Perhaps the largest population to be threatened by naturally occurring groundwater contamination by arsenic is in West Bengal, India, and adjacent areas of Bangladesh. Arsenic, in the form of insoluble salts, occurs naturally in the bedrock that underlies much of this region. Under normal conditions, the groundwater stays relatively free of arsenic in a soluble form. Rapid population growth, industrialization, and intensification of agricultural irrigation, however, have put increasing stresses on the limited surface water supplies of this region. Groundwater has all but replaced other water sources for most people in West Bengal, especially in the dry season.