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

Evaluation of a general model for multimodal unsaturated soil hydraulic properties

Many soils and other porous media exhibit dual- or multi-porosity type features. In a previous study (Seki et al., 2022) we presented multimodal water retention and closed-form hydraulic conductivity equations for such media. The objective of this study is to show that the proposed equations are practically useful. Specifically, dual-BC (Brooks and Corey)-CH (common head) (DBC), dual-VG (van Genuchten)-CH (DVC), and KO (Kosugi)$_1$BC$_2$-CH (KBC) models were evaluated for a broad range of soil types. The three models showed good agreement with measured water retention and hydraulic conductivity data over a wide range of pressure heads. Results were obtained by first optimizing water retention parameters and then optimizing the saturated hydraulic conductivity (K_s) and two parameters (p, q) or (p, r) in the general hydraulic conductivity equation. Although conventionally the tortuosity factor p is optimized and (q, r) fixed, sensitivity analyses showed that optimization of two parameters (p+r, qr) is required for the multimodal models. For 20 soils from the UNSODA database, the average $R^2$ for log (hydraulic conductivity) was highest (0.985) for the KBC model with r=1 and optimization of (Ks, p, q). This result was almost equivalent (0.973) to the DVC model with q=1 and optimization of (Ks, p, r); both were higher than $R^2$ for the widely used Peters model (0.956) when optimizing (Ks, p, a, $\omega$). The proposed equations are useful for practical applications while mathematically being simple and consistent.Comment: To be published in Journal of Hydrology and Hydromechanic

Closed-form hydraulic conductivity equations for multimodal unsaturated soil hydraulic properties

Closed-form expressions of the hydraulic conductivity function for linearly superposed subretention (multimodal) functions were derived for arbitrary sets of the Brooks and Corey (BC), van Genuchten (VG), and Kosugi (KO) water retention models. The generalized Mualem hydraulic conductivity model was evaluated using the mathematical approach of Priesack and Durner. Three types of modification to the multimodel were also proposed. Firstly, the derived conductivity equations can be simplified when the submodel parameters, hbi for the BC model, alpha i-1 for the VG model, and hmi for the KO model have the same (common) value (denoted as CH). Secondly, as in the case of the modified single VG and KO models, a hypothetical air-entry head near saturation can be introduced for the multimodal VG and KO models to prevent unrealistic reductions in the hydraulic conductivity near saturation when the VG n parameter approaches its lower limit of n = 1. Furthermore, the multimodal hydraulic conductivity functions become a simple sum of conductivity subfunctions when the exponent r is unity (such as for Burdine's model), which leads to independent tortuosity effects for each submodel. The models are illustrated for two soils: a highly aggregated Kumamoto Andisol and a relatively unimodal dune sand. The dual-(BC, VG, KO) and the VG(1)BC(2) models equally represented the water retention data of the Andisol, with similar hydraulic conductivity curves. The dual-BC-CH, dual-VG-CH, and VG(1)BC(2)-CH models fitted the water retention data of the dune sand well, with the hydraulic conductivity curves of the dual-porosity model being similar to those of the Fayer and Simmons (FS) model

A control volume scheme using compact integrated radial basis function stencils for solving the Richards equation

A new control volume approach is developed based on compact integrated radial basis function (CIRBF) stencils for solution of the highly nonlinear Richards equation describing transient water flow in variably saturated soils. Unlike the conventional control volume method, which is regarded as second-order accurate, the proposed approach has high-order accuracy owing to the use of a compact integrated radial basis function approximation that enables improved flux predictions. The method is used to solve the Richards equation for transient flow in 1D homogeneous and heterogeneous soil profiles. Numerical results for different boundary conditions, initial conditions and soil types are shown to be in good agreement with Warrick's semi-analytical solution and simulations using the HYDRUS-1D software package. Results obtained with the proposed method were far less dependent upon the grid spacing than the HYDRUS-1D finite element solutions

The effects of preferential flow and soil texture on risk assessments of a NORM waste disposal site

This paper investigates the environmental fate of radionuclide decay chains (specially the 238U and 232Th series) being released from a conventional mining installation processing ore containing natural occurring radioactive materials (NORMs). Contaminated waste at the site is being disposed off in an industrial landfill on top of a base of earth material to ensure integrity of the deposit over relatively long geologic times (thousands of years). Brazilian regulations, like those of many other countries, require a performance assessment of the disposal facility using a leaching and off-site transport scenario. We used for this purpose the HYDRUS-1D software package to predict long-term radionuclide transport vertically through both the landfill and the underlying unsaturated zone, and then laterally in groundwater. We assumed that a downgradient well intercepting groundwater was the only source of water for a resident farmer, and that all contaminated water from the well was somehow used in the biosphere. The risk assessment was carried out for both a best-case scenario assuming equilibrium transport in a fine-textured (clay) subsurface, and a worst-case scenario involving preferential flow through a more coarse-textured subsurface. Results show that preferential flow and soil texture both can have a major effect on the results, depending upon the specific radionuclide involved.Indisponível

Analytical solution of the advection–diffusion transport equation using a change-of-variable and integral transform technique

This paper presents a formal exact solution of the linear advection–diffusion transport equation with constant coefficients for both transient and steady-state regimes. A classical mathematical substitution transforms the original advection–diffusion equation into an exclusively diffusive equation. The new diffusive problem is solved analytically using the classic version of Generalized Integral Transform Technique (GITT), resulting in an explicit formal solution. The new solution is shown to converge faster than a hybrid analytical–numerical solution previously obtained by applying the GITT directly to the advection–diffusion transport equation.Indisponível

Bayesian estimation of the hydraulic and solute transport properties of a small-scale unsaturated soil column

In this study the hydraulic and solute transport properties of an unsaturated soil were estimated simultaneously from a relatively simple small-scale laboratory column infiltration/outflow experiment. As governing equations we used the Richards equation for variably saturated flow and a physical non-equilibrium dual-porosity type formulation for solute transport. A Bayesian parameter estimation approach was used in which the unknown parameters were estimated with the Markov Chain Monte Carlo (MCMC) method through implementation of the Metropolis-Hastings algorithm. Sensitivity coefficients were examined in order to determine the most meaningful measurements for identifying the unknown hydraulic and transport parameters. Results obtained using the measured pressure head and solute concentration data collected during the unsaturated soil column experiment revealed the robustness of the proposed approach.Indisponível

Improving wheat (Triticum aestivum L.) antioxidative defense mechanisms against salinity stress by exogenous application of potassium silicate

The primary objective of this study was to investigate the beneficial effects of seed priming and foliar spray of potassium silicate on antioxidant activities under different salinity levels, thereby potentially improving wheat growth. Seeds were soaked into solutions containing potassium silicate (K2SiO3, 1.5 mM) for 6 h, while foliar spray with K2SiO3 (4 mM) was applied at the early and the late stages of tillering. Lake Urmia water was used to prepare salinity levels of 0, 3, 5, 8, 10, 12, and 14 dS m −1. For such traits as anthocyanin, catalase, ascorbate peroxidase, guaiacol peroxidase, and superoxide dismutase activity, an initial increase was observed at lower salinity levels; higher salinities subsequently decreased these traits or they remained mostly constant. Salinity also increased phenol, malondialdehyde, hydrogen peroxide, and polyphenol oxidase, but decreased flavonoid, nitrate content, and nitrate reductase activity. Seed priming and foliar spray provided effective approaches to reduce reactive oxygen species (ROS) manifestation in wheat grown under saline conditions. The improved antioxidant defense abilities by seed priming and foliar spray alleviated the oxidative damage of proteins and lipids and improved nitrate content and nitrate reductase activity

Environmental Impact Assessment of Liquid Waste Ponds in Uranium Milling Installations

A detailed environmental impact assessment is required in many countries when considering the disposal of wastes containing radioactive materials. In this paper we present hybrid numerical-analytical solutions for the subsurface transport of radioactive contaminant decay chains that may be used for such an assessment. The model involves the advective–dispersive transport of multiple radionuclide species within separate but coupled saturated and unsaturated soil domains. The resulting partial differential equations were solved using the Generalized Integral Transform Technique to yield analytical expressions for the concentration distributions versus distance, and analytical or numerical solutions as a function of time. The potential of the hybrid modeling approach is illustrated by means of an environmental impact assessment of an uranium milling liquid waste pond near Caetité, Brazil. Calculated radionuclide concentration distributions were for this purpose used in subsequent radiation dose calculations. Several scenarios were analyzed, including operational, human intrusion, farmer/road builder, and leaching scenarios. Numerical results show that for all scenarios analyzed the total dose is less than the Brazilian regulatory dose limit of 0.3 mSv/year above natural background for a period of 10,000 years.Indisponível