Some of the substances that make up Earth's crust are elements, substances that cannot be naturally broken down into simpler substances. A few of these elements are poisonous even if present in a low concentration. These are known as heavy metals. Examples of heavy metals include mercury, cadmium, arsenic, chromium, thallium, and lead.

A school of bluefin tuna snagged during a mattanza (a mass tuna catch) that periodically occurs off the coast of Sicily, Italy. © Jeffrey L. Rotman/Corbis. Reproduced by permission.

The Heavy Metal-Water Connection

There is a connection between heavy metals and water. Because heavy metals are part of Earth's crust, they can be worn away by the action of weather. When they are worn off of rock, they can collect in surface or groundwater (fresh water in rock and soil layer beneath Earth's surface). Depending on the chemistry of...