the larvae of many other insects. Mollusks, such as mussels and clams, crustaceans, such as crayfish, and many fish and amphibians live in, on, or near the sediment, or consume smaller organisms that do, and therefore sensitive to the contaminants associated with that sediment.

Importance to Stream Ecology
The USGS Regional Stream Quality Assessment (RSQA) is assessing the importance of a wide-range of stressors to aquatic ecosystems in small streams, including sediment-associated contaminants. The assessment includes evaluation of toxicity of stream sediment in urban, agricultural, and undeveloped settings to aquatic organisms.  For example, the pesticide bifenthrin in sediment was found to reduce the abundance of insect larvae and to affect the timing of the metamorphosis of those larvae to adult insects.

At what concentration is a sediment-associated contaminant likely to be toxic?  Sediment benchmarks provide two levels: the concentration below which the sediment is unlikely to be toxic (the threshold effects concentration, or TEC) and the concentration above which the sediment is likely to be toxic (the probable effects concentration, or PEC).  These benchmarks provide a handy context to evaluate the potential toxicity of sediment-associated contaminants.

Trends in Sediment-Associated Contaminants
The fact that some contaminants adhere to sediment offers a unique opportunity to track how their concentrations have changed over time. Sediment cores—essentially long tubes of mud pulled from a lake or reservoir—allow us to look back in time at contaminant trends in a watershed.  Concentrations at the top of the core reflect recent inputs of contaminants, and concentrations farther down the core reflect older and older inputs.

Sediment cores have allowed us to see the effectiveness of bans on some chemicals, such as DDT, with their concentrations decreasing toward the top of the core. Sediment cores also allow us to identify concerns about chemicals, such as PAHs, whose concentrations increase toward the top of the core.

Contaminant Transport on Suspended Sediment
Not all contaminated sediment is bed sediment.  Hydrophobic contaminants—contaminants that do not dissolve in water—can bind to sediment as well and be transported down streams and rivers, ultimately to be deposited in the bed sediment of a downstream lake or the ocean. By measuring contaminants on suspended sediment in conjunction with flow, the loads and yields of hydrophobic contaminants can be estimated. Stormwater is a particularly important source of contaminants sorbed to and transported by suspended sediment.

Although we typically think of suspended sediment as being associated with surface water—streams, rivers, and lakes—suspended sediment and the contaminants associated with it can also be transported with groundwater through the subsurface conduits found in karst aquifers.