## 1 Introduction
(**a**) redox potential (Eh)–pH diagram [9; 17], (**b**) arsenite species and (**c**) arsenate species [16]. Operating conditions: 25 \({}^{\circ}\)C and 1 bar total pressure. Reproduced with permission from Elsevier and RSC. Reported cases of groundwater contamination by arsenic are more diverse than commonly realized and extend from under-developed to developed countries, including Argentina, Bangladesh, Canada, Chile, China, Hungary, India, Japan, Mexico, Nepal, Poland, Taiwan and the USA [15]. Figure 2 shows where groundwater or surface water contamination by arsenic has been reported. Croatia, Hungary and Serbia are three of the countries in Europe where very high concentrations of arsenic in groundwater have been reported. In Hungary, the arsenic concentrations in groundwater were found to exceed the WHO (10 ug/L) and EC guidelines (7.5 ug/L, EC Directive 2006/118/EC) by several times [19]. The concentrations of arsenic in various groundwater samples collected in Hungary and Romania ranged from <0.5 to 240 ug/L [20]. More distinctly, the highest arsenic concentrations (23 to 208 ug/L, mean 123 ug/L) were obtained in waters dominated by methanogenesis, whereas waters dominated by sulfate reduction gave lower arsenic concentrations (<0.5 to 58 ug/L, mean 11.5 ug/L) [20]. In Serbia, the extent of arsenic contamination is not fully resolved and yet to be determined [15]. As illustrated in Figure 2, the most affected countries in the Americas are Argentina, Chile, Mexico and the United States. In Latin America, the estimated number of people affected by arsenic contamination >50 ug/L is at least four million [21]. Some wells in Argentina, Bolivia and Peru were found to contain extremely high arsenic concentrations at high mg/L levels [21]. The global variation of arsenic concentrations in groundwater is summarized in Table 1. Among the affected countries, Bangladesh and West Bengal in India have the largest population at risk of exposure to arsenic contamination [7]. This is why we chose Bangladesh as a case study to highlight the extent of groundwater contamination by arsenic. The people most affected in Bangladesh and West Bengal reside where there is no access to integrated urban water supply systems, mostly in rural areas. Consequently, millions of people living in rural areas in these countries are regularly drinking water contaminated with arsenic. A relatively large number of people in this region have already been identified as having arsenic related disease symptoms [15]. UNICEF reported that 1.4 million tube wells out of the 4.75 million (around 30%) in Bangladesh contained arsenic concentration above 50 ug/L [22]. A household drinking water quality survey conducted by UNICEF in Bangladesh found that 12.6% of drinking water samples exceeded the country's arsenic standard for drinking water [22]. This represents approximately 22 million Bangladesh people at risk of adverse arsenic exposure [22]. Figure 2: Countries affected by arsenic contamination of groundwater or surface waters [15]. Reproduced with permission from IJAET. Furthermore, in West Bengal (India), an estimated 6 million people were found to have been exposed to relatively high arsenic concentrations, ranging from 50 to 3,200 \(\upmu\)g/L [23; 24]. Usually after a few years of extraction of groundwater, a considerable increase in the concentration of arsenic in wells has been observed [15]. Future increases in arsenic contamination issues in drinking water are expected despite the establ