Arsenic in groundwater — south and east Asia

Abstract

Naturally-occurring high arsenic concentrations are a recognised problem in groundwater from many regions of eastern Asia, including parts of Bangladesh, West Bengal (India), China, Vietnam, Taiwan, Thailand and Nepal. A small number of tubewells with high arsenic concentrations have also been found in Pakistan, Myanmar and Cambodia, although sampling in these areas is so far limited and the scale of potential problems is uncertain. Chronic arsenic-related health problems are recognised in some, though not all, of these regions. The high-As groundwaters in eastern Asia typically occur under strongly reducing conditions in young, Quaternary, aquifers in alluvial and deltaic sediments. Under such conditions, arsenic is commonly accompanied by high concentrations of iron, manganese and ammonium and low concentrations of nitrate and sulphate. Arsenic speciation is variable but the element occurs principally as arsenite. High alkalinity values and phosphorus concentrations are also often a feature of the high-As groundwaters. Although high concentrations of dissolved humic matter are also found in some groundwaters from Inner Mongolia (China) and Taiwan, they are not observed in all regions. A notable exception to these conditions is that of the Ron Phibun area of Thailand. Here, localised arsenic contamination of surface and shallow groundwaters is due to the oxidation of arsenopyrite in an area impacted by former tin mining operations. These groundwaters tend to be more oxidising with relatively low iron concentrations, higher sulphate and with arsenic present more typically as arsenate, although high manganese concentrations occur in some. High concentrations of aluminium, cadmium and zinc are also observed in the worst-affected waters from the area. Much has been learned about the scale, distribution and causes of arsenic contamination in Asian aquifers over the last few years. However, still many uncertainties remain over the areas affected and the precise mechanisms involved. Doubtless, other areas with contaminated groundwater will be discovered in future, but an appreciation of the aquifers most at risk and the common indicators of likely arsenic contamination should help to identify new areas more quickly and promote more rigorous water testing during well installation programmes.