Arsenic is found in groundwater in many areas but the conditions producing contaminated water vary. The depth from which the water is drawn is a factor. While a well of a certain depth might yield contaminated water, a deeper well nearby might be arsenic-free, according to Tommy Ngai Ka-kit, lecturer at Massachusetts Institute of Technology's department of civil and environmental engineering. Two wells of the same depth and only 10 metres apart can have also very different arsenic concentrations. The reason why is still unknown. It may have to do with the way the wells are constructed and slight differences in soil composition. The arsenic concentration of groundwater in a well can also vary over time. Arsenic concentrations are usually higher in the dry season, and lower in the wet months, when rain water soaks into the ground and dilutes the arsenic-contaminated water. In a severely-arsenic-affected village, 80 per cent or more of its wells may be contaminated. Experts warn that the movement of groundwater means the problem might shift from one location to another. Another cause of arsenic poisoning on the mainland is the burning of arsenic-rich coal in some parts of Guizhou and Shaanxi . About 2,800 people been poisoned from the practice and another 48,000 people are in danger. People are poisoned by eating food and breathing air contaminated by the arsenic-rich coal they use for cooking, heating and drying food such as corn and hot peppers.