Underground production of radionuclides in the Milk River aquifer, Alberta, Canada

Abstract Most cosmogenic radionuclides that may be used to estimate groundwater ages in excess of 100 ka are also produced underground by in situ nuclear reactions. Radioactive equilibrium between in situ production and decay would be approached within such long residence times. Radionuclide production by in situ nuclear reactions within the groundwater and within its host rock has been calculated theoretically and compared with some measured radionuclide contents of the groundwater. The fractional transfer of in situ produced 37 Ar, 39 Ar and 222 Rn from rock to groundwater necessary to account for the total measured groundwater contents were found to be similar, although site specific. The implications for radionuclide release mechanisms and for groundwater dating by the use of cosmogenic radionuclides, including those which are applicable for very old groundwaters ( 36 Cl, 81 Kr, 129 I), are discussed.