Evaluation of the (3)H-(3)He dating technique in complex hydrologic environments

The {dollar}sp3{dollar}H-{dollar}sp3{dollar}He dating technique is used in a variety of complex hydrogeologic systems to explore the potential of the method when significant challenges arise, including disturbed flow systems, large-scale study areas, the presence of high terrigenic helium concentrations, the presence of tritium contamination, and heterogeneity. Four field sites are addressed in this thesis. At the first site, tritium and helium isotopes are used in a super-anoxic fjord to assess the stability of CFCs in reducing environments. A one-dimensional model is used to simulate the vertical distributions of tritium and its decay product, tritiogenic {dollar}sp3{dollar}He, in order to determine the physical parameters defining processes other than degradation of CFCs; CFC degradation rates are then estimated by comparing model simulations using various degradation rates and scenarios and the CFC concentration profiles. At the second site, the movement of a sewage plume in a homogeneous, small-scale sand and gravel aquifer is characterized. With a high density of sampling in a given area and with depth, the {dollar}sp3{dollar}H-{dollar}sp3{dollar}He dating technique provides detailed information beyond average aquifer properties. The third site is a heterogeneous, large-scale buried-valley aquifer which is highly disturbed, has known tritium contamination, and has high concentrations of radiogenic helium. Based on analyzing the {dollar}sp3{dollar}H-{dollar}sp3{dollar}He data for self-consistency, for consistency with chlorofluorocarbon ages, and for consistency with the current understanding of local hydrologic processes, the {dollar}sp3{dollar}H-{dollar}sp3{dollar}He method provides reliable age estimates throughout the aquifer despite major complications. The final site is a large-scale, karstic and fractured limestone aquifer in which mixing is significant due to sampling from open-hole wells and to the extreme heterogeneity resulting from cavernous openings and diffuse fracturing, terrigenic helium concentrations are high, and the tritium input to the system is complex due to the nature of the recharge to the flow system. {dollar}sp3{dollar}H-{dollar}sp3{dollar}He ages have little direct meaning in such complex flow systems, but were used in conjunction with tritium and helium concentrations to aid in hydrogeologic interpretations. Finally, the results from the four field sites are compared to frame a discussion of the limits in the use of the method determined in this thesis.