Sediments are regarded as a river’s long-term memory. They mainly comprise particles that are eroded from the ground, ending up at some point in river deltas or the sea. However, sediments can also remain stable for a relatively long time—and bind pollutants which, for example, have entered the rivers through mining or industrial wastewater. As a consequence, many old river sediments contain pollutants as “chemical time bombs,” such as heavy metals or dioxins and dioxin-like compounds that are not easily degradable.
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Source: Phys.org