Prof. Chongxi Chen has long been engaged in teaching courses, personnel training and scientific research in the field of hydrogeology, working diligently and innovatively, and has achieved fruitful academic research results.

This article makes a systematic review of Prof. Chongxi Chen's academic contributions in the field of hydrogeology, mainly including three aspects: ① Carrying out innovative and theoretical researchon groundwater hydraulics. He corrected the historical model error of "steady-state well flow within the influence radius", developed a series of analytical models for special hydrogeological problems, and improved and extended classical well flow models. ② Continuously exploring hydrogeological simulation methods. He proposed a number of techniques and methods to improve the emulational level of groundwater simulation, such as the equivalent submarine discharge boundary of coastal groundwater, the coupling seepage-pipe flow model, the linear-nonlinear flow model in triple Karst media of cave-fracture-porous and the groundwater withdrawal-land subsidence model, etc. He also developed a unique and advantageous software for groundwater flow simulation, PGMS. ③ Proposing a theory of recharge-discharge changes for the evaluation of the sustainable groundwater extraction. He completed groundwater modelling in several typical regions in China, which played a demonstration role for the scientific assessment of groundwater resources.

Prof. Chongxi Chen's innovative research has played an important role in promoting the development of hydrogeology in China.

The Dupuit model of well flow is a classical steady-state well flow model for a homogeneous unconfined aquifer in a round island. However, it does not consider the widely existing infiltration recharge from precipitation, and is also inapplicable for stratified heterogeneous aquifer systems. Therefore, it must be modified to address these issues.

On the basis of the revised Dupuit well flow model which incorporates infiltration recharge, this study further extended its application to a stratified heterogeneous unconfined aquifer. The Girinskii's potential function was used to construct the differential equations for the radial groundwater flow according to the water balance principle, and the analytical solutions satisfying the boundary conditions are then obtained as formulas of the flow rate, water table and groundwater divide. Taking the bilayer structure as an example, typical groundwater level curves with respect to 30 scenarios of different parameter values were investigated. A special phenomenon was found in which the curves of different hydraulic conductivities intersect at a single point, which could also be proven in theory. This analytical model still adopted the Dupuit assumption and did not consider the "hydraulic jump" phenomenon on the wall of the pumping well. To check the impact of these constraints on the applicability of the analytical formulas, a two-dimensional numerical model for axially symmetric seepage was built for comparison.

As indicated by the results, the relative error of the groundwater level estimated from the analytical solution is generally less than 4%, except for the zone near the pumping well. On the groundwater divide, where the Dupuit assumption is mostly invalid, the relative errors of the analytical solution to both the distance and height of the divide are smaller than 0.1%.

The Dupuit assumption does not significantly influence applicability of the analytical model.

Both the classical Dupuit model and the modified Dupuit model including infiltration are influenced by Dupuit's assumption and have potential systematic errors. Building a numerical model of well flow in an unconfined aquifer by characterizing the three-dimensional or axisymmetric two-dimensional (2D) flow is an essential approach to verify the performance of Dupuit-type models.

In this study, a 2D numerical model is proposed for the steady state well-flow in an unconfined aquifer, in which the control equation of seepage in the cylindrical coordinates is transformed equivalently to the Cartesian coordinates through parameter transformation, and the sectional 2D modelling is implemented via the MODFLOW finite-difference grid of cubic blocks. In the numeric model, the water level in the pumping well is a given condition, the flux across the seepage face is estimated by difference equation according to Darcy's law, the phreatic surface is identified by the treatment of dry and wet cells in MODFLOW, and the pumping rate is determined from the water debug calculation.

Fine grids are constructed innumerical models of typical cases to obtain high-precision results, in which the relative error of the backwards estimated pumping rate is no more than 0.2%. This numerical model is used to check the Dupuit-type well-flow models. As indicated, the groundwater level estimated from the analytical formulas generally agrees well with the numerical modelling results, except that near the well, where the analytical solution underestimates the groundwater level due to ignoring the waterjump and the errors depend on the anisotropic permeability of the aquifer. When infiltration exists, the flow in the vicinity of watershed does not follow Dupuit's assumption. However, the estimated groundwater level on the watershed by modified Dupuit well-flow equation has a low level of relative error, which is less than 1%.

This numerical method is simple and practical however it is also influenced by limitations in MODFLOW.

Monitoring natural attenuation (MNA) is a widely used, economical and effective remediation technique for soil and groundwater contamination. The migration of nonaqueous phase liquid (NAPL) in heterogeneous strata is an important element in the assessment for the efficiency of MNA.

Based on three consecutive years of dynamic groundwater quality monitoring data, the purpose of this study is to accurately characterize the biodegradation processes of multiphase fluids at a typical phenol contaminated site in northern China. A multiphase flow numerical model of phenol transport considering Monod biodegradation processes based on TOUGH3/TMVOCBio software was developed and applied. The model depicted the spatial distribution and temporal variation of phenol under the current conditions well and analysed the sensitivities of the adsorption and biodegradation parameters. The paper also discussed the removal contributions of dissolution, volatilization, adsorption and biodegradation effects under uncertainty of adsorption and microbial degradation parameters and predicted two different natural attenuation scenarios for source disposal.

The contribution of phenol removal varies over a range under the influence of parameter uncertainty, with 17.91% to 58.02% for biodegradation, and precipitation conditions affect the seasonal variation in phenol concentrations. In the future 20 years, under the conditions with complete removal of the phenol source and the present leakage model, the total mass removal rate of phenol will arrive at 98% and 80% at the end of the 20th year, respectively.

This paper identifies the biodegradation parameters with high sensitivity in the multiphase flow model, which provides a reference for the numerical simulation of the organic matter biodegradation process at petrochemical sites and can also provide a theoretical basis for the application of MNA technology in China.

As a tectonic phenomenon with important hydrogeological significance, fault zones mostly show a significant impact on groundwater migration.

It is important to include fault zones in water resource management and evaluation. This paper focuses on the permeability of the fault zone, the quantitative characterization and the evolution of the hydraulic characteristics of the fault zone, the dynamic evolution of the hydraulic characteristics of fault zones and the simulation of groundwater flow in the fault zone. The permeability of the fault zone is the key to the current study, and different permeability characterization methods have different representative scales. The permeability of the fault zone has complex spatiotemporal distribution characteristics, which challenges the description of groundwater migration in the fault zone. The selection of appropriate numerical simulation methods is an important way to quantitatively describe the groundwater movement in the fault zone and verify the suitability of the other methods. How to consider the complexity of the hydraulic characteristics of the fault zone at the spatiotemporal scale in the simulation process is the premise of accurately characterizing the groundwater flow in the fault zone.

We suggest that a special hydrogeological observation base for the fault zone is the key to further improve the level of fault zone groundwater study, and data from geological, hydrogeological, heat flow, geophysical and remote sensing should be integrated, and interdisciplinary cooperation should be strengthened.

Groundwater flow in fractured rocks has strong heterogeneity and anisotropy. The discrete fracture network (DFN) method has been internationally considered as one of the most reasonable and effective methods to describe the fracture water transport.

In this work, we focused on granite rock from an underground research laboratory site for the geological disposal of high-level radioactive waste. In addition, a high-performance numerical computing system and parallel codes were employed to develop a groundwater flow simulation method in fractured rocks based on the DFN model.

The results indicated that the proposed method could conduct the groundwater flow simulation of DFN with thousands of mesh elements. This improved the computational efficiency and ability of the parallel codes to deal with complex models. We established the DFN model structure optimization and parameter setting methods of boundary conditions in a complex condition. This could ensure that the hydraulic heads were continuous at different scale models. Furthermore, in the model area, the hydraulic head is distributed as a network structure along fractures. For the connected fractures, the water level was continuous and changed from high to low. However, the water level in the nonconnected fractures was discontinuous. Groundwater flows along the fracture from the high water level area to the low water level area. The connectivity and permeability of the fracture network have an obvious influence on the groundwater flow characteristics.

Therefore, we could conclude that the parallel groundwater flow simulation method based on the DFN model could more reasonably reflect the groundwater flow in a fractured rock mass. It was of great significance to further improve the simulated prediction ability and deepen the understanding of groundwater flow characteristics in a fractured medium.

Pumping tests are an important method for determining the hydrogeological parameters of aquifers and evaluating groundwater resources. For the analysis of single-well pumping tests, existing models assume that the well diameter remains constant with depth and that the aquifer is either unconfined or confined. They do not consider situations where the well diameter varies or when multiple aquifers are encountered.

In this study, we developed a steady-state mixed-well flow model to account for a pumping well with a variable diameter that penetrates multiple aquifers with the assumption of horizontal flow within the aquifers. The analytical solutions for the relation between pumping discharge and drawdown were derived. This study explores the methodology of using single-well steady-state pumping tests with variable-diameter wells to obtain aquifer parameters. Additionally, accurate solutions for the equivalent radius of a confined aquifer segment and alternative radius calculation formulas are proposed.

The mixed-well flow model was applied to analyze pumping tests near the Zihe River. Based on the data from three stepped pumping tests, a parabolic relationship between pumping discharge Q_w and drawdown s_w was established.The model predicted a pumping discharge of 4 093.8 m³/d when the maximum drawdown s_w was 25 m. The hydraulic conductivity of the unconfined aquifer was estimated to be 1.88 m/d, and the hydraulic conductivity of the confined aquifer was estimated as 0.43 m/d, with a relative error of less than 5%.

The mixed-well model serves as a theoretical foundation for predicting water yield from pumping wells in multiple aquifers system. However, it is important to acknowledge that the model's applicability is constrained by certain assumptions. In situations where rivers fully penetrate aquifers, there is a possibility of overestimating the hydraulic conductivity. On the other hand, when rivers only partially penetrate aquifers, the analytical solution may underestimate the hydraulic conductivity.

Groundwater numerical models often have uncertainties due to the complexity of the hydrogeological conditions and the economic and time constraints in collecting a sufficiently large dataset as inputs for conducting modelling exercises. In the past 50 years, stochastic methods have been one of the main methods of uncertainty analysis. The interval uncertainty is different from the stochastic uncertainty, and it considers the hydrogeological parameters as the intervals (ranges) without considering their stochastic properties.

From the perspective of interval uncertainty, a numerical simulation method based on first-order perturbation expansion was proposed for simulating unsteady two-dimensional confined flow with known hydrogeological parameters as intervals in this paper.The proposed method is implemented based on GFModel, a three-dimensional (3D) numerical simulation platform for groundwater flow and pollutant migration.

The analysis shows that the relative error can be controlled within 10% when the parameter change rate is less than 0.1. The computational efficiency of the proposed method is obviously higher than that of the continuous sampling method with equal spacing.

This method allows the interval of the head or water budget to be calculated without the requirement for detailed statistical information (which is usually unavailable in advance) if the intervals of hydrogeological parameters are known.It provides a theoretical basis for decisions on the use and protection of groundwater resources.

Geothermal resources are valuable clean resources, and the Enshi Basin contains abundant geothermal resources. It is very important to expore the origin of geothermal resources.

Starting from the chemical and isotopic characteristics of geothermal water, combined with the hydrological and geothermal geological conditions of the Enshi Basin, the following works have been performed. The geochemical characteristics and main ion recharge sources of geothermal water in the Enshi Basin are discussed and analyzed by the Piper diagram method and main ion correlation analysis method. In this study, the appropriate silica temperature scale method is used to solve the problem that it is difficult to accurately measure the heat storage temperature. Using hydrogen and oxygen isotope testing technology, the recharge source, circulation depth and recharge elevation of geothermal water are determined.

Results show that the hydrochemical type of geothermal water in the Enshi Basin is mainly SO₄·Cl-Na water, and the main ions in the groundwater are SO₄^2-, Cl^- and Na⁺. There is a good positive correlation between TDS in geothermal water and Na⁺, Ca²⁺, Mg²⁺, Cl^- and SO₄^2-, while TDS in geothermal water is much higher than that in cold springs.It is due to the large burial depth, long runoff path and strong dissolution leaching action, which makes it easier to extract relevant ions from the surrounding rock and results in ion concentrations much higher than those of surface water. The hot water in the study area is mainly supplied by atmospheric precipitation, and the ¹⁴C and ³⁴S isotopic characteristics of geothermal water show that the storage environment of geothermal water from the edge of the basin to the center of the basin is gradually closed, the retention time of geothermal water is gradually longer, and the degree of water rock reaction is gradually stronger. The results of the water-rock balance show that the concentration of SiO₂ in hot water is controlled by the dissolution balance of quartz.Using the SiO₂ geothermal temperature scale, the estimated thermal storage temperature is 55.74-58.24 ℃, the burial depth of thermal storage is 1 793-1 906 m, and the circulating depth of hot water is 1 823-1 936 m. The recharge elevation of geothermal water is estimated to be 1 022.64-1 109.00 m, according to the elevation effect of δO of atmospheric precipitation.

According to the elevation range of the study area, the geothermal water recharge area is mainly the low and middle mountain area of Cambrian Ordovician carbonate rocks in the western part of the basin.

The alpine region is the source area of many large rivers globally, and understanding river-groundwater interactions in the region is critical to the scientific management of watershed water resources. Due to the widespread occurrence of permafrost, the distribution and dynamics of riverbed taliks play a vital role in controlling river-groundwater exchange in alpine regions, leading to the complex, unique characteristics of the hydraulic relationship between them. However, there have been few investigations on river-groundwater interactions in alpine regions due to the harsh field conditions, and these available studies have dominantly used isotope and hydrochemical tracing methods, which are expensive and not accurate.

In this study, the low-cost and accurate temperature signal was used as a tracer to quantify the exchange between river and groundwater. A vertical 1D transient heat transport analytical model was used to quantify river-groundwater exchange rates at different depths. The spatiotemporal variations in river-groundwater interactions were analyzed using the temperature data measured by a distributed optical fibre sensing system.

Results show a substantial spatiotemporal variation in the exchange between river and groundwater in alpine regions. Season and climate can control the exchange rate between river and groundwater, and even the direction of exchange. It is also found that the exchange rate of river and groundwater increases with the depth of the active layer.

The study indicates that the temperature tracing method is suitable for studying river-groundwater interactions in alpine areas dominated by permafrost. Furthermore, the combination of two temperature tracing methods can effectively improve the accuracy and provide a feasible research framework for alpine regions where hydrogeological data are scarce.

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.

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.

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.

Vadose zone well (VZW) injection is an effective method of managed aquifer recharge (MAR). To improve VZW injection management, it is of great importance to accurately describe the unsaturated zone properties. Several analytical models have been developed for VZW injection based on the two-parameter constitutive model (Gardner model). As the three-parameter model (MB model) and four-parameter model (MN model) have been proposed, it is of interest to know whether the application of more flexible constitutive models is able to improve the analysis of unsaturated-saturated flow induced by VZW injection.

In this study, the MN model (which includes the Gardner model and MB model as subsets) was employed to establish a VZW injection model. The model was solved using COMSOL Multiphysics. The results are utilized to investigate the unsaturated-saturated flow induced by VZW injection for different values of the unsaturated zone properties to analyse the influences of the ground surface flux (GSF), and to compare the hydraulic responses based on different exponential constitutive models.

The analysis demonstrates that the hydraulic response induced by VZW injection and the influence of ground surface flux are affected by the hydraulic conductivity and the water storage capacity of the unsaturated zone. The relative hydraulic conductivity exponent ω_k affects the change in hydraulic conductivity of the unsaturated zone. The moisture retention exponent ω_c affects the water storage capacity of the unsaturated zone. The approximation of ω_k=ω_c=ω will result in some errors in the calculation and prediction of the hydraulic response caused by VZW injection. When the absolute value difference of the pressure head threshold b₁=ψ_a-ψ_k is small, its effect on the water head increment is small. In this case, it is reasonable to assume that b₁=ψ_a-ψ_k=0.

This study can help scholars improve the understanding of the VZW injection process and has important practical significance for the design, implementation and management of injection schemes.

The conceptual model of the single-well push test is a hot topic in groundwater hydrogeology.

In this study, a new mathematical model was developed for radial solute transport in an aquifer near injection wells. The heterogeneity of the aquifer was considered, and the MIM (Mobile-Immobile) convective diffusion model was used to describe the solute transport process in the aquifer. The skin effect and mixing effect are also included in this conceptual model. The semi-analytical solution was derived by using the Laplace transform and Stehfest numerical inverse transform methods. The influence of effective porosity and radial dispersion of the skin zone and the radius of the wellbore on the solute breakthrough curves (BTCs) of a fixed observation point and solute concentration distribution curves at given times were investigated.

Results show that wellbore mixing and skin effects have significant impacts on BTCs, solute radial transport processes and the influence area. The larger the radius of the wellbore is, the more obvious the wellbore mixing effect is. For the skin zone, a larger porosity leads to a smaller velocity of solute migration. The larger the radial dispersion is, the steeper the solute concentration curve of the observation point is, indicating that the solute concentration changes at a faster rate and can reach a stable value earlier.

Compared with previous studies, this model can better describe the solute radial dispersion process near the injection wells.

In order to understand groundwater systems, it is useful to study the changing characteristics and mechanisms of hydrogeological parameters with time. Responses of groundwater level microfluctuations to natural periodic loadings, such as earth tides and barometric pressure, serve as low-cost and effective investigation technique to calculate aquifer hydrogeological parameters.

In this paper, we systematically reviewed theoretical methods of hydrogeological parameters estimation based on groundwater level response to earth tides, barometric pressure, and their combination. We presented earthquake-related and mining-related parameters change with time in well-aquifer system. The barometric pressure response method also applies to the assessment of aquifer vulnerability.

We conclude that studying the microfluctuation of groundwater level may provide insight into the interaction among hydrogeological processes, tectonic activities, and artificial influences in the shallow crust at spatial and temporal scales.

This paper also proposes three scientific problems to be solved in the future: the application of skin and wellbore storage effects, the improvement of computation accuracy of hydrogeological parameters by combining multiple methods for determining basic geological parameters, and the investigation of other artificial influences such as groundwater overdraft and subsidence to regional aquifer systems.

Karstic spring water is an important source of water supply for the karst areas of northern China. To rationally evaluate and scientifically exploit karstic water resources, it is important to identify the recharge, runoff and discharge conditions of spring water and to clearly depict the boundary of the spring area.

Targeting the issues of ambiguous border and unknown replenishment situations of the Gudui-Nanliang spring groups, this paper investigated and analysed the karst hydrogeological conditions of the spring area and the distribution characteristics of δD, δ¹⁸O and ⁸⁷Sr/⁸⁶Sr isotopes in the karstic water. The water-rock interactions and the hydraulic connection along the recharge-runoff path of karstic water were systematically examined by using isotope technique.

The results show relatively large range of δ¹⁸O values in the karstic water. This is mainly attributed to the impacts of altitude effect, isotopic shift of oxygen in hot water, evaporation-induced enrichment and mixing of ancient sealed water. The distribution characteristics of δD, δ¹⁸O and ⁸⁷Sr/⁸⁶Sr in the karstic water provide strong evidence for the identification of the hydraulic connection of karstic groundwater. The formation of the Fenyangling geothermal field may be related to the water-rock interactions of the Mesozoic magmatic rocks that invaded the carbonate rocks. Also, the exploitation of karstic water in the north of the spring area will seize the karstic water resources in the spring area.

Overall, the investigation outcomes of karstic water isotopes in the Gudui-Nanliang spring area can provide a scientific basis for the management and protection of regional karstic water resources.

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.

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.

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).

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.

To study the transport of pollutants in typical hydraulic sedimentary units such as river terraces or alluvial fans, solute transport experiments were carried out in an indoor seepage tank.

NaNO₃ solution were introduced into the tank to simulate the point-pollution in hydraulic sediments. By measuring the concentration of the main ion components at different positions over time, the migration law of pollutants and the ion exchange process are analysed.

The results show that NO₃^2- is a conservative ion, and its breakthrough curve (BTC) is sharp and thin. The transport behaviour of Na⁺ is significantly affected by cation exchange, its concentration rising sharply and decreasing slowly. Cation exchange reduces the dispersion of Na⁺, and the effect becomes more obvious as the distance increases. In the early stage, the high concentration of Na⁺ can exchange Ca²⁺, Mg²⁺, and K⁺ in the sand layer. Cation exchange reduces the Na⁺ dispersion concentration. Due to the adsorption by sediment, the concentrations of Ca²⁺, Mg²⁺, and K⁺ will be lower in the later stage. The change in the reaction direction of cation exchange makes the BTCs of Na⁺ wider and gentler under the action of advective dispersion, and the phenomenon of "tailing" is more obvious. The water chemistry types in different areas in the seepage sank have different properties in space.

The research results have guiding significance for preventing and controlling groundwater pollution in hydraulic sedimentary units.

To quantitatively study the impact of various nonlinear factors in the coastal zone on groundwater flow and solute transport processes, MARUN (Marine Unsaturated) software for simulating groundwater flow in the coastal zone was developed.

This paper reviews the research and application progress of MARUN from the aspects of model principles, algorithm characteristics, literature review, and case analyses. The software is suitable for numerical simulations of two-dimensional vertical profiles of groundwater flow and solute transport reactions in the coastal zone. MARUN implements a finite element algorithm for variable saturation and density flow and solute transport, mass-consistent implicit difference discretization, and Picard, Newton, and Newton-Picard numerical solution methods, covering many nonlinear factors such as tides, waves, evaporation, rainfall, and load effects. It is characterized by its high professionalism, good flexibility, and high fault tolerance, but it does not have a graphical user interface. MARUN has achieved a number of highlighted research results in scenarios such as bioremediation of crude oil pollution on beaches, seawater-groundwater circulation processes in shallow aquifers in intertidal zones, and the evolution of regional groundwater flow systems.

In the future, MARUN needs to be further expanded into three-dimensional space and coupled with physicochemical processes with random characteristics to improve its performance level of simulating groundwater flow and solute transport in coastal aquifers.

The Kuncheng Tunnel of the Central Yunnan Water Diversion Project crosses the Heilongtan and Bailongtan areas, the major headwater regions in the Chenggong District of Kunming City.

Tunnel water inrush may seriously threaten the safety of the urban water supply. In this paper, based on hydrodynamic data and hydrochemical data during tunnel construction, the characteristics of the karst water system, the origin of the springs, and the impact of tunnel construction were analysed. Then, a three-dimensional groundwater flow model for the Heilongtan-Bailongtan Section was developed and calibrated to simulate and predict groundwater level changes during the construction of the Kuncheng Tunnel, and the water environment effect of tunnel construction was evaluated.

The results show that both the Heilongtan and Bailongtan springs are mainly recharged by the same karst P₁q+m aquifer. However, controlled by aquitard P₁d and the Hunshuitang fault downstream of the Sanjiacun Depression, the two springs actually belong to two relatively independent karst water systems. In addition, the excavation of the Kuncheng Tunnel has changed the regional groundwater flow field and cut off the flow of Bailongtan, while it has negligible impact on Heilongtan.

This study discusses the genetic relationship of karst spring, and quantitatively analyzes the impact of tunnel excavation on water environment, which has reference significance for preventing water inrush in tunnel construction.

The permeability of coral sand is the key factor determining the groundwater reserves of coral islands. For a long time, the research on the influence of different salinity filtrates on the permeability of coral sand during the formation of freshwater lenses is quiet few.

In this study, three kinds of solutions with different salinities were designed to represent freshwater, transition zone water and seawater to conduct dissolution and permeation tests on coral sand. Based on X-ray diffraction analysis and PHREEQC reverse simulation, the possible causes of water-rock interactions and permeability changes in coral sand in different salinity solutions were discussed.

Results showed that the permeability of coral sand was highly related to its dissolution in water bodies. The higher the salinity of the solution, the stronger the dissolution and the greater the permeability coefficient of the coral sand. The permeability coefficient of coral sand was almost unchanged in pure water. In saltwater, coral sand has undergone water-rock interactions, including carbonate dissolution and Na⁺-Ca²⁺ ion exchange, and the water-rock interaction is stronger with increasing salinity. After 360 hours of reaction under NaCl solutions with concentrations of 0.2 mol/L and 0.4 mol/L, the coral sand permeability coefficient changed from 0.58 m/d to 0.64 m/d and 0.74 m/d, respectively. It was inferred that dissolution could increase the overall porosity and thus the permeability coefficient by changing the sand particle size.

The results provide a scientific reference for the accurate assessment of freshwater reserves and the sustainable development of water resources on coral islands.

Baseflow recession analysis is an effective approach to estimating watershed-scale hydrogeological parameters. However, the traditional baseflow recession model did not consider the effects of semipervious riverbanks, and their influence on parameter estimation is unclear.

To address this issue, a mathematical model for groundwater flow in an unconfined aquifer with time-dependent recharge and river stages is presented. The effects of the semipervious riverbank are specifically taken into consideration. The analytical solutions of the hydraulic head and discharge are derived by using Green's function method, and their validities are tested by numerical simulations.

The results show that, forced by the fluctuating recharge rate, the lower riverbank permeability leads to a higher peak of hydraulic heads, a lower baseflow, and slower baseflow recessions. For the case forced by the fluctuating river stages, the lower riverbank permeability leads to the weaker responses of water flow to the fluctuated river stage and the lower fluxes of surface water-groundwater interaction. The riverbank permeability significantly affects the baseflow recession curves. During early stage, the low riverbank permeability caused the power index of the recession curve to be larger than 3. For later stage, the power index approaches 1, which is not affected by the riverbank permeability.

For a low riverbank permeability, the traditional model will overestimate the baseflow and underestimate the hydraulic conductivities of aquifers because it neglects the effects of riverbank permeability.

To determine the reliability and applicability of the soil water characteristic curve and unsaturated permeability coefficient combined measuring instrument designed on the evaporation method, three soil samples were selected: silty clay loam, loam and loamy sand.

Under natural evaporation, the evaporation flux, water content, and matric suction at 5 and 10 cm below the surface of the soil column were obtained by using the combined measuring instrument. The measured data were processed based on the linear hypothesis. HYDRUS-1D was used for simulation and inversion, and measured matric suction and flux data were input to determine soil hydraulic parameters. VG-Mualem model was fitted to the processed data by using RETC program.

The results show that the moisture content accords with the theory of the linear hypothesis, and the linear variation in matric suction is not ideal. However, soil samples other than sandy soil can solve this problem by reducing the evaporation rate and shortening the interval between two measurements. The fitting effect of the test data is good, and the error is small with the simulation result. The unsaturated permeability coefficient measured in the early stage of the test has a large error with the simulated value, which is mainly due to the inability to ensure the accuracy of the hydraulic gradient when the water content is close to saturation. These data can be ignored for improvement.

It is reliable to use the evaporation method and combined measuring instrument to measure the soil water characteristic curve and unsaturated permeability coefficient.

The South-to-North Water Transfer Project (SNWTP) has improved water use in northern China and further reduced deep groundwater extraction in the North China Plain. However, its impact on the evolution of regional groundwater hydrochemistry is still unknown.

In this paper, Cangzhou, which has experienced severe land subsidence due to groundwater overexploitation and has received water resources from the SNWTP since 2015, was selected as the study area to investigate the effects of the SNWTP on groundwater chemistry. In 2017 and 2021, groundwater samples from Ⅲ and Ⅳ confined aquifers were collected to determine the hydrochemical characteristics. Moreover, the average annual land subsidence of the regional layer was further evaluated by using SBAS-InSAR to identify the correlation between variations in land deformation and changes in groundwater chemistry.

The results showed that in comparison with those before the SNWTP, the groundwater fluoride concentration was slightly decreased, and the area of the high value zone was reduced after the SNWTP. The groundwater environment characterized by high pH, TDS and HCO₃^- concentrations and low Ca²⁺ concentrations favors fluoride enrichment in groundwater. The water chemistry type did not change, and the salinity concentrations in groundwater increased after the SNWTP. Groundwater receives more dissolution of halite and fluorite. Meanwhile, the amount and rate of land subsidence slowed after the SNWTP. A small degree of land uplift was even observed in the southeastern part of the region. The inhibition of land subsidence constrains clay compaction and the release of high fluoride porewater trapped in the clay layer. As a result, aquifers Ⅲ and Ⅳ received more effective lateral recharge, thereby causing a slight decrease in groundwater fluoride. However, it needs to be noted that the longer hydraulic residence time and stronger water-rock interaction after the SNWTP interaction promote the dissolution of sediment evaporites into groundwater, leading to an increase in groundwater salinity.

The results of the study provide scientific support for drinking water safety and water resource management in Cangzhou.

Dissolved organic matter (DOM) is an important carbon source in the biogeochemical process of groundwater.

To reveal the impact of the seasonal variation in DOM on the migration and transformation in groundwater on N in the Jianghan Plain, long-term water level and hydrochemical data of groundwater and surface water at the Shahu monitoring site were obtained, and the hydrogeochemistry analysis was carried out. The seasonal variation characteristics of DOM were analysed by combing with three-dimensional fluorescence spectroscopy and UV-V is spectroscopy, to explore the role of DOM in groundwater in N migration and transformation under the influence of hydrological conditions.

The results show that DOM in groundwater and surface water includes three components: terrestrial humic-like component (C1), microbial tryptophan-like component (C2) and microbial humic-like component (C3). The input of microbial tryptophan-like components increases in dry season and terrestrial humic-like components increase in wet season. The strong reducibility and high dissolved organic carbon(DOC) content of groundwater provide conditions for the nitrate reduction, and low humification and low molecular weight C2 components are preferentially utilized in N migration and transformation. In dry season, the groundwater level decreases, the aquifer is partial to oxidation, the unstable protein-like components quickly degrade and release NH₄-N, the nitrification and organic nitrogen mineralization rates are higher, and the denitrification and dissimilatory nitrite reduction to ammonium(DNRA) reaction rates are lower. In the wet season, the groundwater level rises, the aquifer tends to be reductive, and nitrification is inhibited. The presence of a large amount of DOM that is not easy to be degraded reduces the mineralization reaction rate of organic nitrogen in the aquifer, and the denitrification and DNRA processes are promoted.

In summary, the seasonal variation in DOM in the study area is an important factor in controlling the migration and transformation of N in groundwater.

The leakage of dense nonaqueous phase liquids (DNAPL) with a density greater than that of water into the underground environment becomes a long-term pollution source. Previous researchers have studied the effects of flushing fluid flow rate, cosolvent concentration, and media properties on DNAPL removal efficiency through methods such as column experiments, sandboxes, and numerical simulations.

However, the effect of pore-scale flow rate on the dissolution rate of residual DNAPL in the pores remains unclear. In this study, ethanol flushing solution was injected into a micropore model to simulate the removal process of residual PCE from pores. Microscale particle image velocimetry (PIV) was used to obtain the distribution of the water phase velocity field in the pore channel, and the factors affecting the dissolution and removal rate of residual PCE in different pore structures were analysed.

The experimental results indicate that the dissolution rate (R) of residual PCE in unconnected pores is influenced by several factors, including the water phase flow rate, the cross-sectional flux (q) within the pore, the angle (α) between the direction of pore opening and the direction of water phase flow, and the flux gradient (I). Based on fitting the experimental data, the quantitative relationship between dissolution rate and influencing factors is obtained as follows: R=3 876.79q^{(-0.016a+2.28/I)2}. A larger value of q corresponds to a higher renewal rate of the water phase in pore channels near unconnected pores, leading to an increased dissolution rate of residual PCE. When α is larger (α>90°), more flushing fluid enters the unconnected pores, further enhancing the dissolution rate of residual PCE. On the other hand, a larger value of I leads to a faster attenuation of the vertical component of the water phase flux entering the unconnected pores. This results in lower flow velocities near the interface, causing a decrease in the dissolution rate of residual PCE.

Based on micropore PIV technology, multiple factors, such as pore flow rate and medium pore structure, have been quantitatively revealed to jointly affect the dissolution rate of DNAPL in pores. This finding offers a novel approach to enhance our the understanding of the dissolution mechanism of residual DNAPL in pores and to quantitatively assess the removal efficiency of residual DNAPL under real-site conditions.

In the set of 3D seepage simulation programs of MODFLOW, the Conduit Flow Process (CFP) module has been widely used to represent karst conduits and karst aquifers and provides an important tool for groundwater simulation in karst areas.

When using CFP module to conceptualize the subterranean river conduits and their outlets, there are two schemes for the conceptualization of the outlets: setting the subterranean river outlet in the conduit model (FH scheme) or in the equivalent porous medium (Dr scheme), whose simulation effects are to be evaluated. In this paper, a groundwater numerical model with conduit flow was constructed, taking the Jiangjia Subterranean River in Jinyun Mountain, Chongqing as an example. The outlets of the subterranean river were conceptualized by the aforementioned two schemes, and then the differences between the subsequent two simulations were analyzed and compared.

The results show that the FH scheme is better than the Dr scheme. The fixed head setting in the FH scheme can achieve the effect of groundwater discharge in all segments of the subterranean river, while the drain setting in the Dr scheme only discharges groundwater in the upstream of the subterranean river conduit, then recharges the aquifer in the downstream, and finally, the groundwater is discharged in the drain unit.

The water balance analysis also demonstrates the advantage of the FH scheme in terms of the capacity of groundwater discharge and the function of aquifer storage.

It is very important to study the upscaling transformation of hydraulic conductivity in fracture media for accurately characterizing the seepage field characteristics.

Based on the fracture medium statistics of crystalline rock in an underground water-sealed cavern, a 2-dimensional discrete fracture network (DFN) model is generated using the Monte-Carlo stochastic simulation technique. The hydraulic conductivity parameters of the variable size simulation domain and the equivalent hydraulic conductivity of each grid element after meshing in different sizes are calculated. The hydraulic conductivity representative elementary volume (REV) of the study area was analysed by the variation in hydraulic conductivity parameters with the size of the simulation area, and the equivalent hydraulic conductivity of grid cells smaller than REV was calculated by upscaling.

The results show that the simulation domain can be regarded as an equivalent continuum when the size of the REV reaches 22 m×22 m. After meshing treatment, the equivalent hydraulic conductivity of the mesh cells which is smaller than REV calculated by the upscaling operation is significantly smaller than that of the corresponding composite mesh cells calculated by the crack network model.

Therefore, when the size of the seepage calculation unit reaches the REV size, its hydraulic conductivity parameters can effectively represent the hydraulic conductivity characteristics of a larger area in the study area. However, when the size of the seepage calculation unit is smaller than the REV size, its hydraulic conductivity parameters cannot effectively represent the hydraulic conductivity characteristics of the larger area in the study area. As a consequence, parameter upscaling calculation on the hydraulic conductivity parameters often has underestimated errors and is not of practical significance. For fractured media study areas with insufficient data, it is often difficult to determine the hydraulic conductivity REV of the study area. In this case, it can be considered that there is usually an underestimation error when the hydraulic conductivity of the small-scale area obtained from the hydrogeological test is upscaled. This conclusion provides a theoretical basis for the numerical simulation of the seepage field in fracture media in various related engineering projects.

Monitoring natural attenuation (MNA), as a low cost remediation method for groundwater pollution that does not produce secondary pollutants and has little impact on the polluted site environment, has high application value and development prospects, and is worth practicing and studying.

This paper adopts a groundwater-contaminated site in Baiyun District of Guangzhou to assess the applicability of MNA. Based on the analysis of hydrogeological conditions and pollution status, a groundwater numerical simulation program MODFLOW was used to establish a groundwater flow model for contaminated sites. The pollutant migration numerical simulation program MT3DMS was used to establish a pollutant migration model for the site. The migration processes of groundwater flow, main pollutants total petroleum hydrocarbons (TPH), and heavy metal nickel (Ni) were simulated, respectively. Based on the model, the performances of the MNA alone and the MNA combined with pump and treat methods were compared.

The results show that TPH and Ni are both sensitive to changes in the Freundlich constant and Freundlich exponent. The TPH shows good natural attenuation effect and can be substantially attenuated by the MNA alone. The concentration of TPH decreased from the initial value of 1.52 mg/L to the target (0.3 mg/L) after 850 days. Ni decay is relatively slow and it is suitable to adopt to a natural attenuation scheme under monitoring combined with the pump & treat method. The concentration of Ni decreased from the initial value of 0.13 mg/L to the target (0.02 mg/L) after 300 days. For a contaminated site with large natural attenuation capacities and/or low groundwater flow velocities, MNA alone is an appropriate remediation strategy. In contrast, for a contaminated site with low natural attenuation capacities and/or high groundwater flow velocities, the MNA combined with pump and treat may be a better remediation strategy.

This study provides an appropriate reference for the application of MNA to groundwater pollution remediation.

The morphological structure of the fractures in the rock mass is complex, and the fissures rough characteristics of the rock have a great influence on the permeability of the fractures. The current traditional numerical simulation software is mainly based on the macroscopic evaluation of equivalent continuous media, which cannot simulate the mesoscopic fluid flow characteristics within the tiny structure of the fracture. Although there exist models for assessing the permeability of rough fractures considering fracture roughness characteristics, it lacks physical meaning and has limitations in taking standard deviation of the profile height of rough fractures as the quantitative representation of rough characteristics.

Firstly, the W-M (Weierstrass-Mandelbrot) function was applied to construct a numerical model of a two-dimensional rough single fracture with different fractal dimensions. Secondly, the simulation of fluid flow at the mesoscopic scale was realized by programming based on the lattice Boltzmann method theory and analysed by combining the cubic law formulation with the value of standard deviation of the fracture profile height as a quantitative characterization of roughness.

The results show that the cubic law formula using standard deviation value of the fracture profile height as a quantitative characterization of the roughness feature is inadequate. It is feasible to use the fractal dimension as a local modified cubic law formulation for the quantization of rough features.

The study of rock fracture fluid flow has important engineering practical significance for groundwater pollution control and groundwater resource assessment.

Comprehensive analysis of fracture geometries and seepage characteristics is one of the prominent components in site suitability assessment of high-level radioactive waste (HLW) disposal repositories.

To provide a sufficient basis for the construction of the Xinchang underground research laboratory (URL) site in the Beishan preselected area of the HLW disposal repository in China, this paper analyzed the development and distribution of fractures and the seepage characteristics in fractured granite. Based on the borehole tests of BS32, BS36 and BS39 carried out in the middle of the Xinchang site and the field investigation of the surrounding fractures, the orientations and linear density of fractures were used to quantitatively classify the homogeneous zones of the BS32, BS36 and BS39 boreholes. Supplemented by the theory of the hydraulic conductivity tensor, the hydraulic conductivities of fractured rock and the principal directions of fluid seepage in fractured rock with different burial depths were obtained.

The results show that four groups of dominant fractures are developed around BS32, BS36 and BS39 at the Xinchang URL site, with orientations of 279°∠79°, 98°∠76°, 227°∠79° and 36°∠76°, which are especially dominant in the EW and NNE directions. The fractures are mainly shear-stress formed with steep dips (>60°), accompanied by a few tensile fractures, and the fractures are normally distributed. Compared with the field hydraulic test results, the overall comprehensive hydraulic conductivities of the boreholes are in the range of 10^-13-10^-9 m/s. The main seepage directions are NNE, nearly EW and SE, where NNE and nearly EW are the dominant seepage channels of the fractured rock mass, with larger main permeability tensor values. Two nearly E-W-trending F₆ and F₇ faults and their corresponding NNE-E-trending secondary faults play a macrocontrolling role in fracture development at the Xinchang site. The permeability of fractured rock is mainly affected by the fracture spacing and aperture, demonstrating a great anisotropy.

The results can provide necessary data support for the construction of disposal repositories and the numerical simulation of nuclide migration at the Xinchang URL site. In addition, the presented research idea could provide an alternative and practical method for effectively studying the properties of deep fractured rock masses with deep geological disposal of HLW.