Understanding the effects of mixing-driven precipitation on solute transport behavior is critical for reactive transport predictions, yet its complexity, arising from the interplay of flow dynamics, solute transport, and geochemical reactions, remains a significant challenge. In particular, mineral precipitation modifies the hydraulic properties of porous media. The impact of this process on the solute transport behavior remains largely unexplored and is crucial for accurate reactive transport predictions. This study presents a controlled laboratory investigation of mixing-driven calcite precipitation (MDP) in an intermediate-scale Hele-Shaw cell, simulating a coarse-sand porous medium. The experiment allowed for direct visualization of the spatiotemporal evolution of precipitation while continuously monitoring hydraulic properties. Self-organized heterogeneities in the precipitate structure were observed, with calcite layers forming symmetric patterns aligned with the main flow, contrasting with the asymmetry predicted by a semi-analytical model under idealized conditions. Tracer tests conducted before and after precipitation demonstrated significant impacts on solute transport, including the emergence of strong anomalous transport features, such as earlier solute arrival, a distinct double peak, and pronounced tailing. These findings highlight the critical role of precipitation-induced heterogeneities in shaping transport behavior, emphasizing the need to integrate these dynamics into reactive transport models for improved predictive accuracy.

In many geological, geothermal, and hydrocarbon engineering operations, fluids are typically injected at a temperature different from that of the surrounding subsurface formation. The pressurization resulting from injection induces mechanical expansion of the porous medium. Moreover, injected fluids may be colder or hotter than the surrounding formation, thereby generating thermal contraction or expansion of the geological medium, respectively. Although numerical simulations are essential to predict and analyse the coupled thermo-hydro-mechanical (THM) behaviour, rapid evaluation tools are often required to explore multiple scenarios and perform inverse assessments efficiently. Here, we analytically estimate ground surface displacements due to point non-isothermal injections by combining an existing solution for the deformation due to steady state hydraulic head variations with a novel solution for the deformation due to temperature variations. This solution has been verified satisfactorily against results from a coupled THM numerical model and field measurements (levelling and PS-INSAR) from a leakage case at the geothermal power plant in Landau, Germany. In terms of computational cost, the analytical solution is significantly less demanding than fully coupled THM numerical simulations, allowing systematic sensitivity analyses. The results provide insight into the contribution of hydraulic head and temperature to the ground surface displacements, and allow a better understanding of the effect of the natural geothermal gradient. The solution can be used not only to predict ground displacement under different injection scenarios, but also to perform inverse analyses and infer subsurface parameters from uplift observations.

Geochemistry of groundwater is affected by temperature among other things. We propose a novel method that can be used to develop analytical and semi-analytical solutions for calculating reaction rates for non-isothermal cases, to verify numerical models and give a better understanding of thermo-hydro-chemical (THC) processes. Aqueous and mineral reactions are assumed in equilibrium. The method decouples the chemistry from the thermo-hydraulic (TH) processes. The chemical part of the method consists of batch calculations in which minerals dissolve or precipitate and water chemistry varies as a result of changing temperature. The thermohydraulic part consists of calculating temperature and spatial and temporal derivatives of temperature. From this, chemical composition of groundwater and precipitation or dissolution rates of minerals can be calculated straightforwardly. We applied the method to a simple 1D steady state case, for which an analytical solution could be obtained, and to a 2D Aquifer Thermal Energy Storage (ATES) system of the Forsthaus pilot project near Bern (Switzerland), for which we developed a semi-analytical solution. The use of the method for the simulation of this ATES system reduced computational costs seven-fold in comparison with a standard numerical code. Moreover, the method has provided understanding on the dominant reactive transport processes (which we have divided into mixing, heat retardation and heat conduction terms), mineral reaction rates and porosity changes of the two cases. At interfaces with abrupt changes in temperature gradients, reaction rates tend to infinity. A comparison of thermodynamic databases reveals that not only the temperature dependencies of chemical properties are important, but also first and second derivatives with respect to temperature.

Subterranean Estuaries (STEs) have been recognized for their role in the transport and fate of chemical compounds that discharge to the coastal ocean. The enrichment of coastal groundwater with nutrients is affected by different sources and mechanisms. Moreover, the distribution of these substances discharging to the sea is highly affected by the reactions produced at the mixing zone between the fresh and saline groundwater. In this research, we aim to identify the nutrient sources and biogeochemical processes that are actively playing a role in the subterranean estuary located in the alluvial aquifer of Argentona, in the northeast of Barcelona, Catalonia (Spain). Coastal groundwater in the area has been continuously explored since 2014 with the development of a unique experimental site. The site is 100m long inland from the coastline and 30m wide. It is being monitored with 25 piezometers consisting of 5 nests with 4 piezometers each (with intervals at 10m, 20m, 15m, and 25m) and 4 individual piezometers, equipped with different sensors that collect data every 15 minutes. This study integrates various approaches such as the N-isotopes (δ15N-NO3-, δ18O-NO3-, δ15N-NH4+), hydrogeochemistry, dissolved organic matter (DOM), and bacteria concentration that has been measured in all piezometers during two sampling campaigns (winter and summer). The results show potential sources of ammonium and nitrate and the biogeochemical transformations that have a main role in the subterranean estuary dynamic. Acknowledgments This work was funded by the Spanish Government under the project MUCHOGUSTO (grant no. PID2022-140862OB-C21/C22) and the SENACYT – BID Scholarship by the Panamanian Government.

Dissolution trapping of CO2 in brine can mitigate the risk of supercritical CO2 leakage during long-term geological carbon sequestration (GCS). The dissolution of overlying supercritical CO2 into brine increases the density of brine in its upper portion, which causes gravity-driven convection (GDC) and thus significantly increases the rate of CO2 dissolution. To date, most studies on GDC-enhanced dissolution are based on homogeneous media, and only few studies exist on the effect of heterogeneity on GDC-enhanced dissolution. Here, we study the effect of heterogeneity and anisotropy on GDC-enhanced dissolution rate using numerical simulations with randomly obtained permeability fields. Dissolution rates calculated by these simulations are related to properties of the permeability field using least-squares regression. We obtained two empirical formulas for predicting the asymptotic GDC-enhanced dissolution rate. In the first formula the dissolution rate is almost linearly proportional to the dimensionless equivalent vertical permeability. In the second one the dissolution rate is linearly proportional to a dimensionless vertical finger-tip velocity. This indicates that the GDC-enhanced dissolution can be predicted using either the equivalent vertical permeability or the vertical finger-tip velocity. Furthermore, both formulas demonstrate that higher-permeability anisotropy results in lower dissolution rates, suggesting that pronounced horizontal stratification can inhibit the dissolution of CO2.

El Cabril is the low- and intermediate-level radioactive waste disposal facility in Spain. Water is collected from the drains situated in the concrete cells storing radioactive waste, indicating flow of water into these concrete cells. 2D numerical models, together with temperature and humidity measurements, suggested that this flow of water was caused by a combination of thermo-hydraulic processes occurring in the unsaturated concrete, such as capillary rise from the groundwater, evaporation, and condensation due to temperature gradients caused by seasonal temperature fluctuations outside. The objective of this work is to study the effect of these processes on the mineralogy of the concrete. Therefore, 1D reactive transport models have been developed following the 2D thermo-hydraulic conceptual model. When minerals are controlled by fast kinetics, the model results show that the cement phases precipitate and dissolve, clearly following the yearly fluctuations of condensation and evaporation. However, when minerals are controlled by slow kinetics, the reactions are less affected by the hydraulic processes. The models suggest that precipitation-dissolution could be particularly important near the air gap in the concrete, where water condenses and evaporates more easily.

Within the framework of the GTS-LTD project (Grimsel Test Site - Long -Term Diffusion), a radionuclide transport experiment in unfractured granitic rock matrix was performed. Grimsel groundwater containing several radionuclide tracers (3H as HTO, 36Cl-, 22Na+, 134Cs+, 133Ba2+) was continuously circulated through a packed -off borehole interval. The decrease in tracer concentrations in the solution was monitored for a period of 1266 days (March 05, 2014-August 22, 2017). Additionally, tracer breakthrough was monitored in an observation borehole at a distance of 18.6 cm. Initial modeling of the experiment (1D radial), considering transport only by diffusion, showed that the evolution of tracer concentrations departed from the expected trend after some time, with concentrations in the injection borehole decreasing faster than expected. Additional 2D calculations (section normal to the boreholes) were performed to check the possible effect of advection through the rock matrix. Advection could explain the evolution of concentrations in the injection borehole, but concentrations in the observation borehole were overestimated. Core samples from new boreholes were collected immediately after the end of the experiment, allowing the measurement of tracer distributions in the rock. The observed patterns for the non -sorbing tracers (HTO, 36Cl-) showed clear preferential transport directions, consistent with advective flow towards the gallery from which the boreholes were drilled. Final 3D modeling of the experiment can explain the measured concentrations in the boreholes and in the rock. Tracer transport for the conservative tracers (HTO, 36Cl-) is affected by both diffusion and advection through the granitic rock matrix. Also, in situ accessible porosities deduced from the modeling (0.0014) are smaller than those measured in rock samples (about 0.009), pointing to unloading and destressing of the rock samples after drilling. At the spatial and temporal scales of the experiment, the effect of advection for the weakly sorbing 22Na+ is only minor, and it is practically negligible for the strongly sorbing tracers (134Cs+, 133Ba2+).

The modeling of reactive transport through porous media is a challenging numerical problem. Methods of solution have leveraged the stoichiometry of chemical reactions to address the transport of multiple aqueous species by expressing them in terms of an equivalent, linearly independent variable (component). This approach effectively decouples advection-dispersion transport from the source terms associated with equilibrium reactions. A common assumption found in the literature is that all species disperse with the same transport coefficients. Recent experimental studies have discussed that this is not necessarily the case, particularly for transverse mixing, which is limited by the species-specific molecular diffusion. This article presents a formulation of multicomponent reactive transport that takes into account the differences in dispersion coefficients. These differences lead to a nonlinear transport equation for the components, from where an expression for evaluating reaction rates is derived. It is demonstrated that this expression simplifies to the well-known equations assuming the same dispersion for all species. Numerical simulations of a binary chemical system under diffusion- and advection-dominated transport conditions are used to evaluate the influence that differential transport coefficients have upon the output of chemical reactions. Results indicate that differences in transport coefficients are particularly relevant when the chemical signature of the input solutions is not strongly dominated by one of the species in the component. Unexpectedly, this opens the possibility to mineral dissolution coexisting with precipitation during the mixing of two waters in equilibrium. This phenomenon can be explained by nonlinear mixing processes proportional to the differences in transport coefficients. New formulation of multicomponent reactive transport with species-specific dispersion properties and nonlinear transport for the components A general expression for evaluating reaction rates is derived considering the transport properties of all species Differences in coefficients can lead to dissolution coexisting with precipitation for a equilibrium binary reaction in dilute systems

Biofilm growth in porous media changes the hydrodynamic properties of the medium: porosity and permeability decrease, and dispersivity increases. However, the first arrival of breakthrough curves (BTCs) is more reduced than derived from the reduction in porosity, and the BTC tail becomes heavier. These observations suggest the need for multicontinuum models (Multi-Rate Mass Transfer, MRMT) which describe reactive transport in heterogeneous porous media and facilitate the simulation of localized reactions often observed within biofilms. Here, we present a conceptual model of biochemical reactive transport with dynamic biofilm growth based on MRMT formulations. The model incorporates microbial growth by updating the porosity, dispersivity, and local mass exchange between mobile water and the immobile biofilm according to the stoichiometry and kinetic rate laws of biochemical reactions. This model has been successfully tested using two sets of laboratory data. We found that (1) the basic model based on the growth of uniformly sized biofilm aggregates (memory function with 1/2 slope), fails to reproduce laboratory tracer tests and rate of biofilm growth, while the fractal growth model, which we obtain by integrating the memory functions of biofilm aggregates with a power law distribution, does; (2) The biofilm memory function evolves as the biofilm grows in response to the varying aggregate size distribution; and (3) the early time portion of eluted volume BTCs are independent of flow rate, whereas the tail becomes heavier when the flow rate is decreased, that both advection controlled and diffusion controlled mass exchange coexist in biofilms.

Submarine Groundwater Discharge (SGD) is recognized as a relevant source of pollutants to the sea, but little is known about its relevance as a source of chemicals of emerging concern (CECs). Here, both the presence and distribution of a wide range of CECs have been evaluated in the most comprehensive manner to date, in a well-characterized Mediterranean coastal aquifer near Barcelona (Spain). Samples from coastal groundwater and seawater allowed for the unique spatial characterization of the pollutants present in the land-ocean interface, an outstanding research gap that required attention. The main goals were (1) to determine CECs in the aquifer, so as to evaluate the SGD as a relevant source of marine pollution, and (2) to identify new tracers to improve our understanding of SGD dynamics. To this end, 92 CECs were located in the aquifer by using wide-scope analytical target methodologies (>2000 chemicals). Among them, the perfluoroalkyl and polyfluoroalkyl substances (PFAS), along with the pharmaceuticals carbamazepine and topiramate, were revealed to be good markers for tracing anthropogenic contamination in ground- and seawater, in concrete situations (e.g., highly contaminated sites). Additionally, non-target analysis expanded the number of potential tracers, making it a promising tool for identifying both the source and the fate of pollutants.

The present study indicated the hydrodynamics and the characteristics geothermal systems of Bouhanifia and Saida by means of hydrochemistry and isotopic data collected from different springs. Because of the lack of data (relying only on regional studies), our work defined the local geology and hydrogeochemistry behavior, following a small scale of investigation. These hot springs were produced by the ascent of thermal water in quaternary age faults marked by the Travertine formations created by the discharge of this groundwater. The geostatistical techniques used to characterize the geochemistry assert that the main contributors of mineralization are Na+, Cl−, Ca2+, and HCO3−, confirming various origins of the chemical elements, from the carbonate formations and interaction with saline shallower water shown through the excess of Na+ and Cl−. The isotopic results confirm the low enthalpy of geothermal water and refute a magmatic origin from calc-alkaline volcanism activity during the Neogene.

• Denitrification is the dominant nitrate reduction process in the woodchip bioreactor. • Woodchips promote localized biochemical reactions enabling aerobic and anaerobic reactions to occur simultaneously. • MRMT is needed to represent the heterogeneity of porous woodchips. • MRMT is required to reproduces biochemical localization. • Conservative tracer tests and qualitative understanding can yield MRMT.

We present a reactive multi-component multi-phase flow program for simulating Geological Carbon Seques-tration (GCS), an approach that reduces carbon emissions by storing CO2 in deep subsurface formations. The program, called MRST_CO2, is implemented in the library Matlab Reservoir Simulation Toolbox (MRST). It can simulate multi-phase flow and transport of species undergoing chemical reactions and mass exchanges among gas, liquid and solid phases. Equations for flow, mass transport and chemical reactions are solved with a sequential iteration approach. The program was tested with a 1D benchmark. It has also been applied to heterogeneous 2D and 3D cases with a structured or unstructured grid.

Within the GTS-LTD project (Grimsel Test Site – Long-Term Diffusion), a field radionuclide tracer transport experiment in unfractured granitic rock was performed. Grimsel groundwater containing several tracers (3H as HTO, 36Cl-, 22Na+, 134Cs+, 133Ba2+) was continuously circulated through a packed-off borehole and the decrease in tracer concentrations in the liquid phase was monitored for a period of 1266 days (05/03/2014 – 22/08/2017). Additionally, tracer breakthrough was monitored in an observation borehole a few cm away.Initial modeling of the experiment (1D radial) showed that the evolution of tracer concentrations seemed to depart from the expected trend after some time, with concentrations in the injection borehole decreasing faster than expected from pure diffusive transport. Additional 2D calculations (section normal to the boreholes) were performed to check the possible effect of advection through the rock matrix. Advection could explain the evolution of concentrations in the injection borehole, but could not fully explain the measurements in the observation borehole.Rock samples around the experimental section were collected right after the end of the experiment, allowing the measurement of tracer distributions in the rock. The observed patterns for the non-sorbing tracers (HTO, 36Cl-) showed clear preferential transport directions, consistent with advective flow towards the gallery from which the boreholes were drilled. Final 3D modeling of the experiment can explain the measured concentrations in the boreholes and in the rock. Tracer transport is affected by both diffusion and advection through the granitic rock matrix. Also, in situ accessible porosities (about 0.0014) are smaller than those measured in rock samples (about 0.009), pointing to the destressing of the rock samples after drilling.

Coastal aquifers are characterized by their unique land-sea interaction and the main difficulty in studying these systems is the complexity of their biogeochemical cycles. Important processes in the “mixing zone” of coastal systems are associated with the ecosystem’s diversity, and supply and exchange of chemical compounds. This coastal body known as a “subterranean estuary” (STE), is characterized by a free connection to the sea, creating an interface between freshwater and seawater. The distribution and composition of substances flowing from land to sea change because the submarine groundwater discharges (SGD) considerably dilute the seawater that has invaded the aquifer through the free connection to the sea (SWI, Sea Water Intrusion).These processes were studied in an area 100 m long inland from the coastline and 30 m wide, in the alluvial aquifer of Argentona, Mataró, northeast of Barcelona, Catalonia (Spain). This experimental site was established in 2015 and has been equipped for intensive monitoring of coastal aquifer processes such as SWI and SGD. Currently, it is monitored with 25 piezometers (2 meters screened) consisting of 5 nests with 4 piezometers each (at 10m, 20m, 15m, and 25m intervals) and 4 individual piezometers. In this system described as a “multi-aquifer and reactive system”, we are characterizing the physicochemical and hydrogeochemical conditions associated with biogeochemical processes at different depths and seasonal variations. For this presentation, we will show new results of hydrogeochemistry, nitrogen isotopes, and microbiological parameters associated with different periods of monitoring that characterize the dynamics in the subterranean estuary. Acknowledgments: This work was funded by the Spanish Government (grant no. PID2019-110212RB-C21 and PID2019-110212RB-C22), the project TerraMar (grant no. ACA210/18/00007) of the Catalan Water Agency, and the SENACYT – IFARHU – BID Scholarship.

Thermotropia, barotropia and hygrotropia of the Ca-SO4-H2O system is reviewed and discussed and a three-dimensional representation of the phase diagram for system at atmospheric pressure is derived. It is concluded that the behaviour of the system is a coupled thermo-hydro-chemical phenomenon in which water activity is the key factor for existence of high concentration of Ca2+ and SO2-4 and confinement pressure plays a secondary role.

High-temperature aquifer thermal energy storage (HT-ATES) systems can help in balancing energy demand and supply for better use of infrastructures and resources. The aim of these systems is to store high amounts of heat to be reused later. HT-ATES requires addressing problems such as variations of the properties of the aquifer, thermal losses and the uplift of the surface. Coupled thermo-hydro-mechanical (THM) modelling is a good tool to analyse the viability and cost effectiveness of HT-ATES systems and to understand the interaction of processes, such as heat flux, groundwater flow and ground deformation. The main problem of this modelling is its high computational cost. We propose a dimensional and numerical analysis of the thermo-hydro-mechanical behaviour of a pilot HT-ATES. The results of this study have provided information about the dominant thermo-hydraulic fluxes, evolution of the energy efficiency of the system and the role of the hydraulic and thermal loads generated by the injection and extraction of hot water.

Submarine Groundwater Discharge (SGD) is recognized as a relevant source of pollutants to the coastal ocean, but little is known about its relevance as a source of chemicals of emerging concern (CECs). Here, both the presence and distribution of a wide range of CECs have been evaluated in the most comprehensive manner to date, in a well characterized Mediterranean coastal aquifer (Barcelona, Spain). Samples from the groundwater-seawater continuum allowed for a unique spatial characterization of the pollutants present in the land-ocean interface, which remained as a research gap so far. The main goals were to (1) determine CECs in the aquifer, and so evaluate the SGD as a relevant source of marine contamination, and (2) identify new chemical tracers which could improve the understanding of the SGD dynamics. Hence, 93 CECs were found in the aquifer by using widescope analytical target methodologies (>2,000 chemicals). Among them, the perfluoroalkyl and polyfluoroalkyl substances (PFAS), as well as the pharmaceuticals carbamazepine and topiramate, revealed to be good markers to trace anthropogenic contamination in ground- and seawater. Additionally, non-target analysis expanded the number of potential tracers. As a result, a PFAS, which was never detected before in environmental water, was tentatively identified.

Aquifer Thermal Energy Storage (ATES) is a technology that injects heat into aquifers during low energy demand and extracts it during high demand. ATES generates variation of temperature in the underground that can lead to chemical reactions. These reactions can affect the efficiency of ATES and modify the groundwater chemistry and the properties of soils and rocks. We present a novel method that allows calculation of complex numerical models and understanding the thermo-hydro-chemical processes in ATES in a simple way. Aqueous and mineral reactions must be assumed in equilibrium. The method decouples the chemistry from the thermo-hydraulic processes. The chemical part of the method consists of chemical batch calculations in which minerals dissolve/precipitate and water chemistry varies as a result of changing temperature. The thermo-hydraulic part consists of calculating temperature and spatial and temporal derivatives of temperature. From this, chemical composition of groundwater and precipitation/dissolution rates of minerals can be calculated straightforwardly. We have applied the method to a HEATSTORE benchmark case, which is inspired by an ATES pilot project located in Bern, Switzerland. We used PHREEQC for the chemical calculations and for the thermo-hydraulic modelling we used the finite element code CODE_BRIGHT. The results permit us to understand better the reactive transport processes in the aquifer (which we divided into mixing, heat retardation and heat conduction), changes in the porosity of the rocks and the precipitation and dissolution of minerals. Acknowledgements: This work was financed by the ERANET project HEATSTORE (170153-4401). This project has been subsidized through the ERANET cofund GEOTHERMICA (Project n. 731117), from the European Commission, RVO (the Netherlands), DETEC (Switzerland), FZJ-PTJ (Germany), ADEME (France), EUDP (Denmark), Rannis (Iceland), VEA (Belgium), FRCT (Portugal), and MINECO (Spain). Also, the first author is supported by a grant from the Department of Research and Universities of the Generalitat de Catalunya (2021 FI_B 00940).

We describe a formulation to solve reactive transport (RT) problems. The basic idea is to represent transport as mixing water instead of individual solute concentrations, hence the Water Mixing Approach (WMA) name. This conceptual representation simplifies RT calculations as it decouples transport from chemistry. Transport is first solved in terms of water mixing ratios (lambda), which we show is feasible for any transport formulation. Chemical calculations can then be written as a set of reactive mixing calculations, which are local, so that they do not need to iterate with transport. We have implemented the WMA for a mixed Eulerian-Lagrangian transport solver with streamline-oriented grid and constant travel time between sequential cells (isochronal grid), which is free of numerical dispersion. We test the WMA on three RT cases. First, a binary system case is used to test the accuracy of the method. Second, a calcite dissolution case is used to compare the WMA to the Direct Substitution Approach to test both accuracy and computational cost (CPU). Third, the accuracy and simplicity of the proposed approach is tested on a reactive pulse injection case, solved in 2D on a spreadsheet. Results confirm the accuracy, simplicity and efficiency (low CPU cost) that result from decoupling transport and chemical steps. Transport through highly heterogeneous media remains a challenge, but the definite separation and simplification of mixing processes in WMA opens a new path for reactive transport modeling in general, and for the search of an effective transport equation.