A general description of the mathematical and numerical formulations used in modern numerical reactive transport codes relevant for subsurface environmental simulations is presented. The formulations ...are followed by short descriptions of commonly used and available subsurface simulators that consider continuum representations of flow, transport, and reactions in porous media. These formulations are applicable to most of the subsurface environmental benchmark problems included in this special issue. The list of codes described briefly here includes PHREEQC, HPx, PHT3D, OpenGeoSys (OGS), HYTEC, ORCHESTRA, TOUGHREACT, eSTOMP, HYDROGEOCHEM, CrunchFlow, MIN3P, and PFLOTRAN. The descriptions include a high-level list of capabilities for each of the codes, along with a selective list of applications that highlight their capabilities and historical development.
In this paper we describe the OpenGeoSys (OGS) project, which is a scientific open-source initiative for numerical simulation of thermo-hydro-mechanical-chemical processes in porous media. The basic ...concept is to provide a flexible numerical framework (using primarily the Finite Element Method (FEM)) for solving multifield problems in porous and fractured media for applications in geoscience and hydrology. To this purpose OGS is based on an object-oriented FEM concept including a broad spectrum of interfaces for pre- and postprocessing. The OGS idea has been in development since the mid-eighties. We provide a short historical note about the continuous process of concept and software development having evolved through Fortran, C, and C++ implementations. The idea behind OGS is to provide an open platform to the community, outfitted with professional software-engineering tools such as platform-independent compiling and automated benchmarking. A comprehensive benchmarking book has been prepared for publication. Benchmarking has been proven to be a valuable tool for cooperation between different developer teams, for example, for code comparison and validation purposes (DEVOVALEX and CO
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BENCH projects). On one hand, object-orientation (OO) provides a suitable framework for distributed code development; however, the parallelization of OO codes still lacks efficiency. High-performance-computing efficiency of OO codes is subject to future research.
•Fate of nitrate in physically and chemically heterogeneous aquifer is investigated.•The trend of nitrate removal changes and influential aquifer factors are discussed.•The most influential aquifer ...factor affecting nitrate reduction can change over time.•Physical heterogeneity substantially influences the nitrate removal for a short-term.•Nitrate removals can be sustained by chemical heterogeneity for a longer time scale.
Nitrate reduction reactions in groundwater systems are strongly influenced by various aquifer heterogeneity factors that affect the transport of chemical species, spatial distribution of redox reactive substances and, as a result, the overall nitrate reduction efficiency. In this study, we investigated the influence of physical and chemical aquifer heterogeneity, with a focus on nitrate transport and redox transformation processes. A numerical modeling study for simulating coupled hydrological-geochemical aquifer heterogeneity was conducted in order to improve our understanding of the influence of the aquifer heterogeneity on the nitrate reduction reactions and to identify the most influential aquifer heterogeneity factors throughout the simulation. Results show that the most influential aquifer heterogeneity factors could change over time. With abundant presence of electron donors in the high permeable zones (initial stage), physical aquifer heterogeneity significantly influences the nitrate reduction since it enables the preferential transport of nitrate to these zones and enhances mixing of reactive partners. Chemical aquifer heterogeneity plays a comparatively minor role. Increasing the spatial variability of the hydraulic conductivity also increases the nitrate removal efficiency of the system. However, ignoring chemical aquifer heterogeneity can lead to an underestimation of nitrate removals in long-term behavior. With the increase of the spatial variability of the electron donor, i.e. chemical heterogeneity, the number of the “hot spots” i.e. zones with comparably higher reactivity, should also increase. Hence, nitrate removal efficiencies will also be spatially variable but overall removal efficiency will be sustained if longer time scales are considered and nitrate fronts reach these high reactivity zones.
The open-source scientific software packages OpenGeoSys and IPhreeqc have been coupled to set up and simulate thermo-hydro-mechanical-chemical coupled processes with simultaneous consideration of ...aqueous geochemical reactions faster and easier on high-performance computers. In combination with the elaborated and extendable chemical database of IPhreeqc, it will be possible to set up a wide range of multiphysics problems with numerous chemical reactions that are known to influence water quality in porous and fractured media. A flexible parallelization scheme using MPI (Message Passing Interface) grouping techniques has been implemented, which allows an optimized allocation of computer resources for the node-wise calculation of chemical reactions on the one hand and the underlying processes such as for groundwater flow or solute transport on the other. This technical paper presents the implementation, verification, and parallelization scheme of the coupling interface, and discusses its performance and precision.
The International Water Research Alliance Saxony (IWAS) is addressing the global challenges concerning water quality in the areas of drinking water and sanitation, agricultural irrigation and the ...quality of surface and ground waters, as well as developing specific ecosystem-relevant services to be implemented on an exemplary basis in selected model regions. Locations (model regions) have been selected in Eastern Europe (R1), Central and Southeast Asia (R2 and R3), the Middle East (R4) and Latin America (R5) that are representative international regions with respect to climate, land use and demographic change (Ibisch et al., Helmholtz Centre for Environmental Research—UFZ, Department of Aquatic Ecosystem analysis and management, 2013). The causes of water problems and the relevant boundary conditions vary from region to region (Borchardt and Ibisch, Integrated water resources management in a changing world : lessons learnt and innovative perspectives, pp 225, 2013). Mongolia and Vietnam were selected model regions in the first IWAS phase; the research was transferred and continued in one of the cross-cutting projects in IWAS II (Vietnam → capacity development) or in the frame of related project activities in Central Asia (R2 Mongolia, Karthe et al., Environ Earth Sci, doi: 10.1007/s12665-014-3789-1 , 2014). The IWAS consortium exists on scientific institutions like the Technische Universität Dresden and the Helmholtz Centre for Environmental Research—UFZ, Leipzig, as well as partners from industry like the Stadtentwässerung Dresden GmbH (SE-DD), DREBERIS—Dresden consulting for international strategies, and itwh—institute for technical-scientific Hydrology, Hanover. This thematic issue compiles the most important scientific results of the second phase of the IWAS project. The project itself and findings of the first phase were already introduced in a previous special issue by Kalbus et al. (Environ Earth Sci 65:1363–1366, 2012). Main results: The IWAS project is structured by the model regions (R) as well as by cross-cutting activities scenario analysis (Q1), technology development (Q2), governance (Q3) and capacity development (Q4).
Numerical modeling of interacting flow processes between roots and the soil is essential for understanding the influence of different root geometries and types on the hydrosystem. The coupling of two ...software tools enables the analysis of water uptake of plant communities, one modeling the water flow along a network of resistances from the bulk soil along radial soil disks toward the root system up to the root collar, while the second tool is covering the nonlinear dynamics of water flow within soil by a three-dimensional Richards model. Appropriate methods for geometric coupling, fast coupled parameter exchange, and coordinated parallelization have been developed to ensure an efficient functionality. An adaptive time stepping with automatic control guarantees the stability of the solution of nonlinear problems.
The paper presents a modeling strategy as well as simulation results of a designed conservative tracer test in the depleted Altensalzwedel natural gas reservoir in order to know the tracer ...concentration and breakthrough time at the production wells. Krypton is considered as a suitable tracer. The production wells are located in several hundred meters to a few kilometers away from the injection well. The numerical simulation has been performed by using the newly implemented compositional gas flow module of OpenGeoSys (OGS), a scientific open-source simulator for calculation of coupled geohydraulic (single and multiphase flow), transport (heat and mass transfer), geomechanical and geochemical processes. The tracer breakthrough curves at the production wells show that the breakthrough times vary between 2 and 3 years. The model verification is presented by comparing the OGS results with a one-dimensional analytical solution. The numerical results have been verified by code comparison, additionally.
Mechanical aeration is commonly used to improve the overall treatment efficacy of constructed wetlands. However, the quantitative relationships of air flow rate (AFR), water temperature, field oxygen ...transfer and treatment performance have not been analyzed in detail until today. In this study, a reactive transport model based on dual–permeability flow and biokinetic formulations of the Constructed Wetland Model No. 1 (CWM1) was developed and extented to 1) simulate oxygen transfer and treatment performance for organic carbon and nitrogen of two pilot–scale horizontal flow (HF) aerated wetlands (Test and Control) treating domestic sewage, and, 2) to investigate the dependence of oxygen transfer and treatment performance on AFR and water temperature. Both pilot–scale wetlands exhibited preferential flow patters and high treatment performance for chemical oxygen demand (COD) and NH4–N at AFRs of 128–700 L m−2 h−1. A reduction of the AFR in the Test system from 128 to 72 L h−1 m−2 substantially inhibited NH4–N removal. Conservative tracer transport as well as reactive transport of dissolved oxygen (DO), soluble and total chemical oxygen demand (CODs, CODt), NH4–N and NOx–N measured in pilot–scale experiments were simulated with acceptable accuracy (E1¯=0.39±0.26). An equation to estimate the volumetric oxygen transfer coefficient was found to be: kLa,20=0.511ln(AFR). Simulated treatment performance depended on kLa,20 in a non–linear manner. A local sensitivity analysis of the calibrated parameters revealed porosity, hydraulic permeability and dispersion length of the fast flow field as well as kLa,20 as most important. An optimal AFR for a spatially and temporally continuous aeration pattern for treatment wetlands treating similar influent was estimated to 150–200 L h−1 m−2. This study provides insights into aeration mechanisms of aerated treatment wetlands and highlights the benefits of process modeling for in–depth system analysis.
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•Development of a process model for aerated horizontal flow wetlands treating sewage.•Model calibration and validation by pilot–scale experiments.•Aeration controls location and gradients of treatment performance.•O2–transfer coefficient depended logarithmically on air flow rate.•Optimal air flow rate to treat similar influent strengths ≈150–200 L h−1m−2.
Groundwater travel time distributions (TTDs) provide a robust description of the subsurface mixing behavior and hydrological response of a subsurface system. Lagrangian particle tracking is often ...used to derive the groundwater TTDs. The reliability of this approach is subjected to the uncertainty of external forcings, internal hydraulic properties, and the interplay between them. Here, we evaluate the uncertainty of catchment groundwater TTDs in an agricultural catchment using a 3-D groundwater model with an overall focus on revealing the relationship between external forcing, internal hydraulic properties, and TTD predictions. Eight recharge realizations are sampled from a high-resolution dataset of land surface fluxes and states. Calibration-constrained hydraulic conductivity fields (Ks fields) are stochastically generated using the null-space Monte Carlo (NSMC) method for each recharge realization. The random walk particle tracking (RWPT) method is used to track the pathways of particles and compute travel times. Moreover, an analytical model under the random sampling (RS) assumption is fit against the numerical solutions, serving as a reference for the mixing behavior of the model domain. The StorAge Selection (SAS) function is used to interpret the results in terms of quantifying the systematic preference for discharging young/old water. The simulation results reveal the primary effect of recharge on the predicted mean travel time (MTT). The different realizations of calibration-constrained Ks fields moderately magnify or attenuate the predicted MTTs. The analytical model does not properly replicate the numerical solution, and it underestimates the mean travel time. Simulated SAS functions indicate an overall preference for young water for all realizations. The spatial pattern of recharge controls the shape and breadth of simulated TTDs and SAS functions by changing the spatial distribution of particles' pathways. In conclusion, overlooking the spatial nonuniformity and uncertainty of input (forcing) will result in biased travel time predictions. We also highlight the worth of reliable observations in reducing predictive uncertainty and the good interpretability of SAS functions in terms of understanding catchment transport processes.
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