RETRASO, a code for modeling reactive transport in saturated and unsaturated porous media

The code RETRASO (REactive TRAnsport of SOlutes) simulates reactive transport of dissolved and gaseous species in non-isothermal saturated or unsaturated problems. Possible chemical reactions include aqueous complexation (including redox reactions), sorption, precipitation-dissolution of minerals and gas dissolution. Various models for sorption of solutes on solids are available, from experimental relationships (linear KD, Freundlich and Langmuir isotherms) to cation exchange and surface complexation models (constant capacitance, diffuse layer and triple layer models). Precipitation-dissolution and aqueous complexation can be modelled in equilibrium or according to kinetic laws. For the numerical solution of the reactive transport equations it uses the Direct Substitution Approach. The use of the code is demonstrated by three examples. The first example models various sorption processes in a smectite barrier. The second example models a complex chemical system in a two dimensional cross-section. The last example models pyrite weathering in an unsaturated medium

The HPx software for multicomponent reactive transport during variably-saturated flow: Recent developments and applications

Abstract HPx is a multicomponent reactive transport model which uses HYDRUS as the flow and transport solver and PHREEQC-3 as the biogeochemical solver. Some recent adaptations have significantly increased the flexibility of the software for different environmental and engineering applications. This paper gives an overview of the most significant changes of HPx, such as coupling transport properties to geochemical state variables, gas diffusion, and transport in two and three dimensions. OpenMP allows for parallel computing using shared memory. Enhancements for scripting may eventually simplify input definitions and create possibilities for defining templates for generic (sub)problems. We included a discussion of root solute uptake and colloid-affected solute transport to show that most or all of the comprehensive features of HYDRUS can be extended with geochemical information. Finally, an example is used to demonstrate how HPx, and similar reactive transport models, can be helpful in implementing different factors relevant for soil organic matter dynamics in soils. HPx offers a unique framework to couple spatial-temporal variations in water contents, temperatures, and water fluxes, with dissolved organic matter and CO2 transport, as well as bioturbation processes

VS2DRT: Variably saturated two dimensional reactive transport modeling in the vadose zone

Contaminate transport in vadose is a huge concern since the vadose zone is the main passage way for ground water recharge. Understanding this process is crucial in order to prevent contamination, protect and rehabilitate ground water resources. Reactive transport models are instrumental for such purposes and there are numerous solute transport simulation programs for both ground water and vadose zone but most of this models are limited to simple Linear, Langmuir and Freundlich sorption models and first order decay and fail to simulate more complex geochemical reactions that are common in the vadose zone such as cation exchange, surface complexation, redox reaction and biodegradation. So it is necessary to enhance capabilities of solute transport models by incorporating well tested hydrogeochemical models like PHREEQC in to them to be able closely approximate the geochemical transport process in the subsurface. In this PhD research a new reactive transport model called VS2DRT was created by coupling existing public domain solute and heat transport models VS2DT, VS2DH with hydro-chemical model PHREEQC using non-iterative operator splitting technique. VS2DRT was compiled using MinGW compiler using tools like autotools and automake. A graphical user interface was also created using QT creator and Argus ONE numerical development tools. The new model was tested for one dimensional conservative Cl transport, surface complexation, cation exchange, dissolution of calcite and gypsum, heat and solute transport as well as for two dimensional cation exchange cases. Their results were compared with VS2DT, VS2DH, HP1 and HP2 models and the results are in good agreement

Thermo-Hydro-Mechanical analysis of evaporation from unsaturated soils

ABSTRACT Estimating evaporation from unsaturated soil is important for many applications including agriculture, climate, hydrology, water resources, saturated and unsaturated groundwater flow, slope stability, and soil covers. As an example, the long term performance of soil covers, which are widely used in mining and landfill applications to protect the environment, strongly depends on evaporation from their surfaces. Evaporation depends on both moisture flow within an unsaturated soil mass, which is generally coupled with heat flow, and void ratio and hydraulic and air conductivities, which are in turn affected by stress and strain and resulting soil settlement. This makes thermo-hydro-mechanical (THM) analysis of evaporation necessary. Evaporation also depends on environmental parameters, including air temperature, humidity, net radiation and wind speed. Therefore, considering atmospheric coupling in predicting evaporation is also necessary. The stress-strain behavior of the soil affects its settlement, which changes void ratio and porosity, and in turn permeability of the soil. This can alter the evaporation characteristics significantly and should be accounted for in any reliable evaluation of the actual evaporation and the performance of soil covers. However, existing soil-atmospheric models such as SOILCOVER(1994) and VADOSE/W(2002), which attempt to represent the soil-atmosphere continuum by linking the subsurface and the atmosphere, do not couple the equilibrium equation. Therefore, they can estimate evaporation but cannot estimate stress, strain and soil settlement. In fact, they perform thermo-hydraulic (TH) analysis. On the other hand, thermo-hydro-mechanical models such as 2D finite element program θ-Stock (Gatmiri et al., 1999) can perform THM analysis of unsaturated soil by coupling equilibrium equation with moisture and heat flow equations, but only work under soil surface, and do not have the capability to consider the environment and to incorporate atmospheric parameters. Therefore, they cannot estimate evaporation. The purpose of this study is to bring together the advantages of the above-mentioned programs by using an approach including both ideas and employing a formulation coupling THM analysis with soil-atmosphere modeling. Therefore, the resulting program EVAP1 numerically estimates evaporation from unsaturated soil using THM analysis and at the same time estimates stress, strain and soil settlement. In other words, the program can estimate evaporation considering soil settlement, which occurs in real world. The program EVAP1 was verified with published experimental and numerical studies on evaporation, including Wilson (1990) and Yang & Yanful (2002). Then, it was used to compare evaporation with and without considering soil settlement. The results showed that soil settlement alters the evaporation characteristics significantly and should be accounted for in any reliable evaluation of the evaporation from unsaturated soil. Neglecting settlement causes an overestimation of evaporation. A parametric study was also performed to evaluate the effects of environmental parameters on evaporation, in order to identify the parameter that affects evaporation the most. It was found that the most important parameters, in order, are humidity, net radiation, temperature and wind speed. The sensitivity of evaporation to these parameters was also evaluated, and the trend of the change in evaporation due to the change in each of the parameters was noted. The results showed that the effects of these parameters on evaporation are mostly nonlinear

Phase field modeling of partially saturated deformable porous media

A poromechanical model of partially saturated deformable porous media is proposed based on a phase field approach at modeling the behavior of the mixture of liquid water and wet air, which saturates the pore space, the phase field being the saturation (ratio). While the standard retention curve is expected still to provide the intrinsic retention properties of the porous skeleton, depending on the porous texture, an enhanced description of surface tension between the wetting (liquid water) and the non-wetting (wet air) fluid, occupying the pore space, is stated considering a regularization of the phase field model based on an additional contribution to the overall free energy depending on the saturation gradient. The aim is to provide a more refined description of surface tension interactions. An enhanced constitutive relation for the capillary pressure is established together with a suitable generalization of Darcy's law, in which the gradient of the capillary pressure is replaced by the gradient of the so-called generalized chemical potential, which also accounts for the \lq\lq force\rq\rq\, associated to the local free energy of the phase field model. A micro-scale heuristic interpretation of the novel constitutive law of capillary pressure is proposed, in order to compare the envisaged model with that one endowed with the concept of average interfacial area. The considered poromechanical model is formulated within the framework of strain gradient theory in order to account for possible effects, at laboratory scale, of the micro-scale hydro-mechanical couplings between highly-localized flows (fingering) and localized deformations of the skeleton (fracturing)

Inclusion of chemical effect in a fully coupled THM finite element code

Bentonite-rich clays can be used as a buffer / backfill material in deep geological repositories for nuclear waste. The prediction of the long-term performance of a buffer / backfill in such a complex environment, where the temperature, humidity and chemistry of water change, requires a fully thermo-hydro-mechanical-chemical (THMC) coupled numerical code. This paper presents a simple extension of a THM coupled finite element code to include chemical effects. After deriving the governing salt mass balance equation and discussing its implementation into the code, the paper verifies the extended framework against an analytical solution for 1D salt transport. In addition, the article presents a validation example in which the code replicates experimental data. The numerical results obtained from the extended THMC coupled finite element code encourage further investigation of the chemical effects on the mechanical and thermal behaviour of the material, which would serve the ultimate goal of achieving a safer design of the nuclear waste storage facility.Postprint (published version