Papers by Maarten W. Saaltink
Waste Management, 2011
Lightweight tire derived aggregate (TDA) fills are a proven recycling outlet for waste tires, req... more Lightweight tire derived aggregate (TDA) fills are a proven recycling outlet for waste tires, requiring relatively low cost waste processing and being competitively priced against other lightweight fill alternatives. However its value has been marred as several TDA fills have self-combusted during the early applications of this technique. An empirical review of these cases led to prescriptive guidelines from the ASTM aimed at avoiding this problem. This approach has been successful in avoiding further incidents of self-combustion. However, at present there remains no rational method available to quantify selfcombustion risk in TDA fills. This means that it is not clear which aspects of the ASTM guidelines are essential and which are accessory. This hinders the practical use of TDA fills despite their inherent advantages as lightweight fill. Here a quantitative approach to self-combustion risk evaluation is developed and illustrated with a parametric analysis of an embankment case. This is later particularized to model a reported field self-combustion case. The approach is based on the available experimental observations and incorporates well-tested methodological (ISO corrosion evaluation) and theoretical tools (finite element analysis of coupled heat and mass flow). The results obtained offer clear insights into the critical aspects of the problem, allowing already some meaningful recommendations for guideline revision.
Journal of Contaminant Hydrology
This paper describes the modeling of the hydrogeochemical effects of deep well recharge of oxic w... more This paper describes the modeling of the hydrogeochemical effects of deep well recharge of oxic water into an anoxic pyrite-bearing aquifer. Kinetic expressions have been used for mineral dissolution-precipitation rates and organic matter oxidation. Hydrological and chemical parameters of the model were calibrated to field measurements. The results showed that oxidation of pyrite (FeS(2)) and, to a lesser extent, organic matter dominate the changes in quality of the recharged water during its passage through the aquifer. The recharge leads to the consumption of oxygen and nitrate and the formation of sulfate and ferrihydrite. Complexation reactions, cation exchange and precipitation and dissolution of calcite, siderite and rhodochrosite were also identified through the modeling. Despite problems of non-uniqueness of the calibrated parameters, the model was used successfully to depict the geochemical processes occurring in the aquifer. Non-uniqueness can be avoided by constraining th...
Journal of Contaminant Hydrology, 2011
We present an experimental and modeling study of solute transport in porous media in the presence... more We present an experimental and modeling study of solute transport in porous media in the presence of mixing-induced precipitation of a solid phase. Conservative and reactive transport experiments were performed in a quasi-two-dimensional laboratory flow cell, filled with homogeneous and heterogeneous porous media. Conservative experiments were performed by injecting solutions containing sodium chloride and calcium chloride into the domain. In reactive transport experiments, inlet solutions of calcium chloride and sodium carbonate were injected in parallel, resulting in calcium carbonate precipitation where the solutions mix. Experimental results were used as a benchmark to examine the performance of a reactive transport numerical model. Good agreement between model predictions and experimental results was obtained for the conservative transport experiments. The reactive transport experiments featured the formation of a calcium carbonate mineral phase within the mixing zone between the two solutions, which controlled the spatial evolution of calcium carbonate in the domain. Numerical simulations performed on high resolution grids for both the homogeneous and heterogeneous porous systems underestimated clogging of the system. Although qualitative agreement between model results and experimental observations was obtained, accurate model predictions of the spatial evolution of calcium concentrations at sample points within the flow cell could not be achieved.
The code RETRASO (REactive TRAnsport of SOlutes) simulates reactive transport of dissolved and ga... more The code RETRASO (REactive TRAnsport of SOlutes) simulates reactive transport of dissolved and gaseous species in non-isothermal saturated or unsaturated problems. Possible chemical reactions include aqueous complexation (including redox reactions), sorption, precipitation-dissolution of minerals and gas dissolution. Various models for sorption of solutes on solids are available, from experimental relationships (linear KD, Freundlich and Langmuir isotherms) to cation exchange and
Developments in Water Science, 2002
ABSTRACT
Waste Management, 2011
Lightweight tire derived aggregate (TDA) fills are a proven recycling outlet for waste tires, req... more Lightweight tire derived aggregate (TDA) fills are a proven recycling outlet for waste tires, requiring relatively low cost waste processing and being competitively priced against other lightweight fill alternatives. However its value has been marred as several TDA fills have self-combusted during the early applications of this technique. An empirical review of these cases led to prescriptive guidelines from the ASTM aimed at avoiding this problem. This approach has been successful in avoiding further incidents of self-combustion. However, at present there remains no rational method available to quantify selfcombustion risk in TDA fills. This means that it is not clear which aspects of the ASTM guidelines are essential and which are accessory. This hinders the practical use of TDA fills despite their inherent advantages as lightweight fill. Here a quantitative approach to self-combustion risk evaluation is developed and illustrated with a parametric analysis of an embankment case. This is later particularized to model a reported field self-combustion case. The approach is based on the available experimental observations and incorporates well-tested methodological (ISO corrosion evaluation) and theoretical tools (finite element analysis of coupled heat and mass flow). The results obtained offer clear insights into the critical aspects of the problem, allowing already some meaningful recommendations for guideline revision.
Reactive transport equations may become cumbersome to solve when a large number of species are co... more Reactive transport equations may become cumbersome to solve when a large number of species are coupled through fast (equilibrium) and slow (kinetic) reactions. Solution is further encumbered when both mobile (solutes) and immobile (minerals) species are involved. In these cases, sequential iteration approaches (SIA) may become extremely slow to converge. Direct substitution approaches (DSA), which solve all transport equations together, may become extremely large. Here we propose a formulation for optimally decoupling the reactive transport equations. The procedure is described sequentially using a paradigm system. We start by a tank paradigm in which all species are mobile and undergo equilibrium reactions. In this case, all components are fully decoupled. If some of the reactions are kinetic (canal paradigm), then we can still decouple components corresponding to equilibrium reactions. The same can be said regarding a system in which immobile species only react kinetically (river). The number of components can be reduced in the case that both types of reactions and species are present (aquifer). In short, the number of coupled equations that need to be solved simultaneously is, at most, equal to the number of kinetic reactions. This benefits both SIA and DSA solution methods. SIA should improve convergence because most components are linear and effectively decoupled, thus reducing nonlinearity to kinetic terms only in transport equations for kinetic components. DSA systems become reduced as the number of components that need to be solved together is, at most, equal to the number of independent kinetic reactions.
Water Resources Research, 2007
Chemical reactions are driven by disequilibrium, which is often caused by mixing. Therefore quant... more Chemical reactions are driven by disequilibrium, which is often caused by mixing. Therefore quantification of the mixing rate is essential for evaluating the fate of solutes in natural systems, such as rivers, lakes, and aquifers. We propose a novel mixing ratios-based formulation to evaluate solute concentrations and reaction rates when equilibrium aqueous reactions and precipitation/dissolution of minerals are driven by mixing of different end-members. Each end-member corresponds to a water from a given source with a specific chemical signature. The approach decouples the solute transport and chemical speciation problems, so that mixing ratios can be first obtained from the solution of conservative transport and then be used in general speciation codes to obtain the concentration of reacting species. One key finding is a general expression for reaction rates which demonstrates that the amount of reactants evolving into products depends on the rate at which solutions mix. Our formulation constitutes a general framework according to which one can design and interpret experimental analyses devoted to study mixing-driven reactive processes and obtain transverse dispersion coefficients. The formulation is also proposed as a useful tool to derive analytical solutions of reactive transport problems and may result computationally advantageous when compared to previous approaches to reactive transport modeling. We apply the developed formulation to provide an analytical solution of the reactive transport process resulting from mixing different CaCO 3-saturated waters in a two-dimensional setup.
Water Resources Research, 1998
We present a concise and general mathematical formulation for reactive transport in groundwater f... more We present a concise and general mathematical formulation for reactive transport in groundwater for general applications. By means of linear algebraic manipulations of the stoichiometric coefficients of the chemical reactions we are able to reduce the number of unknowns of the equations to be solved to the number of degrees of freedom according to thermodynamic rules. We present six formulations that differ from each other by number and type of unknowns and discuss their advantages and disadvantages with respect to the two most important numerical solution methods, the Sequential Iteration Approach (SIA) and the Direct Substitution Approach (DSA). Our conclusion is that the proposed reduction of the number of variables is of special interest for the DSA. We have applied one of these formulations to an example of the flushing of saline water by fresh water.
Radiochimica Acta, 2004
SummaryWe analyze remediation alternatives for a soil contaminated with
Journal of Hydrology, 1998
Dedolomites are rocks formed by the replacement of dolomite with calcite which is driven by the i... more Dedolomites are rocks formed by the replacement of dolomite with calcite which is driven by the infiltration of Ca-rich water. This process has been described in the literature as either increasing, preserving or decreasing porosity of the initial dolostone. According to textures observed in Prades, NE Spain, dedolomitization took place due to two simultaneous processes: non-pseudomorphic replacement of dolomite with calcite at the wall-rock of fractures; and pseudomorphic replacement in the bulk rock. A multicomponent reactive transport model is used to analyze the two dedolomitization processes observed. Diffusion of ions from the fractures was the driving force for non-pseudomorphic replacement. In the wall-rock, the dissolution of dolomite and precipitation of calcite took place in accord with a 1:2 stoichiometry. As a consequence of the mineral molar volume, the porosity was sealed and replacement limited to a rim on both sides of the fractures. According to calculations, the development of a rim took less than 500 years. Advection of ions from the top of the dolomite bed was the driving force for pseudomorphic replacement in the bulk rock. Due to the absence of empty volume for expansion, replacement was governed by the condition of equal volumetric rate for dolomite dissolution and calcite precipitation. According to calculations, pseudomorphic replacement of 1 m of dolostone was completed after about 10 5 years for reasonable values of flow parameters. As the infiltrating water was also undersaturated with respect to calcite, one stage of calcite dissolution postdated dedolomitization. This process was responsible for the widening of open fractures, where the dissolution front followed the dedolomitization front into the dolomite matrix. Intensive calcite dissolution was also developed, following the advective flux from the top of the bed downwards. Drusy calcite is observed to precipitate in the pore spaces which consist of intergrains, fractures, and dissolution cavities. This process is not predicted in detail by reactive transport calculations, although it may be due to calcite supersaturation of the solution resulting from pseudomorphic dedolomitization.
Journal of Hydrology, 2011
Evaporation from a salty soil generates salt accumulation near the surface with the subsequent de... more Evaporation from a salty soil generates salt accumulation near the surface with the subsequent deterioration of the soil quality. Salinization mechanisms are poorly understood despite their global impact. Vapor flux and solute transport were studied under evaporation conditions. Laboratory experiments consisted of open sand and silt columns initially saturated with epsomite (MgSO 4 Á7H 2 O) or halite (NaCl) solutions. Salt precipitation occurred only above the evaporation front, which occupied a very narrow region. Vapor flowed both upwards and downwards from this front. The downward vapor flow condensed further down the column, diluting the solution. This gave rise to two areas: a high salinity area above the evaporation front, and a diluted solution area below it. The effects of thermal, suction and osmotic gradients on water fluxes were studied in order to better understand the underlying mechanisms of this phenomenon.
Journal of Hydrology, 2011
ABSTRACT Evaporation controls the salinity of many natural and anthropic brine systems, but the r... more ABSTRACT Evaporation controls the salinity of many natural and anthropic brine systems, but the reverse is also true. By controlling water activity, salinity affects evaporation rates. We present a method to compute the evolution of water activity in high salinity systems so as to evaluate evaporation rates. We place special emphasis on the assessment of invariant points, where activity is controlled by the set of precipitated minerals. The algorithm is tested on a natural Mg-SO4 rich brine evaporation experiment. In accordance with the experiments, the model predicts two intervals (invariant points) in which water activity, the concentration of all species and the amount of liquid water remain constant, because evaporating water comes from the dissolution of hydrated minerals. This suggests that mineral paragenesis might have a considerable influence on shallow brine lake evolution by fixing chemical composition for a significant portion of time. This conjecture was tested with a simplified model of a perennial saline playa lake. An analytical solution was developed to illustrate the evolution of the proposed system. According to the calculations, the system tends to a cyclical steady state for both lake level and chemical composition. The latter remains fixed at invariant points during long time intervals, where hydrated minerals act as the water source for evaporation. This situation is to be expected in epsomite lakes where two invariant points can be found in equilibrium with relative humidities of 57% and 50%.
Journal of Contaminant Hydrology, 2011
We present an experimental and modeling study of solute transport in porous media in the presence... more We present an experimental and modeling study of solute transport in porous media in the presence of mixing-induced precipitation of a solid phase. Conservative and reactive transport experiments were performed in a quasi-two-dimensional laboratory flow cell, filled with homogeneous and heterogeneous porous media. Conservative experiments were performed by injecting solutions containing sodium chloride and calcium chloride into the domain. In reactive transport experiments, inlet solutions of calcium chloride and sodium carbonate were injected in parallel, resulting in calcium carbonate precipitation where the solutions mix. Experimental results were used as a benchmark to examine the performance of a reactive transport numerical model. Good agreement between model predictions and experimental results was obtained for the conservative transport experiments. The reactive transport experiments featured the formation of a calcium carbonate mineral phase within the mixing zone between the two solutions, which controlled the spatial evolution of calcium carbonate in the domain. Numerical simulations performed on high resolution grids for both the homogeneous and heterogeneous porous systems underestimated clogging of the system. Although qualitative agreement between model results and experimental observations was obtained, accurate model predictions of the spatial evolution of calcium concentrations at sample points within the flow cell could not be achieved.
Journal of Contaminant Hydrology, 2003
This paper describes the modeling of the hydrogeochemical effects of deep well recharge of oxic w... more This paper describes the modeling of the hydrogeochemical effects of deep well recharge of oxic water into an anoxic pyrite-bearing aquifer. Kinetic expressions have been used for mineral dissolution-precipitation rates and organic matter oxidation. Hydrological and chemical parameters of the model were calibrated to field measurements. The results showed that oxidation of pyrite (FeS(2)) and, to a lesser extent, organic matter dominate the changes in quality of the recharged water during its passage through the aquifer. The recharge leads to the consumption of oxygen and nitrate and the formation of sulfate and ferrihydrite. Complexation reactions, cation exchange and precipitation and dissolution of calcite, siderite and rhodochrosite were also identified through the modeling. Despite problems of non-uniqueness of the calibrated parameters, the model was used successfully to depict the geochemical processes occurring in the aquifer. Non-uniqueness can be avoided by constraining the model as much as possible to measurements and/or data from literature, although they cannot be considered always as fixed values and should be considered as stochastic variables instead.
Journal of Contaminant Hydrology, 2001
... The model code CHEMFRONTS (Bäverman et al., 1999) uses this approach. ... 2. Numerical approa... more ... The model code CHEMFRONTS (Bäverman et al., 1999) uses this approach. ... 2. Numerical approaches. 2.1. Sequential iteration approach (SIA). 2.1.1. Equations. Reactive transport can be formulated in different ways, leading to effectively different mathematical problems. ...
Ecological Engineering, 2008
Computers & Geosciences, 2009
Accurate prediction of contaminant migration in surface and ground water bodies, including intera... more Accurate prediction of contaminant migration in surface and ground water bodies, including interaction with aquifer and hyporheic zone materials requires reactive transport modeling. The increasing complexity and the procedure-oriented type of programming often used in reactive transport hinder codes reuse and transportability. We present a Fortran 90 module using object-oriented concepts that simulates complex hydrobiogeochemical processes (CHEPROO, CHEmical PRocesses Object-Oriented). CHEPROO consists of a general structure with two classes. The Nodal Chemistry class accounts for the description of local chemistry and geochemical state variables. As such, it provides many functions related to basic operations (evaporation, mixing, etc.) and can easily grow on this direction (extreme dry conditions, biochemical state variables, etc.). The Chemical System class includes kinetic and thermodynamic models that describe reactions between and within phases. As such, it can grow in the direction of increasingly complex chemical systems (solid solutions, microorganisms as individual phases, etc.), without loss in the handling of simple problems. These two classes are overlaid by CHEPROO, a general structure designed for interaction with other codes. CHEPROO can be used as a geochemical tool for the modeling of complex processes such as biodegradation or evaporation at high salinities. However, many functions CHEPROO are devoted to coupling a broad range of chemical processes to other phenomena (flow, transport, mechanical). We have shown that reactive transport (based on either DSA or SIA approaches) could be easily implemented into existing conservative transport code with a minimal number of changes.
Chemical Engineering Journal, 2011
Constructed Wetland Model No. 1 (CWM1) processes were implemented within RetrasoCodeBright (RCB) ... more Constructed Wetland Model No. 1 (CWM1) processes were implemented within RetrasoCodeBright (RCB) to simulate hydraulics and reactive transport as well as the main biodegradation and transformation processes in horizontal subsurface flow constructed wetlands (SSF CWs). New values for some stoichiometric and kinetic coefficients were determined in the calibration step in order to obtain more realistic biochemical transformation and degradation processes. The model was checked and then tested for a horizontal SSF CW operating with different hydraulic loading rates [20, 36 and 45 mm/d]. Modifications to the CWM1 formulation had a negligible effect on the good fitting of measured and simulated data. However, changes in stoichiometric and kinetic parameters positively affected performance. Bacterial concentrations defined as initial conditions proved to be a variable requiring attention in the calibration. In terms of pollutant concentrations in effluent, simulated data corresponded well with data measured in most cases evaluated. The quality of the results obtained suggests that CWM1-RETRASO, the resulting model, is a potential tool for studying hydraulics, reactive transport and the main biochemical transformation and degradation processes for organic matter, nitrogen and sulphur in horizontal SSF CWs.
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Papers by Maarten W. Saaltink