Mining of rare earth elements (REEs) in regolith-hosted ion-adsorption clay deposits generates REE-rich, high-nitrogen mining drainage (REEs-HNMD), yet whether this coupled stressor weakens riverine nitrogen (N)-sink function remains unclear. We investigated a river network draining an ion-adsorption rare-earth mining region across a contamination gradient during wet and dry seasons. Sediment denitrification and anammox were quantified using 15N isotope-tracer incubations and integrated these rates with water and sediment chemistry, microbial community profiles, denitrification genes (nirS, nirK, nosZ), and dual nitrate isotopes (δ15N-NO3- and δ18O-NO3-). Denitrification dominated benthic N loss (0.01-63.50 nmol N g-1 h-1) but responded non-linearly to REEs-HNMD, peaking at moderately polluted sites and declining sharply under high contamination, especially in the wet season. Anammox was rarely detectable (0-0.83 nmol N g-1 h-1) and contributed little. Under high contamination, denitrification-gene abundance declined, the (nirS+nirK)/nosZ ratio decreased, and nitrate isotopes showed a weak watershed-scale expression of denitrification. Structural equation modeling further indicated that REEs-HNMD constrained denitrification mainly through microbial bottlenecks, with stronger net inhibition in the wet season than in the dry season. These results show that REEs-HNMD not only increases N loading, but also weakens riverine N-sink function by suppressing sediment denitrification.

Reduction of As(V) to As(III) under anaerobic conditions significantly increases arsenic (As) toxicity and bioavailability, making it a crucial process that drives As contamination. Simultaneously, co-occurring microbial nitrogen (N) transformations may accelerate As(V/III) conversion through complex interactions and competition, yet their competing effects remain insufficiently resolved. To address this, we applied six N-addition treatments with varying total N input and form to anaerobic microcosms established with As-contaminated soil. N treatments receiving NH4+ only (MN(NH4+)) and NO3- only (MN(NO3-)) were included to examine how specific N forms and their associated transformations affect As(V/III) speciation. Additionally, low (LN), medium (MN), and high (HN) N levels were applied as NH4NO3 to increase N availability and intensify competition among N transformations. Results showed that MN(NH4+) increased As(III) in soil (1.4-36.0 mg kg(- 1)) and porewater (0.1-138.7 mu g L- 1) by enhancing anammox (similar to 40 %) and promoting DOC and Fe(II) accumulation. Conversely, MN(NO3- ) lowered As(III) by stimulating denitrification and restricting DOC and Fe(II) accumulation. Increasing N input from LN to HN decreased denitrification rates by 19.9-51.5 % while enhancing anammox rates by 51.9-199.2 % and the transcriptional activity of the anammox gene hzs (up to 1.5). It also increased the abundance of the Asreducing gene arrB, DOC accumulation, and Fe(III) reduction, ultimately elevating As(III) by 22.3-31.4 mg kg(-1) and 35.0-130.8 mu g L- 1 in soil and porewater, respectively. Structural equation modeling (SEM) and linear mixed-effects models (LMM) identified the largest standardized effect (0.79) and importance (19.1 %) for anammox, highlighting anammox as the dominant driver of As(V/III) speciation. This study provides novel insights into N-As interactions.

Land use and rainfall are key drivers of dissolved organic matter (DOM) dynamics in rivers, yet their coupling effects on DOM sources in heterogeneous urbanized watersheds remain unclear. Here we combine fluorescence spectroscopy and stable carbon isotopes within end-member mixing analysis (EMMA) framework to trace DOM sources along the Guangzhou reach of the Pearl River under contrasting land use and rainfall conditions. The optimized three-dimensional subregional EMMA model (BIX-HIX-delta 13C-DOC) substantially outperformed twodimensional and whole-watershed alternatives, reducing prediction error by 45-50%. Upstream agricultural areas were dominated by soil-derived, humic-rich DOM, whereas midstream and downstream urbanized reaches showed greater wastewater contributions and microbial characteristics. Rainfall increases DOM loads across all zones but variably alters source composition depending on land use. The strongest coupling effect occurs in the midstream agricultural-urban transition zone, where rainfall enhances the surface soil input but decreases the wastewater contribution. By comparison, downstream reaches show minimal source shifts after rainfall, reflecting buffering by continuous wastewater inputs. This study evaluates land use-rainfall interactions on DOM sources, and provides an optimized subregional EMMA approach for DOM source apportionment in tropical and subtropical urban rivers.

Hydrogen and oxygen isotopes in atmospheric water vapor (δv) and precipitation (δp or δr) were continuously measured using a laser-based water isotope spectrometer in Guangzhou, southeastern China, from March 2016 to February 2018. The measurements were conducted to investigate the variations in water isotopes in the hydrological cycle under the subtropical monsoon climate. The isotopic composition ranged from −24.4‰ to −11.1‰ for δ18O in water vapor (δ18Ov) and from −11.5‰ to 2.3‰ for δ18O in precipitation (δ18Or). The values of δv and δr were enriched during the dry season and depleted during the wet season, exhibiting systematic seasonal variation. A negative correlation was observed between monthly δv and precipitation amount, indicating that the values of δv exhibits an ‘amount effect’. However, a corresponding amount effect was not observed in the values of δr. The mean difference between δv and δr was −9.7‰ for δ18O and −76‰ for δD, suggesting that equilibrium fractionation is the dominant process during precipitation. The local meteoric vapor line (LMVL) for Guangzhou (δD = 6.6δ18O − 6.4) exhibited a slope similar to that of the equilibrium local meteoric vapor line (ELMVL) but with an intercept difference of 8.6. This difference in intercepts can be attributed to the vertical profile of δv. The δD-q (q refers to water vapor concentration) relationship is useful for identifying water vapor sources and tracking isotopic changes during atmospheric transport and precipitation. The local water vapor was found to originate primarily from the mixing of oceanic air masses. Data points falling between the oceanic source mixing line and the Rayleigh curve likely reflect post-condensation processes, such as raindrop re-evaporation or mixing with surrounding ambient vapor. Short periods of heavy precipitation were observed to cause severe depletion in δv, resulting in values falling below the Rayleigh curve.

Insects are critical indicators of environmental health, providing a valuable insight into pollutant dynamics within mangrove ecosystems. Despite the recognized use of fatty acids (FAs) as biomarkers in various organisms, the effects of organohalogen contaminants (OHCs) on FA profiles in insects have not been extensively investigated. This study analyzed 54 OHCs and 36 FAs across 16 insect species from the Pearl River Estuary to assess the viability of FAs as biomarkers for OHC exposure. Results indicated that insects from Shenzhen (160 ± 5.1 ng/g lw) exhibited significantly higher concentrations of OHCs than those from Zhuhai (100 ± 4.9 ng/g lw). Carnivorous insects (190 ± 6.1 ng/g lw) accumulated higher OHC levels but had lower proportions of polyunsaturated fatty acids than herbivores (80 ± 4.2 ng/g lw), with OHCs exerting stronger effects on lipid metabolism among carnivores. Significant correlations were observed between OHC levels and desaturase activity/lipogenesis indices in both regions, indicating potential lipid metabolism disruptions. While correlations between OHCs and FAs in insects from Shenzhen and Zhuhai suggest the potential of FAs as biomarkers of OHC exposure, the limited overlap in OHC-FA correlations (5 out of 279) between the regions highlights the influence of spatial heterogeneity. These findings underscore the promise of FAs as biomarkers for OHC exposure but emphasize the need to account for species- and habitat-specific factors.

As an integral component of the global nitrogen cycle, nitrate are readily transferred from urban sewage discharge, agricultural activities, and atmospheric sedimentation to surface water. This paper introduces an innovative framework that combines multi-source remote sensing technology with stable nitrate nitrogen (δ15N-NO3−) and oxygen (δ18O-NO3−) isotopes mixing model, to identify nitrate sources quantitatively in surface water for the first time (R2 range from 0.50 to 0.99, RMSE range from 0.05‰ to 2.31‰, MAE range from 0.03‰ to 1.35‰). By reconstructing the historical nitrate isotopes from 2006 to 2023, we found that manure and sewage were the main contributing sources, followed by soil nitrogen, fertilizer and atmospheric deposition (contribution ratio of 3.5:2.5:2.5:1.5), wastewater discharge and fertilizer application in Xijiang river had a significant impact on this. This framework fills a gap in the research pertaining to remote sensing technology’s identification of surface nitrate sources, facilitating straightforward and user-friendly forecasting of nitrate source spatio-temporal sequences.

Urban rivers are critical sources of nitrous oxide (N2O) emissions, necessitating the use of distinct indirect emission factor (EF5r) values in N2O emission assessments based on IPCC methodology. However, the high nitrogen (N) loads, accelerated N transformations, and intricate N cycling in urban rivers result in dynamic changes in EF5r, introducing uncertainties in N2O emission estimation. This study examined highly urbanized and suburban rivers in Guangzhou, China, to investigate the mechanisms of EF5r variability. By combining dissolved N2O concentrations, EF5r values, isotopic signatures (delta 15N-NO3-, delta 18O-NO3-, delta 15N-NH4+ and delta 15N-PN), N2O-related microbial features, and 15N isotope-based rates of nitrification, denitrification, and assimilation, we found that urban rivers exhibit a higher EF5r value (0.0044) compared to other river types and the IPCC default (0.0026), underscoring their role as "hotspots" for indirect N2O emissions. Seasonal variations were observed, with higher N2O concentrations and EF5r values during the dry season, linked to increased nitrification and denitrification. In contrast, algal assimilation during the wet season suppressed nitrification and promoted complete denitrification, reducing N2O production. A nonlinear "U-shaped" relationship between EF5r values and NO3--N concentrations was observed. At low NO3--N levels, denitrifying bacteria showed limited response, reducing N2O production; however, above a threshold concentration, both nitrifying and denitrifying bacteria were stimulated, enhancing N2O production. These findings highlight the microbial mechanisms driving N2O emissions and the role of microbe-algae interactions, offering valuable insights for improving global N2O emission estimates and informing climate change mitigation strategies.

Mangrove ecosystems are increasingly threatened by halogenated organic contaminants (HOCs), posing a significant threat to resident arthropods, including spiders and insects. Spider webs, due to their distinctive chemical composition and adsorption properties, have emerged as promising tools for monitoring environmental pollutant. This study quantified 58 HOCs in insects, spider tissues, and webs of Nephila pilipes across mangroves with varying pollution levels, using quantitative fatty acid signature analysis (QFASA) to estimate spider diets. Significantly elevated concentrations of dichlorodiphenyltrichloroethane (DDT), hexachlorocyclohexanes (HCHs), polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), alternative halogenated flame retardants (AHFRs), hexabromocyclododecanes (HBCDs), and tetrabromobisphenol A (TBBPA) were observed in all matrixes from heavily polluted areas, driven by habitat contamination and dietary intake. Concentrations of HCHs, PCBs, PBDEs, and AHFRs in spider webs were highly correlated with those in spider tissues (cephalothorax and abdomen; p < 0.05), whereas DDTs and HBCDs exhibited weak correlations (only in the abdomen; p < 0.05), and TBBPA showed no significant correlation, potentially attributed to compound-specific properties and web characteristics. Integration of HOC concentrations in spider webs with chemical properties (log KOW, log KOA, and molecular weight) yielded significant correlations with biomagnification factors (R2 = 0.415-0.539, p < 0.01) and with tissue HOC levels (R2 = 0.511-0.795, p < 0.001), supporting predictive models for biomagnification and tissue partitioning. These findings underscore the potential of spider webs as noninvasive, sustainable bioindicators that integrate pollutant data to enhance ecological risk assessments and pollution monitoring in sensitive ecosystems, providing a viable alternative to destructive sampling.

Lignin, as the most abundant recalcitrant organic carbon in terrestrial ecosystems, plays a crucial role in the Earth’s carbon cycle. After lignin entering aquatic environments, portion of it tends to accumulate in sediments, forming a stable carbon relatively reservoir. However, the increasing temperature caused by human activities may impact microbial-mediated lignin decomposition, thereby affecting sedimentary carbon reservoirs. Therefore, revealing how temperature affects microbial-mediated lignin decomposition in river sediment, a topic that remains elusive, is essential for comprehending the feedbacks between river carbon reservoirs and climate. To address this, we conducted stable isotope probing of river surface sediment using 13C-lignin and 13C-vanillin, and utilized a series of techniques, including CO2 production analysis, 16S rRNA gene amplicon sequencing, metagenomics, and metatranscriptomics, to identify the lignin-decomposing microbes and the effects of temperature on microbial-mediated lignin decomposition. We found that elevated temperatures not only increased the total sediment respiration (total CO2) and the CO2 emissions from lignin/vanillin decomposition, but also enhanced priming effects. The 13C-labled taxa, including Burkholderiales, Sphingomonadales, and Pseudomonadales, were identified as the main potential lignin/vanillin decomposers, and their abundances and activity significantly increased as temperature increased. Furthermore, we observed that increasing temperature significantly increased the activity of lignin decomposing pathways, including β-aryl ether fragments and 4,5-PDOG pathway. Additionally, as temperature increases, the transcriptional abundances of other carbon cycling related genes, such as pulA (starch decomposition) and xyla (hemicellulose decomposition), also exhibited increasing trends. Overall, our study elucidated the potential lignin-decomposing microbes and pathways in river sediment and their responses to temperature increasing. Our study demonstrated that the temperature increasing can increase the rate of lignin/vanillin decomposition via affecting the activity of lignin-decomposing microbes. This finding indicates that the ongoing intensification of global warming may enhance the decomposition of recalcitrant organic carbon in river sediment, thereby impacting global carbon cycling.

The aim of this study is to explore the feasibility of applying bipolar membrane electrodialysis (BMED) to treat coal chemical waste salt with different pretreatments of organic pollutants. The organic content in the real waste salt reached 0.6 +/- 0.1 % with a weight averaged molecular weight of 641 g/mol. Only 26.5 % of the total COD in a 250 g/L waste salt solution could be removed by the coagulation pretreatment, resulting in no apparent improvement in BMED performance. Thermal pretreatment at 850 degrees C for 180 min completely removed organics from the waste salt, resulting in efficient control of membrane fouling in the BMED. At the current density of 50 mA/cm2 and 250 g/L waste salt solution, the base concentration in the BMED with 850 degrees C pretreatment of waste salt was higher than that with 550 degrees C pretreatment and without any pretreatment (58.2 +/- 1.0 vs. 56.0 +/- 2.4 and 53.6 +/- 2.0 g/L) within 180 min, respectively. The average current efficiency based on base productions with 850 degrees C pretreatment of the waste salt reached 91 +/- 5 % in the BMED within 180 min. Our results should be useful for the resource utilization of coal chemical waste salt.

In the Karst Critical Zone (KCZ), mining and urbanization activities produce multiple pollutants, posing a threat to the vital groundwater and surface water resources essential for drinking and irrigation. Despite their importance, the interactions between these pollutants in the intricate hydrology and land use of the KCZ remain poorly understood. In this study, we unraveled the transformation mechanisms and sources of nitrogen, sulfate, and carbon using multiple isotopes and the MixSIAR model, following hydrology and surface analyses conducted in spatial modelling with ArcGIS. Our results revealed frequent exchange between groundwater and surface water, as evidenced by the analysis of δD-H2O and δ18O-H2O. Nitrification predominantly occurred in surface water, although denitrification also made a minor contribution. Inorganic nitrogen in both groundwater and surface water primarily originated from soil nitrogen (48 % and 49 %, respectively). Sewage and manure were secondary sources of inorganic nitrogen in surface water, accounting for 41 % in urban and 38 % in mining areas. Notably, inorganic sulfur oxidation displayed significant spatial disparities between urban and mining areas, rendering groundwater more susceptible to sulfur pollution compared to surface water. The frequent interchange between groundwater and surface water posed a higher pollution risk to groundwater. Furthermore, the primary sources of CO2 and HCO3- in both groundwater and surface water were water‑carbonate reactions and soil respiration. Sulfide oxidation was found to enhance carbonate dissolution, leading to increased CO2 release from carbonate dissolution in the KCZ. These findings enhance our understanding of the transformation mechanisms and interactions of nitrogen, sulfur, and carbon in groundwater and surface water. This knowledge is invaluable for accurately controlling and treating water pollution in the KCZ.

Serious arsenic (As) contaminations could commonly result from the oxidative dissolution of As-containing sulfide minerals, such as arsenopyrite (FeAsS). Pyrite (Py) and calcite (Cal) are two typically co-existing reactive minerals and represent different geological scenarios. Previous studies have shown that a high proportion of Py can generate a stronger galvanic effect and acid dissolution, thereby significantly promoting the release of arsenic. However, this conclusion overlooks calcite's antagonistic effect on the release of As in the natural environment. That antagonistic effect could remodel the linear relationship of pyrite on the oxidative dissolution of arsenopyrite, thus altering the environmental risk of As. We examined As release from arsenopyrite along a gradient of Py to Cal molar ratios (Py:Cal). The results showed that the lowest As release from arsenopyrite was surprisingly found in co-existing Py and Cal systems than in the singular Cal system, let alone in the singular Py system. This phenomenon indicated an interesting possibility of Py assistance to Cal inhibition of As release, though Py has always been regarded as a booster, also evidenced in this research, for As release from arsenopyrite. In singular systems of Py and Cal, As continued to be released for 60 days. However, in co-existing Py and Cal systems, As was released non-linearly in three stages over time: initial release (0-1 Day), immobilization (1-15 Days), and subsequent re-release (>15 Days). This is a new short-term natural attenuation stage for As, but over time, this stage gradually collapses. During the re-release stage (> 15 Days), a higher molar ratio of Py:Cal (increasing from 1:9 to 9:1) results in a lower rate constant k (mgL-1h(-1)) of As release (range from 0.0011 to 0.0002), and a higher abundance of secondary minerals formed (up to 26 mg/g goethite and hematite at Py: Cal=9:1). This demonstrates that increasing the Py:Cal molar ratio results in the formation of more secondary minerals which compensate for the higher potential antagonistic mechanisms generated by pyrites, such as acid dissolution and galvanic effect. These results explain the mechanisms of the high-risk characteristics of As both in acidic mine drainage and karst aquifers and discover the lowest risk in pyrite and calcite co-existing regions. Moreover, we emphasize that reactive minerals are important variables that can't be ignored in predicting As pollution in the future.

High-nitrogen mining drainage (HNMD) is a significant source of watershed nitrogen pollution, influencing the nitrogen distribution in streams through various pathways, including surface runoff (HNMDs) and subsurface runoff (HNMDg). In this study, the nitrogen contributions of HNMDs and HNMDg were characterized by using water chemistry analysis, isotope analysis, and a Bayesian stable isotope mixing model. The combined effects of HNMDs, HNMDg, and domestic sewage (DS) were found to substantially impact nitrogen dynamics in the study area. On average, HNMDs and HNMDg contributed 60.5 ± 8.8% and 19.8 ± 12.5%, respectively, to riverine nitrogen. After accounting for the exclusion of DS, the dominance of HNMDs became more pronounced, contributing 67.0 ± 4.1% and 81.9 ± 0.1% of the HNMD nitrogen in the Chakeng and Caiyang Rivers, respectively. HNMDs and HNMDg displayed distinct nitrogen discharge behaviors within the watershed, which influenced the observed variations in nitrogen fluxes. Precipitation had a stronger influence on nitrogen discharge from HNMDs compared to HNMDg. Furthermore, NH4+-N from HNMD was more likely to enter streams via surface runoff, while HNMDg served as a critical and relatively stable source of nitrogen discharge.

A low dissolved oxygen (DO) concentration in summer has been observed in river–estuary systems worldwide. Many studies have caused our stereotype that biochemical oxygen depletion was higher in summer than in winter; however, there was no direct evidence particularly in the tidal river with complex hydrological and biochemical processes. This study employed natural-abundance and labeled isotopes to quantify seasonal apportionment of biochemical oxygen depletion. In this study, apparent oxygen utilization (AOU) and carbon (C) and nitrogen (N) turnover potentials (nitrification rates and δ13C signals) were higher in the wet season than in the dry season. However, calculation results of the nitrification flux demonstrated that actual N turnover was constrained by shorter river residence time in the wet season. Similarly, the δ13C end-member mixing and Rayleigh fractionation models revealed that the conservative C behavior was more pronounced than degradation in situ in the river channel. Overall, C- and N-driven oxygen depletion accounted only for ~8% of AOU in the wet season. This substantiated that the hydrological control regulated C and N behaviors to “the conservative transport” to mitigate O2 depletion in the wet season. In contrast, a good correspondence between C and N turnover and low oxygen was recorded in the dry season. Therefore, the “nutrient- and non-nutrient-constrained DO cold spots” during the dry and wet seasons provided new insights into oxygen deficits in tidal rivers. Our study provided compelling evidence that seasonal apportionment of C- and N-driven oxygen depletion in situ has changed in tidal rivers. Biochemical oxygen depletion was more evident in the dry season than in the wet season; thus, it had been previously overestimated in the wet season, which will provide implications for using different water management strategies in different seasons.

The sources and distribution characteristics of three phenolic endocrine-disrupting compounds (EDCs), e.g., alkylphenols (APs) (including nonylphenols (NPs) and 4-t-octylphenol (OP)) and Bisphenol A (BPA), were investigated in the rivers of the Pearl River Delta Region (PRDR) with complex land-use types. The mean concentrations of NPs, OP, and BPA in river water including wet and dry seasons were 87, 6, and 74 ng/L in the agricultural regions ( n = 10), 135, 7, and 61 ng/L in the transitional regions ( n = 8), and 249, 15, and 152 ng/L in the urban regions ( n = 28). Contents of NPs and BPA were high in the river sediments (ranged from 7 to 3048 ng/g and 2 to 271 ng/g, respectively). Equilibrium analysis results suggested that sediment release was not the main source of the river EDCs. Principal component analysis (PCA) showed that sewage was the major source of EDCs in the dry season, while the leaching effect of rainfall on the agricultural soils, urban roads, and commercial products was an important source in the wet season. Furthermore, the ratio of APs and total concentration of phenolic EDCs (ΣEDCs) was used to characterize the agricultural regions and urban regions in the PRDR. The ratio was less than 0.6 in the agricultural regions while the ratio was large than 0.6 in the dry season and less than 0.6 in the wet season in urban regions. BPA and NPs in transitional region and urban region had small/medium potential risk to aquatic organisms.

In closed mines,the rebounded and risen groundwater level and the changed groundwater level hydrodynamics influenced the evolution of groundwater hydrochemistry,leading to serious problems in groundwater environment.In this research,the SLW Mine,a typical closed mine in Qingyuan,Guangdong province was selected as the study area to elucidate the hydrochemical characteristics and origins of the closed mine,using a field survey,multivariate statistical analysis,stoichiometry analysis and isotope tracing technology.The results showed that （1） the hydrogen and oxygen isotope composition (δ 18 O and δD) analysis indicated that groundwater and mine drainage both had precipitation origins and a close hydraulic connection between groundwater and mine drainage was also found due to similar δ 18 O and δD signals,which was also approved by multivariate statistical analysis.（2） There were significant differences between background water,groundwater and mine drainage hydrochemic.The TDS values ranged from44.18 to 138.86 mg·L -1 in background water,from 43.39 to 6 917.6 mg·L -1 in the groundwater,and from 3 329.22 to 4 174.12 mg·L -1 in the mine drainage;while,their p H values ranged from 4.39 to 8.2,from 2.75 to 7.9,and from 2.87 to 2.92,respectively.The hydrochemical patterns of background water,groundwater and AMD were Ca-Mg-HCO 3 ,Ca-SO 4 -HCO 3 ,and Ca-SO 4 .（3） The stoichiometric analysis showed that water-rock interaction was the main factor in controlling the evolution of hydrochemistry in the study area.In the background area,the weathering of silicate rock was the main controlling process.For groundwater in the mining area,the water-rock interaction was strengthened,and the neutral or slightly alkaline groundwater was formed by sulfide oxidation and the weathering of silicate and carbonate rocks.The dissolution of sulfide minerals was the main process forming typical acid mine drainage with low p H and high SO 4 2- .This study revealed the hydrochemical characteristics and main controlling factors of a closed mine,which is highly important for heavy metal pollution prevention and control in regional water environment.

The evaluation of groundwater environmental quality and the identification of recharge sources are very important for groundwater utilization. In this study, hydrochemistry and isotope analysis methods are used to investigate the recharge sources and hydrochemical processes of groundwater in Zhanjiang City. The results show that all samples of groundwater were drawn on the left of the global meteoric water line (GMWL: δD = 8δ18O + 10) and local meteorological water lines (LMWL1: δD = 8.17δ18O + 11.74 and LMWL2: δD = 7.50δ18O + 6.18), indicating that the groundwater was mainly recharged by meteoric precipitation and influenced by the effect of evaporation. In the middle and deep confined aquifers, the isotope data depleted with the depth, indicating that there is a relatively weak hydraulic connection between them. In addition, compared with unconfined groundwater, the isotope data of confined groundwater showed relative depletion, indicating that the confined aquifer may be partially recharged from other confined aquifers. The main chemical types in the groundwater were Na*Ca-HCO3. There are three major natural hydrochemical processes controlling the source of groundwater ions: silicate weathering, carbonate dissolution, and the cation exchange reaction. In addition, the differences in physical and chemical properties between unconfined groundwater and confined groundwater are significant. Due to the differences in anthropogenic activities and land-use types, the nitrate of the unconfined groundwater exceeds the groundwater standards. Due to the geological background of Zhanjiang City, iron and manganese exceed the third standard of groundwater in confined groundwater. Due to groundwater exploitation, TDS levels in confined groundwater have been increasing. Closed groundwater extraction is not sustainable, and it is depleting ancient water reserves. This study highlights the effectiveness of hydrochemistry and isotope analysis methods for identifying the recharge area and recharge mode of groundwater, andit is significant for fully understanding groundwater hydrochemistry and scientifically managing and protecting groundwater.

Elucidating the sources of particulate organic matter (POM) is the foundation for understanding their fates and the sea-sonal variation of their movement from the land-to-ocean aquatic continuum (LOAC). The POM from different sources has different reactivity, which determines their fates. However, the key link between the sources and fates of POM, es-pecially in the complex land use watersheds in bays is still unclear. Stable isotopes and contents of organic carbon and nitrogen were applied to reveal them in a complex land use watershed with different gross domestic production (GDP) in a typical Bay, China. Our results showed that the POMs preserved in suspended particulate organic matter (SPM) were weakly controlled by assimilation and decomposition in the main channels. Source apportionments of SPM in the rural area were controlled by soil (46 % -80 %), especially inert soils eroded from land to water due to precipi-tation. The contribution of phytoplankton resulted from slower water velocity and longer residence time in the rural area. The soil (47 % -78 %) and manure and sewage (10 % -34 %) were the two major contributors to SOMs in the developed and developing urban areas. The manure and sewage were important sources of active POM in the ur-banization of different LUI, which showed discrepancies in the three urban areas (10 % -34 %). Due to soil erosion and the most intensive industry supported by GDP, the soil (45 % -47 %) and industrial wastewater (24 % -43 %) were the two major contributors to SOMs in the industrial urban area. This study demonstrated the close relationship between the sources and fates of POM with complex land use patterns, which could reduce uncertainties in future es-timates of the LOAC fluxes and secure ecological and environmental barriers in a bay area.

Carbonate aquifers are critical for the health and well-being of human communities. Despite considerable efforts on the structure of karst aquifers, the variable and invisible flow paths or the vertical and spatial heterogeneity of karst systems remain too complex to be fully understood. An artificially constructed small limestone aquifer with an extensive network of observation wells in the Miyako Island, Japan, provides a unique opportunity to illustrate the vertical and spatial hydrological connectivity for water movement and storage. Water chemical data at two different depths over six years, including electrical conductivity (EC), dissolved oxygen (DO), oxidation-reduction potential (ORP), major ions (Na+, K+, Ca2+, Mg2+, SO42-, and HCO3- ), 818O and 82H capture water mixing processes during groundwater recharge in the aquifer influenced by irrigation pumping and recharge by precipitations throughout drying and wetting cycles. Results suggest that the intensive weathering of coral limestone under the impact of agricultural fertilization, as indicated by high EC values and high concentrations of Ca2+ and HCO3- . The high DO and low ORP values imply a lack of organic decomposition activity, which is typical in the karst groundwater condition. More importantly, groundwater recharge in the small aquifer was found to be influenced by following three processes. Spatially, both Cl- and NO3- -N indicate the large heterogeneity of water mixing as rainfall or drainage water percolates differently to various parts of the aquifer. Vertically, there are apparent differences in the concentration of Cl- and NO3- -N between the top and bottom layers in several wells, suggesting that, even within a single well, the vertical profile can be weakly connected between the top and the bottom layers. The bottom layer should have been recharged by lateral water inflow from fractures or conduits. And thirdly, the even distribution of Cl- over the long precipitation season from October 2016 and March 2021 and another nearly even distribution of NO3- -N in the low fertilization season in March 2021, demonstrate that the aquifer system is basically connected and porous. In summary, the insights gained from our study reveal that: 1) coral limestone can be treated as a well-developed porous aquifer and 2) the hydrological connectivity is highly heterogeneous and influenced by rock properties, including formation structure, degree of weathering, and clay content. Such findings are essential for understanding the hydrogeological properties of coral limestone aquifers globally.

Compound-specific stable isotope analysis (CSIA) is an efficient method for source apportionment and the identification of the transformation process in organic compounds. However, most studies of CSIA are still limited to laboratory experiments. Few studies used have CSIA in an in situ environment due to the complexity of environmental samples. Therefore, a purification method for analyzing the carbon isotope ratios of three phenolic endocrine disrupting compounds (EDCs) (nonylphenols (NPs), octylphenol (OP), and bisphenol A(BPA)) in sediment and water samples was developed in this study. The silica gel column was used to isolate EDCs from complex matrices with multiple organic solvents. Gas chromatography/mass spectrometry was used to quantify the targeted EDCs and analyze the purity of the extracts in full-scan mode. The interfering peaks disappeared, the baseline was sharply reduced, and all the target compounds appeared as single peaks in the chromatogram after purification. Analyzing the standard samples with known isotope ratios showed that the purification treatment did not cause isotope fractionation. The isotopic difference before and after purification was less than 0.04. The method was successfully used to analyze the isotope composition of BPA, OP, and NPs in river water and sediments in the Guangzhou River, Pearl River Delta, South China. Sewage discharge significantly affected the carbon isotope values of BPA, OP and NPs in Guangzhou rivers, suggesting that sewage discharge is the main source of EDCs in the Guangzhou rivers. There is a significant correlation between the isotopic values and concentrations of OP and NPs in sediments, indicating that they may undergo chemical transformation.