Using Oxygen-18 and Deuterium to Delineate Groundwater Recharge at Different Spatial and Temporal Scales

Since the 1960s, the stable isotopes oxygen-18 and deuterium have become widely used for delineating sources and timing of precipitation, identifying effects of evaporation and apportioning end-member contributions in streamflow and groundwater. During the last decade, the development of cavity ring-down spectroscopy has revolutionized monitoring by allowing rapid, relatively inexpensive analyses of large numbers of samples. This paper reviews the use of water isotopes together with other parameters, particularly chloride (a conservative solute) and temperature, to delineate sources and timing of recharge in (1) regional sedimentary aquifer systems and (2) mountainous karst terrains. Case (1) studies focus on spatial variability in recharge, with implications for delineating diffuse versus focused recharge to surficial aquifers, paleorecharge to deep aquifers and sources of salinization. Case (2) studies focus on temporal variability in individual spring basins, which may be particularly susceptible to climate change, and identify recharge at event to seasonal time scales.