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This paper addresses the behaviour of several technology critical metals (TCMs), i.e., rare earth elements (REEs), Y, Sc, Ga and Tl, in the Tinto River (SW Spain), quantifying their fluxes to the Atlantic Ocean and unravelling the governing geochemical processes controlling their solubility. To accomplish this goal, a high-resolution (2–24 h) sampling was performed during the hydrological year 2017/18. Mean dissolved concentrations of 380 μg/L of REE, 99 μg/L of Y, 15 μg/L of Sc, 9.2 μg/L of Ga and 4.8 μg/L of Tl were found. Most TCMs followed a behaviour similar to that of sulphate and base metals throughout the year, exhibiting a quasi-conservative behaviour due to acidic conditions. However, dissolved Tl concentrations seem to be strongly controlled by Tl incorporation onto secondary minerals and diatoms deposited on the riverbed, especially during the dry season. The remobilization of riverbed sediments led to the transport of significant amounts of TCMs associated with particulate matter, especially Al oxy-hydroxy-sulphates or Al-silicates rather than Fe precipitates (except for Tl and Ga). Around 5.8 t of REE, 1.3 t of Y, 248 kg of Sc, 139 kg of Ga and 138 kg of Tl were delivered annually in their dissolved forms by the Tinto River to the Atlantic Ocean, which constitutes around 0.09% of the dissolved global flux into the oceans of Y, 0.02% of the REE flux, 0.01% of the Ga flux and 0.001% of the Sc flux., This work was supported by the Spanish Ministry of Economy and Competitiveness through the research projects SCYRE (CGL2016- 78783-C2-1-R),CAPOTE (CGL2017-54686050- R) and VALOREY (RTI2018 -101276-J-I00).The authors thank to Dr. Damiá Barceló (Co-Editor- in-Chief) and three anonymous reviewers for their helpful comments that notably improved the quality of the original manuscript., This work was supported by the Spanish Ministry of Economy and Competitiveness through the research projects SCYRE (CGL2016-78783-C2-1-R), CAPOTE (CGL2017-86050-R) and VALOREY (RTI2018-101276-J-I00). The authors thank to Dr. Damiá Barceló (Co-Editor-in-Chief) and three anonymous reviewers for their helpful comments that notably improved the quality of the original manuscript.

The effect of natural CO2 leakage through water wells on groundwater chemistry from alluvial aquifers of Hamadan, Iran, has been investigated through analysis of water samples from 5 springs and 19 wells. The average CO2 partial pressure in gas charged groundwater has increased about 32 times with respect to background groundwater, leading to an increase in alkalinity and in the concentration of all ions, except for SO4, and to a decrease in pH and DO. Due to a high pH buffering capacity, pH of gas charged groundwater has decreased only one unit. The increase in salinity of the gas charged groundwater cannot be attributed to in situ weathering of aquifer materials because of (1) the lack of correlation between DIC vs δ13CDIC and TDS vs pH, (2) the high concentration of SiO2 and F and (3) the 87Sr/86Sr ratio in the range from 0.7085 to 0.7118. Instead, it can be attributed to saline CO2-rich waters from deep sources, which can dissolve a variety of minerals during their migration towards the surface. Although it is not clear the role of CH4 as electron donor, the association of δ18OSO4 and δ34SSO4 with SO4 concentration suggests that sulfate reduction could occur in the environment. The salinity of Mesozoic gas-rich springs, which present higher CO2 pressure and lower pH, is five times lower than that of Cenozoic ones because of the different degrees of metamorphism, which lead to an increase in grain size and slower reaction rate in Mesozoic than in Cenozoic carbonate rocks., B.D. acknowledges the financial support received from the “Iran’s Ministry of Science, Research and Technology, Iran” (Ph.D. students’ sabbatical grants), Hamadan regional water company and Geological survey of Iran. V.V. acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Program through the Starting Grant GEoREST (www.georest.eu), grant agreement No. 801809. The authors thank for technical and human support provided by SGIker of UPV/EHU and European funding (ERDF and ESF).

Aluminum precipitates control the hydrochemistry and mineralogy of a broad variety of environments on Earth (e.g., acid mine drainage, AMD, coastal wetlands, boreal and alpine streams, tropical acid sulfate soils, laterites and bauxites, …). However, the geochemical and mineralogical processes controlling Al (and other associated metals and metalloids) transport and removal in those environments are not fully understood. The geochemical system of Paradise Portal (Colorado, USA) comprises sulfate-rich mildly acidic waters, the hydrochemistry of which is directly controlled by the massive precipitation of hydrobasaluminite Al4(SO4)(OH)10·12-36H2O. Three connected but discernible aluminum precipitation stages were identified and described: 1) nanoparticle formation and size decrease along the creek, 2) hydrobasaluminite neoformation on the riverbed, and 3) precipitate accretion and accumulation on the riverbed leading to Al and Fe banded formations. The co-occurrence of Al and Si in the system was observed, recording significant amounts of Si accompanying the three different components of the system (i.e., nanoparticles and fresh and aged Al-precipitates). Also, abrupt and minor changes in the sedimentary record were described and proposed to be the response of the system to seasonal and interannual changes in AMD chemistry. Concerning the mobility of other metals and metalloids, P, Th, V, W, Ti and B showed a tendency to be preferentially incorporated into hydrobasaluminite, while others like Be, As, Se or Ba tend to remain dissolved in the water. © 2019 Elsevier B.V., This study was partially financed by the program Fondecyt Iniciación N°11150002 and U-Inicia from the University of Chile. M.A.C. was financially supported by the Spanish Ministry of Education through the Post-doctoral International Mobility Subprogramme I+D+i 2008–2011. M.A.C. gratefully acknowledges the support from the Advanced Mining Technology Center of the University of Chile. The authors thank M. Cabañas and R. Bartrolí (IDAEA-CSIC), Chris Winkler and Stephen McCartney (NanoEarth-NCFL), Cristobal Cantero (Servicios Centrales I+D-UHU) for their analytical assistance. RBW and PLV were supported by the Mineral Resources Program of the U.S. Geological Survey. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement of the U.S. Government. We would also like to thank Dr. Karen Johannesson (Editor) and two anonymous reviewers for their suggestions and comments that significantly improved the quality of the original manuscript.

Scandium, yttrium and lanthanides (REE) are critical raw materials in increasing demand for modern technology, so identifying and developing new sources of REE has become a pressing need. REE concentrations in acid mine drainage (AMD) are several orders of magnitude higher than those in natural water, and their recovery is of economic interest. Passive remediation systems designed to minimize AMD impact on the ecosystem retain REE in solid waste, where basaluminite, Al4SO4(OH)10·5H2O, is the mineral responsible for the scavenge. However, no information about the retention mechanisms of REE is currently available in the literature. The objective of the present work is to study the adsorption of lanthanides, yttrium and scandium onto synthetic basaluminite over a pH range of 4–7 at room conditions. Since sulfate is ubiquitous in AMD, the adsorption has been investigated with variable sulfate concentrations. Experimental results show that sorption onto basaluminite is strongly dependent on pH, starting at pH 5 for lanthanides and yttrium and at pH 4 for scandium. At any given pH values, sorption increases with sulfate concentration. Distribution coefficients, defined as KD = [REEsorbed]/[REEsolution], are higher for Sc, and across the lanthanide series, the distribution coefficients increase from La to Lu according to decreasing ionic radius, where yttrium is considered close to Ho. Experimental results were modeled using a sorption model that considers mass law equations where the strong sulfate aqueous complex, MSO4 +, is adsorbed by exchanging a proton with the mineral surface. The dependence of the experimental results on pH suggests the formation of monodentate binding for Y and lanthanides. The bidentate complex for Sc is deduced by the two proton exchange per mol of Sc extracted from the experiments. The thermodynamic constants for the surface complexation reactions were obtained from experiments with high sulfate concentration and were successfully applied to the experiment with low sulfate content and different solid-liquid ratios. Therefore, the model can be applied to interpret the REE geochemistry in natural systems with variable pH and sulfate concentrations. © 2019 Elsevier Ltd, This work was funded by the European EIT ‘Morerecovery’ and the Spanish SCYRE (CGL2016-78783-C2-R)] projects. A. L. was also funded by the FPI grant (BES-2014-069978) Ministry of Science, Innovation and Universities (Spain). The authors wish to thank J. Bellés, M. Cabañas, R. Bartrolí and N. Moreno (IDAEA-CSIC) for their analytical assistance. The manuscript has been greatly improved with the comments of Prof. Villalobos and three anonymous reviewers.

The Salar de Atacama (SdA) is the largest Li reserve globally. The origin of Li, together with the rest of solutes, has been object of debate. Thus, rock weathering at low temperature, hydrothermal leaching or magmatic origin together with subsequent evaporation has been hypothesized. However, the extreme Li enrichment (>4000 mg/L) and the location of the Li-Mg-rich brines around the Salar Fault System (SFS) that crosses the nucleus of the SdA in half remain unexplained. The objective of this work is to define the thermohaline groundwater flow in the SdA basin to account for the genesis of its extreme Li enrichment. Thermohaline flow modelling has demonstrated the critical effect of the minimum hydraulic head (MHH) of the regional water table on the groundwater flow of salt flats. The MHH divides the basin into two isolated hydrodynamic systems and constitutes the endpoint towards which the most evaporated brines converge. The spatial mismatch between the locations of the Li-Mg-rich brines in the central-western zone of the nucleus (in the SFS) and the MHH in the easternmost zone of the nucleus discards recent evaporative concentration of the recharge water as the main mechanism of Li enrichment. Moreover, the persistence of a saline interface surrounding the nucleus at depth, regardless of the temperature gradient, also precludes lateral recharge (predominantly from the east) to ascend along the SFS. On the other hand, the computed thermohaline flow is compatible with the remobilization of buried layers of Li-Mg-enriched salts and/or clays by dilute recharge waters coming from the west or southwest of the basin. Here, the role of faults and density-driven flow is key to allow efficient downward and upward flow rates that favour the remobilization of Li and Mg., The authors acknowledge Fabien Magri for sharing the BrineDensity plug-in and FEFLOW for sponsoring the license. The authors thank Craig Simmons and James Wards for sharing their experience during the modelling work and Juan Hidalgo for his fruitful contribution to the discussion. Finally, David Dempsey and an anonymous reviewer are greatly acknowledged for their comments and corrections which significantly improved the original version.

Rare earth elements (REE) concentrations in acid mine drainage (AMD) are several orders of magnitude higher than those in other surface waters and could be a future supplementary source of highly valuable REE. Therefore, knowing the mobility constraints of REE in natural waters, particularly in AMD, is of interest. Precipitation of schwertmannite and basaluminite occurs due to mixing of REE-enriched AMD and neutral streams. The REE fractionation between AMD and solid phases has been investigated in several localities in the Odiel-Tinto (SW Spain) and Arroyo del Val (N Spain) basins. The mobility of REE is related to the precipitation of Fe and Al phases that occurs at different pH values. The behavior of REE during the mixing is conservative under low pH conditions (pH, This work has been funded by the European EIT-Raw Materials ‘Morecovery’ and the Spanish SCYRE (CGL2016-78783-C2-R) and CAPOTE (CGL2017-86050-R) projects. A. L. was also funded by the FPI grant (BES-2014-069978). The authors wish to thank J. Bellés, M. Cabañas, R. Bartrolí and N. Moreno (IDAEA-CSIC) for their analytical assistance.

The Chaco-Pampean Plain (CPP), Argentina, is one of the most extensive loess areas in the world, higlhly productive in terms of agriculture and livestock and with a generalized groundwater signature of Na-HCO3 water type. High As concentrations have been often reported in aquifers throughout the entire CPP related to higher pH and alkalinity values. The processes leading to the groundwater from the humid Matanza-Riachuelo Aquifer System (MRAS) to acquire its alkalinity and pH have been investigated by means of detailed sampling of water and solid phases and geochemical modeling (Phreeqc code). The model has also been applied to two extra areas of the CPP with different climates: the semi-arid Río Dulce Alluvial Aquifer (RDA), and the arid Northern area of the La Pampa province (NLP). The analyses of groundwater from the three different regions (from humid to arid) confirm the positive correlation between arsenic and the high pH and HCO3 concentrations. The model has offered insights into key soil-rock interaction parameters that determine the groundwater chemistry of the CPP. Thus, in addition to the already known calcite precipitation, Ca retention by the volcanic glass exchange complex has appeared as crucial for groundwater to acquire the characteristic Na-HCO3 signature. Moreover, the model has also highlighted the role of low CO2 pressures in attaining the high pH values measured in poorly vegetated soils. Of particular significance is the unexpected role played by the formation of secondary alumino-silicates in increasing (rather than decreasing) pH. Further investigations on the role of volcanic glass in the soil exchange properties and the chemical composition of the exchange complex together with the accurate determination of the clay mineralogy are needed to understand the high pH and alkalinity values of groundwater of this vast region., This work has been funded by the Autoridad de Cuenca Matanza Riachuelo (ACUMAR). S.A. The first author thanks the National Council of Scientific and Technical Research (CONICET, Argentina) for doctoral fellowship.

The Salar de Atacama is one of the most well-known saline endorheic basins in the world. It accumulates the world main lithium reserves and contains very sensitive ecosystems. The objective of this work is to characterize the hydrodynamics of the Salar de Atacama, and to quantify its complex water balance prior to the intense brine extraction. The methodology and results can be extrapolated to the groundwater flow and recharge of other salt flats. A three-dimensional groundwater flow model using low computational effort was calibrated against hundreds of hydraulic head measurements. The water infiltrated from the mountains ascends as a vertical flux through the saline interface (mixing zone) produced by the density contrast between the recharged freshwater and the evaporated brine of the salt flat nucleus. This water discharges and is largely evaporated from lakes or directly from the shallow water table. On the other hand, the very low hydraulic gradients, coupled with the presence of the mixing zone that operates as barrier, leads the salt flat nucleus to act as a hydrodynamically quasi-isolated area. The computed water table shows the lowest hydraulic head in the salt flat nucleus near the discharge at the mixing zone. The groundwater balance of the Salar de Atacama in its natural regime was quantified resulting in an inflow/outflow of 14.9 m3·s−1. This balance considers the basin as an endorheic system. The very low infiltration values that are generally assumed for hyperarid basins are not consistent with the hydrogeology of the Salar de Atacama. Indeed, very high infiltration rates (up to 85% of rainfall) occur because of the high degree of fracturing of rocks and the scarce vegetation. This high infiltration is consistent with the light isotopic composition of the water from the recharge area (Altiplano). Therefore, the existence of additional inflows outside the basin is unlikely. © 2018, The authors acknowledge Sociedad Química y Minera de Chile S.A. (SQM) for their support and sharing data throughout the hydrogeological characterization of the SdA site. M.A. Marazuela gratefully acknowledges the financial support from the AGAUR (Agència de Gestió d'Ajuts Universitaris I de Recerca, Generalitat de Catalunya) and the European Union (grant number 2017FI B1 00194). Finally, we thank three anonymous reviewers for their valuable comments.

The Salar de Atacama is used as a case study to analyse and quantify coupled natural (evaporation and recharge) and anthropogenic processes (pumping of lithium-rich brine) to abstract their patterns to other salt flats using a three-dimensional groundwater flow model. Important changes in the dynamics of the water table between the pre-operational period (1986–1994) and operational period (1994–2015) are observed. The water table exhibited a gradual drawdown during the pre-operational period because the evaporation was greater than the recharge for most of these periods. This negative balance was counteracted by some sharp rises that were produced by direct rainfall events on the salt flat. The deep lateral recharge that arrived from the mountains did not produce abrupt changes in the water table because the rainfall events in the mountains were damped by the distance of the recharge zone and great thickness of the unsaturated zone. The natural evolution of the water table was modified by the intensive brine pumping that was performed in the south-western Salar de Atacama during the operational period. As evaporation depends on the water table depth, the pumping caused a drawdown of the water table, resulting in an evaporation rate reduction that partially compensated for the pumped brine in the water balance of the basin. This effect is defined as the damping capacity of salt flats. Thus, salt flats have a high capacity for dampening oscillations in their water table in response to both natural and anthropogenic disturbances which is of great importance for the management of lake and wetland ecosystems and brine exploitation. The limit of the dampening capacity of salt flats is defined by the evaporation extinction depth, which is in the range of 0.5–2 m. © 2018 Elsevier B.V., The authors acknowledge Sociedad Química y Minera de Chile S.A. for their support and sharing data throughout the hydrogeological characterization of the SdA site. M.A. Marazuela gratefully acknowledges the financial support from the AGAUR (Agència de Gestió d’Ajuts Universitaris i de Recerca, Generalitat de Catalunya) and the European Union (grant number 2018FI B2 00068). Finally, we thank four anonymous reviewers for their valuable comments.

The Odiel River Basin (SW Spain) drains the central part of the Iberian Pyrite Belt (IPB), a world-class example of sulfide mining district and concomitantly of acid mine drainage (AMD) pollution. The severe AMD pollution and the incipient state of remediation strategies implemented in this region, coupled with the proximity of the deadline for compliance with the European Water Framework Directive (WFD), urge to develop a restoration and water resources management strategy. Furthermore, despite the presence of some reservoirs with acid waters in the Odiel basin, the construction of the Alcolea water reservoir has already started. On the basis of the positive results obtained after more than 10 years of developing a specific passive remediation technology (dispersed alkaline substrate (DAS)) for the highly polluted AMD of this region, a restoration strategy is proposed. The implementation of 13 DAS treatment plants in selected acid discharges along the Odiel and Oraque sub-basins and other restoration measurements of two acidic creeks is proposed as essential to obtain a good water quality in the future Alcolea reservoir. This restoration strategy is also suggested as an economically and environmentally sustainable approach to the extreme metal pollution affecting the waters of the region and could be considered the starting point for the future compliance with the WFD in the Odiel River Basin. © 2016, Springer-Verlag Berlin Heidelberg., This work was supported by the Spanish Ministry of Economic and Competitiveness through the projects EMPATIA (CGL2013-48460-C2-1-R) and SCYRE (CGL2016-78783-C2-1-R) and LIFE-ETAD project (LIFE12 ENV/ES/000250) and partially financed by Program U-INICIA VID 2014, grant 193, University of Chile, and by CONICYT, Program Fondecyt Iniciacion, Project Nº 111500002. R. Pérez-López also thanks the Spanish Ministry of Science and Innovation and the “Ramón y Cajal Subprogramme” (MICINN-RYC 2011). M.A. Caraballo gratefully acknowledges the support from the Advanced Mining Technology Center of the University of Chile. C.R. Cánovas was funded by the European Union’s Seventh Framework Program, Marie Skłodowska-Curie actions, and the Ministry of Economy, Innovation, Science and Employment of the Junta de Andalucía by the program Talent Hub (COFUND-Grant Agreement 291780). We would also like to thank Dr. Philippe Garrigues (editor in chief) and an anonymous reviewer for the support and comments that significantly improved the quality of the original manuscript.

Extensive extraction of technology critical elements (TCEs) from the lithosphere and their use results in a growing dispersion and remobilisation of these elements within the environmental compartments. We investigated the concentration and mobility of different TCEs (rare earth elements (REEs), Sc, Y, Ga and Tl) in acid mine drainage (AMD) outflows from a massive sulfide underground mine in south-west Spain for around 2 years. High levels of TCEs were observed; average concentrations of 8.2 mg L-1 of REEs, 1.5 mg L-1 of Y, 80 g L-1 of Ga, 53 g L-1 of Sc and 42 g L-1 of Tl were reported, several orders of magnitude higher than those observed in natural waters. The TCEs source in the study site is primarily accessory minerals in the host rocks, although the contribution of Ga and Tl by sulfides cannot be discarded. A seasonal variability in TCEs is observed in AMD waters, although their maximum concentrations do not coincide with those of sulfide-related elements. TCEs seem not to be controlled by the precipitation of secondary minerals, but by the intensity of chemical weathering inside the mined zone. A positive correlation between REEs and the Si/Na+K ratio seems to indicate that these elements are linked to resistant minerals to weathering. © 2019 CSIRO., This work was funded by the H2020 EIT Raw Materials program through the project MORECOVERY “Modular recovery services for hydrometallurgy and water treatment (H2020-EIT-PN 18190)” and by the Spanish Ministry of Science, Innovation and Universities through the SCYRE project (CGL2016-78783-C2-R). C.R. Cánovas was funded by the Talent Consolidation Program of the University of Huelva. MD Basallote also thanks the Spanish Ministry of Science, Innovation and Universities for the Postdoctoral Fellowship granted under application reference FJCI-2015-24765.

Understand the origin of the exotic strontium-rich calcite/magnesium-rich clay/silica deposits formed during the Early Cretaceous in the South Atlantic Pre-salt lakes is challenging our ability to comprehend the chemical evolution of alkaline volcanic lakes. Here we present a new hydrochemical model based on an open basin concept, thermodynamic equilibrium, and chemical data from Lake Baringo (East African Rift) to shed light on the mechanisms facilitating carbonate-clay-silica precipitation. This model explores the effects that the leakage to adjacent waterbodies, the combination of lake evaporation and lake recharge (with river, hydrothermal and marine waters), and the bathymetric effects of the partial CO2 pressure have in the stationary precipitation of Pre-salt lacustrine mineral assemblages. Sediment thickness calculations suggest that the facies cyclicity in the Pre-salt and elsewhere are likely reflecting temporal fluctuations in the type of waters sourcing the lake, and in the degree of leakage to aquifers over multiple evaporative-freshening cycles. A plausible link between enhanced strontium uptake into Pre-salt calcite allochems and [Ca2+]/[CO3 2−] lake water stoichiometries allows to infer different scenarios for strontium-rich calcite formation with or without magnesium clays. Our conceptual model allows to quantify the role that hydrology (groundwater charge, discharge to aquifers; evaporation) exert on carbonate-clay-silica precipitation providing with a refined framework to understand the basin-scale chemical sedimentation in alkaline lakes. Furthermore, this model represents a robust template upon which critically test the superimposed kinetic factors likely attached to the genesis of specific mineral factories such as those encountered in the South Atlantic Pre-salt alkaline lakes. © 2019 Elsevier B.V., Gernot Arp, Marcelo Rezende, and Mateu Esteban are warmly thanked for stimulating discussions on Pre-salt spherulites-shrubs. Repsol Exploración and Jeferson Luiz Dias are kindly acknowledged for permission to publish the Pre-salt and borehole images respectively. Support from Origins Center project190438131, NWO and Dutch National Science Agenda (NWA) program is greatly appreciated. Albert Nardi (Barcelona Science) is thanked for yielding us permission to use a developing version of GibbsStudio software. Franco Pirajno and two anonymous reviewers are kindly acknowledged for their constructive and detailed comments which improved the final version of this paper. Joan-Albert Sánchez-Cabeza is thanked for his effective editorial work.

The skin effect is an important issue associated with a loss of hydraulic efficiency in pumping systems. In this paper, a comprehensive study is conducted to determine how the evolution of the skin effect over time affects the productivity and functionality of brine exploitation systems in a crystalline halite aquifer. Several double-packer system tests have been interpreted via groundwater modeling to hydrogeologically characterize the investigated aquifer. A numerical groundwater flow model that accounts for the heterogeneity of the multilayer halite aquifer and the transient well skin effect on the brine exploitation system is presented to explain the continuous drawdown during three months of constant-rate brine extractions. Numerical results obtained suggested that an exponential decay function of permeability over time was required in a wellbore skin zone to reproduce numerically experimental observations during brine exploitation. The empirical exponential function obtained specifically for this case study was generalized, and coefficients considered were discussed to infer their physical and geochemical dimensions related to the mixing process triggering heterogenous reactions responsible for the hydraulic loses. Our results will be useful for predicting the spatial and temporal losses of hydraulic efficiency and for evaluating the lifetimes of brine exploitation infrastructure. © 2019 Elsevier B.V., Personnel of the Sociedad Química y Minera de Chile (SQM) generously provided access to data, accommodations and logistical support in the field. The authors would like to extend gratitude to the associate editor and two anonymous reviewers for their valuable and constructive comments that led us to an improvement of the work.

Underground Pumped Storage Hydropower (UPSH) is an alternative to manage the electricity production in flat regions. UPSH plants consist of two reservoirs of which at least one is underground. For this last reservoir, abandoned mines could be considered. UPSH related activities may induce hydrochemical variations, such as the increase of the oxygen (O2) partial pressure (pO2), which may entail negative consequences in terms of environment and efficiency, especially in coal mined areas where the presence of sulfide minerals is common. This work assesses the main expected environmental impacts that UPSH using abandoned coal mines may induce. © 2017 The Authors. Published by Elsevier Ltd., E. Pujades and A. Jurado gratefully acknowledge the financial support from the University of Liège and the EU through the Marie Curie BeIPD-COFUND postdoctoral fellowship programme (2014/16 and 2015/17 fellows from “FP7-MSCA-COFUND, 600405”). This research has been supported by the Public Service of Wallonia – Department of Energy and Sustainable Building.

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. © Author(s) 2016., This work was funded by the U.S. Federal Aviation Administration (FAA) Office of Environment and Energy as a part of ASCENT Project 13-C-AJFE-GIT-008 under FAA Award No. 27A.