The presence of high levels of arsenic in the groundwater of West Bengal and Bangladesh has become a very serious health problem. Recently there has been increased use of groundwater sources in an attempt to reduce the infectious diseases that result from bacteria-laden surface water (Sarkar 2009). Unfortunately the underground aquifers that supply this water for agriculture and domestic use are contaminated with arsenic. When consumed over long periods of time, low levels of arsenic found in water and food can cause skin disorders, cancer, and cardiovascular disease (Dissanayake, Rao, and Chandrajith, 2010). The World Health Organization recommends that drinking water contain less than 10 μg/L of arsenic, but arsenic concentration in the wells of West Bengal and Bangladesh is frequently above this level (Sarkar). About half of the hand pumps and irrigation wells in these areas are contaminated (Sarkar). In West Bengal, 95% of people extract their drinking water from the ground (Sarkar). In the Murshidabad district of West Bengal 73% of irrigation wells and 62% of domestic wells are contaminated (Sarkar). In Bangladesh only 20% of wells are characterized as arsenic free, and 97% of the population relies on 8.6 million tube wells, so the availability of clean drinking water is paramount (Dissanayake, Rao, and Chandrajith).
Potable water and food grown with arsenic contaminated water are the main sources of arsenic poisoning (Sarkar). Food consumption accounts for 20-40% of human arsenic intake, with rice and vegetables grown underground being the most susceptible to retaining high levels of arsenic (Sarkar). Because rice is a major dietary staple in these areas, arsenic poisoning is a wide-reaching problem. Arsenic is even found in processed foods, because the manufacturing plants use groundwater without filtering it to remove the arsenic (Sarkar). I thought this was an interesting example of the omnipresent character of water. It affects our lives in countless ways, some obvious and some not. Sarkar reported that studies have found a link between poor nutrition and the effect of arsenic intake. Poor people who are unable to eat protein and nutrient rich foods are more likely to be affected by arsenic than the rich who can eat more protein, legumes, vegetables, and fruits (Sarkar). The poor also suffer because they cannot afford the technology to filter the harmful substance out of their drinking water and because they do not receive adequate health care.
Most arsenic found in groundwater naturally accumulates within the sediment of the aquifer (Dissanayake, Rao, and Chandrajith). The arsenic in West Bengal and Bangladesh is thought to have originated in the rock formations of the Himalayas (Dissanayake, Rao, and Chandrajith). It was probably transported via sedimentary particles traveling in rivers originating in the Himalayas, before settling in the aquifer (Dissanayake, Rao, and Chandrajith). I found it really interesting and somewhat counterintuitive that the spatial distribution of arsenic within an aquifer can vary widely. Normally I would expect the arsenic to be equally concentrated throughout the aquifer, but in reality water may have areas of very high arsenic concentration surrounded by spaces where low to no arsenic is present (Dissanayake, Rao, and Chandrajith).
The aspect of this issue that most interested me was the mechanism of arsenic release within contaminated aquifers. Generally arsenic within these wells exists in a form where it is adsorbed onto the surface of iron oxyhydroxide (FeOOH), which coats sedimentary particles (Nickson, McArthur, Ravenscroft, Burgess, Ahmed, 2000). The reductive dissolution of arsenic-rich FeOOH is driven by bacteria, which degrade sedimentary organic matter (Nickson, et.al.). In the degradation process bacteria consume dissolved oxygen and this loss of oxygen in the water produces a reducing environment (Nickson, et.al.). Arsenic and iron are reduced and released to groundwater. However, microbiological mediated reduction of iron oxyhydroxide only occurs after reducing conditions are reached and all free molecular oxygen and NO3-, which are more easily reduced than iron or arsenic, is reduced (Nickson, et.al.).
Observable relationships between substances involved in this process allowed the authors to show that their proposed mechanism and conditions required for arsenic reduction were likely to accurately reflect the true arsenic release process. Water with high levels of dissolved oxygen has low levels of free arsenic (< 50 μg/L) because the presence of oxygen creates an oxidizing environment, and the arsenic remains adsorbed to the iron oxyhydroxide (Nickson, et.al.). Only when bacteria consume the oxygen present does the system become a reducing environment.
Wells with high levels of NO3- and almost no detectable amounts of dissolved arsenic indicate that reduction of arsenic and iron must not take place until the more easily reduced nitrate ions are removed from the system (Nickson, et.al.).
The reduction-oxidation reaction for this process (shown below) shows that bicarbonate ions are produced thus explaining why the concentration of bicarbonate ions is positively correlated to the concentration of arsenic in the well (Nickson, et.al.)
Although iron is also released in this process, Fe2+ levels are poorly coordinated to arsenic levels because the free iron tends to participate in further reactions (Nickson, et.al.).
There are many proposed solutions to the issue of arsenic contamination. Nickson et.al. suggest that because dissolved arsenic is so closely associated with the presence of dissolved iron, it may be possible to remove arsenic by the aeration of iron rich water, which will precipitate FeOOH (Nickson, et.al.) The precipitation of iron oxyhydroxide co-precipitates some of the arsenic from solution, so that it returns to an adsorbed state (Nickson, et.al.). The solid could then be removed from the drinking water. Other suggestions include placing shallow wells closer to the surface within a range that will maintain oxidizing conditions so that arsenic release is less likely (Dissanayake, Rao, and Chandrajith). Sarkar recommends a long-term goal encompassing the reduction of groundwater dependence through better water management. To provide safe drinking water in the short term, communities should consider investing in arsenic filters, although these require long-term maintenance and toxic waste disposal (Sarkar). In addition to shallow wells, deep bore wells that draw water from below the level of arsenic contamination might be implemented (Sarkar; Dissanayake, Rao, and Chandrajith). Other methods include collection of rainwater and filtration of river water (Sarkar). Regardless of the solution chosen, government involvement in water management and community awareness and initiative are crucial to providing clean drinking water to these areas.
References:
Dissanayake, C. B., Rao, C. R. M., and Chandrajith, R., (2010). Some Aspects of the Medical Geology of the Indian Subcontinent and Neighboring Regions. In Selinus, O., Finkelman, R. B., and Centeno J. A., (Eds.). Medical Geology: A Regional Synthesis. Springer.
Nickson, R.T., McArthur, J.M., Ravenscroft, P., Burgess, W.G., Ahmed, K.M. (2000). Mechanism of Arsenic Release to Groundwater, Bangladesh and West Bengal. J Appl Geochem. 15(4): 403-413.
Sarkar, A. (2009). Sustainable Solutions to Arsenic Contamination of Groundwater: The Ganga-Maghna-Brahmaputra Basin. In Pascual, U., Shah, A., and Bandyopadhyay, J., (Eds.). Water, Agriculture, and Sustainable Well-Being.
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