Chromium (Cr) is the 21st most abundant element in the earth's crust, which mostly occurs in its trivalent form Cr~(3+), with the ferrous chromite (Fe(Mg)Cr_2O_4) to be the principal ore. It enters the water from both natural and anthropogenic sources. Leather tanneries, textile industry, cooling tower blow-down and plating factories are the most often anthropogenic sources. Since Cr(Ⅲ) solubility at the common pH range 6.5 - 8.5 of potable water was determined to be around 10~(-7)M (～5μg/L) [1], higher naturally occurring chromium concentrations were attributed to Cr(Ⅵ) [2) due to natural oxidation of Cr(Ⅲ) in ultramafic derived soils [3] and ophiolitic rocks [4]. Ultramafic soils and ophiolitic rocks contain grains (0,2 -1 mm) of Cr_2O_3 in combination with high pH values. Thermodynamically, these parameters favor, although they don't warrant (Fig. 1, cross lines), the Cr(Ⅵ) formation in a natural water.
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