Natural mobile colloids strongly facilitate the subsurface transport of radioactive Americium(III) (Am(III)), considerably extending the transport distance of Am(III) in porous media. Assessing the long-term transport behavior of colloidal Am(III) and its potential subsurface risk is critical for the safety evaluation and emergency management of radioactive disposal sites. In this study, laboratory column experiments combined with numerical simulations were conducted to investigate colloid-facilitated Am(III) transport in unsaturated porous media. A one-dimensional unsaturated column model was constructed using Hydrus-1D to inversely determine key retention and transport parameters, followed by a parameter sensitivity analysis to quantitatively evaluate the influencing intensity of hydrodynamic and geochemical factors. On this basis, an unsaturated disposal site model was established to predict the environmental transport characteristics of colloidal Am(III) under varied leakage and rainfall scenarios. Sensitivity analysis identified highly sensitive parameters such as the medium dispersion (0.141 ≤ S ≤ 0.258) and the saturated volumetric water content (-0.165 ≤ S ≤ -0.139), as well as low sensitivity parameters such as the saturated permeability coefficient (-0.044 ≤ S ≤ -0.012) and medium density (-0.0030 ≤ S ≤ -0.0026). The scenario simulation results indicated the vertical transport depth of colloidal Am(III) in the vadose zone increased with increasing time, rainfall intensity, and leakage amount. Short term heavy rainfall (100-300 mm/d) remarkably accelerated downward transport of colloidal Am(III), leading to a maximum breakthrough depth of 0.60-1.26 m without reaching the groundwater table. Nevertheless, long-term leakage under Case S1 (> 224 years) or large volume accidental leakage under Case S3 (> 7.66 m3) may eventually lead to groundwater contamination, posing severe threats to subsurface environmental safety.
The assessment of nitrate pollution in groundwater is crucial for ensuring the safety of groundwater quality and related human health risks. This study conducted a comprehensive hydrogeochemical survey in an area strongly influenced by mining areas and human activities. A total of 236 groundwater samples were collected. The range of nitrate concentration was 0.020 mg/L to 658.0 mg/L, with 78.4% of the samples exceeding the recommended limit of 10 mg/L by the Chinese Guidelines. As the nitrate content increases, the chemical type of groundwater shifted from HCO₃-Ca type to ClCa type. The human input generated by residential land was the driving factor for nitrate enrichment and salinity increase. The calculation results of Entropy Weighted Water Quality Index (EWQI) showed a range of 4.9 to 650.8, with a mean of 41.6. Based on the current situation of land use, the causes of various excessive hydrochemical parameters were analyzed, and corresponding improvement suggestions were proposed for the local situation. The non-carcinogenic risk assessment indicated that the total risk range for infants was 0.0034 to 8.7 (mean 0.78), children ranged from 0.0021 to 5.4 (mean 0.48), adult females ranged from 0.0015 to 3.9 (mean 0.34), adult males ranged from 0.0018 to 4.6 (mean 0.41). The groundwater health risk of nitrate ranged from 0.0012 to 8.7, and the threat mainly came from oral ingestion of nitrates, with minors (infants, children) being more susceptible. The results can provide a scientific perspective for the sustainable development and utilization of groundwater resources and public health protection in adjacent mining areas.
27 sampling sites were set up to study the concentration levels, seasonal and spatial distribution characteristics of typical steroid hormones in Qinjiang River, and to assess their ecological risks. 33 Steroid hormone compounds were detected during both the wet and dry seasons, with concentration ranges of ND ∼ 13.25 ng/L (α-E2) and ND ∼ 4.9 ng/L (1,4-AE), respectively. The total concentration of steroid hormones in Qinjiang River generally exhibited a trend of being higher in the upstream areas with less urbanization than in the downstream areas with more urbanization, increasing from the upstream to the midstream or downstream, and then decreasing towards the estuary. The investigation indicated that the spatial distribution of steroid hormone concentrations in the Qinjiang River basin closely matches the geographic locations of riparian anthropogenic activities, aquaculture, and livestock and poultry farming, with the high pollution areas lying within the intensive distribution zones of these activities. The ecological risk assessment indicated that β-E2 and 17-MT posed high ecological risks at certain sites in the region during both the wet and dry seasons. E1, E2, and E3 might lead to high estrogenic potency, and these compounds should be regarded as priority pollutants in the river. Relevant authorities should pay increased attention and enhance environmental management efforts around the Qinjiang River Basin to further mitigate the ecological risks posed by steroid hormones.
Precise characterization of dense non-aqueous phase liquid (DNAPL) source zone architecture (SZA) is crucial for effective site remediation and environmental protection. To achieve accurate SZA characterization, the traditional uniform sampling method requires considerable intrusive boreholes, resulting in high costs and low feasibility. Therefore, optimizing the monitoring network is essential for minimizing the number of wells while maximizing information yield. However, DNAPL SZA exhibits a highly irregular morphology, posing great challenges to conventional optimization methods that determine all well locations in a single step. To address these challenges, this study proposes an adaptive sampling strategy that sequentially and jointly optimizes horizontal well placement and vertical sampling depths, while iteratively updating SZA estimates with measurements collected from newly selected locations. Relative entropy within a Bayesian experimental design framework is used to determine the next most informative sampling configuration. We evaluated the proposed approach through a 2-D aquifer model representing site-relevant conditions, and compared the performance of traditional uniform and adaptive sampling strategies under the same number of wells. Results show that the adaptive sampling strategy outperformed the traditional uniform sampling in characterizing the SZA, achieving a 40% reduction in DNAPL mass estimation error. This improved characterization further enabled more accurate simulation of DNAPL depletion dynamics and longevity, providing a more reliable initial condition for predictive remediation modeling. This study develops a novel framework for monitoring network optimization, offering a practical strategy for more efficient contaminated site management and remediation.
This study investigated the relationship between temporal variability of microbial contamination at a karst spring and the different flow components of a karst groundwater system in western Ireland. A high frequency tryptophan like fluorescence (TLF) concentration time series was collected over 12 months as a surrogate indicator of microbial contamination. The study evaluated whether temporal variations in TLF were associated with the dominant spring flow components and assessed the relative influence of internal aquifer characteristics and rainfall intensity. Flow duration curves, master recession curves, Fast Fourier Transform spectral analysis, and continuous and discrete wavelet analyses were applied. Results characterize the spring as a triple-porosity karst system comprising fast conduit, intermediate fracture, and slow diffuse matrix flow domains. TLF variability was weakly associated with slow/baseflow but more strongly associated with intermediate and fast flow components. TLF was also strongly correlated with E. coli abundance (Pearson r = 0.703; Spearman r = 0.806; Kendall r = 0.644; p < 0.05), supporting its use as a surrogate indicator of microbial contamination. TLF temporal patterns were associated with the aquifer's internal flow architecture, although rainfall intensity or recharge alone did not fully explain the observed variability. Collectively, these findings suggest that TLF variability, and by extension microbial contamination signatures, is more closely associated with shorter timescale groundwater flow responses and internal flow partitioning than with rainfall forcing alone, highlighting the value of flow component-based monitoring for managing contamination risk in karst groundwater systems.
During the groundwater recharge to reclaimed water, dissolved organic matter (DOM) might be affected by Ca2+-induced aggregation and coprecipitation with CaCO3. In this study, humic acid (HA) was taken as the representative of DOM. Batch experimental methods combined with three-dimensional fluorescence and FT-ICR MS were used to investigate the changes in DOM components during the aggregation and coprecipitation in water at the molecular level. The results showed that Ca2+ tended to bind with high aromatic and carboxyl-rich components in HA, leading to aggregation and precipitation. During this process, the contribution of number of carboxyl groups in alicyclic molecules was more significant than that of the hydrophobicity of aromatic molecules. Among alicyclic molecules, those containing CHONS exhibit a stronger tendency to aggregate and precipitate than those containing CHO, CHON, or CHOS. As the initial HA concentration decreased from 24 to 6 mg C/L, the CaCO₃-normalized partition coefficient of residual HA in the supernatant decreased after aggregation and precipitation, due to the removal of highly unsaturated oxygen-containing components prone to coprecipitation with CaCO3. In this process, the effects of pH decrease and Ca-induced further aggregation were negligible. These findings are helpful to understand the effects of typical hydrogeochemical processes on HA components during reclaimed water recharge to groundwater.
For composite cutoff walls (CCWs) containing geomembranes, the geomembrane is commonly found with defects, and the engineering properties of barrier materials (e.g., soil-bentonite) usually alter with depth due to the consolidation behavior. However, the impacts of these factors on the transport laws of pollutants remain unclear. In this study, aiming at the CCW-aquifer system containing a defective geomembrane, a novel theoretical model for two-dimensional transport of organic pollutant considering the effect of consolidation behavior is established. Then, this model is numerically solved and is fully validated through several comparisons, including the comparison with the experimental measurements. Following this, the investigation finds that, compared with ignoring the consolidation behavior, the upper concentration in the CCW increases but the bottom concentration declines when this behavior is considered. The concentration is higher when accounting for the presence of geomembrane defects as compared to the scenario where a geomembrane is thought to be intact, and their difference grows with an increasing hydraulic head difference. Furthermore, the influences of several factors on the CCW's barrier performance are quantitatively assessed. Eventually, a defective geomembrane's anti-seepage function is visually exhibited through the comparative study with the single-layered cutoff wall's barrier performance. Overall, this study helps to reasonably evaluate the CCW's barrier performance, and offers guidance for the application of geomembranes in pollution-control engineering.
This study assessed the water quality of the Toledo River through monthly monitoring over one year at five sites, spanning from its headwaters to its mouth. Principal Component Analysis (PCA) and Analyses of Variance (ANOVA) indicated a progressive increase in physicochemical parameters along the river course and occasional elevated concentrations of copper and iron. This deterioration in water quality is correlated with a transition in land use, evolving from a preserved headwater to the influx of agropastoral nutrients in rural areas and industrial and domestic wastewater in urban zones. Consequently, a critically degraded condition was observed at the river mouth. Given this scenario, there is an urgent need to implement mitigation and sustainable management strategies to restore this vital water resource.
In industrialized regions, groundwater contamination by heavy metals exhibits significant spatial heterogeneity and poses a sustained threat to human health. This study focuses on a typical heavy industrial agglomeration area in northern Hebei Province. Based on 199 groundwater samples, a comparative health-risk assessment is conducted for pore water (PW) and fissure water (FW) using the entropy-weighted water quality index (EWQI), positive matrix factorization (PMF), and Monte Carlo simulation. The framework enables the identification of potential contamination sources, characterization of their spatial patterns, and probabilistic assessment of associated health risks. The results indicate that localized PW samples exhibit very poor quality. These samples show a marked spatial concentration in the industrial agglomeration zone in the southeastern part of the region. In this area, significant exceedances of iron and manganese are observed. The PMF model identifies five distinct pollution sources. The geogenic dust composite source exhibits the highest contribution rate, dominated by iron and manganese, concentrated in the southeast, and controlled by natural reduction processes. Monte Carlo simulations reveal distinct health-risk patterns among population groups. Children exhibit higher non-carcinogenic risks, with a 1.23% probability of HI > 1, whereas adults exhibit higher carcinogenic risks, with a low probability (<1%) of ILCR exceeding 1.0 × 10-4. These results indicate that children are more vulnerable to non-carcinogenic effects, while carcinogenic risks are mainly associated with upper-tail exposure scenarios. This study provides a transferable approach for linking groundwater contamination sources with spatial health risks under hydrogeological heterogeneity.
This study develops a one-dimensional numerical model to investigate back diffusion of the Th-230-Ra-226 decay chain in fractured crystalline rock with a stagnant water zone, an altered fracture-skin layer, and an intact host-rock matrix. Radionuclide-specific boundary conditions are applied: Th-230 is supplied from the stagnant zone via time-dependent source strength functions (step, exponential, and linear depletion), whereas Ra-226 is constrained to a zero-concentration boundary so that all Ra-226 originates from in situ matrix decay. The finite-difference model, validated against two analytical solutions for layered diffusion with decay, resolves diffusion, sorption, and mass-flux continuity across sharp contrasts in porosity and retardation between skin and host rock. Results show that a thin, high-porosity skin layer stores substantially more Th-230 than a homogeneous matrix and thereby amplifies subsequent back-diffusive fluxes, while source-depletion kinetics exert a first-order control on the onset and magnitude of flux reversal. Exponential depletion prevents Th-230 flux reversal but still drives sustained Ra-226 back diffusion, indicating that weakly sorbing daughters can be released even when the parent remains trapped. Spatial profiles and cumulative mass budgets further reveal Ra-226 hotspots at the skin-host-rock interface and scenarios where the long-term back-diffused mass of Ra-226 exceeds that of Th-230, underscoring the need to account for layered interfaces and decay-chain coupling when assessing contaminant persistence in fractured low-permeability media.
Railway operations have long relied on herbicides to keep tracks free of vegetation and ensure safety and track stability. In Germany, regulatory changes introduced in 2020 have increased concern about the long-term environmental effects of herbicide use along railway corridors. This study examines the fate and transport of glyphosate, its metabolite aminomethylphosphonic acid (AMPA), flumioxazin, and flazasulfuron beneath railway embankments using HYDRUS-2D simulations and field monitoring data from five German sites. Simulations were carried out under identical application scenarios, focusing on the influence of local hydrogeological conditions, precipitation patterns, and groundwater depth. Results show that the standardized embankment geometry produces a consistent internal flow regime, characterized by vertical percolation through the ballast, lateral redirection at the subgrade protective layer (PSS), and convergent flow toward the embankment slopes. The ballast layer was identified as the dominant retention zone, while site-specific differences in contaminant fate were primarily controlled by the geological boundary conditions beneath the embankment. Three distinct hydrogeological regimes emerged: sites without a geological barrier showed retention comparable to that of the reference scenario; a site with a low-permeability silt layer exhibited anomalously high retention; and a site with a shallow water table over permeable deposits showed the highest potential groundwater vulnerability. The interaction between climatic forcing and geological conditions was multiplicative, implying that site-specific risk assessments must consider both factors simultaneously. AMPA shows highest peak concentrations, exceeding those of glyphosate, flazasulfuron the deepest plumes and railway-specific sorption parameters was essential to reproduce observed concentration patterns.
Microplastics (MPs) are increasingly recognized not only as persistent particulate pollutants but also as active sources and vectors of phthalate esters (PAEs) in aquatic environments. Because PAEs are physically incorporated rather than chemically bonded within polymer matrices, they can be progressively released through leaching, photooxidation, mechanical abrasion, and biological interactions. This review synthesizes current understanding of MP-mediated PAE release, their interactions with co-contaminants, and the implications for ecological exposure. Environmental aging enhances polymer fragmentation and surface oxidation, accelerating additive desorption and altering sorption behavior. Field evidence from major river systems shows that high-molecular-weight PAEs, particularly DEHP and DBP, frequently dominate contamination profiles and contribute most significantly to ecological risk, with cumulative risk quotients (∑RQ) reaching up to 59.22 in heavily impacted watersheds. Ingestion of MPs by aquatic organisms further promotes in situ chemical transfer under gastrointestinal conditions, increasing internal exposure beyond dissolved-phase concentrations alone. Moreover, MPs act as multi-contaminant platforms, facilitating the co-transport of PAEs with metals and hydrophobic organic pollutants, thereby intensifying mixture toxicity and trophic transfer. Collectively, these processes demonstrate that MP-associated PAE release represents a dynamic and sustained pathway of chemical pollution requiring integrated risk assessment and source-control strategies.
Natural source zone depletion (NSZD) has become central to the management of hydrocarbon-contaminated sites, and the methods established to quantify it rely on the gaseous or thermal expression of biodegradation in the vadose zone. These methods were developed in temperate settings, where the water table fluctuates modestly and typically where the LNAPL remains within reach of the air phase. Tropical and subtropical environments differ in two respects that bear directly on this premise: they experience large-amplitude seasonal water-table fluctuations, and their soils sustain high natural CO₂ partial pressures. Here we reinterpret fifteen years of investigation (2002 to 2017) at a jet-fuel-contaminated site in a subtropical climate, combining previously published hydrological, geochemical, isotopic and microbiological records with new analyses of the LNAPL n-alkane distribution, the joint dynamics of water table and LNAPL thicknesses, and laser-induced fluorescence profiles referenced to the water-table range. Seasonal fluctuation commonly reaches 2 m and multi-annual variation approaches 4 m, against 0.9 and 0.5 m at the low-rainfall sites on which much of the NSZD framework was built. The LNAPL-bearing interval extends over more than 3 m and its base lies below the lowest recorded water level, so that a large proportion of the oil remains submerged. A geochemical model reproduces the source-zone water, matching the measured ferrous iron and methane and accounting for its alkalinity to within about 18%, through the degradation of about 4 mg L-1 of toluene by methanogenesis and goethite reduction, and requires a CO₂ partial pressure matching the value independently inferred from the Keeling-plot intercept. Because aerobic degradation alters none of these parameters, this figure is a lower bound, and the aliphatic fraction is depleted without leaving any aqueous trace. Assessments based solely on vadose-zone CO₂ may therefore substantially underestimate the depletion at such sites.
Co-contaminants can alter per- and polyfluoroalkyl substance (PFAS) mobility by modifying aqueous association, phase transfer, and retention by aquifer solids, yet how protonated amines influence these processes remains poorly constrained. Here, we investigated interactions between diisopropylamine (DIPA) and two perfluoroalkyl carboxylates with contrasting chain lengths, trifluoroacetate (TFA) and perfluorooctanoate (PFOA), using bottle tests, diffusion-ordered spectroscopy (DOSY) nuclear magnetic resonance (NMR), retention experiments with montmorillonite, and conformer-ensemble COSMO-RS and qualitative molecular-mechanics cluster calculations. At 20-25 °C and 0.1 wt% per solute, aqueous DIPA/TFA mixtures remained optically clear, and DOSY NMR showed no convergence toward a common diffusion coefficient, consistent with weak and/or fast-exchanging association in bulk water rather than a dominant long-lived contact species. BP86/TZP COSMO-RS calculations for independently solvated ions predicted greater aqueous compatibility for DIPA+/TFA- than for DIPA+/PFOA- and a demixing tendency for the PFOA-containing conformer ensemble. In contrast, higher equimolar DIPA/PFOA loadings produced flocs or precipitates; DIPA/PFOS mixtures also precipitated. Montmorillonite retention experiments showed strong removal of DIPA, limited retention of TFA with or without DIPA, and measurable co-retention of PFOA in the presence of DIPA. These results show that favorable PFAS-amine interactions in bulk water do not necessarily translate into co-retention or reduced mobility. Instead, PFAS chain length and interfacial interactions govern whether association remains labile, as for TFA, or promotes phase transfer and mineral co-retention, as for PFOA. Thus, amine co-contaminants can differentially affect PFAS migration in clay-rich subsurface environments.
Microplastics are increasingly recognised as persistent contaminants in fluvial sediments, yet the processes governing their storage and redistribution remain poorly constrained. Here, we use a numerical sediment-transport model to investigate the accumulation, mobilisation, and redistribution of microplastics in two contrasting rivers within the Irwell-Mersey catchment in Greater Manchester, United Kingdom. Simulations are initialised with a uniform sedimentary microplastic distribution and exclude ongoing external inputs, allowing the role of internal river processes to be isolated. Despite these simplifying assumptions, the model generates pronounced spatial heterogeneity in bed-sediment contamination and large event-driven reductions following extreme floods, consistent with patterns observed between 2015 and 2016. Model results demonstrate that high-flow events exert primary control on microplastic mobility, driving bed scour, downstream transfer, and reorganisation of particle concentrations. Particle density strongly modulates these responses: low-density microplastics are readily mobilised during moderate floods, whereas higher-density particles require larger discharges for significant entrainment. Differences between the Irwell and Upper Mersey reflect contrasts in hydraulic forcing rather than mean channel slope, with higher peak discharges and shear stresses in the Irwell promoting localised accumulation in mild reaches, and stronger slopes in the Upper Mersey favouring more effective flushing, with accumulation mainly in the lower reach. By explicitly resolving flow, sediment transport, and particle properties, the model provides a process-based explanation for observed spatial and temporal variability in sedimentary microplastic contamination. The results show how event magnitude, particle density, and channel morphology interact to control redistribution and retention, offering a quantitative framework for interpreting monitoring data and for assessing the sensitivity of sedimentary microplastic inventories to hydrological extremes.
Reactive groundwater systems often involve strongly coupled hydrological, geochemical, and electrochemical processes, complicating the interpretation of hydrogeophysical observations. Karst carbonate aquifers are particularly vulnerable to contamination by acidic, sulfate-and phosphate-rich leachates derived from phosphogypsum landfills. In this study, laboratory column experiments were conducted to investigate spectral induced polarization (SIP) responses during phosphogypsum leachate‑carbonate interactions under static, dissolution-dominated, and dynamic breakthrough conditions representative of karst environments. SIP measurements were performed synchronously with geochemical monitoring of pH, fluid electrical conductivity, and major ion concentrations. Multivariate statistical analyses were applied to explore relationships between SIP parameters and evolving geochemical conditions and to aid interpretation of overlapping electrochemical behavior. Results indicate that the conductivity reflects pore-fluid ionic strength but exhibits weak linear correlations with individual ions due to the superposition of electrolytic conduction and reaction-induced pore structure modifications. In contrast, polarization signals and their frequency dependence are primarily sensitive to interface associated with carbonate dissolution and the formation or removal of secondary sulfate and phosphate-bearing mineral phases. Dual-frequency polarization features suggest the coexistence and temporal evolution of matrix-scale interfacial polarization and smaller-scale polarization associated with secondary mineral particles or surface coatings. Principal component analysis separates the dominant variability associated with bulk geochemical conditions from variations in SIP parameters related to interfacial polarization, while independent component analysis identifies statistically independent patterns that are consistent with dissolution- and precipitation-influenced electrochemical responses under heterogeneous experimental conditions. These results indicate that SIP, when interpreted in a multivariate manner and constrained by geochemical and mineralogical evidence, can provide process-sensitive information beyond bulk conductivity and help distinguish hydrologically and geochemically different reaction regimes in laboratory carbonate systems.
Non-aqueous phase liquids (NAPLs) in aquifers act as persistent sources of groundwater contamination, while conventional flushing and sparging methods often suffer from inefficient sweep and excessive reagent consumption. This study proposes a novel strategy to hydraulically isolate the contaminant source zone using a temporally constructed alginate gel barrier (AGB) to enhance remediation efficiency. A two-dimensional flow chamber model packed with a sand was used for this bench-scale study. The AGB was formed in the flow chamber by sequential injection of sodium alginate and thickener solutions, followed by Ca2+-induced crosslinking, and subsequently removed using a sodium-rich solution. A series of experiments were conducted to evaluate ozone sparging, surfactant flushing (SDS + n-butanol), and cosolvent flushing (ethanol) within the confined source zone containing either a dissolved contaminant (fluorescein) or NAPL (mixture; PCE/hexane/oil-soluble fluorescence agent). The AGB effectively confined gas and liquid flow to the target region, significantly improving contact between remedial agents and contaminants. Ozone sparging removed >96% of the fluorescence from the oil-soluble fluorescence agent dissolved in the NAPL, surfactant flushing eliminated >90% within 12 h, and ethanol flushing achieved >97% removal within 5 h, with 78% PCE recovery in the effluent. Notably, required reagent volumes were substantially lower than those estimated for full-domain treatment. These results demonstrate that temporary source-zone encapsulation using AGB can markedly reduce chemical usage while enhancing remediation performance in NAPL-contaminated aquifers.
