**9. Conclusions**

The presence of different species of elements at different redox levels makes assessments of the associated health risks complicated. In addition, the species have different levels of toxicity. A number of trace elements may pose risks to human health, arsenic, manganese, chromium as chromate, nitrate, selenium and uranium. Except for manganese, these species are either uncharged or in the form of anions and their adsorption to soils and aquifer materials is weaker than for cations such as most heavy metals like Cu, Pb and Zn. The main adsorbents for the lat‐ ter are clay minerals and organic matter, and the adsorption of those increases with pH while the adsorption for anions, by aluminium and ferric oxyhydroxides, decreases with pH and approaches zero, close to ZPC at just above pH 8. Uncharged species are particularly mobile like arsenite.

Arsenic is mobilised into groundwater both under reducing and oxidising conditions. With reduction of ferric oxyhydroxides, ferrous iron and arsenite are released into groundwater. This is common especially in south and southeast Asian delta regions. While removal by filter is possible, but not always functioning due to social reasons and in addition gives a waste problem, an alternate way of supplying safe water is to identify aquifers with more oxidising environments. This can be done by examining the colour of the sediments. White, off-white and red sediments are likely to yield a low arsenic groundwater. It is also possible by the colour code to identify low manganese groundwater. In oxidising environments at neutral pH, arse‐ nate (As(VI) is immobilised into ferric hydroxides. However, at pH above 8, these adsorbents lose their positive charge and arsenic becomes mobile.

Chromium is carcinogenic and mobile in oxidising environments. Chromium comes from both natural and anthropogenic sources. Ultramafic rocks are high in chromium. Main anthropogenic sources are electroplating and leather tanning. Manganese oxides in soils and on mafic minerals can act as oxidants of Cr(III) to form mobile chromate (Cr(VI)).

Selenium, an essential element, seldom reaches toxic levels in groundwater, but in areas where irrigation is practised with groundwater, with elevated selenium concentrations, the intake via the crops cultivated can be too high. Nitrate may act as the oxidant of Se(IV) and form mobile Se(VI).

The nitrogen cycle has been radically changed by human action mainly through the industrial production and use of nitrogenous fertilisers. The natural symbiotic nitrogen fixation was exceeded by the anthropogenic in the 1980s. Nitrate or rather nitrite formed in the human intestinal tracts could be carcinogenic by forming nitrosamines. However, the main reason for the guideline value at 50 mg/l is the risk of methaemoglobinemia in bottle-fed children below the age of 1 year. Breastfeeding efficiently counteracts this.

Uranium can be toxic from radiological point of view but more often due to its chemical action on the re‐adsorption of water and salts from the primary urine formed in the kidney cortex. Uranium speciation is important both for its mobility and for its toxicity. Uncharged species are mobile. Calcium‐carbonate complexes do not seem to be taken up by humans. U(IV) is largely immobilised while U(VI) is mobile. In the search for safe repositories for radioactive waste, reducing environments are preferred. These immobilise many other radioactive elements.
