A Stable Isotope Investigation of Vapor Transport During Ground-Water Recharge in New Mexico

New Mexico (especially the southern part of the state) is heavily dependent on ground water for agricultural irrigation, municipal supply, and economic development. The quantification of recharge is thus very important to scientists, engineers, and water resource planners.

We present a technique to model the transport of the stable isotopes 2H and 18O. This technique is based upon work by Barnes and Allison (1984). The model developed here can predict the shape of the stable isotope profiles in the vadose zone, and also quantify the vapor flux portion of soil-water movement. Through a non-linear least-squares approach, the model is matched to an observed isotope profile by optimizing the values of specified variables. The optimized parameters are then used to numerically solve for the vapor flux as a percentage of the deep flux of water below the root zone. This technique of modeling the isotope species transport is much more robust than conventional heat and moisture flow modeling techniques. This conclusion stems from the fact that the effective isotopic diffusivities vary little with water content, whereas hydraulic diffusivity is strongly dependent on water content.

Based upon the model results, in the top 2.7 meters of the soil profile a depth-averaged estimate of 15 to 33 percent of the total soil-water movement is in the vapor phase. This implies that the liquid flux of water is most important in the recharge process. However, the vapor flux is substantial enough to strongly influence the solute transport characteristics of the system. The strong downward vapor flux is also confirmed qualitatively by the pronounced minimum in the isotope profiles. The lighter isotopes, 16O and 1H, are preferentially transported in the vapor phase and condensed at depth, thereby causing the depletion of the isotope ratios.

A vacuum distillation procedure has been developed to extract soil water from soil samples without isotopic fractionation. This new technique is an alternative to the azeotropic toluene distillation method used by other investigators.