**5. Selenium**

This is common in the Andes in South America [16]. The arsenic is then present as arsenate,

Manganese in groundwater has been considered as a technical problem causing blackish flushes in tap water after oxidation of the Mn2+ to Mn oxides in the distribution networks. Manganese inhaled from air has neurological effects known from metallurgical industries [18]. Recently, however, reports have come to indicate that chronic exposure of children to manga‐ nese in drinking water may affect their intellectual capacity [19]. So far, there are three reports that are comparable, from Quebec in Canada [19], from Mexico [20] and from Brazil [21], and that give concern regarding the neurotoxicity of manganese in drinking water. A summary of the findings regarding toxicity is published from a conference [22]. Manganese is an essential element and the major intake is via food. Elevated intake via drinking water by women may be positive for foetal survival [23] but foetal growth may be impaired [24]. The intake of man‐ ganese via drinking water is smaller but seems unregulated. This is mirrored in hair in which the concentration increases with that in drinking water but not with the intake via food. The Mn2+ seems to be taken up by the divalent metal transporter 1 (DMS-1) and be affected by, for instance, iron status [25]. A hypothesis, as humans have not developed a regulation, could be that moderately reduced groundwater from larger depth has, in the history of mankind, not

been a common way to get drinking water but rather from surface water and springs.

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].

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

2+ + 1 .5 MnO2 ↔ HCrO4<sup>−</sup> +1 .5 Mn2+ (1)

type [17].

and the high pH of groundwater tends to be of the Na‐HCO3

**3. Manganese**

230 Redox - Principles and Advanced Applications

**4. Chromium**

Cr (OH)

Selenium which is an essential metalloid is a part of selenoproteins in the human body. Selenium is ingested via food and drinking water. The concentration varies largely when seen in a global context. The lowest contents are seen in Sweden in the order of 0.1–0.2 μg/l while as high as 480 μg/l are seen [37, 38]. The current guideline is seldom exceeded in drinking water [39]. Nevertheless, these authors demand more studies and especially what concerns specific spe‐ cies of selenium in groundwater. Inorganic species are, as per their results, considerably more toxic than organic species [39]. Selenium deficiency is common in, for instance, Sweden and Finland. In Finland, action has been taken by adding selenium to commercial fertilisers [40]. This selenium is considered to reach humans mainly via the food and the serum‐Se has increased while so far no obvious positive health effects are seen in humans but they are seen in animals. However, in Punjab, in northwest India, groundwater concentrations above 300 μg/l have been recorded [41]. In part, the selenium is likely to come from food as groundwater irrigation is com‐ mon which has caused the accumulation of selenium in cultivated soils. Selenosis in the form of malformed nails is observed [41]. In the area, the main source of selenium is food, contributing about 90% of the total intake [42].

Selenium has a complicated redox pattern with the main species in groundwater being Se(IV) and Se(VI) [1]. Oxidising conditions increased the leaching from soils in the form of Se(VI).
