3H-3HE DATING: A CASE FOR MIXING OF YOUNG AND OLD GROUNDWATERS

Abstract

3 He / 4 He and 20 Ne / 4 He ratios were measured in shallow underground waters (opened by water-supplying wells) of the Large Vud-Javr intramountain artesian basin in the Khibiny alkaline massif, the Kola Peninsula. The ratios vary from 1.321 × 10 -6 to 2.065 × 10 -6 and from 1.412 to 2.941, respectively, and a well-defined correlation is observed between them. Both these ratios in aquifers are known to be time-dependent, the former increases with time due to accumulation of 3 He, produced in waters by 3 H -decay; the latter decreases due to migration of helium from water-bearing rocks into the waters. The correlation is interpreted as a result of the mixing of two different types of waters. The approximation line enables us to estimate the isotopic ratios for the endmembers participating in the mixing and the mean residence time ( ) of tritigenic helium-3 in the water: 1. (1) 3 He / 4 He = 3.655 × 10 -6 , 20 Ne / 4 He = 4.03, and taking into consideration 3 H concentrations in the well waters, 3 H = 31.1 TU (practically the same for all samples), = 15.8 ± 1.5 years for the young water; 2. (2) 3 He / 4 He = 0.20 × 10 -6 , 20 Ne / 4 He = 0.18 and T = 0.11 Ma for the old one, the contribution of the old water being less than 10%. In one well a considerable contribution of modern-day meteoric water, about 16%, is observed.