Modelling of natural attenuation processes in groundwater using adaptive and parallel numerical methods.

Biodegradation is an important process contributing to the natural attenuation (NA) of organic contaminants in groundwater. A numerical model was created to describe anaerobic phenol biodegradation data from an aquifer-derived laboratory scale microcosm. The dynamic behaviour of the system was simulated by considering a two-step syntrophic biodegradation model with fermentation and respiration steps, both simulated kinetically, and with hydrogen and acetate as intermediate species, and additionally, other geochemical reactions including aqueous speciation, surface complexation, mineral dissolution and precipitation. The model suggested microbial competition between respiration processes using different electron acceptors was important. In contrast, a partial equilibrium approach, considering only thermodynamics, and not kinetics, for respiration, did not explain the data. The laboratory scale biodegradation model was transferred to a field scale reactive transport model of the phenol plume at Four Ashes, UK. The effects of acclimatisation, toxicity, and bioavailability on microbial kinetics were considered. The simulations suggest that plume core processes are much more important than previously thought, possibly with a greater impact than plume fringe processes. The field scale model was computationally demanding due to the biogeochemical complexity. Two strategies for dealing with high computational demands are (i) parallel processing, where the workload is shared between multiple processors, and (ii) locally adaptive remeshing, where a refined area of the grid tracks moving plume fringes through the domain. A new code was developed using the partial differential equation software toolbox, UG, and tested against other biodegradation simulators. The relative efficiency of parallel, adaptive methods for multispecies biodegradation simulations was measured. It appears, in general, that relatively complex models are required for the realistic, quantitative assessment of NA at field scale, and that parallel, adaptive numerical methods provide appropriate efficiency benefits for such simulations

Scalable parallel simulation of variably saturated flow

In this thesis we develop highly accurate simulation tools for variably saturated flow through porous media able to take advantage of the latest supercomputing resources. Hence, we aim for parallel scalability to very large compute resources of over 105 CPU cores. Our starting point is the parallel subsurface flow simulator ParFlow. This library is of widespread use in the hydrology community and known to have excellent parallel scalability up to 16k processes. We first investigate the numerical tools this library implements in order to perform the simulations it was designed for. ParFlow solves the governing equation for subsurface flow with a cell centered finite difference (FD) method. The code targets high performance computing (HPC) systems by means of distributed memory parallelism. We propose to reorganize ParFlow's mesh subsystem by using fast partitioning algorithms provided by the parallel adaptive mesh refinement (AMR) library p4est. We realize this in a minimally invasive manner by modifying selected parts of the code to reinterpret the existing mesh data structures. Furthermore, we evaluate the scaling performance of the modified version of ParFlow, demonstrating excellent weak and strong scaling up to 458k cores of the Juqueen supercomputer at the Jülich Supercomputing Centre. The above mentioned results were obtained for uniform meshes and hence without explicitly exploiting the AMR capabilities of the p4est library. A natural extension of our work is to activate such functionality and make ParFlow a true AMR application. Enabling ParFlow to use AMR is challenging for several reasons: It may be based on assumptions on the parallel partition that cannot be maintained with AMR, it may use mesh-related metadata that is replicated on all CPUs, and it may assume uniform meshes in the construction of mathematical operators. Additionally, the use of locally refined meshes will certainly change the spectral properties of these operators. In this work, we develop an algorithmic approach to activate the usage of locally refined grids in ParFlow. AMR allows meshes where elements of different size neighbor each other. In this case, ParFlow may incur erroneous results when it attempts to communicate data between inter-element boundaries. We propose and discuss two solutions to this issue operating at two different levels: The first manipulates the indices of the degrees of freedom, While the second operates directly on the degrees of freedom. Both approaches aim to introduce minimal changes to the original ParFlow code. In an AMR framework, the FD method taken by ParFlow will require modifications to correctly deal with different size elements. Mixed finite elements (MFE) are on the other hand better suited for the usage of AMR. It is known that the cell centered FD method used in ParFlow might be reinterpreted as a MFE discretization using Raviart-Thomas elements of lower order. We conclude this thesis presenting a block preconditioner for saddle point problems arising from a MFE on locally refined meshes. We evaluate its robustness with respect to various classes of coefficients for uniform and locally refined meshes

Numerische-stochastische Simulationen für Sanierungsmaßnahmen und Risikobetrachtungen eines urbanen Grundwasserleiters

Groundwater systems are under enormous hazards in urban areas. Urbanization influences the behavior and compositions of the subsurface system. This leads to adverse hydrological, aquifer quality and socio-economic effects which compromise sustainability. The European Environmental Agency (EEA) estimates a number of 100,000 polluted sites in European countries, many of them contaminated with chlorinated hydrocarbons (EEA 2005). To reduce these impacts, an application of a groundwater risk assessment management is essential. The present dissertation contains a contribution to groundwater risk identification of an urban aquifer contaminated with chlorinated ethenes, which dischares to adjacent hydrosystems. In this process, the hydrodynamic impact on a regional chlorinated ethenes dispersal of an urban groundwater and surface water system is quantified. The aim is a determination of spatial probability of concentration occurrences isolines (spcois) on a regional scale under average and extreme conditions. The computation of the spcois is based on steady-state and transient 3D multi-species transport simulations of an unconfined aquifer. For solving the formulated problem, an aquifer reconstruction by coupling of conventional and geo-stochastic simulations were performed to estimate parameter uncertainties. Furthermore, a Direct push technique was applied for a downscale part of the model domain to evaluate the implemented hydraulic parameters of the reconstructed subsurface model. A hydrograph analysis was performed to identify appropriate initial and boundary conditions. The calibrated multi-species transport model was subjected to a Monte Carlo simulation. Seven flow and transport-relevant parameters were ranged n-times from a probability distribution to compute spcoi of the contaminated site. The thesis shows that hydrodynamics represent a crucial risk factor in the field of urban groundwater risk identification. Especially, the pollutant dispersal pattern is affected spatially and temporally. Even through the Monte Carlo approach, a future pollutant passage into the adjacent ecosystems could be identified including its occurrence probabilities. The PhD research is assigned to a source-pathway-receptor approach according to McKnight et al. (2010). Indeed, this approach contains a 3D multi-species transport model including different hydrological dynamics. This approach was implemented in cooperation within the framework of the International Graduate College 802 “Risk Management of Natural and Civilization Hazards on Buildings and Infrastructure”.Die Dissertation befasst sich mit der Beschreibung sowie mit einer mittelfristigen Prognose einer CKW-Kontaminationsgenese in einem urbanen Gebiet (Stadt Braunschweig). Zur Analyse, Beschreibung und Prognose werden in Kombination stochastische und numerische Verfahren eingesetzt, wobei die notwendigen Eingabeparameter direkt im Untersuchungsfeld ermittelt werden. Ziel der Dissertation ist die Ermittlung von räumlichen Auftretenswahrscheinlichkeiten von Schadstoffkonzentrationen im Untergrund und benachbarter Hydrosysteme. Risikobetrachtungen oder Durchführung von Sanierungen setzen voraus, dass die Schadstoffausbreitungen räumlich und zeitlich hinreichend genau prognostiziert werden können. Die räumlichen Auftretenswahrscheinlichkeiten der einzelnen Schadstoffe im Untergrund oder benachbarter Hydrosysteme basiert auf einer Vielzahl von numerischen Realisationen von Strömungs- und Transportberechnungen unter Verwendung von stochastisch generierten kontinuierlichen Parameterfeldern und Untergrundstrukturen unter mittleren und extremen hydrologischen Verhältnissen. Mit der vorliegenden Dissertation wird der Versuch unternommen, mittel- und langfristige Ausbreitungsmuster einer komplexen CKW-Kontamination zu erfassen und zu prognostizieren, wobei ausschließlich im Untersuchungsfeld erhobene Strukturen und Modellparameter verwendet werden. Die Rekonstruktion der komplexen Untergrundstruktur basiert auf einer klassischen Indikator-Variographie, angewandt auf Schichtenverzeichnisse aus Bohrungen. Diese geostatistische Strukturanalyse wurde für einen ausgewählten Bereich mit der HPT-Sondierung (Hydraulic Profiling Tool) abgeglichen. Ziel der geostatistischen Analyse nach dem Ansatz von Journel ist die Beschreibung der räumlichen Struktur für die Implementierung in das Finite-Elemente Berechnungsgitter und der räumlichen Korrelation der hydraulischen und transportrelevanten Parametern für den stochastischen Monte-Carlo-Ansatzes zur Erzeugung von Berechnungs-Ensembles. Basierend auf der Anwendung der Monte Carlo Methode zur Erzeugung von n- Aquiferrealisationen mit veränderten Eingangsparameterfeldern (für zwei Strömungsparameter und fünf Transportparameter) erfolgt eine Ausweisung von Auftretenswahrscheinlichkeiten für ausgewählte CKW-Schadstoffe