ENVIRONMENTAL TRACERS FOR WATER-MOVEMENT IN DESERT SOILS OF THE AMERICAN SOUTHWEST

Tritium and Cl-36 from nuclear weapons testing fallout can serve as tracers for water and solute movement in desert soils on the time scale of decades. Chloride deposited from the atmosphere can provide a tracer for much longer periods. We reviewed and compared studies using these tracers at locations across the southwestern USA in order to assess the controls on water and solute movement across time scales ranging from decades to millennia. The displacements of all the tracers were quite similar despite variations in location, climate, and soil properties. The time-averaged Cl-36 fallout transport rate velocity is 23 +/- 17 mm yr-1. The Cl concentration profiles indicate that, prior to about 15 000 yr ago, soil-water fluxes were approximately 20 times the magnitude of modern fluxes, and that the soil water shifted rapidly (on a geological time scale) to the dry regime between 16 000 and 13 000 yr ago. The remarkable uniformity of the water and solute transport processes across the region may be dictated by the water extraction efficiency of the vegetation.