Microplastics (MPs) are increasingly recognized as pervasive contaminants in terrestrial environments, yet their redistribution through erosion and accumulation within sediment-retention structures remains poorly quantified. This study quantified MPs in sediments retained behind check dams and examined their relationships with sediment storage and erosion-related sediment yield at the catchment scale. Twenty check dams in a semi-arid mountainous catchment in northeastern Iran were investigated. Sediment storage ranged from 531 to 9828 Mg (mean: 2683 Mg), while specific sediment yield ranged from 16.2 to 855.4 Mg km[-2] yr[-1] (mean: 247.1 Mg km[-2] yr[-1]). MPs were detected in all sediment samples, with concentrations ranging from 800 to 5240 particles kg[-1] dry sediment (mean: 2702 particles kg[-1]). Estimated MP inventories ranged from 9.0 × 10[8] to 2.4 × 10[10] particles per check dam. Strong positive relationships were observed between specific sediment yield and estimated MP yield (r = 0.99, p < 0.01) and between accumulated sediment mass and estimated MP inventory (r = 0.80, p < 0.01), indicating a close association between sediment dynamics and MP redistribution. Fibrous MPs dominated the assemblage (86.6%), and the 500-1000 μm size class accounted for the largest proportion of detected particles (44.7%). Raman spectroscopy confirmed several polymer types within the analyzed subset. Overall, the findings show substantial accumulation of sediment-associated MPs behind check dams and indicate that erosion-driven sediment mobilization may represent an important pathway for MP redistribution in erosion-prone catchments. However, the persistence of this retention may depend on the stability of accumulated sediments, which can potentially be remobilized during high-energy hydrological events.

This study investigates the processes governing tracer transport in a stratified porous medium, which produces asymmetric breakthrough curves (BTCs) with multiple inflection points. Such complex BTCs are often analyzed using multi-porosity or multi-permeability flow and transport models. However, these models introduce increased parameter complexity and associated sensitivity, necessitating thorough evaluation. In this context, we coupled the Global Sensitivity Analysis (GSA) and Parameter Estimation (PEST) with the DualPerm (DP) HYDRUS module to quantify parameter sensitivity and enhance BTC representation. A micro-aquifer experiment was conducted on a stratified porous medium using a non-reactive tracer for testing the capabilities of the developed framework. The sensitivity analysis aimed to examine how different model parameters contribute to different segments of the asymmetric BTC. This analysis revealed that the fracture and immobile domain parameters dominate early tracer breakthrough (64% of the total sensitivity). In comparison, the matrix parameters and the porosity of the immobile region contributed 48% of the total sensitivity during the rising limb period. The prolonged tailing limb was primarily influenced by the matrix parameters (porosity and permeability, 91% sensitivity), with the porosity of the immobile region adding only 4%. It was also established that BTC-based parameter estimation emphasizes peak concentration, but often overlooks critical features like mass recovery, solute residence time, and spatial variance. An alternate approach, which is based on cumulative solute mass recovery, showed efficiency in capturing complex features of experimental BTCs such as inflection points, peak timing, and long-tailing behavior. The proposed framework enhances the capability of the HYDRUS (DualPerm) module to investigate various flow and transport processes, improving mass recovery estimation compared to conventional BTC-based methods.

The migration of non-aqueous phase liquids (NAPL) in unsaturated fractures involves NAPL-water-air three-phase displacement. Understanding the impact of entrapped water on this migration process is crucial for predicting NAPL flow and solute transport in the fractured vadose zone. This study presents visualized flow experiments to systematically investigate NAPL (n-octane) invasion behaviors in initially dry and water-entrapped unsaturated single fractures by varying fracture inclination angle and injection flow rate. The experimental results reveal that entrapped water fundamentally shifts NAPL flow patterns from fingering flow in dry fractures to film flow propagating along entrapped water droplets in water-entrapped fractures. Compared to fingering flow in dry fractures, film flow in water-entrapped fractures exhibits enhanced lateral expansion, longer fracture penetration time, higher breakthrough saturation, larger perimeter-area ratio, and stronger ability to invade large-aperture channels. The morphology of the film flow is jointly regulated by injection flow rate and fracture inclination angle. Lower flow rates and larger inclination angles result in thinner films, while sufficiently high flow rates at small inclination angles can trigger a transition from film flow to fingering flow in the main flow channels. We also find that the distribution of NAPL-occupied apertures consistently follows a normal distribution, independent of flow rate and fracture inclination angle, and aligns with the statistical properties of the fracture aperture field. These findings elucidate the regulatory role of entrapped water on NAPL migration and advance the mechanistic understanding of NAPL invasion in unsaturated single fractures, thereby informing groundwater contamination risk assessment and remediation design.

The combined influence of soil properties, irrigation water chemistry, and transport processes on antibiotic fate remains poorly understood. Here, ciprofloxacin (CIP) retention, transport, and remobilization were investigated in three agricultural soil types under four different irrigation water matrices using batch, column, thermodynamic, mechanistic, and machine-learning approaches. At elevated concentrations (10 mg/L), all soils exhibited comparable retention capacities, whereas substantial differences emerged at extremely elevated concentrations (150 mg/L), with maximum adsorption capacities following the order black soil (≈17.6 mg/g) > alluvial soil (≈15.6 mg/g) > red soil (≈10.4 mg/g). Transport behaviour differed markedly among soils, with retardation factors following the order black soil (7.9) > alluvial soil (2.24) > red soil (1.4). After 180 pore volumes, relative breakthrough remained below 0.03 in black soil but reached 0.25 and 0.45 in alluvial and red soils, respectively. Rainfall flushing further revealed contrasting environmental outcomes, with black soil strongly immobilizing CIP, red soil promoting rapid transport, and alluvial soil acting as a persistent contaminant leaching reservoir. Thermodynamic analyses indicated predominantly endothermic adsorption with enhanced spontaneity at elevated temperatures. FTIR and zeta potential analyses identified surface complexation, hydrogen bonding, electrostatic interactions, and cation bridging as the primary retention mechanisms. Random Forest modelling identified soil type as the dominant control on CIP retention, accounting for ~92% of total feature importance (R[2] = 0.96-0.97). These findings demonstrate that soil properties govern CIP fate and determine contrasting pathways of sequestration, persistence, and groundwater contamination risk.

Groundwater contamination at the site is worsening, and in-situ chemical oxidation (ISCO) has been widely adopted for contaminated groundwater remediation due to its low environmental risk and low cost. Through laboratory experiments and field simulations, we optimized the system for TCE degradation using activated sodium persulfate and explored how injection pressure and formation permeability affect TCE remediation. The results show that both sodium citrate (TCD) and citrate (CA) chelating agents can increase TCE oxidation by persulfate, with optimal molar ratios of Na2S2O8:TCD:Fe[2+]:TCE of 20:10:10:1, and Na2S2O8:CA:Fe[2+]:TCE of 20:5:10:1, achieving high TCE removal rates of 97% and 98%, respectively. Pressurized injection increases the hydraulic gradient and flow velocity, thereby enhancing advection and mechanical dispersion, especially in high-permeability media. In low-permeability media, pressurization also modifies porosity and tortuosity and increases the effective diffusion coefficient, which is relatively more important for solute transport. Pressure-induced permeability changes were limited under the tested 0.1-1 MPa range and were secondary to advection, mechanical dispersion, and effective diffusion. In layered subsurface systems, placing horizontal wells at the interface between high- and low-permeability layers was more effective in delivering oxidants to contaminated regions than vertical wells. In summary, higher-pressure oxidant injection improved remediation in the simulated low-permeability formations by increasing advection, mechanical dispersion, and effective diffusion. The relative contributions were site-specific, and diffusion enhancement was not the sole controlling mechanism. The numerical model can effectively simulate ISCO of TCE by accounting for the multi-field-coupled stress field. It can provide valuable insights for designing groundwater remediation projects.

Natural Source Zone Depletion (NSZD) is widely used to manage petroleum-contaminated sites, but estimating NSZD rates is challenging because most methods rely on short period, averaged or point-in-time measurements. These can be misleading due to seasonal variability in biological activity, moisture and water table fluctuations influencing NSZD rates. Here we utilise continuous inground sensor measurements and light non-aqueous phase liquid (LNAPL) distributions from cores to understand seasonal processes affecting NSZD rates. Linked to multiphase modelling these data are used to test the hypothesis that seasonal NSZD variations result from LNAPL distributions and water table fluctuations. Using ~3.5 years of continuous oxygen and temperature data across four fuel types, NSZD rates were shown to vary by up to an order of magnitude seasonally. Rates ranged from 0.1 to 0.8 kg/m[2]/y for diesel to ~5 to 13 kg/m[2]/y for aviation gasoline, with broadly comparable trends between oxygen- and temperature-based methods. Rising water tables generally reduced NSZD rates at the diesel, gasoline, and crude oil sites by inundating the LNAPL depth distribution and disconnecting it from oxygen in the vadose zone. In contrast, at the aviation gasoline site, where LNAPL persisted above the maximum water table, NSZD rates remained high or increased. Modelling reproduced observed trends at the diesel site and showed that NSZD rates are influenced by the exposure of LNAPL to the vadose zone air phase (i.e., oxygen), with inundation suppressing rates, while exposure enhanced it. This is further supported by soil core TPH measurements that show loss of shorter chain TPH ranges at shallower depths above and within the zone of water table fluctuation relative to deeper in the formation. Continuous monitoring and modelling have provided insights into the dynamic NSZD rate estimates, indicating that care is needed to align field measurements with conceptualisation of LNAPL distributions and water table dynamics.

In natural subsurface systems, Dissolved Organic Matter (DOM) is a potent reducing agent, as it sustains and catalyzes a number of microbially-mediated reactions. One of such reactions, namely the reduction of sulfate to sulfide mediated by sulfate-reducing bacteria (SRB) is particularly relevant for the evaluation of the long-term stability of deep geological repositories (DGRs) for spent nuclear fuel. Some of these proposed DGRs will be hosted at great depth in fractured crystalline rocks where sulfide, potentially produced by SRB, is practically the only oxidant of relevance for the copper canisters used to safely store and keep isolated the radionuclides from the natural environment. One important limitation to this reaction is substrate availability, as the DOM that typically infiltrates, e.g. from overlying surficial sediments, is typically bound in complex polymeric organic compounds (cellulose, starch, proteins, fats, etc.) that cannot be directly used by SRB. A further intermediate step is therefore needed which consists in the degradation of 'complex' organic matter into 'simpler' organic substrates. This intermediate step is performed by a different microbial group (typically acetogens and/or fermenters) that acts on a substrate characterized by significant heterogeneity. Previous studies have documented that the organic carbon substrate typically comprises thousands of species depending on the type of subsurface environment. In this study we argue that the evaluation of microbially mediated sulfate reduction by means of reactive transport modeling should explicitly account for the substrate heterogeneity. However, explicitly accounting for thousands of chemical species of complex organic matter presents two significant challenges. The first is how to parameterize each individual reaction. The second is how to include this large number of species in the mass balance calculations. The first challenge was addressed through the development of a Generalized Kinetic Model (GKM) that formulates all rates as a function of a universally accepted rate of electron transfer along with the underlying stoichiometry of each individual metabolic reaction. The computational challenge was overcome by using weight-based reaction rates. In practice, we solve transport and mass balance for the equivalent total concentration of complex organic carbon while retaining information on the underlying substrate and associated microbial population through the use of two weighting functions, which are updated on lagged time based on mass balance. The numerical framework is first verified and then applied to simulate sulfide production in a single fracture-matrix system. Important modeling outcomes, such as the dependence of SRB activity on the degradation of a complex organic substrate and the role of the matrix as a temporary storage term for substrates, are derived.

The geological disposal of radioactive waste is under investigation in several countries, especially in Europe. In this field, clay-rich sedimentary rocks are promising natural barriers to limit and delay the release of radionuclides toward the biosphere, due to their diffusion-driven transport and retention properties. In addition to laboratory studies, field studies in Underground Research Laboratories (URL) represent a significant value to strengthen safety demonstrations. In this context, the DIR experiment (Diffusion of Inert and Reactive tracers) was developed and implanted in the Meuse/Haute-Marne URL located in the Callovo-Oxfordian clay-rich formation. Over 6 years, several diffusion experiments were performed with radioactive tracers: HTO (tritiated water), [36]Cl[-], [125]I[-], [22]Na[+], [134]Cs[+], assessing the behaviour of neutral, anionic and cationic species. The evolution of each radiotracer activity was monitored in the injection borehole, from at least 100 days up to 3 years. Overcoring of the rock at the end of the experiments allowed the accurate characterization of radiotracer profiles. An overview of the DIR experiment is provided here, illustrating the large amount of data gathered in 4 specific boreholes at two different depths: -445 and - 495 m, within the Callovo-Oxfordian geological layer. The modelling of experimental results allowed the quantification of diffusion properties parallel to the bedding of the rock for the studied radionuclides. Diffusion parameters for HTO are in agreement with those determined on samples using the conventional through diffusion method. Anions are partially excluded from porosity with quantified values 0.3 < εa(anion)/ε(H2O) < 0.6. On contrary, the diffusion of cations is limited by their adsorption on clay surfaces with the evidence of surface diffusion processes. The sensitivity analysis on adjusted parameters provided additional insights on the local properties, such as homogeneity of the diffusion plume and the limited extension of the damaged zone surrounding boreholes: LEDZ ~ 1-3 cm. Still, further field experiments are necessary to measure diffusion in such EDZ and to improve accuracy on the diffusion anisotropy ratio.

This study investigates the source-sink dynamics and hydrogeochemical controls governing arsenic fate and transport within a fractured crystalline aquifer system. The hydrogeologic framework is characterized by spatial heterogeneity in redox conditions and persistent undersaturation with respect to key Fe-, sulfate-, and Ca-bearing mineral phases. Mineralogical characterization identifies arsenopyrite and cobaltite as the primary geogenic sources; oxidative weathering of these sulfide phases drives the continuous release of arsenic. Aqueous speciation modeling reveals a predominance of As(V) and Fe(III) species, with minor As(III) occurring within reducing microenvironments. Persistent undersaturation of Fe- and As-bearing minerals, together with other attenuation-controlling phases, suggests limited potential for secondary mineral precipitation, while allowing arsenic to remain predominantly in the dissolved phase. Weak correlations between arsenic and hydrochemical gradients indicate that contaminant distribution is highly heterogeneous and controlled by the occurrence of mineralized zones. The results demonstrate that persistent arsenic mobility in fractured crystalline aquifers can occur under predominantly oxidizing conditions through sustained sulfide weathering combined with limited attenuation capacity within the aquifer system.

This study examined the environmental fate, source apportionment, and associated ecological and human health risks of polycyclic aromatic hydrocarbons (PAHs) in the coral reef ecosystems of four inhabited islands in the Lakshadweep Archipelago (Kadmat, Kiltan, Kalpeni, and Minicoy). Water, suspended particulate matter (SPM), and sediments were collected from reef lagoons during April 2025 and analysed for 16 PAHs congeners designated as priority by the United States Environmental Protection Agency (USEPA). PAHs were in the range of 20.9-240 ng/L in the dissolved phase, 146-356 ng/L in the SPM, and 2.05-22.2 ng/g in sediments. PAH transport and sedimentary accumulation differ markedly from those in siliciclastic coastal environments due to the dominance of carbonate-rich, organic-matter-deficient, highly permeable sediments and strong hydrodynamics that promote rapid particle export to adjacent offshore waters. As a result, reef lagoons act as dynamic conduits that promote contaminant exchange with the Arabian Sea, thereby limiting long-term contaminant accumulation in the lagoon sediments. Source apportionment studies indicated mixed petrogenic and pyrogenic PAHs sources, primarily associated with navigational activities, fuel handling, diesel-powered energy generation, and combustion-related activities. Ecological risk assessment indicated negligible risks from sedimentary PAHs, while dissolved and SPM-bound PAHs posed low to moderate ecological risks. Incremental lifetime cancer risk (ILCR) values for all age groups were within the potential low-risk category, with dermal contact identified as the dominant exposure pathway. This study provides critical baseline information on transport behaviour and ecological implications of PAHs in the coral reef ecosystems of Lakshadweep and recommends sustained environmental monitoring and targeted management strategies to protect the ecological integrity of these coral reef systems under mounting anthropogenic perturbations.

Antibiotic resistance genes (ARGs), as emerging pollutants, have become a growing environmental concern owing to their wide distribution, durability, and potential adverse effects on humans, animals and the surrounding environments. Livestock and poultry manure and related wastewater (LPMW), the major reservoir of antibiotics, has higher abundance of extracellular ARGs (eARGs) compared with other environments. This review summarizes the abundance of intracellular ARGs (iARGs) and eARGs in LPMW in China. The absolute abundance of iARGs and eARGs in LPMW were 10[3]-10[12] and 10[2]-10[10] copies/mL or copies/g, respectively, with sul1, sul2, tetQ, tetW, tetG, ermB, ermF, blaTEM and blaampc as the main detected i/eARGs types. Subsequently, the study summarized the intrinsic molecular and biological properties, extrinsic drivers, and methodological influences that may influence the abundance of i/eARGs. Discrepancies in the absolute abundance of i/eARGs could be due to factors such as genetic location, sample sources, antibiotic residues and manure treatment methods. Besides, the efficiencies of extracellular DNA(eDNA) extraction methods commonly used for LPMW samples were compared and quantitative methods of i/eARGs were evaluated, which could also introduce subjective bias into i/eARGs abundance. Limitations related to the current eDNA extraction methods and quantitative methods, the establishment of the safety thresholds for eARGs, in situ monitoring methods, and the role of phages in the spread of ARGs were discussed as future prospects. This review highlights the importance of environmental risk assessment for i/eARGs, lay theoretical basis for the effective removal of i/eARGs for further researches and provides advice for future governance and regulatory measurement.

Reactive fronts form at interfaces where remediation agents encounter contaminant plumes in subsurface environments. The contaminant degradation is largely confined to this narrow mixing zone, but the transient reaction dynamics within it remain poorly understood. Existing models often assume uniform flow or constant dispersion, failing to capture how the spatial deceleration of radial velocities alters interfacial mixing. To address this, two-dimensional sandbox experiments were conducted by radially injecting potassium permanganate into a Naphthol Green B-dyed perchloroethylene (PCE) plume. Image analysis, advection-dispersion modeling, and breakthrough-curve measurements were combined to characterize reactive mixing zone evolution and product generation dynamics. The results indicate that (i) the 1/r decay of radial velocity drove a spatial transition from a proximal reaction-limited regime (Damköhler number, Da = 0.14-0.99) to a distal mixing-limited regime (Da = 3.98-27.38), producing a bimodal chloride breakthrough pattern; (ii) the sampling-based effective mixing-zone width exhibited a pronounced non-monotonic evolution, increasing rapidly during the early stage, reaching a maximum at approximately τ ≈ 0.10, and subsequently decreasing, with a more pronounced decline at higher injection rates; and (iii) the apparent reaction rate similarly increased during the early stage and then declined, while cumulative product mass continued to increase but its growth slowed markedly after τ ≈ 0.07. These results suggest that while higher injection rates enhance mechanical dispersion and initial migration, they can reduce local contact time, highlighting the need to balance physical mixing and reaction residence time in radial injection strategies.

Acid mine drainage (AMD) affects the mobility of naturally occurring radioactive materials (NORM), but its influence on the distribution of U- and Th-series radionuclides in polymetallic mining districts and downstream estuaries remains poorly understood. Here, we characterize the spatial evolution and isotopic signatures of U- and Th-series radionuclides in the dissolved fraction along the Oraque-Meca-Odiel river-estuary system (SW Spain). This system shows a strong geochemical gradient, from highly acidic AMD headwaters (locally pH < 3; high sulfate and metal concentrations) through weakly acidic reaches (pH ≈ 4.5) to near-neutral estuarine waters (pH ≈ 7). [238]U and [232]Th activity concentrations were strongly enriched in AMD-impacted headwaters, reaching values in the order of 10[3] and 10[2] mBq L[-1], respectively, with [238]U exceeding global riverine background by more than two orders of magnitude. PHREEQC modeling indicated that modeled aqueous U(VI) was dominated by free uranyl and uranyl-sulfate species under strongly acidic conditions. In contrast, dissolved [232]Th decreased more strongly than U as pH increased, producing a clear UTh geochemical decoupling. This pattern is consistent with the generally higher particle reactivity of Th reported in previous studies and suggests transfer from the dissolved phase during downstream evolution. [234]U/[238]U ratios were generally >1, indicating isotopic disequilibrium and preferential [234]U mobilization. The [230]Th/[232]Th activity ratios remained close to unity, indicating limited isotopic fractionation within the dissolved Th pool. These results show that the spatial variations of dissolved U and Th across the investigated river-estuary system coincided with hydrochemical gradients and changing source contributions, while the selected confluences showed nominal element-specific departures from the adopted sulfate-based mixing predictions.

Karst aquifers are recharged both by diffuse infiltration through soil and rock (autogenic recharge) and by concentrated inflow through swallow holes and sinking streams (allogenic recharge). Differences in the origin of water are reflected in rapid changes in its physicochemical and microbiological characteristics. Various approaches are used to monitor these changes and analyse the data obtained. In this paper, concentration-discharge hysteresis analysis is used to define and compare the recharge characteristics of two karst springs. The study was conducted in a binary karst aquifer in the catchment of the Malenščica and Unica springs in south-western Slovenia, an area known for complex hydrodynamics with both autogenic and allogenic recharge. The area offers extensive prior hydrogeological knowledge and is equipped with a dense monitoring network that has been operating since 2016. High-frequency monitoring of temperature, specific conductance, and discharge in springs, water-active caves, and sinking streams was supplemented by event-based sampling for chemical and microbiological analyses. The data obtained were first analysed using descriptive statistics to compare the characteristics of individual monitoring sites and to infer connections between them. In the next step, individual storm events under various hydrological conditions were analysed in more detail. Hysteresis plots between discharge and specific conductance of the springs were compared with those of the sinking streams and underground channels in their catchments to characterise the mixing of groundwater originating from different recharge sources. Additionally, for a selected storm event, hysteresis plots between discharge and water quality parameters (specific conductance, temperature, turbidity, [Ca[2+]/Mg[2+]] ratio, Cl[-], and bacteria) were analysed. Significant differences in recharge characteristics, water transfer through the system, and the chemical and microbiological composition between the two springs were detected. These findings provided information about the differences in the spatial and temporal distribution of autogenic and allogenic recharge of the two springs, which is very important for planning the protection of these karst water sources.

The role of microbial communities in regulating the transport and retention of per- and polyfluoroalkyl substances (PFAS) in the subsurface environment, particularly under water-unsaturated conditions, remains poorly understood. Here, miscible-displacement experiments were conducted with saturated and unsaturated sand-packed columns to investigate the influence of biofilms on the transport of 6:2 fluorotelomer sulfonate (6:2 FTS), considering biofilm age, bacterial species, and flow rate. The underlying mechanisms were further elucidated through characterization of extracellular polymeric substances (EPS), zeta potential, and cell surface hydrophobicity, while microbial community responses to 6:2 FTS exposure were also evaluated. Compared with clean sand, biofilm-coated sand enhanced 6:2 FTS retention under both saturated and unsaturated conditions. Under saturated conditions, retention differences among bacterial species were primarily governed by EPS content and protein-to-polysaccharide ratios. In contrast, species-level effects became less pronounced under unsaturated conditions, indicating a shift from species dependent controls to coupled biofilm interface processes. Quantitative analysis of these specific experiments showed that biofilms-associated processes contributed 42% of the additional apparent retention observed upon transition from saturated to unsaturated conditions, compared with 58% attributed to air-water-interfacial adsorption. These results demonstrate that biofilm-associated processes constitute an important and previously overlooked retention domain contributing substantially to the enhanced retention of 6:2 FTS under unsaturated conditions. Biofilm maturation further increased retention through EPS accumulation and associated changes in surface physicochemical properties, including surface potential, roughness, and hydrophobicity. Moreover, 6:2 FTS exposure altered bacterial community composition, suggesting reciprocal interactions between PFAS transport and microbial ecology. These findings provide new mechanistic insights into PFAS fate in the subsurface and improve the basis for predicting PFAS transport and designing remediation strategies for PFAS-impacted sites.