Inflection Points on Groundwater Age and Geochemical Profiles Along Wellbores Light up Hierarchically Nested Flow Systems

Hierarchically nested groundwater flow systems have been widely investigated by numerical modeling and laboratory experiments but seldom recognized from in situ observations and testing. Groundwater age and geochemical profiles were obtained by the authors using a packer system along two wellbores drilled in the Ordos Plateau, China. Groundwater age profiles were constructed from ³H, ⁸⁵Kr, ¹⁴C, and ⁸¹Kr data, with the apparent age generally increasing with depth, while exhibiting several inflection points. These inflection points were compared with geochemical profiles and numerical modeling results to delineate groundwater flow systems. Inflection points with significant increases in the gradients of age and Cl⁻ concentration versus depth may indicate interfaces among flow systems. A concurrent decrease-increase turning of groundwater age and Cl⁻ concentration is influenced by a stagnation point. The groundwater age profiles are quite different from previous studies, indicating that the method can greatly reduce the uncertainty in model construction.