**4. Chromium**

Chromium is a genotoxic carcinogen and Cr(VI) is due to its similarity with sulphate taken up by the same pathways. While excess arsenic in groundwater is often a natural phenomenon, the occurrence of chromium is natural as well as anthropogenic. In Greece, a groundwater plume up to 160 μg/l was studied [26]. It was found to be essentially natural but with an anthropogenic component in one area. Chromium is used in stainless steel but major pollu‐ tion by chromium is from leather tanning and electroplating. Cr(III) forms solid phases in a reducing environment in groundwater but is mobile as Cr(VI) under oxidising conditions. Cr(III) can be oxidised to Cr(VI) by manganese oxides commonly present in soils [27–30].

$$\text{Cr(OH)}\_{2} + 1.5 \text{MnO}\_{2} \leftrightarrow \text{HCrO}\_{4} + 1.5 \text{Mn}^{2+} \tag{1}$$

Kazakis et al. [31] have studied the oxidation of Cr(III) on surfaces of mafic minerals and con‐ cluded that it was mediated by manganese oxides. Chromium tends to be present at higher levels not only in ultramafic rocks present in Greece but also elsewhere in the world [32]. Cr(VI) is mobile in groundwater but its mobility is pH dependant, being higher at elevated pH levels depending on the fact that the main adsorbents aluminium and iron oxyhydroxides lose their positive charge at pH above 8 [28]. Soluble organic complexes decrease the oxidation rate due to complexation with Cr(III) [33]. These authors studied the use of tannery waste for increasing the organic matter content in soils. Cr(VI) may be oxidised to chromate even in an organic matrix like tannery sludge provided the oxygen is high enough [34]. Chromate may then be leached, and it may pollute the groundwater. A number of actions to decrease chromium leaching from leather tanning are proposed [35]. In Punjab, in northwestern India, a sand delta was detected at levels of 5 mg/l at 60 m depth, in this case from electroplating activities [36]. At a level below 2 mg/l in drinking water, chromium can be reduced to Cr(III) which is not taken up by humans.
