Pétrola Lake in southeast Spain is one of the most representative examples of hypersaline wetlands in southern Europe. The rich ecosystem and environmental importance of this lake are closely associated with the hydrogeological behaviour of the system. The wetland is fed by the underlying aquifer with relatively fresh groundwater—1 g L⁻¹ of total dissolved solids (TDS)—with a centripetal direction towards the wetland. In addition, the high evaporation rates of the region promote an increase in the concentration of salts in the lake water, occasionally higher than 80 g L⁻¹ TDS. The density difference between the superficial lake water and the regional groundwater can reach up to 0.25 g cm⁻³, causing gravitational instability and density-driven flow (DDF) under the lake bottom. The objective of this study was to gain an understanding of the geometry of the freshwater–saltwater interface by means of two-dimensional mathematical modelling and geophysical-resistivity-profile surveys. The magnitude and direction of mixed convective flows, generated by DDF, support the hypothesis that the autochthonous reactive organic matter produced in the lake by biomass can be transported effectively towards the freshwater–saltwater interface areas (e.g. springs in the lake edge), where previous research described biogeochemical processes of natural attenuation of nitrate pollution.

Coastal groundwater flow is driven by an interplay between terrestrial and marine forcings. One of the distinguishing features in these settings is the formation of a freshwater lens due to the density difference between fresh and saline groundwater. The present study uses data collected on Sable Island, Canada—a remote sand island in the northwest Atlantic Ocean—to highlight the potential of exploiting freshwater lens geometry for calibration of numerical groundwater flow models in coastal settings. Three numerical three-dimensional variable-density groundwater flow models were constructed for different segments of the island, with only one model calibrated using the freshwater–saltwater interface derived from an electromagnetic geophysical survey. The other two (uncalibrated) models with the same parameterisation as the calibrated model successfully reproduced the interpreted interface depth and location of freshwater ponds at different parts of the island. The successful numerical model calibration, based solely on the geophysically derived interface depth, is enabled by the interface acting as an amplified version of the water table, which reduces the relative impact of the interpreted depth uncertainty. Furthermore, the freshwater–saltwater interface is far more inertial than the water table, making it less sensitive to short-term forcings. Such “noise-filtering” behaviour enables the use of the freshwater–saltwater interface for calibration even in dynamic settings where selection of representative groundwater heads is challenging. The completed models provide insights into island freshwater lens behaviour and highlight the role of periodic beach inundation and wave overheight in driving short-term water-table variability, despite their limited impact on the interface depth.

Dryland inland freshwater lenses (IFLs) that have been topographically induced are represented using physically modeled laboratory simulations, to characterize the stages of IFL evolution (i.e. formation, migration, degradation) as a function of recharge rate. Arid regions with shallow brackish to saline groundwater possess IFLs. The position and geometry (i.e. thickness, length) of IFLs over varying temporal and spatial scales is poorly understood due to their transient nature. The physically modeled IFLs in this study formed from an initial recharge pulse, after which IFL geometry was measured over time as it flowed in the direction of simulated groundwater flow. The time required for an IFL to reach the maximum thickness exhibited a negative exponential correlation to recharge rate. At IFL formation, thickness and length were positively correlated, and the ratio of IFL thickness to length exhibited a positive exponential correlation to recharge rate. After IFL formation, the central position of the simulated IFLs migrated laterally in the direction of groundwater flow at a velocity less than the range of applied recharge rates and greater than the groundwater flow velocities. The time required for the IFL to reach a minimum thickness, or IFL degradation, exhibited a positive exponential correlation to recharge rate. The Dupuit-Ghyben-Herzberg solution used to model coastal freshwater lens thickness was tested against the physically modeled IFLs and deemed invalid. A correction factor and modified solution are provided to predict IFL thickness, providing motivation for future analytical and numerical studies on inland variable-density groundwater systems in arid regions globally.

Permafrost in Arctic watersheds limits soil biological activity to a thin, seasonally thawed active layer that contributes water to streams. In many hillslopes, relatively wet drainage features called water tracks have distinct freeze-thaw patterns that affect groundwater flow and storage, and thus the export of heat and solutes to Arctic streams. This study uses groundwater flow and energy transport models to examine potential controls on the timing and duration of freeze–thaw conditions and the magnitude of temperature fluctuations within water tracks and their adjacent hillslopes. The simulated length of the active-layer thaw season varies by 1 month over the range of snow-cover and mean annual air-temperature scenarios simulated. The timing and duration of freezing is particularly sensitive to depth and duration of snow cover. Thus, the deeper snowpack covers that can accumulate in water tracks contribute to their more persistent thaw conditions and their ability to conduct groundwater downslope. A three-dimensional simulation shows that during the summer thaw season, the water track captures groundwater laterally from half way across the hillslope. The models presented here elucidate key mechanisms driving small-scale variation in the active-layer thermal regime of tundra hillslopes, which may be responsible for changes in drainage-network geometry and Arctic biogeochemical fluxes under a warming climate.

Three-dimensional geological and groundwater flow models of a submarine groundwater discharge (SGD) site at Hanko (Finland), in the northern Baltic Sea, have been developed to provide a geological framework and a tool for the estimation of SGD rates into the coastal sea. The dataset used consists of gravimetric, ground-penetrating radar and shallow seismic surveys, drill logs, groundwater level monitoring data, field observations, and a LiDAR digital elevation model. The geological model is constrained by the local geometry of late Pleistocene and Holocene deposits, including till, glacial coarse-grained and fine-grained sediments, post-glacial mud, and coarse-grained littoral and aeolian deposits. The coarse-grained aquifer sediments form a shallow shore platform that extends approximately 100–250 m offshore, where the unit slopes steeply seawards and becomes covered by glacial and post-glacial muds. Groundwater flow preferentially takes place in channel-fill outwash coarse-grained sediments and sand and gravel interbeds that provide conduits of higher hydraulic conductivity, and have led to the formation of pockmarks on the seafloor in areas of thin or absent mud cover. The groundwater flow model estimated the average SGD rate per square meter of the seafloor at 0.22 cm day⁻¹ in autumn 2017. The average SGD rate increased to 0.28 cm day⁻¹ as a response to an approximately 30% increase in recharge in spring 2020. Sensitivity analysis shows that recharge has a larger influence on SGD rate compared with aquifer hydraulic conductivity and the seafloor conductance. An increase in recharge in this region will cause more SGD into the Baltic Sea.