Importance of tritium-based transit times in hydrological systems

Disappearance of bomb tritium from the atmospheres of both hemispheres in recent years means that tritium is becoming more effective for determining transit times in hydrological systems. In particular, increasing use of tritium is being made to determine transit times of water through catchments from rainfall to rivers in order to derive vital information for the protection and management of river systems. Tritium reveals the transit times of the older water in streamflow, transit times which are too long to be seen by stable isotope or major chemical variation methods (this has been characterized as 'hidden streamflow'). Documenting these longer timescales is vital for understanding the flow and water quality responses of rivers to changes in land use, diffuse or point-source chemical pollution, ecological degradation, and climate change, responses which have proven to be considerably longer than previously expected. Including the longer timescales in the models allows more realistic approaches to management of catchments and emphasizes interdisciplinary differences between surface water and groundwater hydrologists' views of river basins. Contrasting research agendas are proposed for effective use of tritium in the Northern and Southern Hemispheres based on their different tritium input histories since the advent of nuclear testing in the atmosphere. (C) 2016 Wiley Periodicals, Inc.