Chromium (Cr) is a polyvalent element and can exist in several distinct oxidation states, but only trivalent chromium [Cr(III)] and hexavalent chromium [Cr(VI)] occur with any frequency in the natural environment. The mobility, bioavailability, and toxicity of chromium largely depend on which of these two chemical species is prevalent. The speciation of chromium is determined by the processes that control reduction/oxidation reactions in the environment. As can be seen by the adjacent simplified Pourbaix diagram of chromium species in water, species conversion can occur with only fairly minor changes in pH and Eh.

Chromium is a naturally occurring metal found in small quantities associated with other metals, particularly iron. It is commonly used for making steel and other alloys, bricks in furnaces, dyes and pigments, chrome plating, leather tanning, and wood preserving. Due to its extensive use in industrial processes, large quantities of chromium compounds are discharged into the environment.

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