Isotope Fractionation of Water Adsorption on Clay Minerals

点击翻译按钮获取中文摘要

The soil-water system of terrestrial environments is located at a very critical interface within land-atmosphere-vegetation continuum of the energy-water-carbon dioxide balance and ecological systems. The outcome from this project can contribute to our better understanding of the land-atmosphere-vegetation system on plot to regional to global scales. By extension, the improved understanding can lead to more accurate prediction of regional to global water cycles and likely climate changes. Such capability is increasingly required to meet societal needs such as water resources development, global primary production, and agriculture/food production and security. The investigator will conduct an outreach activity designed to enhance middle school girls' interest from rural West Texas in STEM topics through a one-day, interactive "Water Planet" workshop on water-related projects.The isotopic compositions of soil waters have been extensively utilized as a very effective tool to study vadose-zone hydrology, including evapotranspiration, water movement, and groundwater recharge. Recently, isotope-based studies found that mobile water (expressed by stream water) and tightly bound water (represented by soil and plant water) in watersheds were isotopically distinct and the notion of ?ecohydrological separation? has been proposed. Other explanations of the observed isotopic differences include a mixing of evaporated and unevaporated soil waters during percolation. The proposed activities and expected outcome from this project will test the hypothesis that isotope fractionation associated with the adsorption and condensation of water on the surface and in pores of soil minerals differ from that for bulk liquid water. This proposal seeks an integrated approach of detailed, systematic experiments and basic modeling to quantify, understand and predict the isotope fractionation of adsorbed water on various non-expansive clay (kaolinite), expansive clays (Na- and Ca-rich montmorillonite), and soils in general as a function of physicochemical properties (surface chemistry, pore size/interlayer distance, type of interlayer cations) and the relative vapor pressure. The potential of the outcome has far-reaching implications for our understanding of the water cycle at the land-atmosphere-vegetation interface, because the isotopic compositions of soil water has been increasingly and widely used. Knowledge that can be directly impacted and further advanced include: a) analytical methods for the isotopic compositions of soil water, b) multiple subsurface water pools in soil pores and fractures with different isotopic compositions, and c) Craig-Gordon evaporation model for calculating the isotopic compositions of evaporation and transpiration fluxes from soils and vegetation, respectively.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.