RETRASO, a code for modeling reactive transport in saturated and unsaturated porous media

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 surface complexation models (constant capacitance, diffuse layer and triple layer models). Precipitation-dissolution and aqueous complexation can be modelled in equilibrium or according to kinetic laws. For the numerical solution of the reactive transport equations it uses the Direct Substitution Approach. The use of the code is demonstrated by three examples. The first example models various sorption processes in a smectite barrier. The second example models a complex chemical system in a two dimensional cross-section. The last example models pyrite weathering in an unsaturated medium

Modeling evaporation processes in a saline soil from saturation to oven dry conditions

Thermal, suction and osmotic gradients interact during evaporation from a salty soil. Vapor fluxes become the main water flow mechanism under very dry conditions. A coupled nonisothermal multiphase flow and reactive transport model was developed to study mass and energy transfer mechanisms during an evaporation experiment from a sand column. Very dry and hot conditions, including the formation of a salt crust, necessitate the modification of the retention curve to represent oven dry conditions. Experimental observations (volumetric water content, temperature and concentration profiles) were satisfactorily reproduced using mostly independently measured parameters, which suggests that the model can be used to assess the underlying processes. Results show that evaporation concentrates at a very narrow front and is controlled by heat flow, and limited by salinity and liquid and vapor fluxes. The front divides the soil into a dry and saline portion above and a moist and diluted portion below. Vapor diffusses not only upwards but also downwards from the evaporation front, as dictated by temperature gradients. Condensation of this downward flux causes dilution, so that salt concentration is minimum and lower than the initial one, just beneath the evaporation front. While this result is consistent with observations, it required adopting a vapor diffusion enhancement factor of 8. © Author(s) 2011

Long term water flow scenario in low-level waste disposal vaults, with particular regard to concrete structures in El Cabril, Cordoba, Spain

This paper deals with the main durability design objectives adopted for the Spanish Low-level waste disposal facility of El Cabril. The presentation summarizes the studies and models developed to represent the performance of the reinforced concrete vaults. Particular attention is paid to recent developments in modelling the water flow through the disposal system and its humidity saturation and their relation to the long term behaviour of the concrete barriers. It also describes the work being carried out to improve the existing models as a part of the required effort to maintain up to date the performance assessment of the facility

Application of a mixing-ratios based formulation to model mixing-driven dissolution experiments

We address the question of how one can combine theoretical and numerical modeling approaches with limited measurements from laboratory flow cell experiments to realistically quantify salient features of complex mixing-driven multicomponent reactive transport problems in porous media. Flow cells are commonly used to examine processes affecting reactive transport through porous media, under controlled conditions. An advantage of flow cells is their suitability for relatively fast and reliable experiments, although measuring spatial distributions of a state variable within the cell is often difficult. In general, fluid is sampled only at the flow cell outlet, and concentration measurements are usually interpreted in terms of integrated reaction rates. In reactive transport problems, however, the spatial distribution of the reaction rates within the cell might be more important than the bulk integrated value. Recent advances in theoretical and numerical modeling of complex reactive transport problems [12, 13] result in a methodology conducive to a simple exact expression for the space-time distribution of reaction rates in the presence of homogeneous or heterogeneous reactions in chemical equilibrium. The key points of the methodology are that a general reactive transport problem, involving a relatively high number of chemical species, can be formulated in terms of a set of decoupled partial differential equations, and the amount of reactants evolving into products depends on the rate at which solutions mix. The main objective of the current study is to show how this methodology can be used in conjunction with laboratory experiments to properly describe the key processes that occur in a complex, geochemically-active system under chemical equilibrium conditions. We model three CaCO3 dissolution experiments reported in Singurindy et al. [48, 49], in which saltwater and freshwater were mixed in different proportions. The integrated reaction rate within the cell estimated from the experiments are modeled independently by means of (a) a state-of-the-art reactive transport code, and (b) the uncoupled methodology of [12, 13], both of which use dispersivity as a single, adjustable parameter. The good agreement between the results from both methodologies demonstrates the feasibility of using simple solutions to design and analyze laboratory experiments involving complex geochemical problems

New perspectives on the use of 224Ra/228Ra and 222Rn/226Ra activity ratios in groundwater studies

The naturally occurring Ra isotopes (223Ra; T1/2 = 11.4 d, 224Ra; T1/2 = 3.66 d, 226Ra; T1/2 = 1,600 y, and 228Ra; T1/2 = 5.7 y) and Rn (222Rn; T1/2 = 3.82 d) have been widely applied as environmental tracers. The application of these radioactive tracers has mainly been restricted to the evaluation of oceanographic and land-ocean interaction processes, although in recent years their use has also been extended to the study of groundwater systems. In this context, the activity ratios of 224Ra/228Ra and 222Rn/226Ra can be instrumental in providing key information on groundwater transit times in aquifers and those processes governing groundwater discharge into the coastal sea (often referred to as Submarine Groundwater Discharge or SGD). This work evaluates the potential use of these activity ratios as proxies for investigating groundwater systems through an advective transport model that integrates the radionuclides involved in these activity ratios (224Ra, 228Ra, 226Ra, and 222Rn) and their immediate parents into a single formulation. The results provided by the transport model indicate that the main factors controlling the 224Ra/228Ra and 222Rn/226Ra activity ratios are the alpha recoil supply, the retardation factor of Ra, and the groundwater transit times. The advective transport model and the activity ratios are used to present novel applications that interrelate the disciplines of hydrogeology and coastal oceanography. The main applications include the determination of groundwater transit times and the assessment of pathways and end-members related to submarine groundwater discharge processes. These applications were tested in a Mediterranean coastal aquifer.This work was partly funded by the projects PID2019-110212RB-C22, CGL2016-77122-C2-1-R/2-R and PID2019-110311RB-C21 of the Spanish Government and the project TerraMAr ACA210/18/00007 of the Catalan Water Agency. The authors want to thank the support of the Generalitat de Catalunya to MERS (2017 SGR-1588) and GHS (2017 SGR 1485) for additional funding. We would like to thank all the colleagues from the Laboratori de Radioactivitat Ambiental (Universitat Autònoma de Barcelona). We would also like to thank Leon Humphries for his detailed English corrections. We would like to thank SIMMAR (Serveis Integrals de Manteniment del Maresme) and the Consell Comarcal del Maresme in the construction of the research site. M. Diego‐Feliu acknowledges the economic support from the FI‐2017 fellowships of the Generalitat de Catalunya autonomous government (2017FI_B_00365). V. Rodellas acknowledges financial support from the Beatriu de Pinós postdoctoral program of the Generalitat de Catalunya autonomous government (2017‐BP‐00334). A. Alorda‐Kleinglass acknowledges financial support from ICTA “Unit of Excellence” (MinECo, MDM2015‐0552‐17‐1) and PhD fellowship, BES‐2017‐080740. T. Goyetche acknowledges PhD fellowship (BES‐2017‐080028) from the FPI Program by the Spanish Ministry of Economy, Industry and Competitiveness. A. Folch is a Serra Húnter Fellow.Peer reviewe

Combinig fiber optic DTS, cross-hole ERT and time-lapse induction logging to characterize and monitor a coastal aquifer

The characterization of saline water intrusion (SWI) and its hydrodynamics is a key issue to understand sub-marine groundwater discharge (SGD) and manage groundwater resources in coastal areas. To test and comparedifferent methods of characterization and monitoring, a new experimental site has been constructed in a coastalalluvial aquifer north of Barcelona city (Catalonia, Spain). The site is located between 30 and 90 m from theseashore and comprises 16 shallow piezometers organized in nests of three with depths ranging between 15 and25 m and 4 solitary piezometers. The objective of this paper is to combine different recently developed mon-itoring techniques to evaluate temporal variations in the aquifer hydrodynamics of the site at different spatialscales before and after the dry season of 2015. At the site scale,fibre optic distributed temperature sensing (FO-DTS), for thefirst time applied to study SWI, and cross-hole electrical resistivity tomography (CHERT) has beenapplied. At the meter/borehole scale, electrical conductivity of the formation has been applied not only in arepeated manner (¿time lapse¿), but also for thefirst time at relatively high frequency (1 sample every 10 min).CHERT has provided a better characterization of the seawater intrusion than electrical conductivity data ob-tained from piezometers. The combination of techniques has allowed improving the understanding of the systemby: 1) characterizing the extent and shape of SWI; 2) differentiating two different dynamics in the aquifer; and 3)identifying preferentialflow paths over different time and spatial intervals. Future challenges and the appli-cation of these techniques in other areas are also discussed.This work was funded by the projects CGL2013-48869-C2-1-R/2-R and CGL2016-77122-C2-1-R/2-R of the Spanish Government. We would like to thank SIMMAR (Serveis Integrals de Manteniment del Maresme) and the Consell Comarcal del Maresme in the construction of the research site. The authors want to thank the support of the Generalitat de Catalunya to MERS (2018 SGR-1588). This work is contributing to the ICTA ‘Unit of Excellence’ (MinECo, MDM2015-0552). Part of the funding was provided by the French network of hydrogeological observatories H+ (hplus/ore/fr/en) and the ANR project EQUIPEX CRITEX (grant ANR-11-EQPX-0011). V Rodellas acknowledges financial support from the Beatriu de Pinós postdoctoral program of the Generalitat de Catalunya (2017-BP-00334). M. Diego‐Feliu acknowledges the economic support from the FI‐2017 fellowships of the Generalitat de Catalunya autonomous government (2017FI_B_00365). This project also received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Grant Agreement No 722028