At one time, ± up to the 1970's, the rural people who lived on the Bengali alluvial plain only used surface water for drinking. The faeco-oral transmission of various pathogens was enormous and the diarrhoea problem in Bangladesh was notorious. In the early 1970s the decision was taken to drill a large number of tubewells throughout the country, which were used to draw pure water from deeper levels by means of handpumps. This meant that the local population was no longer dependent upon surface water and the diarrhoea problem was contained. Using tubewells and handpumps protects the groundwater from bacterial contamination. No one knows precisely how many wells there are in Bangladesh, but there are certainly more than a million. At present over 95% of the Bangladeshi population drinks well water. Analysis subsequently revealed that the water from many of these wells contained high concentrations of arsenic. The risks vary considerably from one well to the next, but it is a serious national problem affecting a large percentage of the population. In a study of 25,000 wells in Bangladesh, 20% were found to have a high arsenic content. At present, there is a lawsuit against the British Geological Survey for not spotting this problem earlier. The WHO has set a safety limit of 10 ppb for natural arsenic.