Hydrogeochemical evidence for fluoride behavior in groundwater and the associated risk to human health for a large irrigation plain in the Yellow River Basin

Biogeochemical Mechanisms of HCO3-Ca Water and NO3- Pollution in a Typical Piedmont Agricultural Area: Insights from Nitrification and Carbonate Weathering

Water hardening and NO3- pollution have affected water quality globally. These environmental problems threaten social sustainability and human health, especially in piedmont agricultural areas. The aim of this study is to determine the biogeochemical mechanisms of HCO3-Ca water and NO3- pollution in a typical piedmont agricultural area (Qingshui River, Zhangjiakou, China). Here, an extensive biogeochemical investigation was conducted in a typical piedmont agricultural area (Qingshui River, China) using multiple hydrochemical, isotopic (δ2H-H2O, δ18O-H2O and δ13C-DIC) and molecular-biological proxies in combination with a forward model. In the region upstream of the Qingshui River, riverine hydrochemistry was dominated by HCO3-Ca water, with only NO3- concentrations (3.08-52.8 mg/L) exceeding the acceptable limit (10 mg/L as N) for drinking water quality. The riverine hydrochemistry responsible for the formation of HCO3-Ca water was mainly driven by carbonate dissolution, with a contribution rate of 49.8 ± 3.96%. Riverine NO3- was mainly derived from agricultural NH4+ emissions rather than NO3- emissions, originating from sources such as manure, domestic sewage, soil nitrogen and NH4+-synthetic fertilizer. Under the rapid hydrodynamic conditions and aerobic water environment of the piedmont area, NH4+-containing pollutants were converted to HNO3 by nitrifying bacteria (e.g., Flavobacterium and Fluviimonas). Carbonate (especially calcite) was preferentially and rapidly dissolved by the produced HNO3, which was attributed to the strong acidity of HNO3. Therefore, higher levels of Ca2+, Mg2+, HCO3- and NO3- were simultaneously released into river water, causing riverine HCO3-Ca water and NO3- pollution in the A-RW. In contrast, these biogeochemical mechanisms did not occur significantly in the downstream region of the river due to the cement-hardened river channels and strict discharge management. These findings highlight the influence of agricultural HNO3 on HCO3-Ca water and NO3- pollution in the Qingshui River and further improve the understanding of riverine hydrochemical evolution and water pollution in piedmont agricultural areas.

Two-dimensional Monte Carlo simulation coupled with multilinear regression modeling of source-specific health risks from groundwater

Effective protection of groundwater requires an accurate health risk assessment of contaminants; however, the diversity of pollution sources, variability, and uncertainties in exposure parameters present significant challenges in this assessment. In this study, groundwater risk estimates associated with NO3-, and F-, along with fourteen heavy metal(loid)s (V, Cr, Mn, Fe, Ni, Cu, As, Co, Cd, Se, Pb, Hg, Zn, and Al) in an agricultural area were optimized by implementing positive matrix factorization (PMF), multilinear regression, and two-dimensional Monte Carlo simulations to characterize source-specific health risks. Groundwater pollution was analyzed considering regional variations, including differences in elevation, land use and land cover, and soil types. Three pollution sources were identified: agricultural practices, traffic, and natural processes. Moreover, the results revealed NO3- from an agricultural source as the primary control contaminant. Additionally, both adults and children in the study area face significant non-carcinogenic health risks. To mitigate these risks, this study recommends maximum consumption levels of 1.44 L/day for adults and 0.35 L/day for children. Furthermore, adults weighing > 68.1 kg and children weighing > 15.9 kg are likely to be at reduced risk of experiencing adverse health effects. Compared to deterministic health risk assessment and one-dimensional Monte Carlo simulation of health risks, two-dimensional Monte Carlo simulation showed improved performance, providing better accuracy and higher precision in health risk assessment results. Thus, this research is expected to enhance the understanding of health risk assessment related to groundwater and to provide valuable guidance for managing groundwater pollution.

Multi-isotope tracer for identifying nitrate sources in shallow groundwater in a large irrigation area, China

Nitrate pollution of groundwater associated with anthropogenic activities and social development is a global issue, especially in agricultural regions. This study investigated the source and fate of nitrate in groundwater of the Jiaokou Irrigation Area using chemical and isotopic fingerprints (isotope tracing, graphical analysis, and d-excess) in combination with end-member mixing analysis (EMMA). The hydrochemical analysis revealed that the NO3- concentration exceeds 20 mg/L (calculated as N) in approximately 66.67% of groundwater samples. The highest NO3- concentration was observed in Cl·SO4 type water, followed by the HCO3 and SO4 types. Areas with serious nitrate pollution are mainly distributed in cultivated land. Groundwater mineralization is driven by silicate weathering and evaporite dissolution. Ion exchange and mixing processes are also major factors affecting the chemical composition of groundwater in the study area. The largest contributors to NO3-N in groundwater were found to be manure and sewage input, followed by soil N (SON) and NH4-N fertilizer (NF) application. The high nitrogen content in groundwater is mainly affected by mixing and nitrification processes. Interestingly, although evaporation is the main factor affecting groundwater mineralization in the study area, it has little effect on the high nitrate concentration in groundwater. Based on the results, a conceptual model was constructed to describe the nitrogen cycle in the irrigation area. The findings help clarify the mechanism of nitrate pollution and deepen our understanding of the role of nitrogen cycling in the soil-water-gas system.

Occurrence and enrichment mechanisms of groundwater hexavalent chromium in typical loess area of China

Understanding the geochemical mechanisms governing hexavalent chromium (Cr(VI)) in groundwater is essential for mitigating health risks. However, the processes driving Cr(VI) accumulation and migration in loess regions remain insufficiently understood. This study investigated the occurrence, release, and migration mechanisms of Cr(VI) across different groundwater environmental units (GEUs) in the south-central Loess Plateau, China. This study used combined approach of isotopic analysis, multivariate statistical methods, hydrochemical graphical methods, and GIS technology to reveal the origins and processes influencing Cr(VI) hydrogeochemistry within these GEUs. The results revealed significant spatial variability in Cr(VI) concentrations among the GEUs, ranging from below the detection limit to 300 μg/L, with nearly 40% of samples exceeding the WHO limit. Pronounced enrichment of Cr(VI) was observed in the fissure-pore water of the loess tableland and pore water of the alluvial plain. Cr(VI) enrichment and release in the GEUs were facilitated by oxidative conditions (high Eh, SO42-/HCO3-, Mn-oxide presence) and cation exchange processes under slightly alkaline conditions (pH > 7.80). Key hydrogeological processes and geomorphological factors, including lateral runoff recharge, slow groundwater flow in the loess tableland, vertical recharge, extensive water-rock interactions, and hydraulic gradients were identified as critical divers of Cr(VI) migration and enrichment across different GEUs. Under reductive conditions, Cr(VI) was reduced to Cr(III), particularly in the pore water of the alluvial plain, but competitive adsorption with nitrate allows the enrichment of Cr(VI) in groundwater, particularly in the fissure-pore aquifer. A conceptual model was developed to elucidate Cr(VI) sources and migration mechanisms in groundwater, offering a framework for risk mitigation and management of groundwater in loess regions.

Hydrochemistry characteristics and genesis of shallow groundwater in diverse industrial agglomeration areas in typical alluvial plain of the Yellow River

The groundwater environment in the middle and upper reaches of the Yellow River in China has attracted extensive attention. But, the hydrochemistry characteristics and genesis of shallow groundwater in diverse industrial agglomeration areas in typical alluvial plain of the Yellow River are still unknown. In this study, geochemical analysis methods, the positive matrix factorisation model, and the geodetector model were used to multidimensional analyze. The results showed that 78 % of the groundwater samples had high-alkalinity-risk and high-salinity-risk, and the water quality was poor and unsuitable for irrigation. "Very poor" and "undrinkable" samples accounted for 26.31 %. The weathering of calcium magnesium minerals and the dissolution of soluble sulfate were important sources of the main chemical components of the groundwater. However, evaporation and concentration gradually dominated from the inclined plain of the piedmont to the alluvial plain of the Yellow River. Domestic and agricultural activities were important sources of nitrogen species (NO3-, NO2-, and NH4+). The presence of Al, Mn, Zn, and Pb was caused by the production activities of various enterprises. The pollution contributors of natural sources, domestic and agricultural activities, industrial production and other sources were 29.49 %, 17.05 %, 31.41 %, and 22.05 %, respectively. Groundwater velocity and enterprise type were the main factors driving groundwater pollution by these four metal substances. The interaction between enterprise type and groundwater velocity had the greatest influence on the concentrations of Al, Mn, and Zn. Moreover, the interaction between enterprise type and the distance from enterprise had the greatest influence on Pb levels. Overall, groundwater pollution in the study area was the result of both natural and human factors. The findings of this study can offer valuable insights and theoretical support for preventing groundwater pollution in other industrial agglomeration areas in the alluvial plain of the Yellow River.

A multi-perspective exploration of the salinization mechanisms of groundwater in the Guanzhong Basin, China

A comprehensive understanding of the salinization of groundwater in the Guanzhong Basin, China, is crucial for ensuring sustainable groundwater development. However, the mechanism driving salinization in different regions of the basin remains unclear. Therefore, this study employed multivariate statistical methods, hydrochemical analysis, isotope studies, and hydrochemical modeling to uncover the factors and processes influencing groundwater salinization. The results indicate significant regional variations in total dissolved solids (TDS), with concentrations exceeding 1000 mg/L predominantly occurring to the north of the Weihe River and the east of the Jinghe River. The correlations of groundwater chloride (Cl-) with Cl/Br molar ratio and stable isotopes show that groundwater salinity in the Guanzhong Basin is mainly controlled by mineral dissolution, and evaporation. In addition, human activities, such as vertical irrigation recharge and excessive fertilizer use, exacerbate local salinity levels. Irrigation activities worsen the shallow groundwater salt enrichment in the runoff zone of the central basin, revealed by the high salinity (TDS>3000 mg/L), high Cl/Br ratios (>2000), moderate δ2H (-57.5 to -67.5 ‰) and moderate δ18O (-8.1 to -8.9 ‰). High salinity (TDS>1000 mg/L), high nitrate concentration (>100 mg/L), and moderate Cl- (100 to 500 mg/L) indicate the impact of excessive fertilizer use. It is worth noting that intensive groundwater withdrawal disrupts the dynamic balance within the aquifer, causing shallow high-saline groundwater to percolate downward, thereby increasing the risk of deep groundwater pollution. The research enhances the understanding of groundwater salinity transport and provides insights into the effects of groundwater salinization in the irrigation area.

Enrichment mechanism and probabilistic health risk assessment of high-fluoride groundwater in Gaomi City, China

Fluoride (F) is the most important inorganic pollutant in groundwater that affects human health, and analyzing the causes of high-fluoride groundwater is a prerequisite for protecting the health of residents. To comprehensively understand the enrichment characteristics of groundwater in the high-fluoride areas, this study systematically investigated the concentrations of fluoride in Gaomi City, a typical study area in the Jiaolai Plain and explored the spatiotemporal distribution patterns, enrichment mechanisms, and the probabilistic health risk associated with F-. The results indicate that there is serious fluorine pollution in groundwater, which is mainly concentrated in the alluvial plain in the north and affected by topographical and aquifer characteristics. Favorable runoff conditions effectively improve the fluoride status of shallow groundwater on both sides of rivers and in hilly areas. Hydrogeochemical methods reveal the mechanism of fluoride enrichment. The relative contributions rates of different hydrogeochemical processes to the fluoride enrichment are as follows: dissolution and precipitation (39.02%) > cation exchange (25.25%) > competitive adsorption (19.48%) > seawater intrusion (3.14%) > evaporative and concentration (1.99%). Health risk assessment based on Monte Carlo simulation shows that health risk susceptibilities of different populations are infants (76.07%), children (66.59%), teenagers (44.54%), and adults (5.68%), respectively. In addition, targeted management suggestions are put forward regarding the enrichment mechanisms of fluoride in groundwater and its impact on health. These findings have significant implications for controlling regional diffuse F- contamination in groundwater, protecting public health, and promoting social development in regions with a high risk of groundwater fluoride contamination.

Geogenic fluorine-contaminated groundwater increases fluorosis risk in communities of northern cold regions: Genesis mechanism and exposure pathways

Geogenic fluorine-contaminated groundwater (F >1 mg/L) prevails in cold Mollisol regions of the world. Seasonal variation of F concentration in groundwater likely renders multiple pathways of toxic-level F exposure, posing unrecognized health risk to many economically challenged communities. Herein, different types of samples within the groundwater-soil-crop-human hair network were collected from the Mollisol regions of northeastern China and assessed by joint approach of medical geochemical assay, hydrogeochemical modeling, and health risk indexation. The results unravel that infiltration of dissolved organic matter from Mollisols induced by vertical infiltration led to seasonal variation of F concentration and speciation in groundwater. This is attributable primarily to biogenic dissolution-precipitation equilibria of Ca-containing minerals and altered hydrochemical types of groundwater over season, causing dynamic F- partitioning between water and minerals in aquifers. Further risk assessment suggests greater adverse effects of groundwater use on human health in summer than in autumn. Especially, two major pathways of F exposure, i.e., groundwater drinking-human and groundwater irrigation-soil-crop-human, accounted for 57.3 % of F accumulation in hair of local residents. This research highlights the underestimated health impacts of seasonal variability of F in exploited groundwater and the urgency of groundwater quality management to prevent endemic fluorosis in communities of cold regions.

Fluoride and nitrogen contamination and potential health risks in the groundwater of a typical agricultural region

Fluoride and nitrogen contamination is a global concern and has been a serious problem in agricultural areas. This study aims to identify the source of fluoride and nitrogen in the groundwater and assess groundwater quality and human health risks in the Guanzhong Plain, northwest China. The results showed that the concentrations were 0.15-4.74 mg/L for F-, 0.02-89.89 mg/L for NO3--N, and BDL-2.40 mg/L for NH4+-N in groundwater. Distinct area-dependent distributions of fluoride and nitrogen were observed in the study region. Higher F- and NO3--N concentrations in groundwater were detected in the northern part, and higher NH4+-N levels were observed in the southern part. Water-rock interaction and agricultural activities were the controlling factors for fluoride and nitrogen distribution in groundwater. About 80% of samples are considered to have good water quality with WQI < 100. Exposure to fluoride and nitrogen through drinking should require more attention. The total non-carcinogenic risks through oral ingestion of groundwater were 0.22-3.19 for adults and 0.51-7.44 for children, respectively. The order of pollutants in the groundwater in terms of their hazard to residents was F- > NH4+-N > NO3--N > NO2--N. The findings of this study could provide more insights into groundwater management.

Hydrogeochemical properties, source provenance, distribution, and health risk of high fluoride groundwater: Geochemical control, and source apportionment

This study evaluated high fluoride (F-) levels, source distribution, provenance, health risk, and source apportionment in the groundwater of Sargodha, Pakistan. Therefore, 48 groundwater samples were collected and analyzed by ion-chromatography (DX-120, Dionex). The lowest concentration of F- was 0.1, and the highest was 5.8 mg/L in the aquifers. In this study, 43.76% of the samples had exceeded the World Health Organization's allowable limit of 1.5 mg/L. The hydrogeochemical facies in Na-rich and Ca-poor aquifers showed NaCl (66.6%), NaHCO3 (14.5%), mixed CaNaHCO3 (8.3%), CaCl2 (8.3%), mixed CaMgCl2 (2%), and CaHCO3 (2%) type water. Alkaline pH, high Na+, HCO3- concentrations, and poor Ca-aquifers promoted F- dissolution in aquifer. The significant positive correlations between Na⁺ and F- suggested cation exchange, where elevated Na⁺ occurs in Ca-poor aquifers. The cation exchange reduces the availability of Ca2+ would lead to higher F- concentrations. Meanwhile, the correlation between HCO₃- and F- indicates that carbonate minerals dissolution helps in increasing pH and HCO₃- as a result F- triggers in aquifers. Groundwater chemistry is primarily governed by the weathering of rock, water-rock interaction, ion-exchange, and mineral dissolution significantly control groundwater compositions. Cluster analysis (CA) determined three potential clusters: less polluted (10.4%), moderately polluted (39.5%), and severely polluted (50%) revealing fluoride toxicity and vulnerability in groundwater wells. Mineral phases showed undersaturation and saturation determining dissolution of minerals and precipitation of minerals in the aquifer. PCAMLR model determined that high fluoride groundwater takes its genesis from F-bearing minerals, ion exchange, rock-water interaction, and industrial, and agricultural practices. The health risk assessment model revealed that children are at higher risk to F- toxicity than adults. Thus, groundwater of the area is unsuitable for drinking, domestic, and agricultural needs.

Hydrochemical assessment of groundwater utilizing statistical analysis, integrated geochemical methods, and EWQI: a case study of Laiwu region, North China

The investigation of groundwater quality and hydrochemical assessment holds immense significance in safeguarding and ensuring the rational utilization of groundwater resources. This study utilizes groundwater sampling and testing data from the Laiwu region (LWR), encompassing both dry and wet seasons, to delve into the hydrochemical characteristics, ion sources, and overall groundwater quality. The research findings indicate that the groundwater in LWR exhibits weak alkalinity, with the dominant ions being Ca2+ followed by Mg2+, Na+, and K+, and HCO3-, SO42-, NO3-, Cl-, and F-. The average total dissolved solids (TDS) concentrations during the dry and wet seasons are recorded as 683 mg/L and 679 mg/L, respectively, classifying LWR's groundwater primarily as hard-fresh water. The spatial pattern of TDS concentration in LWR displays consistency throughout both the dry and wet seasons, with relatively low TDS levels observed in the northern and southeastern regions and higher concentrations in the lower reaches of the Dawen River and nearby Gangcheng. Predominantly, Ca2+, Mg2+, and HCO3- ions in groundwater originate from the dissolution of calcite and dolomite, with the hydrogeochemical process of carbonate rock weathering involving the presence of sulfuric acid. It is noteworthy that human activities significantly impact the chemical composition of groundwater in LWR. Notably, during the dry and wet seasons, the average concentration of NO3- in groundwater is 102.81 and 106.61 mg/L respectively, and the analysis shows that agricultural practice is the main source. Furthermore, the calculated average values of the entropy water quality index (EWQI) during those seasons are 58.44 and 57.24, respectively. The EWQI shows good to moderate water quality in most areas, except for a few poor-quality spots in the west. It is worth mentioning that LWR's groundwater is deemed suitable for agricultural irrigation. These research findings provide valuable insights and serve as a significant reference for the rational development and sustainable utilization of groundwater resources in the LWR region.

Source-oriented health risk assessment of groundwater based on hydrochemistry and two-dimensional Monte Carlo simulation

Accurately assessing the health risks posed by major contaminants is essential for protecting groundwater. However, the complexity of pollution sources and the uncertainty of parameters pose challenges for quantitative health risk assessment. In this study, a source-oriented groundwater risk evaluation process was improved by screening key pollutants, employing a combined hydrochemical and positive matrix factorization (PMF) approach for source apportionment, and incorporating two-dimensional Monte Carlo simulation for risk characterization. The application of this process to groundwater assessment in Central Jiangxi Province identified NO3-, F-, Se and Mn as the key pollutants. The pollution sources were anthropogenic activities, rock dissolution, regional geological processes, and ion exchange. Anthropogenic sources contributed 36.8 % and 28.8 % of the pollution during the wet season and dry season, respectively, and accounted for more than half of the health risks. NO3- from anthropogenic sources was the primary controlling pollutant. Additionally, the risk assessment indicated that children were at the highest health risk during the dry season, with ingestion rate suggested to be controlled below 1.062 L·day-1 to make the health risk within an acceptable range. The improved assessment methodology could provide more accurate results and recommended intakes.

Hydrogeochemical controls on contrasting co-occurrence of geogenic Arsenic (As) and Fluoride (F-) in complex aquifer system of Upper Indus Basin, (UIB) western Himalaya

Arsenicosis and fluorosis have become severe health hazards associated with the drinking of Arsenic (As) and Fluoride (F-) contaminated groundwater across south-east Asia. Although, significant As and F- concentration is reported from major Himalayan river basins but, the hydrogeochemical processes and mechanisms controlling their contrasting co-occurrence in groundwater is still poorly explored and understood. In the present study, groundwater samples were collected from phreatic and confined aquifers of Upper Indus Basin (UIB), India to understand the hydrogeochemical processes controlling the distribution and co-occurrence of geogenic As and F- in this complex aquifer system. Generally, the groundwater is circum-neutral to alkaline with Na+-HCO3-, Ca2+-Na+-HCO3- and Ca2+-Mg2+-HCO3- water facies signifying the dominance of silicate and carbonate dissolution. The poor correlation of As and F- in groundwater depicted that these geogenic elements have discrete sources of origin with distinct mechanisms controlling their distribution. As enrichment in groundwater is associated with high pH, Fe, Mn and NH4-N suggesting dominance of metal oxide/hydroxide reduction with organic matter degradation. However, F- enrichment in groundwater is associated with high pH, HCO3- and Na+, which is assisted by the incessant dissolution of fluorinated minerals. The study also revealed that high HCO3- facilitates the exchange of hydroxides (OH-) with As and F- on sediment surfaces that contribute to As and F- enrichment in groundwater through desorption. 70% groundwater samples have As and F- concentration above the permissible limit given by WHO. Therefore, continuous exposure to these contaminants may pose severe health hazard of arsenicosis and fluorosis to people living in the region and downstream. The study provides insights into geological sources, hydrogeochemical processes and mechanisms controlling distribution of As and F- in groundwater that will help in developing the appropriate measures to mitigate the impact these contaminants on human health.

Comprehensive evaluation and prediction of groundwater quality and risk indices using quantitative approaches, multivariate analysis, and machine learning models: An exploratory study

Assessing and predicting quality of groundwater is crucial in managing groundwater availability effectively. In the current study, groundwater quality was thoroughly appraised using various indexing methods, including the drinking water quality index (DWQI), pollution index of heavy metals (HPI), pollution index (PI), metal index (MI), degree of contamination (Cd), and risk indicators, like hazard quotient (HQ) and total hazard indicator (HI). The assessments were augmented through multivariate analytical techniques, models based on recurrent neural networks (RNNs), and integration of geographic information system (GIS) technology. The analysis measured physicochemical parameters across 48 groundwater wells from El-Menoufia region, revealing distinct water types influenced by ion exchange, rock-water interactions, and silicate weathering. Notably, the groundwater showed elevated levels of certain metals, particularly manganese (Mn) and lead (Pb), exceeding the drinking water limits. The DWQI deemed the bulk of the tested samples suitable for consumption, assigning them to the "good" category, whereas a small number were considered inferior quality. The HPI, MI, and Cd indices indicated significant pollution in the central study region. The PI revealed that Pb, Mn, and Fe were significant contributors to water pollution, falling between classes IV (strongly affected) and V (seriously affected). HQ and HI analyses identified the central area of the study as particularly prone to metal contamination, signifying a high risk to children via oral and dermal routes and to adults through oral exposure alone (non-carcinogenic risk). The adults had no health risks due to dermal contact. Finally, the RNN simulation model effectively predicted the health and water quality indices in training and testing series. For instance, the RNN model excelled in predicting the DWQI, with three key parameters being crucial. The model demonstrated an excellent fit on the training set, achieving an R2 of 1.00 with a very low root mean of squared error (RMSE) of 0.01. However, on the testing set, the model's performance slightly decreased, showing an R2 of 0.96 and an RMSE of 2.73. Regarding HPI, the RNN model performed exceptionally well as the primary predictor, with R2 values of 1.00 (RMSE = 0.01) and 0.93 (RMSE = 27.35) for the training and testing sets, respectively. This study provides a unique perspective for improving the integration of various techniques to gain a more comprehensive understanding of groundwater quality and its associated health risks, with a strong focus on feature selection strategies to enhance model accuracy and interpretability.

Arsenic-contaminated groundwater of the Western Banat (Pannonian basin): Hydrogeochemical appraisal, pollution source apportionment, and Monte Carlo simulation of source-specific health risks

Due to rapid urbanization and industrial growth, groundwater globally is continuously deteriorating, posing significant health risks to humans. This study employed a comprehensive methodology to analyze groundwater in the Western Banat Plain (Serbia). Using Piper and Gibbs plots, hydrogeochemistry was assessed, while the entropy-weighted water quality index (EWQI) was used to evaluate groundwater quality. Pollution sources were identified using positive matrix factorization (PMF) accompanied by Pearson correlation and hierarchical cluster analysis, while Monte Carlo simulation assessed health risks associated with groundwater consumption. Results showed that groundwater, mainly Ca-Mg-HCO3 type, is mostly suitable for drinking. Geogenic pollution, agricultural activities, and sewage were major pollution sources. Consumption of contaminated groundwater poses serious non-carcinogenic and carcinogenic health risks. Additionally, arsenic from geogenic source was found to be the main health risks contributor, considering its worryingly elevated concentration, ranging up to 364 μg/L. These findings will be valuable for decision-makers and researchers in managing groundwater vulnerability. PRACTITIONER POINTS: Groundwater is severely contaminated with As in the northern part of the study area. The predominant hydrochemical type of groundwater in the area is Ca-Mg-HCO3. The PMF method apportioned three groundwater pollution sources. Monte Carlo identified rock dissolution as the primary health risk contributor. Health risks and mortality in the study area are positively correlated.

Implications of atrazine concentrations in drinking water from Ijebu-North, Southwest Nigeria on the hypothalamic-pituitary-adrenal axis

There is an increasing overdependence and use of atrazine herbicide for the control of pre-and post-emergence broad leaf weeds on maize farms in rural agricultural communities in Nigeria. We carried out a survey of atrazine residue in 69 hand-dug wells (HDW), 40 boreholes (BH) and 4 streams from all the 6 communities (Awa, Mamu, Ijebu-Igbo, Ago-Iwoye, Oru and Ilaporu) in Ijebu North Local Government Area, Southwest Nigeria. The effect of the highest concentration of atrazine detected in the water from each of the communities on the hypothalamic-pituitary-adrenal (HPA) axis of albino rats was investigated. Varying concentrations of atrazine were detected in the HDW, BH and stream waters sampled. The highest concentration of atrazine recorded in the water from the communities ranged from 0.01 to 0.08 mg/L. Although there were no significant differences (p > 0.05) in serum levels of corticosterone, aldosterone and ROS of rats exposed to 0.01, 0.03 and 0.04 mg/L concentrations of atrazine compared to control, a significant increase (p < 0.05) was observed at 0.08 mg/L. Catalase activity increased significantly (p < 0.05) only at 0.03 and 0.04 mg/L of atrazine exposure. Butyrylcholinesterase activity, lipid peroxidation and serum ACTH of rats exposed to all the atrazine concentrations were not significantly different (p > 0.05) compared to control. Atrazine at environmentally relevant concentrations of 0.01, 0.03 and 0.04 mg/L detected in the water may not affect the HPA axis, attention should be given to 0.08 mg/L, which increases the serum corticosterone and aldosterone of the exposed rats.

Analysis of the distribution across media, migration, and related driving factors of fluoride in cold and arid lakes during the freezing period

Fluoride pollution in water has become a global challenge. This challenge especially affects China as a country experiencing serious fluoride pollution. While the have been past studies on the spatial distribution of fluoride, there has been less attention on different forms of fluoride. This study collected 176 samples (60, 40, and 76 ice, water, and sediment samples, respectively) from Lake Ulansuhai during the freezing period. The occurrence and spatial distribution characteristics of fluoride in lake ice-water-sediment were explored using Kriging interpolation, Piper three-line diagram, and Gibbs diagram analysis methods. The migration and transformation of fluoride during the freezing period were revealed and the factors influencing fluoride concentration in the water body were discussed considering the hydrochemical characteristics of lake surface water. The results showed that the average fluoride concentrations in the upper ice, middle ice and lower ice were 0.18, 0.09, and 0.12 mg/L, respectively, decreasing from north to south in the lake. The average concentrations of fluoride in surface water and bottom water were 0.63 and 0.83 mg/L, respectively. The concentrations of fluoride in ice and water were within the World Health Organisation drinking water threshold of 1.50 mg/L and the Class III Chinese surface water standard (GB3838-2002). The average sediment total fluorine was 1344.38 ± 200 mg/kg, significantly exceeding the global average (321 mg/kg) and decreasing with depth. The contents of water soluble, exchangeable, Fe/Mn bound, organic bound, and residual fluorides were 40.22-47.18, 13.24-43.23, 49.52-160.48, and 71.59-173.03 mg/kg, respectively. There was a significant positive correlation between fluoride concentration in ice and that in water. The change in fluoride concentration in water was mainly due to specific climatic and geographical conditions, pH, hydrochemical characteristics and ice sealing. This study is of great significance for the management of high-fluorine lakes in arid and semi-arid areas.

From fuzzy-TOPSIS to machine learning: A holistic approach to understanding groundwater fluoride contamination

Fluoride (F-) contamination of groundwater is a prevalent environmental issue threatening public health worldwide and in India. This study targets an investigation into spatial distribution and contamination sources of fluoride in Dhanbad, India, to help develop tailored mitigation strategies. A triad of Multi Criteria Decision Making (MCDM) models (Fuzzy-TOPSIS), machine learning algorithms {logistic regression (LR), classification and regression tree (CART), Random Forest (RF)}, and classical methods has been undertaken here. Groundwater samples (n = 283) were collected for the purpose. Based on permissible limit (1.5 ppm) of fluoride in drinking water as set by the World Health Organization, samples were categorized as Unsafe (n = 67) and Safe (n = 216) groups. Mean fluoride concentration in Safe (0.63 ± 0.02 ppm) and Unsafe (3.69 ± 0.3 ppm) groups differed significantly (t-value = -10.04, p < 0.05). Physicochemical parameters (pH, electrical conductivity, total dissolved solids, total hardness, NO3-, HCO3-, SO42-, Cl-, Ca2+, Mg2+, K+, Na+ and F-) were recorded from samples of each group. The samples from 'Unsafe group' showed alkaline pH, the abundance of Na+ and HCO3- ions, prolonged rock water interaction in the aquifer, silicate weathering, carbonate dissolution, lack of Ca2+ and calcite precipitation which together facilitated the F- abundance. Aspatial distribution map of F- contamination was created, pinpointing the "contaminated pockets." Fuzzy- TOPSIS identified that samples from group Safe were closer to the ideal solution. Among these models, the LR proved superior, achieving the highest AUC score of 95.6 % compared to RF (91.3 %) followed by CART (69.4 %). This study successfully identified the primary contributors to F- contamination in groundwater and the developed models can help predicting fluoride contamination in other areas. The combination of different methodologies (Fuzzy-TOPSIS, machine learning algorithms, and classical methods) results in a synergistic effect where the strengths of each approach compensate for the limitations of the other.

Seawater intrusion-triggered high fluoride groundwater development on the eastern coast of China

High-fluoride groundwater is commonly found in coastal areas worldwide, while its formation mechanism remains elusive. Herein, a comprehensive study was performed to identify the major controlling factor of high-fluoride groundwater occurrence along the eastern coast of China. Hydrogeochemical methods were employed to examine the distribution patterns of seawater intrusion and fluoride concentration and the impact of seawater intrusion on the fluoride concentration. The results indicate that seawater intrusion significantly influences the groundwater evolution process in the study areas. The groundwater in Laizhou Bay was affected by brine, and the groundwater in Tianjin and Jiangsu was affected by seawater with a mixing ratio lower than 40% and 20%, respectively. And the fluoride concentration in groundwater from Tianjin, Laizhou Bay, and Jiangsu generally exceeded 1 mg/L, with the average of 2.3 mg/L, 24.9 mg/L, and 34.6 mg/L, respectively. Both the degree of seawater intrusion and the fluoride concentration exhibit a consistent pattern: Laizhou Bay > Tianjin > Jiangsu. Cl- concentration in groundwater varies positively with the F- concentration (y = 0.66x - 1.31). Moreover, the spatial distribution of areas with high-fluoride groundwater mirrors that of seawater intrusion. The high-fluoride groundwater varies spatially and is related to the degree, stage, and type of seawater intrusion. In other words, when seawater intrusion intensifies more or groundwater in the freshwater renewal phase with higher Na+/Ca2+ or the presence of paleo-seawater intrusion with higher fluoride concentration of brine, the concentration of fluoride in groundwater is higher. As seawater intrusion intensifies, the high-fluoride groundwater in the study areas generally poses a higher health risk to human. These findings enhance our comprehension of the mechanisms underpinning high-fluoride groundwater in coastal regions and the environmental ramifications of seawater intrusion.

Spatio-temporal variability of public water supply characteristics and associated health hazards for children and adults in selected locations of Ambala, India

The contamination of public water supply and groundwater resources is a major concern in many parts of developing nations. Polluted water poses serious health risks to humans and the environment. This research was conducted to investigate the seasonal variations of the water quality parameters in the public water supply. To assess the supply water quality in different blocks of Ambala District, hydro-chemical analysis was conducted after a series of systematic sampling in various locations. The statistical tools for water quality indexing including water quality indexing (WQI), heavy metal pollution indexing (HMPI), pollution indexing (PI), overall pollution indexing (OPI), metal indexing (MI), and hazard indexing (HI) were used for data as well as the health hazard analysis through water pathway. Overall, 40 water samples were taken from the public water supply systems covering winter and summer seasons, and the levels of pH, total dissolved solids (TDS), EC, F- , Cl- , NO3 - , SO4 2- , HCO3 - , As, B, Cd, Co, Pb, Zn, Cr, Fe, and Mn were investigated. The weight arithmetic index method was used for WQI, and water pollution indices such as HMPI, PI, OPI, and MI were calculated using different models to check the severity of contamination. The mean hazard quotient and hazard index values calculated using the concentration levels of As, B, Cd, Co, Pb, Cr, Fe, Mn, Zn, F- , and NO3 - reveal that supply water may pose a significant health risk to both adults and children that further varies with temporal and spatial changes. During both seasons, a high carcinogenic risk for both adults and children was observed in the studied area because of high levels of As, Pb, Cd, and NO3 - . PRACTITIONER POINTS: The quality of public supply water was assessed at the selected sites of Ambala, India. High levels of NO3 - , As, Cd, and Pb were observed posing a health risk to adults and children via water pathway. 95% of the samples qualified for the excellent water quality category with respect to the levels of F- , Cl- , NO3 - , SO4 2- , HCO3 - , pH, EC, and TDS. Statistical analysis (HMPI, PI, MI, OPI, HI) using different models revealed water contamination with reference to the levels of NO3 - , As, Pb, Cr, Ni, and Cd. Immediate measures are needed to uphold the safety and health of the natives.

Source identification and potential health risks of fluoride and nitrate in groundwater of a typical alluvial plain

A comprehensive understanding of the role of natural and anthropogenic factors in groundwater pollution is essential for sustainable groundwater resource management, especially in alluvial plains with intensive anthropogenic activities. Numerous studies have focused on the contribution of individual factors on groundwater pollution in alluvial aquifers, but distinguishing the effects of natural and anthropogenic factors is limited. In this study, 64 wells were sampled in different seasons from the Yellow River alluvial plain in China for hydrochemical and isotopic analysis to investigate the spatiotemporal distribution, sources and health risks of fluoride and nitrate in alluvial aquifers. Results showed that fluoride contamination was widely distributed without significant seasonal variation, and 78.1 % of the dry season samples and 65.6 % of the wet season samples showed fluoride concentrations above the permissible limit (1.5 mg/L). High-F- groundwater was generally accompanied by Na-HCO3 and Na-HCO3·SO4 water types. Fluoride was from a natural origin mainly associated with mineral dissolution, competitive adsorption, cation exchange, and evaporation. Groundwater nitrate contamination was spatially sporadic and showed significant seasonal differences. Only 13.6 % of the dry season samples and 3.2 % of the wet season samples had NO3- concentrations exceeded the permissible limit of 50 mg/L. The hydrochemical phase evolved from bicarbonate or sulfate type to chloride type with increasing nitrate concentration. Manure and sewage attributed to agricultural activities contributed the most nitrogen to groundwater, followed by soil organic nitrogen and chemical fertilizers, revealing the anthropogenic origin of nitrate. Nitrification was the dominant nitrogen transformation process in the wet season, and denitrification was prevalent in the dry season. Oral ingestion of high fluoride groundwater was a major threat to human health, especially for infants. This study provided a significant reference for water resources management in alluvial aquifers.

Hydrochemical Assessment of Groundwater from the Harrat Khyber Flood Basalts, Northwest Saudi Arabia

The present study was carried out in the Harrat Khyber region of the Madinah province in Western Saudi Arabia. The objective of the study was to determine the main factors affecting groundwater chemistry and assess the impact of groundwater quality on human health with respect to nitrate, fluoride, and heavy metals (arsenic, chromium, copper, nickel, selenium, and zinc). Hydrochemical data for 70 groundwater samples from dug wells and bore wells from Harrat Khyber were interpreted to achieve the research objective. The groundwater chemistry is influenced by ion-exchange and the evaporation process. A wide variation in the concentration of various major ions is observed primarily due to the varied nature of the aquifer system which includes the unconsolidated wadi deposits, weathered-fractured basaltic aquifer system, and subbasaltic sedimentary aquifer system. The total hazard quotient (THQ) with respect to NO3 and F was determined. Although the F values are well within the limits of human consumption in water prescribed by WHO, the values of NO3 exceed the allowable limits in 50% of the groundwater samples. The average THQ values are 2.16, 2.92, and 2.34 for adults, children, and infants, respectively, which makes the water unsafe for human consumption. Six heavy metals (arsenic, chromium, copper, nickel, selenium, and zinc) were used to calculate the heavy-metal contamination index (HCI). The average HCI value is 19.505. Overall HCI calculation shows that the groundwater is unpolluted with respect to heavy metals. The heavy metals in water are mainly of geogenic origin.

Identifying the hydrochemical features, driving factors, and associated human health risks of high-fluoride groundwater in a typical Yellow River floodplain, North China

Fluoride enrichment (> 1.5 mg/L) in groundwater has become a global threat, particularly given the hazards to human health. This study collected 58 unconfined groundwater samples from Fengpei Plain in June 2022 for hydrochemical and stable isotope analyses combined with multiple methods to explore sources, influencing factors, and potential health hazards of groundwater F-. The results showed that groundwater F- concentration ranged from 0.08 to 8.14 mg/L, with an average of 1.91 mg/L; over 41.4% of them exceeded the acceptable level of 1.5 mg/L prescribed by the World Health Organization (WHO). The dominant hydrochemical facies changed from Ca·Mg-HCO3 and Ca·Mg-SO4·Cl type in low-F- groundwater to Na-HCO3 and Na-SO4·Cl water types in high-F- groundwater. The Self-Organizing Map (SOM) and ionic correlation analysis indicated that F- is positively correlated to pH, EC, Na+, K+, SO42-, and TDS, but negatively to Ca2+ and δ18O. Groundwater F- accumulation was primarily driven by F--bearing minerals dissolution such as fluorite. Simultaneously, the carbonates precipitation, positive cation exchange processes, and salt effect were conducive to groundwater F- enrichment. However, competitive adsorption between OH-/HCO3- and F-, evaporation, and anthropogenic activities only had a weak effect on the F- enrichment in groundwater. The hazard quotient (HQ) assessment results show that 67.2% of groundwater samples pose a non-carcinogenic risk (HQ > 1) for infants, followed by 53.4% for children, 32.8% for females, and 25.9% for males. The Monte Carlo simulation results agreed with those of the deterministic model that minors are more susceptible than adults. These findings are vital to providing insights into the geochemical behavior, driving factors, and drinking water safety of high-F- groundwater worldwide.

Hydrogeochemical evidence for fluoride sources and enrichment in desert groundwater: A case study of Cherchen River Basin, northwestern China

The identification of fluoride (F-) sources and enrichment mechanisms is imperative to understand the multiple fluorine (F) pathways, and further, to control regional diffuse F- contamination in groundwater. However, the factors that control high-F- groundwater are not fully understood in desert climate regions. Hence, a sampling campaign was conducted from 71 desert groundwater sites and six river water sites in the Cherchen River Basin (CRB), northwestern China. This study combined hydrochemical compositions with an optimized forward model, with the aim of determining the potential sources and enrichment mechanisms in F--contaminated desert groundwater. Approximately 58.46% of the samples had F- concentrations over the national standard of 1.0 mg/L. More severe F- contamination was found in the multi-layered structured confined aquifer (MCA) of the alluvial plain (1.42 ± 1.11 mg/L). The primary contributors of desert groundwater F- were the dissolution of F-bearing minerals containing evaporite (∼58.80%), silicate (∼15.89%), and carbonate (∼12.94%), followed by the river water input (∼12.08%). In contrast, anthropogenic activities (∼0.16%) and precipitation contributed less to desert groundwater F-. The dissolution equilibrium of CaF2 was important for F- enrichment in desert groundwater. Compared with the piedmont plain, intensive evaporation and salinization were more conducive to F- enrichment in the alluvial plain. Under alkaline condition, the dissolutions of evaporite and fluorite allowed extra F- to release into desert groundwater when Ca2+ and Mg2+ were up to oversaturation. Moreover, the desorption of F- was promoted by competitive adsorption of OH- and HCO3-, and the adsorption capacity of F- was weakened by cation exchange of K++Na+ with Ca2++Mg2+. As a result, desert groundwater had a higher concentration of F- in the alluvial plain. Our study provided a comprehensive understanding of multiple F pathways in desert groundwater. This study also highlights the effect of hydrogeochemical background on high-F- desert groundwater.

Assessment of groundwater hydrochemistry, water quality, and health risk in Hainan Island, China

Groundwater is an important source of water for human sustenance. The determination of groundwater quality at island sites is an urgent priority in China, but there are lacking systematic reports relating to them. Here, 63 groups of groundwater samples were collected and analyzed of Hainan Island. The groundwater in the study area is weakly alkaline, mainly comprising hard and soft freshwater. The predominant anions and cations are HCO₃^-, and Ca²⁺ and Na⁺, respectively, and the main water chemistry types are HCO₃-Cl-Na and HCO₃-Cl-Na-Ca. The chemical evolution of groundwater is mainly affected by water-rock interactions, cation exchange, and human activity. The groundwater is mostly of high quality and, in most areas, is suitable for drinking and irrigation. Contrastingly, the water quality in the west of the island is relatively poor. The spatial distribution of the risk coefficient (HQ) is consistent with the spatial variation in the NO₃^- concentrations in the groundwater. Notably, there are unacceptable health risks for different groups of people, with infants having the greatest level of impact, followed by children, teenagers, and adults. This study provides a valuable reference for the development and utilization of groundwater resources, as well as the improvement of aquatic ecological conditions on Hainan Island and other island areas worldwide.

Fluoride disrupts intestinal epithelial tight junction integrity through intracellular calcium-mediated RhoA/ROCK signaling and myosin light chain kinase

Fluoride is a common contaminant of groundwater and agricultural commodity, which poses challenges to animal and human health. A wealth of research has demonstrated its detrimental effects on intestinal mucosal integrity; however, the underlying mechanisms remain obscure. This study aimed to investigate the role of the cytoskeleton in fluoride-induced barrier dysfunction. After sodium fluoride (NaF) treatment of the cultured Caco-2 cells, both cytotoxicity and cytomorphological changes (internal vacuoles or massive ablation) were observed. NaF lowered transepithelial electrical resistance (TEER) and enhanced paracellular permeation of fluorescein isothiocyanate dextran 4 (FD-4), indicating Caco-2 monolayers hyperpermeability. In the meantime, NaF treatment altered both the expression and distribution of the tight junction protein ZO-1. Fluoride exposure increased myosin light chain II (MLC2) phosphorylation and triggered actin filament (F-actin) remodeling. While inhibition of myosin II by Blebbistatin blocked NaF-induced barrier failure and ZO-1 discontinuity, the corresponding agonist Ionomycin had effects comparable to those of fluoride, suggesting that MLC2 serves as an effector. Given the mechanisms upstream of p-MLC2 regulation, further studies demonstrated that NaF activated RhoA/ROCK signaling pathway and myosin light chain kinase (MLCK), strikingly increasing the expression of both. Pharmacological inhibitors (Rhosin, Y-27632 and ML-7) reversed NaF-induced barrier breakdown and stress fiber formation. The role of intracellular calcium ions ([Ca²⁺]_i) in NaF effects on Rho/ROCK pathway and MLCK was investigated. We found that NaF elevated [Ca²⁺]_i, whereas chelator BAPTA-AM attenuated increased RhoA and MLCK expression as well as ZO-1 rupture, thus, restoring barrier function. Collectively, abovementioned results suggest that NaF induces barrier impairment via Ca²⁺-dependent RhoA/ROCK pathway and MLCK, which in turn triggers MLC2 phosphorylation and rearrangement of ZO-1 and F-actin. These results provide potential therapeutic targets for fluoride-induced intestinal injury.

Distribution, enrichment mechanisms, and health risk assessment of high-fluorine groundwater in the Yudong Plain, Henan Province, China

The Yudong Plain is in the eastern part of Henan Province, China, where there is little rain and high evaporation. Compared to other areas in Henan Province, the groundwater fluorine content is generally high, which affects the health of residents. Based on the systematic analysis of water chemistry data of shallow and mid-depth groundwater samples in the Yudong Plain, the causes of shallow and mid-depth high-fluorine groundwater in the Yudong Plain were explored using mathematical statistics, spatial interpolation, and ion ratios. The results show that the fluorine contents of both shallow and mid-depth groundwater in the study area are high. The shallow samples had fluorine contents ranging from 0.1 to 4.89 mg/L, with an exceedance rate of 48% and an average content of 1.15 mg/L. The fluorine content of mid-depth samples ranged from 0.14 to 3.32 mg/L, with an exceedance rate of 68% and an average content of 1.33 mg/L. The shallow high-fluorine groundwater is mainly distributed in the central low-lying area, and its main hydrochemical type is HCO₃-Na·Mg; the mid-depth high-fluorine groundwater is mainly distributed in strips in the north and east of the study area, and its main water chemistry type is HCO₃-Na. Fluorine enrichment in shallow groundwater in the study area is controlled by rock weathering, evaporation concentration, and competitive adsorption, while leaching and dissolution of fluorine-containing minerals in sedimentary strata are the main factors influencing fluorine enrichment in mid-depth groundwater. The results of the human health risk assessment (HRA) showed that the mean non-carcinogenic hazard quotients (HQs) in shallow groundwater were 0.95, 0.64, 0.57, and 0.55 for infants, children, teenagers, and adults, respectively, while the mean non-carcinogenic HQs in mid-depth groundwater were 1.11, 0.74, 0.66, and 0.63, respectively. The study provides a scientific basis for the rational development and use of groundwater in the area and offers theoretical support for the prevention and control of groundwater pollution.

Geochemical evidence of fluoride behavior in loess and its influence on seepage characteristics: An experimental study

High background levels of fluorine in groundwater and soil in arid and semi-arid loess regions pose a severe threat to socio-economic development and human health, necessitating the evaluation of fluorine migration in loess. In this study, static leaching and dynamic seepage tests as well as scanning electron microscopy, mercury intrusion porosimetry, and X-ray fluorescence analyses were conducted using loess as the porous medium. Additionally, simulations using PHREEQC software were performed. The results indicated that the studied loess had a high background level of fluorine. Geochemical processes closely related to fluorine include dissolution of gypsum and dolomite, precipitation/dissolution of calcite and fluorite, and ion exchange between CaX and NaX. Under seepage of water with high fluorine levels, soil particles flocculated and formed aggregates; furthermore, the contact area between soil particles reduced, resulting in point-to-point contact between particles. Consequently, pores changed from small (intra- and inter-particle pores) to large (intra- and inter-granular pores) scale pores. Permeability initially decreased rapidly, then remained relatively stable, and subsequently rapidly increased with the passage of time. This was attributed to the dissolution of calcium minerals in loess, yielding Ca²⁺, which induced the precipitation of fluorite and promoted the dissolution of carbonates. Cation exchange and dissolution of other components were also important factors influencing permeability. The findings of this study can elucidate the coupling between loess microstructure, seepage behavior, and geochemical actions under the influence of high‑fluorine water, and are of great significance for in-situ regional research in loess areas.

Natural-human driving factors of groundwater salinization in a long-term irrigation area

Salinization of groundwater is a major challenge for groundwater management in long-term irrigation areas, decoupling its complex influencing factors can provide insights for the sustainable development of irrigation areas. In this study, the natural-human driving factors of groundwater salinization in the Yinchuan Plain, a typical irrigated area, were identified using isotope analysis, information entropy, and self-organizing map. Results show that groundwater in the study area is seriously salinized with obvious spatial heterogeneity. Multiple natural conditions and frequent human activities complicate the salinization characteristics of groundwater. On this basis, four typical natural influence units of groundwater were identified, namely, an evaporation and upward leakage zone, a runoff zone, an evaporation zone, and a runoff and upward leakage zone. Information entropy was proposed to quantify the complexity of groundwater resulting from human activities: The complexity difference between densely populated areas and natural dominant areas is mainly reflected in Na⁺, SO₄^2-, and Cl^-. Multiple human-made drivers of complex water environment were further separated into three patterns by the SOM model: blockage-evaporation type, leakage-evaporation type, and irrigation type. The blockage of drainage ditches and obstruction of salt discharge has the highest impact on the salinization of groundwater, followed by irrigation activities and transportation losses. Water excessive stagnation caused by blockage or irrigation is the root cause of groundwater salinization in the irrigated area, and its impact is greater than that of the traditional understanding of groundwater level rise. Based on the evaluation of irrigation water quality, management initiatives for irrigated areas should prioritize dredging and maintaining a healthy soil and groundwater environment in tandem.

Spatiotemporal variability and control factors of NO3- in a polluted karst water system of an agricultural wetland in South China

Nitrate (NO₃^-) pollution in karst water is an important environmental issue in intensive agricultural regions worldwide. The integrated understanding of the spatiotemporal variability and control factors of NO₃^- pollution in karst water is imperative for controlling the diffuse pollution caused by agricultural activities. In this study, 49 water samples were collected from surface water (SW) and groundwater (GW) in the Huixian karst wetland (HKW) and analyzed using hydrogeochemical and isotopic data (δ¹⁸O-NO₃^-, δ¹⁵N-NO₃^- and δ¹³C_DIC) in combination with a Bayesian mixing model to investigate the spatiotemporal distribution and control factors in NO₃^--polluted karst water. The results showed that approximately 40.82% of the karst water samples exceeded the natural threshold value of 3 mg/L for NO₃^--N, and 32.14% of the GW samples exceeded the permissible limit for drinking water established by WHO (10 mg/L as NO₃^--N), indicating that high levels of NO₃^- were mainly found in GW samples from the agricultural core area, especially in the dry season. The NH₄⁺-synthetic fertilizer (NHF) and soil organic nitrogen (SON) were the dominant factors controlling pollution sources in the HKW, accounting for 36.13% ± 4.66% and 28.68% ± 4.75% of the karst GW NO₃^- concentration, respectively. However, the seasonal differences in NO₃^- pollution sources were not significant in GW. Microbial nitrification was the main process affecting the NO₃^- levels in GW, whereas the occurrence of denitrification did not significantly affect NO₃^- concentration in the HKW due to the relatively low rate. Moreover, the HNO₃ produced from NH₄⁺ via microbial nitrification facilitated carbonate weathering, thereby controlling NO₃^- enrichment in karst GW. Our results suggest that NHF should be controlled to prevent further GW pollution in the HKW. Our study also provides a scientific basis for understanding the factors controlling the NO₃^- concentrations in karst water systems.

Hydrogeochemical characterization of groundwater and their associated potential health risks

The present study assessed the human health risk exposure from the consumption of poor quality groundwater in the Lucknow area, a part of Central Ganga alluvial plain in India. Around 27 (n = 27) groundwater samples were collected from the study area. The analytical results of the samples (n = 27) collected indicate silicate and carbonate weathering is the dominant process along with cation exchange, sulfide oxidation, and reverse ion exchange. The type of groundwater is Ca²-Na-HCO₃^- type having all cations and anions within permissible WHO limits except for iron (Fe²⁺) and nitrate (NO₃^-). The high concentrations of Fe² and NO₃^- in samples indicate the possibility of a non-geogenic point source for the same in an urban-influenced environment. The ionic concentration of dissolved constituents is used in weighted overlay analysis to generate the water quality index (WQI). WQI indicates that most urban areas (~ 98.52%) have fallen in the good to excellent category except few situated in the highly populated parts of Lucknow. The ionic concentrations of Fe²⁺ and NO₃^- have been further used to estimate human health risk by integrating regional urban population density data in Lucknow. The risk map shows alarming risks in the west-central part, where nearly ~ 35% of the total area is at moderate to high health risk.

Identifying the geochemical evolution and controlling factors of the shallow groundwater in a high fluoride area, Feng County, China

Understanding how groundwater is formed and evolves is critical for water resource exploitation and utilization. In this study, hydrochemistry and stable isotope tracing techniques were adopted to determine the key factors influencing groundwater chemical evolution in Feng County. A total of fourteen wells and five surface water samples were investigated in November 2021. The δD and δ¹⁸O compositions show that both surface water and groundwater are recharged from atmospheric precipitation. The dominating order of cations and anions in groundwater appears to be Na⁺ > Mg²⁺ > Ca²⁺ > K⁺ and HCO₃^- > SO₄^2- > Cl^- > NO₃^- > F^-, respectively. The groundwater hydrochemical facies are mainly characterized by HCO₃-Ca-Mg and SO₄-Cl-Na types. The chemical evolution of groundwater is dominated by water-rock interaction and cation exchange reactions. The major ions in groundwater are mainly controlled by various geogenic processes including halite, gypsum, calcite, dolomite, Glauber's salt, feldspar, and fluorite dissolution/precipitation. Furthermore, the abundant fluoride-bearing sediments, together with low Ca²⁺, promote the formation of high F^- groundwater. Approximately 85.7% and 28.6% of groundwater samples exceeded the permissible limit for F^- and NO₃^- respectively. Apart from geogenic F^-, human interventions (i.e., industrial fluoride-containing wastewater discharge and agricultural phosphate fertilizer uses) also regulate the F^- enrichment in the shallow groundwater. Nitrate pollution of the groundwater may be attributed to domestic waste and animal feces. Our findings could provide valuable information for the sustainable exploitation of groundwater in the study area and the development of effective management strategies by the authorities.

Contrasting behaviors of groundwater arsenic and fluoride in the lower reaches of the Yellow River basin, China: Geochemical and modeling evidences

Genesis of the contrasting distributions of high arsenic (>10 μg/L) and fluoride (>1 mg/L) groundwater and their negative correlations remain poorly understood. We investigated spatial distributions of groundwater arsenic and fluoride concentrations in the lower reaches of the Yellow River basin, Henan Province, China, using bivariate statistical analyses and geochemical simulations. Results suggest that high arsenic and fluoride groundwater showed contrasting distributions with few overlapped area. Groundwater arsenic concentrations were significantly negatively correlated with oxidation-reduction potential (ORP) values and positively with NH₄⁺ and Fe(II) concentrations, while the opposites were true for groundwater fluoride concentrations. These may suggest that high arsenic groundwater is related to stronger organic matter degradation and Fe(III) oxide reduction, while groundwater fluoride enrichment occurs with less extent of organic matter degradation. Geochemical calculations supported that groundwater fluoride enrichment was governed by extent of fluorite dissolution, which was constrained by varied saturation indices of fluorite in groundwater. However, groundwater arsenic mobility may be explained by different solubility of Fe(III) oxides. Higher Fe(III) oxide solubility corresponding to goethite and lepidocrocite was related to higher arsenic concentrations, while hematite was too low in solubility to produce high arsenic groundwater. The study presented both geochemical and modeling evidences for the contrasting behaviors of groundwater arsenic and fluoride concentrations in anoxic aquifers.

Characteristics of fluoride migration and enrichment in groundwater under the influence of natural background and anthropogenic activities

Excessive enrichment of fluoride threatens ecological stability and human health. The high-fluoride groundwater in the Chagan Lake area has existed for a long time. With the land consolidation and irrigation area construction, the distribution and migration process of fluoride have changed. It is urgent to explore the evolution of fluoride under the dual effects of nature and human. Based on 107 groundwater samples collected in different land use periods, hydrogeochemistry and isotope methods were combined to explore the evolution characteristics and hydrogeochemical processes of fluoride in typical high-fluoride background area and elucidate the impact of anthropogenic activities on fluoride migration. The results indicate that large areas of paddy fields are developed from saline-alkali land, and its area has increased by nearly 30%. The proportion of high-fluoride groundwater (>2 mg/L) has increased by nearly 10%, mainly distributed in the new irrigation area. Hydrogeochemical processes such as dissolution of fluorine-containing minerals, precipitation of carbonate minerals and exchange of Na⁺, Ca²⁺ on the water-soil interface control the enrichment of fluoride. The groundwater d-excess has no obvious change with the increase of TDS, and human activities are one of the reasons for the increase of fluoride. The concentration of fluoride is diluted due to years of diversion irrigation in old irrigation area, whereas the enrichment of δ²H, δ¹⁸O and Cl^- in new irrigation area indicates that the vertical infiltration of washing alkali and irrigation water brought fluoride and other salts to groundwater. Fertilizer and wastewater discharges also contribute to the accumulation of fluoride, manifesting as co-increasing nitrate and chloride salts. The results of this study provide a new insight into fluoride migration under anthropogenic disturbance in high-fluoride background areas.

Variation and multi-time series prediction of total hardness in groundwater of the Guanzhong Plain (China) using grey Markov model

Total hardness (TH) is an important index representing the water suitability for domestic purpose. TH is represented mainly by Ca²⁺ and Mg²⁺ which are essential elements for human bone development. Between 2000 and 2015, the TH values of groundwater in major cities of the Guanzhong Plain varied significantly. The study was carried out to investigate TH variation over 16 years and to examine how effective the grey Markov model was in predicting TH concentrations in time series datasets. The hydrochemical parameters determining TH concentration and their origins were investigated using statistical analysis and geochemical models. The grey Markov model, which is effective in short time series prediction, was used to forecast the multi-time series of TH. The findings demonstrated a prevalence of HCO₃^- and SO₄^2- in the groundwater types combined with calcite precipitation, gypsum, and dolomite dissolution that increased the concentration of Ca²⁺, Mg²⁺, and HCO₃^-, influencing TH variation. The predicted TH values of the eight monitoring wells for the year 2016 were 1213.66, 124.30, 203.66, 103.01, 349.56, 251.23, 453.31, and 471.81 mg/L, respectively. Datasets with low TH variation were more accurately predicted than datasets with high TH variation. This was especially observed on sample B557 where TH concentration in 2010 was 400.33 mg/L and suddenly dropped to 90.1, 82.6, 85.1, 87.6, and 75.1 mg/L in 2011, 2012, 2013, 2014, and 2015, respectively. The study also shows that the Markov chain model can optimize the GM(1,1) model and improve the prediction accuracy significantly. All samples in Weinan City and one sample in Xi'an City showed a significant decrease in TH concentration. Except one sample in Xi'an City, TH concentrations tended to rise in the other cities (Baoji, Xianyang) of the Guanzhong Plain. This study verified the reliability of the grey Markov model in terms of forecasting time series datasets with high variability, and the results can be referential to similar studies in the world.

Application of data-mining technique and hydro-chemical data for evaluating vulnerability of groundwater in Indo-Gangetic Plain

Vulnerability of groundwater is critical for the sustainable development of groundwater resources, especially in freshwater-limited coastal Indo-Gangetic plains. Here, we intend to develop an integrated novel approach for delineating groundwater vulnerability using hydro-chemical analysis and data-mining methods, i.e., Decision Tree (DT) and K-Nearest Neighbor (KNN) via k-fold cross-validation (CV) technique. A total of 110 of groundwater samples were obtained during the dry and wet seasons to generate an inventory map. Four K-fold CV approach was used to delineate the vulnerable region from sixteen vulnerability causal factors. The statistical error metrics i.e., receiver operating characteristic-area under the curve (AUC-ROC) and other advanced metrices were adopted to validate model outcomes. The results demonstrated the excellent ability of the proposed models to recognize the vulnerability of groundwater zones in the Indo-Gangetic plain. The DT model revealed higher performance (AUC = 0.97) followed by KNN model (AUC = 0.95). The north-central and north-eastern parts are more vulnerable due to high salinity, Nitrate (NO₃^-), Fluoride (F^-) and Arsenic (As) concentrations. Policy-makers and groundwater managers can utilize the proposed integrated novel approach and the outcome of groundwater vulnerability maps to attain sustainable groundwater development and safeguard human-induced activities at the regional level.

Fluoride enrichment mechanisms and related health risks of groundwater in the transition zone of geomorphic units, northern China

Although groundwater is the primary drinking water source in northern of China, little is known about generation mechanisms and related health risks of high fluoride groundwater at the geomorphic transition zones. Thus, 419 groundwater samples were collected from Zhangjiakou region, where is a typically geomorphic transition zone of the North China Plain and the Inner Mongolia Plateau, to conduct the hydrochemical analysis, geochemical modeling, multivariate statistical analysis, and health risks assessment. From the results, F^- concentration in groundwater had a range of 0.05-9.71 mg L^-1. About 37.1% and 26.2% of groundwater samples from Bashang region (BSR) and Baxia region (BXR), respectively, were over the 1.50 mg L^-1, which were mainly distributed in the groundwater flow retardation area and/or evaporation discharge area. Thermodynamic simulations demonstrated that F-bearing minerals dissolution and Ca²⁺/Mg²⁺ removal via calcite/dolomite precipitation primarily governed high-F^- groundwater formation in the whole study area. Competitive adsorption, evaporation, evaporites dissolution and salt-effect also affected F^- enrichment in BSR. Desorption in alkaline environment, ion exchange and human activities played a vital role in F^- enrichment at BXR. The multivariate statistical analysis revealed that the origin of F^- contamination was geogenic in BSR; whereas, it was geogenic and anthropogenic in BXR. Besides, more than 71.8%, 51.0%, 36.1% and 25.5% of the study area exceeded the acceptable level (health index>1) for infants, children, adult males, and females, respectively. The health risks for different groups of people varied significantly and ranked: infants > children > males > females, suggesting that younger people were more susceptible to fluoride contamination. Meanwhile, females were more resistant to fluoride contamination than males. These findings are vital to providing insights on high-F^- groundwater formation, investigate the situation of health risks, and conduct the integrated management for high fluoride groundwater in geomorphic transition zones at northern China.

Characterization of soil salinization and its driving factors in a typical irrigation area of Northwest China

Salinization of irrigation areas is a global environmental challenge. The uncertainty in the distribution of salinization is increased by the complexity of the natural environment. This study adopted Yinchuan Plain as a typical irrigation area to study the relationship between soil salinity and the environment from the perspective of macro-environmental elements and micro-ion composition. A Geographic Weighted Regression model (GWR) was used to predict the risk of salinization in the Yinchuan Plain. The results showed obvious spatial variation in soil salinization in the Yinchuan Plain. Farmland accounted for the largest proportion of salinized land area, followed by woodland and "other" land use categories. The main characteristic ions in the salinized area of the Yinchuan Plain were SO₄^2-, K⁺+Na⁺and Cl^-. The rank of ions in terms of change rate with increasing soil salinity was: SO₄^2- > K⁺ + Na⁺ > Cl^- > Ca²⁺ > HCO₃^- > Mg²⁺ > CO₃^2-. However, the rank of ions in terms of their sensitivity to salinization was: HCO₃^- > Ca²⁺ > Mg²⁺ > SO₄^2- > Cl^- > K⁺ + Na⁺. On this basis, the geographical indicators of DEM and NDVI, groundwater indicators of groundwater depth and TDS, climate indicators of SPEI, as well as soil indicators of PH and organic matter were taken as the representative ecological drivers of salinization in irrigation areas. These environmental factors were found to control the distribution of salinization, whereas human activity affected the degree of change in salinization. The enrichment of SO₄^2- in the Yinchuan Plain was mainly related to agricultural activities (such as pesticides application and irrigation evaporation), and followed by phreatic evaporation. The salt ions carried by irrigation and rainfall further polluted phreatic water. In the end, the measures of optimizing drainage, combined irrigation, and improving planting layout were recommended for the effectively and economically controlling of salinization.

Combinations of multivariate statistical analysis and analytical hierarchical process for indexing surface water quality under arid conditions

Novel methods for water quality indexing increase insight into the fitness of water bodies for different uses. We hypothesized that integrating multivariate statistical analysis (MSA) with the analytical hierarchical process (AHP) may provide a reliable estimation of water quality status. Hence, twenty water samples from canals and drains in the northern Nile Delta, Egypt were collected during summer, autumn, winter, and spring and analyzed. Data were subjected to MSA, including correlation analysis, principal component analysis (PCA), and hierarchal cluster analysis (HCA). The AHP was applied to derive weights of parameters implied in developing water quality indices for irrigation (IWQI) and fish farming (FFWQI). Human health risks due to exposure to potentially toxic elements (PTEs) via dermal contact were also considered. The average concentrations of water constituents were acceptable for irrigation, except sodium adsorption ratio (SAR) and Cl^-. The dissolved oxygen, total dissolved solids, Cl^-, NO₂-N, NO₃-N, NH₃, and PTEs (except Zn) did not meet standard limits for fish production. The MSA revealed that water contamination resulted from human activities (agriculture, industry, and domestic wastes) and hydrochemical processes. The PCA indicated that SAR, Cu, and pH could adequately represent water quality for irrigation, while temperature, NO₂-N, Cr, and Zn could reflect fish farming requirements. The AHP provided consistent weights for the original and shortlisted parameters. The water quality varied from good to poor for irrigation and from excellent to low for fish farming. The minimum IWQI could adequately represent the IWQI (R² = 0.83) and thus reduce the time, effort, and cost for monitoring water quality. However, the minimum FFWQI showed moderate consistency (R² = 0.51) with FFWQI, implying that increasing the sampling size is essential for better performance. The hazard quotient of all PTEs was below 1.0 for both adults and children, indicating a safe limit. The potential cancer risk was acceptable (1.36E-06) for adults and safe (8.03E-07) for children. Results of this work would be a start point for efficient quality control programs in arid regions.

Source and enrichment mechanism of fluoride in groundwater of the Hotan Oasis within the Tarim Basin, Northwestern China

In arid inland irrigated areas, the role of human activities on fluoride enrichment in groundwater is not fully understood. There is an extremely arid climate, high-intensity irrigation, and severe soil salinization in the Hotan Oasis within the Tarim Basin, Northwestern China. In this study, hydrogeochemistry and environmental isotope methods were combined to explore the distribution characteristics and controlling processes of fluoride enrichment in groundwater. The F^- concentration in groundwater had a range of 1.12-9.4 mg/L. F^- concentrations of all the groundwater samples were higher than 1.0 mg/L (Chinese Standards for Drinking Water Quality), and about 89% were higher than 1.5 mg/L (WHO Guidelines for Drinking Water Quality). High fluoride groundwater was mainly distributed downstream of the river and in the middle of the interfluvial zone. Vertically, the fluoride concentration was higher when the sampling depth was less than 15 m. There was a significant positive correlation between F^- concentration and salinity in groundwater. F^- in groundwater was mainly derived from river water fluoride, which could be imported to groundwater with infiltration of rivers and irrigation canals as well as irrigation return flow. Anthropogenic inputs may be partly responsible for fluoride enrichment in groundwater. Fluoride accumulated in the vadose zone by strong evapotranspiration and then leached into groundwater with irrigation return flow was the main mechanism of F^- enrichment in groundwater in the study area. This work is a clear example of how human activities together with natural processes can affect the chemical quality of groundwater, which is essential to safeguard the sustainable management of water and soil resources inland arid oasis areas.

Application of the hydrochemistry, stable isotopes and MixSIAR model to identify nitrate sources and transformations in surface water and groundwater of an intensive agricultural karst wetland in Guilin, China

Karst water as the vital water supply source is generally suffered from NO₃^- contamination in intensive agricultural areas worldwide. Identifying NO₃^- sources and transformations is the key for understanding nitrogen pathways, and also for effectively controlling diffuse NO₃^- pollution. In this study, chemical variables and stable isotopes (δ²H-H₂O, δ¹⁸O-H₂O, δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) were measured in 10 surface water (SW) samples and 13 groundwater (GW) samples collected from the Huixian karst wetland, with the application of a Bayesian stable isotope mixing model (MixSIAR) to identified NO₃^- sources and biogeochemical transformations. The results showed that the NO₃^- concentrations ranged from the below detection limit to 117 mg/L, with 30.8% of GW samples obtained from the north central part of the study area exceeding the maximum permissible limit for drinking water, and posing significant non-carcinogenic health risks for native people through drinking water pathway. Moreover, based on characteristics of the hydrochemistry and stable isotopes, different biogeochemical fates were evaluated in SW and GW: nitrification process was a dominant factor in GW, as a result of high NO₃^- levels, and this microbial process was unlikely occurred in SW associated with relatively anaerobic condition and low NO₃^- levels; however, the denitrification might not be a main process of degradation NO₃^- levels throughout the study area. The MixSIAR outputs revealed that the long-term application of synthetic NH₄⁺ fertilizer (36.6%) and soil organic nitrogen (28.0%) were the main contributors to NO₃^- pollution, followed by synthetic NO₃^- fertilizer (16.8%) and domestic sewage and manure (15.1%), whereas NO₃^- in precipitation (3.44%) played a less important role. Additionally, NO₃^- concentration was significantly influenced by agricultural activities rather than NO₃^- source's contribution between SW and GW. This work suggests that synthetic NH₄⁺ fertilizer should be the primary target for control to prevent further NO₃^- pollution of the karst groundwater.

Synergistic removal of fluoride from groundwater by seed crystals and bacteria based on microbially induced calcium precipitation

A new hypothesis that seed crystals (SC) and bacteria based on microbially induced calcium precipitation (MICP) synergistically remove fluoride (F^-) from groundwater was proposed, with a focus on evaluating the defluoridation potential of this method and revealing its F^- removal mechanism. The crucial conditions were optimized to reduce preparation and operation costs. SC furnished more available binding sites due to the existence of bacteria, and the reuse experiments showed that the defluoridation efficiency of SC still remained a high level after 14 cycles (70.10%), with a residual F^- concentration of 0.96 mg L^-1. The SEM-EDS, FTIR and XRD analyses indicated the predominant F^- removal mechanism of SC could be ascribed to the chemisorption, ion exchange, and co-precipitation. Moreover, ion exchange and co-precipitation (PO₄^3- involvement) were validated more contributive than chemisorption (CaCO₃ and CaSO₄ involvement). As a feasible, reusable, and eco-friendly technique, SC suggests promising applications in the treatment of fluoride-contaminated groundwater.

The occurrence of geogenic fluoride in shallow aquifers of Kenya Rift Valley and its implications in groundwater management

Widespread concerns about high-fluoride groundwater and their health risks have been raised worldwide. Weathering of volcanic minerals is regarded as a principal source of groundwater fluoride in regions with volcanic bedrocks. However, how does the volcanic minerals control fluoride occurrence, if it induces other hydrogeochemical processes participating in and how this relates to human health still remain unclear. This study takes Kenya Rift Valley, which has volcanic geological formations, as an example to delineate the occurrence and origins of high-fluoride shallow groundwater with analysis of hydrochemistry, graphical and multivariate statistical methods. Over 40% of shallow groundwater (F^-: up to 23.5 mg/L) show elevated fluoride values over the WHO standards of 1.5 mg/L. High fluoride groundwater are generally Na-rich and Ca-poor with high pH and HCO₃^¯ concentrations. Hydrogeochemical and principal component analysis indicate that weathering of hyper-alkaline volcanic rocks could release accumulated fluoride in melts and volatile fractions, as well as in clay minerals. Alkaline condition and high HCO₃^- contents lead to the competitive desorption of F^- from clay minerals and Fe-hydroxides into groundwater. Clay minerals also provide abundant exchange sites where cation exchange happens and promotes the release of F^- from the sediments by controlling the dissolution/precipitation of calcite and fluorite. Health risk assessment results show that chronic health risks by groundwater geogenic fluoride ingestion are identified to various individuals, with highest threats in children. Finally, a conceptual model has been developed to demonstrate the formations of high geogenic fluoride groundwater in regions with volcanic bedrocks and its relation with human health.