It can take hundreds or thousands of years for underground water to percolate through the mud before reaching the sea. All the while it is absorbing arsenic. This, says Kinniburgh, helps explain the diverse pattern of arsenic concentrations in tubewell waters. The contaminated wells almost all take water from a depth of 20 to 100 metres. Shallower wells are clean because they contain mostly recent rainwater or water flowing swiftly through the sediments.
Deeper wells tap water in older sediments which have by now been flushed clean of arsenic.
It will take thousands of years, says Kinniburgh, before the rest of the arsenic will wash away into the Indian Ocean. Many underground water sources around the world contain arsenic. Parts of Taiwan, Argentina, Chile and China have all suffered epidemics of skin diseases, gangrene and cancer as a result. Smith s analysis of the Taiwan epidemic in particular helped set the WHO arsenic standards for water and is the basis for his current predictions. Bangladesh, he says, is quite unprecedented.