Groundwater age persistence in topography-driven groundwater flow over paleohydrogeologic time scales

The Pleistocene-Holocene climate transition resulted in a dramatic reduction in groundwater recharge in many aquifers in arid and semiarid regions throughout the world. This study conducted numerical experiments to compare the evolution of groundwater hydraulics and age patterns in arid and semiarid aquifers in response to transient conditions associated with recharge decline from the Pleistocene to the Holocene. Our results show that after a rapid reduction in recharge, the amplitude of water-table undulations and regional groundwater slope both reduced. This resulted in a general, and relatively rapid, contraction of local flow systems and an increase in the extent of intermediate and regional systems. The previous hierarchy of local, intermediate, and regional flow systems was completely replaced by largely horizontal and regional flow patterns after similar to 10,000 yr. However, in stark contrast, we observed that the original Pleistocene age patterns have remained almost unchanged throughout the 10,000 yr Holocene period. Thus, groundwater age is more likely to be indicative of past rather than current flow systems. Consequently, due to this age persistence, the use of modern groundwater age data to calibrate models or compute recharge with methods that do not account for this potentially significant spatial and temporal mismatch between age and hydraulics will be misleading and erroneous. This has significant implications for hydrogeologic analyses. The findings of this study may also apply to areas that have undergone dramatic changes in land cover or land use that strongly influence transient groundwater recharge processes.