Michael J. Singleton

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

and diffusive transport). Developing tractable analytical equations for these processes requires simplifying asNumerical simulations of transport and isotope fractionation prosumptions, which lead to analytical methods that are not vide a method to quantitatively interpret vadose zone pore water stable isotope depth profiles based on soil properties, climatic condieasily adapted to field conditions. Previous numerical tions, and infiltration. We incorporate the temperature-dependent models have relied on assumptions such as neglecting equilibration of stable isotopic species between water and water vapor, the temperature dependence of isotope fractionation and their differing diffusive transport properties into the thermodyand treating the isotopic species as nonreactive tracers namic database of the reactive transport code TOUGHREACT. with concentrations defined by fixed partition coeffiThese simulations are used to illustrate the evolution of stable isotope cients. profiles in semiarid regions where recharge during wet seasons disPrior approaches to predicting the impact of infiltraturbs the drying profile traditionally associated with vadose zone pore tion water on stable isotope profiles include a semiemwaters. Alternating wet and dry seasons lead to annual fluctuations pirical model (Barnes and Allison, 1988), a mixing in moisture content, capillary pressure, and stable isotope composischeme (Mathieu and Bariac, 1996b), and an analytical tions in the vadose zone. Periodic infiltration models capture the effects of seasonal increases in precipitation and predict stable isotope model to predict overall average pore water isotope profiles that are distinct from those observed under drying (zero compositions (DePaolo et al., 2004). However, a more infiltration) conditions. After infiltration, evaporation causes a shift general approach is needed to link observed isotope to higher 18O and D values, which are preserved in the deeper pore compositions with dynamic hydrological processes, waters. The magnitude of the isotopic composition shift preserved in where precipitation events or temperature changes afdeep vadose zone pore waters varies inversely with the rate of infilfect the isotopic profile with depth. tration. We use the thermodynamic framework of the TOUGHREACT transport code (Xu and Pruess, 2001; Xu et al., 2003) to develop a general transport model for stable T fraction of precipitation that reaches the deep isotopes in vadose zone soil water and consider the vadose zone, or the net infiltration, is difficult to impact of infiltration processes on measured stable isopredict in arid regions, but important for understanding tope profiles from the Hanford Site. These reactive groundwater recharge and contaminant transport. At transport models of stable isotope transport provide a the USDOE’s Hanford Site in south-central Washingquantitative method to link the observed isotopic proton State, where a large amount of radionuclide contamfiles to soil properties, climatic conditions, and net infilination is present in the vadose zone, it is critical to tration into the vadose zone. know the net water infiltration flux, as this determines how rapidly radionuclides or other contaminants may Background: Stable Isotope Measurements reach groundwater. The vadose zone hydrological proThe isotopic compositions discussed here are meacesses that control net infiltration rate also affect the sured relative to a well-defined standard material (Stanratios of stable isotopes (i.e., 18O/16O and 2H/1H) in water dard Mean Ocean Water [SMOW]). Stable isotope comand water vapor. positions (‰) are calculated as delta values from the The transport of stable O and H isotopes in water isotopic ratio (R 18O/16O or 2H/1H), where within drying soil columns has been studied extensively (e.g., Barnes and Allison, 1983, 1984; Allison et al., 1994; RSample RStandard 1 1000 [1] Shurbaji et al., 1995; Mathieu and Bariac, 1996a; Melayah et al., 1996). Approaches used to predict stable isotope profiles in drying soils must consider the comBased on this system, typical ocean waters have D plex interaction of multiple processes (e.g., drainage, and 18O values near 0‰ relative to SMOW. Meteoric temperature effects on flow and isotope fractionation, precipitation over land varies as a function of temperature, latitude, and altitude, but generally has D and 18O values that are shifted to values less than zero M.J. Singleton, E.L. Sonnenthal, M.E. Conrad, D.J. DePaolo, Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, because of the fractionation of lighter isotopes into the CA 94720; and G.W. Gee, Hydrology Group, Environmental Techvapor phase during the change from liquid to vapor. nology Division, Pacific Northwest National Laboratory, Richland, Craig (1961) documented a linear relationship, known WA. Received 30 Aug. 2003. Special Section: Research Advances in as the global meteoric water line (GMWL), between Vadose Zone Hydrology through Simulations with the TOUGH Codes. *Corresponding author (MJSingleton@lbl.gov). Abbreviations: GMWL, global meteoric water line; LBNL, Lawrence Berkeley National Laboratory; LMWL, local meteoric water line; Published in Vadose Zone Journal 3:775–785 (2004).  Soil Science Society of America PNNL, Pacific Northwest National Laboratory; SMOW, standard mean ocean water. 677 S. Segoe Rd., Madison, WI 53711 USA