The U.S. Geological Survey is a government agency that collects data related to water sources around the country. A recent USGS study of randomly sampled private bedrock wells concluded that nearly one-fifth of them contain arsenic in concentrations greater than the maximum contamination level allowed for public water supplies. From this data, it can be assumed that approximately 41,000 in Hillsborough, Rockingham, and Strafford Counties have private wells that exceed that level. Private wells are not regulated in the state of New Hampshire and are subsequently seldom tested for arsenic, although it occurs naturally in the area. The State of New Hampshire does recommend individuals test for well water arsenic and other contaminants that affect human health.