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A Transient Model of Vadose Zone Reaction Rates Using Oxygen Isotopes and Carbon Dioxide

^a Dep. of Geological Sciences, Univ. of Saskatchewan, Saskatoon, SK, Canada, S7N 5E2
^b National Water Research Inst., Environment Canada, 11 Innovation Blvd., Saskatoon, SK, Canada, S7N 3H5
^c Dep. of Earth and Atmospheric Sciences, 126 Earth Sciences Bldg., Univ. of Alberta, Edmonton, AB, Canada, T6G 2E3

View larger version (20K): [in this window] [in a new window]   Fig. 1. Depth profiles of mean volumetric moisture contents ± 1 SD. Data from Diviner-1 and -2 are represented by open circles and star symbols. Data from the neutron probe are represented by solid squares. Horizontal lines represent the depth to the water table measured on, from top to bottom, 16 June 2002, 2 Oct. 2003, 4 Nov. 2003, 29 Oct. 2002, 27 Aug. 2002, 10 June 2003, and 20 Feb. 2003. Shaded area represents the range of moisture contents used for numerical modeling of O2, CO2, 16O16O, and 18O16O concentrations.

View larger version (21K): [in this window] [in a new window]   Fig. 2. Spatial and temporal trends in temperature. Dotted line represents daily mean surface temperature. Solid boxes, triangles, and diamonds represent measured soil temperatures at 1, 2, and 3 m, respectively. Solid lines represent simulated temperatures at, from largest to smallest amplitude, 1, 2, and 3 m.

View larger version (26K): [in this window] [in a new window]   Fig. 3. Temporal trends in field-measured and best-fit modeled (A) CO2 concentration, (B) O2 concentration, and (C) 18O of O2. Sampling depths and corresponding symbols for field-measured data are: solid diamonds-0.5 m, open boxes-1.0 m, solid triangles-1.5 m, crosses-2.0 m, open diamonds-2.5 m, and stars-3.0 m. Solid lines represent, from bottom to top, (A) and (B) best-fit modeled concentrations (C) and 18OO2 for depths of 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 m.

View larger version (21K): [in this window] [in a new window]   Fig. 4. Selected concentration-depth profiles of field-measured and best-fit modeled (A) CO2, (B) O2, and (C) 18OO2. In (A) and (B), sampling dates 16 June, 13 July, and 26 Aug. 2002 are represented by solid diamonds, stars, and open triangles, respectively. In (C), sampling dates 13 July and 26 Aug. 2002, and 19 Feb. 2003 are represented by solid diamonds, open triangles, and open circles, respectively. Associated best-fit modeled profiles are represented by solid lines.

View larger version (12K): [in this window] [in a new window]   Fig. 5. Effective diffusion coefficient (using the Millington [1959] formulation) as a function of volumetric moisture content. Sand and near-surface clayey sand are represented by stars and solid boxes, respectively.

View larger version (13K): [in this window] [in a new window]   Fig. 6. Comparison of modeled and measured 18O of O2. Solid line represents a 1:1 correlation between modeled and measured 18O of O2.