A trace element is a chemical element found in low or small amounts, also known as a trace amount. Trace elements is a more general term that applies to metals as well as other elements found in aquatic systems. Metals are a diverse group of elements that can occur naturally in water or be contributed to water from human activities. Some of these elements are required in very small (trace) amounts by things that live in water, but at higher concentrations they can be toxic. Some metals are toxic to living things in any amount, like mercury. Metals do not degrade, but they can change form and move between water and sediments in aquatic systems.

In this chapter
A metal-like element that does not break down in the environment
A metal that is toxic and extremely persistent in the environment
An element in many different kinds of minerals