YOUNG AND OLD WATER IN GLOBAL RIVERS AND GROUNDWATER WELLS (Invited Presentation)

The age of water is defined by the time that rain and snow take to flow into a river channel or to a location under the ground. These water ages impact a number of earth systems processes, including nutrient cycling, crustal cooling and bedrock weathering, but have remained unknown for global river and groundwater well waters. Here, stable and radioactive isotope compositions of rain, snow, groundwater and streamflow are compiled and analyzed in an effort to partition young and old water fractions that are (i) flowing in rivers, or (ii) pumped from groundwater wells. First (i), we show that ~1/3 of global streamflow is precipitation that fell and flowed to the stream in less than ~2.3 months. This young streamflow is detectable in the great majority (~90%) of surveyed streams, implying that most catchments can convey precipitation to the stream channel quite quickly, leaving little time for pollutant detection and dispersion. Second (ii), we survey tritium and radiocarbon in groundwater samples pumped from wells screened in ~200 globally-distributed aquifers. We show that modern groundwater (less than ~50 years old) constitutes a small fraction of global groundwater, and that paleowaters (more than ~12 thousand years old) are not an anomaly but, rather, the norm for flow paths that are more than ~300m below the land surface. Calculated fractions of young and old groundwater pumped from global wells can be used to assess water supply vulnerabilities to contamination and to better quantify groundwater renewal fluxes under modern and ancient Pleistocene climates.