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With the increase of nitrogen (N) input in vadose zones-groundwater systems, N contamination in groundwater has become a global environmental and geological issue that has a profound impact on the ecological environment and human health. N migration in the vadose zone is the most significant means of contaminating the groundwater aquifer. However, the current research on the control of groundwater N contamination focuses solely on the content change of certain indicators and is unable to comprehend the cause and subsequent development of groundwater N contamination. These factors pose significant environmental management challenges in areas where groundwater is contaminated with nitrate. In recent years, research on the migration and transformation behavior of various N forms in vadose zones-groundwater systems has yielded some breakthroughs but also encountered some roadblocks. The biogeochemical behavior of nitrogen consists of a series of intricate chain reaction cycles (called N-cycle). The crucial role of microorganisms in the N biogeochemical process has attracted the interest of soil carbon- and N-cycle researchers and become a hot topic of study. Nonetheless, the role of microbial regulation in groundwater systems has been largely neglected and needs to be summarized immediately. Consequently, this review summarizes recent advancements, mechanisms, and challenges, and proposes a dynamic perspective on microbial regulation. On the basis of these findings, we propose a dynamic and comprehensive groundwater N system centered on microbial regulation. In addition, we critically summarized the migration and transformation behavior of the most recent N indicators, the impact of global environmental change on each N component, and the non-negligible effects of these factors on the control of groundwater N contamination. Future research must focus on the migration and transformation behavior of nitrogen in the deep vadose zone, based on the dynamic regulation of microorganisms, and complete the missing pieces of the developed N-cycle index system. These are essential for providing scientific guidance for global N management and effectively mitigating N contamination in groundwater.

Generally, Zn in stormwater runoff is considered as low toxicity, but in the senarios of roads and zinc-based materials roof runoff, the concentration of Zn becomes extremely high and cannot be ignored. Bioretention systems are used to remove heavy metals from stormwater runoff, while Zn adsorption is insufficient by conventional filler and is prone to secondary release when exposed to acid rain or high salinity runoff. This study integrated batch experiments and density functional theory calculation to investigate the mechanisms of how KOH-modified biochar (KBC) influences the removal and release of Zn in bioretention systems. The results revealed that KBC adsorbed 89.0-97.5% Zn in the influent, the main adsorption mechanism were complexation and precipitation, and precipitation is more important. In addition, 67% of Zn was immoblized as the residual form by KBC. In acidic and saline runoff, KBC reduced Zn secondary release by 43.6% and 37.08% compared to the results in the absence of KBC, which was attributed to the convertion of most dissolved Zn in acidic and saline runoff into residual Zn. Therefore, KBC has a considerable application potential not only to decontaminate the runoff of roads and Zn-containing roofs, but also to deal with secondary Zn release in acid rain or under the treatment of snow-melting agents.

Freezing and thawing of soil are dynamic thermo-hydro-mechanical (THM) interacting coupled processes and have attracted more and more attention due to their potentially severe consequences in geotechnical engineering. In this paper, a fully coupled thermo-hydro-mechanical freezing (THM-F) model is established for advanced system design and scenario analysis. The model is derived within the framework of the theory of porous media and solved numerically using the finite-element method. Particularly, the derivation of theoretical aspects pertaining to the governing equations, including in particular the thermo-mechanical decomposition treatment of the solid phase, is presented in detail. Verification examples are provided from purely freezing, THM and THM-F perspectives. Attention is paid to the heat and mass transfer, thermodynamic relations and the formation of frost heave. The migration of pore fluid from the unfrozen zone to the freezing area and the blockage of pore space by ice lenses within the porous media are studied. The model is able to capture various coupled physical phenomena during freezing – for example, the latent heat effect, groundwater flow alterations and mechanical deformation.

To investigate the effect of pH on bioclogging process during managed aquifer recharge, three laboratory-scale column experiments were conducted and the relative hydraulic conductivity, bacterial cell number and the concentrations of polysaccharide, protein and EPS were measured under pH 5, 7, and 9, respectively. High-throughput sequencing was also used to determine the characteristics of bacterial community under different pH conditions. The development of bioclogging was rather different for the case of pH 5. 7, and 9; i.e., the growth process and number of bacteria differed with pH. The shortest growth period and lowest number of bacteria were observed at pH 5. In addition, the difference in bacterial EPS concentration was mainly associated with the polysaccharides. The variation in pH led to different bacterial community composition and diversity. The acid-resistant Elizabethkingia and Bacillus were abundant at pH 5, while Chryseobacterium and Klebsiella had relatively high abundances at pH 7. In contrast, the basophilic Exiguobacterium accounted for80% of the total bacterial abundance at pH 9. This work is of great significance to explore bioclogging mechanism during MAR process, and provides insights and guidances for field-based managed aquifer recharge.

The effects of underground reservoir operation have important scientific significance and application value for the development, utilisation, and management of groundwater resources. Based on the investigation of hydrogeological conditions and operation management of the Dagu River underground reservoir, a new assessment system for the operation effects of the underground reservoir was established. In this study, water quantity, water quality, pollution source, vulnerability, and economic benefits of the underground reservoir were considered assessment indexes.. And we used a coupled set pair analysis and variable fuzzy set (SPA-VFS) to evaluate the Dagu Rive underground reservoir. The assessment results show that the assessment indexes of groundwater exploitation rate, groundwater exploitation modulus, wastewater treatment rate, and economic benefit ratio are good, but the water quality of the reservoir area is usually poor, the fertilizer application rate is high, and the aquifer vulnerability is weak.

The construction of subsurface dams for controlling seawater intrusion triggers the accumulation of nitrate upstream of a dam. This is raising the concerns about nitrate contamination in those regions of coastal aquifers that are supposed to be used as a fresh groundwater source behind a subsurface dam. Research on this subject has been mostly restricted to the use of a simplified sea boundary (e.g., static and no slope), ignoring sea level fluctuations driven by tides. In this study, the combined effect of tides and subsurface dams on nitrate pollution in upstream groundwater was examined through laboratory experiments and numerical simulations. The results revealed that the difference in the extent of nitrate contamination under various conditions (i.e., static, tidal, static with a dam, and tidal with a dam) was related to the temporal pollution behavior. In the early stage, nitrate contamination in upstream groundwater was essentially identical for different scenarios. Both tides and subsurface dams were found to increase nitrate contamination in upstream aquifers. The extent of nitrate contamination increased with higher tidal amplitudes, whereas the increment was more evident for a large tidal amplitude. The effects of tides and subsurface dams on nitrate contamination were also regulated by the locations and infiltration rates of the pollution source. Interestingly, under the joint action of tides and subsurface dams, the increment in the extent of nitrate pollution was greater than the sum of their individual effects. The increased pollutions caused by subsurface dams and tides were quantified as 9.47% and 37.22%, respectively, whereas the increased value caused by their joint action was measured as 51.10%. These findings suggest that tidal activity should not be overlooked when assessing nitrate contamination in upstream groundwater.

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.

Nitrate (NO

These data contains experimental and simulated results related to our latest work—Fang, Y., Zheng, T., Zheng, X., Yang, H., Wang, H., Walther, M. Influence of tide–induced unstable flow on seawater intrusion and submarine groundwater discharge, under review.

The subsurface barrier is one of various engineering measures used to prevent seawater intrusion in coastal regions which has been widely applied. However, its two common types, the cut-off wall and the subsurface dam, are both found to cause nitrate (NO3-) accumulation in landward aquifers. In this study, numerical simulations were conducted to investigate the mechanism of NO3- accumulation caused by the two types of subsurface barriers, as well as the influence of several key parameters, i.e. the infiltration NO3- concentration, the inflow DOC concentration, the barrier height and the barrier location on the performance of the subsurface barriers. The results showed that the cut-off wall generally requires a large height to ensure a satisfactory seawater prevention effectiveness, and it is more likely to cause NO3- accumulation compared to a subsurface dam. On the other hand, despite the subsurface dam may not result in the significant increase of NO3- concentration in groundwater upstream, it cannot be applied to the areas where SI has occurred due to the residual seawater problem. Moreover, the construction of a cut-off wall results in a stagnation zone appeared at the upper corner of the barrier, where accumulated NO3- significantly. With the increase of the barrier height, the stagnation zone expanded, leading to further increase of mean NO3- concentration in the landward aquifer. Since the construction of a subsurface dam will not generate such a zone, the subsurface dam generally has little impact on NO3- accumulation.

Subsurface dams are widely used to prevent saltwater intrusion. Once a subsurface dam is established, a large amount of residual saltwater (RSW) may trap behind the dam and negatively affect the groundwater exploitation over the long term. Based on monitoring hydrological information and field-scale numerical models, we proposed three kinds of RSW clean-up strategy coupling the applications of a pit lake and wells in a coastal aquifer. We firstly investigated the spontaneous natural removal of RSW in an actual coastal aquifer affected by a pit lake, assessed the impact of pit lake drainage on the removal of RSW, and designed a feasible strategy for reducing residual saltwater utilizing both pit lake and wells. Our results indicated that RSW remediation is a slow process under natural conditions. The area of the RSW decreased from 15.17 to 11.62 km2 after 20 years, and the Cl− concentration of RSW around the pit lake increased to 1750 mg/L. The RSW cleaning efficiency can be significantly enhanced by the pit lake drainage process. With the pit lake drainage rate of 1.6E5 m3/d, the area of the RSW decreased by 10.14 km2 in five years being 17-times larger than that of the natural restoration scheme (0.58 km2). With the combined well-lake drainage system, the total residual saltwater area decreased by more than 90% within five years, and the Cl− concentration of RSW almost reached WHO standards for drinking-water quality (250 mg/L). RSW upstream the subsurface dam was thoroughly cleaned up adopting the well–lake drainage strategy. Therefore, a well–lake drainage strategy can be strongly recommended for removing the RSW in coastal aquifers.

A method which combines electronegativity different, CALPHAD (CALculation of PHAse Diagrams) and machine learning has been proposed to efficiently screen the high yield strength regions in Co-Cr-Fe-Ni-Mo multi-component phase diagram. Firstly, the single-phase region at a certain annealing temperature is obtained by combining CALPHAD method and machine learning, to avoid the formation of brittle phases. Then high yield strength points in the single-phase region are selected by electronegativity different. The validation experiments indicate this method is effective to predict high yield strength points in the whole compositional space. The method is instructive to property-oriented compositional design for high entropy alloys.

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.

Lignocellulosic agricultural residues were utilized as denitrification carbon substrates to improve the purification capacity of unsaturated soil and alleviate nitrate pollution of groundwater. In this study, corncob and wheat straw were treated by calcium hydroxide to improve biodegradability and enhance denitrification potential. Calcium hydroxide treatment decreased the contents of lignin (i.e., from 16.7 wt% to 15.2 wt% in corncob and from 21.9 wt% to 20.6 wt% in wheat straw), increased potential biodegradable carbon by 4.4-5.3 times, reached complete nitrate removal 7-14 days earlier and decreased N2O/(N2O+N2) ratios by 85-99%. The results provide an insight into the application of alkali-treated agricultural residues as denitrification carbon sources to alleviate nitrate transport to groundwater and reduce potential greenhouse effect.

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.

Physical barrier is a coastal engineering widely used to prevent seawater intrusion. However, previous studies have not evaluated the combined influence of type and structure of the physical barrier on nitrate accumulation in upstream aquifers. Furthermore, the mechanisms of nitrate accumulation caused by the physical barriers are still unclear. In this study, numerical simulations were conducted to investigate the mechanisms and influence of the physical barrier on seawater intrusion and NO 3 − accumulation. The results show that constructing physical barrier can result in the reduction of nitrate discharge to the sea and accumulation of nitrate in upstream aquifers. The accumulation degree is significant if the barrier height is large; the barrier location is near the sea; the nitrate infiltration rate is large; the infiltration nitrate concentration is large; the inflow is weak, and the inflow DOC concentration is low. A cut-off wall is more likely to cause nitrate accumulation than a subsurface dam. It can result in mean nitrate concentration in groundwater upstream increasing by more than 30 %. Because a nitrate accumulation zone is formed behind the cut-off wall where the flow is slow and dissolved oxygen carbon is difficult to be replenished so that the denitrification is weak. Despite the subsurface dam may not result in a significant increase in nitrate concentration, it cannot be applied to the areas where SI has occurred due to the residual seawater problem. The nitrate accumulation in the upstream aquifer is a long-term process that lasts for more than three years to reach a pseudo-steady state. Seasonal variations of inflow and infiltration lead to fluctuation of mean nitrate concentration, thereby the nitrate accumulation rate increased after April and weakened between July and December.

For nitrogen (N) migration and transformation from unsaturated soil to groundwater, the N stable isotope (δ15N) was modified due to the isotope fractionation effect. To quantitatively evaluate the N cycle in groundwater systems, the determination of isotope fractionation is decisive. In this research, for the first time, incubation experiments were conducted to quantitatively investigate the N isotope enrichment factor (ϵp/s) associated with ammonification in unsaturated soil. Under weak isotopic fractionation, the Rayleigh function cannot be directly applied during ammonification. Thus, we proposed a different method calculating the ϵp/s values during ammonification, which were −0.03‰ for 15 °C and −2.34‰ for 30 °C. Moreover, for the first time, experimental equipment is presented to explore the isotopic fractionation effects under the co-occurrence of nitrification and volatilization. The results indicated that the isotope effect of volatilization during nitrification can be ignored in this study, and the ϵp/s values during nitrification were −10.59 and −6.81‰ at 15 and 30 °C, respectively. This work provides a novel arrangement determining the crucial parameters for identifying nitrate pollution sources in groundwater systems.

Numerical modeling of CO2 injection in the deep saline aquifer is computationally expensive due to the large spatial and temporal scales. To address the computational challenge, reduced-dimensional models (e.g., vertical equilibrium (VE) and dynamic reconstruction (DR) models) based on vertical integration of the full-dimensional governing equations have been developed. VE models assume rapid segregation of the injected and the resident fluids due to strong buoyancy. Conversely, DR models employ a multiscale framework that relaxes the VE assumption and captures the vertical dynamics of CO2 and brine by solving the vertical two-phase flow dynamics as one-dimensional fine-scale problems. Although DR models relax the VE assumption while maintaining much of the computational efficiency of VE models, they are thus far limited to homogeneous and layered heterogeneous formations. We present a novel hybrid framework that couples a multilayer dynamic reconstruction model and a full-dimensional model. The new hybrid framework allows simulation of CO2 injection in geological formations with local heterogeneities. It employs a full-dimensional model in local heterogeneous regions (where the full-dimensional model should be used for accuracy), while applying the dynamic reconstruction model in the rest of the domain. Numerical simulations of CO2 injection in three heterogeneous reservoirs show that the hybrid model maintains the accuracy of the conventional full-dimensional models with significantly reduced computational cost.

Uncertainties of aging after the injection of microscale zero valent iron (mZVI) into subsurface hinder the widespread application of this technology into in-situ groundwater remediation. As TCE removal by mZVI is usually accompanied by mZVI anaerobic reaction with water, this study investigated the hydrogen evolution kinetics, TCE removal performance, and electron efficiency (EE) of mZVI particles under single and combined effects of organic geochemical constituents (humic acid (HA)), inorganic constituents (hardness (Ca2+) and alkalinity (HCO3−)) in groundwater. The results showed that both H2 generation and TCE removal were enhanced in the presence of HA (5–20 mg L−1 as DOC), and significant delay of aging precipitates formation was supported by SEM-EDS, XRD and XPS depth-profiling characterization. With increasing Ca2+-HCO3− concentrations (0.4–2 mM), the mZVI corrosion were inhibited for the formation of a thin compact passive film on mZVI surface. Furthermore, in co-presence of organic and inorganic constituents, the negatively-impacted iron corrosion kinetics suggested that promotional effects of organic constituents were dwarfed by the inhibitory effects of inorganic constituents. In addition, the EE values in different systems ranged among 1.31–7.40%, and decreased significantly with time. Hence, these findings provide insight into aging mechanism and performance prediction of mZVI under different groundwater geochemical conditions.

A microscale zero-valent iron (mZVI)-based in situ reactive zone is a promising technology for contaminated groundwater remediation. Estimation of mZVI aging behavior after its injection into the subsurface is essential for efficiency and longevity assessments. In this study, batch tests were conducted to investigate the effect of initial pH on mZVI aging dynamics, as well as the formation and evolution of aging products over 112 days. Results indicated that mZVI aging accelerated with decreasing initial pH. Corrosion rates of mZVI particles under pH 6.0 and 7.5 were approximately two orders of magnitude higher than those observed at pH 9.0. The morphological, structural, and compositional evolution of mZVI particles in three systems (pH = 6.0, 7.5, and 9.0) were investigated using scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. In acidic and neutral solutions, a thick passivation layer with loosely and unevenly distributed aging precipitates was observed, and Fe3O4 was the final aging precipitate. Nevertheless, in alkaline solutions, minute aging precipitates were detected on the mZVI surface at 112 day. Characterization results suggested that mZVI was oxidized via the Fe0–Fe(OH)2–Fe3O4 route. These findings shed new light on mZVI aging mechanisms, particularly its physicochemical characteristics and the structural evolution of mZVI in field-scale groundwater remediation applications.