Dissolving multicomponent NAPL as a source of contamination in subsurface water is considered. In particular, two processes are analyzed with regard to how they alter inter-species concentration ratios at remote monitoring locations relative to inter-species molar ratios in the NAPL: nonlinear dissolution governed by Raoult’s Law and differential sorption during subsurface transport. An analytic solution for Raoult’s Law-governed dissolution is presented. Separately, it is shown how a variety of 1D analytic transport models for simple boundary conditions may be adapted to use arbitrary time-varying concentrations by use of some properties of Laguerre series. This is combined with the analytic solution so that Raoult’s Law-governed multicomponent NAPL dissolution may be employed as the boundary condition for analytic transport models. A new computer model implementing this technique in an environment of discrete, parallel fractures is presented, and its accuracy verified for specific conditions against an existing code. The new code is applied to a parametric study on the plume separation of PAH and phenolic compounds from the dissolution of creosote. Narrow fracture spacing as well as significant values of matrix organic carbon are seen as particularly conducive to separation of these types of plumes, which in some circumstances may be entirely disjoint. Concentration ratios downgradient are shown largely unrelated to concentration ratios at the source. Finally, a study of PCB speciation is undertaken in fractured rock with known parameters, for which a rigorous, least squares speciation approach is developed. Even at distances of 5 m from the source, given perfect information about the subsurface, it is found not possible to chemically fingerprint a source PCB mixture from a list of three absent a model of the weathering of the NAPL. Both the PCB and creosote studies demonstrate that forensic inference of source compositions from field data is unreliable and the need for coupled dissolution and transport models like the one developed here. / Thesis (Ph.D, Civil Engineering) -- Queen's University, 2012-09-25 21:43:29.04

This international workshop entitled “How to integrate geochemistry at affordable costs into reac-tive transport for large-scale systems” was organized by the Institute of Resource Ecology of the Helmholtz-Zentrum Dresden Rossendorf in Feb-ruary 2020. A mechanistic understanding and building on that an appropriate modelling of geochemical processes is essential for reliably predicting contaminant transport in groundwater systems, but also in many other cases where migration of hazardous substances is expected and consequently has to be assessed and limited. In case of already present contaminations, such modelling may help to quantify the threads and to support the development and application of suitable remediation measures. Typical application areas are nuclear waste disposal, environmental remediation, mining and milling, carbon capture & storage, or geothermal energy production. Experts from these fields were brought together to discuss large-scale reactive transport modelling (RTM) because the scales covered by such pre-dictions may reach up to one million year and dozens of kilometers. Full-fledged incorporation of geochemical processes, e.g. sorption, precipitation, or redox reactions (to name just a few important basic processes) will thus create inacceptable long computing times. As an effective way to integrate geochemistry at affordable costs into RTM different geochemical concepts (e.g. multidimensional look-up tables, surrogate functions, machine learning, utilization of uncertainty and sensitivity analysis etc.) exist and were extensively discussed throughout the workshop. During the 3-day program of the workshop keynote and regular lectures from experts in the field, a poster session, and a radio lab tour had been offered. In total, 40 scientists from 28 re-search institutes and 8 countries participated.

Natural attenuation and biotransformation are processes that can potentially control the transport and enhance the remediation of contaminants in groundwater. It is necessary to develop computer simulations that not only model the physical transport (advection and dispersion) of contaminants, but that can also accurately depict chemical reactions and some of these more complex processes, in order to determine the type and extent of contaminant plumes and to analyze potential remediation strategies. Modeling these systems effectively is becoming possible with a growing understanding of the chemical and biological processes that occur in groundwater. However, more accurate and more involved models come with much higher memory and computational requirements. Parallel processing provides the computational resources needed to employ reactive transport simulations effectively and more efficiently. N2D-H2 is a FORTRAN code that simulates two-dimensional reactive solute transport in groundwater. More specifically, it simulates the biotransformation of nitrate into the end products of denitrification. A parallel version of the N2D-H2 code is developed using the Message-Passing Interface (MPI), a library of sequences and routines that can be called from FORTRAN programs. Using MPI to develop the parallel version of the code involves decomposing the computational domain among processors, defining the computational roles of each processor, and implementing the required communication between processors by using the message-passing procedures that allow the processors to exchange data. Several test problems are developed to analyze the performance of the parallel code. The test problems are used in the benchmarking procedure to demonstrate that the parallel code returns results identical to the sequential code. The CPU time required and the speedup achieved by running the simulation on parallel processors is presented for multiple test problems with varying physical processes and computational grid sizes. For a two-dimensional plume simulation of five solutes, with a finite difference grid of 490 nodes x 99 nodes, the total CPU time is decreased from 410 seconds on one processor to 220 seconds on two processors, and 75 seconds on ten processors. The speedup achieved gets closer to the ideal speedup as the problem size increases. Although the speedup observed with the parallel version of N2D-H2 is not 100% of the ideal speedup because of communication requirements, the parallel simulation demonstrates the benefits of parallel processing and the possibility of expansion that it provides for modeling reactive transport in groundwater. / Master of Science

Leakage of CO₂ saturated fluid along wellbores has critical implications for the feasibility of geologic CO₂ storage. Wells, which are ubiquitous in locations ideal for CO₂ storage, develop leaks (e.g. fractures) for many reasons and at different points in their age. Small leaks pose the most significant risk to geological CO₂ sequestration because they are difficult to detect and provide a direct pathway through which fluid can escape the storage formation. This dissertation shows that due to complex coupling between reaction and flow, leaking wells will tend to self-seal via secondary precipitation of calcium carbonate in the open pathway. Residence time, fluid reactivity, and initial fracture aperture all play a key role in determining the time required to seal the leakage pathway. To test the self-sealing hypothesis, laboratory experiments were conducted to inject reactive fluids into naturally fractured cement. Restriction of the leakage pathway, i.e., the fracture, was inferred from the relationship between flow rate and pressure differential. Precipitation was observed in both constant flow rate and constant pressure differential experiments. In the former precipitation resulted in an increasing pressure differential, while precipitation caused a decrease in flow rate in the latter. Analysis by electron microprobe and x-ray diffraction, and corroborated with effluent chemical analysis, showed that the reacted channel was depleted in calcium and enriched in silicon relative to the original material. The remaining silicon rich material prevents widening of the reacted channel and development a self-enhancing (e.g. wormhole) behavior. Self-limiting behavior is caused by calcium mixing with carbonate ions in high pH slow flow regions where local residence time is large and calcium carbonate is insoluble. Secondary precipitation initially develops next to the reacted channel and then across the fracture surface and is the source of pathway restriction and the self-sealing behavior. Results from the experiments are used to develop a simple analytical model to forecast well scale leakage. Future work is needed to test a broader range of experimental conditions (e.g. brine salinity, cement formulations, cement-earth interface, effect of CO₂ saturation, pressure, and temperature), to improve our understanding of both the fundamental behavior and the leakage model. / text

[pt] O presente trabalho apresenta o desenvolvimento de uma ferramenta para o estudo de problemas de transporte de contaminantes na água subterrânea. No desenvolvimento desta ferramenta foram utilizados métodos de estabilização na solução da equação de transporte e o método dos gradientes conjugados. Os métodos de estabilização buscam superar as limitações impostas pelo número de Peclet. O método dos Gradientes Conjugados, por sua vez, tem como objetivo melhorar a eficiência da solução do sistema de equações decorrente da utilização do método dos elementos finitos na solução do problema de transporte. Desenvolveu-se um modelo de reação química que fosse capaz de lidar com compostos que possuem propriedades de transporte diferentes. O desenvolvimento deste modelo levou à idealização de uma metodologia que abrange outros modelos químicos de uma forma única. Para descrever essa metodologia foi utilizado o paradigma de Orientação a Objetos que deu origem a uma hierarquia de classes capaz de suportar o desenvolvimento de modelos químicos das mais variadas formas. Por fim utilizou-se a ferramenta desenvolvida para avaliar alguns problemas decorrentes da contaminação por HCH da Cidade dos Meninos no município de Duque de Caxias, estado de Rio de Janeiro, e a avaliação da técnica de barreiras reativas como alternativa de remediação de águas subterrâneas contaminadas. / [en] This work presents the development of a tool to study the water contamination problem. This tool involves the utilization of stabilization methods and the utilization of the Conjugate Gradient method. The stabilization methods aims at overcoming the limitations imposed by the grid Peclet number. The Conjugate Gradient method aims at providing an eficient method to solve the system of equations which arises from the application of the finite element method in the solution of the contamination problem. As part of this work a chemical model was developed to simulate the interaction among chemical species involved in the transport process. These species sometimes have different physical properties that have to be taken under consideration by the chemical model. Taking advantage of the Object Oriented paradigm, we proposed a class hierarchy (OOP) to support the development of a chemical model. Finally the developed program was used to study a contamination problem with HCH at Cidade dos Meninos city in Duque de Caxias township, Rio de Janeiro, and to evaluate the reactive barrier design as a remediation method. / [es] Este trabajo presenta el desarrollo de una herramienta para el estudio de problemas de transporte de contaminantes en el agua subterránea. En el desarrollo de esta herramienta se utilizaron métodos de estabilización en la solución de la ecuación de transporte y el método del gradiente conjugado. Los métodos de estabilización intentan superar las limitaciones impuestas por el número de Peclet. El método del Gradiente Conjugados, por su vez, tiene como objetivo mejorar la eficiencia de la solución del sistema de ecuaciones que resulta de utilizar el método de los elementos finitos en la solución del problema de transporte. Se dearrolló un modelo de reacción química para compuestos que poseen propriedades de transporte diferentes. El desarrollo de este modelo tiene como resultado a formulación de una metodología que contiene otros modelos químicos de una forma única. Para describir esa metodología fue utilizado el paradigma de Orientación a Objetos que dió origen a una jerarquía de clases capaz de soportar el desarrollo de modelos químicos de las más variadas formas. Se utilizó la herramienta desarrollada para evaluar algunos problemas provocados por la contaminación por HCH de la Ciudad de los Meninos en el municipio de Duque de Caxias, estado de Rio de Janeiro, y la evaluación de la técnica de barreras reactivas como alternativas de remediación de aguas subterráneas contaminadas.

Carbonate diagenesis encapsulates a wide range of water rock interactions that can occur within many environments and act to modify rock properties such as porosity, permeability, and mineralogical composition. These rock modification processes occur by the supply of reactant-laden fluids to areas where geochemical reactions are thermodynamically and kinetically favoured. As such, understanding the development of diagenesis requires an understanding of both palaeohydrology and geochemistry, both of which have their own complexities. However, within geological systems, both the conditions that control fluid migration and the distribution of thermodynamic conditions can change through time in response to external factors. Furthermore, they are often coupled, with rock modification exercising a control on fluid flow by altering the permeability of sediments. Numerical methods allow the coupling of multiple complex processes within a single mathematical formulation. As such, they are well suited to investigations into carbonate diagenesis, where multiple component subsystems interact. This thesis details the application of four separate types of numerical forward modelling to investigations of diagenesis within two Carboniferous carbonate platforms, the Derbyshire Platform (Northern England) and the Tengiz Platform (Western Kazakhstan). Investigations of Derbyshire Platform diagenesis are primarily concerned with explaining the presence of Pb-mineralisation and dolomitisation observed within the Dinantian carbonate succession. A coupled palaeohydrology and basin-development simulation and a series of geochemical simulations was used to investigate the potential for these products to form as a result of basin-derived fluids being driven into the platform by compaction. The results of these models suggest that this mechanism is appropriate for explaining Pb-mineralisation, but dolomitisation requires Mg concentrations within the basin-derived fluids that cannot be attained. Geothermal convection of seawater was thus proposed as an alternative hypothesis to explain the development of dolomitisation. This was tested using an advanced reactive transport model, capable of considering both platform growth and dolomitisation. The results of this suggests that significant dolomitisation may have occurred earlier on in the life of the Derbyshire Platform than has previously been recognised. An updated framework for the development of diagenesis in the Derbyshire Platform is proposed to incorporate these new insights. The Tengiz platform forms an important carbonate oil reservoir at the northeastern shore of the Caspian Sea. The effective exploitation of any reservoir lies in an understanding of its internal distributions of porosity and permeability. Within carbonate systems, this is critically controlled by the distribution of diagenetic products. A model of carbonate sedimentation and meteoric diagenesis is used to produce a framework of early diagenesis within a sequence stratigraphic context. The studies mentioned above provide a broad overview of the capabilities and applicability of forward numerical models to two data-limited systems. They reveal the potential for these methods to guide the ongoing assessment and development of our understanding of diagenetic systems and also help identify key questions for the progression of our understanding in the future.

Carbon capture and sequestration (CCS) is an engineering-based approach for mitigating excess anthropogenic CO2 emissions. Deep brine aquifers and basalt reservoirs have shown outstanding performance in CO2 storage based on their global widespread distribution and large storage capacity. Capillary trapping and mineral trapping are the two dominant mechanisms controlling the distribution, migration, and transportation of CO2 in deep brine aquifers and basalt reservoirs. Understanding the behavior of CO2 in a storage reservoir under realistic conditions is important for risk management and storage efficiency improvement. As a result, numerical simulations have been implemented to understand the relationship between fluid properties and multi-phase fluid dynamics. However, the physics-based simulations that focus on the uncertainties of fluid flow dynamics are complicated and computationally expensive. Machine learning method provides immense potential for improving computational efficiency for subsurface simulations, particularly in the context of parametric sensitivity. This work focuses on parametric uncertainty associated with multi-phase fluid dynamics that govern geologic CO2 storage. The effects of this uncertainty are interrogated through ensemble simulation methods that implement both physics-based and machine learning modeling strategies. This dissertation is a culmination of three projects: (1) a parametric analysis of capillary pressure variability effects on CO2 migration, (2) a reactive transport simulation in a basalt fracture system investigating the effects of carbon mineralization on CO2 migration, and (3) a parametric analysis based on machine learning methods of simultaneous effects of capillary pressure and relative permeability on CO2 migration. / Doctor of Philosophy / Carbon capture and sequestration (CCS) has been proposed as a technological approach to mitigate the deleterious effects of anthropogenic CO2 emissions. During CCS, CO2 is captured from power plants and then pumped in deep geologic reservoirs to isolate it from the atmosphere. Deep sedimentary formations and fractured basalt reservoirs are two options for CO2 storage. In sedimentary systems, CO2 is immobilized largely by physical processes, such as capillary and solubility trapping, while in basalt reservoirs, CO2 is transformed into carbonate minerals, thus rendering it fully immobilized. This research focuses on how a large range of capillary pressure variabilities and how CO2-basalt reactions affect CO2 migration. Specifically, the work presented utilizes numerical simulation and machine learning methods to study the relationship between capillary trapping and buoyancy in a sandstone formation, as well as the combined effects of capillary pressure and relative permeability on CO2 migration. In addition, the work also identifies a new reinforcing feedback between mineralization and relative permeability during reactive CO2 flow in a basalt fracture network. In aggregate, the whole of this work presents a new, multi-dimensional perspective on the multi-phase fluid dynamics that govern CCS efficacy in a range of geologic formations.

This thesis focuses on solutions of reactive transport problems in porous media. The principle mechanisms of flow and reactive mass transport in porous media are investigated. Global implicit approach (GIA), where transport and reaction are fully coupled, and sequential noniterative approach (SNIA) are implemented into the software OpenGeoSys (OGS6) to couple chemical reaction and mass transport. The reduction scheme proposed by Kräutle is used in GIA to reduce the number of coupled nonlinear differential equations. The reduction scheme takes linear combinations within mobile species and immobile species and effectively separates the reaction-independent linear differential equations from coupled nonlinear ones (i.e. reducing the number of primary variables in the nonlinear system). A chemical solver is implemented using semi-smooth Newton iteration which employs complementarity condition to solve for equilibrium mineral reactions. The results of three benchmarks are used for code verification. Based on the solutions of these benchmarks, it is shown that GIA with the reduction scheme is faster (ca. 6.7 times) than SNIA in simulating homogeneous equilibrium reactions and (ca. 24 times) in simulating kinetic reaction. In simulating heterogeneous equilibrium mineral reactions, SNIA outperforms GIA with the reduction scheme by 4.7 times. / Diese Arbeit konzentriert sich auf die numerische Berechnung reaktiver Transportprobleme in porösen Medien. Es werden prinzipielle Mechanismen von Fluidströmung und reaktive Stofftransport in porösen Medien untersucht. Um chemische Reaktionen und Stofftransport zu koppeln, wurden die Ansätze Global Implicit Approach (GIA) sowie Sequential Non-Iterative Approach (SNIA) in die Software OpenGeoSys (OGS6) implementiert. Das von Kräutle vorgeschlagene Reduzierungsschema wird in GIA verwendet, um die Anzahl der gekoppelten nichtlinearen Differentialgleichungen zu reduzieren. Das Reduzierungsschema verwendet Linearkombinationen von mobilen und immobile Spezies und trennt die reaktionsunabhngigen linearen Differentialgleichungen von den gekoppelten nichtlinearen Gleichungen (dh Verringerung der Anzahl der Primärvariablen des nicht-linearen Gleichungssystems). Um die Gleichgewichtsreaktionen der Mineralien zu berechnen, wurde ein chemischer Gleichungslaser auf Basis von ”semi-smooth Newton-Iterations” implementiert. Ergebnisse von drei Benchmarks wurden zur Code-Verifikation verwendet. Diese Ergebnisse zeigen, dass die Simulation homogener Equilibriumreaktionen mit GIA 6,7 mal schneller und bei kinetischen Reaktionen 24 mal schneller als SNIA sind. Bei Simulationen heterogener Equilibriumreaktionen ist SNIA 4,7 mal schneller als der GIA Ansatz.

Reactive transport modeling is an important approach to understand water dynamics, mass transport and biogeochemical processes from the hillslope to the catchment scale. It has a wide range of applications in the fields of e.g. water resource management, contaminanted site remediation and geotechnical engineering. To simulate reactive transport processes at a hillslope or larger scales is a challenging task, which involves interactions of complex physical and biogeochemical processes, huge computational expenses as well as difficulties in numerical precision and stability. The primary goal of the work is to develop a practical, accurate and efficient tool to facilitate the simulation techniques for reactive transport problems towards hillslope or larger scales. The first part of the work deals with the simulation of water flow in saturated and unsaturated porous media. The capability and accuracy of different numerical approaches were analyzed and compared by using benchmark tests. The second part of the work introduces the coupling of the scientific software packages OpenGeoSys and IPhreeqc by using a character-string-based interface. The accuracy and computational efficiency of the coupled tool were discussed based on three benchmarks. It shows that OGS#IPhreeqc provides sufficient numerical accuracy to simulate reactive transport problems for both equilibrium and kinetic reactions in variably saturated porous media. The third part of the work describes the algorithm of a parallelization scheme using MPI (Message Passing Interface) grouping concept, which enables a flexible allocation of computational resources for calculating geochemical reaction and the physical processes such as groundwater flow and transport. The parallel performance of the approach was tested by three examples. It shows that the new approach has more advantages than the conventional ones for the calculation of geochemically-dominated problems, especially when only limited benefit can be obtained through parallelization for solving flow or solute transport. The comparison between the character-string-based and the file-based coupling shows, that the former approach produces less computational overhead in a distributed-memory system such as a computing cluster. The last part of the work shows the application of OGS#IPhreeqc for the simulation of the water dynamic and denitrification process in the groundwater aquifer of a study site in Northern Germany. It demonstrates that OGS#IPhreeqc is able to simulate heterogeneous reactive transport problems at a hillslope scale within an acceptable time span. The model results shows the importance of functional zones for natural attenuation process. / Modellierung des reaktiven Stofftranports ist ein wichtiger Ansatz um die Wasserströmung, den Stofftransport und die biogeochemischen Prozesse von der Hang- bis zur Einzugsgebietsskala zu verstehen. Es gibt umfangreiche Anwendungsgebiete, z.B. in der Wasserwirtschaft, Umweltsanierung und Geotechnik. Die Simulation der reaktiven Stofftransportprozesse auf der Hangskala oder auf größeren Maßstäbe ist eine anspruchsvolle Aufgabe, da es sich um die Wechselwirkungen komplexer physikalischer und biogeochemischen Prozesse handelt, die riesigen Berechnungsaufwand sowie numerischen Schwierigkeiten bezogen auf die Genauigkeit und die Stabilität nach sich ziehen. Das Hauptziel dieser Arbeit besteht darin, ein praktisches, genaues und effizientes Werkzeug zu entwickeln, um die Simulationstechnik für reaktiven Stofftransport auf der Hangskala und auf größeren Skalen zu verbessern. Der erste Teil der Arbeit behandelt die Simulation der Wasserströmung in gesättigten und ungesättigten porösen Medien. Das Anwendungspotential und die Genauigkeit verschiedener numerischer Ansätze wurden mittels einiger Benchmarks analysiert und miteinander verglichen. Der zweite Teil der Arbeit stellt die Kopplung der wissenschaftlichen Softwarepakete OpenGeoSys und IPhreeqc mit einer stringbasierten Schnittstelle dar. Die Genauigkeit und die Recheneffizienz des gekoppelten Tools OGS#IPhreeqc wurden basierend auf drei Benchmark-Tests diskutiert. Das Ergebnis zeigt, dass OGS#IPhreeqc die ausreichende numerische Genauigkeit für die Simulation reaktiven Stofftransports liefert, welcher sich sowohl auf die Gleichgewichtsreaktion als auch auf die kinetische Reaktion in variabel gesättigten porösen Medien beziehen. Der dritte Teil der Arbeit beschreibt zuerst den Algorithmus der Parallelisierung des OGS#IPhreeqc basierend auf dem MPI (Message Passing Interface) Gruppierungskonzept, welcher eine flexible Verteilung der Rechenressourcen für die Berechnung der geochemischen Reaktion und der physikalischen Prozesse wie z.B. Wasserströmung oder Stofftransport ermöglicht. Danach wurde die Leistungsfähigkeit des Algorithmus anhand von drei Beispielen getestet. Es zeigt sich, dass der neue Ansatz Vorteile gegenüber die konventionellen Ansätzen für die Berechnung von geochemisch dominierten Problemen bringt. Dies ist vor allem dann der Fall, wenn nur eingeschränkter Nutzen aus der Parallelisierung für die Berechnung der Wasserströmung oder des Stofftransportes gezogen werden kann. Der Vergleich zwischen der string- und der dateibasierten Kopplung zeigt, dass die erstere weniger Rechenoverhead in einem verteilten Rechnersystem, wie z.B. Cluster erzeugt. Der letzte Teil der Arbeit zeigt die Anwendung von OGS#IPhreeqc für die Simulation der Wasserdynamik und der Denitrifikation im Grundwasserleiter eines Untersuchungsgebietes in NordDeutschland. Es beweist, dass OGS#IPhreeqc in der Lage ist, reaktiven Stofftransport auf der Hangskala innerhalb akzeptabler Zeitspanne zu simulieren. Die Simulationsergebnisse zeigen die Bedeutung der funktionalen Zonen für die natürlichen Selbstreinigungsprozesse.

L’anomenat drenatge àcid de mina (AMD) ve generat per l’oxidació de sulfurs i és causa major de contaminació d’aigües a nivell mundial. L’arsènic és un del princiapls contaminants laconcentració del qual pot assolir centenars de mgL-1, és a dir, de 5 a 6 ordres de magnitud més gran que el límit de potabilitat per a l’aigua (10μg L-1) establert per la UE en 1998. En aquesta tesi, s’estudia l’impacte de la mobilització de l’arsènic al llarg de descàrregues de drenatge àcid de mina. L’oxidació de sulfurs que contenen arsènic (tal com l’arsenopirita (AsFeS), la pirita rica en arsènic (FeS2) o la marcassita (FeS2) és una de les principals fonts d’alliberament d’arsènic a l’aigua. En la primera part de la tesi, s’ha estudiat la cinètica de dissolució de l’arsenopirita i de la marcassita a pHs àcids i neutre, utilitzant reactors de flux continu, i s’han valorat els efectes del pH, de l’oxigen dissolt i de la temperatura en la dissolució d’ambdós sulfurs. A partir de les velocitats en estat estacionari establertes, es proposen les respectives lleis de dissolució que tenen en compte el lleu i el fort efecte del pH i de l’oxigen dissolt, respectivament, en llur dissolució. La incorporació d’aquestes lleis cinètiques en les bases de dades del codis geoquímics i de transport reactitu permeten fer prediccions molt més realistes. L’impacte mediambiental causat per l’arsènic alliberat a les aigües depèn de la seva atenuació natural. El principal procés que controla el destí i la mobilitat de l’arsènic aquós és l’adsorció de l’arsenat en fases de ferro precipitades. Per tant, cal tenir en compte el paper que juga l’estat d’oxidació de l’arsènic. En la segona part de la tesi, shan estudiat tant l’oxidació de l’arsènic com l’adsorció de l’arsènic. L’oxidació s’estudia en condicions abiòtiques i biòtiques a pH i composició típics d’aigües àcides de mina, fent servir experiments de tipus batch. S’hi mostra com en condicions biòtiques tenen lloc simultàniament l’oxidació de Fe(II) a Fe (III) i d’arsenit a arsenat, de manera que mentre els bacteris governen la primera, el contingut de Fe(III) domina la segona. En condicions abiòtiques, l’oxidació d’arsenit a arsenat en presència de Fe(III) és lenta, tot i que augmenta augmentant la presència de Fe(III) i de clorur amb llum de dia. L’adsorció d’arsènic en llocs d’AMD, i per tant l’atenuació d’arsènic, ocorre mitjançant l’adsorció d’arsenat en precipitats formats per oxi-hidròxids i oxi-hidròxid-sulfats de ferro (principalment schwertmannita (Fe8O8(OH)5.5(SO4)1.25), K-jarosita (KFe3(SO4)2(OH)6) i goetita (FeOOH)). S’han estudiat les capacitats d’adsorció de la jarosita i de la goetita i s’han comparat amb la de la schwertmannita. Amb aquest propòsit es van fer experiments de tipus batch a pH molt àcid i amb mostres sintetitzades de K-jarosita i de goetita. Sense la competència d’altres anions, la capacitat de la jarosita per eliminar arsenat és més alta que la de la goetita. També s’ha vist que la força iònica té un escàs efecte en l’adsorció d’ambdós minerals, però que la presència de sulfat, que és l’anió més abundant en aigües àcides de mina, minva llurs capacitas d’adsorció. Cal conèixer bé els mecanismes dominants que controlen el contingut d’arsènic en les aigües, no només en condicions de laboratori, sinó també en les condicions de camp. Per tant, en la tercera part de la tesi s’han estudiat el processos d’atenuació de l’arsènic en un sistema natural. Amb aquest objetiu s’han caracterizat exhaustivament l’aigua i els sediments del rieron provinent de la mina abandonada Tinto Santa Rosa, situada a la Faixa Pirítica Ibèrica (IPB). La característica dominant de l’aigua del rierol és un descens del pH aigües avall que va acompanyat d’un decreixement sistemàtic de les concentracions de ferro ferrós i de ferro total, d’arsenit i d’arsenat, així com d’arsènic total. A més a més, els sediments de llit mostren contiguts alts d’arsènic. Els principals mecanismes que dominen el destí i la mobilitat de l’arsènic en aquestes aigües de camp són l’oxidació del ferro i de l’arsènic i la precipitatió de compostos de Fe(III) que adsorbeixen l’arsenat. S’ha proposat un model unidimensional de trasnport reactiu, utilitzant el codi PHREEQC, per explicar i quantificar els processos mencionats que han estat estudiats en condicions de laboratori. / Acid mine drainage (AMD) generated by sulfide oxidative dissolution is a major cause of water contamination world-wide. Arsenic is one of the main AMD pollutants whose concentration can reach up to hundreds of mg L-1, i.e. 5-6 orders of magnitude higher than the limit of 10 μg L-1 for potable water established by the European Union in 1998. This thesis is concerned with the impact of arsenic mobilization along AMD discharges. Oxidation of As-bearing sulfides such as arsenopyrite (AsFeS), As-rich pyrite (FeS2) or marcasite (FeS2) is one of the main sources of arsenic release. The first part of this thesis is focused on the dissolution kinetics of arsenopyrite and marcasite at acidic to neutral pH using long term flow-through experiments. The effects of pH, dissolved oxygen and temperature on their dissolution were assessed. The respective dissolution rate laws were proposed on the basis of the steady-state rates, taking into consideration the slight pH effect and the strong dissolved oxygen effect on dissolution. The incorporation of these rate laws into the kinetic databases of geochemical and reactive transport codes allows us to obtain better realistic simulations. The environmental impact of released arsenic into waters depends on its natural attenuation. The arsenic oxidation state is considered given that the main process that controls the fate and mobility of aqueous arsenic is arsenate sorption onto precipitated Fe-phases. The second part of the thesis discusses arsenic oxidation and arsenic sorption. Oxidation was studied by means of batch experiments under abiotic and biotic conditions at typical AMD water pH and water composition. Simultaneous oxidation of Fe(II) to Fe(III) and arsenite to arsenate occurs under biotic conditions, the former mediated by bacteria, and the latter by the presence of Fe(III). Under abiotic conditions, oxidation of arsenite to arsenate in the presence of Fe(III) is slow, but is enhanced by increasing dissolved Fe(III) and chloride concentrations in the presence of light. Arsenic sorption at AMD sites, and hence arsenic attenuation, occurs via arsenate sorption on new iron-oxyhydroxide and iron-oxyhydroxide-sulphate precipitates (mainly, schwertmannite (Fe8O8(OH)5.5(SO4)1.25), jarosite (KFe3(SO4)2(OH)6) and goethite (FeOOH)). The sorption capacity of goethite and jarosite was studied and compared with the one reported for schwertmannite. To this end, batch experiments were conducted using synthetic powders of K-jarosite and goethite at highly acidic pH. In the absence of competitive effects of other anions, K-jarosite presented better removal efficiency for arsenate, and ionic strength and pH had little effect on the sorption capacity of the two minerals. In contrast, these sorption capacities diminished considerably in the presence of sulfate, which is the main anion in AMD waters. A deeper understanding of the dominant mechanisms controlling arsenic content in waters demands the study of the processes not only under laboratory but also under natural conditions. Accordingly, the third part of this thesis deals with the arsenic attenuation processes in a natural system. To this end, the acidic water and sediments of the abandoned Tinto Santa Rosa mine discharge, located in the Iberian Pyritic Belt, were studied. The most striking feature of the water was a pH decrease accompanied by a systematic decrease in ferrous iron, total iron, arsenite, arsenate and total arsenic concentration. Additionally, bed-stream sediments showed high arsenic contents. The main processes that control the fate and mobility of arsenic in waters in the field were iron and arsenic oxidation, precipitation of Fe(III)- minerals and sorption of As(V) onto them. A 1-D reactive transport model using the PHREEQC code was used to explain and quantify the aforementioned processes that had been studied previously under laboratory conditions.