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 H-3, He-4, Cl-36, Ar-37, Ar-39, Ar-40, Kr-85, I-129, and Rn-222 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 H-3 and C-14 could also take place in aquifers having high U and Th concentrations. Some groundwater which has been recharged during the past 40 years will have H-3, He-3, Cl-36, Kr-85, and I-129 concentrations which are dominantly anthropogenic. Dating methods with the least complications in interpretation appear to be H-3-He-3 for postbomb waters and Kr-81 for waters in the range of 5 x 10(4) to 1 x 10(6) a.