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Multiphase Reactive Transport Modeling of Seasonal Infiltration Events and Stable Isotope Fractionation in Unsaturated Zone Pore Water and Vapor at the Hanford Site

^a Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720
^b Hydrology Group, Environmental Technology Division, Pacific Northwest National Laboratory, Richland, WA

View larger version (23K): [in a new window]   Fig. 1. Hydrogen and O isotope compositions of soil waters collected from the Hanford vadose zone. The value of average precipitation used for model input waters is shown as a gray box. Pore water stable isotope data from DePaolo et al. (2004) and unpublished data from ongoing studies are shown as blue diamonds. Also shown are the global meteoric water line of Craig (1961) and the Hanford local meteoric water line, based on data from Graham (1983) and Early et al. (1986).

View larger version (13K): [in a new window]   Fig. 2. Soil water 18O values with depth in the Hanford vadose zone. Pore waters in the upper 2 m are most strongly affected by evaporation (Fig. 2) and have 18O values up to –3.8. Below the evaporation zone, soil water 18O values are shifted by approximately +2 (dashed line) higher than typical winter precipitation (–16.5). Pore water stable isotope data from DePaolo et al. (2004) and unpublished data from ongoing studies are shown as blue diamonds.

View larger version (23K): [in a new window]   Fig. 3. Zero infiltration model results for pore water 18O in three hypothetical soil types: clayey silt, silty sand, and medium sand. Starting from an initial pore water 18O value similar to winter precipitation at Hanford (–16.5), the model soils were subjected to 20 yr of evaporation and drainage under an atmosphere with h = 40% and 18Oa = –21. The value of 18OMAX that is reached under these atmospheric conditions is shown as a vertical line.

View larger version (17K): [in a new window]   Fig. 4. Predicted 18OMAX vs. relative humidity for unsaturated soils at 10, 20, and 30°C from numerical model results with zero infiltration. Model calculations are based on reservoir and atmospheric vapor isotopic compositions of 18Ores = –16.5 and 18Oa = –21.

View larger version (20K): [in a new window]   Fig. 5. Model steady-state pore water 18O values for a constant infiltration of 50 mm yr–1 of water with a 18O value of –16.5. The soil types and atmospheric conditions (h = 40% and 18Oa = –21) are the same as for Fig. 3. The value of 18OMAX that would be reached under these atmospheric conditions, but without infiltration, is shown as a vertical line.

View larger version (31K): [in a new window]   Fig. 6. Capillary pressures respond to the annual cycle of wet and dry seasons in the VZFS300N lysimeter at Hanford (solid lines), and in a periodic infiltration model with silty-sand soil properties (dashed lines). Data from Sisson et al. (2002) and online at http://vadose.pnl.gov (verified 3 June 2004). (1 mbar = 0.0001 MPa).

View larger version (47K): [in a new window]   Fig. 7. A sequence of 0.1-yr time steps from the periodic infiltration model illustrates (A) disturbances to the isotopic profile during the wet season, Steps 1 through 2, and (B) the effects of drainage and evaporation during the dry season, Steps 3 through 9. The atmospheric conditions (h = 40% and 18Oa = –21) are the same as those for Fig. 3 and 5. The soil type is silty sand for Parts A and B. (C) the 18O profiles at Time Step 3 for the three soil types considered in this study. Water input (18O of –16.5) occurs such that annually all of the infiltration (50 mm) comes during the first 0.3-yr wet period and is followed by a 0.7-yr dry period.

View larger version (22K): [in a new window]   Fig. 8. Stable isotope profiles at the beginning of the dry season for pulsed infiltration models. The time-dependent input rates of winter infiltration of 16.7, 58.3, and 150 mm yr–1 result in calculated (Eq. [19]) steady-state net infiltration rates of 14.3, 55.0, and 147 mm yr–1, respectively. Water input (input water 18O of –16.5) occurs such that annually all of the infiltration comes during the first 0.3-yr wet period and is followed by a 0.7-yr dry period. The soil type is silty sand, and atmospheric conditions (h = 40% and 18Oa = –21) are the same as those for Fig. 3, 5, and 7. Model results are compared with data collected as part of an ongoing study of the VZFS300N lysimeter in March 2003, shortly after the wet season (diamonds). The lysimeter has a measured drainage rate of 55 ± 10 mm yr–1 (Sisson et al., 2002).