Environmental and Applied Tracers as Indicators of Liquid and Vapor Transport in the Chihuahuan Desert, Texas

Chemical and physical approaches are used to study unsaturated flow; however, few studies include an in-depth analysis of data from both approaches. Detailed chemical tracer studies were conducted at a site in the Chihuahuan Desert of Texas, and the results were compared with hydraulic attributes of the system. Estimated soil-moisture fluxes from both chemical and physical approaches were compared to better understand the hydrologic processes in the unsaturated zone.Chemical tracer studies included mass balance of the chloride ion. Chloride profiles were measured in ephemeral stream and interstream settings. Moisture fluxes were calculated from measured chloride concentrations on the basis of a steady-state flow model. Chlorine 36 and tritium had been enriched in the atmosphere by weapons tests conducted in the 1950's and 1960's, and the distribution of these radionuclides in the shallow (<2 m) unsaturated zone was used to quantify flux in an ephemeral stream during the past 35 yr. In addition to environmental tracers, a bromide pulse was applied over a 100-m2 area, leached by natural precipitation for 1 yr, then recovered and measured in soil cores.The chloride profiles are characterized by low chloride concentrations near the surface, which increase to a maximum of 1,900 to 9,300 g m-3 at depths of 1.3 to 4.7 m and gradually decrease with depth below the peak. Calculated moisture fluxes are inversely proportional to chloride concentrations in the soil water because a constant chloride accession rate was assumed throughout the study area, Reductions in chloride concentrations below the peak are attributed to differences in moisture flux as a result of paleoclimatic variations. Moisture fluxes based on the 36Cl and 3H peak depths were 1.4 and 7 mm yr-l, respectively. The artificially applied bromide pulse penetrated to a depth of 0.3 m during the 1-yr period; however, much of this water movement may have occurred during the initial application of the tracer with irrigation water.The chemical tracer data record cumulative downward-directed fluxes that contrast with the generally upward driving forces for liquid water movement indicated by the hydraulic data. If water movement were upward, the highest tracer concentrations should occur at land surface; however, maximum 36Cl/Cl ratio and 3H and chloride concentrations occur at depths of 0.5 m, 1.4 m, and 1.3 to 4.7 m, respectively. The discrepancy between the chemical and hydraulic data could be resolved if the tracers moved down by concentration-driven diffusion. Analysis of the 36Cl profile suggests that diffusion alone cannot account for the distribution of 36GI and advection is also involved. Downward percolation is episodic and is better estimated by long-term average fluxes from the chemical tracer data rather than from the limited time period represented by the soil physics monitoring.The higher moisture flux indicated by the 3H profile relative to that indicated by the 36Cl profile is attributed to enhanced downward movement of 3H in the vapor phase and suggests a vapor flux of approximately 6 mm yr-I. The vapor flux hypothesis was tested using nonisothermal liquid and vapor flow simulations with the computer code SPLaSHWaTr2. Simulations of 5-day periods in the winter and summer were conducted to represent extremes in the temperature gradients. The calculated vapor flux was 2 to 8 orders of magnitude greater than the liquid flux for the periods simulated. Predicted vapor fluxes were upward in the top 0.04 m of the unsaturated none in the summer and winter periods in response to steep water potential gradients induced by surface evaporation. Below the evaporation front, from depths of 0.15 to 1 m, downward vapor fluxes in the summer were much greater than generally upward vapor fluxes in the winter. These results suggest a net downward vapor flux on an annual basis that is consistent with the chemical tracer results.