There is little disagreement that the potential effects of water contamination on human health and the environment should not be ignored, even though the exact nature of those effects is not yet fully understood. That permanently incapacitating and even lethal substances (asbestos, for example) have, in ignorance, been introduced into the environment may become apparent only decades after their introduction. A new principle in water quality regulation is emerging in response to awareness of these dangers: An individual or organization can be held accountable for hazards to human health or for degradation of the environment created by the introduction of a substance, even if the individual or organization is not the source of that substance, even if no regulation of the substance currently exists, and even if the substance is not known to be hazardous or to degrade the environment at the time its release occurs. This book outlines the scientific aspects of the control of natural radioactivity in water supplies, as well as the labyrinthine uncertainties in water quality regulation concerning natural radiocontamination of water. The author provides an introduction to the theory of natural radioactivity, addresses risk assessment, describes sources and effects of natural radiocontamination of water, surveys federal water law concerning natural radiocontamination, and presents an account of how one city dealt with the perplexities that mark this rapidly evolving area of water quality regulation.