A Critical Review of Groundwater Table Fluctuation: Formation, Effects on Multifields, and Contaminant Behaviors in a Soil and Aquifer System

Sources and fate of nitrate in the unsaturated zone in an alluvial-lacustrine plain

Nitrate pollution in terrestrial and aquatic ecosystems in global agricultural areas poses an environmental concern. However, there is limited understanding of hydrogeological controls on the behavior of nitrogen compounds in unsaturated zones. Here, Self-Organizing Map and multiple isotopes approaches (δ15N-NO3-, δ18O-NO3-, and δ15N-NH4+) were used to investigate the sources, transport and transformation of N-species in the unsaturated zone in an alluvial-lacustrine plain, southeast China. The results revealed significant spatial heterogeneity in soil texture and physicochemical properties with vertically four soil geochemical and N-species zones (high NO₃⁻, high Fe(Ⅲ) and Mn, low ionic, and high NH₄⁺ contents), dominated by agricultural input, soil minerals and redox conditions. Nitrate in the unsaturated zone primarily originated from fertilizers and soil nitrogen. Excess nitrogen fertilizers infiltrated into the soil, where mineralization, nitrification, and dissimilatory nitrate reduction to ammonium (DNRA) acted as key mechanisms for nitrogen transformation. The change in the depositional environment from the plain to the lakeshore area led to nitrification gradual decrease and DNRA significant increase. Consequently, a conceptual model of reactive transport of N-species, influenced by hydrogeologic conditions and biogeochemical processes, was proposed. This study provides a new insight into the nitrate behaviors in unsaturated zone and contributes to groundwater nitrogen management strategies.

Development of a process-based modeling framework for harnessing hydrometeorological datasets to estimate groundwater recharge

A better understanding of groundwater recharge rates is needed for sustainably managing freshwater resources. The goal of this effort is to develop a calibration-free, simple, process-based modeling framework for estimating groundwater recharge rates at a yearly timescale. To accomplish this, we performed water balance calculations to estimate recharge as the difference between precipitation and the combined losses due to evapotranspiration and runoff. The SCS method was used to compute runoff at the daily timescale and at the HUC-12 spatial scale and was later mapped over the desired 800-m grid scale. The Penman-Monteith model was used to estimate the ET value for every grid at the daily scale. The daily values of runoff, ET, and precipitation were then averaged to estimate annual averages, and these values were then used to obtain groundwater recharge at the annual timescale. We compared our results with the USGS recharge product for the Tombigbee-Black-Warrior River Basin located in the Southeastern United States. Results show that at the annual timescale, about 55 % of the average annual rainfall that fell over the catchment is lost as evapotranspiration and 24 % as runoff, thus yielding about 21 % as groundwater recharge over the entire river basin. The study provides a simple water balance approach for computing recharge values using publicly available hydrometeorological data and demonstrates its application for a large river basin in Alabama, USA.

CO2 geological sequestration can remove emerging contaminants in groundwater: The important role of secondary mineral carbonates

Carbon dioxide (CO2) injection has been proposed as a strategy for carbon sequestration, while uncertainties persist regarding its effects on groundwater. Concerns have been raised that CO2 mineralization and sequestration could potentially lead to groundwater contamination. However, our study demonstrates its capability to mitigate pollution. The injection of CO2 facilitates the rapid dissolution of minerals, releasing Ca(II), Mg(II), and Fe(II) and forming secondary carbonate minerals, such as CaCO3, MgCO3, and FeCO3. The in-situ generated FeCO3 can activate oxygen to produce hydroxyl radicals (•OH) under oxic condition, thereby enhancing the degradation of emerging organic contaminants in groundwater, such as 2,4,6-tribromophenol, flurbiprofen, diclofenac, carbamazepine, phenol, and sulfamethoxazole. Mechanism studies suggest that this process is enhanced by the conversion of in-situ formed FeCO3 into a two-dimensional goethite nanosheet structure, which provides a larger specific surface area and enables more Fe(II) to be adsorbed on the mineral surface. The formation of Fe-O coordination bonds effectively reduces the loss of •OH at the interfacial reaction layer. The study further distinguishes and quantifies the contributions of different Fe(II) forms to •OH generation. The transformation pathways of the six contaminants and the toxicity of their intermediates are also analyzed. CaCO3 and MgCO3 do not exhibit the ability to degrade pollutants, but play a role in carbon mineralization. This work reveals that secondary minerals generated through the CO2 mineralization and sequestration process display simultaneous capabilities of contaminant degradation and carbon fixation. Such activities are pivotal not only for the environmental fate and transformation of emerging contaminants in groundwater but also for regulating the carbon cycle.

Stable Interpenetrated Zirconium-Based Metal-Organic Framework for the Fluorescence Detection of MnO4. Inorg Chem 2025;64:6648-6655. [PMID: 40128184 DOI: 10.1021/acs.inorgchem.5c00200] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

In this work, a novel stable zirconium-based metal-organic framework (Zr-MOF) with the formula [Zr6O4(OH)4(PVDC)6]4·66DMF (Zr-1, H2PVDC = (E,E)-2,5-dimethoxy-1,4-bis[2-(4-carboxylatestyryl)]benzene; DMF = N,N-dimethylformamide) was synthesized by introducing a linear phenylenevinylene-based carboxylate ligand to react with ZrCl4 under solvothermal conditions. According to single-crystal X-ray diffraction measurement, complex Zr-1 featured a 2-fold interpenetrated framework, in which the single coordination framework possessed a structure similar to that of the well-known Zr-MOF, UiO-66, constructed from [Zr6O4(OH)4]12+ clusters and carboxylate ligands PVDC2-. Due to the introduction of the phenylenevinylene-functionalized ligand, complex Zr-1 exhibited a unique fluorescence sensing performance toward permanganate (MnO4-) with different concentrations. At low concentrations, the fluorescence emission intensity of Zr-1 around 510 nm was enhanced significantly with an increase in the concentration of MnO4- in an aqueous suspension. However, while excess MnO4- was added into the suspension, the fluorescence emission intensity decreased significantly, and the single emission peak turned into five emission peaks upon the addition of MnO4-. Such a phenomenon has been scarcely reported in previous MOF-based fluorescence sensors. Moreover, complex Zr-1 showed high anti-interference capability for the detection of MnO4- both at low and high concentrations. This work may pave a new way for the development of MOF-based fluorescence sensing platforms.

Migration and transformation of Pb and Cd in the unsaturated zone induced by water table fluctuations

Heavy metal migration poses serious threats to soil and groundwater environments, especially under fluctuating water table conditions, where the transport and transformation behaviors of heavy metals are more complex and difficult to predict. However, there are still limited studies on the effects of groundwater level fluctuations on the migration and transformation behavior of Pb and Cd in an unsaturated zone. This study conducted column experiments to investigate the migration, transformation, and interaction of Pb and Cd in an unsaturated zone. The results showed that Pb and Cd generally migrate to the upper sand with increasing water levels owing to their higher activity under low pH conditions. After many water table fluctuations, Pb and Cd accumulated in the sand and the Cd content in the lower layer was reduced compared to that in the upper layer. This may be due to the coexistence with Pb, leading to the conversion of some of the Cd in the sand solids to unstable (Cd(OH)2), which is susceptible to migration. The mechanism of the migration and transformation of Pb and Cd may be based on the synergistic effect of single or multiple processes, such as surface adsorption, surface precipitation, homocrystalline substitution, and interlayer cation exchange. The results show that hydrodynamic conditions affect the migration and transformation of Pb and Cd by changing the soil-groundwater hydrochemical environment, and this finding not only deepens the understanding of the heavy metal migration mechanism, but also provides an important scientific basis and technical support for the remediation of contaminated sites in areas of frequent fluctuations of the groundwater table.

Nutrients and metal(loid)s in surface sediments of the Chishui River: A DGT-based assessment of the last natural tributary of the upper Yangtze River (China)

This study investigates the distribution and probabilistic ecotoxicological risk assessment of nutrients and metal(loid)s in the Chishui River, the last natural tributary of the upper Yangtze River, which plays a crucial role in maintaining regional biodiversity and water quality. Understanding the impact of contaminants in this ecologically significant river is essential for effective environmental management. Sediment samples were analyzed using diffusive gradients in thin films (DGT) to measure labile concentrations of nutrients and metal(loid)s, revealing significant spatial variability. Concentrations of PO4-P, NH4-N, NO3-N, and metal(loid)s such as Mn, Fe, Cu, and Zn varied notably across sampling sites. Risk quotient (RQ) analysis identified Mn as posing the highest ecological risk, followed by Cu and Fe. A combined probabilistic risk assessment using the SPI (Species Sensitivity Distribution-Probabilistic Risk Assessment-Inclusion-Exclusion Principle) model indicated a 32.46 % probability of toxic effects from nutrient and metal(loid) mixtures on aquatic organisms. This study underscores the effectiveness of DGT technology in assessing bioavailable contaminants and highlights the need for targeted risk management strategies to mitigate ecological impacts in the Chishui River.

Sources and Pathways of PFAS Occurrence in Water Sources: Relative Contribution of Land-Applied Biosolids in an Agricultural Dominated Watershed

This study evaluated PFAS occurrence in rural well water and surface water relative to land application of biosolids in a tile-drained agriculture-dominated watershed. Spatial data were used to identify potentially vulnerable rural wells based on their proximity to biosolid-permitted land and location with respect to groundwater flow. Water was collected from 103 private wells in Greater Tippecanoe County Indiana and 168 surface water locations within the Region of the Great Bend of the Wabash River watershed. Overall, results indicate that surface water (∑PFAS ≤ 169.4 ng/L) is more vulnerable to PFAS contamination than well water (∑PFAS ≤ 15.7 ng/L). Short-chain perfluoroalkyl acids made up 72% of the ∑PFAS in both water sources. Nonetheless, long-chain homologues were detected more frequently in surface water (94%) than well water (82%). Hierarchical cluster analysis identified biosolid-applied fields, WTTPs, and industrial discharges as PFAS sources in first-order streams with high ∑PFAS. Temporal trends revealed an inverse relationship between streamflow and concentrations in surface water sites impacted by point discharges and vice versa for diffuse sources, thereby providing complementary evidence of potential sources. The well water data set did not show a distinct spatial trend between ∑PFAS and distance from biosolid application or well characteristics.

Interface mechanism of Cd bioavailability by porewater prediction in paddy field system

Cadmium (Cd) contamination poses a significant threat to human health. Predicting the risk of Cd in rice grains is challenging due to the heterogeneity and complexity of bioavailable Cd in paddy soils. We proposed that porewater during the grain-filling period can effectively predict Cd bioavailability in rice (R2 > 0.5, p < 0.05). The prediction mechanism was elucidated through soil-porewater interface characterization analysis and DFT calculations. Key factors determining Cd bioavailability included Cd2+ and SO42- concentration, pH, and ORP of porewater, with pH showing the highest correlation. As porewater pH increased from 5 to 9, typical mineral surfaces gradually deprotonated and formed complexation bonds {SOCd+} instead of {SOH} or {SOH2+}. Additionally, the complexation energy between montmorillonite and Cd was 1.57-1.78 eV higher than that between goethite and Cd, while the protonation activation energy barrier on montmorillonite was 1.33-1.52 eV lower than on goethite. Therefore, {H+} concentration in porewater had the potential to quantify {SOCd+} content and binding capacity, aiding Cd bioavailability prediction. This study outlines the mechanism of predicting heavy metal health risks in rice grains through porewater and offers a potential regulation approach for agricultural product safety.

Hydrologic changes induced by groundwater abstraction lead to arsenic mobilization in shallow aquifers

Intensive groundwater abstraction leads to hydrologic changes of groundwater. Nevertheless, the effects of hydrologic change on groundwater arsenic (As) mobilization remain controversial. Here, we investigated fluctuations in water levels and their effects on As mobilization in the shallow aquifer of the Hetao Basin. Results showed that large groundwater level fluctuations and high horizontal hydraulic gradients occurred in irrigation seasons. In the groundwater near the wetland with higher surface water levels than groundwater levels, biological index values of dissolved organic matter (DOM) ranged from 0.54 to 0.72, and a positive correlation between δ18O values and dissolved organic carbon (DOC) was observed, indicating that groundwater DOM was mainly sourced from surface water. The degradation of allochthone labile DOM drove the reductive dissolution of As-bearing Fe(III) oxides to Fe(II). Both DOC and humification indices of DOM exhibited positive correlations with horizontal hydraulic gradients downstream of the study area, implying that the humified organic matter flushed from aquifer sediments contributed to groundwater DOM. The humified DOM controlled by hydraulic conditions participated in the redox reactions mainly by shuttling electrons to As-bearing Fe(III) oxides. These findings highlight distinct roles of hydrologic changes induced by groundwater abstraction in As mobilization.

Colloid mobilization and transport in response to freeze-thaw cycles: Insights into the heavy metal(loid)s migration at a smelting site

Smelting sites often exhibit significant heavy metal(loid)s (HMs) contamination in the soil and groundwater, which are inevitably subjected to environmental disturbances. However, there is limited information available regarding the migration behaviors of HMs in a disturbed scenario. Thus, this work explored the migration of HMs-bearing colloids in response to freeze-thaw treatments by laboratory simulation and pore-scale study. Ultrafiltration results of soil effluents revealed that 61.5 %, 47.6 %, 68.0 %, and 59.2 % of Zn, Cd, Pb, and As were present in colloidal phase, and co-transported during treatments. Nanoparticle tracking analysis (NTA) further confirmed that freeze-thaw cycles were conducive to the generation of colloidal particles and showed the heteroagglomeration among different particles. Pore-network model (PNM) was used to quantify the soil macropore characteristics (macropore diameter, macropore number, coordination number, and Euler value) after treatments. It is evident that freeze-thaw cycles induced the formation of larger macropores while simultaneously enhancing macropore connectivity, thereby establishing an optimal pathway for colloid migration. These findings underscored the importance of environmental disturbances as a trigger for the release and migration of HMs in the smelting site, offering valuable insights for controlling HMs pollution. ENVIRONMENTAL IMPLICATION: The contaminated site has been subjected to prolonged environmental disturbances, causing the exacerbation of pollutants leaching and frequent occurrences of unstable pollution situations. This work explored the migration of HMs-bearing colloids in response to freeze-thaw treatments by laboratory simulation and pore-scale study. The distinct effects of freeze-thaw treatment on colloidal particle number concentration and macropore characteristics may explain the generation and migration of colloid-associated HMs driven by environmental disturbances. This work revealed the underlying mechanisms driving the redistribution of HMs under freeze-thaw cycles, offering valuable insights for risk assessment of soil and groundwater associated with HMs migration.

The successive reduction of iodate to iodide driven by iron redox cycling

Ferrous iron (Fe(II)) produced by microbial Fe(III) reduction and reactive oxygen species (ROS) generated from aerobic Fe(II) oxidation can mediate iodate (IO3-) reduction and iodide (I-) oxidation, respectively. Nevertheless, how Fe redox cycling under redox fluctuating conditions drives transformation of iodine species remain unclear. In this study, Shewanella oneidensis MR-1 wildtype (WT) and its mutant △dmsEFAB, which lost the ability to enzymatically reduce IO3-, were chosen to conduct ferrihydrite/goethite/nontronite culture experiments under consecutive cycles of anoxic reduction of Fe(III) and re-oxidation of Fe(II) by O2 to reveal the role of Fe redox cycling in the transformation of iodine species. The results showed that both surface-adsorbed and mineral structural Fe(II) chemically reduced IO3-. Chemical IO3- reduction by biogenic Fe(II) was slower than enzymatic IO3- reduction by WT. Compared to △dmsEFAB cultures, WT cultures all showed higher Fe(II) concentrations under anoxic conditions but lower cumulative •OH under oxic conditions, which imply the chemical reaction between I- and ROS. I- oxidation by ROS, however, did not lead to a significant production of IO3- compared with I- formed under anoxic conditions. Consequently, Fe redox cycling successively reduced IO3- to I-, which highlights vital roles of Fe(III)-reducing bacteria in I- formation and mobilization in environments.

Distinguishing contributions of diverse sediment components to vanadium transport, immobilization and transformation in aquifer

Vanadium (V) occurs in environment naturally and anthropogenically, but little has been understood about its environmental behavior in groundwater aquifer with sediments. This study investigated the pentavalent V [V(V)] transport and transformation under the influence of different sediment components (minerals, organic matter, and microorganisms) through column experiments. All these components played pivotal roles in V immobilization. The synergistic effects of sediment components enhanced V retention compared to individual component. Mineral components, particularly those containing carbonates and metal oxides, predominantly influenced V(V) transport as indicated by XRD analysis. Organic matter, especially under low pH conditions, induced particle aggregation, thereby inhibiting the transport of V(V). The V K-edge X-ray absorption near-edge structure spectroscopy revealed the formation of tetravalent V[V(IV)] in treatments involving organic matter and microorganisms. Notably, organic matter exhibited the capability to directly reduce V(V). The introduction of microorganisms restricted V(V) transfer. V(V) reducing genera (e.g., Brevundimonas, Arenimonas, Xanthobacter) were detected, achieving V(V) reduction to insoluble V(IV). V(V) bioreduction was improved by minerals that promote microbial metabolism with enhanced electron transfer, or by organic matter that increases levels of intracellular nicotinamide adenine dinucleotide and extracellular polymeric substances. This study specifies the contributions of different sediment components to the transportation and transformation of V, deepening our understanding of V biogeochemistry in groundwater aquifer.

Recent Progress in Molecular Oxygen Activation by Iron-Based Materials: Prospects for Nano-Enabled In Situ Remediation of Organic-Contaminated Sites

In situ chemical oxidation (ISCO) is commonly used for the remediation of contaminated sites, and molecular oxygen (O2) after activation by aquifer constituents and artificial remediation agents has displayed potential for efficient and selective removal of soil and groundwater contaminants via ISCO. In particular, Fe-based materials are actively investigated for O2 activation due to their prominent catalytic performance, wide availability, and environmental compatibility. This review provides a timely overview on O2 activation by Fe-based materials (including zero-valent iron-based materials, iron sulfides, iron (oxyhydr)oxides, and Fe-containing clay minerals) for degradation of organic pollutants. The mechanisms of O2 activation are systematically summarized, including the electron transfer pathways, reactive oxygen species formation, and the transformation of the materials during O2 activation, highlighting the effects of the coordination state of Fe atoms on the capability of the materials to activate O2. In addition, the key factors influencing the O2 activation process are analyzed, particularly the effects of organic ligands. This review deepens our understanding of the mechanisms of O2 activation by Fe-based materials and provides further insights into the application of this process for in situ remediation of organic-contaminated sites.

Modeling of Microplastics Migration in Soil and Groundwater: Insights into Dispersion and Particle Property Effects

Migration of microplastics (MPs) in soil-groundwater systems plays a pivotal role in determining its concentration in aquifers and future threats to the terrestrial environment, including human health. However, existing models employing an advection-dispersion equation are insufficient to incorporate the holistic mechanism of MP migration. Therefore, to bridge the gap associated with MP migration in soil-groundwater systems, a dispersion-drag force coupled model incorporating a drag force on MPs along with dispersion is developed and validated through existing laboratory and field-scale experiments. The inclusion of the MP dispersion notably increased the global maximum particle velocity (vmaxp) of MPs, resulting in a higher concentration of MPs in the aquifer, which is also established by sensitivity analysis of MP dispersion. Additionally, increasing irrigation flux and irrigation areas significantly accelerates MP migration downward from soil to deep saturated aquifers. Intriguingly, vmaxp of MPs exhibited a nonlinear relationship with MPs' sizes smaller than 20 μm reaching the highest value (=1.64 × 10-5 m/s) at a particle size of 8 μm, while a decreasing trend was identified for particle sizes ranging from 20 to 100 μm because of the hindered effect by porous media and the weaker effect of the drag force. Moreover, distinct behaviors were observed among different plastic types, with poly(vinyl chloride), characterized by the highest density, displaying the lowest vmaxp and minimal flux entering groundwater. Furthermore, the presence of a heterogeneous structure with lower hydraulic conductivity facilitated MP dispersion and promoted their migration in saturated aquifers. The findings shed light on effective strategies to mitigate the impact of MPs in aquifers, contributing valuable insights to the broader scientific fraternity.

The heterogeneous distribution of heavy metal(loid)s at a smelting site and its potential implication on groundwater

The downward migration of soil heavy metal(loid)s (HMs) at smelting sites poses a significant risk to groundwater. Therefore, it is requisite for pollution control to determine the pollution characteristics of soil HMs and their migration risks to groundwater. 198 soil samples collected from a Pb-Zn smelting site were classified into 6 clusters by self-organizing map (SOM) and K-means clustering. Cd, Zn, As, and Pb were identified as the characteristic contaminants of the site. The driving factors for the heterogeneous distribution of HMs have been validated through the implementation of K-means clustering and multiple-hits calculation. Using ultrafiltration extraction and microscopic analysis, the soil colloids were identified as crucial carriers facilitating the migration of HMs. Specifically, the colloidal fractions of Cd, Zn, and As, Pb in deep soil (3-4 m) accounted for 91 %, 78 %, 88 %, and 82 %, respectively, consistently surpassing those found in topsoil (0-0.5 m). It was primarily attributed to the strong affinity of HMs toward soil colloids (franklinite, PbS, and kaolinite) and dissolved organic matter (humic acids and protein). The research findings highlight the potential risk of colloidal HMs to groundwater contamination, providing valuable insights for the development of targeted management and remediation strategies.

Enhanced Arsenate Immobilization by Kaolinite via Heterogeneous Pathways during Ferrous Iron Oxidation

Clay minerals are ubiquitous in subsurface environments and have long been recognized as having a limited or negligible impact on the fate of arsenic (As) due to their negatively charged surfaces. Here, we demonstrate the significant role of kaolinite (Kln), a pervasive clay mineral, in enhancing As(V) immobilization during ferrous iron (Fe(II)) oxidation at near-neutral pH. Our results showed that Fe(II) oxidation alone was not capable of immobilizing As(V) at relatively low Fe/As molar ratios (≤2) due to the generation of Fe(III)-As(V) nanocolloids that could still migrate easily as truly dissolved As did. In the presence of kaolinite, dissolved As(V) was significantly immobilized on the kaolinite surfaces via forming Kln-Fe(III)-As(V) ternary precipitates, which had large sizes (at micrometer levels) to reduce the As mobility. The kaolinite-induced heterogeneous pathways for As(V) immobilization involved Fe(II) adsorption, heterogeneous oxidation of adsorbed Fe(II), and finally heterogeneous nucleation/precipitation of Fe(III)-As(V) phases on the edge surfaces of kaolinite. The surface precipitates were mixtures of amorphous basic Fe(III)-arsenate and As-rich hydrous ferric oxide. Our findings provide new insights into the role of clay minerals in As transformation, which is significant for the fate of As in natural and engineered systems.

Active anaerobic methane oxidation in the groundwater table fluctuation zone of rice paddies

Rice paddies are globally important sources of methane emissions and also active regions for methane consumption. However, the impact of fluctuating groundwater levels on methane cycling has received limited attention. In this study, we delved into the activity and microbial mechanisms underlying anaerobic oxidation of methane (AOM) in paddy fields. A comprehensive approach was employed, including 13C stable isotope assays, inhibition experiments, real-time quantitative reverse transcription PCR, metagenomic sequencing, and binning technology. Geochemical profiles revealed the abundant coexistence of both methane and electron acceptors in the groundwater table fluctuation (GTF) zone, at a depth of 40-60 cm. Notably, the GTF zone exhibited the highest rate of AOM, potentially linked to the reduction of iron oxides and nitrate. Within this zone, Candidatus Methanoperedens (belonging to the ANME-2d group) dominated the Archaea population, accounting for a remarkable 85.4 %. Furthermore, our results from inhibition experiments, RT-qPCR, and metagenome-assembled genome (MAG) analysis highlighted the active role of Ca. Methanoperedens GTF50 in the GTF zone. This microorganism could independently mediate AOM process through the intriguing "reverse methanogenesis" pathway. Considering the similarity in geochemical conditions across different paddy fields, it is likely that Ca. Methanoperedens-mediated AOM is prevalent in the GTF zones.

Critical Role of Mineral Fe(IV) Formation in Low Hydroxyl Radical Yields during Fe(II)-Bearing Clay Mineral Oxygenation

In subsurface environments, Fe(II)-bearing clay minerals can serve as crucial electron sources for O2 activation, leading to the sequential production of O2•-, H2O2, and •OH. However, the observed •OH yields are notably low, and the underlying mechanism remains unclear. In this study, we investigated the production of oxidants from oxygenation of reduced Fe-rich nontronite NAu-2 and Fe-poor montmorillonite SWy-3. Our results indicated that the •OH yields are dependent on mineral Fe(II) species, with edge-surface Fe(II) exhibiting significantly lower •OH yields compared to those of interior Fe(II). Evidence from in situ Raman and Mössbauer spectra and chemical probe experiments substantiated the formation of structural Fe(IV). Modeling results elucidate that the pathways of Fe(IV) and •OH formation respectively consume 85.9-97.0 and 14.1-3.0% of electrons for H2O2 decomposition during oxygenation, with the Fe(II)edge/Fe(II)total ratio varying from 10 to 90%. Consequently, these findings provide novel insights into the low •OH yields of different Fe(II)-bearing clay minerals. Since Fe(IV) can selectively degrade contaminants (e.g., phenol), the generation of mineral Fe(IV) and •OH should be taken into consideration carefully when assessing the natural attenuation of contaminants in redox-fluctuating environments.

Experimental research on the transport-transformation of organic contaminants under the influence of multi-field coupling at a site scale

Organic-contaminated shallow aquifers have become a global concern of groundwater contamination, yet little is known about the coupled effects of hydrodynamic-thermal-chemical-microbial (HTCM) multi-field on organic contaminant transport and transformation over a short time in aquifers. Therefore, this study proposed a quick and efficient field experimental method for the transport-transformation of contaminants under multi-field coupling to explore the relationship between organic contaminants (total petroleum hydrocarbon (TPH), polycyclic aromatic hydrocarbons (PAHs), benzene-toluene-ethylbenzene-xylene (BTEX) and phthalates acid esters (PAEs)) and multi-field factors. The results showed that hydrodynamics (affecting pH, p < 0.001) and temperature (affecting dissolved oxygen, pH and HCO3-, p < 0.05) mainly affected the organic contaminants indirectly by influencing the hydrochemistry to regulate redox conditions in the aquifer. The main degradation reactions of the petroleum hydrocarbons (TPH, PAHs and BTEX) and PAEs in the aquifer were sulfate reduction and nitrate reduction, respectively. Furthermore, the organic contamination was directly influenced by microbial communities, whose spatial patterns were shaped by the combined effects of the spatial pattern of hydrochemistry (induced by the organic contamination pressure) and other multi-field factors. Overall, our findings imply that the spatiotemporal patterns of organic contaminants are synergistically regulated by HTCM, with distinct mechanisms for petroleum hydrocarbons and PAEs.