Changes in hydrodynamic, structural and geochemical properties in carbonate rock samples due to reactive transport

Reactive transport plays an important role in the development of a wide range of both anthropic and natural processes affecting geological media. To predict the consequences of reactive transport processes on structural and hydrodynamic properties of a porous media at large time and spatial scales, numerical modeling is a powerful tool. Nevertheless, such models, to be realistic, need geochemical, structural and hydrodynamic data inputs representative of the studied reservoir or material. Here, we present an experimental study coupling traditional laboratory measurements and percolation experiments in order to obtain the parameters that define rock heterogeneity, which can be altered during the percolation of a reactive fluid. In order to validate the experimental methodology and identify the role of the initial heterogeneities on the localization of the reactive transport processes, we used three different limestones with different petrophysical characteristics. We tracked the changes of geochemical, structural and hydrodynamic parameters in these samples induced by the percolation of an acid fluid by measuring, before and after the percolation experiment, petrophysical and hydrodynamic properties of the rocks.Peer ReviewedPostprint (published version

Estimation of hydraulic parameters for heterogeneous coastal aquifer systems from pumping test analysis and tidal response

Postprint (published version

Remediación agua contaminada por mercurio y portlandita en Flix

Hace unos años se decidió descontaminar el embalse de Flix, afectado por vertidos de residuos por un empresa química durante todo el siglo pasado. •Los residuos formaron 4 lóbulos de sedimentos en el río, con elevadas concentraciones de Mercurio, compuestos orgánicos volátiles (COV´s), residuos de producción de acetileno (Ca(OH)2) y pesticidas organoclorados. •Se ha realizado un modelo 0D de transporte reactivo de la concentración en el agua conforme se excavan los sedimentos transitorio y explícito en excel, calibrado hasta Septiembre 2014 con datos semanales de la empresa extractora FCC. •El modelo concluía dos graves problemas al terminar la excavación: Elevada concentración de Hg y pH (12.4), equilibrado con portlandita •Desde Marzo de 2015 se observó un acusado descenso de pH y Hg. Nuestro objetivo ha sido encontrar la causa de estos descensos, para ello se han realizado simulaciones, muestreos y experimentos de laboratorio. La precipitación CaCO3 debida a la difusión de CO2 atmosférico explica una parte de la disminución de pH, pero resulta insuficiente aun considerando la oxidación de materia orgánica. •El Hg se adsorbido en CaCO3 es alrededor de un 30%, otro 60% parece haberse volatilizado. •La electrolisis en laboratorio reproduce un descenso del Hg de 80% en 3 días. Sólo un 10% de Hg ha sido recuperado en el cátodo. • Se cree que el Hg2+ se reduce a Hg0 y volatiliza. •El pH descendió hasta 3.8, se cree que debido a hidrólisis del agua. •Se esta trabajando en comprobar el Hg0 volatilizado en próximos experimentos.Postprint (published version

Temperature driven vapor fluxes in soils cause a net recharge

Temperature gradients can drive vapor diffusion by controlling vapor pressure in the soil. We studied vapor diffusion for soils in two different climates: A semiarid climate at El Cabril (Córdoba, Spain) and a subarctic climate in the Upper Tuul River basin (Mongolia). For El Cabril vapor diffusive fluxes were studied by means of the measured temperatures and an analytical model. For the second site (Upper Tuul) a physically based soil water and energy balance model was developed accounting for relevant processes such as melting-freezing of water and vapor diffusion in the soil. Results of both sites show that vapor diffuses downwards during summer and upwards during winter, while yearly averaged fluxes diffuse downwards. The overall amount is small for El Cabril, but significant for the Upper Tuul. The latter large values can be explained by the large temperature oscillations of the Mongolian climate and the freezing/thawing of subsoil layer.Postprint (published version

Stochastic simulation of daily rainfall fields conditioned on atmospheric circulation patterns and orographic effects.

The objective of the current work is to present a methodology for simulation of stochastic spatial distributed rainfall fields at the daily time step. For this purpose, we develop a geo-stochastic rainfall generating process (SRGP) to generate spatially distributed rainfall fields at daily time scale, that respect the spatial correlation structure of historically observed precipitation, while taking into account important factors that influence the development of observed spatial patterns. For each day, a spatially distributed rainfall field is generated from a pre-specified SRGP, selected based on atmospheric synoptic conditions relevant for that day. Each SRGP is simulated by applying the concept of double kriging, as the product of the spatial amount of rainfall and the spatial occurrence of rainfall by sequential simulation (sequential Gaussian simulation and sequential indicator simulation respectively). The SRGP can account for spatial rainfall nonstationarity related to orographic effects, and can be incorporated as part of a downscaling technique in the context of climate change impact studies. A case study for the Upper Guadiana basin (Spain) is presented that shows the ability of the method to reproduce various spatio-temporal characteristics of precipitation.Peer ReviewedPostprint (published version

Temporal scaling of groundwater discharge in dual and multicontinuum catchment models

This paper presents a multicontinuum approach to model fractal temporal scaling of catchment response in hydrological systems. The temporal scaling of discharge is quantified in frequency domain by the transfer function (), which is defined as the ratio between the spectra of catchment response and recharge time series. The transfer function may scale with frequency as ()approximate to-. While the classical linear and Dupuit models predict exponents of =2 and =1, observations indicate scalings with noninteger exponents . Such behaviors have been described by multifractal models, which, however, often lack a relation to the medium characteristics. We revisit and extend the classical linear Dupuit aquifer models and discuss their physical meanings in the light of the resulting aquifer dynamics. On the basis of these classical models, we derive a multicontinuum approach that provides physical recharge models which are able to explain fractal behaviors with exponents 1/2<<2. Furthermore, this approach allows to link the fractal dynamics of the discharge process to the physical aquifer characteristics as reflected in the distribution of storage time scales. We systematically analyze the catchment responses in the proposed multicontinuum models, and identify and quantify the time scales which characterize the dynamics of the catchment response to recharge.Peer ReviewedPostprint (published version

Numerical formulation for a simulator (CODE_BRIGHT) for the coupled analysis of saline media

Salt rocks are being considered as potential recipients for geologic disposal of nuclear waste because of their favourable hydraulic and mechanical properties. In essence, processes controlling the behaviour of salt rocks are similar to those occurring in other media, except that they take place at unusually fast rates. The high solubility of salt in water is one of the causes of these high rates. In fact, creep deformation of wet salts takes place much faster than under dry conditions. This can be explained by means of a mechanism of creep deformation based on salt dissolution, molecular diffusion and precipitation at the pore scale caused by stress concentration[...]. The complex behaviour of saline media requires new theoretical and numerical developments. The study of the basic mechanisms has revealed that coupling between thermal, hydraulic and mechanical problems may be required in some cases. We have developed the governing equations for nonisothermal multiphase flow of brine and gas in deformable saline media. These include mass balance equations for the species in the system (salt, water and air), energy balance equation and stress equilibrium equation. Equilibrium restrictions complete the theoretical formulation. This paper presents a numerical formulation required for solving this complex problem in a practical way. An example of its application is also included.Peer Reviewe

Reactive transport: a review of basic concepts with emphasis on biochemical processes

Reactive transport (RT) couples bio-geo-chemical reactions and transport. RT is important to understand numerous scientific questions and solve some engineering problems. RT is highly multidisciplinary, which hinders the development of a body of knowledge shared by RT modelers and developers. The goal of this paper is to review the basic conceptual issues shared by all RT problems, so as to facilitate advancement along the current frontier: biochemical reactions. To this end, we review the basic equations to indicate that chemical systems are controlled by the set of equilibrium reactions, which are easy to model, but whose rate is controlled by mixing. Since mixing is not properly represented by the standard advection-dispersion equation (ADE), we conclude that this equation is poor for RT. This leads us to review alternative transport formulations, and the methods to solve RT problems using both the ADE and alternative equations. Since equilibrium is easy, difficulties arise for kinetic reactions, which is especially true for biochemistry, where numerous challenges are open (how to represent microbial communities, impact of genomics, effect of biofilms on flow and transport, etc.). We conclude with the basic eleven conceptual issues that we consider fundamental for any conceptually sound RT effort.This work is part of grants MEDISTRAES III funded by MCIN/AEI/ PID2019-110212RB-C22 and MCIN/AEI/PID2019-110311RB-C21 and Water JPI project MARadentro (PCI2019-103603), and by the Catalan Water Agency through the project RESTORA (CA210/18/00040). IDAEA-CSIC is a Center of Excellence Severo Ochoa (Grant CEX2018-000794-S funded by MCIN/AEI/ 10.13039/501100011033).Peer ReviewedPostprint (published version

Tracer test modeling for characterizing heterogeneity and local-scale residence time distribution in an artificial recharge site

Artificial recharge of aquifers is a technique for improving water quality and increasing groundwater resources. Understanding the fate of a potential contaminant requires knowledge of the residence time distribution (RTD) of the recharged water in the aquifer beneath. A simple way to obtain the RTDs is to perform a tracer test. We performed a pulse injection tracer test in an artificial recharge system through an infiltration basin to obtain the breakthrough curves, which directly yield the RTDs. The RTDs turned out to be very broad and we used a numerical model to interpret them, to characterize heterogeneity, and to extend the model to other flow conditions. The model comprised nine layers at the site scaled to emulate the layering of aquifer deposits. Two types of hypotheses were considered: homogeneous (all flow and transport parameters identical for every layer) and heterogeneous (diverse parameters for each layer). The parameters were calibrated against the head and concentration data in both model types, which were validated quite satisfactorily against 1,1,2-Trichloroethane and electrical conductivity data collected over a long period of time with highly varying flow conditions. We found that the broad RTDs can be attributed to the complex flow structure generated under the basin due to three-dimensionality and time fluctuations (the homogeneous model produced broad RTDs) and the heterogeneity of the media (the heterogeneous model yielded much better fits). We conclude that heterogeneity must be acknowledged to properly assess mixing and broad RTDs, which are required to explain the water quality improvement of artificial recharge basins.Peer ReviewedPostprint (published version

Modeling mixing in stratified heterogeneous media: the role of water velocity discretization in phase space formulation

Modeling solute transport in heterogeneous porous media faces two challenges: scale dependence of dispersion and reproducing mixing separately from spreading. Both are crucial since real applications may require km scales whereas reactions, often controlled by mixing, may occur at the pore scale. Methods have been developed in response to these challenges, but none has satisfactorily characterized both processes. In this paper, we propose a formulation based on the Water Mixing Approach extended to account for velocity variability. Velocity is taken as an independent variable, so that concentration depends on time, space and velocity. Therefore, we term the formulation the Multi-Advective Water Mixing Approach. A new mixing term between velocity classes emerges in this formulation. We test it on Poiseuille’s stratified flow using the Water Parcel method. Results show high accuracy of the formulation in both dispersion and mixing. Moreover, the mixing process exhibits Markovianity in space even though it is modeled in timePeer ReviewedPostprint (published version