The groundwater flow and the transport of solutes and contaminants in fractured geologic media play a very important role in various hydrogeological and geological processes. Fractures are discontinuities that occur in practically all types of rocks, consolidated and semi-consolidated sediments, in which groundwater flows at different scales of space and time. This article reviews more than 20 years of research in the CGEO of different selected examples in Mexico, from local to regional scales, associated with 1) gravitational Groundwater Flow Systems, 2) hydrogeochemical interaction of groundwater with fractured rocks through which it circulates, 3) instrumentation and coupled numerical analysis of flow parameters and time -varying geomechanics, during consolidation associated with pumping, 4) analysis of fracture generation with the development and application of coupled flow and geomechanical equations, 5) formation of new minerals, 6) sustenance of ecosystems, 7) artificial fracturing of soils for their conservation and infiltration of rainwater improvement; and on the issue of transport of natural solutes, 8) used as a tracers, 9) toxic elements to health and environment, 10) spills of hydrocarbon derivatives in low permeability and double porosity media due to fracturing and 11) heat. The results show the importance of the physical -chemical interaction between fractured and granular geological media at both local and regional scales, where groundwater residence times range from a few days to thousands of years; which implies modifying the criteria for water management and the permanence of ecosystems in the country. The complexity of these processes requires different methodologies for their evaluation, among them the instrumentation and calibration of numerical models from 1D to 3D for analysis, predictions and the proposal of restoration, sustainability and management solutions; they also help to prevent, control and mitigate the negative impacts on health and the environment caused by the induction of geogenic elements and by various types of pollutants; fractured geologic media also support numerous terrestrial and marine ecosystems; in the case of damaged agricultural soils, artificial fracturing allows increasing rainwater infiltration and improving productivity in adaptation to climate change and reducing the extraction in aquifers where safe capacity has been exceeded; unfortunately, excessive extraction in closed basins is causing fracturing of the aquitards, both hydraulic and due to differential settlement, which favors the migration of pore water rich in elements harmful to human health and the environment, whose natural function was its protection. All this requires designing mechanisms for the transfer of scientific knowledge to society and decision makers to propose novel restoration and sustainability strategies, under the new paradigm of Gravitational Groundwater Flow Systems.

Extreme precipitation events (EPEs) are projected to become more frequent and intense due to global warming. Understanding how coastal groundwater levels respond to and recover from these severe events is important for estuarine ecosystems to adapt to global change. Numerical model and non-EPE scenario simulation were used to examine groundwater level recovery time (RT) after Super Typhoon Lekima, which triggered EPEs that resulted in groundwater rise and widespread flooding in the Yellow River Delta (YRD). The three-day rainfall during Lekima totaled 290.9 mm, equivalent to 50 % of the annual rainfall for 2019 (581.5 mm), leading to a general rise in groundwater levels. Groundwater recovery to EPE can be divided into two types: inland and coastal. The RT of groundwater levels in monitoring wells in inland areas ranged from 12 to 89 days, with an average of 56.2 days, and there was spatial variation. However, groundwater levels in monitoring wells close to the coast may not recover. Differences in recovery are reflected in the land-sea gradient, with RT gradually increasing from inland highlands to coastal depressions and lowlands. The results showed that inland aquifers were more resistant to EPEs, while coastal aquifers were less resistant. In addition, EPE can cause groundwater flooding, and areas at lower altitude and close to the sea are more sensitive to flooding. Estuarine groundwater and the ecological processes on which it depends are profoundly affected by the direct and legacy effects of EPEs, including salt contamination, widespread flooding, crop damage, and reduced biodiversity. The study of this event provides case support for the response of estuarine groundwater to EPEs, while highlighting the importance of continuous monitoring.

The mapping of water content at a landslide sensitive area is important in order to identify the potency of ground motion. In such an area a minuscule amount of movement may lead to a catastrophic event. Water, which may act as a precursor of ground motion, changes the mechanical properties of the land, hence, changing the ability of the ground to resist gravitational force. In order to identify the water containment, as well as the flow of groundwater, we apply a geophysical method, namely the Self Potential (SP) measurement. Based on the analysis of Darcy's law the measurement result is related directly to the flow velocity. Although the measurement was performed on top of soil, the measured quantity is a response due to the amount of water infiltration into the soil. The mapped profile of the measurement identifies the flow pattern of groundwater. The result can be used to estimate the soil instability and the potency of landslide events. Our data shows the distribution of the groundwater in the observed area, which can be used as a hint to design the drainage system, in order to divert water from the landslide prone areas. The main goal of this work is to minimize the risk to the community by preventing groundwater flow from targeting inhabited regions, thus ensuring the safety of the residents.

Under a warming climate, permafrost degradation has resulted in profound hydrogeological consequences. Here, we mainly review 240 recent relevant papers. Permafrost degradation has boosted groundwater storage and discharge to surface runoffs through improving hydraulic connectivity and reactivation of groundwater flow systems, resulting in reduced summer peaks, delayed autumn flow peaks, flattened annual hydrographs, and deepening and elongating flow paths. As a result of permafrost degradation, lowlands underlain by more continuous, colder, and thicker permafrost are getting wetter and uplands and mountain slopes, drier. However, additional contribution of melting ground ice to groundwater and stream-flows seems limited in most permafrost basins. As a result of permafrost degradation, the permafrost table and supra-permafrost water table are lowering; subaerial supra-permafrost taliks are forming; taliks are connecting and expanding; thermokarst activities are intensifying. These processes may profoundly impact on ecosystem structures and functions, terrestrial processes, surface and subsurface coupled flow systems, engineered infrastructures, and socioeconomic development. During the last 20 years, substantial and rapid progress has been made in many aspects in cryo-hydrogeology. However, these studies are still inadequate in desired spatiotemporal resolutions, multi-source data assimilation and integration, as well as cryo-hydrogeological modeling, particularly over rugged terrains in ice-rich, warm (>-1 degrees C) permafrost zones. Future research should be prioritized to the following aspects. First, we should better understand the concordant changes in processes, mechanisms, and trends for terrestrial processes, hydrometeorology, geocryology, hydrogeology, and ecohydrology in warm and thin permafrost regions. Second, we should aim towards revealing the physical and chemical mechanisms for the coupled processes of heat transfer and moisture migration in the vadose zone and expanding supra-permafrost taliks, towards the coupling of the hydrothermal dynamics of supra-, intra- and sub-permafrost waters, as well as that of water-resource changes and of hydrochemical and biogeochemical mechanisms for the coupled movements of solutes and pollutants in surface and subsurface waters as induced by warming and thawing permafrost. Third, we urgently need to establish and improve coupled predictive distributed cryo-hydrogeology models with optimized parameterization. In addition, we should also emphasize automatically, intelligently, and systematically monitoring, predicting, evaluating, and adapting to hydrogeological impacts from degrading permafrost at desired spatiotemporal scales. Systematic, in-depth, and predictive studies on and abilities for the hydrogeological impacts from degrading permafrost can greatly advance geocryology, cryo-hydrogeology, and cryo-ecohydrology and help better manage water, ecosystems, and land resources in permafrost regions in an adaptive and sustainable manner.

As a result of global warming induced permafrost degradation in recent decades, thermokarst lakes in the Qinghai-Tibet plateau (QTP) have been regulating local hydrological and ecological processes. Simulations with coupled moisture-heat numerical models in the Beiluhe basin (located in the hinterland of permafrost regions on the QTP) have provided insights into the interaction between groundwater flow and the freeze-thaw process. A total of 30 modified SUTRA scenarios were established to examine the effects of hydrodynamic forces, permeability, and climate on thermokarst lakes. The results indicate that the hydrodynamic condition variables regulate the permafrost degradation around the lakes. In case groundwater recharges to the lake, a low-temperature groundwater flow stimulates the expansion of the surrounding thawing regions through thermal convection. The thawing rate of the permafrost underlying the lake intensifies when groundwater is discharged from the lake. Under different permeability conditions, spatiotemporal variations in the active layer thickness significantly influence the occurrence of an open talik at the lake bottom. A warmer and wetter climate will inevitably lead to a sharp decrease in the upper limit of the surrounding permafrost, with a continual decrease in the duration of open talik events. Overall, our results underscore that comprehensive consideration of the relevant hydrologic processes is critical for improving the understanding of environmental and ecological changes in cold environments.

A two-dimensional (2D) cryo-hydrogeological numerical model of groundwater flow, coupled with advective-conductive heat transport with phase change, has been developed to study permafrost dynamics around an ice-rich permafrost mound in the Tasiapik Valley near Umiujaq, Nunavik (Quebec), Canada. Permafrost is degrading in this valley due to climate warming observed in Nunavik over the last two decades. Ground temperatures measured along thermistor cables in the permafrost mound show that permafrost thaw is occurring both at the permafrost table and base, and that heat fluxes at the permafrost base are up to ten times higher than the expected geothermal heat flux. Based on a vertical cross- extracted from a 3D geological model of the valley, the numerical model was first calibrated using observed temperatures and heat fluxes. Comparing simulations with and without groundwater flow, advective heat transport due to groundwater flow in the subpermafrost aquifer is shown to play a critical role in permafrost dynamics and can explain the high apparent heat flux at the permafrost base. Advective heat transport leads to warmer subsurface temperatures in the recharge area, while the cooled groundwater arriving in the downgradient discharge zone maintains cooler temperatures than those resulting from thermal conduction alone. Predictive simulations incorporating a regional climate-change scenario suggest the active layer thickness will increase over the coming decades by about 12 cm/year, while the depth to the permafrost base will decrease by about 80 cm/year. Permafrost within the valley is predicted to completely thaw by around 2040.

This study simulates and quantifies the exchange and the pathways of deep and shallow groundwater flow and solute transport under different climate and permafrost conditions, considering the example field case of the coastal Forsmark catchment in Sweden. A number of simulation scenarios for different climate and permafrost condition combinations have been performed with the three-dimensional groundwater flow and transport model MIKE SHE. Results show generally decreasing vertical groundwater flow with depth, and smaller vertical flow under permafrost conditions than under unfrozen conditions. Also the overall pattern of both the vertical and the horizontal groundwater flow, and the water exchange between the deep and shallow groundwater systems, change dramatically in the presence of permafrost relative to unfrozen conditions. However, although the vertical groundwater flow decreases significantly in the presence of permafrost, there is still an exchange of water between the unfrozen groundwater system below the permafrost and the shallow groundwater in the active layer, via taliks. 'Through taliks' tend to prevail in areas that constitute groundwater discharge zones under unfrozen conditions, which then mostly shift to net recharge zones (through taliks with net downward flow) under permafrost conditions.