PREDICTING THE GLOBAL DISTRIBUTION OF NATURAL ARSENIC POLLUTION IN GROUNDWATER

Well over a hundred instances of arsenic pollution of groundwater due to natural causes have been recognised from 47 countries in five continents. Arsenic is mobilised into groundwater through four common mechanisms: (1) reductive-dissolution, (2) alkali-desorption, (3) sulphide oxidation, and (4) geothermal action. Evaporation may increase concentrations of arsenic initially generated by any of these mechanisms. Two compensatory processes, adsorption and sulphate reduction, act to remove arsenic from groundwater. The occurrence of actual health impacts further depends on the particular socio-economic conditions of a region, such that groundwater may be contaminated but is not part of an active pathway to the human food-chain. The four mobilisation mechanisms operate in systematic geological – climatic associations. Although specific factors may act to prevent arsenic contamination at individual locations, these associations allow global predictions of where there is a high-risk of encountering arsenic-contaminated groundwater. On a global scale, the distribution of arsenic-contamination by each mechanism appears to be irrational, with entire continents apparently lacking any examples of some mechanisms. Based on the geological – climatic associations, regions judged to be at high risk have been identified. The extent to which the health risk in these regions remains unknown is partly because of the absence of testing, but may also be partly a function of our incomplete knowledge of the factors controlling the mobilisation and immobilisation of arsenic. Testing groundwater in these regions should be a high priority.