FIMAR: A new Fluoride Index to mitigate geogenic contamination by Managed Aquifer Recharge

Distribution characteristics and risk assessment of fluoride in surface water of urban typical rivers

Recent research on fluoride in water primarily focuses on groundwater; however, the potential environmental risks of fluoride in urban rivers should not be overlooked. In 2023, this study collected 135 surface water samples from the Ershibu River in Hefei, China, during various flood periods. Through descriptive statistical analysis, correlation analysis, principal component analysis-multiple linear regression (PCA-MLR) modeling, hazard quotient (HQ) assessment, and Monte Carlo simulation analysis, the spatial and temporal distribution, potential sources, and health risks of fluoride were investigated. The results showed that fluoride concentrations in the Ershibu River ranged from 0 to 1.38 mg/L. According to the PCA-MLR calculations, industrial pollution (73.92 %) was identified as the main source, followed by hydrogeochemical evolution (16.10 %) and agricultural activities (9.98 %). The HQ analysis revealed that the average exceedance rates of HQ for the five exposed populations were as follows: infants (64.45 %) > young children (2.22 %) = adults (2.22 %) > children (0) = teenagers (0). Therefore, relevant authorities should improve defluoridation facilities to reduce fluoride levels in industrial and agricultural wastewater and implement measures to protect public health. Future research should investigate the migration processes and toxicity mechanisms of fluoride more thoroughly.

Identification of hydrochemical fingerprints, quality and formation dynamics of groundwater in western high Himalayas

Identifying hydrochemical fingerprints of groundwater is a challenge in areas with complex geological settings. This study takes the Gilgit-Baltistan, a complex geological area in west high Himalayas, Pakistan, as the study area to get insights into the hydrochemcial genesis and quality of groundwater in complex geological mountainous regions. A total of 53 samples were collected across the area to determine the hydrochemical characteristics and formation of groundwater. Results revealed groundwater there is characterized by slightly alkaline and soft fresh feature. Groundwater is dominated by the hydrochemical facies of HCO3·SO4-Ca·Mg type. The factor method yields three components (PCs) of principal component analysis, which together explain 75.71% of the total variances. The positive correlation of EC, TDS, Ca2+, SO42-, K+ in PC1, and NO3-, Cl- in PC2 indicate that a combination of natural and anthropogenic activities influences groundwater hydrochemistry. Water-rock interaction is the main mechanism governing the natural hydrochemistry of groundwater. The negative correlation of Cl-, SO42-, Ca2+, and Na+ with NDVI attributes to inorganic salt uptake by plant roots. Groundwater chemical composition is also affected by the type of land use. Groundwater is characterized as excellent and good water quality based on the entropy-weighted water quality index assessment, and is suitable for drinking purposes except for very few samples, while aqueous fluoride would pose potential health threats to water consumers in western high Himalayas, and infants are most at risk compared to other populations. This study will help to deepen the hydrochemial formation mechanism and exploitation suitability of groundwater resources in the mountainous areas that undergone the combined actions of nature and human activities, and provide insights into the characteristics of water environmental quality in western Himalayas area.

Evaluating hydrogeochemical controls and noncarcinogenic health risk assessment of fluoride concentration in groundwater of Palacode and Pennagaram taluk, Dharmapuri district, Tamil Nadu, India

This study focuses on assessing hydrochemical characteristics and non-carcinogenic health risks associated with fluoride contamination in groundwater within the Palacode and Pennagaram taluks of Dharmapuri district. The presence of fluoride in drinking water is a significant concern due to its potential health impacts on both adults and children. We collected a total of 158 groundwater samples during both the summer (SUM) and monsoon (MON) seasons in 2021 to evaluate the suitability of water for drinking purposes in this region. During the SUM season, groundwater exhibits alkaline characteristics with a pH range of 6.70 to 8.73 and a mean value of 7.43, while the MON season falls within the neutral pH range with values ranging from 6.60 to 7.60 and a mean of 7.00. Hydrogeochemical analysis reveals that fluoride concentrations during the SUM season range from 0.13 to 2.7 mg/L, with a mean of 0.82 mg/L, whereas the MON season exhibits concentrations ranging from 0.08 to 1.6 mg/L, with a mean of 0.5 mg/L. Spatial distribution analysis indicates a gradual increase in fluoride concentrations from the northeast to the central and southern parts of the study area during both seasons. Residents in these areas have been exposed to high fluoride levels for an extended period, leading to health issues related to fluorosis. Our hydrogeochemical analysis attributes fluoride dominance to the Cl--SO42- water type in both seasons. Furthermore, the relationship between fluoride and pH, HCO3-, Ca2+, and Na+ suggests the influence of geological factors in fluoride dissolution under alkaline conditions, while a reverse cation exchange process and increasing calcium concentration inhibit fluoride concentration. Saturation indices indicate that the unsaturated state of gypsum dissolution contributes to elevated fluoride levels in groundwater. Additionally, Gibbs plots highlight rock-water interactions as a significant factor influencing groundwater chemistry in the study area. Based on our hazard quotient (HQ) investigation, children are at a higher risk during both seasons compared to adults, with the central and northern regions showing alarming HQ values. These findings underscore the urgent need for enhanced groundwater quality monitoring and a comprehensive assessment of health risks, providing valuable insights for groundwater safety management in vulnerable areas of this region.

Groundwater hydrogeochemistry and non-carcinogenic health risk assessment in major river basins of Punjab, India

The Indian Punjab state is drained by the four rivers, along with a well-connected network of canals, and is now dealing with a slew of water quality issues and problems. In this study, basin-wise hydrogeochemical modelling of 323 groundwater samples and identification of NO3- and F- enrichment pathways in aquifer systems of Punjab were studied using different plots and multivariate statistics. To evaluate the groundwater quality and human health risks, an entropy-based water quality index and Monte Carlo simulation were used, respectively. Spatial distribution of NO3- indicated that its very high values were prominent in parts of southwestern Punjab falling under LSRB, along with few pockets in eastern and northeastern Punjab falling under MSRB and GRB. High NO3- values (> 45.0 mg/L) were found in 15.0% of Ravi River Basin (RRB) groundwater samples, 22.86% of Beas River Basin (BRB), 23.52% of Middle Sutlej River Basin (MSRB), 36.9% of Lower Sutlej River Basin (LSRB), and 21.31% of Ghaggar River Basin (GRB). The spatial distribution of NO3- revealed elevated concentrations (> 100 mg/L) in the southwestern part of Punjab, particularly in LSRB and localized pockets in the eastern and northeastern areas of Punjab within MSRB and GRB. High F- concentration (> 1.5 mg/L) was observed in 15.12% and 21.31% groundwater samples of LSRB and GRB, respectively. Spatially southern parts falling under LSRB and GRB reflected high F- content (> 1.5 mg/L) in groundwater. In LSRB, evaporative and anthropogenic processes influence the groundwater quality. The results of interionic relationships and statistical analysis revealed that NO3- has anthropogenic origin and that is being aggravated by leaching, the evaporation processes, animal excreta, septic tanks and irrigation return flows in LSRB and GRB, while F- is geogenic in nature. Hazard index (HI) values in 14.63%, 22.2%, 24.6%, 49.58%, and 34.42% samples for adults and 21.95%, 27.7%, 42.0%, 72.3%, and 52.46% samples for children were higher than unity in RRB, BRB, MSRB, LSRB, and GRB, respectively. The basin-wise demarcation of various groundwater quality parameter and assessment of human health risk would be of significance for the management of water resources.

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.

Classification of groundwater using multivariate statistical methods: a case study from a part of Haryana, northwestern India

This article aimed to estimate the local underground water associations, which will, in turn, be applied to discuss the location of each underground water cluster in the flow system. Additionally, this investigation intended to evaluate underground water's aptness from aquifers of the study area for domestic agricultural activities and the prime sources of alteration in the water chemistry. Geographically, the region does not have the privilege of the river (except the Yamuna in the eastern part) running through it and thus, has to rely heavily on groundwater. Therefore, it is necessary to study the groundwater characteristics in this region. This investigation manifested two sub-surface water associations (groups) showing two prime underground water types in the study area: the calcium-magnesium-bicarbonate water-types, groups (clusters) 1 member; and the sodium-bicarbonate-chloride water-types consisting of Group 2 members during post-monsoon and pre-monsoon periods. Group 1 is typical of underground water in recharge regions of the underground water flow system. It is the cleanest underground water type in part except for a few groundwater samples. The highest mean salinity was vested in group 2, the signature of underground waters in discharge regions of the underground water flow system. This investigation observes that three principal elements controlled the hydro-chemistry of underground water in the study area: chemical fertilizers from farms, carbonate mineral weathering, calcite, fluorite, silicate minerals, and exchange of cations in the region. All the underground water groups had high sodium (Na) concentrations and will cause the Na-hazard when applied for agricultural activities. Most samples of groups 1 and 2 were within the class II and I area of Doneen's plot during both periods and were therefore acceptable for agricultural activities in the investigation region.

State-of-the-art of research progress on adsorptive removal of fluoride-contaminated water using biochar-based materials: Practical feasibility through reusability and column transport studies

Fluoride (F^-) is one of the essential elements found in soil and water released from geogenic sources and several anthropogenic activities. Fluoride causes fluorosis, dental and skeletal growth problems, teeth mottling, and neurological damage due to prolonged consumption, affecting millions worldwide. Adsorption is an extensively implemented technique in water and wastewater treatment for fluoride, with significant potential due to efficiency, cost-effectiveness, ease of operation, and reusability. This review highlights the current state of knowledge for fluoride adsorption using biochar-based materials and the limitations of biochar for fluoride-contaminated groundwater and industrial wastewater treatment. Biochar materials have shown significant adsorption capacities for fluoride under the influence of low pH, biochar dose, initial concentration, temperature, and co-existing ions. Modified biochar possesses various functional groups (-OH, -CC, -C-O, -CONH, -C-OH, X-OH), in which enhanced hydroxyl (-OH) groups onto the surface plays a significant role in fluoride adsorption via electrostatic attraction and ion exchange. Regeneration and reusability of biochar sorbents need to be performed to a greater extent to improve removal efficiency and reusability in field conditions. Furthermore, the present investigation identifies the limitations of biochar materials in treating fluoride-contaminated drinking groundwater and industrial effluents. The fluoride removal using biochar-based materials at an industrial scale for understanding the practical feasibility is yet to be documented. This review work recommend the feasibility of biochar-based materials in column studies for fluoride remediation in the future.

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.

Spatial and seasonal variability, control factors and health risk of fluoride in natural water in the Loess Plateau of China

Hundreds of millions of people around the world are currently exposed to excessive amounts of fluoride (F^-) in drinking water. Although the factors controlling the spatiotemporal distribution of F^- contents have been analyzed, their contributions have rarely been quantified. In this study, 510 water samples were collected in the dry and wet seasons in China's Loess Plateau to investigate the spatial and seasonal distribution, controlling factors, and potential health risks of F^- in natural water. High-F^- waters were mainly distributed in valley areas of the Loess Plateau, and more severe fluoride pollution of streamwater and groundwater was found in the wet and dry seasons, respectively. Mineral dissolution, competitive adsorption, adsorption/desorption and cation exchange jointly controlled F^- enrichment. Spatiotemporal distribution of high-F^- levels was mainly determined by climate and streamwater-groundwater connectivity in the dry season, with contribution rates of 41.7% and 37.6%, and by terrain and anthropogenic activities in the wet season, with contribution rates and 49.9-55.6% and 30.7%, respectively. Fluoride in groundwater through oral intake posed the greatest health risks to infants, followed by children, teenagers and adults in the dry and wet seasons. This study provides a scientific basis for the effective management of high-F^- water in arid regions.

Increase in fluoride concentration in mine water in Shendong mining area, Northwest China: Insights from isotopic and geochemical signatures

Mine water poses severe threats to the quality of the water supply and ecological environment of the Shendong mining areas owing to its excessive fluoride (F^-) content. However, the geochemical behaviours and enrichment mechanisms responsible for F⁻ enrichment during mining activities are not fully understood. In total, 18 Yanan groundwater and 45 mine water samples were collected to analyse the spatial distribution, hydrogeochemical behaviours, and formation mechanisms related to elevated F^- levels by analysing the stable isotopes and water-rock interactions. In this study, F^- concentrations in mine water samples varied from 0.16 to 12.75 mg/L, with a mean value of 6.10 mg/L, and 77.78% of the mine water samples had a concentration that exceeded China's national standards (1.00 mg/L) for drinking water. The F^- concentration was markedly high in the mine water samples, with the mean F^- concentration being 1.58 times of that in the Yanan groundwater samples. The results of stable isotopes (¹⁸O_H2O, D, ³⁴S_SO4, and ¹⁸O_SO4) and water-rock interaction analyses suggested that cation exchange and competitive effects were the dominant factors responsible for elevated F^- concentration in mine water during mining activities. Thus, the weathering of F-bearing minerals, agriculture, and domestic activities do not play a significant role in the secondary enrichment of F^- concentration.

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.

Hydrogeochemical characterizations and quality evaluation of groundwater in the major river basins of a geologically and anthropogenically driven semi-arid tract of India

Access to clean drinking water has been acknowledged as a human right and assessing the hydrogeochemistry and groundwater quality status plays an important role in proving cleaner and safer water for human consumption. This study evaluated the sources and driving factors of the groundwater facies in the five major river basins (viz. Ajay, Mayurakshi, Kopai, Brahmani and Dwarka) of an agroeconomic semi-arid Indian tract through hydrogeochemical and principal component analyses based on 2200 groundwater samples (N_s = 2200) obtained during the pre- and post-monsoon cycles from 1100 wells (N_w = 1100). The results revealed that minerals weathering, ion/reverse ion exchange, mixing and evaporation processes along with anthropogenic inputs are responsible for the deteriorated groundwater quality of the river basins. The study has considered the cokriging approach that uses geostatistical and multivariate statistical techniques to interpolate a dataset. To determine the spatio-seasonal variabilities of the groundwater facies more accurately, the estimation accuracies of different interpolation techniques viz. inverse distance weighting, kriging/cokriging and splines techniques were compared and kriging/cokriging was found to represent the variability more accurately. Shannon's entropy theory was employed to assess the groundwater quality of the river basins as it eliminates the subjective bias and inherent uncertainties of the groundwater systems. Groundwater in ~37.45-38.42% of the total area was moderate to extremely poor for human consumption where 10.40-12.14%, 9.09-12.40%, 21.18-22.35%, 15.20-19.93% and 6.48-8.80% samples from the Ajay (N_w = 175), Brahmani (N_w = 175), Dwarka (N_w = 180), Kopai (N_w = 350) and Mayurakshi (N_w = 220) river basins exhibited unfit to drink water quality. The sensitivity of the water quality model was analyzed to identify the influences of the individual parameters which revealed that the outcome does not depend solely on one parameter. The study recommends adaptation of the treatment techniques to ensure clean drinking water for the residents. Managed aquifer recharge techniques might also improve the groundwater quality in certain areas.

Assessment of shallow aquifer vulnerability to fluoride contamination using modified AHP-DRASTICH model as a tool for effective groundwater management, a case study in Yuncheng Basin, China

Vulnerability assessment is an effective tool for the precaution of groundwater quality to specific pollutants. Groundwater fluoride pollution is severe and universal in the world, however, the method of groundwater risk assessment to fluorine is never built. The objective of this study was to establish an effective method to assess the potential level of groundwater vulnerability to fluoride by a modification of the typical DRASTIC model. The hydrogeochemical (H) factor (pH and TDS) was designed as a complementary parameter in the DRASTICH model. The weights of the parameters were revised by Analytical Hierarchy Process (AHP) method. The built AHP-DRASTICH model was applied to evaluate the groundwater vulnerability to fluoride contamination in Yuncheng basin, northern China, where groundwater with high fluoride concentration occurred extensively. The assessment result indicates that about 40 percent of the area belonged to relatively high and high vulnerability zones, which mainly distributed at the central part of the basin with strong evaporation and longer water-rock interaction time. The AHP-DRASTICH model shows a stronger positive correlation between risk scores and F concentration, giving better vulnerability assessment to F pollution compared with DARSTIC and DRASTICH models. The AHP-DRASTICH model is reliable and useful for guiding government and policy maker to take effective actions protecting groundwater from specific pollution.

Groundwater quality assessment and hydrogeochemical processes in typical watersheds in Zhangjiakou region, northern China

It is of significance to elucidate the groundwater quality and hydrogeochemical processes for sustainable utilization of groundwater resources in water shortage regions. A total of 256 groundwater samples were collected in typical watersheds in Zhangjiakou, northern China. The hydrochemical parameters, conventional ions, and trace elements were measured, and δD and δ18O data were collected to delineate the groundwater quality and hydrogeochemical processes. The results showed that 32.91% of the groundwater could be directly used for drinking water sources in the Bashang Plateau, north of the study area. The F- and NO3--N were the main parameters above the standard threshold for drinking water. In contrast, the groundwater quality in the Baxia River Basins, south of the study area, was of a better scenario. Nonetheless, high concentrations of F-, total hardness, and SO42- were still observed. Most samples in the Bashang Plateau had relatively higher salinity than the Baxia River Basins. Both surface water and groundwater in the study area originated from local meteoric water with considerable hydraulic connections. The high-fluoride groundwater was primarily formed by dissolution of fluoride-rich minerals under conditions of high pH and Na+, low Ca2+, and rich in HCO3-. The dissolution of carbonate and silicate minerals accompanied by strong cation exchange and weak evaporation was the dominant water-rock interaction affecting the hydrochemical composition of groundwater, and anthropogenic NO3- input had an extra influence on hydrochemical process. This study provides a scientific guideline for the protection and allocation of local groundwater resources.

Fluoride and nitrate contamination of groundwater in the Loess Plateau, China: Sources and related human health risks

Fluoride (F^-) and nitrate (NO₃^-) in groundwater have caused serious health problems worldwide. However, in the Chinese Loess Plateau where groundwater is the primary source of drinking water, previous studies have rarely reported the health risks from fluoride and nitrate in groundwater. Therefore, we collected 105 groundwater samples (78 from shallow aquifers and 27 from deep aquifers) from the western district of the Loess Plateau for physicochemical and isotopic analysis to investigate the sources of F^- and NO₃^- in groundwater and associated health risks. Fluoride concentration in 73.1% of shallow groundwater and 22.2% of deep groundwater exceeds 1.5 mg/L, while NO₃^- content in 76.3% of shallow groundwater and 51.9% of deep groundwater surpasses 50 mg/L. High-F^- groundwater is associated with HCO₃-Na, SO₄-Na·Mg and Cl-Na·Mg types water. Fluorine-bearing minerals dissolution, cation exchange, calcite precipitation, evaporation, and anthropogenic activities contribute significantly F^- in groundwater. Mixing with shallow groundwater is an important source of F^- in deep groundwater. The NO₃^- content is highest in Cl type water, followed by SO₄ type and HCO₃ type water. NO₃^- mainly originates from soil organic nitrogen (SON), chemical fertilizers (CF), and manure and sewage (M&S). Nitrification is the dominant transformation process of nitrogen nutrients in groundwater. The hazard index (HI) values for shallow groundwater are 0.203-9.232 for adults, 0.253-11.522 for teenagers, 0.359-16.322 for children, and 0.507-23.043 for infants, while those for deep groundwater are 0.713-5.813 for adults, 0.890-7.254 for teenagers, 1.261-10.277 for children, and 1.780-14.508 for infants. Approximately 96.2% of shallow groundwater poses non-carcinogenic risks to infants and children, followed by 92.3% to teenagers, and 89.7% to adults. All deep groundwater poses non-carcinogenic risks to infants and children, followed by 92.6% to teenagers, and 74.1% to adults. This study is helpful to develop strategies for the integrated management of high fluoride or nitrate groundwater in arid areas.

Hydrogeochemistry and health hazards of fluoride-enriched groundwater in the Tarim Basin, China

Fluoride (F^-) enrichment reduces the availability of groundwater resources in the arid region, and it is thus important to investigate the hydrogeochemistry and health hazards of fluoride-enriched groundwater. Seventy-two groundwater samples (20 unconfined samples from the piedmont plain, 22 unconfined samples and 30 shallow confined samples from the alluvial plain) were collected in the Tarim Basin of China to illustrate the geochemical processes driving the F^- enrichment and the incidence of dental fluorosis. The patterns of average ions contents in groundwater are Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ and SO₄^2- > Cl^- > HCO₃^- > NO₃^- > F^-. The highest F^- concentration (average 2.16 mg/L) is observed in unconfined groundwater in the alluvial plain, while the lowest (average 0.63 mg/L) is recorded in unconfined groundwater in the piedmont plain. Approximately 5.0% of unconfined groundwater in the piedmont plain, 90.9% of unconfined groundwater and 33.3% of shallow confined groundwater in the alluvial plain contain F^- concentrations exceeding 1.0 mg/L (Chinese drinking water standard). Mineral dissolution, cation exchange, and evaporation play a significant role in the formation of solutes in groundwater. High-F^- groundwater is mostly associated with SO₄·Cl-Na·Ca, SO₄·Cl-Na·Mg, and SO₄·Cl-Na types water. Thermodynamic simulations reveal that the dissolution of F-bearing minerals (e.g., fluorite) significantly controls the F^- contents in groundwater. High concentrations of F^- are closely related to high HCO₃^-, high Na⁺, high salinity, cation exchange, and evaporation. This demonstrates that high F^- concentrations are caused by the increase in fluorite solubility due to high ionic strength, Ca²⁺ consumption and the desorption of F^- from solid surfaces under alkaline conditions. Mixing with the upper unconfined groundwater plays a vital role in the enrichment of F^- in shallow confined groundwater in the alluvial plain. The health risk assessment based on Dean's classification indicates that the percentage prevalence of fluorosis for boys aged 6 to 18 is 15.5% for Yecheng (YC), 18.4% for Zepu (ZP), 33.3% for Shache (SC), 29.8% for Maigaiti (MG), and 44.9% for Bachu (BC), while that for girls of the same age is 14.3% for YC, 24.3% for ZP, 42.2% for SC, 41.4% for MG, and 45.3% for BC. For male and female adults aged between 19 and 68, the percentage prevalence of fluorosis is: YC (11.5%, 12.0%), ZP (18.3%, 20.0%), SC (35.4%, 35.0%), MG (32.5%, 39.7%), and BC (42.4%, 44.3%). It is obvious that younger generation, especially girls, suffers from more severe dental fluorosis. This study has implications for the effective management of high-F^- groundwater in arid regions.

Assessment of geochemical modeling applications and research hot spots-a year in review

Geochemical modeling has been employed in several fields of science and engineering in recent years. This review seeks to provide an overview of case studies that applied geochemical modeling in the 2019 year, which includes over 250 articles. This review is intended to inform new users on the possibilities that geochemical modeling brings, while also informing existing and past users on its latest developments. The survey of studies was conducted with an emphasis on the modeling techniques, the objective of studies, the prevalent simulated variables and the use of specific software packages. The analysis showed that geochemical modeling is still predominantly employed in experimental projects and in the form of equilibrium modeling. PHREEQC and Visual MINTEQ were recognized as the most popular software packages for simulating a wide range of processes, using equilibrium or other geochemical modeling forms. The study of fluid-rock interactions and pollution and remediation processes can be regarded as the principal geochemical modeling objectives, constituting 37% and 36% of the reviewed studies, respectively. Focusing on fluid-rock interactions, hydrogeochemical processes, carbon capture and storage and enhanced oil recovery have been the main topics examined with geochemical modeling. Assessments of the toxicity of metals in terms of leachate and mobilization, as well as their removal from soil and water systems, have been major topics investigated with the aid of geochemical modeling in terms of pollution and remediation research. It was found that the scholars benefit from geochemical modeling in their research both as a main technique and as an accessory tool. Saturation index, elemental concentration and speciation, mineral mass and composition and pH were among the most common variables modeled in reviewed studies. Geochemical modeling has gained a wider user base in recent years, and many research groups have used it in consecutive studies to deepen knowledge. However, much potential for further dissemination still remains.

Mobilization of Arsenic and Other Naturally Occurring Contaminants during Managed Aquifer Recharge: A Critical Review

Population growth and climate variability highlight the need to enhance freshwater security and diversify water supplies. Subsurface storage of water in depleted aquifers is increasingly used globally to alleviate disparities in water supply and demand often caused by climate extremes including floods and droughts. Managed aquifer recharge (MAR) stores excess water supplies during wet periods via infiltration into shallow underlying aquifers or direct injection into deep aquifers for recovery during dry seasons. Additionally, MAR can be designed to improve recharge water quality, particularly in the case of soil aquifer treatment and riverbank filtration. While there are many potential benefits to MAR, introduction of recharge water can alter the native geochemical and hydrological conditions in the receiving aquifer, potentially mobilizing toxic, naturally occurring (geogenic) contaminants from sediments into groundwater where they pose a much larger threat to human and ecosystem health. On the basis of the present literature, arsenic poses the most widespread challenge at MAR sites due to its ubiquity in subsurface sediments and toxicity at trace concentrations. Other geogenic contaminants of concern include fluoride, molybdenum, manganese, and iron. Water quality degradation threatens the viability of some MAR projects with several sites abandoning operations due to arsenic or other contaminant mobilization. Here, we provide a critical review of studies that have uncovered the geochemical and hydrological mechanisms controlling mobilization of arsenic and other geogenic contaminants at MAR sites worldwide, including both infiltration and injection sites. These mechanisms were evaluated based on site-specific characteristics, including hydrological setting, native aquifer geochemistry, and operational site parameters (e.g., source of recharge water and recharge/recovery cycling). Observed mechanisms of geogenic contaminant mobilization during MAR via injection include shifting redox conditions and, to a lesser extent, pH-promoted desorption, mineral solubility, and competitive ligand exchange. The relative importance of these mechanisms depends on various site-specific, operational parameters, including pretreatment of injection water and duration of injection, storage, and recovery phases. This critical review synthesizes findings across case studies in various geochemical, hydrological, and operational settings to better understand controls on arsenic and other geogenic contaminant mobilization and inform the planning and design of future MAR projects to protect groundwater quality. This critical review concludes with an evaluation of proposed management strategies for geogenic contaminants and identification of knowledge gaps regarding fate and transport of geogenic contaminants during MAR.

Hydrogeochemistry and fluoride contamination in Jiaokou Irrigation District, Central China: Assessment based on multivariate statistical approach and human health risk

Too little and too much fluorine are potentially hazardous for human health. In the Jiaokou Irrigation District, ionic concentrations, hydrogeochemistry, and fluoride contaminations were analyzed using correlation matrices, principal component analysis (PCA), and health risk assessment. The patterns for the average cation and anion concentrations were Na⁺ > Mg²⁺ > Ca²⁺ > K⁺ and SO₄^2- > HCO₃^- > Cl^- > NO₃^- > CO₃^2-. The fluoride concentrations ranged between 0.29 and 8.92 mg/L (mean = 2.4 mg/L). 5% of the samples displayed lower than the recommended limit of 0.5 mg/L fluoride content, while 69% exceeded the allowable limits of 1.5 mg/L for drinking. The low F^- content is distributed in a small part of the southeast, while elevated F^- mainly in the central area of the study region. The PCA results indicated three principal components (PC), PC1 having the greatest variance (45.83%) and affected by positive loadings of TDS, Cl^-, SO₄^2-, Na⁺, and Mg²⁺, PC2 accounting for 17.03% and dominated by Ca²⁺, pH, HCO₃^-, and K⁺, and PC3 representing 12.17% and mainly comprising of CO₃^2-. High fluoride groundwater is of the SO₄-Cl-Na type, followed by HCO₃-Na type. Evaporation and ion exchange play important roles in producing high fluoride groundwater. Furthermore, saturation index and anthropogenic activities also promote the high fluoride concentrations. The values of the total hazard quotient of 93% groundwater samples were greater than 1 for infants, followed by 85% for children, 68% for teenagers, and 57% for adults. Non-carcinogenic health risks to infants may occur over the entire study area, while for adults, health risks are mainly found in Weinan and Pucheng. High fluorine may have a potential negative influence on neurodevelopment, especially for infants and children. Adults in this region have serious dental fluorosis and skeletal fluorosis because of long-term drinking of high fluoride groundwater. Therefore, measures, including using organic fertilizers, strengthening defluoridation process, and optimizing water supply strategies, are necessary in this area.

Hydrogeochemical and statistical analysis of high fluoride groundwater in northern China

Understanding the formation of high fluoride (F^-) groundwater in water-scarce northern China is critical for the sustainable development of the region. This study investigates the effects of F^- enrichment in groundwater from seven typical regions of northern China, including Datong, Guide, Junggar, Yinchuan, Taiyuan, and Tarim basins and the North China Plain. A literature survey of 534 samples of selected regions showed that 45.13% of groundwater F^- exceeded the 1.0 mg/L of Chinese drinking water guideline. Based on the geological background and hydrogeochemical analysis, in Datong and Yinchuan basins and part of the North China Plain, the main types of groundwater are soda water and controlling processes of F^- enrichment are salinization, mineral dissolution, and desorption. In Taiyuan and Guide basins with Cl-Na water type, F^- enrichment is mainly affected by salinization, cation exchange, and evaporation. The hydrogeochemical characteristics of high F^- groundwater in Tarim and Junggar basins reflect the extent of salinization and weathering dissolution of minerals in groundwater. According to PCA, the contribution of salinization and mineral dissolution to F^- enrichment is relatively high. Under the alkaline condition, groundwater with high Cl^-, HCO₃^-, and Na⁺ concentration favors F^- enrichment. Based on HCA, index clustering category I explains the influence of pH and buried depth on F^- enrichment, and category II explains the effect of different ions. It is concluded that F^- enrichment in groundwater is related to hydrogeochemical processes and hydrogeological conditions. The hydrogeochemical and alkaline conditions of groundwater are regulated by mineral dissolution, ion exchange, and evaporation, resulting in different degrees of F^- enrichment.

Two distinct mechanisms of fluoride enrichment and associated health risk in springs' water near an inactive volcano, southeast Iran

Groundwater fluoride contamination is a major issue of water pollution in the world with health hazards such as dental and skeletal fluorosis. This research focused on exposure to the high concentration of fluoride in the springs water in the Bazman volcanic area, southeast Iran. The combination of chemical/isotopic analysis, geochemical modeling, health risk assessment and multivariate statistical methods were applied to investigate the contamination and sources of fluoride in the samples. Groundwater samples were collected from cold and thermal springs. Major ions, fluoride, trace elements and stable isotopes δ¹⁸O and δD were measured in the samples using standard methods, ICP-MS and OA-ICOS, respectively. Fluoride content in springs varied from 0.5 to 3.75 mg/L with an average value of 1.66 mg/L. The highest fluoride concentrations were observed in the eastern cold springs while thermal springs showed the minimum fluoride contents. The majority of samples showed F contents higher than the calculated optimal concentration of fluoride (0.75 mg/L). Reaction of fluorite mineral with HCO₃ and replacement of F in clay minerals and metal oxy-hydroxides with OH^- in water were likely cause fluoride enrichment in the eastern springs. Whereas, in the western springs and thermal springs, origin of fluoride was related to weathering of muscovite, cryolite, apatite and fluoroapatite minerals. The δ¹⁸O and δ²H of the water samples displayed the impact on evaporation on fluoride enrichment in all spring water samples. The average value of contamination index (C_d) in the water samples was 1.94 categorizing medium risk level while springs S7, S8, S9 and S4 were above the threshold value of C_d index. The fluoride hazard quotient (HQ) showed that 25%, 44%, 56% and 0% of springs' water resources had high risk level for age group of adults, teenager, children and infants, respectively. Therefore, health risk of fluoride in drinking water resources were in the following order: children > teenager > adults > infants.

Fluoride network and circular economy as potential model for sustainable development-A review

Fluorine is the most reactive elements among the halogen group and commonly and ubiquitously occurs as fluoride in nature. The industrial processes produce fluoride by-products causing the increase of unwanted environmental levels and consequently posing risk on human and environmental health worldwide. This review gives a fundamental understanding of fluoride networks in the industrial processes, in the geological and hydrological transport, and in the biological sphere. Numerous biological pathways of fluoride also increase the risk of exposure. Literature shows that various environmental levels of fluoride due to its chemical characteristics cause bioaccumulation resulting in health deterioration among organisms. These problems are aggravated by emitted fluoride in the air and wastewater streams. Moreover, the current waste disposal dependent on incineration and landfilling superpose to the problem. In our analysis, the fluoride material flow model still follows a linear economy and reuse economy to some extent. This flow model spoils resources with high economic potential and worsens environmental problems. Thus, we intend a shift from the conventional linear economy to a circular economy with the revival of three-dimensional objectives of sustainable development. Linkages between key dimensions of the circular economy to stimulate momentum for perpetual sustainable development are proposed to gain economic, environmental and social benefits.

Geochemical factors controlling the occurrence of high-fluoride groundwater in the western region of the Ordos basin, northwestern China

Hydrogeochemistry and isotope hydrology were carried out to investigate the spatial distribution of fluoride (F^-) and the mechanisms responsible for its enrichment in the western region of the Ordos basin, northwestern China. Sixty-two groundwater samples from the unconfined aquifer and fifty-six from confined aquifer were collected during the pre-monsoon (June 2016). Over 77% of groundwater samples from the unconfined aquifer (F^- concentration up to 13.30 mg/L) and approximately 66% from confined aquifer (with a maximum F^- concentration of 3.90 mg/L) exhibit F^- concentrations higher than the Chinese safe drinking limit (1.0 mg/L). High-F^- groundwater presents a distinctive hydrochemical characteristic: a high pH value and HCO₃^- concentration with Ca-poor and Na-rich. Mineral dissolution (e.g., feldspar, calcite, dolomite, fluorite), cation exchange and evaporation in the aquifers predominate the formation of groundwater chemistry, which are also important for F^- enrichment in groundwater. Mixing with unconfined groundwater is a significant mechanism resulting in the occurrence of high-F^- groundwater in confined aquifer. These findings indicate that physicochemical processes play crucial roles in driving F^- enrichment and that may be useful for studying F^- occurrence in groundwater in arid and semi-arid areas.