Countermeasures against saltwater intrusion (SWI) are critical to prevent coastal groundwater deterioration. Among different measures to prevent SWI, subsurface dams have shown to be an effective ...approach, but it is likely to produce residual saltwater behind the dam in a landward aquifer. This study investigated the influences of subsurface dam design and aquifer properties on the dynamic characteristics of residual saltwater in a field‐scale aquifer and for the first time revealed the desalinization mechanism of residual saltwater behind the dams from the point of mixing zone. It was found that the low‐concentration mixing zone (LCMZ) (for the area between 10% and 50% of seawater salinity) was a major channel for the saltwater to flow over the dam to the ocean boundary while the residual salt was continuously dispersed to the LCMZ from the high‐concentration mixing zone (HCMZ) (for the area between 50% and 90% of seawater salinity) under high‐concentration gradients. Moreover, we developed two formulas of the reduction rate of saltwater wedge length (RSWL*) and the removal rate of total residual salt mass (RTSM*) to evaluate the desalination effectiveness of high‐ and low‐concentration residual saltwater, respectively. The results showed that it took much longer time for a taller dam and a dam at a closer position to the sea boundary to desalinize the high‐concentration residual saltwater in the upstream aquifer, more than 50 years for the cases of dam height beyond 16 m. On the contrary, only a slightly shorter time was needed to remove the low‐concentration saltwater behind the dams with the decrease of the distance from the sea boundary. Aquifer properties including the hydraulic gradient, hydraulic conductivity, and dispersivity strongly altered the desalinization time of the residual saltwater. The dispersivity was found to be the most critical factor influencing the removal effectiveness of saltwater retained in the landward aquifer. Increase of dispersivity from 1 to 3 m can dramatically reduce the desalinization time from more than 30 to 4 years.
Key Points
The saltwater desalinization mechanism is firstly revealed from the point of mixing zone
Desalinization timescale and effectiveness of residual saltwater are quantified in field scale
Dam height and dispersivity are found to be the key factors controlling the desalinization
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
•The nitrate accumulation induced by cut-off wall was firstly revealed considering layered heterogeneity.•Wall height and location could significantly affect nitrate enrichment in layered ...aquifers.•Stratified patterns were found to be the key factors controlling the nitrate contamination.
Cut-off walls have proven to be an effective countermeasure for mitigating seawater intrusion (SI). However, construction of cut-off walls may lead to upstream accumulation of nitrate contaminants in coastal aquifers. While stratified geological heterogeneities are widely present in coastal areas, how they may influence nitrate enrichment induced by cut-off walls remains poorly understood. In the present study, we employ variable-density coupling with reactive transport numerical models to understand and quantify the dynamic changes of salinity and NO3− in layered heterogeneous coastal aquifers in the presence of cut-off walls. Our comprehensive simulation results indicate that aquifer heterogeneities are a primary factor in regulating nitrate accumulation. For example, the presence of a middle low-permeability layer between two high-permeability layers reduces the influence of cut-off wall height on SI if the bottom of the wall is placed inside the low-permeability layer. While the thickness of the middle low-permeability layer has a minor impact on SI, a thicker middle low-permeability layer leads significantly aggravated nitrate contamination. Moreover, our simulations reveal that the degree of nitrate accumulation changes non-monotonically with changes in the hydraulic conductivity of low-permeability layer—the total nitrate mass (TNM) first increases and then decreases as the hydraulic conductivity of the low-permeability layer increases. We also found that the impact of heterogeneities is a function of the location and depth of the cut-off wall. With the increases of the distance between wall and the ocean, the nitrate enrichment degree of nitrate pollution in the upstream aquifer decreases while seawater intrudes further in layered aquifer (i.e. case HLH). TNM increases as the wall depth increases, but the growth rates of TNM becomes lower when the bottom of the wall is placed inside the low-permeability layer. These quantitative analyses have critical implications for the planning and design of cut-off walls to effectively control SI while at the same mitigating nitrate pollution.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Accurate identification of nitrate (NO3−) sources is critical to address the issue of groundwater pollution. The nitrogen (N) isotopic enrichment factor (ɛp/s) is an important parameter to explain ...the N cycle and determine the proportional contribution of NO3− sources. Considering the isotopic fractionation effects in N transformation processes, this study quantitatively analyzed the NO3− sources in groundwater using stable isotopes (δ15N-NO3− and δ18O-NO3−) and the Bayesian isotope mixing model (SIAR). For the first time, the ɛp/s values (0.0‰, −8.7‰, −8.7‰, and 14.7‰) of atmospheric deposition (AD), soil nitrogen (SN), chemical fertilizers (CF), and manure and sewage (M&S) were calculated to determine the NO3− source apportionment in groundwater. It was proved that the isotopic fractionation effect could produce a more accurate NO3− source apportionment. We also found that the NO3− source contributions were closely related to the cropping system. In the vegetable cultivation area, CF (54.32%) and SN (37.75%) were the dominant NO3− source, while in the grain cultivation area, NO3− pollution was largely influenced by SN (33.67%), CF (33.27%), and M&S (30.16%). According to this study, the isotope fractionation is strongly recommended for NO3− source apportionment in groundwater system.
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•Obvious isotope fractionation effect occurred in nitrogen transformation processes.•Explored nitrogen isotope enrichment factor of various sources for the first time.•A Bayesian isotope mixing model was applied to the NO3− source apportionment.•Made the source apportionment more accurate considering isotopic enrichment factor.•NO3− pollution in groundwater was closely related to land use and cropping system.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Subsurface dams are rather effective and used for the prevention of saltwater intrusion in coastal regions around the world. We carried out the laboratory experiments to investigate the elevation of ...saltwater wedge after the construction of subsurface dams. The elevation of saltwater wedge refers to the upward movement of the downstream saltwater wedge because the subsurface dams obstruct the regional groundwater flow and reduce the freshwater discharge. Consequently, the saltwater wedge cannot further extend in the longitudinal direction but rises in the vertical profile resulting in significant downstream aquifer salinization. In order to quantitatively address this issue, field‐scale numerical simulations were conducted to explore the influence of various dam heights, distances, and hydraulic gradients on the elevation of saltwater wedge. Our investigation shows that the upward movement of the saltwater wedge and its areal extension in the vertical domain of the downstream aquifer become more severe with a higher dam and performed a great dependence on the freshwater discharge. Furthermore, the increase of the hydraulic gradient and the dam distance from the sea boundary leads to a more pronounced wedge elevation. This phenomenon comes from the variation of the freshwater discharge due to the modification of dam height, location, and hydraulic gradient. Large freshwater discharge can generate greater repulsive force to restrain the elevation of saltwater wedge. These conclusions provide theoretical references for the behaviour of the freshwater–seawater interface after the construction of subsurface dams and help optimize the design strategy to better utilize the coastal groundwater resources.
The elevation of saltwater wedge refers to the upward movement of the downstream saltwater wedge after the construction of subsurface dams. The subsurface dams obstruct the regional groundwater flow and decrease the freshwater discharge. Then the saltwater wedge cannot advance in the longitudinal direction due to the block of the dam but rises in the vertical profile because of the reduced freshwater discharge, which eventually increases the saltwater contaminated area in the downstream aquifer.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
•A new barrier is proposed to control seawater intrusion and nitrate contamination.•Effectiveness of seawater intrusion and nitrate accumulation are investigated.•An optimization method is developed ...to design the variable permeability cut-off wall.
Seawater intrusion (SI) is an increased threat to the groundwater resource in coastal regions. The cut-off wall is widely used to arrest the advance of a seawater wedge, but it is likely to cause groundwater contamination in landward aquifers. In order to alleviate the contamination problem, we proposed a new variable-permeability cut-off wall (VPFW) with a semi-permeable section at the bottom of the physical barrier. This study is the first to investigate the spatial distributions and transient behaviors of SI and nitrate pollution at field scale under the joint effects of physical barriers and denitrification. Results show the dissolved oxygen (DO) decreases almost synchronously with the intrusion of SW near the sea boundary, and the denitrification of NO3− and retreat of the dissolved organic carbon (DOC) wedge occurred in the inland aquifer. Notably, the freshwater discharge flux and nitrate discharge flux for the developed VPFW increase by 35% and 20%, respectively, while the enrichment ratio of specific volume of nitrate contamination zone (SVNR) went down by 15% on average compared with the traditional cut-off wall. Four indices were proposed to quantify the impervious barrier: heights (H), relative hydraulic conductivities (K*), and control effectiveness of SI and nitrate pollution. Moreover, we found the control effectiveness for SI and nitrate pollution was continually enhanced with the reduction of K* and increase of H; however, excessive reduction and increase may result in the oppositeimpacts. The optimal K* and H for the VPFW can be determined with the required value of the reduction rate of saltwater wedge length (SWR) and SVNR; but all cut-off wall cases fail to meet the demand. The results show that the VPFW is much more effective in controlling SI and nitrate pollution in comparison with the traditional cut-off wall.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•The minimum effective dam height to prevent SWI was proposed.•The fresh groundwater discharge was used to assess the environmental performance of the subsurface dam.
Subsurface dams are widely used ...to prevent saltwater intrusion around the world. A subsurface dam blocks the groundwater movement both from and towards the sea. This blockage often leads to an accumulation of pollutants and salt on the inland and sea-side of the dam, respectively. While the latter is intended, the former effect is not desired and poses a huge problem in groundwater management. Herein, we propose the use of dams of minimum effective height to prevent saltwater intrusion, and the use of the fresh groundwater discharge to assess the environmental performance of the subsurface dam. Laboratory tests and numerical simulations were used to study the effects of dam height, distance from the saltwater boundary, and head difference on the subsurface dam’s saltwater intrusion prevention efficiency and fresh groundwater discharge. We found that i) the fresh groundwater discharge reaches its peak at the minimum dam effective height, and ii) the minimum effective dam height is shorter than the height of SWI without the dam. This means, under the premise of effectively preventing SWI, we can reduce both construction costs and increase fresh groundwater discharge through constructing the dam with the minimum effective dam height. When the dam height was less than the minimum effective dam height, the subsurface dam had little effect in preventing saltwater intrusion. As the dam distance to the shoreline increased, the minimum effective dam height and the peak fresh groundwater discharge decreased simultaneously. A reduction of the dam height is conducive to saving engineering cost, while a reduction of the peak fresh groundwater discharge favors the accumulation of land-based pollutants and salt. Although an increased distance of the dam to the coast seems more economic during construction, this also implies a larger inland soil salinization and accumulation of pollutants. The site selection of subsurface dams is therefore an optimization task that should consider both the engineering cost and ecological environmental effects.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•The influence of a subsurface cut-off wall on nitrate contamination in groundwater upstream is discussed.•The relative height of the wall and the nitrate infiltration rate are the two most ...significant factors determining the degree of nitrate contamination in groundwater.•The subsurface cut-off walls should be designed such that their height lies below the critical height to reduce the impact on nitrate enrichment.
Due to shallow groundwater tables and easy water availability, many people live in coastal areas. Additionally, alluvial sediments from fluviatile and marine deposits are highly suitable for agricultural use, making coastal settlement even more desirable. However, groundwater nitrate pollution resulting from widespread application of fertilizers creates difficulties for water management officials. Cut-off walls are used to control seawater intrusion into coastal aquifers. However, until now, the effect of cut-off walls on the nitrate contamination of groundwater has been largely unknown. In this paper, the first investigation of the influence of a subsurface cut-off wall on nitrate contamination in groundwater is discussed. Two sets of laboratory experiments – one with and one without a subsurface cut-off wall – were performed to validate the effectiveness of the constructed numerical model. The effects of the subsurface cut-off wall height, distance between the subsurface cut-off wall and the saltwater boundary, and infiltration rates on nitrate distribution and contamination in groundwater were analyzed. Results showed that after the construction of the subsurface cut-off wall, the domain mean velocity decreased, the migration of nitrate pollutants to the saltwater boundary was reduced, and the nitrate-contaminated area expanded. Further, the morphology of the nitrate pollution plumes changed, an indentation appeared directly above the subsurface cut-off wall, the widths of the transition zones increased significantly, and the transition zones became more diffuse as one approached the saltwater wedge. Three indices were developed to quantify the degree of nitrate contamination. The relative height of the subsurface cut-off wall (H′) and the nitrate infiltration rate (Ni) were the two most significant factors determining the degree of nitrate contamination in groundwater near the subsurface cut-off wall. Therefore, we suggest that subsurface cut-off walls be designed such that their height lies below an experimentally determined critical height; thus reducing the infiltration rate of the contaminants produced during fertilization through water-saving irrigation techniques.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Recent studies have confirmed the instability of tide‐induced upper saline plume (USP) in tidal beach aquifers. A systematic understanding of how the unstable USP contributes to groundwater flow and ...salinity distribution is still lacking. Here we used laboratory experiments and numerical simulations to examine the responses of the salinity distribution and submarine groundwater discharge (SGD) to tide‐induced unstable flow. The results revealed that changes in physical forcing conditions (tides and inland freshwater input) drove the transition of the USP from stable to unstable states. Meanwhile, the phase‐averaged SGD and its components fluctuated over time, and their oscillation amplitudes depended on the physical forcing conditions. There was a correlation between USP instability and the fluctuation of water exchange across the aquifer‐sea interface. The fluxes across the aquifer‐sea interface under unstable conditions usually varied considerably over time with a lower frequency in comparison to stable conditions. The extent of seawater intrusion (SWI) measured by the total salt mass stored in the aquifer also varied over time, especially under unstable conditions. The vibration tendency of the total salt mass was opposite to that of the sum of inland freshwater discharge and tide‐induced circulation efflux. For coastal aquifers affected by tides, SWI was dominated by the sum of inland freshwater discharge and tide‐induced circulation efflux, followed by the sum of inland freshwater input and tide‐induced circulation influx. These findings lay the foundation for gaining a better understanding of tide‐induced unstable flow and its potential implications.
Key Points
The phase‐averaged submarine groundwater discharge (SGD) and seawater intrusion extent fluctuate with time, especially under unstable conditions
There is a correlation between the instability of tide‐induced upper saline plume and the fluctuation of SGD
The extent of seawater intrusion is dominated by the sum of freshwater discharge and tide‐induced efflux
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Groundwater optimization models coupling Non-dominated Sorting Genetic Algorithm II (NSGAII) with surrogate models have shown great success in recent years. However, most previous models adopted ...global optimization over time steps while ignoring the time-varying properties of optimization objectives, which may lead to an unrealistic strategy for specific months. Moreover, employing global optimization for time-varying optimization may result in computation inefficient. To address these issues, We developed EGN combining Expert knowledge, Graph Attention Networks and the NSGAII algorithm. Specifically, EGN defines a deterministic policy based on the encoded Expert knowledge to efficiently select the optimal strategy from the Pareto Front given by NSGAII. The framework significantly reduced the asymptotic complexity from O(NT) (global optimization) to O(N2). We evaluate EGN on a real-world dataset and the results show that EGN outperforms baselines and global optimization.
•EGN combines Expert knowledge, Graph Attention Network and NSGAII algorithm.•EGN is efficient for solving time-varying multi-objective optimization task.•Encoding Expert knowledge makes EGN achieve efficiency and physical significance.•Graph algorithm is first employed for groundwater optimization problem.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Subsurface dam possess an optimal range of dam height for water resources protection.•Tidal vibration alters the optimal range and reduces the lower and upper thresholds.•The minimum effective dam ...height is close to the 20% saltwater wedge height.•Salt-freshwater mixing critically affects the minimum effective dam height.
Subsurface dam is a technically feasible and widely used solution to prevent seawater intrusion (SWI) in coastal areas. The effectiveness of subsurface dams and the related environmental problems have been individually established, although static oceanic conditions are usually considered. In this study, sea boundary conditions, including static and tidal scenarios, were compared to comprehensively assess subsurface dams for SWI control and upstream groundwater pollution, thus extending previous findings. The index of fresh groundwater discharge was used as a proxy to assess the environmental problems associated with subsurface dams. Laboratory experiments and numerical simulations were performed to analyze the effects of dam height, location, and dispersivity. The results revealed that the effectiveness of subsurface dams could be described through a so called effective and increased discharge height range (E&IDHR), which was depending on dam location. Showing properties within the E&IDHR, subsurface dams could not only effectively prevent the invasive saltwater, but also increase the amount of fresh groundwater discharge relative to the scenario prior to dam construction. Tidal fluctuation significantly altered the E&IDHR, and the lower and upper thresholds of the E&IDHR were lower than those of the no-tide cases. The lower threshold was the minimum effective height, which was close to the saltwater wedge (SW) height determined by 95% isohaline (95% SW height) for the no-tide cases and the 20% SW height for the tide cases. This difference was attributed to intensive salt-freshwater mixing driven by tides. The upper threshold of the E&IDHR was usually close to 116% of the 5% SW height. Acknowledging the E&IDHR, applied studies may benefit from numerical and parametric uncertainties when designing subsurface dams, while still attaining optimal effectiveness to prevent SWI and upstream groundwater pollution.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
