Chromium (Cr) is the first element in Group VIB in the periodic table. It has an atomic number of 24, an atomic weight of 51.99, and valences of 0 and 2 through 6. The average abundance of Cr in the earth’s crust is 122 ppm; in soils Cr ranges from 11 to 22 ppm; in streams it averages about 1 μg/L, and in groundwaters it is generally 100 μg/L. Chromium is found chiefly in chrome-iron ore (FeO · Cr2O3). Chromium is used in alloys, in electroplating, and in pigments. Chromate compounds frequently are added to cooling water for corrosion control.
In natural waters trivalent chromium exists as Cr3+, Cr(OH)2+, Cr(OH)2+, and Cr(OH)4−. Chromium in the form of Cr3+ would be expected to form strong complexes with amines and would be adsorbed by clay minerals. In the hexavalent form chromium exists as CrO42− and as Cr2O72−.
Chromium is considered nonessential for plants, but an essential trace element for animals. Hexavalent compounds have been shown to be carcinogenic by inhalation and are corrosive to tissue. The chromium guidelines for natural water are linked to the hardness or alkalinity of the water (i.e., the softer the water, the lower the permitted level for chromium). The United Nations Food and Agriculture Organization recommended maximum level for irrigation waters is 100 μg/L. The U.S. EPA primary drinking water standard MCL is 100 μg/L for total chromium.