Health Stream Literature Summary - Issue 55 - September 2009

Public health strategies for western Bangladesh that address arsenic manganese, uranium, and other toxic elements in drinking water.
Frisbie, S.H., Mitchell, E.J., Mastera, L.J., Maynard, D.M., Yusuf, A.Z., Siddiq, M.Y., Ortega, R., Dunn, R.K., Westerman, D.S., Bacquart, T. and Sarkar, B. (2009) Environmental Health Perspectives, 117(3); 410-416.

Arsenic (As) contamination of tube wells in Bangladesh is a major public health problem which has prompted a number of large scale surveys to identify wells with high arsenic levels. Two national surveys of tube well water have indicated that other potentially toxic elements including manganese (Mn), uranium (U), boron (B), barium (Ba), chromium (Cr), molybdenum (Mo), nickel (Ni) and lead (Pb) are often present at concentrations that exceed World Health Organisation (WHO) health-based drinking water guidelines. There is a need to consider exposure to these elements in addition to arsenic in order to provide safe drinking water. If the presence of these elements is positively correlated with the presence of arsenic, then testing wells for arsenic alone may be sufficient to identify safe drinking water sources. However if these elements occur independently of arsenic then a broader testing program would be needed.

Groundwater samples were collected from four randomly selected neighbourhoods in western Bangladesh. This area was selected because it has some of the greatest ranges of groundwater As concentrations in the country. There were 71 samples collected from 67 randomly selected tube wells in these four neighbourhoods. All sampled tube wells were purged by pumping for 10 minutes immediately before sample collection. Samples were analysed immediately after collection for pH and then analysed for Ba, Cr, Mn, Mo, Ni, Pb, Se (selenium), U and Zn (zinc) by inductively coupled plasma mass spectrometry. Samples were also analysed for B, Fe (iron), and Sb (antimony). The owner or principal user of each well was interviewed to determine the depth, age and number of well users. Contour maps were drawn to show the concentrations of As, B, Ba, Cr, Fe, Mn, Mo, Ni, Pb, Sb, Se, U and Zn as well as pH, depth of tube well, age of tube well and number of users per tube well for each of the four neighbourhoods.

The percentage of tube wells that had concentrations exceeding WHO health-based drinking water guidelines was 78% for Mn, 48% for U, 33% for As, 1% for Pb, 1% for Ni and 1% for Cr. Levels of B, Ba and Mo did not exceed these guidelines. Of the 22 tube wells with unsafe As concentrations (above 10 microg/L), the percentage exceeding guidelines for other elements was 59% (Mn), 14% (U), 5% (Pb), 5% (Ni) and 5% (Cr). This suggests that drinking water wells with unsafe concentrations of As may also have unsafe concentrations of these or possibly other elements. For the entire region, increases in As concentration correlated with statistically significant increases in Mn, Pb, Cr, B, Ba and Mo. Nearly all of the home-scale drinking water treatment systems used currently in Bangladesh have been designed to remove As but may not remove these other toxic elements. These treatment systems should be further evaluated for the removal of Mn, Pb, Ni, Cr, B, Ba, Mo and possibly other elements.

Of the 45 tube wells with safe As concentrations, 87% and 64% had unsafe concentrations of Mn and U, respectively. It was found that 42 of the 45 (93%) tube wells had unsafe concentrations of Mn, U or both Mn and U; however, tube wells seldom (4%, 3 of 67 tube wells) had unsafe levels of both As and U together. Drinking water with safe As concentrations therefore may have unsafe concentrations of Mn, U or possibly other elements. The current practice of testing wells only for As will therefore not identify drinking water with safe concentrations of other toxic elements.

In this study 96% of tube wells tested exceeded WHO health-based guidelines for at least one of these toxic elements, with As being the single greatest risk to public health. On the basis of the results found here, the authors propose a three-step testing program to provide safe drinking water. The program involves: 1) testing all wells for As; 2) if a sample meets the WHO guideline for As then it should be retested for Mn and U; 3) if a sample meets the WHO guideline for As, Mn and U, then it should be retested for B, Ba, Cr, Mo, Ni and Pb. Tube wells that do not exceed WHO guidelines for these nine elements could be used as public drinking water supplies. These safe tube wells need to be periodically monitored for As, Mn, U, Pb, Ni, Cr, B, Ba and Mo. If a tube well becomes unsafe then an alternative drinking water supply must be identified or the unsafe water treated.

Comment The large scale exposure of people in Bangladesh and a number of other countries to arsenic-contaminated tube wells was caused in part by well-intentioned efforts to provide a microbiologically safe alternative to polluted surface water supplies. Unfortunately the potential chemical risks from these new water sources were not assessed, resulting in a public health catastrophe. The findings of this study suggest that interventions to alleviate the arsenic problem need to consider a broader range of chemical hazards to adequately address health risks.

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