Ensuring safe drinking water in Bangladesh

I n the early 1980s, K. C. Saha from the School of Tropical Medicine in Kolkata attributed skin lesions in West Bengal, India, to exposure to arsenic in groundwater pumped from shallow tube wells (1). Despite these findings, millions of tube wells have been installed across the Bengal Basin, the geological formation that includes West Bengal and Bangladesh, and across river floodplains and deltas in southern Asia. The popularity of tube wells reflects the reduced incidence of diarrheal disease when drinking groundwater, instead of untreated surface water, and the modest cost of installation (about 1 month of household income). Today, perhaps 100 million people in India, Bangladesh (see figure, right), Vietnam, Nepal, and Cambodia (and possibly other countries) are drinking water with arsenic concentrations up to 100 times the World Health Organization (WHO) guideline of 10 μg per liter (2–4). Whereas technologies for treating either surface water or groundwater periodically receive considerable attention, the record to date suggests that more widespread testing of wells to identify those aquifers that do not require treatment is presently far more promising. Arsenic can occur in groundwater naturally, without an anthropogenic source. There is broad agreement that arsenic release into groundwater of the Bengal Basin is facilitated by microbial metabolism of organic matter contained in river floodplain and delta deposits (3–6). Elevated concentrations of arsenic in Bangladesh groundwater probably predate agricultural practices that could plausibly have caused the composition of groundwater to change, such as the use of phosphate fertilizer or large groundwater withdrawals for irrigation (3, 7). This does not rule out the possibility that irrigation is affecting the distribution and mobilization of arsenic today (7–9). Although there are remaining questions, current understanding of the occurrence of arsenic is sufficient to direct national strategies for lowering exposure. After studies established the scale of the problem (2, 3), a massive campaign was initiated in Bangladesh in 1999 to test tube wells in the most affected portions of the country. The field kits that were used had limitations, but were reliable enough (10) to identify the vast majority of tube wells that did not meet the local standard for arsenic in drinking water of 50 μg per liter. By 2005, the spouts of 1.4 million cast-iron pumps that draw groundwater with >50 μg per liter arsenic according to the field test had been painted red. Another 3.5 million wells with up to 50 μg per liter arsenic had been painted green (11). Such testing did not reduce the rate of private well installations, at least within areas that have been recently resurveyed (12, 13). Sadly, most tube wells that were installed after the national testing campaign remain untested today. The two interventions that have so far most effectively lowered human exposure in Bangladesh rely on the spatial heterogeneity of the distribution of arsenic in groundwater, which is controlled principally by the local geology (3, 14). Testing alone had the biggest impact as ~29% of the millions of villagers informed that their tube well was elevated in arsenic have changed their water source (see the chart on page 1688). Large variations in the proportion of well-switching across villages reflect in part the availability of safe wells that are within walking distance. A recent comparison has shown, however, that both additional education and periodic reinforcement of the message that arsenic is a health hazard can overcome existing obstacles to nearly double the proportion of switching (12, 13). The intervention with the second largest impact (~12% of users with unsafe wells) has been the installation of tens of thousands of deep wells by the government and by nongovernmental organizations. Such wells supply groundwater from deeper, usually older, aquifers that generally do not contain elevated levels of arsenic (3, 14, 16). They are often shared or community wells that require walking ~100 m several times a day. Yet these wells can be very popular when placed in a central location that, for instance, does not discourage use by women. These community installations have also had an indirect impact because numerous households followed suit by reinstalling their own well to greater depth (12, 13). In 2004, Bangladesh issued a National Policy for Arsenic Mitigation (NPAM) accompanied by a more detailed Implementation Plan for Arsenic Mitigation (17). Well-switching was recognized by the NAPM in the sense that alternative water supply was not proposed for villages where <40% of tube wells are unsafe. However, the NPAM considered deep tube wells a low-priority option. Instead, the document encouraged a return to the use of surface or very shallow groundwater without paying sufficient attention to the increased likelihood of exposure to microbial pathogens. Five other mitigation approaches promoted by the NPAM have had a limited impact, each reaching <1% of the population at risk (see the chart on page 1688). The early record of Excessive levels of arsenic in drinking water is a vast health problem in Southeast Asia. Several viable approaches to mitigation could drastically reduce arsenic exposure, but they all require periodic testing. Ensuring Safe Drinking Water in Bangladesh