Production, Accumulation, and Transport of Helium in the Subsurface - Possibilities and Limits for Groundwater Dating

Natural rocks are radioactive. They contain e.g. uranium and thorium and their daughter products in very small quantities and emit alpha-particles at well-defined rates. Neutralized alpha-particles are helium atoms and therefore natural rocks produce helium. The production rates are easy to calculate if the U and Th concentrations are known. In a closed system helium will accumulate with time; a measured helium concentration represents a certain accumulation time.