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Unconfined coastal aquifers are a main pathway for land‐sourced solutes to enter the oceans. The migration of these solutes in aquifers is highly affected by the groundwater flow and salt transport processes, which are, to a great extent, controlled by tides. While many studies have examined how tidal oscillations would influence the subsurface hydrodynamics in coastal aquifers, most of them ignored the potential impact of groundwater pumping, a common practice in coastal areas to satisfy the demand for freshwater. This study, by means of laboratory experiments and numerical simulations, explored the combined effects of tides and groundwater pumping on the pore water flow and salinity distributions in an unconfined coastal aquifer. The results show that, in a tide‐controlled aquifer, the addition of groundwater pumping would exacerbate the degree of seawater intrusion and lead to wider spreading and deeper penetration of the upper saline plume. Moreover, groundwater pumping would enhance the tide‐driven circulation in the upper saline plume and weaken the density‐driven circulation in the saltwater wedge, ultimately leading to the reduction in total submarine groundwater discharge. These findings may promote a deep insight into the complex coastal groundwater systems experiencing human activities, and provide guidance for better evaluating the environmental impact of groundwater pumping. [ABSTRACT FROM AUTHOR]

The saltwater‐freshwater mixing zones in coastal aquifers can host complex physical exchange and biogeochemical transformations. The land‐sourced dense contaminant plumes could be transferred into the mixing zone of the saltwater wedge due to the density effect prior to discharge to the sea. However, the mixing process between dense contaminants and the saltwater wedge has not received much attention, largely due to the lack of physical evidence. This study used laboratory experiments and numerical simulations to investigate the transport and discharge behaviors of variable‐density contaminant plumes in tidally influenced unconfined coastal aquifers. Results demonstrate that the highly dense contaminants mix with the underlying saltwater and finally merge with the saltwater wedge. This process significantly extends the contaminant discharge durations, thereby reducing the peak value of contaminant efflux. The dense contaminants are elongated along the landward margin of the saltwater wedge, leading to a larger spreading area (Ms) than that of constant‐density contaminants. The sensitivity analysis indicates that the high density of contaminants acts as a trigger to induce the mixing of them and wedges. The higher hydraulic conductivity, lower dispersivities and reduced inland freshwater flux significantly increase the residence times (Rt) and discharge duration (Dt) by enhancing the mixing of dense contaminants with seawater. In contrast, both Rt and Dt values are not only non‐monotonic functions of tidal amplitudes but also less sensitive to tidal effects. Compared with the non‐tidal condition, however, the addition of tides significantly increases both Rt and Ms values of dense contaminant plumes. The results presented herein provide valuable insights into the mechanisms of dense contaminants mixing into saltwater wedges, which could guide practitioners in designing effective strategies to protect coastal environments from land‐sourced contaminants. Key Points: The process of dense contaminant plumes mixing into the saltwater wedge is experimentally observed for the first timeThe high density of contaminants triggers their mixing with the saltwater wedge, thereby extending their residence timesThe gravity and modified Rayleigh numbers assess how hydrogeological parameters affect the mixing of dense contaminants into saltwater wedges [ABSTRACT FROM AUTHOR]

The freshwater lenses ("lenses" hereafter) within saline floodplain aquifers are sensitive to river flooding events. However, the effects of extensive floodplain inundation on saline aquifers are rarely considered and have not been examined previously under controlled laboratory conditions. We conducted laboratory experiments within a two‐dimensional (cross‐section) sand tank (i.e., representing a saline floodplain aquifer) and built both laboratory‐ and field‐scale numerical models to examine lens responses to flood events. Three sets of experiments were performed to explore different lateral extents of floodplain inundation. The temporal behavior of experimental lenses was quantified and compared to variable‐density numerical models that adopted calibrated laboratory parameters, showing good agreement. Results show that more extensive floodplain inundation leads to larger lenses (as expected). The sensitivity analysis was performed based on field‐scale numerical models, demonstrating that the floodplain inundation extent, hydraulic conductivity, and dispersivity are key factors controlling the post‐flood recession in lens extent and volume. In field‐scale simulations of floodplain inundation, the entire lens was significantly salinized during flood recession due to enhanced dispersion accompanying higher groundwater velocities, which may further split into several isolated freshwater bodies before eventually returning to steady‐state conditions. Importantly, field‐scale numerical results indicated that the salt load to the adjacent river increased immediately following the flood event, consistent with reporting of the River Murray (South Australia). These results provide critical new insights into relationships between flood events and the behavior of lenses, highlighting the significance of flooding events on both intermediate and long‐term conditions of saline floodplains. Plain Language Summary: In semi‐arid and arid regions, river‐fed freshwater lenses within saline floodplain aquifers can sustain salt‐sensitive riparian vegetation and the health of floodplain ecosystems, especially during periods of low river flow. The dynamics of these lenses may be significantly altered by the river flooding events that often cause widespread floodplain inundation. Thus, the quantitative analysis of how riparian lenses respond to river flooding events is crucial for the sustainable management of salt‐affected floodplains. In this study, the lens behavior under flooding conditions was directly observed and quantified in laboratory experimentations for the first time. Flood‐induced lens dynamics in real‐world settings were further investigated numerically. The results indicate that the occurrence of floodplain inundation leads to a complex relationship between flood events and the behavior of lenses. The salt load to the adjacent river increased immediately following the flood event. These previously unarticulated outcomes are expected to broaden the understanding of riparian lens dynamics and river salinization associated with flooding events. Key Points: Response of riparian freshwater lenses to flooding is examined using laboratory experimentation and numerical modelingRiparian freshwater lenses formed during floodplain inundation may split into several isolated lenses after flood recessionA rapid increase in saltwater discharge to adjacent rivers occurs following a river flooding event [ABSTRACT FROM AUTHOR]

Excess export of nitrate to streams affects ecosystem structure and functions and has been an environmental issue attracting worldwide attention. The dynamics of catchment-scale solute export from diffuse nitrogen sources can be explained by the changes of dominant flow paths, as solute attenuation (including the degradation of nitrate) is linked to the age composition of outflow. Previous data-driven studies suggested that catchment topographic slope has strong impacts on the age composition of streamflow and consequently on in-stream solute concentrations. However, the impacts have not been systematically assessed in terms of solute mass fluxes and solute concentration levels, particularly in humid catchments with strong seasonality in meteorological forcing. To fill this gap, we modeled the groundwater flow and nitrate transport for a small agricultural catchment in Central Germany. We used the fully coupled surface and subsurface numerical simulator HydroGeoSphere (HGS) to model groundwater and overland flow and nitrate transport. We computed the water ages using numerical tracer experiments. To represent various topographic slopes, we additionally simulated 10 synthetic catchments generated by modifying the topographic slope from the real-world scenario. Results suggest a negative correlation between the young streamflow fraction and the topographic slope. This correlation is more pronounced in flat landscapes with slopes <1:60. Flatter landscapes tend to retain more N mass in the soil (including mass degraded in soil) and export less N mass to the stream, due to reduced leaching and increased degradation. The mean in-stream nitrate concentration shows a decreasing trend in response to a decreasing topographic slope, suggesting that a large young streamflow fraction is not sufficient for high in-stream concentrations. Our results improve the understanding of nitrate export in response to topographic slope in a temperate humid climate, with important implications for the management of stream water quality. [ABSTRACT FROM AUTHOR]

Global‐change drivers that alter tidal amplitude or sea level could directly impact coastal aquifers' seawater–microbe interactions. The semidiurnal or diurnal tide can have consequences for chemical redistribution, because it deepens oxygen and nutrients penetration depth, thereby increasing O2 depletion via microbial respiration in sediments during groundwater circulation. However, chemical and microbial distributions in tidal aquifers remain unclear, in deference to their link with depth and lateral location. By combining the microbes‐accumulated equation with the traditional groundwater flow model in coastal aquifers, we explicitly unravelled the heterotrophic zonation, the salinity zonation, and the oxic zonation of tidal aquifers. According to the model estimates, salt penetrates deeper than reactive chemicals, and the heterotrophic high biomass zone is presumably 2–3 m deep. Such depth of heterotrophic distribution is 2–3 orders of magnitude higher than phototrophic distribution. Additionally, heterotrophic biomass can exhibit faint oscillations, even under the tide forces, whereas dissolved chemicals fluctuate significantly with tidal pumps. Due to the transport of nutrients by tidal penetration, intertidal sediments have a higher respiration rate than subtidal sediments, converting more organic carbon from seawater to atmospheric CO2 via microbes. We demonstrate that heterotrophic biomass may play the dominant role in the chemical reactions of nearshore sediments. Our results confirm the efficacy of unvegetated tidal sediments that rely on ubiquitous heterotrophs as a driver of oxygen consumption and DOC transformation. These steps above will allow for better field investigations of coastal aquifers within a framework of zonations and oscillations. [ABSTRACT FROM AUTHOR]

The main computational costs of gradient‐based inverse methods for high‐resolution groundwater flow inverse problems include costly forward model simulations and the large number of such simulations required to determine the Jacobian matrix. We develop an upscaling‐based inverse approach, named upscaled principal component inverse approach (UPCIA), which achieves dimensionality reduction and reduces computational cost of forward model simulations by evaluating the Jacobian through upscaled effective models on a coarse‐resolution grid constructed from upscaled principal components. UPCIA integrates downscaling into the inverse problem by estimating principal component coefficients based on the coarse‐resolution forward model, which are then used to generate high‐resolution parameter fields. Various numerical experiments demonstrate the effectiveness and efficiency of UPCIA, including 2‐D/3‐D high‐dimensional steady‐state and transient hydraulic tomography with known storativity to estimate multi‐Gaussian transmissivity or hydraulic conductivity fields. Results show that the hydraulic head is insensitive to small‐scale variability of conductivity, and UPCIA provides high‐quality inversion results similar to inverse methods with high‐resolution forward model simulations and significantly reduces computation time by orders of magnitude. In addition to supporting the characterization of heterogeneity in sufficient detail, UPCIA can also be used to examine whether finer resolution is necessary and possibly to determine an optimal inverse resolution. Key Points: Upscaled principal component inverse approach (UPCIA) achieves dimensionality reduction and reduces forward model computation by a grid‐coarsening methodDownscaling and inverse modeling are integrated by evaluating projections on upscaled principal components to invert high‐resolution fieldUPCIA can also be used to examine whether finer resolution is needed and possibly to determine an optimal inverse resolution [ABSTRACT FROM AUTHOR]

Submarine groundwater discharge via coastal aquifers is an important pathway for land‐derived chemicals to enter the oceans. While previous studies have mostly focused on natural coastal aquifers, little attention has been paid to those modified by land reclamation. This study used laboratory experiments and numerical simulations to investigate the effects of land reclamation on flow and mixing processes in a nearshore subterranean estuary. The results show that, regardless of the permeability, the addition of reclamation soil will shift the tide‐driven upper saline plume (USP) seaward and reduce the USP size. When the aquifer is reclaimed by low‐permeability soil, the seawater intrusion length is reduced for small tidal amplitudes but extended as tidal amplitude further increases. Depending on the tidal amplitude, high‐ and low‐permeability reclamation soils prolong or shorten the average particle travel time in the USP, the freshwater discharge zone, and the saltwater wedge. Compared to the case without reclamation, high‐/low‐permeability reclamation soil enhances/weakens the water discharge rate across the aquifer‐ocean interface, narrows/expands the freshwater discharge zone, and increases the tide‐/density‐driven recirculation percent. Findings from this study highlight the complexity of nearshore groundwater systems subject to human activities, and have significant implications for better understanding the fate of terrestrially sourced chemicals in subterranean estuaries before discharging into the coastal water. Plain Language Summary: A deep insight into the groundwater flow and salt transport processes in coastal aquifers is the precursor to effectively managing and rationally exploiting limited fresh groundwater resources. While many studies attempted to understand these processes in natural coastal aquifers, little attention has been paid to the aquifers subjected to human activities, especially land reclamation. In the context of many countries worldwide carrying out land reclamation projects to increase land availability, understating such anthropic impact on subsurface hydrodynamics in these coastal areas is necessary. This study, via laboratory experiments and numerical modeling, examined how land reclamation would alter the groundwater flow and salt transport patterns in coastal aquifers. The results show that land reclamation would shift the upper saline plume and saltwater wedge seaward, altering the particle travel times. Also, high‐/low‐permeability reclamation soil would greatly enhance/weaken the overall water exchange rate between the aquifer and the ocean. Findings from this study have important implications for nearshore groundwater systems experiencing human activities. Key Points: The addition of reclamation soil would shift the upper saline plume seaward and shrink its extentParticle travel times are either increased or decreased by land reclamation, depending on the tidal amplitudeHigh‐permeability reclamation soil enhances the overall water exchange rate, which is weakened by low‐permeability reclamation soil [ABSTRACT FROM AUTHOR]

Aquifer storage and recovery (ASR) involves the injection, and later extraction, of freshwater into aquifers, which often contain saline groundwater. Mixing between the fresh injectant and native saltwater often leads to part of the injectant becoming unrecoverable, thereby impacting ASR performance. This study explores freshwater‐saltwater mixing within ASR operations arising from aquifer heterogeneity using Monte Carlo analysis of 2D‐axisymmetric, density‐dependent flow and transport models. Logarithmic hydraulic conductivity (lnK) distributions are generated using either two‐point statistics or higher‐order connectivity features. Results show that higher variance in lnK leads to stronger freshwater‐saltwater mixing, which lowers the recovery efficiency (RE; i.e., ratio of extracted to injected freshwater). On average, the lowest RE values were obtained from lnK fields with connected high‐K features, across all ASR cycles. In contrast, RE values from lnK fields with connected low‐K features were typically the highest, at least where buoyancy was considered. The impact of aquifer heterogeneity on RE reduces with subsequent ASR cycles. Buoyancy was a major factor in lowering RE regardless of the adopted heterogeneity model. Heterogeneity tended to mitigate the adverse impacts of buoyancy, leading to some heterogeneous cases having higher RE values than the corresponding homogeneous case. These results highlight the importance of understanding buoyancy effects and subsurface heterogeneity (including the connectivity of geological structures), and interrelationships thereof when assessing the feasibility of multi‐cycle ASR in heterogeneous saline aquifers. Key Points: Aquifer storage and recovery (ASR) is assessed in heterogeneous, saline aquifers using Monte Carlo simulationsASR performance is influenced by hydraulic conductivity variance, correlation length, anisotropy, number of ASR cycles, and buoyancyConnected high hydraulic conductivity zones lower injectant recovery, highlighting the importance of subsurface characterization [ABSTRACT FROM AUTHOR]

Previous studies of freshwater lenses in saline aquifers adjoining gaining rivers ("riparian lenses") have so far considered only rivers that fully penetrate the aquifer, whereas in most cases, rivers are only partially penetrating. This paper presents a new methodology for obtaining the saltwater discharge and the shape of a steady‐state, non‐dispersive riparian lens, where the river is partially penetrating, combining two previous analytical solutions. The resulting analytical solution is compared to numerical modeling results to assess assumptions and the methodology adopted to approximate the "turning effect," which is the change in groundwater flow direction (horizontal to vertical) near the partially penetrating river. Model parameters were taken from previous studies, representing simplified conditions in the River Murray floodplains (Australia). Consistency between analytical and numerical results and field observations highlights the capability of the proposed analytical solution to predict the riparian lens geometry and saltwater discharge into partially penetrating rivers. Sensitivity analysis indicates that larger riparian lenses are produced adjacent to the deeper and wider rivers, as expected. The change in width or depth of the river has more influence on the saltwater discharge and the horizontal extent of the riparian lens (and less effect on the vertical extent of the lens adjacent to the river) for shallower and narrower rivers. This research highlights the utility of the new method and demonstrates that the assumption of a fully penetrating river likely leads to significant overestimation of the saltwater discharge to the river and the riparian lens horizontal extent and vertical depth. Key Points: A new method is developed to find saltwater discharge and steady‐state, non‐dispersive riparian lens shape close to a partially penetrating riverConsistency between analytical and numerical results and available field data highlights the capability of proposed methodologyAssumption of a fully penetrating river likely leads to significant overestimates of lens size and saltwater discharge to the river [ABSTRACT FROM AUTHOR]

• The NFVMSFEM can compute continuous x, y directional velocities and heads simultaneously. • The NFVMSFEM is a combination of the FVM, MSFEM and Yeh's FEM, based on the FVMSFEM. • The NFVMSFEM velocity matrixes can present velocity unknowns by head unknowns. • The NFVMSFEM achieves a high computation efficiency. This paper proposes a new finite volume multiscale finite element model (NFVMSFEM) for groundwater flow and velocity simulation in porous media. The main goal of this method is to simultaneously obtain the heads and continuous x , y direction Darcian velocities, thereby simplifying the simulation process and improving the efficiency. This goal is accomplished by introducing a novel velocity matrix in the control volume boundary flux estimation of the finite volume multiscale finite element method (FVMSFEM) framework. The velocity matrix allows for the NFVMSFEM to directly represent continuous fine-scale velocities by coarse-scale heads, thus merging the two kinds of unknowns into one. Through a modified Yeh's Galerkin model, the velocity matrix is constructed in each local coarse element before the head computation; thus, the computational cost is very little. Furthermore, the NFVMSFEM inherits the advantageous features of the FVMSFEM, which ensures the local conservation of heads and further improves the efficiency. The results of the comparison between the NFVMSFEM and certain classical methods for head and velocity computation demonstrate its effectiveness and high efficiency. [ABSTRACT FROM AUTHOR]

We here propose a two-step approach-based simulation-optimization model for multi-objective groundwater remediation using enhanced random vector functional link (ERVFL) and evolutionary marine predator algorithm (EMPA). In this study, groundwater flow and solute transport models are developed using MODFLOW and MT3DMS. The ERVFL network is used to approximate the flow and transport models, enhancing the computational performance. This study also improves the robustness of the ERVFL network using a kernel density estimator (KDE) based weighted least square approach. We further develop the EMPA by modifying the marine predator algorithm (MPA) using elite opposition-based learning, biological evolution operators, and elimination mechanisms. In the multi-objective version of EMPA, the non-dominated/Pareto-optimal solutions are stored in an external repository using an archive controller and adaptive grid mechanism to promote better convergence and diversity of the Pareto front. The proposed methodologies are applied for multi-objective groundwater remediation of a hypothetical unconfined aquifer based on the two-step method. The first step directly integrates flow and transport models with EMPA and finds the optimal locations of pumping wells by minimizing the percent of contaminant mass remaining in the aquifer. In the second step, the ERVL-based proxy model is integrated with EMPA and used for multi-objective optimization while explicitly using the pumping well locations obtained in the first step. The multi-objective optimization generates a Pareto-optimal solution representing the relationship between the rate of pumping and the amount of contaminant mass in the aquifer. Further analyses show a significant advantage of the two-step approach over a traditional method for multi-objective groundwater remediation. • A novel two-step approach is proposed for multi-objective groundwater remediation. • A random vector functional link is used for remediation of contaminated groundwater. • The random vector functional link is enhanced using weighted least square method. • Multi-objective groundwater remediation by evolutionary marine predator algorithm. • Archive controller and adaptive grid mechanism are used to store Pareto solutions. [ABSTRACT FROM AUTHOR]

We introduce a simple correction to coastal heads for constant-density groundwater flow models that contain a coastal boundary, based on previous analytical solutions for interface flow. The results demonstrate that accurate discharge to the sea in confined aquifers can be obtained by direct application of Darcy's law (for constant-density flow) if the coastal heads are corrected to (( α + 1)/ α) h s − B/2 α, in which hs is the mean sea level above the aquifer base, B is the aquifer thickness, and α is the density factor. For unconfined aquifers, the coastal head should be assigned the value [ABSTRACT FROM AUTHOR]

Summary: Stagnation points in groundwater flow fields delineate different flow regions by the separation streamlines passing through them. Stagnation-point analysis can hereby provide a useful tool in streamline delineation as well as in hydraulic control. In the present work, we present a stagnation-point analysis for a flow field created by a pumping well in recharge areas. This scenario is of great interest since it is very common in practice for pumping wells to be located inside or near a recharge area. A typical case is that an irrigation system is fed by groundwater pumped from an inside well. By performing stagnation-point analysis, it is found that the behavior of the created flow field under study is not only determined by the magnitudes of specific parameters such as pumping rate, regional flow rate, infiltration rate, recharge area, and well location, but also related to the interrelation of these parameters. Applying the properties of the potential Hessian matrix at stagnation points, we also identified the nature of stagnation points (e.g., maximum, minimum, or saddle point) assuming that the pumping well locates at the center of the recharge area. In addition, we delineated the streamlines by tracing streamlines starting from the stagnation points. The orientation of the separation streamlines is determined by the potential Hessian matrix. Generally, for a well with a relatively high pumping rate, there is one, and only one stagnation point outside the recharge area, since all infiltration and partial regional flow are pumped by the well. For a well inside the recharge area with a relatively low pumping rate, however, it is found that there are always three stagnation points, because infiltration is surplus for providing the well extraction, which results in the separation streamlines outside the recharge area and eventually forms a third stagnation point. Under certain field conditions, cases of two stagnation points can occur, and these critical conditions have also been identified. [Copyright &y& Elsevier]

• Unstable density-driven convection is assessed in heterogeneous aquifers with connected structures present using Monte Carlo simulation. • Free convection is promoted by connected high-permeable structures and retarded by connected low-permeable ones in the early stage. • Ignoring connectivity features leads to risks of biased salt transport assessment. Free convection usually happens in variable-density groundwater flow systems, and it favors contaminant transport by enlarging length scales and shortening timescales compared to advection and diffusion/dispersion alone. Previous studies have demonstrated that heterogeneity with multi-variate Gaussian distribution for logarithmic permeabilities (log 10 k) plays an important role in the onset, growth, and/or decay of instability during the density-driven convective process. Nevertheless, the connectivity features (i.e., connected structures of extremely high/low- k values), which are common in natural aquifers, have received little attention. In this study, the classical problem of transient free convection has been modified and numerically simulated by Monte Carlo approach to investigate the effects of connectivity features in heterogeneity on the unstable convective processes. Results show that free convection is promoted by the connected high- k structures and retarded by the connected low- k structures during mainly the early-stage mass loading. The impacts of connectivity features tend to be amplified by higher variation in log 10 k distributions, and can be secondarily influenced by correlation lengths and anisotropy. Under the multi-variate Gaussian assumption, the existence of connected high- k structures leads to underestimation of density-driven instability, in which the risk differs based on the statistics of permeability fields, metrics of interest and timeframe. This study highlights the importance of understanding connectivity features in heterogeneous geological media when assessing density-dependent solute transport in groundwater systems. [ABSTRACT FROM AUTHOR]

• We developed a novel analytical model for regional groundwater transit time distributions s accounting for the impact of stormwater infiltration systems. • Global sensitivity analysis successfully identifies the most important parameters for urban groundwater TTDs. • The urban groundwater flow regime cannot be fully restored to the pre-urban conditions even with the aid of stormwater infiltration systems. The stormwater infiltration system (SIS) has been recognized as a feasible way to manage stormwater and restore streamflow. As a lumped representation of the flow and transport processes, the groundwater transit time distribution (TTD) is highly relevant to this restoration goal. However, the influence of the SIS on groundwater TTDs has rarely been studied. In this paper, we propose a novel analytical model for groundwater TTDs in regional urban aquifers accounting for the impact of the SIS. This analytical solution is developed for a homogeneous 2-D cross-section of an idealized unconfined aquifer following the Dupuit-Forchheimer assumption. The analytical solution is verified through the comparison with simulated TTDs using the Lagrangian particle tracking algorithm. The global sensitivity analysis reveals that the stormwater infiltration rate, the number of infiltration facilities, the size of the urban area, and the evapotranspiration are the most important factors for groundwater mean transit times (MTTs). Although groundwater discharge and MTT can be restored to the pre-urban one, groundwater TTDs will inevitably deviate from the pre-urban ones due to the alteration from diffuse recharge to infiltration-focused recharge, indicating that the urban groundwater flow regime cannot be fully restored to the pre-urban conditions. Overall, as a first-order approximation of the real-world urban groundwater system, the proposed analytical model provides decision-making support for urban water resources management and the design of the SIS. [ABSTRACT FROM AUTHOR]

• The saltwater desalinization mechanism using negative hydraulic barriers is revealed from the point of mixing zone. • The timescale and effectiveness of saline groundwater removal using negative hydraulic barriers are quantified at the field scale. • SGD is closely related to the structure of negative hydraulic barriers. • Effect of negative hydraulic barriers weakens and SGD fluxes increase as barriers move seaward. • Optimization of the structure of negative hydraulic barriers considering the environmental impact is proposed. Submarine groundwater discharge (SGD) plays a critical role in local and regional hydrological cycles, and has a significant impact on the quality and ecology of coastal waters. However, our current understanding of the influence of negative hydraulic barriers (NHB), commonly used in coastal aquifers to mitigate seawater intrusion, on SGD and its components is still lacking. This study aims to fill this gap by developing a three-dimensional variable density groundwater flow model to assess the contribution of NHB to the fresh and saline SGD fluxes. The stability and reliability of our model were validated through rigorous comparisons with previous work by our group. The simulation results indicate that although the NHB is effective in desalinating areas with high salt concentrations, it does not sufficiently eliminate low-concentration saline groundwater within the 20-year simulated period. The saltwater wedge initially advances landward and then recedes, with the rate of retreat decreasing over time. In general, the implementation of NHB disrupts the exchange flux between the aquifer and the ocean, resulting in reduced fluxes of SGD compared to scenarios without barriers. The reduction in fresh SGD (Q f) ranges from 3.12 % to 21.87 %, while for density-driven circulation fluxes (Q s), it varies from 2.22 % to 29.86 %. Moreover, the sensitivity of Q s to changes in NHB location decreases in the case of inland pumping wells. Specifically, when the distance between the well and the shoreline (dis well) increases from 30 to 40 m, Q s experiences a decrease of 10.82 %. However, as the dis well increases from 60 to 70 m, the decrease in Q s is only 2.81 %. In addition, it was found that the time required for Q f to reach a stable state is shorter than that of Q s. As the pumping intensity and distance from the ocean increase, both Q s and Q f decrease, resulting in an overall reduction in total discharge. [ABSTRACT FROM AUTHOR]

• Radionuclides can intrude into the studied coastal aquifer from ocean up to 3.3 km landward from shore after a 30 years' exposure. • Depending on exposure duration, full recovery of the coastal aquifer may take up to 217 years for 14C contamination. • Ocean surge event is more likely to exacerbate the intrusion, but it does not significantly change the full recovery time. • Shorter exposure duration should be favored to minimize the influences on the coastal aquifer. Human activities, such as discharging nuclear wastewater into the ocean, can put coasts in the risk of being exposed to radioactive contaminations. The radionuclides may potentially intrude into coastal aquifers from the ocean and endanger the groundwater resources, either laterally through subsurface or vertically through land surface during ocean surge induced inundation. This study attempted to investigate the temporal and spatial scales of these intrusion processes using an experimental site near Bremerhaven, northern Germany. A fully coupled surface–subsurface flow and transport model was employed to simulate the groundwater flow and the transport of TDS (total dissolved salt), 3H and 14C, under conditions with or without an ocean surge event. Scenarios of different exposure schemes were designed by varying the exposure duration and radionuclide concentrations. The intrusion and post-exposure recovery were assessed via contaminated volume and area, intrusion length, and recovery time. Results show that, depending on exposure duration, the intrusion of 3H and 14C can cause serious contamination in the coastal aquifer, with up to 18.9 km2 land area being contaminated and reaching up to 3.3 km landward from the shore. 3H shows a 'harder intrusion, easier recovery' pattern due to its higher decay rate. The full recovery may take up to 70 years and 217 years (considering 30-years-exposure) for 3H and 14C, respectively. An ocean surge event is more likely to exacerbate the intrusion, but it does not significantly change the full recovery time. Shorter exposure duration should be favored to minimize the influences on the coastal aquifer when the total exposure quantity is fixed. This study highlights the severity of radioactive contamination in coastal aquifers and the difficulty of recovery. Thus, cautious decision should be made concerning the possible impacts of releasing radioactive wastewater on coastal life and human beings. [ABSTRACT FROM AUTHOR]