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As a widely employed method for in situ remediation of groundwater contamination, the pump-and-treat (PAT) system involves the management of water recirculation between the extraction and injection wells. The recirculation zone (RZ) of an extraction-injection well pair in a confined aquifer has been well known. However, PAT systems are more frequently used in unconfined aquifers with a natural regional flow and may not only include one injection well. We develop comparable analytical models for an unconfined aquifer treated by two different system settings, including an extraction well and one injection well (1e/1i system) or two injection wells (1e/2i system). The role of regional groundwater flow is highlighted. Analytical solutions of RZs and recirculation ratios are obtained using complex potential functions, with a new treatment of the jump of the stream function at a branch cut. Results indicate that the shape of RZs and the recirculation ratio nonlinearly depend on several dimensionless parameters linked to the pumping rate and direction of regional flow. Compared to the 1e/1i system, the two injection wells in the 1e/2i system may reduce the integrity of RZs and decrease the recirculation ratio; however, they lead to a higher allowable pumping rate in satisfying the limitations of the water table in wells. This study suggests a useful methodology for analyzing PAT systems with multiple injection wells and provides new insights into RZs between extraction and injection wells. [ABSTRACT FROM AUTHOR]

• Stream-function-based (SFB) method is developed for 2D particle tracking along exact streamlines. • The branch cut effect of multivariable stream function around wells is avoided. • Unlike existing methods, the SFB method does not lead to accumulative errors in the particle position. Particle tracking is an essential process in analyzing groundwater flow and solute transport in aquifers. A drawback of existing particle tracking methods, which are based on approximate estimations of the velocity in a time step, is the accumulative errors of numerous time steps would lead to deviation of particles away from the exact streamlines, especially when pumping and injecting wells exist. In this study, to improve the accuracy of particle tracking for the two-dimensional (2D) steady-state flow around wells, we propose a stream-function-based (SFB) particle tracking method in which a particle is traced along the exact streamline with a given value of the stream function, and the branch cut effect caused by the multivalued stream function around a well is eliminated. Three synthetic cases are used to demonstrate the advantages of the SFB method over the existing Pollock method and the fourth-order Runge-Kutta method. In the cases of a single pumping well and an injecting-pumping well pair, the Pollock and Runge-Kutta methods yield pathways that may be far away from the exact streamline, but the SFB method performs much better. In the case with a pumping well and two injecting wells, the SFB method successfully delineated a narrow capture zone of the pumping well, which requires a high accuracy particle tracking process. Moreover, the accuracy of particle positions obtained by the SFB method is insensitive to the step-travel distance. The study would be useful for accurate investigation of flow fields in aquifers disturbed by wells. [ABSTRACT FROM AUTHOR]

• 3D pathways of IGF are identified with an assessment on flow rates. • IGF contributes 36%∼59% to the baseflow and 32%∼52% to the streamflow in the studied river. • Infiltration recharge and aquifer properties affect the development of IGF. Inter-basin groundwater flow (IGF), defined as groundwater flows across the topographic divides, occurs in nature as a common phenomenon, and seriously affect water balance and solute transport in a basin. Although the existence of IGF has been checked in drainage basins with indirect methods, such as using information of hydrogeochemistry, few studies are conducted to directly identify and quantify the IGF in the three-dimensional (3D) space using numerical modeling of groundwater flow. In this study, we identify, partition and quantify IGF based on a regional-scale 3D groundwater flow model covering several traditional surface water basins. The investigation is focused on the Hailiutu River, a groundwater-fed stream in the Ordos Plateau, China. It is the first time that the source zones and 3D paths of IGF as well as its contribution to the streamflow are fully captured for a groundwater-fed stream. Results indicate that the IGF contributes 36%∼59% to the baseflow and 32%∼52% to the streamflow of the Hailiutu River, through different groups of circulation paths with the maximum depth up to ∼ 1000 m. Uncertainty analyses of the model show that the contribution of IGF to the streamflow is positively correlated to the infiltration recharge and hydraulic conductivities of the aquifer media. Several indirect methods are also checked for comparison, but they could only be used to assess the potential contribution of IGF, not the IGF paths. This study will facilitate in deeply understanding the evolution mechanism of the inter-basin groundwater circulation and the interaction between surface water and groundwater. [ABSTRACT FROM AUTHOR]

Hierarchically nested groundwater flow systems have been widely investigated by numerical modeling and laboratory experiments but seldom recognized from in situ observations and testing. Groundwater age and geochemical profiles were obtained by the authors using a packer system along two wellbores drilled in the Ordos Plateau, China. Groundwater age profiles were constructed from 3H, 85Kr, 14C, and 81Kr data, with the apparent age generally increasing with depth, while exhibiting several inflection points. These inflection points were compared with geochemical profiles and numerical modeling results to delineate groundwater flow systems. Inflection points with significant increases in the gradients of age and Cl− concentration versus depth may indicate interfaces among flow systems. A concurrent decrease‐increase turning of groundwater age and Cl− concentration is influenced by a stagnation point. The groundwater age profiles are quite different from previous studies, indicating that the method can greatly reduce the uncertainty in model construction. Plain Language Summary: The provision of renewable groundwater, a crucial source of drinking water worldwide, relies on a circulation path from recharge to discharge zones with a subsurface flow in reservoirs (aquifers). Basin‐scale groundwater circulation is speculated to form hierarchically nested flow systems with differing capabilities for renewal. However, flow systems are invisible and difficult to detect through in situ observations on groundwater. In this study, nested groundwater flow systems were identified from groundwater age and geochemical profiles along wellbores. Inflection points on these profiles provide significant insights into the interfaces and stagnation points among nested groundwater flow systems. Uncertainties could be brought by the aquifer heterogeneity in the hydraulic conductivity and localized soluble salts. This adopted method is expected to be improved by incorporating geophysical survey and high‐resolution geochemical analyses to eliminate uncertainties in model construction. Key Points: Groundwater age and geochemical profiles were obtained to investigate groundwater flow systemsInflection points on profiles are used to infer interfaces or stagnation points among different groundwater flow systemsGroundwater flow models constrained by inflection points greatly reduce the uncertainty of flow system characterization [ABSTRACT FROM AUTHOR]

Freezing-induced groundwater-level decline is widely observed in regions with a shallow water table, but many existing studies on freezing-induced groundwater migration do not account for freezing-induced water-level fluctuations. Here, by combining detailed field observations of liquid soil water content and groundwater-level fluctuations at a site in the Ordos Plateau, China, and numerical modeling, we showed that the interaction of soil water and groundwater dynamics was controlled by wintertime atmospheric conditions and topographically driven lateral groundwater inflow. With an initial water table depth of 120 cm and a lateral groundwater inflow rate of 1.03 mmd-1 , the observed freezing and thawing-induced fluctuations of soil water content and groundwater level are well reproduced. By calculating the budget of groundwater, the mean upward flux of freezing-induced groundwater loss is 1.46 mmd-1 for 93 d , while the mean flux of thawing-induced groundwater recharge is as high as 3.94 mmd-1 for 32 d. These results could be useful for local water resources management when encountering seasonally frozen soils and for future studies on two- or three-dimensional transient groundwater flow in semi-arid and seasonally frozen regions. By comparing models under a series of conditions, we found the magnitude of freezing-induced groundwater loss decreases with initial water table depth and increases with the rate of groundwater inflow. We also found a fixed-head lower boundary condition would overestimate freezing-induced groundwater migration when the water table depth is shallow. Therefore, an accurate characterization of freezing-induced water table decline is critical to quantifying the contribution of groundwater to hydrological and ecological processes in cold regions. [ABSTRACT FROM AUTHOR]

• Croplands greatly altered the annual E and Q , while have little effect on annual C. • Croplands increased the evapotranspiration sensitivity to E p but decreased to P. • Croplands significantly increased the runoff sensitivity to climate change. Croplands can significantly influence the hydrologic response of catchments to climate change in a region but such influence has not been well investigated and quantified. In this study, we firstly use the conceptual hydrological model to investigate the annual water balance for croplands-altered catchments, and the "natural" state without croplands of the catchment can be traced back by setting a zero area of croplands in the calibrated model. The model results are then applied to a hydrologic sensitivity framework with the Budyko-type relationship between hydrologic ratios (i.e. evapotranspiration ratio and water storage change ratio) and the aridity index (E p / P) to yield the hydrologic sensitivity coefficients for the croplands-altered and "natural" catchments. The method is applied to investigate three catchments that are rarely affected by inter-basin groundwater flow in Illinois. The results show that croplands significantly increase the actual evapotranspiration (E) and decrease the runoff (Q), while changes in water storage (C) are rarely affected by croplands at the annual time scale. The actual evapotranspiration at the annual timescale is much more sensitive to potential evapotranspiration when the croplands exist. Croplands in Illinois significantly increase the sensitivity of streamflow on the climate change, to at least twice of the sensitivity in the "natural" state. The effectiveness of inter-annual hydrologic sensitivity framework is also demonstrated by hydrological model. This study provides a new simple insight to quantify the inter-annual stability with varying climate and land use at the catchment scale. [ABSTRACT FROM AUTHOR]

A coupled regional and local model is required when groundwater flow and solute transport are to be simulated in local areas of interest with a finer grid while regional aquifer boundary and major stresses should be retained with a coarser grid. The coupled model should also maintain interactions between the regional and local flow systems. In the Beijing Plain (China), assessment of managed aquifer recharge (MAR), groundwater pollution caused by rivers, capture zone of well fields, and land subsidence at the cone of depression requires a coupled regional and local model. This study evaluates three methods for coupling regional and local flow models for simulating MAR in the Chaobai River catchment in the Beijing Plain. These methods are the conventional grid refinement (CGR) method, the local grid refinement (LGR) method and the unstructured grid (USG) method. The assessment included the comparison of the complexity of the coupled model construction, the goodness of fit of the computed and observed groundwater heads, the consistency of regional and local groundwater budgets, and the capture zone of a well filed influenced by the MAR site. The results indicated that the CGR method based on MODFLOW-2005 is the easiest to implement the coupled model, capable of reproducing regional and local groundwater heads and budget, and already coupled with density and viscosity dependent model codes for transport simulation. However, the CGR method inherits shortcomings of finite difference grids to create multiple local models with inefficient computing efforts. The USG method based on MODFLOW-USG has the advantage of creating multi-scale models and is flexible to simulate rivers, wells, irregular boundaries, heterogeneities and the MAR site. However, it is more difficult to construct the coupled models with the unstructured grids, therefore, a good graphic user interface is necessary for efficient model construction. The LGR method based on MODFLOW-LGR can be used to create multiple local models in uniform aquifer systems. So far, little effort has been devoted to upgrade the LGR method for complex aquifer structures and develop the coupled transport models. [ABSTRACT FROM AUTHOR]

• Sustainable groundwater development is achievable in Beijing plain. • Substituting groundwater abstraction with transferred surface water has stopped depletion. • Managed aquifer recharge is very effective for sustaining groundwater abstraction. • Climate variation has larger impact on groundwater resources than the anticipated future climate change. To cope with the groundwater depletion problem and achieve sustainable groundwater development, groundwater conservation measures and managed aquifer recharge (MAR) have been implemented worldwide. However, knowledge gaps exit how does the aquifer system respond to these interventions differently and if these interventions are adequate to lead to long-term sustainable groundwater development under future climate change. In Beijing Plain, two measures have been implemented: reduction of groundwater abstraction by substituting groundwater abstraction with transferred surface water and implementation of managed aquifer recharge (MAR) in two major rivers. This study aims to assess how do the shallow and deep aquifers respond to these measures and if these measures can lead to long-term sustainable groundwater development in Beijing Plain under future climate change. A 3-D transient groundwater flow model was calibrated and used to simulate groundwater level and budget changes from 2021 to 2050. The monthly groundwater recharge was estimated using the projected monthly precipitation from three downscaled regional climate models under two scenarios (RCP4.5 and RCP8.5). The results show that declines in groundwater head and storage can be reversed with the combined two measures, thereby contributing to achieve sustainable groundwater development. The reduction of abstractions is a deciding measure to reverse the trend of groundwater depletion, especially in the deep confined aquifers, while large scale MAR schemes can restore the cones of depressions in shallow aquifers and maintain the groundwater abstraction. Climate variation has large impacts on groundwater resources, especially, consecutive dry years can cause rapid groundwater storage depletion. The projected monthly precipitation from 2021 to 2050 is not significantly different from the past. Therefore, the projected future precipitation has minor impacts on groundwater resources in the next 30 years. The findings from the study will support the Beijing municipality to maintain the tight control on groundwater abstraction and to implement large-scale MAR schemes in two rivers. This successful example will encourage managers of other heavily exploited aquifers to take similar measures to achieve sustainable groundwater development. [ABSTRACT FROM AUTHOR]

• We summarize field observations of 14C age showing obvious GW aging in two basins. • Pumping causes not only rejuvenation, but also aging of GW in layered basins. • Although GW rejuvenation is larger in area, aging is greater in magnitude. • We identify two new mechanisms of pumping-induced GW aging in layered basins. • Topography-driven flow is the main cause of GW aging in the discharge area. As an indicator of the renewability of a groundwater basin, the spatial distribution of groundwater age has attracted intensive research. As reported in the literature, 3H measurements in 74 US aquifers showed pumping-induced groundwater rejuvenation, whereas 14C ages in two heavily developed aquifers in France and China indicated pumping-induced groundwater aging. Unfortunately, the mechanisms leading to the seemingly opposite behavior of pumping-induced aging and rejuvenation remain unclear. In this study, we conducted numerical simulations of groundwater age in layered aquifer-aquitard systems under different pumping scenarios and aquitard permeability to show that pumping-induced groundwater rejuvenation and aging always occur simultaneously. Groundwater rejuvenation widely occurs throughout the basin wherever the well is located, whereas groundwater aging mainly occurs to the downgradient parts of the basin. Although groundwater rejuvenation is larger in area, groundwater aging is greater in magnitude, and the magnitude of aging increases with pumping intensity and duration. Moreover, when pumping occurs in the discharge area, aging could occur in almost the entire aquifer to the downgradient of the pumping well, and the magnitude of aging is larger. The large area and large magnitude of aging in the discharge area is a direct result of upward leakage of old groundwater from underlying aquitards/aquifers, which is controlled by the topography-driven groundwater flow. By demonstrating the importance of topography-driven groundwater flow and underlying formations in groundwater age redistribution, this study has implications for determining the bottom of a groundwater flow and solute transport model and interpretation of various tracers. [ABSTRACT FROM AUTHOR]

• Groundwater mounds resulting from nearshore recharge is identified in a sandy hillslope. • The thickness of vadose zone determines fluxes across water table along the hillslope. • Nearshore recharge temporarily changes streamlines and increases tortuosity of flowpaths. • Flowpaths near the wetland are mainly controlled by regional GW discharge and strong ET. Groundwater mounds and depressions induced by rainfall and evapotranspiration near wetlands/lakes have been widely observed, but their effects on groundwater flow field are poorly known. Here, a sandy hillslope near a wetland in the Ordos Plateau, China, which has a semi-arid climate, is selected for investigation. After identifying temporary groundwater mounds/depressions based on three water level observation wells, a HYDRUS-2D model is setup to obtain the highly transient daily flow field. The variable thickness of unsaturated zone near the wetland, which controls soil water content and unsaturated hydraulic conductivity, is found to be responsible for groundwater mounds resulting from focused recharge and groundwater depressions resulting from nonuniform evapotranspiration. On nonrainy days, evapotranspiration in the lowland is supported by flowpaths from regional groundwater discharge, and in the slope is supported by a flow cell from the upland to the upper part of the slope, which causes water table recession in the upland. On rainy days, focused recharge in the lowland leads to two temporary local flow cells, which break the regional groundwater discharge flowpaths into two temporary flow cells. The two temporary flow cells toward the upland contribute to water table rises in the uplandMoreover, we find focused recharge in rainy days does not change the overall direction of flowpaths controlled by regional groundwater discharge and strong evapotranspiration, but only increase the tortuosity of flowpaths. By coupling daily atmospheric conditions and regional groundwater discharge, this study has implications for groundwater recharge estimation and future groundwater modelling in arid regions. [ABSTRACT FROM AUTHOR]

• Field data show the control of soil texture on freezing-induced water table recession. • Ku below the frozen zone is the limiting factor of freezing-induced water flow. • We obtain extinction depths of freezing-induced GW migration for different soils. • The extinction depths of freezing-induced and evaporation-induced GW migration. Although water table recessions associated with seasonal freezing have been widely reported, the conditions that groundwater could respond to seasonal freezing are poorly known. Based on 30 records of winter-time water table recessions reported in the literature, we find the mean water table recession increases from sand to loam to silt to clay. To obtain a quantitative understanding of the conditions of freezing-induced groundwater migration and the role of soil texture, we construct a one-dimensional SHAW model with a dynamic water table that could respond to winter-time atmospheric conditions. By simulating freezing-induced groundwater migration under a series of pre-freezing water table depths, soil textures and freezing indices, we find the extinction depth of freezing-induced groundwater migration, i.e., the limit water table depth beyond which there is negligible freezing-induced groundwater flow, increases from coarse to fine soils and increases with freezing index. Moreover, we find there is a good linear relationship between the distance from the freezing front to the water table corresponding to the extinction depth of freezing-induced groundwater migration and the distance from the land surface to the water table corresponding to the extinction depth of evaporation-induced groundwater migration for different soils. To demonstrate the importance of defining extinction depth, we also show the overestimation of freezing-induced water gain in the frozen zone by the widely used fixed water table condition when the water table depth is shallower than the extinction depth. This study has implications for future hydrologic and engineering studies in critical zones with a cold climate. [ABSTRACT FROM AUTHOR]

• We separate the roles of upward discharge and aquitards in flowing artesian zone. • Basinal flow are coupled with well hydraulics to examine hydraulics of flowing wells. • A low K aquitard causes a large flowing artesian zone but low stable discharge rate. • Baseflow reduction is the main source of groundwater derived in flowing wells. Causes and water sources of flowing artesian wells attracted the interest of many hydrogeologists throughout history, however, a quantitative model that satisfactorily considers the roles of topography, groundwater recharge/discharge and aquitards on hydraulics of flowing wells is still lacking. In this study, a three-layer river-valley basin with a recharge boundary is used to obtain the basinal flow field, spatial distribution and transient discharge rates of flowing wells. Even if there is an aquitard separating the unconfined and the confined aquifers, groundwater discharge to the river still plays a critical role in occurrence of flowing wells. An aquitard with lower permeability would enlarge the zone with flowing wells, implying that flowing wells are more likely to occur in basins with continuous aquitards. By comparing flowpaths and inter-aquifer leakages before and after numerically installing a flowing well in a three-layer basin, we find decreased upward leakage in the discharge limb plays a much larger role than increased downward leakage in the recharge limb in the stable discharge rate of a flowing well. This recognition is different from previous studies where the stable discharge rate is supported only by downward leakage from the overlying aquifer. By considering well hydraulics in a basinal background flow field, this study improves understanding of the mechanisms of flowing artesian wells and the interaction of surface water and deep groundwater, indicating that flowing wells are valuable for sampling deep groundwater representative of deep and old groundwater in the discharge area of a basin. [ABSTRACT FROM AUTHOR]