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In western Mediterranean areas, seawater intrusion (SWI) used to be a major issue during the 1970’s decade due to the agricultural exploitation of the fertile lands located near to the coast. Proper pumping management nearly solved the problem, but the later economic development carried out by industries and tourism added an extra pressure to these sensitive systems, reactivating SWI in some cases. A good example of this situation can be found in the northeastern coast of Spain. In this area, the topographic configuration is controlled by medium reliefs and sloped piedmonts that discharge precipitation through ephemeral streams towards the sea. These torrential flows have a groundwater component that generates a coeval submarine groundwater discharge (SGD) with the intrusion of seawater. SGD strongly determines the health of marine environments, reason why is classically neglected in SWI research: in this area, SGD has been characterized measuring natural radioactive tracers (223Ra, 224Ra, 226Ra, 228Ra) in open sea campaigns, but the impact of mixing of this seaward flow and SWI within the aquifer remains unknown. The presence of different lithologies also determines the kind of geochemical reactions that modify the conservative behavior of the tracers used to measure SGD, as well as the localization of preferential flowpaths, making hard to quantify and predict both types of reciprocal flows (SGD and SWI). To be able to understand these interactions, a detailed characterization of the solid and liquid phase of the aquifer at different scales has been performed. To do so, we built a dedicated experimental site in a coastal alluvial aquifer in the Maresme coast line (Barcelona, Spain): the Argentona experimental site. Different types of geophysical logging (induction, spectral gamma ray and magnetic susceptibility) were deployed in the drilled wells to characterize the salinity gradient and distinguish sedimentary bodies. Also, the lithology and the geochemistry of discrete samples recovered from the drilling stage, were determined at smaller scale using X-ray diffraction, rock total analysis for chemical composition, cation exchange capacity, surface area, 226Ra and 228Ra content and grain size distribution. Moreover, groundwater composition was characterized performing mayor and minor elements analysis and radium and radon isotopes quantifications. This characterization of the geological heterogeneity will serve as a basis for future interpretation of field tests and modelling studies focused on the Argentona experimental site.

The steady increase of geothermal systems using groundwater is compromising the renewability of the geothermal resources in shallow urban aquifers. To ensure sustainability, scientifically-based criteria are required to prevent potential thermal interferences between geothermal systems. In this work, a management indicator (balanced sustainability index, BSI) applicable to groundwater heat pump systems is defined to assign a quantitative value of sustainability to each system, based on their intrinsic potential to produce thermal interference. The BSI indicator relies on the net heat balance transferred to the terrain throughout the year and the maximum seasonal thermal load associated. To define this indicator, 75 heating-cooling scenarios based in 23 real systems were established to cover all possible different operational conditions. The scenarios were simulated in a standard numerical model, adopted as a reference framework, and thermal impacts were evaluated. Two polynomial regression models were used for the interpolation of thermal impacts, thus allowing the direct calculation of the sustainability indicator developed as a function of heating-cooling ratios and maximum seasonal thermal loads. The BSI indicator could provide authorities and technicians with scientifically-based criteria to establish geothermal monitoring programs, which are critical to maintain the implementation rates and renewability of these systems in the cities. © 2018 Elsevier Ltd, This work was performed by the Geological Survey of Spain (IGME) under the Cooperation Agreement framework between IGME and the Ebro Hydrographic Confederation (CHE) for the specific arrangement entitled “The application of a groundwater flow and heat transport numerical model for the simulation of management strategies of geothermal installations in the city of Zaragoza”. Alejandro García-Gil gratefully acknowledges MIKE by DHI for the sponsored FEFLOW license.

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.