title: The effects of dual-domain mass transfer on the tritium-helium-3 dating method.
abstract: Diffusion of tritiated water (referred to as tritium) and helium-3 between mobile and immobile regions in aquifers (mass transfer) can affect tritium and helium-3 concentrations and hence tritium-helium-3 (3H/3He) ages that are used to estimate aquifer recharge and groundwater residence times. Tritium and helium-3 chromatographically separate during transport because their molecular diffusion coefficients differ. Simulations of tritium and helium-3 transport and diffusive mass transfer along stream tubes show that mass transfer can shift the 3H/3He age of the tritium and helium-3 concentration ([3H + 3He]) peak to dates much younger than the 1963 peak in atmospheric tritium. Furthermore, diffusive mass-transfer can cause the 3H/3He age to become younger downstream along a stream tube, even as the mean water-age must increase. Simulated patterns of [3H + 3He] versus 3H/3He age using a mass transfer model appear consistent with a variety of field data. These results suggest that diffusive mass transfer should be considered, especially when the [3H + 3He] peak is not well defined or appears younger than the atmospheric peak. 3H/3He data provide information about upstream mass-transfer processes that could be used to constrain mass-transfer models; however, uncritical acceptance of 3H/3He dates from aquifers with immobile regions could be misleading.
authors.affiliation: Ralph M. Parsons Laboratory, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.
authors.forename: Rebecca B
authors.initials: RB
authors.lastname: Neumann
authors.forename: Eric M
authors.initials: EM
authors.lastname: Labolle
authors.forename: Charles F
authors.initials: CF
authors.lastname: Harvey
mesh_terms: D000367:Age Factors; D005618:Fresh Water / Q000737:chemistry*; D006371:Helium / Q000032:analysis*; D007554:Isotopes; D008956:Models, Chemical; D014316:Tritium / Q000032:analysis*; D014872:Water Movements
chemical_list: D007554:Isotopes; D014316:Tritium; D006371:Helium