Groundwater age transport modeling results from "The global volume and distribution of modern groundwater" by Gleeson et al. (2015) Nature Geoscience.
http://www.nature.com/doifinder/10.1038/ngeo2590
Gleeson, T., K. M. Befus, S. Jasechko, E. Luijendijk, and M. B. Cardenas (2015), The global volume and distribution of modern groundwater, Nat. Geosci., (November), 1–15, doi:10.1038/ngeo2590.