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A surface water body fed by groundwater is normally known as a terminal place of groundwater flow systems originating from precipitation recharge on highlands. The theory of Tóth predicted that these flow systems form a hierarchically nested structure of groundwater circulation in a composite basin. In this study, we will report new flow paths among groundwater flow systems that were unknown in Tóth's theory, identified as special seepage paths linking different surface water bodies. These seepage paths do not start from the groundwater table but can transmit water between lakes or streams that already serve as discharge zones of traditional local flow systems. As indicated in theoretical models and two real‐world cases, special seepage paths are developed if some parametric conditions are satisfied, especially when surface water bodies cut deeply below the water table or are large enough. Different surface water bodies or different river reaches can directly exchange water, chemicals and heat through deep seepage paths even when both surface and subsurface water divides exist between them. Special seepage paths may play a role in the regional scale hyporheic flow or contribute to inter‐basin groundwater flow. The knowledge of special seepage paths could greatly improve our conventional perception of surface water‐groundwater interaction, groundwater age and geochemical and heat transport at the river basin scale. [ABSTRACT FROM AUTHOR]

Volcanic regions have complicated groundwater flow systems because of their geological structures, and the effects of rainfall on their groundwater recharge/discharge are still largely unknown. A field survey and chemical analyses were conducted on springs near an andesitic volcano in Japan to investigate changes in groundwater flow processes associated with heavy rainfall. After heavy rainfall, some springs showed a decrease in tracer values while others at lava or pyroclastic flow termini were stable. This indicates that the rainfall component rapidly discharges or the bedrock groundwater component discharges from deeper subsurface areas depending on the structure, length, and permeability of the volcanic ejecta. Hence, the groundwater mixing process in areas with ejecta can be approximated by the binary mixing model. In contrast, tracer values increased after heavy rainfall in areas near agricultural land, which suggests that nitrate ions were loaded into groundwater. Thus, the changes in various groundwater flow processes in volcanic regions after rainfall can be attributed to not only geological/geographical differences but also the groundwater stored in mountain bodies. [ABSTRACT FROM AUTHOR]

Investigating and studying groundwater flow systems is important for water resources management, for pollution prevention and for maintaining the ecological balance in arid and semi-arid areas. Systematic geophysics and hydrogeological investigations allow us to define the thickness of the Quaternary sedimentary layer, the lateral boundary of the groundwater system, and the depth and basement of water circulation. Hydrogeochemistry and environmental isotopes are used to gain insights into the recharge process, water–rock interactions, hydraulic characteristics and groundwater retention time and to identify groundwater flow systems at all levels in the Aksu River Basin. Owing to the dissolution of carbonate and gypsum minerals and evaporites, cation exchange between Ca2+ (Mg2+) and Na+ (K+), and the evaporation–concentration effect, concentrations of SO42−, Cl−, Na+ and total dissolved solids gradually increase along the flow direction and decrease with depth, indicating that they belong to different groundwater flow systems. Furthermore, the interpretation of stable isotope concentrations such as δ18O values suggests different degrees of depletion in the horizontal and vertical directions. Combined with the unique structural framework, the particle size variation of loose sediments and the distribution and aggregation of phreatic water with high F and As and soil salinization show the existence of the surface water and groundwater interaction mixing zones and the distribution pattern of multiple local groundwater flow systems. The vertical zonation of 3H and 14C isotope concentrations and unconformable distribution of groundwater residence time further illustrate the hydrodynamic cycle of the local and regional groundwater flow systems. The hydrodynamic and hydrochemical characteristics confirmed the distribution of groundwater flow systems and the complex mixing relationships between groundwater flow systems in the Aksu River Basin under the tectonic conditions since the Neogene in the South Tianshan Mountains. [ABSTRACT FROM AUTHOR]

Significance Groundwater flow paths may cross the surface divide at the regional scale, resulting in interbasin groundwater circulation that affects hydrological relationships and solute transport process between basins. However, research on interbasin groundwater circulation is still in its infancy internationally, and the progress achieved is a matter of concern. Progress This study systematically tracks and analyses the literature on interbasin groundwater circulation at home and abroad in the past 20 years and summarizes the existing research progress from three perspectives: formation mechanism, identification methods, and impact assessment. In terms of the hydrodynamic formation mechanism, the study theoretically determines the deviation characteristics between the surface divide, the highest point of the water table and the divide point of groundwater flow systems. Based on the deviation characteristics, multiple interbasin groundwater circulation paths can be separated between rivers.In terms of identifying the interbasin groundwater circulation, a series of real basin cases provide available methods, including the water balance method, basin-scale hydrological model and hydrogeochemical end element mixed model. The methods identify the existence of interbasin groundwater circulation and even evaluate the circulation fluxes, which can improve the recognition of the water balance in the basin. It is also found that the location, size, climate and geological conditions of the basin affect the occurrence and flux of interbasin groundwater circulation.In terms of impact assessment, it is preliminarily found that the interbasin groundwater circulation has an important impact on the assessments of climate sensitivity, state parameters of the Budyko framework and carbon source/sink in the basin. Ignoring its role may lead to obviously incorrect conclusions. Conclusions and Prospects At present, research on the dynamic process and material transport effect of interbasin groundwater circulation is relatively weak. Accurate and quantitative evaluation methods are also lacking. The focuses of future research are to reveal the circulation paths of interbasin groundwater in three-dimensional aquifer space and accurately assess the various impacts of interbasin groundwater circulation.

Objective Geological, climatic, and topographical conditions control regional groundwater flow systems. Previous researchers have made significant advancements in developing the theory of groundwater flow systems under steady-state climatic conditions. However, there has been limited progress in comprehensively understanding transient groundwater flow systems. Methods To address this gap, we constructed a two-dimensional numerical model that couples groundwater and surface water using HydroGeoSphere. We then examined the relationships among the subsystems of transient groundwater flow systems in response to variations in rainfall. Results The results demonstrate that the areas occupied by subsystems change with rainfall fluctuations. Local groundwater flow systems may neither expand during wet seasons nor contract during dry seasons. The relationships among the penetration depths, which indicate the elevation of the lowest point in a local flow system, can be positive, negative, or unrelated. This variation mainly arises from the high activity of intermediate groundwater flow systems under transient conditions, where their inputs, outputs, flow paths, and areas of recharge and discharge vary with rainfall fluctuations. These relationships are also sensitive to factors such as geology (hydraulic conductivity, specific storage), climate (rainfall rate), and topography (local and regional). Conclusion Based on a sensitivity analysis of five scenarios, changes in local flow systems are more influenced by aquifer anisotropy. Future research should prioritize conducting a more comprehensive analysis of the non-steady-state response patterns exhibited by groundwater flow systems, considering climate fluctuations on a seasonal scale, over multiple years, and even across centuries.

Thoroughly investigating the evolution of groundwater circulation and its controlling mechanism in the Aksu River Basin, where human activities are intensifying and the groundwater environment is increasingly deteriorating, is highly urgent and important for promoting the theory, development and implementation of groundwater flow systems (GFSs) and protecting groundwater resources. Based on a detailed analysis of the sediment grain size distribution, chronology, electrofacies, glacial sedimentary sequence, palaeoclimate indicators and existing groundwater age, this paper systematically reconstructs the palaeosedimentary environment of the basin-scale aquifer system in the study area and scientifically reveals the evolutionary pattern and formation mechanism of the GFS. The results showed that the later period of the late Pleistocene experienced a rapid downcutting erosional event caused by tectonic uplift, and the sedimentary environment transitioned from a dry–cold deep downcutting environment in the Last Glacial Maximum (LGM) to a coarse-grained fast-filling fluvial facies sedimentary environment in the Last Glacial Deglaciation (LDP) as the temperature rose; then, it shifted to an environment of fine-grained stable alternating accumulation of fluvial facies and lacustrine facies that was dominated by the warm and arid conditions of the Holocene megathermal period (HMP); this process changed the previous river base level via erosion, glacier elongation or shortening and river level, thus resulting in a complex coupling relationship between the palaeosedimentary environment, palaeoclimate and basin GFS. Furthermore, the existing GFS pattern in the basin exhibits a vertically unconformable groundwater age distribution, which indicates that it is the outcome of the complex superposition of groundwater flow controlled by the palaeosedimentary environment in different periods. Therefore, neotectonic movement and climate fluctuation have jointly acted on the variation in the river level, resulting in the "seesaw" effect, thereby fundamentally controlling the strength of the driving force of groundwater and resulting in the gradual evolution of the GFS from the fully developed regional GFS pattern during the LGM to the current multihierarchy nested GFS pattern. [ABSTRACT FROM AUTHOR]

Groundwater vulnerability assessments are vital for protecting valuable resources by revealing susceptibility to contamination. This study developed an enhanced index model to assess the intrinsic vulnerability of a supplied karst aquifer in Qingduo, Henan Province. The model considered the 3-D geological structure and modified indices to account for Northern China's mild karstification. Emphasizing the absolute infiltration capacity of surface contaminants, the model also integrated the groundwater sources and sinks (SS) index. The vulnerability map revealed that over 60% of the aquifers, including the Qingduo wellfield, exhibited very low to low vulnerability. Conversely, only small areas (<5%) along the Kejing (KJ) fault's southern wall were classified as highly vulnerable. These findings highlighted the significant role of groundwater flow alongside aquifer conditions. The upward groundwater flow through the Fengmenkou (FMK) faults slowed the downward infiltration of surface contaminants into the lower karst aquifer, effectively reducing vulnerabilities. Lower levels of dissolved lead (Pb) and nitrate (NO3−) in Qingduo groundwater aligned with PISSR vulnerability mapping. Sensitivity analysis assessed the results' sensitivity to index weight assignment. The inclusion of the sources and sinks (SS) index holds implications for semi-quantitatively assessing dynamic groundwater vulnerability by delineating flow patterns. [ABSTRACT FROM AUTHOR]

The hydrogen and oxygen isotopes in precipitation have the altitude effect, and they participate in the groundwater circulation after the infiltration of precipitation. How does the altitude effect of hydrogen and oxygen isotopes in precipitation transform to the depth effect of hydrogen and oxygen isotopes in groundwater under the influence of groundwater flow systems? The existing research lacks quantification for this problem. In this study, the steady-state groundwater flow theoretical models represented by unidirectional inclined basin and bimodal wavy basin are constructed. The MODFLOW and MT3DMS programs are used to simulate the two-dimensional groundwater flow field in the profile and the convection-dispersion process of heavy isotope molecules to obtain the spatial distribution of D and 18O values in groundwater and discuss the mechanism of altitude effect of the hydrogen and oxygen isotopes transforming to depth effect in the groundwater flow systems. The results indicate that in the monoclinal basin, the altitude effect of D and 18O content in precipitation in the recharge area is transformed to the depth effect of δD and δ18O values in groundwater through the regional groundwater flow system which exponentially decreases with the increasing water table depth in the drainage area. In the bimodal wavy basin, when the permeability of the aquifer is relatively larger than the infiltration intensity (K0/w=1000), only one regional groundwater flow system develops, and the distribution of δD and δ18O presents a S-shaped curve with the increasing water table depth in the discharge area of regional groundwater. When the permeability of the aquifer is relatively smaller than the infiltration intensity (K0/w=250), multiple local groundwater flow systems develop in the bimodal wavy basin. The δD and δ18O indicate an S-shaped curve with the increasing water table depth in the discharge area of regional groundwater, while in the discharge area of local groundwater, they show a monotonic attenuation trend with the increasing water table depth. This study theoretically advances the understanding of the influence mechanism of groundwater flow systems on solute transport, and reveals the indicative role of hydrogen and oxygen isotopes in groundwater flow systems.

Jindong coal basin is one of the 14 large coal basins planned and constructed by the state, and groundwater resources play an important role in supporting the sustainable development of the coal basin. To improve the understanding of deep karst hydrogeological characteristics of the coal basin, the combination of techniques (i.e., 1:50,000 and 1:100,000 surveys, geophysical prospecting, drilling, dynamic monitoring, hydrochemistry and isotopes, etc.) was used to characterize the hydrogeological structures of deep-buried aquifers and analyze the evolution characteristics of groundwater systems under the conditions of long-term and large-scale coal mining. Deep Cambrian Zhangxia formation oolitic limestone water-rich aquifer was newly discovered in this survey, which characterized by the development of karstic fissures and strong water-richness in the effective structural zone. The dissolubility of the Cambrian Zhangxia formation oolitic limestone is weaker than that of the Ordovician Majiagou formation and Carboniferous Taiyuan formation limestone, but stronger than that of the Ordovician and Cambrian dolomite. Controlled by Jinhuo fault zone, there are many large karst groundwater-bearing basins distributed on both sides, such as Jincheng basin, Yangcheng basin, Changzhi basin, etc., and water yield in the center of the basins can reach more than 10,000 m3/days. Main types of karst groundwater storage structures in study area are syncline basin type, fault fracture zone type and permeable-impermeable contact zone type. Affected by coal mining, the dynamic conditions of karst groundwater have changed significantly, mainly manifested in the movement of the boundary of the karst groundwater system, the decline of the groundwater level, the attenuation of karst springs flow, and the complex conversion of multi-source water. The variation characteristics of the spring flow can be subdivided into three stages, namely relatively stable stage, rapid decline stage and slow decline stage. The main controlling factors of these three stages are atmospheric precipitation, coal mining and karst water exploitation, respectively. The regional groundwater circulation pattern under coal mining can be divided into shallow groundwater flow system, deep groundwater flow system and local groundwater flow system. The local groundwater flow system was mainly affected by coal mining, which was manifested as the concentrated discharge of groundwater to goaf. The results of this study will provide scientific basis for groundwater exploration and exploitation and sustainable development of coal basins. [ABSTRACT FROM AUTHOR]

The karst groundwater flow system in the ejective folds area in the eastern Sichuan Basin shows the characteristics of a hierarchically nested structure. However, previous studies were mainly focused on hydrogeological conditions analysis, lacking qualitative evidence directly from hydrochemistry. In this study, the authors collected and sorted the chemistry data of representative karst springs and boreholes in the study area, namely, 41 shallow karst waters and 19 deep ones. The Pipper chart shows that the shallow karst water is HCO3-Ca·Mg and low Mg2+ concentration, while the deep karst water is SO4-Ca·Mg and high Mg2+ concentration. For both shallow and deep karst groundwater, the analysis of the groundwater flow system of a typical profile shows that the Mg2+ concentration value can be used to evaluate the groundwater residence time. The shallow karst groundwater system is highly controlled by geomorphology, among which the shallow incised valley springs are the discharge points of the local groundwater flow system. However, springs in deep gullies correspond to intermediate or even regional flow systems. This pattern is verified by the groundwater residence time evaluation from the Mg2+ concentration, which also reflects the movement characteristics of deep-circulation groundwater flow revealed by hot springs and boreholes.The hydrochemical characteristics can effectively indicate the pattern of groundwater flow systems. This paper provides a reference for identifying the hierarchically nested structure of groundwater flow systems in similar karst areas.

Assessment of the groundwater potential of the OAU Estate involving digital image processing of Landsat 8 imagery, and geophysical and hydrogeological investigations were carried out. Landsat 8 imagery was processed to produce hydrolineament map. Topographic parameters including drainage density and slope gradient were derived from SPOT DEM using ArcGIS 10.3 software, and used to generate drainage density and slope gradient maps. The lithology of the study area was extracted from existing geological map. One hundred and four Vertical Electrical Sounding data were analyzed quantitatively using partial curve matching technique and computer assisted 1-D forward modelling and the results used to generate overburden thickness and coefficient of anisotropy maps. The hydrolineament, drainage density, slope gradient, lithology, overburden thickness and coefficient of anisotropy maps were developed into thematic maps and used to generate the groundwater potential map. The groundwater potential of the study area was classified as low- (61%), moderate- (32%) and high- (7%) with corresponding yields of < 0.5 L/s, 0.5 L/s – 1.5 L/s and > 1.5 L/s, respectively.

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Long-term changes in deep groundwater flow in a coastal sedimentary plain affected by sea-level changes were investigated using a paleohydrological approach with 36Cl. The 36Cl/Cl ratios of the saline component in the Na-Cl type deep saline/brackish waters extensively distributed inland were found to be comparable to the secular equilibrium value of the aquifer. On the other hand, younger saline/brackish waters with low 36Cl/Cl are distributed in coastal areas, particularly in the deep aquifer. Deep freshwaters of the Na-Cl to Na-HCO 3 type found at slightly shallower depth than −1000 m a.s.l. have δD values ∼10 ‰ lower than recent precipitation, suggesting that they were recharged during a colder period than present. The presence of low-δD deep freshwaters inland and the distribution of relatively young saline/brackish waters in coastal areas both indicate that older saline water was flushed out by meteoric water during the glacial period, but to a limited extent and area. [ABSTRACT FROM AUTHOR]

The concept of baseline is crucial for assessing natural water conditions and detecting anthropogenic influences in groundwater systems. This research proposes a methodology to define a geochemical baseline using the groundwater flow theory to develop a conceptual model, distinct from most methodologies that primarily rely on statistical techniques. The proposed methodology comprises three phases: defining a conceptual model, defining a geochemical baseline based on hydrogeochemical characterization, and identifying anthropogenic sources. This approach enhances understanding of hydrogeologic processes such as water-rock interactions, exchanges with other water bodies, redox conditions, and groundwater flow by integrating information about geology, structural geology, base level of discharge, hydrological connectivity, topography, and water table. The methodology was applied in the southern region of Mexico, specifically in the Comitán-Montebello lakes watershed characterized by complex features like the karst system and interactions with surface water bodies, providing relevant insights into the hydrogeological setting. It is a cost-effective approach that improves understanding of hydrogeological processes in data-scarce areas, thereby supporting the establishment of geochemical baselines. • Introduces a replicable methodology for establishing geochemical baselines in groundwater systems. • Provides insights into hydrogeological setting of a karst system that interacts with surface water bodies. • Establish geochemical baselines based on knowledge of geology, hydrogeology and anthropogenic activities. • Identify nitrate as the main anthropogenic indicator in the groundwater system associated with agricultural activities. [ABSTRACT FROM AUTHOR]

As an essential part of hydrological cycle, groundwater circulation is a process including groundwater recharge, flow and discharge, which has been the primary research topic in hydrogeology.The spatial patterns of groundwater circulation are composed of different structure elements.There are two partitioning methods: the media structure determined from aquifer properties and the dynamic patterns determined according to the seepage field.A groundwater flow system (GFS) roles as a groundwater circulation cell within the concept of the dynamic patterns.In the last decade, significant advances have been achieved in the theory of GFSs, revealing groundwater circulation patterns in more comprehensive and deep ways.Studies on GFSs between parallel streams discovered that the top of water table is not the accurate position of groundwater divide.At the basin scale, the integrated impacts of the vale landform, precipitation infiltration rate, depth dependent permeability and basin thickness on the undulating shape of water table and groundwater circulation patterns were systematically investigated, with preliminary discovery on the evolution characteristics of dynamic patterns in periodicity and tendencies.Partitioning methods for three dimensional groundwater circulation cells were developed, according to fine-grained identification of paths or statistic analysis on residence times along a huge number of streamlines.For the hydrogeological effect, it has been found that the groundwater circulation patterns significantly influence the distribution of groundwater age.As an indication of hydraulic connection between recharge and discharge zones, groundwater circulation patterns play a key role in the geochemical processes at the basin scale and eco-hydrological processes at the catchment scale, which should be studied with more attentions on the dynamic and evolution behaviors of the three-dimensional groundwater circulation.

Temperature is an inherent property of groundwater.Groundwater temperature field and dynamic characteristics are objective manifestations of groundwater flow system.Based on multi-point, long series and high precision groundwater temperature monitoring data, the characteristics of shallow groundwater temperature distribution and dynamic type of water temperature in Jiuquan East Basin are studied, in order to reveal the relationship between the characteristics of shallow groundwater temperature field and groundwater flow system under the influence of human activities such as groundwater exploitation in northwest inland basin.The results show that the shallow groundwater temperature ranges from 9.33℃ to 20.77℃, and the average water temperature is 13.54℃.From the recharge area to the discharge area, the shallow groundwater temperature increases gradually along the direction of groundwater runoff, and the average groundwater temperature in January is higher than that in July.The comparison of different groundwater flow systems with similar circulation depth shows that the shallow groundwater temperature is negatively correlated with groundwater dynamic conditions.The average groundwater temperature of the water flow system with large surface water recharge and strong hydrodynamic conditions is lower, and that of the water flow system with small recharge and weak hydrodynamic conditions is higher.The temperature dynamics of shallow groundwater are affected by human activities such as natural groundwater circulation and groundwater exploitation.From groundwater recharge area to oasis area in the middle reaches and drainage area in the lower reaches, the temperature dynamic of shallow groundwater can be divided into four basic types, which are river recharge type, water temperature stability type, groundwater exploitation related type and sinusoidal fluctuation type respectively.

China Geological Survey has organized and implemented 1:50 000 standard map hydrogeological survey with an area of more than 40000 square kilometers in Yimeng Mountain area. And it carried out a large amount of water prospecting and well drilling demonstration work in villages and towns with serious water shortage, obtained rich geological data and gained a new understanding of groundwater flow systems. Under the influence of Mesozoic and Cenozoic structures, the faults and folds, extensional detachment and magmatic intrusion occurred in Yimeng Mountain area, formed a series of graben-semi graben basins characterized by overlapping in the south and faulting in the north, and finally formed today's typical "basin-mountain" karst hydrogeological structure and the relatively independent karst groundwater system, which taking basin as unit. In order to study the development characteristics of groundwater flow system in Yimeng Mountain area, Laiwu Basin was selected as a typical study area. On the basis of the field geological survey, through comprehensively analyze the geomorphic units, the three-dimensional spatial distribution of water bearing rock groups, groundwater level, hydrochemistry, and water isotope results on the north and south sides of the basin, this paper comprehensively discusses the characteristics of multistage karst groundwater flow systems with typical "basin-mountain" structure in Yimeng Mountain area. The results show that, the karst groundwater in Laiwu Basin presents a "centripetal" runoff from the outer periphery to the center of the basin. Affected by human activities, controlled by geological structure and aquifer distribution, the characteristics of groundwater flow systems between the north and south sides of the basin are different. There are mainly intermediate and local groundwater flow systems in the south of the basin, while a single local groundwater flow system is developed in the north of the basin, however, it occupies nearly 1/2 of the groundwater sources that have been explored and demonstrated. In addition, it is found that at the lowest erosion base level of Dawen River, the voids separated by regional detachment structure and hydrothermal mixing, together with rock layer, interlayer fissures and bedding dissolution voids, form a three-dimensional karst groundwater network, which influence the circulation path and depth of karst water. Guided by this understanding, the completion rate of exploration production combined wells has reached 86%, which strengthens the combination of theory and practice of groundwater flow system theory in hydrogeological work in bedrock mountainous areas in North China.

The theory of groundwater flow systems proposed by Tóth, which was based on the two-dimensional profile flow field, has become the theoretical basis for studying basin-scale groundwater circulation.The stagnation points on the two-dimensional profile can be used to accurately divide the spatial distribution of groundwater flow systems.However, the spatial distribution of groundwater flow systems and whether stagnation points exist in the 3D domain remain unknown.In this study, two typical three-dimensional basins, namely foreland-like basins with undulations only in the x direction and dune-type basins with undulations in both x and y directions, are used as typical examples.By using a combination of cosine functions to characterize the undulating water table, analytical solutions of head distribution in the two basins are derived.Moreover, by employing the TECPLOT software, the spatial distribution of the 3D groundwater flow systems is clearly shown.It is found out that stagnation points can develop in two basins.The stagnation points in the foreland-like basin constitute a horizontally extending stagnation line, which controls the spatial distribution of four flow systems in the basin, while the stagnation points in the dune-type basin form an arc-shaped stagnation line, which controls the spatial distribution of six flow systems in the basin.By studying the distribution of stagnation line in 3D basins and successfully dividing flow systems by using the stagnation line, this study leads to a deepened understanding of groundwater circulation in 3D basins.

The theory of regional groundwater flow proposed by Tóth provides a quantitative analysis approach for the study of basin-scale groundwater circulation. This paper summarizes the development of the theory of regional groundwater flow from four aspects, compares the description of the theory of regional groundwater flow in foreign hydrogeology textbooks, and systematically summarizes the evolution of descriptions in the theory of regional groundwater flow in the four editions of the textbook "Fundamentals of hydrogeology" in Chinese published from 1986 to 2018. It is concluded that the textbook "Fundamentals of hydrogeology" plays an important role in promoting the study of regional groundwater flow in China. This paper also puts forward some scientific problems to be solved in the theory of regional groundwater flow and some suggestions to guide more young students to conduct research in this field.

Groundwater age and groundwater residence time contain important information about groundwater circulation and evolutionary processes, and have been widely used in the study of groundwater circulation patterns in basins.In this paper, we simulated a three-stage flow system model through a multi-stage groundwater flow system demonstrator, and simulated the groundwater age distribution and groundwater residence time distribution based, and found that the bottom of the basin, the downstream of the regional flow, and the basin retention area responded the latest.The local flow system in the shallow part has relatively low concentration values after stabilization, the intermediate flow system is also relatively low compared to the regional flow system in the deep part, and the stagnant zone has relatively large concentration values due to salt accumulation.The groundwater age distribution curves are single-peaked, and the circulation time of the regional flow system is greater than that of the intermediate flow system than that of the local flow system.The residence time distribution monitored in the discharge zones shows that different levels of recharge will produce early, middle and late peaks, and the peaks correspond exactly to the level of the groundwater flow system.It can be judged from the peaks in the discharge zones that the groundwater is recharged from the local, intermediate or regional flow system, and thus the source of contaminants can be determined.The present research results have some significance for the evolution of groundwater circulation and the improvement of groundwater flow system theory.

Under the situation of water shortage and increasingly serious groundwater nitrate pollution in Ordos Basin, it is helpful for rational utilization and protection of groundwater resources to identify the distribution and causes of nitrate in different groundwater flow systems in Ordos Basin, so this research was carried out in the typical groundwater-fed lakes area within the basin.Hydrochemistry and multivariate statistical analysis (i.e., hierarchical cluster analysis and principal component analysis) were combined to identify the nested groundwater flow systems.Based on this, the distribution characteristics of nitrate in different groundwater flow systems were compared and analyzed.And both hydrochemistry and environmental isotopes were considered to further identify the sources and potential evolution processes of nitrate in nested groundwater flow systems.The results show that the groundwater samples with nitrate content exceeding the groundwater quality standard (GB/T 14848-2017) were taken from the local and intermediate groundwater flow system, with an over-standard rate reached 28%, and the mean nitrate content in the regional groundwater flow system is about 1 mg/L.The distribution of nitrate in different groundwater flow systems was dominated by the extent of human activities, while the influence of evaporation enrichment and denitrification attenuation processes can be ignored.And within the nested groundwater flow systems, the local and intermediate ones were significantly affected by pollution from human activities, with inorganic ammonium fertilizer and manure and sewage the main pollution sources of nitrate, but the regional one has not been polluted, with mineralization of natural organic nitrogen as its nitrate source.

Groundwater flow in mountainous areas is controlled by climate conditions, topography, geological structure, and other factors.Due to the restrictions such as data acquisition, the groundwater flow pattern and control mechanism in mountainous areas are still not clearly understood.Taking Beishan area as an example, which locates the pre-selected area of the Geological Repository for High-Level radioactive Waste in China, groundwater flow pattern is of great significance for the safety of the repository on the ten-thousand-year time scale.In this study, characteristics of the hydrogeological structure and permeability in Beishan area, Hexi Corridor, and Qilian Mountains were obtained based on regional remote sensing interpretation, geological structure evolution analysis, geophysical exploration, and hydrogeological drilling.A conceptual model was proposed through comprehensive hydrogeological survey, hydrogeochemical and isotope analysis.In addition, regional groundwater flow numerical models and multi-scenario simulations were used to understand the groundwater flow patterns in the Beishan-Hexi Corridor-Qilian Mountains in Gansu.Results show that topography plays a dominant role in the groundwater flow pattern in the area.Groundwater from Qilian Mountains is difficult to pass through the Hexi Corridor to discharge in the Beishan Mountains.With the lowest altitude in the region, the Hexi Corridor can be regarded as the boundary of the groundwater system between the Qilian Mountains and the Beishan Mountain.Under the control of topography and lithology, the groundwater in Beishan mountain area only develops local flow systems and the flow velocity is small.At the same time, due to the blocking effect of the faults in this area, the groundwater from Xinchang, the key pre-selected high level radioactive waste disposal site in Beishan, cannot flow directly to the south into the Huahai Basin.In addition, its length of the flow path is greatly increased.Only the groundwater on the south side of the F95 fault can discharge to the Huahai Basin, but due to the limited catchment basin, slow flow velocity and poor circulation condition, the discharge is weak.This study investigates the mountain-basin groundwater circulation pattern, and provides a scientific basis for the site evaluation of the geological repository for high-level radioactive waste in Beishan, Gansu.

The natural ecological conditions in the Shendong mining area in China are very fragile, and water resources are seriously lacking. As the production scale of the Shendong mine continues to expand, the demand for water for production and living is also increasing; however, the available surface and underground water is decreasing, and the water scarcity in the Shendong mine is becoming increasingly apparent. Obtaining water resources is a major technological challenge for green mining. The underground reservoir is a new type of underground water conservancy project, and the water shortage in China's coal mine pits is resolved by underground reservoirs, which also makes a substantial contribution to the effective utilization of water resources. How the construction of underground reservoirs affects the groundwater system in a mining area has become one of the most important factors to consider when finding sites for underground reservoirs. In this study, we took the Daliuta Coal Mine as an example. A numerical model based on the hydrogeological conditions in the mining area was developed to determine the effects on groundwater using FEFLOW software via the finite element method. The model was used to analyze the impacts of coal seam mining thickness, overlying lithology, water-storage range and the water level of the underground reservoir on the groundwater flow system in the mining area. The results indicate that the thickness of the coal seam mining and the lithology of the overlying reservoir both had a significant effect on the upper aquifer system. The water-storage range and water level of the underground reservoirs were the main influences on the lower aquifer system. The results prove that underground reservoir storage had a good effect on water retention in the groundwater system in the mining area, and was able to achieve the desired result of storing groundwater and reducing water loss. It also had a positive feedback effect on the mining area's environment. [ABSTRACT FROM AUTHOR]

Climate warming is the greatest future challenge to the hydrosphere and the human community, especially in arid and semiarid regions. This study took the Golmud river watershed on the Tibetan Plateau as an example to numerically identify the development of groundwater flow systems in a large arid sedimentary basin and explore what would the dramatic climate warming pose on groundwater flow system. The numerical results show that the Golmud river watershed has developed three hierarchical groundwater flow systems. River seepage is the predominant recharge for the groundwater systems inside the basin. The local groundwater flow system discharges some 82.69% of all groundwater in the basin, followed by the intermediate system with 14.26% and the regional system with 3.05%. The local system is mainly distributed in the shallow area of the alluvial-pluvial fan at the piedmont and provides the dominant water resource for human exploitation and oasis ecological usages. Climate warming would increase about 30.78% of the quantity of the recharge water to the groundwater system inside the basin via river seepage due to the increasing precipitation and increased glacier melt in the headwater region of the watershed. These waters would pose disturbances to all groundwater flow systems but to different degrees. The local flow system exhibits the largest response to the climate warming with more than 90% of increased water cycled in and discharged through it. The significant groundwater level rising leads to the trailing edge of the overflow belt at the piedmont moving ∼5 km towards to the mountain pass, which would potentially pose a water disaster to the local region. The influences of climate warming on the intermediate and regional flow system are relatively limited. This study provides a preliminary understanding of the influences of climate warming on the groundwater flow systems in arid endorheic basins and is essential for tackling future climate change challenges faced by arid and semiarid regions.

Tóthian theory refers to the gravity driven groundwater flow system (GFS) theory represented by Tóth, which mainly expounds the driving and distribution law of groundwater. The establishment and development of this theory not only deepened people’s understanding of the driving and distribution law of groundwater, but also greatly promoted the study of groundwater chemical evolution (GCE). Modern GCE research is mostly based on Tóthian theory, characterized by combining with advanced scientific and technological means. Based on the clue of time, this paper is divided into two parts. The first part mainly summarizes the establishment and development of Tóthian theory, including the exploration of groundwater driving force and distribution form by hydrogeologists before Tóthian theory, and the enrichment, development and application of Tóthian theory by geologists after its establishment. The second part mainly combs the main theories and application progress of GCE mechanism research, including the main theories and findings of GCE research before the emergence of Tóthian theory, as well as the research progresses of GCE after the emergence of Tóthian theory. With the flow of groundwater in GFS, groundwater undergoes continuous chemical evolution, which eventually leads to the transformation of hydrochemical types and the gradual increase of total dissolved solids (TDS). The distribution of GFS and GCE complement each other. The distribution of GFS directly determines the model of GCE, and the results of GCE also play a certain role in the distribution of GFS. GCE mainly includes dissolution, precipitation, cation exchange and adsorption, which is affected by the physical and chemical conditions of permeable media, organic matter content and microorganisms. GCE has the characteristics of universality, sustainability and diversity. With the increasing global population and the progresses of science and technology, the impact of human life, industrial and agricultural production on groundwater is deepening. The aggravation of pollution directly changes the chemical compositions of groundwater, resulting in changes of the law of GCE.

In order to study the groundwater flow system and effective groundwater management in Turpan Basin, a total of 57 samples including 8 spring samples and 8 river samples were collected for isotopic compositions analysis. Based on stable isotopes, the groundwater flow system in Turpan Basin can be divided into several secondary flow systems. Groundwater in the northern sub-basin flows from north to south and recharges southern sub-basin through the bedrock gap of the Flaming Mountain. In the southern sub-basin, groundwater flows from all sides to Ayding Lake which is the lowest point of the basin. Surface water has δ18O − 11.3 to − 8.7‰ and δ2H − 78 to − 56‰. The isotopic compositions of the groundwater samples collected vary from –11.5 to 6.9‰ for δ18O and from –85 to 7‰ for δ2H. The groundwater 14C value is relatively low (22.94–32.92) in the middle part of the southern sub-basin, which indicates groundwater is older; while the groundwater 14C value is relatively high (76.65–77.94) in the eastern part of the northern sub-basin, which indicates the groundwater is younger. As groundwater flows toward south, groundwater becomes older. The Turpan Basin can be divided into three main groundwater flow systems. One is located in the northern sub-basin, where groundwater is recharged from the northern mountainous areas, and flows toward south. The other two are located in the southern sub-basin. Under natural conditions, groundwater on the east and west sides of southern sub-basin drains to Aiding Lake which locates in the center of the basin. In Turpan city, where human activities are concentrated, a variety of water sources have been drawn from other areas for living and irrigation, including river water, groundwater and overflow spring water. These mixed water in Turpan city recharges groundwater by infiltration, which strengthens groundwater vertical movements. [ABSTRACT FROM AUTHOR]

The 'Dynamic System Approach for Geofluids and their Resources' was developed to harmonize research and the sustainable exploration of geofluids (e.g., groundwater, geothermal and hydrothermal fluids) and the related geo-resources (groundwater, geothermal energy and hydrothermal minerals). The conception is based on the basin-scale groundwater flow systems, which behave as a geologic agent, mobilize, transport and deposit matter and heat, whilst it can be evaluated quantitatively and in a distributed manner. Evaluation of these systems combined, for instance, with numerical, stochastic and isotope methods can jointly manage all types of geofluids, the related resources and the environmental consequences of their exploration and utilization. This paper describes the fundamental concepts of the approach and displays the proposed workflow and guidelines for practical applications such as groundwater vulnerability assessment, managed aquifer recharge, geothermal energy utilization, and the evaluation of hydrothermal mineral potential. [ABSTRACT FROM AUTHOR]

A review and assessment of earlier studies shows that the thermal groundwater near Chongqing in the eastern Sichuan Basin of China has a unique occurrence called the ‘basin-anticline outcropping’ type. Its occurrence and emergence are strongly controlled by the nearly north–south trending anticlines. The basin-anticline outcropping type groundwater is similar to that of the basin type but also has the characteristics of the outcropping type because of the anticlines. The natural hot springs in the study area exist mainly in the outcropping areas of the carbonates, in the middle and the plunging ends of the anticlines where the topography was cut by rivers. They can also rise through the overlying sandstones and form up-flow springs. Geothermal wells tapping the carbonate reservoirs on the flanks of the anticlines also produce thermal groundwater. The groundwater flow can be divided into three levels: (1) shallow circulation system with groundwater of HCO3-Ca type and low TDS discharging through normal temperature springs, (2) middle circulation system with groundwater of SO4-Ca type and TDS of 2–3 g/L discharging through hot springs and (3) deep circulation system with groundwater of Cl-Na type and high TDS discharging through hot springs or wells.

The groundwater stored in the aquifer is a kind of moving fluid, which is continuously distributed in space and can transmit pressure.It is driven by the potential energy difference to move from high to low head.Usually in the recharge area (high head), the groundwater moves vertically from top to bottom, while in the drainage area (low head), the groundwater moves from bottom to top.The movement of groundwater in aquifers is like the same life process, in which it always moves from the source to the sink in the direction of energy reduction, whether it is in a single or multiple permeable rock formations.Hydrogeological stratified exploration technology (HSET) makes it possible to monitor different depths of groundwater elements such as head, temperature and quality in the vertical direction, so as to realize the comparison of groundwater elements between the vertical points of multi-layer aquifer rock formations.At the same time, HSET can judge the potential source and sink conditions of different vertical sections of a full well or a single well, laying a foundation for finely describing the spatial movement of groundwater flow and delimiting the groundwater system.This paper discusses the principle, system structure and function of groundwater layered exploration based on the theory and experimental observation of groundwater flow system and expounds the application method of layered exploration technology in the identification of groundwater recharge or discharge area in Zhangye Basin.

Study region: Olézoa watershed, Yaoundé-Cameroon, Central Africa Study Focus: This study aimed to determine the hydrogeological characteristics of shallow hard rock aquifer and the environmental levels of heavy metals in an urbanized equatorial region. Physico-chemical parameters and heavy metals were determined on 25 groundwater samples and analyzed using conventional hydrochemical methods, multivariate analysis, heavy metal pollution index (HPI) and natural background levels (NBL). New Hydrological Insights for the Region: Hydrogeological functioning is not marked by a mass of basal groundwater with a low hydraulic gradient Hydrochemical facies were predominantly Ca-Mg-Cl, HCO3-Ca-Mg and Ca-Cl type, which were controlled water-rock interaction, cation exchange, dissolution of silicates and anthropogenic activities. Multivariate analysis did not show very significant hydrochemical spatial zonation but the first major component can be used as an indication of the groundwater flow direction. Vertically, a three-part structure around depths of 1 m and 3−7 m were identified and associated with local and intermediate flow systems at the Olézoa watershed-scale.HPI showed short range variations similar to that of EC, suggesting compartmentalization of the shallow aquifer on a small scale. Thus, a preliminary conceptual model describing groundwater dynamics has been proposed. NBL and median HPI values indicate a serious concern in terms of groundwater qualitative potential and the urgency of regular monitoring of heavy metals in groundwater in this urbanized part of sub-Saharan Africa.

• An integrated groundwater–surface water model is developed in an agricultural region. • Microtopography plays a pivotal role in the determination of groundwater patterns. • Ignoring microtopography causes overestimation of groundwater response times. • The filtration of signals is impacted by the hydrologic controls of microtopography. • Microtopography is a pre-requisite for accurately quantifying non-zero exchange fluxes. Groundwater–surface water interaction is greatly affected by surface topography and agricultural activities in managed agricultural regions. Such an interaction increases the complexity of hierarchical groundwater flow systems. However, the combined effects of microtopography and agricultural activities on groundwater flow patterns remain unclear. In this study, an integrated groundwater–surface water model was used to quantify the impact of microtopographic variations on groundwater flow patterns in an agricultural region in northwest China. Numerical experiments were conducted under different scenarios representing various degrees of microtopography. The results indicated that microtopography plays a pivotal role in the transfer of signals from spatiotemporally varying surface processes to the groundwater flow. Without consideration of the microtopography, the stationary fractional Gaussian noise–type infiltration processes in the vadose zone–saturated zone interface may be incorrectly simulated as non-stationary fractional Brownian motion. Furthermore, fractal analysis reveals that the scaling exponents of the pressure head are highly sensitive within the vadose zone and local flow subsystems. Microtopography plays an important role in shaping the groundwater response time (GRT) pattern, which results in a smaller average GRT but a larger core range. Contrarily, neglecting microtopography may lead to overestimation of the water table ratio (WTR), resulting in erroneous identification of groundwater patterns as topography-controlled types. This study highlights the importance of microtopography in the formation and evolution of nested groundwater flow systems and provides guidance for groundwater–surface water interaction simulations in managed regions. [ABSTRACT FROM AUTHOR]

The Okama Crater Lake is located in the highly active Zao Volcano on the boundary of Miyagi and Yamagata Prefectures, Japan. At present, the lake stays relatively calm with neither bubbling, steaming nor gas smell at a pH of 3.2–3.4, though the lake did change color with steaming from the water surface in 1939 as the result of one of Zao’s volcanic activities. In order to clarify the geothermal effect on Okama and the groundwater flow system below or around Okama, field observations were performed in 2019 and 2020. Groundwater inflow and outflow in Okama were separately evaluated by estimating the hydrological and chemical budgets of the lake, based on the hydrometeorology, water temperature and river inflow measured in the field. The average groundwater inflow and outflow were estimated at 0.012 m3/s and 0.039 m3/s during the non-rainfall periods of 2020, respectively. A surplus of groundwater outflow makes the lake level consistently decrease during non-rainfall periods or the completely ice-covered season. In the completely ice-covered periods, the water temperature consistently increased at 0–15 m above the lake bottom, which is probably due to thermal leakage from a hydrothermal reservoir below the lake bottom. The heat fluxes averaged over December 2019–April 2020 and December 2020–March 2021 were calculated at 2.5 and 2.9 W/m2, respectively. A coupling between the estimated groundwater inflow and the calculated geothermal heat flux was used to evaluate the temperature of inflowing groundwater.

Springs have an important role both in groundwater flow system understanding and in maintaining groundwater-related ecosystems. The aim of the research of the lukewarm karst springs in Kistapolca is to understand the origin of the elevated temperature (22–24 °C), i.e., whether it is the result of mixing of cold and thermal waters or it represents the terminal area of intermediate flow systems. This question has also an important relevance to the evaluation of recent cave formation in the area. In case of mixing of different waters dissolution by mixing corrosion could take place. This scenario was proposed for the formation of the Beremend Cave, which is situated in the close vicinity of the springs, and which is the only known cave of the area, where the passages are partly filled by water. Therefore, several methods, including geophysical, geochemical and stable isotope measurements, evaluation of continuous-time series of water level change, temperature, and electrical conductivity data and hydraulic evaluation of vertical flow conditions by pressure–elevation profiles were combined in order to acquire information about the hydrogeological environment and flow conditions in the area. The results show that a higher order flow system derived from the Villány Hills feeds the springs in Kistapolca without significant local influence. In addition, the results related to the Beremend Cave indicate that cave formation by mixing corrosion is not an active process today in the area, the cave is rather influenced by local processes. [ABSTRACT FROM AUTHOR]

This study aims to study hydrochemical characteristics and evolution of groundwater, and to assess suitability for human drinking in Dake Lake Basin, Northwest China. Hierarchical cluster analysis produces two geochemically distinct clusters, C1 and C2, which are dominated by Ca·Mg–HCO3 and Na–HCO3 type, respectively. C1 and C2 belong to shallow local and deep regional flow systems, respectively. Carbonate dissolution and ion exchange process dominate in C1 and C2, respectively. C1 groundwater is not suitable for human drinking due to relatively high NO3− contents caused by agricultural activities; while C2 groundwater is suitable for human consumption. [ABSTRACT FROM AUTHOR]

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Considering the eco-environmental problems such as the shrinkage of lakes and the decline of groundwater level in the Ordos Plateau ,the impact of regional vegetation change on groundwater system and lakes was discussed in view of the hydrodynamic mechanism by developing the two-dimensional groundwater flow model with ideal profile on the regional scale. This paper proposed an improved method for generalizing the upper boundary of the flux by comparing the applicable conditions and shortcomings of the two methods of the specified-head upper boundary and the flux upper boundary conditions. The method was applied to the groundwater simulation of Ordos Plateau. The results show that the effect of regional vegetation conditions on groundwater and lakes is obvious ； the effect is controlled by the evolution of regional groundwater system. [ABSTRACT FROM AUTHOR]

• Water table in the riparian slope rose quickly with a small amount of infiltration. • The capillary fringe induced groundwater ridging and variable flow direction. • Initial water table depth affected the evolution of groundwater flow pattern. Comprehensive investigations of the riparian groundwater flow system response to rainfall based on laboratory experiments and numerical simulation are presented and discussed. The 1D sand column experiment results showed that the capillary fringe of the sand sample was 25 cm, and the supporting capillary water height was about 65 cm. The 2D sandbox was constructed to investigate the changes of hydraulic heads and groundwater flow system in the hypothetical riparian zone under different initial water tables and rainfall intensities. When the initial water table was shallow, a small amount of rainfall infiltration resulted in a rapid and large rise of the water table due to the effect of capillary fringe. When the initial water table was equal to the riverbed elevation, groundwater flow pattern changed once after the rainfall infiltration. In the early rainfall period, a groundwater divide and two opposite local flow systems occurred; one flowed to the upslope unsaturated zone (counter directional local flow system), the other flowed to the river (codirectional local flow system). As the rain continued, the groundwater divide kept moving to the upslope and finally disappeared by forming a stable codirectional regional flow system. When the initial water table was lower than the riverbed, groundwater flow pattern was more complex and changed twice. At the beginning, all the rainfall infiltration recharged the thick unsaturated zone and there was only a counter directional regional flow system. When the water table rose to the riverbed, a groundwater divide and two opposite local flow systems were formed. The groundwater divide was moving to the upland and finally formed a steady codirectional regional flow system. More fine characterization of hydrological variation in a short-time scale in the riparian zone is necessary in the future. [ABSTRACT FROM AUTHOR]

• The prokaryotic density was 103 to 106 cells/mL in groundwater and spring water. • Groundwater with longer residence time contained thermophilic bacteria. • The community constituents had been changed among the groundwater flow processes. • Prokaryotic information can reflect changes in groundwater flow processes. Besides surface water systems, groundwater also provides a habitat for prokaryotes. The prokaryotes inhabiting groundwater are affected by the environment that they grow in. Owing to the environmental specificity, some prokaryotic community constituents might thus provide information on local subsurface growth conditions as well as advective transport processes in case of rapid dislocation. In this study, we investigated the spatiotemporal distribution of prokaryotic communities in groundwater and spring water to examine the relationship between the groundwater flow system and prokaryotic dynamics. We sampled and monitored the water in a mountainous forested catchment in Kawamata Town, Fukushima Prefecture, Japan. Seven piezometers and a spring located across a ridge, hillslope, and valley were monitored. Prokaryotic information, such as density estimated by total direct counting, community constituents and diversity using the next-generation sequencing methods, physicochemical parameters, general water quality, and groundwater mean residence time using sulfur hexafluoride (SF 6) were analyzed. Our results showed that the mean residence time of groundwater and spring water ranged from less than 1 year to 30 years. The prokaryotic density ranged from 5.37 × 103 cells/mL to 2.01 × 106 cells/mL (n = 60). As the mean residence time increased, the density of prokaryotes in the shallow subsurface also increased, whereas that of the prokaryotes in the deep subsurface decreased. The contribution of deep groundwater to the examined groundwater is inferred using the groundwater residence time and prokaryotic information, including density and specific constituents. The stable water isotopes did not suggest the deep groundwater contribution to the groundwater at the ridge in the high-rainfall period; however, the SiO 2 concentration indicated that. Elucidating the prokaryotic information along with existing hydrological tracers may enable a better understanding of groundwater flow processes. [ABSTRACT FROM AUTHOR]

Classical understanding on basin-scale groundwater flow patterns is based on Tóth’s findings of a single flow system in a unit basin (Tóth, 1962) and nested flow systems in a complex basin (Tóth, 1963), both of which were based on steady state models. Vandenberg (1980) extended Tóth (1962) by deriving a transient solution under a periodically changing water table in a unit basin and examined the flow field distortion under different dimensionless response time, τ ∗ . Following Vandenberg’s (1980) approach, we extended Tóth (1963) by deriving the transient solution under a periodically changing water table in a complex basin and examined the transient behavior of nested flow systems. Due to the effect of specific storage, the flow field is asymmetric with respect to the midline, and the trajectory of internal stagnation points constitutes a non-enclosed loop, whose width decreases when τ ∗ decreases. The distribution of the relative magnitude of hydraulic head fluctuation, Δ h ∗ , is dependent on the horizontal distance away from a divide and the depth below the land surface. In the shallow part, Δ h ∗ decreases from 1 at the divide to 0 at its neighboring valley under all τ ∗ , while in the deep part, Δ h ∗ reaches a threshold, whose value decreases when τ ∗ increases. The zones with flowing wells are also found to change periodically. As water table falls, there is a general trend of shrinkage in the area of zones with flowing wells, which has a lag to the declining water table under a large τ ∗ . Although fluxes have not been assigned in our model, the recharge/discharge flux across the top boundary can be obtained. This study is critical to understand a series of periodically changing hydrogeological phenomena in large-scale basins. [ABSTRACT FROM AUTHOR]