Nitrate and nitrite reduction by adsorbed Fe(II) generated from ligand-promoted dissolution of biogenic iron minerals in groundwater.

Chemical denitrification by redox-active Fe(II) species is pivotal in the coupled iron and nitrogen cycles. The reductive dissolution of ferric minerals by ligand can generate Fe(II)-ligand complexes, but their reducing capability for electrophilic pollutants like nitrate and nitrite remains uncertain. Here, biogenic secondary iron minerals (SIM) after dissimilatory iron reduction were reductively dissolved by oxalate and the siderophore desferrioxamine B, and subsequently the partially-dissolved SIM (SIMD) effectively removed NO2- from groundwater via reduction, while exhibiting much lower reactivity towards NO3-. The dissolution and removal processes were well-fitted with the Kabai model and the pseudo-second-order adsorption model, respectively. The equilibrium NO2- removal capacity (qe) of SIMD reached 0.146-0.223 mmol/g, accompanied with the rate constants as 0.433-0.810 g/(mmol·h). The emission of N2O and NO verified the occurrence of chemical denitrification during NO2- removal by SIMD. From the perspective of Fe(II) reactivity, SIMD exhibited higher densities of surface Fe(II) and more negative Eh values than SIM, and these two indicators showed linear correlations with the removal rates. Combined with microscopic, electrochemical and spectral analysis, our results indicated the redox reaction of adsorbed Fe(II)-complexes with NO2- on SIMD surface. The concurrent substance biochar was also considered, as it indirectly influenced dissolution and pollutant removal by shifting the iron mineral phase in SIM from magnetite to goethite. These findings highlight the significant role of reductive dissolution of iron mineral in N transformation, expand the electron pool available to support chemical denitrification, and have implications for Fe and N cycling coupling with pollutant reduction.

Groundwater denitrification enhanced by a hydrogel immobilized iron/solid carbon source: impact on denitrification and substrate release performance.

Encapsulating a solid carbon source and zero-valent iron (ZVI) within a hydrogel can prevent direct contact with groundwater, thereby extending the lifespan of their released active substrates. It is currently unclear whether the solid carbon source and ZVI will mutually influence each other's active substrate release process and the corresponding denitrification patterns, necessitating further investigation. In this study a hydrogel encapsulating different weight ratios of micron-sized zero-valent iron (mZVI, as ZVI) and polyhydroxybutyrate (PHB, as a solid carbon source) was synthesized. The aim was to investigate the influence of PHB on the release of dissolved iron from mZVI and denitrification mechanism. Results indicated that PHB was consumed at a higher rate than mZVI, and more mZVI active sites could be exposed after PHB consumption. Meanwhile, PHB increased the porosity of the hydrogel, allowing more active sites of mZVI to be exposed and thus releasing more dissolved iron. Furthermore, PHB enhanced the rate of microbial corrosion of mZVI, which further increased the release of dissolved iron. Higher PHB content in the hydrogel reduced the oxidation of the released dissolved iron, resulting in a microbial community dominated by heterotrophic microorganisms. Conversely, lower PHB content led to significant Fe(II) oxidation and a considerable relative abundance of mixotrophic microorganisms in the microbial community. Microorganisms with iron reduction potential were also detected. This study provides theoretical support for the precise control of mixed nutrient denitrification based on hydrogel immobilization and lays the foundation for its further practical application in groundwater.

Coupling of polyhydroxybutyrate and zero-valent iron for enhanced treatment of nitrate pollution within the Permeable Reactive Barrier and its downgradient aquifer.

Permeable Reactive Barriers (PRBs) have been utilized for mitigating nitrate pollution in groundwater systems through the use of solid carbon and iron fillers that release diverse nutrients to enhance denitrification efficiency. We conduct laboratory column tests to evaluate the effectiveness of PRBs in remediating nitrate pollution both within the PRB and in the downgradient aquifer. We use an iron-carbon hydrogel (ICH) as PRB filler, which has different weight ratios of polyhydroxybutyrate (PHB) and microscale zero-valent iron (mZVI). Results reveal that denitrification in the downgradient aquifer accounts for at least 19.5 % to 32.5 % of the total nitrate removal. In the ICH, a higher ratio of PHB to mZVI leads to higher contribution of the downgradient aquifer to nitrate removal, while a lower ratio results in smaller contribution. Microbial community analysis further reveals that heterotrophic and mixotrophic bacteria dominate in the downgradient aquifer of the PRB, and their relative abundance increases with a higher ratio of PHB to mZVI in the ICH. Within the PRB, autotrophic and iron-reducing bacteria are more prevalent, and their abundance increases as the ratio of PHB to mZVI in the ICH decreases. These findings emphasize the downgradient aquifer's substantial role in nitrate removal, particularly driven by dissolved organic carbon provided by PHB. This research holds significant implications for nutrient waste management, including the prevention of secondary pollution, and the development of cost-effective PRBs.

Hydrodynamic behavior of freshwater-saltwater mixing zone in the context of subsurface physical barriers.

The seawater intrusion (SWI) process lasts for decades in real world, thus the research on dynamic process of SWI is essential. The freshwater-saltwater mixing zone plays a crucial role in governing the groundwater movement and the solute transport in coastal aquifers. To date, there has been a lack of research on the hydrodynamic behavior of the mixing zone in the presence of subsurface physical barriers. In this work, we employed laboratory experiments and numerical simulations to investigate the dynamics of the mixing zone, comparing scenarios with and without subsurface physical barriers. The findings indicate that the construction of a subsurface physical barrier will not immediately slow down the seawater intrusion velocity and change the salinity distribution of mixing zone. The block effect of subsurface physical barriers with different heights or bottom opening sizes became apparent only when the wedge toe approached the physical barriers. The widening effect of increasing longitudinal dispersivity on the mixing zone width was more pronounced during the dynamic process compared to the steady state. Furthermore, the widening effect of increasing longitudinal dispersivity on the mixing zone was more significant compared to transverse dispersivity in both the SWI and subsurface dam scenarios throughout the intrusion process. However, in the cutoff wall scenarios, the widening effect of increasing transverse dispersivity became more obvious during the later intrusion period. Our conclusions provide a reference for the groundwater management in coastal aquifers. According to the current seawater intrusion situation, the local water bureau can predict the seawater intrusion velocity and the temporal changes of mixing zone after the construction of physical barriers.

Combined effects of aquifer heterogeneity and subsurface dam on nitrate contamination in coastal aquifers.

Subsurface dams are effective for seawater intrusion mitigation, yet they can cause upstream nitrate accumulation. This research examines the interplay between subsurface dam construction and aquifer layering on nitrate pollution in coastal settings, employing numerical models to simulate density-driven flow and reactive transport. The study reveals that while subsurface dams are adept at curbing seawater intrusion, they inadvertently broaden the nitrate accumulation zone, especially when a low-K layer is present. Heterogeneous aquifers see more pronounced nitrate accumulation from subsurface dams. This effect is pronounced as it influences dissolved organic carbon dynamics, with a notable retreat inland correlating with the expansion of the nitrate pollution plume. A critical finding is that controlling seawater intrusion via dam height adjustment within the Effective Damming Region effectively reduces nitrate levels and bolsters freshwater output. However, exceeding the critical threshold-where the dam surpasses the low-K layer's bottom-results in a substantial shift in nitrate concentration, underscoring the need for precise dam height calibration to avoid aggravating nitrate pollution. This study's innovative contribution lies in its quantification of the nuanced effects of subsurface dams in stratified aquifers, providing an empirical basis for dam design that considers the layered complexities of coastal aquifers. The insights offer a valuable framework for managing nitrate contamination, thus informing sustainable coastal groundwater management and protection strategies.

Rhamnolipid-induced alleviation of bioclogging in Managed Aquifer Recharge (MAR): Interactions with bacteria and porous media.

The prevention and treatment of bioclogging is of great significance to the application of Managed Aquifer Recharge (MAR). This study investigated the alleviating effect of biosurfactant rhamnolipid (RL) on bioclogging by laboratory-scale percolation experiments. The results show that the addition of RL greatly reduced bioclogging. Compared with the group without RL, the relative hydraulic conductivity (K') of the 100 mg/L RL group increased 5 times at the end of the experiment (23 h), while the bacterial cell amount and extracellular polymeric substances (EPS) content on the sand column surface (0-2 cm) decreased by 60.8% and 85.7%, respectively. In addition, the richness and diversity of the microbial communities within the clogging matter decreased after the addition of RL. A variety of bacterial phyla were found, among which Proteobacteria were predominant in all groups. At the genus level, RL reduced the relative abundance of Acinetobacter, Bacillus, Klebsiella, and Pseudomonas. These microbes are known as strong adhesion, large size, and easy to form biofilms, therefore playing a critical role during MAR bioclogging. Moreover, RL changed the surface properties of bacteria and porous media, which results in the increase of electrostatic repulsion and decrease of hydrophobic interaction between them. Therefore, RL mediated the bacteria-porous media interaction to reduce biomass in porous media, thereby alleviating bioclogging. This study implies that RL's addition is an environmentally friendly and effective method to alleviate the bioclogging in MAR.

Distinct chromium removal mechanisms by iron-modified biochar under varying pH: Role of iron and chromium speciation.

Iron-modified biochar (Fe-biochar) has been widely developed to attenuate Cr(VI) pollution in both acid and alkaline environments. However, there are few comprehensive studies on how the iron speciation in Fe-biochar and chromium speciation in solution influencing the removal of Cr(VI) and Cr(III) under varying pH. Here, multiple Fe-biochar containing Fe3O4 or Fe(0) were prepared and applied to remove aqueous Cr(VI). Kinetics and isotherms suggested that all Fe-biochar could efficiently remove Cr(VI) and Cr(III) via adsorption-reduction-adsorption. The Fe3O4-biochar immobilized Cr(III) by forming FeCr2O4, while amorphous Fe-Cr coprecipitate and Cr(OH)3 was formed when using Fe(0)-biochar. Density functional theory (DFT) analysis further indicated that pH increase caused more negative adsorption energies between Fe(0)-biochar and the pH-dependent Cr(VI)/Cr(III) species. Consequently, the adsorption and immobilization of Cr(VI) and Cr(III) species by Fe(0)-biochar was more favored at higher pH. In comparison, Fe3O4-biochar exhibited weaker adsorption abilities for Cr(VI) and Cr(III), which were in consistent with their less negative adsorption energies. Nonetheless, Fe(0)-biochar merely reduced ∼70% of adsorbed Cr(VI), while ∼90% of adsorbed Cr(VI) was reduced by Fe3O4-biochar. These results unveiled the importance of iron and chromium speciation for chromium removal under varying pH, and might guide the application-oriented design of multifunctional Fe-biochar for broad environmental remediation.

Dynamic influence of land reclamation on the nitrate contamination and saltwater redistribution.

Previous research concerning the effect of land reclamation on seawater intrusion mostly focused on the modification of the saltwater wedge and the dynamics of freshwater-saltwater interface after land reclamation, utilizing both analytical and numerical models. So far, the impact of land reclamation on the recharging and accumulation of land-based pollutants such as nitrate has been disregarded. In this work, we are the first to examine the impact of land reclamation on the discharge of nitrate together with the movement of saltwater. The influence of reclamation area and filled soil permeability on nitrate pollution and saltwater redistribution is revealed using a series of field-scale simulations based on numerical models including density flow combined with reactive transport. It was discovered that land reclamation might, on the one hand, result in a substantial redistribution based on the initial saltwater-freshwater interface and, on the other hand, significantly modify the nitrate discharge. This in total would drastically alter the distribution of nitrate in the subsurface. The reclamation area and the permeability of the reclamation material are the two elements that determine the amount of variance. For the cases with hydraulic conductivities increasing from 5 to 50 m/d, the salt mass reduction rate showed a trend of first increased (84.78 %-95.58 %) and then slowly decreased (95.58 %-74.01 %). Meanwhile, the nitrate reduction rate decreased from 80.08 % to 12.93 %, when hydraulic conductivities increased from 5 to 50 m/d. It was also found that coastal nitrate accumulation was always intensified with the enlargement of the reclamation area. Finally, we are able to assist engineers in optimizing their land reclamation strategies by taking into account both the degree of saltwater intrusion and nitrate enrichment.

Bidirectional potential effects of DON transformation in vadose zones on groundwater nitrate contamination: Different contributions to nitrification and denitrification.

The main cause of groundwater nitrate contamination is the continual downward migration of dissolved nitrogen (N) in vadose zone with leachate. In recent years it has been found that dissolved organic N (DON) rise to forefront due to its great migration capacity and environmental effects. However, it remains unknown how the transformation behaviors of DONs with different properties in vadose zone profile may impact N forms distribution and groundwater nitrate contamination. To address the issue, we conducted a series of 60-day microcosm incubation experiments to investigate the effects of various DONs transformation behaviors on the distribution of N forms, microbial communities, and functional genes. The results revealed that urea and amino acids mineralized immediately after substrates addition. By contrast, amino sugars and proteins caused less dissolved N throughout entire incubation period. The transformation behaviors could substantially alter the microbial communities. Moreover, we discovered that amino sugars remarkably increased the absolute abundances of denitrification function genes. These results delineated that DONs with unique characteristics (such as amino sugar) promoted different N geochemical processes in distinct ways: different contributions to nitrification and denitrification. This can provide new insights for nitrate non-point source pollution control in groundwater.

Repulsion driven by groundwater level difference around cutoff walls on seawater intrusion in unconfined aquifers.

Cutoff walls have been widely used to prevent seawater intrusion (SWI) in coastal regions. Previous studies generally concluded that the ability of cutoff walls to prevent seawater intrusion depends on the higher flow velocity at the wall opening, which we have shown is not the most critical mechanism. In this work, we implemented numerical simulations to explore the driving force of cutoff walls on the repulsion of SWI in both homogeneous and stratified unconfined aquifers. The results delineated that the inland groundwater level was raised by cutoff walls, which generated a significant groundwater level difference beside two sides of the wall and thus provided a large hydraulic gradient to repel SWI. We further concluded that by increasing inland freshwater influx, the construction of cutoff wall could result in a high inland freshwater hydraulic head and fast freshwater velocity. The high inland freshwater hydraulic head posed a large hydraulic pressure to push the saltwater wedge seawards. Meanwhile, the fast freshwater flow could rapidly carry the salt from the mixing zone to the ocean and induce a narrow mixing zone. This conclusion explained the reason that the cutoff wall can improve the efficiency of SWI prevention through recharging freshwater upstream. With a defined freshwater influx, the mixing zone width and saltwater pollution area mitigated with the increase of the ratio between high and low hydraulic conductivity values (KH/KL) of the two layers. This was because the increase of KH/KL caused a higher freshwater hydraulic head, a faster freshwater velocity in the high-permeability layer, and the prominent change of flow direction at the interface between the two layers. According to the above findings, we deduced that any way to increase the inland hydraulic head upstream of the wall would improve the efficiency of cutoff walls, such as the freshwater recharge, the air injection, and the subsurface dam.

Dynamics of upstream saltwater intrusion driven by tidal river in coastal aquifers.

For the coastal aquifers, recent research have shown that the tidal has a significant effect on saltwater intrusion in the near-shore aquifer. However, it is currently unclear how the tidal river contributes to the groundwater flow and salinity distribution in the upstream aquifer of the estuary. This study examined the effects of a tidal river on the dynamic characteristics of groundwater flow and salt transport in a tidal river-coastal aquifer system using field monitoring data and numerical simulations. It was found that changes in tidal-river level led to the reversal of groundwater flow. For a tidal cycle, the maximum area of seawater intrusion is about 41.16 km2 at the end of the high tide stage. Then the area gradually decreased to 39.02 km2 at the end of the low tide stage. More than 2 km2 area variation can be observed in a tidal cycle. Compared to the low tide stage, the area of SWI increased by 5 % at high tide stage. The SWI region was also spreading landward from the tidal river. In addition, we quantified the water exchange and salt flux between the tidal river and aquifer. When the tidal fell below the level of the riverbed, the water exchange rate was stabilized at about -1.6 m/h. The negative value indicated that the river was recharged by the groundwater. With the increasing of tidal water level, the water exchange rate gradually changes from negative to positive and reached the maximum value of 3.2 m/h at the beginning of the falling tide stage. The presence of a physical river dam can amplify the difference in water level between high and low tides, thereby enhancing the influence of a tidal river on water exchange and salt flux. The findings lay the foundation for gaining a comprehensive understanding of the tidal river on groundwater flow and salt transport in upstream aquifers.

Detoxification mechanisms of biochar on plants in chromium contaminated soil: Chromium chemical forms and subcellular distribution.

The complete pathway of chromium (Cr) transfer from soil to plant tissues and subcellular components under biochar amendment remains to be quantified, as well as the involved diverse detoxification processes in roots and stems respectively. Pot experiments and quantitative analysis were conducted to investigate Cr fixation in soil amended with Enteromorpha prolifera-derived biochar and subsequent phytoprocesses (Cr uptake, transfer, and phytotoxicity) in cultivated Secale cereale L. (rye). The results indicated that adding 5-30 g kg-1 of biochar increased the residual form of Cr (B4) in soil by 8-21% and decreased the bioavailable form of Cr (B1) by 9-29%. For Cr transferred to rye, Cr in the rye was mainly present in the low-toxicity bound state, with the acetic acid-extracted Cr (F4) (45-54%) in roots and the NaCl-extracted Cr (F3) (37-47%) in stems. The subcellular distribution of Cr in both roots and stems was predominantly in the cell wall and residues (T1), followed by the cytoplasm (T4). Partial least squares path model (PLS-PM) was used for quantifying the effect of biochar on the form changes and subcellular detoxification of Cr from soil to roots and stems to sub-cells. In soils, biochar reduced the bioavailability of Cr and decreased the transfer of Cr to rye. In plant roots, Cr was distributed mainly as low-toxicity phosphate complexes in cell walls and vacuoles in sub-cells (with the largest path coefficients of 0.90 and -0.91, respectively). In the stems, Cr was distributed mainly as proteins integrated into the cell walls and vacuoles. This was due to the difference in subcellular compartmentalization of detoxification in the roots and stems. These PLS-PM results provide new insights into the entire process of pollutant detoxification in complex environments.

Causal interpretation for groundwater exploitation strategy in a coastal aquifer.

Machine learning models (ML), as a collection of nonparametric or semiparametric estimation methods, can successfully encode the distribution of the problems into its trainable parameters based on observation data. However, the distributions of hydrological variables may change suddenly under complex environmental conditions, leading to biased estimates when using ML models. This work is the first attempt to solve this issue using structural causal models (SCM). Specifically, two SCM were constructed based on hydrological conditions and monitoring data. Then the Propensity Score estimator and the Double Machine Learning estimator were employed to estimate the causal effects of four treatments on the mean Cl- concentration (MCL) in the coastal aquifer. The results showed that pumping groundwater from area A1 or increasing the river level directly leads to a decrease in MCL, while pumping area A3 directly leads to an increase in MCL. Moreover, MCL can be effectively controlled by cooperative-treatment strategies. Finally, two practical exploitation strategies are derived. In the planting month, it should increase groundwater pumping from area A1, limit groundwater pumping from A2, and prohibit groundwater pumping from A3. For the normal month, it is proposed to increase the height of the rubber dam to raise the river level and reduce groundwater pumping from A1 and A2.

Distinguishing and quantifying the fate of nitrate in irrigation water and nitrate produced by ammonium nitrification.

When the sources of nitrogen include not only ammonium (NH4+) fertilizer (ANF) but also nitrate (NO3-) from groundwater and rainfall (NRI), if the proportions of various types of NO3- are still based on the amount of ANF, the corresponding calculation method may be complicated. This paper established a water flow-nitrogen migration transformation model for the unsaturated zone in grain-planting and vegetable-planting areas, and studied the migration and transformation of NH4+ and NO3- in the unsaturated zone when ANF and NRI coexist. This paper proposed for the first time the proportional coefficient method (PCM) and hypothetical assignment method (HAM) to distinguish and quantify the fate proportions of NO3- from NO3- produced by NH4+ nitrification (NNR) and NRI. The results showed that the PCM was more practical than the HAM in quantifying the fate of NO3- from different sources. If only the root absorption ratio was used to evaluate the degree of nutrient supply to crops, the ratios of root absorption were as high as 40% (44.75-50.85%). NRI provided more nutrients in grain-growing areas than those in vegetable-growing areas. If the sum of the proportion of other fates was regarded as the degree of groundwater NO3- mitigation through irrigation in the unsaturated zone, except for the ratio of NO3- leaching to groundwater, the proportion of NO3- pollution mitigation was as high as 57.89% (57.89-92.99%), and the mitigation ability of groundwater NO3- pollution in grain-growing areas was higher than that in vegetable-growing areas.

A critical review of the central role of microbial regulation in the nitrogen biogeochemical process: New insights for controlling groundwater nitrogen contamination.

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.

Influence of pH on bioclogging in porous media during managed aquifer recharge (MAR): Effectiveness and mechanism.

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 for >80% 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.

Influence of velocity distribution conditions of coastal aquifer on nitrate contamination and seawater intrusion in the presence of subsurface physical barriers.

The velocity distribution is an important factor that affects seawater intrusion (SI) and nitrate (NO3-) pollution. However, there are few studies on the impact of subsurface physical barriers (SPBs) on the velocity distribution of the whole aquifer and the impact of velocity distribution on SI and NO3- pollution. Especially, the quantitative method of velocity distribution has not been studied. By the methods of laboratory experiments and numerical simulations, effects of the NO3- concentrations of the pollution source, hydraulic gradients (HGs), the location of the SPB and relative heights of SPBs (HP') on the SI, NO3- pollution levels and velocity in the presence of SI and SPBs were investigated. The velocity distribution was first quantified to better describe the relationships between the velocity and degrees of SI and NO3- pollution. The results showed that the HG and HP' were the main factors that affected the velocity, NO3- pollution and SI. The higher the HG, the smaller the HP', and the decreased SI inferred a more serious NO3- pollution. The influence of SPBs on NO3- pollution and SI was mainly affected by the changes in the velocity distribution in the aquifer. With increasing HGs, for the region with flow rate less than 0.5 m/d (A0.5), the smaller its distribution area is, the smaller the relative area of SI (TLs') is. With an increase in the HG or decrease in the HP', the relative area of NO3- pollution (Ns') is proportional to the distribution area where the flow velocity is greater than 1 m/d (A1). When the flow velocity distribution condition was A'1 (the relative area of A1) > A'0.5-1 (A'0.5-1 is the ratio of the area where the flow velocities are greater than 0.5 m/d and less than 1 m/d to the total area of the aquifer) > A'0.5 (the relative area of A0.5), NO3- pollution was serious; when the flow velocity distribution condition was A'0.5 > A'0.5-1 > A'1, the levels of NO3- pollution were the lowest.

Nitrate transport behavior behind subsurface dams under varying hydrological conditions.

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.

Quantitative evaluation of the synergistic effect of biochar and plants on immobilization of Pb.

Biochar and plant cooperation in remediation of heavy metal contaminated soil is effective and important, but there still have knowledge gaps of synergistic effect between the two and the synergistic pathway has not been clarified. We prepared the Enteromorpha prolifera biochar at 400 °C and 600 °C (denoted as BC400 and BC600). The Pb fractions changes in soil and Pb toxicity in Brassica juncea were investigated by adding 30 g kg-1 biochar to soil containing 1200 mg kg-1 Pb in a pot experiment. There was a significant synergistic effect between biochar and plants on Pb immobilization in soil, according to the "E > 0" of Pb fractions in the interaction equation. Pb immobilization rates of biochar-plant treatments (BJBC4 and BJBC6) were 12.47% and 11.38% higher than biochar treatment (BC4, BC6), and 17.66% and 16.28% plant treatment (BJ). BJBC4 had a better immobilization effect than BJBC6. Biochar alleviated the phytotoxicity of Pb by increasing the antioxidant enzymes activities of plants. These results indicated two synergistic pathways: (1) The high pH and oxygen-containing functional groups of biochar could immobilize Pb through ion exchange, precipitation, or complexation. (2) Biochar enhanced the activity of the antioxidant enzyme system in plants thus improving the Pb tolerance of plants. Statistical analysis methods such as the partial least squares path modeling (PLS-PM) also confirmed the pathways. In a word, clear synergistic effects and pathways could guide the application of biochar and plants in Pb-contaminated soil.

APCS-MLR model: A convenient and fast method for quantitative identification of nitrate pollution sources in groundwater.

Nitrate (NO3-) contamination in groundwater has diverse sources and complicated transformation processes. To effectively control NO3- pollution in groundwater systems, quantitative and accurate identification of NO3- sources is critical. In this work, we applied hydrochemical characteristics and isotope analysis to determine NO3- source apportionment. For the first time, the NO3- source contributions were calculated using hydrochemical indicators combined with multivariate statistical model (PCA-APCS-MLR). The results interpret that chemical fertilizers (58.11%) and natural sources (22.69%) were the primary NO3- sources in the vegetable cultivation area (VCA) which were rather close to the estimation by Bayesian isotope mixing model (SIAR). In particular, the contributions of chemical fertilizers in the VCA differed by only 3.79% between the two methods. Compared with previous approaches e.g. SIAR, the key advantage of the proposed PCA-APCS-MLR model is that it only requires the hydrochemical indicators which can be easily measured. A series of complicated experiments including measurement of isotope data of NO3- in groundwater, monitoring of in-situ pollution source information and calculation of isotopic enrichment factor can be simply avoided. The PCA-APCS-MLR model offers a much more convenient and faster method to determine the contribution rates of NO3- pollution sources in groundwater.