Variations in Groundwater Recharge and Water Table Elevations Across the Holocene in a Semi‐Arid Alluvial Basin

Many regional aquifer systems undergoing depletion contain “fossil” groundwater recharged under different climate conditions, raising questions on groundwater renewability. We examine the spatial distribution in groundwater ages estimated with multiple tracers covering different timescales, along with differences in paleoclimate and water table depths (WTD) in the Tucson Basin, Arizona, USA, to constrain recharge history. The presence of 3H and 85Kr in upgradient wells indicates modern recharge, while 39Ar, 14C, and 4He values show components of water several hundred to tens of thousands of years old in downgradient wells. In some wells, apparent ages on the orders of decades, centuries, and millennia are simultaneously indicated by 85Kr, 39Ar, and 14C activities, indicating that the flow system cannot be explained by a simple piston-flow model. To disentangle mixing and recharge history signals, we apply an inverse modeling technique that couples lumped parameter modeling of residence times with (stable) noble gas reconstructions of recharge temperature and WTD. A mid-Holocene maximum in WTD, reconstructed from Kr and Xe isotopes, suggests enhanced aridity associated with a reduction of wintertime precipitation, inferred from a shift in δ18O and δD. Colder temperatures were present during the Last Glacial Maximum, reflecting both climatic cooling and cold mountain front recharge. Recharge to the Tucson Basin has been continuous since the LGM, albeit with a likely reduction during the mid-Holocene. Natural fluctuations in groundwater recharge have been small compared to those associated with groundwater extraction, indicating that pumping poses a larger threat to groundwater resources than climate change.