Chromium and water: reaction mechanisms, environmental impact and health effects

Seawater chromium content varies strongly, and is usually between 0.2 and 0.6 ppb.
Rivers contain approximately 1 ppb of chromium, although strongly increased concentrations are possible, for example 5-20 ppb in the River Rhine, and 10-40 ppb in the River Elbe in 1988.
Phytoplankton contains approximately 4 ppm chromium, sea fish contain between 0.03 and 2 ppm, and oyster tissue contains approximately 0.7 ppm (all values dry mass). Phytoplankton has a bio concentration factor of approximately 10⁴ in seawater.
In dissolved form chromium is present as either anionic trivalent Cr(OH)₃ or as hexavalent CrO₄^2-. The amount of dissolved Cr³⁺ ions is relatively low, because these form stable complexes.
Oxidation ranks from Cr(II) to Cr(VI). In natural waters trivalent chromium is most abundant.