DISCONTINUITIES IN RECHARGE THROUGH ARID AND SEMIARID BASIN FLOORS: EVIDENCE FROM THE UNSATURATED ZONE AND LINKAGES TO ENVIRONMENTAL CHANGE

Thick unsaturated zones in arid and semiarid interdrainage environments of the southwestern United States record abrupt shifts in basin floor recharge coincident with the glacial to interglacial transition 10,000 to 16,000 years ago. Compelling evidence of this major hydrologic transition derives from paired porewater chemistry (chloride and stable isotopes) and water potentials measured with depth. Multiphase flow and transport simulations to explain measured unsaturated zone (UZ) data indicate an exponential decline in recharge through the Holocene and a long-term transient drying of the UZ mediated by climate and vegetation change. Changes in recharge have a large impact on groundwater resources, since nearly all decreases in recharge through the basin floor in arid and semiarid settings result in aquifer depletion. In addition to recharge reduction, the early Holocene hydrologic transition supported the accumulation of oxyanions below the root zone in the absence of fluxes sufficient to flush solutes to groundwater. For example, substantial reservoirs of trapped nitrate, sequestered from the active nitrogen cycle, have been observed in some arid basin locations. The extreme disparity between the slow 10,000+ year drying required for the accumulation of such large oxyanion UZ reservoirs and the rapid (decadal scale) wetting induced by natural climate and vegetation shifts or anthropogenic land-use change imparts a certain vulnerability to groundwater due to flushing of these oxyanions to the water table. UZ physical and chemical data offer information about paleorecharge and insight for the outlook of groundwater resources.