Potable water supplies that contain arsenic concentrations high enough to pose a human health hazard is a problem of international proportion. Surface water and ground water are both at risk of arsenic contamination. However, most incidences of high concentrations of arsenic have been reported for ground water, which is the subject of this book. The geochemistry of arsenic in aqueous environments is complex. This book consolidates much of what is known about the geochemistry of arsenic and provides new information on relationships between high concentrations of arsenic in ground water and geochemical environments. The subject matter of this book ranges in scope from molecular-scale geochemical processes that affect the mobility of arsenic in ground water, to arsenic contaminated ground water at the national scale. Chapters were contributed by an international group of research scientists from a broad range of backgrounds.