Superfund Research Program
Arsenic Mobilization in Bangladesh Groundwater
In the early 90's, an epidemic of arsenic poisoning began to emerge in Bangladesh. This epidemic is the indirect consequence of a well-intentioned campaign by donor agencies approximately 20 years ago to shift water consumption patterns in Bangladesh away from microbial disease-ridden surface water to "safe" clean tube well groundwater. It is now estimated that nearly one-third of these wells is contaminated with levels of arsenic greater than 50 ug/L (or 50 ppb), the drinking water standard for Bangladesh. This crisis has now been recognized in other areas of Asia, including India and Vietnam. Health experts estimate that 40 million people have been chronically exposed to arsenic, making this the worst environmental health disaster of the 20th century.
Development of national and international strategies to reduce levels of arsenic in drinking water supplies is hampered by a serious lack of information concerning a complex set of geochemical, public health, and remediation issues. The SBRP supports seven interrelated, multidisciplinary research projects at Columbia University with the goal of addressing some of these issues. Yan Zheng (Queens College, CUNY and Lamont-Doherty Earth Observatory [LDEO], Columbia University) and Martin Stute (Barnard College and LDEO) are leading an investigation to identify the fundamental processes that result in the arsenic enrichment of groundwater in Bangladesh. SBRP-funded research of Alexander van Geen (LDEO) focuses on translating this new knowledge into practical mitigation strategies to provide the rural population of Bangladesh access to safe water. Arsenic occurs in high concentrations under a wide range of hydrological and hydrochemical conditions in Bangladesh - making it an ideal study site for gaining a better understanding of arsenic mobilization.
Zheng and Stute began with a pilot study of the geochemistry of six sites in eastern Bangladesh. These sites represent three distinct geological units (uplifted Pleistocene terrace, fluvial flood plain, and delta plain). Arsenic concentrations in collected groundwater samples ranged from <10 ug/L to ~1200 ug/L. Despite regional differences and spatial variability of groundwater arsenic concentrations, there are systematic patterns between arsenic and other properties of the groundwater such as dissolved oxygen, nitrate, iron and manganese concentrations and sulfate/chloride ratios. The sulfur isotopic composition of dissolved sulfate suggests that oxidation of sulfide is not the dominant process in arsenic release. These observations (soon to be published in Applied Geochemistry) confirm that reduction of iron oxyhydroxides is likely the dominant mechanism of arsenic release to groundwater in Bangladesh. Although the reason groundwater systems in Bangladesh are driven to reducing conditions remains somewhat enigmatic, the researchers suspect the cause may be the oxidation of dispersed organic matter in sandy aquifers formed over the past 20,000 years, as sea level rose by about 100 m. They believe that bacterial decomposition of deltaic sediments consumes all of the oxygen in the water. The resulting "reducing" water causes arsenic to be chemically reduced from arsenate to arsenite, thereby making it soluble in water.
Zheng and Stute suggest that exhaustion of labile organic matter is the likely explanation for the much lower groundwater arsenic levels associated with aquifers made of older sediments (>100,000 years). Although these deeper aquifers are more difficult to reach, they appear to be a promising source of water that is both low in arsenic and free of infectious human pathogens.
Data from the pilot study were also used to identify the area where Columbia University SBRP-funded biomedical studies are being conducted and where major remediation effort has taken place. To set the stage for the epidemiological study conducted by scientists from Columbia's Mailman School of Public Health, van Geen spearheaded an effort to collect and analyze groundwater samples from nearly 6000 contiguous tube wells covering a 25 km2 area of Bangladesh. The location of each well was determined with handheld Global Positioning System receivers. A statistical analysis from this unprecedented geo-referenced data set shows that although only half the resident population of 70,000 has access to water that meets the Bangladesh drinking water standard, nearly 90% live within 100 m of a low-arsenic well. This striking observation (reported in a recent issue of the Bulletin of the World Health Organization) indicates that the extreme spatial variability of groundwater arsenic offers an opportunity for mitigation in the short-term through the sharing of water from the safe wells. Social barriers to such sharing need to be better understood and, if possible, overcome.
The patchiness of the groundwater arsenic distribution is somewhat less bewildering when the depth dimension is added to the picture. In a manuscript which builds on the same 6000 well data set (and was recently accepted for publication by Water Resources Research) the researchers show that the proportion of wells that does not meet the Bangladesh standard for drinking water arsenic increases with depth from 25% between 8-10 m to 75% between 15-30 m, then declines gradually to less than 10% at 90 m. Some villages within the study area do not have a single well that meets the standard, while others have wells that are nearly all acceptable. In most villages, the distribution of arsenic is highly variable in the 8-30 m depth range and does not follow any obvious pattern. The distribution of arsenic in the 30-90 m depth range, instead, appears to be controlled by the depth of the transition from relatively young Holocene to older Pleistocene sand deposits, which typically contain low-arsenic groundwater. In some parts of the SBRP study area, these older deposits are separated by a thick clay layer from shallower aquifers with elevated levels of arsenic. In other parts of the study area where there is no such clay layer, there is concern that tapping into the deeper aquifers may be only a temporary solution if withdrawal leads to downward entrainment of high-arsenic water from the shallower aquifers.
This project is a critical component of the overall SBRP program at Columbia University - it provides the necessary foundation for long term resources management, and in turn, may greatly reduce the population at risk of exposure to arsenic in the United States, Bangladesh and elsewhere. The findings to date, obtained in close collaboration with Dr. Kazi Matin Ahmed of the Geology Department of the University of Dhaka, could assist the Government of Bangladesh in finding safe sources of groundwater for the people of Bangladesh. SBRP-funded researchers, along with the President of Columbia University (Lee Bollinger) and the Director of the Columbia Earth Institute (Jeffrey Sachs), traveled to Bangladesh in January, 2003, to meet the Bangladesh Prime Minister and discuss Columbia's ability and interest in tackling mitigation problems in Bangladesh on a much wider geographic scale.
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To learn more about this research, please refer to the following sources:
- Zheng Y, Stute M, van Geen AF, Gavrieli I, Dhar RK, Simpson HJ, Ahmed KM. 2004. Redox control of arsenic mobilization in Bangladesh groundwater. Appl Geochem 19(2):201-204.
- van Geen AF, Zheng Y, Versteeg RJ, Stute M, Horneman AH, Dhar RK, Steckler MS, Gelman AE, Small C, Ahsan H, Graziano JH, Hussain A, Ahmed KM. 2003. Spatial variability of arsenic in 6000 tube wells in a 25 km2 area of Bangladesh. Water Resources Management 39(5):1140-1151.
- van Geen AF, Ahsan H, Horneman AH, Dhar RK, Zheng Y, Hussain A, Ahmed KM, Gelman AE, Stute M, Simpson HJ, Wallace S, Small C, Parvez F, Slavkovich VN, Lolacono NJ, Becker M, Cheng Z, Momotaj H, Shahnewaz M, Seddique AA. 2002. Promotion of well-switching to mitigate the current arsenic crisis in Bangladesh. Bull World Health Organ 80(9):732-737. PMID:12378292
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