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

Abstract

and diffusive transport). Developing tractable analytical equations for these processes requires simplifying as-Numerical 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 condi-easily 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 coeffi-These simulations are used to illustrate the evolution of stable isotope cients. profiles in semiarid regions where recharge during wet seasons dis-Prior 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 ␦ 18 O 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 TOUGH-REACT transport code (Xu and Pruess, 2001;Xu et al.,

In situ unsaturated zone water stable isotope (<sup>2</sup>H and <sup>18</sup>O) measurements in semi-arid environments: a soil water balance

“…6). Other studies showed similar patterns in the context of a numerical experiment (Singleton et al, 2004). The first transect (Fig.…”

“…Variations in isotope values are attenuated with time by vapor phase redistribution (transect 2) (Fontes et al, 1986) and an evaporation profile begins to develop (transect 3) (Rothfuss et al, 2015). Further, it can be observed that infiltrating water of small, intermittent events compresses the isotope profile (Barnes and Allison, 1988;Singleton et al, 2004). Therefore, the presented in situ measurements have great potential to visualize theoretically discussed processes with feasible measurement trueness under natural field conditions.…”

Mineralization of carbon dioxide sequestered in volcanogenic sandstone reservoir rocks

“…The code couples the hydraulic transport capabilities of TOUGH2 (Pruess, 1991a) with reactive transport based on thermodynamic principles of chemical equilibrium and kinetics (Xu and Pruess, 2001b). Singleton et al (2004) extended TOUGHREACT to simulate oxygen and hydrogen isotope fractionation. In this study a similar approach is employed, allowing for an unlimited number of isotopes for any element to be tracked.…”

“…Multi-component reactive transport models such as TOUGHREACT (Xu et al, 2004) and CrunchFlow (Steefel, 2007) integrate hydrologic transport, aqueous and surface complexation and dissolution/precipitation into a comprehensive network of kinetic and equilibrium pathways. Incorporation of isotopic ratios into such multi-component models has been demonstrated in analysis of vadose zone infiltration rates from temperature dependent equilibrium fractionation of water isotopes (Singleton et al, 2004), plagioclase dissolution from uranium-series isotopes (Maher et al, 2006) and recently the influence of hydrologic transport on measured isotope fractionation in compound-specific stable isotope analysis (Rolle et al, 2010).…”

Hydrologic Resources Management Program and Underground Test Area Project FY2005 Progress Report

“…The conceptual model for this process is based on earlier studies of isotope profiles in soil moisture (e.g. Allison et al, 1983, Singleton et al, 2004. As the soil zone dries following a recharge event, the residual soil moisture gradually develops an evaporative stable isotope profile as a function of depth.…”