Groundwater from deep "tube wells" has been widely used for drinking water in Bangladesh in order to overcome the impact of bacterial contamination of surface water since the 1970s, but it has emerged more recently that naturally occurring arsenic taints the water in many of the country's 10 million wells. A programme of testing wells and closing down those found to be tainted continues, but some 57 million Bangladeshis still drink water with unsafe arsenic levels, which can cause skin lesions and cancer. Although arsenic is native to the sediments, the processes by which it is released to groundwater remains unresolved.
It had been thought that the arsenic came from rock sediments deep underground, and that knowing where they were could lead to a long-term solution. Previous research found that microbes release the arsenic from underground minerals (see 'Microbes to blame' for arsenic threat to millions).
Hopes that the arsenic might naturally fall to safer levels were dashed this week when Scott Fendorf and colleagues at Stanford University in California reported their results from the analysis of core samples that indicate that the arsenic primarily occurs within the top five metres of soil. From there, the contaminated water seeps down through the earth and rock before being brought back up as drinking water by the wells.
Fendorf speculates that arsenic gets into the aquifers when seasonal flood waters trigger its release, transporting it down into the aquifers. "We cannot assume that arsenic will be naturally removed from the aquifer with time or that we only have to deal with a finite quantity," he says.
Charles Harvey, a researcher at the Massachusetts Institute of Technology, United States, who also contributed to the study, says the findings highlight the need to conduct basic research into the water cycle in Bangladesh. The situation is rather complex, because the ponds, rivers and rice fields that cover the surface in a watery "mosaic" all have a different chemistry and deliver different amounts of water underground. It is only by developing a better understanding of this mosaic that we can explain why arsenic levels are high in groundwater, and begin to think about how to tackle the problem. Unfortunately, says Harvey, neither development organisations nor researchers — the two groups looking at arsenic contamination in Bangladesh — have both the expertise and funding to do this.
Development organisations are busy searching for immediate solutions such as alternative clean water sources, while scientists only receive funding for original work — for example, sequencing the genome of the bacteria that liberates the arsenic. "We can't get funding to do the real basic hydrological work that will really help answer this question," says Harvey.