Partitioning young and old groundwater with geochemical tracers

•Average groundwater ages can vary widely when determined with different tracers.•Geochemical tracers can partition fractions of young and old groundwater.•New database of global groundwater dissolved inorganic carbon-13 and carbon-14.

Groundwater age-defined as the time elapsed since the groundwater entered the subsurface-is a helpful indicator of groundwater quality and renewal. Some tracer-based groundwater age calculations require assumptions about the distribution of ages in the groundwater sample that, unfortunately, cannot be validated. Here, straightforward mixing models are developed that do not require a priori knowledge of age distributions and are able to quantify the fraction of a groundwater sample that recharged more recently than calendar year 1953 (using tritium) or within the past ~ 12 thousand years (using carbon-14). Calculations of young and old groundwater can be used to map groundwater velocities in flow systems driven by diverse climate conditions, groundwater extraction rates, rock types, topographic gradients and plant populations.