The Impact of Climate Change on the Chemical Composition of Deep Vadose Zone Waters

Chloride mass balance, and stable (deuterium and 18 O) and radiogenic ( 3 H, 36 Cl) isotope studies of deep vadose zone pore waters have generally concluded that variations in moisture flux can account for the observed variations in abundance of these approximately conservative tracers. It can be inferred, on the basis of these observations and interpretations, that a climate change record is preserved in these vadose zone waters. In arid regions where thick (>100 m) vadose zones persist, it has been concluded that this record may extend back more than 100 000 yr. Consideration of the mechanisms that control reactive transport led to the conclusion that such climate-driven effects will also be evident as chemical reactions involving dissolution and/or precipitation of mineral phases along the flow pathway. As a result, there should also be variations in the concentrations of nonconservative chemical species that correspond to changes in the concentrations of the conservative tracers. Simulations of this reactive transport, in a regime typical of the arid U.S. Southwest, demonstrate that these changes can modify pore water chemistry by factors of up to 200%, but the changes take place slowly, requiring thousands of years to achieve steady-state conditions. This suggests that a very rich archive of climate change history is preserved in this type of setting. However, extracting that history is currently hampered by limitations in data and models (e.g., effective mineral reactive surface areas, fluid flow pathways, and quantified models of wetted fracture surface in unsaturated, fractured systems). This challenge may be overcome if coordinated efforts are undertaken that exploit the power of detailed studies of isotope systematics, microscale rock characterization, and high performance computing.