Zhen Wang

Coastal groundwater systems are highly susceptible to seawater intrusion and salinization. However, complex depositional settings and multiple geochemical processes have made it difficult to understand the mechanism of groundwater salinization. To characterize coastal groundwater salinization and elucidate its controlling mechanisms in Tianjin, China, a total of 228 groundwater samples were collected. The degree of groundwater salinization and associated hydrochemical evolution was systematically investigated using the groundwater quality index for seawater intrusion (GQI _SWI ) and some hydrochemical indicators and evolutionary diagrams. Hydrochemical analyses revealed an overall evolution from HCO₃‐Ca type to Cl‐Na type groundwater, with distinct salinization geochemical evolution pathways among different aquifers. The GQI _SWI assessment indicated that both shallow and deep groundwater in Tianjin are dominated by fresh and brackish waters, with only minor differences in the overall degree of salinization along the vertical profile. Salinization of shallow groundwater is mainly concentrated in coastal areas and generally decreases progressively from the coastline toward inland areas. In contrast, the spatial distribution of salinity in deep groundwater is characterized primarily by localized high‐salinity anomalies, without the development of a continuous salinization zone. Groundwater salinization in Tianjin therefore cannot be attributed solely to modern seawater intrusion; rather, it is jointly controlled by modern coastal salt inputs, freshwater recharge, long‐term water–rock interactions, and ion exchange. These findings provide new insights into the identification of seawater intrusion and the interpretation of salinization mechanisms in complex coastal groundwater systems and offer a scientific basis for groundwater resource protection and management.

Gadolinium (Gd), a typical rare earth element (REE), exhibits high toxicity in its free ionic form. Chelated Gd complexes are widely applied as contrast agents for safe clinical magnetic resonance imaging (MRI). However, the increasing use of these highly stable Gd complexes leads to release of anthropogenic Gd (GdAnth) into aquatic environments, because conventional wastewater treatment plants (WWTPs) remove them inefficiently. Herein, 52 surface water samples were collected from five WWTPs and their receiving rivers in the urban area of Nanchang city, to investigate the concentration and downstream transport of Gd by combining REE-normalization with analyses of MRI device distribution and population density. Results showed distinct positive Gd anomalies in all samples with Gd/Gd* values of 1.35 to 136.55, exceeding the anthropogenic threshold of 1.4. GdAnth concentrations ranged from 2.29 to 364.05 ng/L with higher value in WWTPs effluents. Gd concentrations were positively correlated with the number of MRI devices within the corresponding WWTP service areas, indicating that medical wastewater discharge was the primary source for GdAnth. Proxies of Gd/Gd* and Eu/Eu* served as robust indicators for predicting Gd concentrations. Site-level population density provided negligible linear explanatory power for Gd concentration, whereas downstream distance reflected the attenuation of Gd along the WWTP-affected flow paths. Along the WWTP effluent flow paths, Gd concentrations declined exponentially downstream with varied attenuation coefficients. Nevertheless, the mean Gd/Gd* value remained 2.28 after dilution in the Ganjiang River, indicating the long-distance persistence of GdAnth, which raises environmental and health concerns for Poyang Lake, China’s largest freshwater lake.

Elevated uranium concentrations in acid mine drainage (AMD) and AMD-impacted waters pose a significant environmental concern worldwide. Schwertmannite and basaluminite are common Fe- and Al-oxyhydroxysulfate minerals that spontaneously precipitate during AMD neutralization when acidic waters mix with freshwater in estuarine environments. Although hexavalent uranium [U(VI)] is known to exhibit strong affinity for schwertmannite and basaluminite, the sorption behavior and underlying mechanisms of U(VI) on their surfaces remain insufficiently understood. In this study, U(VI) sorption onto these minerals was investigated over a pH range of 2.5–8.5, sulfate concentrations of 0–1000 mg/L, initial U(VI) concentrations of 1–10 mg/L, and solid-to-liquid ratios of 0.5–3 using batch adsorption experiments combined with XRD, SEM-EDS, FTIR, and XPS analyses. Results showed that basaluminite exhibited a higher affinity for U(VI) than schwertmannite. U(VI) sorption was strongly pH-dependent, with maximum removal occurring at pH 4.5. At an initial U(VI) concentration of 2 mg/L, both minerals achieved complete U(VI) removal within 10–12 h. Sulfate exerted a concentration-dependent influence on U(VI) sorption. At low sulfate concentrations (<2 mg/L), sulfate temporarily enhanced U(VI) adsorption onto schwertmannite through coordination with surface ≡Fe–OH groups and synergistic interactions with structural sulfate ions within the mineral tunnels. In contrast, sulfate consistently inhibited U(VI) adsorption onto basaluminite, and the inhibitory effect became more pronounced with increasing sulfate concentration. Despite the presence of sulfate, U(VI) adsorption on both minerals was well described by the pseudo-second-order kinetic model. Adsorption isotherm analyses revealed that when sulfate concentrations reached or exceeded 500 mg/L, U(VI) adsorption onto basaluminite gradually shifted from Langmuir-type monolayer adsorption to Freundlich-type heterogeneous adsorption. In contrast, U(VI) adsorption onto schwertmannite remained better described by the Langmuir model across all tested sulfate concentrations. These findings would provide new insights into predicting uranium mobility and fate in AMD-impacted aquatic environments.

Cangzhou City on the western coast of the Bohai Sea, a typical coastal plain, was investigated using major ion data from 123 groundwater samples (63 shallow and 60 deep). The Hydrochemical Facies Evolution Diagram (HFE‐D) and the Groundwater Quality Index for Seawater Intrusion (GQI _SWI ) were applied to assess seawater intrusion and its hydrochemical evolution. Groundwater is predominantly brackish, with salinity decreasing inland from the coast, and seawater influence is markedly stronger in the shallow aquifer than in the deep confined aquifer. Compared with the single Cl ⁻ indicator, GQI _SWI reduces the impact of local anomalies and more reliably reflects the regional distribution of intrusion intensity. Hydrochemical facies evolve from Ca‐HCO ₃ to Ca‐Cl and then to Na‐Cl, and HFE‐D indicates that salinization results from freshwater–seawater mixing combined with reverse cation exchange and mineral dissolution rather than simple mixing alone. The shallow aquifer acts as the main zone of salt migration and transformation, whereas deep groundwater chemistry is controlled primarily by long‐term water–rock interaction. The combined application of GQI _SWI and hydrochemical evolution analysis identifies seawater intrusion from both quantitative and process perspectives and provides a scientific basis for groundwater management and zoning in coastal plains.

High-fluoride (F⁻) groundwater is a widespread environmental problem that poses significant risks to human health in many regions worldwide. Understanding the origin, circulation, and evolution of fluoride-rich groundwater is therefore essential for effective groundwater management and mitigation strategies. In recent years, stable isotope techniques have helped to address key gaps in understanding the hydrogeochemical processes governing F⁻ enrichment, particularly regarding the source identification and water-rock interaction mechanisms that remain poorly constrained. This study reviews the applications of hydrogen–oxygen, strontium–calcium, and lithium–boron isotopes in research on high-F⁻ groundwater systems. Hydrogen and oxygen isotopes (δ²H and δ¹⁸O) are widely used to identify groundwater recharge sources, mixing processes, and evaporative effects, thereby providing key constraints on the origin of fluoride-rich groundwater. Strontium and calcium isotopes (⁸⁷Sr/⁸⁶Sr and δ^44/40Ca) serve as effective tracers of water-rock interactions and associated hydrogeochemical processes, including mineral weathering and dissolution, cation exchange, and secondary mineral precipitation, which play critical roles in fluoride mobilization and enrichment. In addition, lithium, and boron isotopes (δ⁷Li and δ¹¹B) provide valuable insights into the influence of geothermal fluids and deep hydrothermal processes on fluoride accumulation in groundwater systems. Overall, the integrated application of these stable isotope systems offers a robust framework for elucidating the formation mechanisms and evolutionary pathways of high-F⁻ groundwater. Moving beyond qualitative source identification, future research should prioritize the development of Bayesian isotope mixing models that explicitly quantify uncertainty in fluoride source apportionment and utilize sensitivity analysis to test competing hydrogeochemical mechanisms.

Groundwater nitrate pollution in freshwater lake basins threatens ecosystems and human health, yet its sources and drivers remain poorly quantified. This study investigates the origin of nitrate contamination, hydrogeochemical controls, and its associated human health risks in groundwater hosted in the sub-basins of five rivers draining the Poyang Lake, China, via hydrogeochemical analysis and modeling approach. Results showed that 52% of groundwater samples exceeded the background value of nitrate, with the highest nitrate exceedance rate observed in the sub-basin of the Rao River, followed by Xin, Fu, Xiu, and Gan Rivers. High-nitrate groundwater primarily occurred as Cl-Ca and Cl-Na water types, while low-nitrate waters were of HCO3-Ca and HCO3-Na. Nitrate sources and transformations were strongly affected by nitrification, especially in areas of artificial surfaces and farmland. Manure/sewage (M&S, 46.9%) and soil nitrogen (SN, 31.4%) were quantified as the dominant nitrate sources, while nitrogen fertilizers (NF, 17%) and atmospheric deposition (AD, 4.8%) made relatively minor contributions. Reconstructedinitial isotopic signatures showed lower δ15N and δ18O than measured values, yet still were within the manure and sewage isotopic ranges, reinforcing identified sources. Variations of uncertainty in nitrate sources tracing process were revealed by UI90, with SN (average 0.67), M&S (average 0.62), CF (average 0.54), and AD (average 0.11), respectively. Infants and children were predicted to be more vulnerable to nitrate exposure, with hazard quotient (HQ) values exceeding 1 in 54% and 46% of cases, respectively. Spatially, groundwater from the sub-basins of Rao and Xin exhibited the highest contamination levels, while the sub-basin of Fu River showed the lowest. This study presents a novel integrated assessment approach to simultaneously investigate the source of groundwater nitrate in the sub-basins of five rivers and the spatial heterogeneity of health risk. These findings underscore a need for targeted mitigation strategies to protect freshwater lake basins globally.

Groundwater nitrate pollution in freshwater lake basins threatens ecosystems and human health, yet its sources and drivers remain poorly quantified. This study investigates the nitrate sources variation, geochemical characteristics of nitrate-contaminated groundwater, and its associated human health risks in the sub-basins of five rivers draining the Poyang Lake, China, via hydrogeochemical analysis and integrated assessment approach. Results showed that 52% of groundwater samples exceeded the background value of nitrate. High-nitrate groundwater primarily occurred as Cl-Ca and Cl-Na water types, while low-nitrate waters were of HCO3-Ca and HCO3-Na. Nitrate sources and transformations were strongly affected by nitrification, were closely associated with artificial surfaces and farmland Manure/sewage (M&S, 46.9%) and soil nitrogen (SN, 31.4%) were quantified as the dominant nitrate sources, while nitrogen fertilizers (NF, 17%) and atmospheric deposition (AD, 4.8%) made relatively minor contributions. Reconstructedinitial isotopic signatures showed lower δ15N and δ18O than measured values, yet still were within the manure and sewage isotopic ranges, reinforcing identified sources. Variations of uncertainty in nitrate sources tracing process were revealed by UI90, with SN (average 0.67), M&S (average 0.62), CF (average 0.54), and AD (average 0.11), respectively. Infants and children were predicted to be more vulnerable to nitrate exposure, with hazard quotient (HQ) values exceeding 1 in 54% and 46% of cases, respectively. Spatially, groundwater from the sub-basins of Rao and Xin rivers exhibited the highest contamination levels, while the sub-basin of Fu River showed the lowest. These findings underscore a need for targeted mitigation strategies to protect freshwater lake basins globally.

Geothermal systems are typically characterized by high-temperature and high-fluoride properties, increasing attention is focused on their environment impacts, yet understanding of fluoride enrichment mechanisms in the Qiabuqia geothermal system (Gonghe Basin, Qinghai, China) remains limited. This study collected 39 geothermal water samples from the Qiabuqia Geothermal Field. Hydrogeochemical and isotopic analyses were conducted to investigate these mechanisms, providing fundamental insights into the geochemical processes governing fluorine distribution patterns. Shallow geothermal waters (GWQ) exhibited a Cl–SO₄·Na hydrochemical type, with a mean fluoride concentration of 0.9 mg/L and an exceedance rate of 25.7%. In contrast, deep geothermal waters (GWN) were predominantly characterized by a Cl–HCO₃·Na hydrochemical type, showing significantly elevated fluoride levels (average 4.2 mg/L) with a 100% exceedance rate against the WHO drinking water standard (1.5 mg/L). Hydrogen (δD) and oxygen (δ¹⁸O) isotopic compositions of GWQ cluster along the Global Meteoric Water Line (GMWL), indicating their predominant recharge sources originate from surface water and atmospheric precipitation. The GWN exhibited significant hydrogen–oxygen isotope anomalies characterized by δ¹⁸O enrichment, indicating their recharge sources originate from both vertical leakage of adjacent aquifers and long-distance groundwater migration through regional deep-seated fault systems that serve as dominant conduits for deep hydrothermal circulation. Fluoride enrichment is governed by a combination of thermal controls, mineral dissolution–precipitation equilibria, cation exchange processes, and competitive adsorption mechanisms, reflecting the coupled physicochemical interactions within the hydrothermal system. Quantitative assessment leveraging the Random Forest (RF) algorithm revealed the relative contributions to enrichment: mineral dissolution–precipitation (38%), cation exchange processes (29%), thermal regime (25%), and competitive adsorption (7%), based on feature importance analysis of the hydrogeochemical dataset. The hazard quotient (HQ) assessment for geothermal water exposure revealed distinct risk gradients across population groups: infants exhibited the highest potential health risk at 47%, followed by children (36%), adult males (32%), and adult females (24%), based on the USEPA exposure model accounting for body weight differentials and ingestion rates. Findings not only advance our understanding of this field, but also provide a scientific basis for the rational exploitation of geothermal energy and environmental protection. Graphical Abstract

Estimation of groundwater resources in plateau continental climate area is difficult due to the minimal groundwater recharge, high evaporation and frequent changes throughout the year. The renewal rate of aquifers can be calculated by constructing a model based on the tritium isotope data of atmospheric precipitation and groundwater. In this study, hydrogeochemical and isotopic methods were employed to study the hydrochemical composition and groundwater renewal processes in unconfined aquifers in Guide Basin. Results indicate that the phreatic water is mainly of the HCO₃–Ca–Na type, with chemical components originating mainly from the silicate weathering and carbonate dissolution in the aquifer. Isotopic analysis (δD and δ¹⁸O) shows that atmospheric precipitation is the main source of recharge for water bodies in the basin. Tritium content variations with depth indicate that groundwater circulation depth affects tritium decay processes. Groundwater age calculations based on tritium content in atmospheric precipitation and groundwater suggest that the hydrolyzed products of silicate gradually entered the groundwater with the increase of phreatic water runoff time. Contour maps of renewal rate and age of groundwater indicate that close hydraulic connection from river water brings the unconfined aquifer fine renewal capacity and a relatively younger age. The research findings not only establish a theoretical basis for groundwater resource management and sustainable development in Guide Basin, but also serve as a reference for the calculating the renewable capacity of groundwater resources in other basins.