Daniel Fernàndez-Garcia

• PhD Environmental Science and Engineering
• Professor (Full) at Polytechnic University of Catalonia

Random walk particle tracking methodologies to simulate solute transport of conservative species constitute an attractive alternative for their computational efficiency and absence of numerical dispersion. Yet, problems stemming from the reconstruction of concentrations from particle distributions have typically prevented its use in reactive transp...

Mixing is increasingly recognized as a critical process for understanding and modeling reactive transport. Yet, mixing is hard to characterize because it depends nonlinearly on concentrations. Visualization of optical tracers in the laboratory at high spatial and temporal resolution can help advance the study of mixing processes. The solute distrib...

We review the basic mathematical concepts of random walk particle tracking (RWPT) and its advantages and limitations. Three different numerical approaches to overcome the local mass conservation problem of the random walk methodology are examined: (i) the interpolation method, (ii) the reflection principle, and (iii) the generalized stochastic diff...

The reconstruction of smooth density fields from scattered data points is a procedure that has multiple applications in a variety of disciplines, including Lagrangian (particle-based) models of solute transport in fluids. In random walk particle tracking (RWPT) simulations, particle density is directly linked to solute concentrations, which is norm...

The reconstruction of smooth density fields from scattered data points is a procedure that has multiple applications in a variety of disciplines, including Lagrangian (particle-based) models of solute transport in fluids. In random walk particle tracking (RWPT) simulations, particle density is directly linked to solute concentrations, which is norm...

Soil moisture measurements are needed in a large number of applications such as hydro-climate approaches, watershed water balance management and irrigation scheduling. Nowadays, different kinds of methodologies exist for measuring soil moisture. Direct methods based on gravimetric sampling or time domain reflectometry (TDR) techniques measure soil...

The particle support volume is crucial for simulating reactive transport with Lagrangian methods as it dictates the interaction among particles. Assuming that it is constant in space, the particle support volume can be selected by means of kernel density estimation theory, an approach that has been shown to provide accurate estimates in simple setu...

Pumping tests are performed during aquifer characterization to gain conceptual understanding about the system through diagnostic plots and to estimate hydraulic properties. Recovery tests consist of measuring head response in observation and/or pumping wells after pumping termination. They are especially useful when the pumping rate cannot be accur...

Soil moisture measurements are needed in a large number of applications such as climate change, watershed water balance and irrigation management. One of the main characteristics of this property is that soil moisture is highly variable with both space and time, hindering the estimation of a representative value. Deciding how to measure soil moistu...

https://authors.elsevier.com/a/1W~Jt16J1mlPA9 . A better understanding of microbial dynamics in porous media may lead to improvements in the design and management of a number of technological applications, ranging from the degradation of contaminants to the optimization of agricultural systems. To this aim, there is a recognized need for predicting...

In recent years a large body of literature has been devoted to study reactive transport of solutes in porous media based on pure Lagrangian formulations. Such approaches have also been extended to accommodate second-order bimolecular reactions, in which the reaction rate is proportional to the concentrations of the reactants. Rather, in some cases,...

Measured (or empirically fitted) reaction rates at groundwater remediation sites are typically much lower than those found in the same material at the batch- or laboratory-scale. The reduced rates are commonly attributed to poorer mixing at the larger scales. A variety of methods have been proposed to account for this scaling effect in reactive tra...

Improving degradation rates of emerging organic compounds (EOCs) in groundwater is still a challenge. Although their degradation is not fully understood, it has been observed that some substances are preferably degraded under specific redox conditions. The coupling of Managed Aquifer Recharge with soil aquifer remediation treatment, by placing a re...

The accumulation of biofilms in porous media is likely to influence the overall hydraulic properties and, consequently, a sound understanding of the process is required for the proper design and management of many technological applications. In order to bring some light into this phenomenon we present a mechanistic model to study the variably satur...

Most methods for hydraulic test interpretation rely on a number of simplified assumptions regarding the homogeneity and isotropy of the underlying porous media. This way, the actual heterogeneity of any natural parameter, such as transmissivity (T), is transferred to the corresponding estimates in a way heavily dependent on the interpretation metho...

When laboratory-measured chemical reaction rates are used in simulations at the field-scale, the models typically overpredict the apparent reaction rates. The discrepancy is primarily due to poorer mixing of chemically distinct waters at the larger scale. As a result, realistic field-scale predictions require accurate simulation of the degree of mi...

We address modern topics of Stochastic Hydrogeology from their potential relevance to real modeling efforts at the field scale. While the topics of stochastic hydrogeology and numerical modelling have become routine in hydrogeological studies, non-deterministic models have not yet permeated into practitioners. We point out a number of limitations o...

A GIS-based software platform was developed to arrange all the available hydrogeochemical data into a comprehensive structure and provide support for its proper storage, management, analysis and interpretation. This platform is composed of a geospatial database and a set of analytical instruments integrated in a graphical user interface that coordi...

In this chapter we take a look at the presence of emerging organic compounds (EOCs) in groundwater, with emphasis in sources of pollution, processes affecting the spatial and temporal concentration of these compounds as they move through the aquifer and interact with the soil. Emphasis is placed on the combination of transport mechanisms and physic...

The interplay between the spatial variability of the aquifer hydraulic properties, mass transfer due to sub-grid heterogeneity and chemical reactions often complicates reactive transport simulations. It is well documented that hydro-biochemical properties are ubiquitously heterogeneous and that diffusion and slow advection at the sub-grid scale typ...

Random walk particle tracking methods are a computationally efficient family of methods to solve reactive transport problems. While the number of particles in most realistic applications is in the order of 106-109, the number of reactive molecules even in diluted systems might be in the order of fractions of the Avogadro number. Thus, each particle...

Facies delineation is defined as the separation of geological units with distinct intrinsic characteristics (grain size, hydraulic conductivity, mineralogical composition). A major challenge in this area stems from the fact that only a few scattered pieces of hydrogeological information are available to delineate geological facies. Several methods...

While particle tracking techniques are often used in risk frameworks, the number of particles needed to properly derive risk metrics such as average concentration for a given exposure duration is often unknown. If too few particles are used, error may propagate into the risk estimate. In this work, we provide a less error-prone methodology for the...

The increasing presence of toxic chemicals released in the subsurface has led to a rapid growth of social concerns and the need to develop and employ models that can predict the impact of groundwater contamination on human health risk under uncertainty. Monitored natural attenuation is a common remediation action in many contamination cases. Howeve...

We monitored oxygen concentration through the unsaturated zone in a mid-scale (1 m high) laboratory soil lysimeter subject to short wetting and drying cycles simulating a high permeable shallow aquifer recharged by river water. Ten cycles of different duration were performed during a period of 85 days. Measurements of oxygen in the liquid and the g...

The often observed tailing of tracer breakthrough curves is caused by a multitude of mass transfer processes taking place over multiple scales. Yet, in some cases it is convenient to fit a transport model with a single-rate mass transfer coefficient that lumps all the non-Fickian observed behavior. Since mass transfer processes take place at all ch...

The analysis of breakthrough curves (BTCs) is of interest in hydrogeology as a way to parameterize and explain processes related to anomalous transport. Classical BTCs assume the presence of a single peak in the curve, where the location and size of the peak and the slope of the receding limb has been of particular interest. As more information is...

A software platform (QUIMET) was developed to improve the sorting, analysis, calculations, visualizations, and interpretations of hydrogeochemical data in a GIS environment. QUIMET is composed of a geospatial database plus a set of tools specially designed for graphical and statistical analysis of hydrogeochemical data. The geospatial database has...

Modeling multi-species reactive transport in natural systems with strong heterogeneities and complex biochemical reactions is a major challenge for assessing groundwater polluted sites with organic and inorganic contaminants. A large variety of these contaminants react according to serial-parallel reaction networks commonly simplified by a combinat...

Infiltration through sediments is linked to complex biogeochemical processes occurring at small spatial scales, often leading to a progressive reduction in infiltration rates due to microbial growth and/or mechanical clogging. Unraveling the linkage between microbial dynamics and water infiltration in a heterogeneous medium is of concern in artific...

Aquifer hydraulic properties such as hydraulic conductivity (K) are ubiquitously heterogeneous and typically only a statistical characterization can be sought. Additionally statistical anisotropy at typical characterization scales is the rule. Thus, regardless of the processes governing solute transport at the local (pore) scale, transport becomes...

Construction of underground structures (e.g., subway lines, railways and highway tunnels) is inherently hazardous, posing risks to both workers and machinery at the site and to surrounding buildings. The presence of groundwater may increase these risks. We develop a general probabilistic risk assessment (PRA) framework to quantify risks driven by g...

Particle tracking methods to simulate solute transport deal with the issue of having to reconstruct smooth concentrations from a limited number of particles. This is an error-prone process that typically leads to large fluctuations in the determined late-time behavior of breakthrough curves (BTCs). Kernel density estimators (KDE) can be used to aut...

We study the time dependent interaction between hydrogeological and exposure parameters in daily dose predictions due to exposure of humans to groundwater contamination. Dose predictions are treated stochastically to account for an incomplete hydrogeological and geochemical field characterization, and an incomplete knowledge of the physiological re...

[1] Anomalous transport in advection-dominated convergent flow tracer tests can occurs due to small-scale heterogeneities in aquifer hydraulic properties. These result in fluctuations of the groundwater velocity field and complex connectivity patterns between injection and extraction wells. While detailed characterization of heterogeneity is often...

Considering complex physical and reactive processes is necessary to a trustable plume behavior prediction. However, complexity is often synonym of inefficiency and numerical problem for existing model. We present an efficient particle method to simulate plumes evolution moved by advection-dispersion and affected by network reactions and multirate-m...

Breakthrough curves (BTCs) usually show asymmetric shapes, with marked tailing. This is particularly relevant in convergent-flow tracer tests (CFTTs). It is found that late-time BTCs frequently scales as a power-law function with a slope of -1 in a log-log plot. In tests performed in heterogeneous sandy aquifers with conservative tracers, the key l...

Memory functions and mass transfer models have been widely used to simulate non-Fickian transport in naturally occurring heterogeneous formations. Yet, the link between the heterogeneous structure and the memory function parameters is largely unknown under convergent flow conditions driven by a pumping well. In this work we evaluate the impact of a...

For typical solute transport applications using particle tracking algorithms, models are run with a limited number of particles and the estimation of the travel time density becomes an error-prone problem. Densities are however needed in groundwater applications, for instance to understand mixing, reactions and other type of phenomena occurring in...

In this work, we evaluate the value of hydrogeological information on the assessment of the risk of contamination of a pumping well operating in a heterogeneous aquifer. Our aim is to statistically characterize the mass fraction of the contaminant recovered at the well and its corresponding arrival time. We do so by investigating the role of the ke...

Technical guide for the evaluation of the subsoil problems associated with organochlorine compounds

The area involving the Lower Valley and Delta of the Llobregat River has a long-standing tradition in Managed Artificial Recharge (MAR) activities. For more than 50 years different MAR activities have taken place, and at present the area provides a catalog of methodologies put into practice. This chapter provides an overview of artificial recharge...

Master Thesis

We addressed the value of hydrogeological information on the assessment of the risk that an operating pumping well is polluted. The work considered a heterogeneous aquifer and focused on the statistical characterization of the contaminant mass fraction from a diffused source recovered at the well and the solute arrival times. We explored the role o...

For risk assessment and adequate decision making regarding remediation strategies in contaminated aquifers, solute fate in the subsurface must be modeled correctly. In practical situations, hydrodynamic transport parameters are obtained by fitting procedures, that aim to mathematically reproduce solute breakthrough (BTC) observed in the field durin...

Facies delineation or reconstruction is defined as the separation of geological units with distinct intrinsic characteristics (i.e. grain size, hydraulic conductivity, mineralogical composition, etc.). It is a major challenge (for scientists, technicians, stake holders, among others) when just a few scattered pieces of information are available. Se...

Since the middle of the previous century, DNAPL contamination is a clear identified problem for the management of urban and suburban groundwater resources quality. Random walk particles tracking methods showed their legitimacy and efficiency for modeling reactive transport especially thanks to the absence of numerical dispersion and computational e...

Capturing the effects of aquifer heterogeneity is key to evaluating risk in groundwater related problems. In this work we analyze the potential failure of remediation due to the rebound of concentrations driven by back diffusion. Withdrawing polluted water from a contaminated aquifer by pumping is one of the most common components of many remediati...

The efficiency of artificial surface ponds (SPs) for managed aquifer recharge (MAR) is mostly controlled by the topmost portion of the soil. The most significant soil property controlling recharge is the infiltration capacity (I-c), which is highly variable in space. Assessing its spatial distribution in detail is prohibitive in practice due to hig...

Aquifer artificial recharge from surface infiltration ponds is often conducted to replenish depleted aquifers in arid and semi-arid zones. Physical and bio-geochemical clogging decreases the host soil’s infiltration capacity, which has to be restored with periodic maintenance activities. We develop a probabilistic modeling framework that quantifies...

Upon their release into the subsurface, non-aqueous phase liquids (NAPLs) dissolve slowly in groundwater and/or volatilize in the vadose zone threatening the environment and public health over extended periods of time. The failure of a treatment technology at any given site is often due to the unnoticed presence of dissolved NAPL trapped in low per...

In this paper the solute retention properties of crystalline fractured rocks due to mixing-induced geochemical reactions are studied. While fractured media exhibit paths of fast flow and transport and thus short residence times for conservative solutes, at the same time they promote mixing and dilution due to strong heterogeneity, which leads to sh...

The performance of managed artificial recharge (MAR) facilities by means of surface ponds (SP) is controlled by the temporal evolution of the global infiltration capacity I c of topsoils. Cost-effective maintenance operations that aim to maintain controlled infiltration values during the activity of the SP require the full knowledge of the spatio-...

Despite the fact that the prognoses on the availability of resources related to different climate scenarios have been already formulated, the complex hydrological and biogeochemical reactions taking place in different compartments in natural environmental media are poorly understood, especially regarding the interactions between water bodies, and t...

Decision makers require long term effective hydraulic criteria to optimize the design of artificial recharge ponds. However, uncontrolled multiscale pore clogging effects on heterogeneous soils determines uncertainties which must be quantified. One of the most remarkable effect is the reduction of infiltration capacity over time, which affect the q...

In the framework of safety assessment studies for geological disposal, site-descriptive models are powerful inter-disciplinary tools aiming at supporting regulatory decision making as well as providing input for repository engineering activities. Striking aspects of these kinds of models are their very large temporal and spatial modeling scales and...

Heterogeneity is key in evaluating risk in environmental related problems. In this work we analyze the potential failure of the endpoint of remediation due to a rebound of concentrations driven by back diffusion. Most typically, pumping out the polluted water is one of the main components of any remediation action. Pumping proceeds until concentrat...

Random walk particle tracking methodologies to simulate solute transport of conservative species constitute an attractive alternative for their computational efficiency and absence of numerical dispersion. Yet, problems stemming from the reconstruction of concentrations from particle distributions have typically prevented its use in reactive transp...

The release of non-aqueous phase liquids (NAPLs) such as petroleum hydrocarbons and chlorinated solvents in the subsurface is a severe source of groundwater and vapor contamination. Because these liquids are essentially immiscible due to low solubility, these contaminants get slowly dissolved in groundwater and/or volatilized in the vadoze zone thr...

We provide a quantitative interpretation of the column experiment reported by Gramling et al. (2002). The experiment involves advection-dominated transport in porous media of three dissolved species, i.e., two reactants undergoing a fast irreversible reaction and the resulting product. The authors found that their observations could not be properly...

Excavation sites such as those associated with the construction of subway lines, railways and highway tunnels are hazardous places, posing risks to workers, machinery and surrounding buildings. Many of these risks can be groundwater related. In this work we develop a general framework based on a probabilistic risk assessment (PRA) to quantify such...

We studied an irreversible bi-molecular reactive transport experiment performed by Gramling et al. [2002]. In this experiment, colorimetric reactions between CuSO4 and EDTA4- were measured in a laboratory column filled with cryolite. After pre-saturating the system with EDTA4- (denoted as species B), CuSO4 (denoted as A) was injected as a step inpu...

The response of coastal aquifers to sea-level fluctuations, notably tides, is known to contain much information about hydraulic parameters. We performed sensitivity analyses to assess how much, about what and where this information can be best obtained. It is well known that the response to harmonic fluctuations (and many harmonics can be superimpo...

We present a method for the stochastic simulation of point-to-point transport connectivity honoring data from three types of information: (a) travel time estimates obtained from field tracer tests; (b) estimates of flow connectivity indicators obtained from the relatively fast or slow flow response that is observed at a point location given the flo...

The release of non-aqueous phase liquids (NAPLs) such as petroleum hydrocarbons and chlorinated solvents in the subsurface is a severe source of groundwater and vapor contamination. Because these liquids are essentially immiscible due to low solubility, these contaminants get slowly dissolved in groundwater and/or volatilized in the vadoze zone thr...

Many groundwater related problems involve artificial/engineered systems build within a complex natural geological medium. Examples include water supply, tunnels, and remediation efforts. Potential failure of a groundwater system can be defined as insufficient quality or quantity of water available for a given use at a given time. Such failures can...

The storage of contaminant material in superficial or sub-superficial repositories, such as tailing piles for mine waste or disposal sites for low and intermediate nuclear waste, poses a potential threat for the surrounding biosphere. The minimization of these risks can be achieved by supporting decision-makers with quantitative tools capable to in...

Construction of subterranean excavations in densely populated areas is inherently hazardous. The number of construction sites (e.g., subway lines, railways and highway tunnels) has increased in recent years. These sites can pose risks to workers at the site as well as cause damage to surrounding buildings. The presence of groundwater makes the exca...

The choice of an adequate large-scale conceptual transport model constitutes a major challenge associated with the upscaling of solute transport. Among the different alternatives to the classical advection-dispersion model, the (multirate) mass transfer model has been proposed as a valuable and convenient alternative to model the large-scale behavi...

1] Heterogeneity of subsurface environments and insufficient site characterization are some of the reasons why decisions about groundwater exploitation and remediation have to be made under uncertainty. A typical decision maker chooses between several alternative remediation strategies by balancing their respective costs with the probability of the...

This paper presents an approach conducive to an evaluation of the probability density function (pdf) of spatio-temporal distributions of concentrations of reactive solutes (and associated reaction rates) evolving in a randomly heterogeneous aquifer. Most existing approaches to solute transport in heterogeneous media focus on providing expressions f...

Transport of reactive species in the subsurface is driven by mixing processes. Whenever all reactions can be considered in chemical instantaneous equilibrium, a multiespecies reactive transport problem can be fully defined in terms of mixing driven conservative quantities, termed components, and the spatio-temporal distribution of reaction rates. M...

The usual observation of anomalous (non-Fickian) transport in the field, manifested by peaked concentration profiles with pronounced tailing, has questioned the use of the classical advection-dispersion equation (ADE) to model transport phenomena at the usual computational scale of a numerical model. In this context, we evaluate the use of (multi-r...

Connectivity of high/low-permeability areas has been recognized to significantly impact groundwater flow and solute transport. The task of defining a rigorous quantitative measure of connectivity for continuous variables has failed so far, and thus there exist a suite of connectivity indicators which are dependent on the specific hydrodynamic proce...

Transport of reactive species in the subsurface is driven by mixing processes. Whenever all reactions can be considered in chemical instantaneous equilibrium, a multiespecies reactive transport problem can be fully defined in terms of mixing driven conservative quantities, termed components, and the spatio-temporal distribution of reaction rates. M...

Chemical species are advected by water and undergo mixing processes due to effects of local diffusion and/or dispersion. In turn, mixing causes reactions to take place so that the system can locally equilibrate. In general, a multicomponent reactive transport problem is described through a system of coupled non-linear partial differential equations...

We analyze the relative importance of the selection of (1) the geostatistical model depicting the structural heterogeneity of an aquifer, and (2) the basic processes to be included in the conceptual model, to describe the main aspects of solute transport at an experimental site. We focus on the results of a forced-gradient tracer test performed at...

Transport of reactive species in the subsurface is driven by mixing processes. Quantification of the mixing rate is, therefore, the basis for a proper characterization of the fate of pollutants in geochemically active environments. We consider the case of an anisotropic correlated random field, with perfect correlation in the horizontal plane, whil...

Efficiency constraints force the use of a coarse discretization of the numerical transport model compared with the detailed scale required for the most adequate description of the physical properties. Upscaling encompasses the methods that transfer small-scale information to the computational scale. The loss of small-scale information of aquifer pr...

We evaluate the benefits of using an alternative upscaled solute transport model in making numerical predictions of contaminant transport in heterogeneous formation. The transport model is based on the concept of mass transfer memory functions, which are used to represent the unresolved mass exchange between highly mobile and less mobile zones occu...

Random walk particle tracking methodologies to simulate solute transport constitute an attractive technique for their computational efficiency and absence of numerical dispersion. Yet, the complexity of implementing mass transfer processes and chemical speciation into transport codes often complicates particle tracking simulations. In this context...

A ventilation experiment (VE) was conducted in a non-lined microtunnel at the Mont Terri Rock Laboratory under well control conditions to evaluate in situ the consequences of desaturation induced by ventilation in consolidated Opalinus Clay rock. These investigations are carried out in the framework of the feasibility studies of radioactive waste u...

Description of solute transport connectivity between different aquifer locations is one of the major challenges for making reliable transport predictions in risk analysis. For instance, accurate representation of the pathway that connects the contamination plume with a given exposure location will largely dominate model predictions. Unfortunately,...

Chemical reactions are driven by disequilibrium, which in several reactive transport problems is caused by mixing. Therefore, quantification of the mixing rate is essential for evaluating the fate of solutes in natural systems, such as rivers and aquifers. Traditionally reactions rates are computed numerically by means of mass balances once the con...

We examine three stochastic transport models of the Macrodispersion Experiment (MADE) site using high-resolution conductivity fields derived from a new geostatistical interpretation of the flowmeter data. Evaluation of the spatial continuity of the hydraulic conductivity data revealed a hole effect structure indicating the occurrence of periodic st...

Efficiency constraints in applying numerical models to real problems force the use of a coarse discretization of the domain compared with the detailed scale required for the most adequate description of the system. Upscaling encompasses the methods that transfer small-scale information to the computational scale. The loss of small-scale information...

The complexity of mass transfer processes often complicates solute transport simulations. We present a new approach for the implementation of the multirate mass transfer model into random walk particle tracking. This novel method allows for a spatially heterogeneous distribution of mass transfer coefficients as well as hydrodynamic parameters in th...

The random walk particle tracking method has become an important tool for the uncertainty assessment of solute transport models due to its computational efficiency and the non-existence of numerical dispersion. Yet, in highly heterogeneous aquifers the smoothness assumption of the velocity field, requisite for a correct solution of the random walk...

Even under the simple linear isotherm adsorption model, the parameters controlling adsorption under field conditions are frequently approximated by the values derived from batch experiments. First, the measurement scale and conditions are very different from those at the model scale. Second, the parameters are heterogeneous in space and, at most, t...

Prediction of the fate and transport of dissolved contaminants in groundwater is required in conducting risk analysis and in decision-making in problems involving hazardous waste management and remediation of contaminated sites. In order to make such predictions, it is necessary to estimate dispersivity, which is the aquifer parameter that measures...

Tracer tests designed to estimate field-scale dispersivities are commonly based upon the interpretation of breakthrough curves. Implicitly, no distinction is made between these dispersivity values and those inferred by analyzing the evolution of tracer plumes. Although this assumption is reasonable in ideal homogeneous media, its applicability to c...

Stochastic simulations of solute transport in heterogeneous log10 K random fields were conducted at two different support scales to assess solute transport upscaling methods in the context of nuclear waste disposal. A very fine grid-scale is used to obtain a reference solution of the real problem, which is based on data from the Sellafield site. A...

Conservative and sorptive tracer experiments were conducted in a highly heterogeneous (sigmalnK2 = 1.79) and anisotropic (lambdaH/lambdaV = 3.52) three-dimensional test aquifer under well-controlled laboratory conditions to evaluate the effective conductivity and temporal moments predicted by stochastic theories. The spatial distribution of lnK in...

Parameters estimated from forced-gradient tracer tests are often used to simulate the transport of large contaminant plumes under natural gradients, yet the difference in the flow system and the plume size is important. Recent intermediate-scale experiments and numerical simulations in two dimensions have suggested that dispersivities from forced-g...

Solute transport is largely influenced by the natural heterogeneity in aquifers properties, as a result, dispersion coefficients estimated from commonly used field tracer tests are a function of the measurement scale. Besides the underlying heterogeneous structure of the aquifer, many different factors also have an impact on the scale-dependence of...