Tritium–helium 3 dating under complex conditions in hydraulically stressed areas of a buried‐valley aquifer

Abstract

The 3 H‐ 3 He dating method is applied in a buried‐valley aquifer near Dayton, Ohio. The study area is large, not all sampling locations lie along well‐defined flow paths, and existing wells with variable screen lengths and diameters are used. Reliable use of the method at this site requires addressing several complications: (1) The flow system is disturbed because of high pumping rates and induced infiltration; (2) tritium contamination is present in several areas of the aquifer; and (3) radiogenic helium concentrations are elevated in a significant number of the wells. The 3 H‐ 3 He ages are examined for self‐consistency by comparing the reconstructed tritium evolution to the annual weighted tritium measured in precipitation; deviations result from dispersion, tritium contamination, and mixing. 3 H‐ 3 He ages are next examined for consistency with chlorofluorocarbon ages; the agreement is poor because of degradation of CFCs. Finally, the 3 H‐ 3 He ages are examined for consistency with the current understanding of local hydrologic processes; the ages are generally supported by hydrogeologic data and the results of groundwater flow modeling coupled with particle‐tracking analyses.