S. H. Farooq

Influence of traditional agricultural practices on mobilization of arsenic from sediments to groundwater in Bengal delta.

In the wake of the idea that surface derived dissolved organic carbon (DOC) plays an important role in the mobilization of arsenic (As) from sediments to groundwater and may provide a vital tool in understanding the mechanism of As contamination (mobilization/fixation) in Bengal delta; a study has been carried out. Agricultural fields that mainly cultivate rice (paddy fields) leave significantly large quantities of organic matter/organic carbon on the surface of Bengal delta which during monsoon starts decomposing and produces DOC. The DOC thus produced percolates down with rain water and mobilizes As from the sediments. Investigations on sediment samples collected from a paddy field clearly indicate that As coming on to the surface along with the irrigation water accumulates itself in the top few meters of sediment profile. The column experiments carried out on a 9 m deep sediment profile demonstrates that DOC has a strong potential to mobilize As from the paddy fields and the water recharging the aquifer through such agricultural fields contain As well above the WHO limit thus contaminating the shallow groundwater. Experiment also demonstrates that decay of organic matter induces reducing condition in the sediments. Progressively increasing reducing conditions not only prevent the adsorption of As on mineral surfaces but also cause mobilization of previously sorbed arsenic. There seems to be a cyclic pattern where As from deeper levels comes to the surface with irrigational water, accumulates itself in the sediments, and ultimately moves down to the shallow groundwater. The extensive and continual exploitation of intermediate/deep groundwater accelerates this cyclic process and helps in the movement of shallow contaminated groundwater to the deeper levels.

Surface Generated Organic Matter: An Important Driver for Arsenic Mobilization in Bengal Delta Plain

An access to safe drinking water is primary human need, but at many places around the globe this primary requirement is not fulfilled and millions of people are forced to drink contaminated water. On a larger scale, various elements such as As, F, V, U, Pb, Hg etc. are contaminating the available drinking water resources (WHO, Guidelines for drinking-water quality, Recommendations. World Health Organization, Geneva, 2006). Among all the above mentioned elements, As is posing a major threat and several countries around the globe are facing problem of As contamination in the groundwater. The problem of As contamination is most severe in the South-East Asian countries (Bhattacharya et al. Int J Water Resour Dev 13:79–92, 1997; Chandrasekharam et al. Proc Water Rock Interact 12:1051–1054, 2001; McArthur et al. Appl Geochem 19:1255–1293, 2004; Nickson et al. Nature 395:338, 1998; Smedley and Kinniburgh, Appl Geochem 17:517–568, 2002; van Geen et al. Appl Geochem 23:3244–3251, 2008). Elevated As concentrations have been reported at places in Nepal, India, Bangladesh, Myanmar, Cambodia, Vietnam and Thailand etc. It is estimated that about 200 million people living in Asia are exposed to various health risks due to consumption of As contaminated water (Sun, Toxicol Appl Pharmacol 198:268–271, 2004). In Bengal delta plain alone more than 50 million people are routinely exposed, thus it is described as the worst case of mass-poisoning in the history of mankind (Ahsan et al. Bull Environ Contam Toxicol 82:11–15, 2009; Chakraborti et al. Environ Health Perspect 111:1194–1201, 2003). The WHO (Guidelines for drinking-water quality, 4th edn. World Health Organization, Geneva, 2011) permissible limit of As in drinking water is 10 μg/L, while the Indian permissible limit is five times higher (50 μg/L). In Bengal delta plain As concentrations more than 1,000 μg/L has been reported at several places. Still it appears that the extent of the contamination is not fully known and many new affected areas are getting discovered on regular basis. Apart from West Bengal, higher As concentrations have been reported from other Indian states including Bihar, Uttar Pradesh, Assam, Jharkhand, Chattisgarh and Madhya Pradesh (Acharyya et al. Environ Geol 49:148–158, 2005; Ahamed et al. Sci Total Environ 370:310–322, 2006; Bhattacharjee et al. Chemosphere 58:1203–1217, 2005; Chakraborti et al. India Curr Sci 77:502–504, 1999, Environ Health Perspect 111:1194–1201, 2003; Das et al. Environ Geochem Health 18:5–15, 1996; Paul and Kar, Environ Ecol 22:588–589, 2004).