Atmospheric and subsurface sources of stable and radioactive nuclides used for groundwater dating

Theoretical calculations together with a review of data from Sweden, Switzerland and Canada indicate that given sufficient isolation time and U and Th in the aquifer matrix, deep subsurface production of 3 H, 4 He, 36 Cl, 37 Ar, 39 Ar, 40 Ar, 85 Kr, 129 I, and 222 Rn can overshadow the normal atmospheric component of these nuclides in recharging groundwater. Although direct evidence is lacking, calculations suggest that small but measurable subsurface production of 3 H and 14 C could also take place in aquifers having high U and Th concentrations. Some ground water which has been recharged during the past 40 years will have 3 H, 3 He, 36 Cl, 85 Kr, and 129 I concentrations which are dominantly anthropogenic. Dating methods with the least complications in interpretation appear to be 3 H‐ 3 He for postbomb waters and 81 Kr for waters in the range of 5×10 4 to 1×10 6 a.