Review of Arsenic

So far, I have discussed the role of Mercury and its affects on our health, I now wish to look briefly at Arsenic. Arsenic is classed as a metalloid which means it shares both metal and non-metal qualities and is ranked the 20th most common element in the Earth's crust.

Arsenic and humans have had a fairly dark relationship for centuries. It was immortalised by Plato when he described the use of Hemlock (a natural resource of arsenic) in the death of Socrates. Due to its lack of odour or smell, and small qunatities being fatal, it is a highly effective killer.

The most infamous aresenic poisoning that occured on a large scale, is the case of Bangladesh. Shallow aquifers, less than 300m deep, supply more than 90% of the drinking water to Bangladesh and West Bengal. It has been known for years that up to one million wells using these water reserves contain dangerous levels of arsenic. In many of these wells the arsenic content reaches 1000 mg l-1, 9950 mg l-1 above the bangladeshi limit and the WHO limit (Nickson et al., 1998).

Arsenic occurs in the Bangladesh and West Bengal aquifers due to reductive disolution of arsenic rich iron oxyhydroxides, which originate from the reduction of sulphate based metals after weathering. Though it is important to note that the arsenic rich wells are primarily isolated to the ganges delta aquifers. This suggests that weathering upstream of Bangladesh transported the arsenic-rich oxyhydroxides in sediments downstream to the delta in the late pliestocene, an epoch covering the last glaciation (Gibbard and van Kolfschoten, 2004). The sediments displaced by glaciation and de-glaciation have accumulated and been chemically reduced for centuries in the genges delta to produce arsenic rich water reserves.

Nickson et al. (1998) discuss how understanding the past geological structure of arsenic depisits and transport allow a better understanding of arsenic distribution in Bangladesh. They propose a predictive model to help influyence and advise future water developments on the aquifer to minimise the risk of future poisoning.

References:

Nickson, R., J. McArthur, W. Burgess, K.M. Ahmed, P. Ravenscroft and M. Rahman (1998) 'Arsenic Poisoning of Bangladesh Grounwater,' Nature 395: 338.

Gibbard, P. and van Kolfschoten, T. (2004) 'The Pleistocene and Holocene Epochs' in Gradstein, Ogg, Smoth and Gilbert (eds.) A geological time scale, Cambridge Press, Cambridge.