ATMOSPHERIC AND SUBSURFACE SOURCES OF STABLE AND RADIOACTIVE NUCLIDESUSED FOR GROUNDWATER DATING

Theoretical calculations together with a review of data from Sweden, S witzerland and Canada indicate that given sufficient isolation time an d U and Th in the aquifer matrix, deep subsurface production of H-3, H e-4, Cl-36, Ar-37, Ar-39, Ar-40, Kr-85, I-129, and Rn-222 can overshad ow the normal atmospheric component of these nuclides in recharging gr oundwater. Although direct evidence is lacking, calculations suggest t hat small but measurable subsurface production of H-3 and C-14 could a lso take place in aquifers having high U and Th concentrations. Some g roundwater 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 dominantl y anthropogenic. Dating methods with the least complications in interp retation appear to be H-3-He-3 for postbomb waters and Kr-81 for water s in the range of 5 x 10(4) to 1 x 10(6) a.