Occurrence and hydrogeochemical characteristics of high-fluoride groundwater in Xiji County, southern part of Ningxia Province, China

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.

Pan India fluoride hazard assessment in groundwater

Fluoride (F¯) contamination in groundwater in India has gained global attention due to human health hazards. India's hydrogeological heterogeneity, spatio-temporal variability of F¯, and health hazards due to geogenic and geo-environmental control pose unique challenges. Addressing these with only a single region-specific study is not possible. Therefore, this study provides an in-depth, holistic analysis of pan India F¯ contamination, controlling factors, and health hazards using a coupled advanced geostatistical and geospatial approach. Alarming F¯ contaminations are identified in Rajasthan, Telangana, Western Andhra Pradesh, Eastern Karnataka, Parts of Haryana, Gujarat, Madhya Pradesh, Tamil Nadu, Uttar Pradesh, Jharkhand, Bihar, and Chhattisgarh. Probabilistic health-risk evaluation using hot-spot, showed similar spatio-temporal distribution of F¯ contamination. The hazard quotient (HQ) for high F¯ shows more adversity to children than adults. Nationally, 8.65 % and 7.10 % of pre- and post-monsoon sites exceed the recommended safe limit of 1.50 mg/L. The highest average F¯ concentration is in Rajasthan. Very high-risk skeletal fluorosis is possible at around ≤ 2 %, whereas dental caries due to deficiency in F¯ concentration is approximately 40 %. A decisive hierarchy of lithology, geomorphology, soils, and lineaments control are identified on F¯ contamination. Climatic conditions are pivotal in governing all these controlling variables. Thus, in arid/semi-arid dry western regions, F¯ contamination is much higher than in the humid areas. Integration of strengths, weaknesses, opportunities, and threats (SWOT) analysis with the results can aid policymakers and government authorities in achieving sustainable remedial measures for future adaptability.

A review on fluoride contamination in groundwater and human health implications and its remediation: A sustainable approaches

Contamination of drinking water due to fluoride (F-) is a major concern worldwide. Although fluoride is an essential trace element required for humans, it has severe human health implications if levels exceed 1.5 mg. L-1 in groundwater. Several treatment technologies have been adopted to remove fluoride and reduce the exposure risk. The present article highlights the source, geochemistry, spatial distribution, and health implications of high fluoride in groundwater. Also, it discusses the underlying mechanisms and controlling factors of fluoride contamination. The problem of fluoride-contaminated water is more severe in India's arid and semiarid regions than in other Asian countries. Treatment technologies like adsorption, ion exchange, precipitation, electrolysis, electrocoagulation, nanofiltration, coagulation-precipitation, and bioremediation have been summarized along with case studies to look for suitable technology for fluoride exposure reduction. Although present technologies are efficient enough to remove fluoride, they have specific limitations regarding cost, labour intensity, and regeneration requirements.

Spatial evolution analysis of groundwater chemistry, quality, and fluoride health risk in southern Hebei Plain, China

The present study investigated ion and fluoride concentrations in groundwater and their associated health risks to local populations in the southern Hebei Plain during 2018-2020. A total of 336 groundwater samples were collected from monitoring wells at 112 different locations. Statistical analysis, Gibbs diagram, principal ion ratio, and saturation index were carried out to clarify the chemical characteristics and control mechanism of groundwater. The results indicated that the groundwater types in the study area were mainly HCO₃-Ca, Cl-Na, and SO₄-Ca. The concentrations of cations and anions were Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ and HCO₃^- > SO₄^2- > Cl^- > NO₃^- > F^-, respectively. Based on the water chemical parameters, the pollution index of groundwater (PIG) was used to comprehensively evaluate the water quality. The results showed that during the study period, 60.41% of groundwater samples were suitable for drinking purposes, and 39.59% needed purification treatment to meet the requirements of drinking water standards. The groundwater quality in the western pre-hill plain areas was good, while the water quality in the northeastern and southeastern areas was poor and contaminated to varying degrees. Groundwater quality was mainly affected by the combined effect of total dissolved solids (TDS), Na⁺, Mg²⁺, Cl^-, SO₄^2- and HCO₃^- concentrations. Fluoride concentrations in the groundwater samples ranged from 0.07 to 8.51 mg/L, with 44% of the samples containing fluoride below the recommended limit of 0.5 mg/L, which would put the population at risk of dental caries. Also, 8% of the samples exceeded the permissible limit for fluoride in drinking water (1.5 mg/L), which would expose the local population to the risk of fluorosis. The human health risk evaluation of fluoride showed significant differences in non-carcinogenic effects between two different groups of children and adults. HI_in values ranged from 0.08 to 10.19 for children and 0.03 to 4.65 for adults, with hazard indices greater than 1 at 29.16% and 10.11%, respectively. This indicates that children have a greater exposure risk than adults, and the entropy of higher risk is mainly distributed in the northeast of the study area. Based on the above analysis of the spatial evolution of groundwater chemistry, water quality, and fluoride health risks in the southern Hebei Plain region, corresponding protection and management measures were proposed, which also provided reference significance for the effective use of drinking water and health risk prevention in the region.

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.

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.

Impact of land use and cover on shallow groundwater quality in Songyuan city, China: A multivariate statistical analysis

The utilization and development of land resources is an important process in which human activities affect groundwater quality. However, the impact of land use on groundwater chemical composition has complex multiple relationships, and is affected by the scale of the buffer zone. Based on these problems, this study used correlation analysis (CA) and principal component analysis (PCA) to discuss the mechanism of the effect of land use/land cover (LULC) on the hydrochemical composition of groundwater in Songyuan City. Samples were divided into two groups, i.e., quaternary unconfined aquifer (0-30 m) and quaternary confined aquifer (30-100 m). By comparing the variation trends of the correlation coefficient and cumulative variance interpretation rate of PCA in different buffer ranges, it was found that the optimal buffer range was 3000 m. Cropland had the greatest impact on groundwater hydrochemistry in the city. The transformation of natural landscapes (such as saline‒alkaline alkali land and grassland) to cropland inhibited salt accumulation in groundwater. This finding is noteworthy since few studies have involved areas where saline‒alkaline land is widely distributed. Compared with CA results, PCA results emphasized the deterioration of groundwater quality by agricultural pollution. Moreover, agricultural pollutants such as NO₃^- and K⁺ were accumulated in areas where cropland transitioned to natural landscapes such as grassland and water bodies. Considering that wide lakes and rivers provide the drainage area for irrigation water in the study area, the groundwater quality in the surrounding area was affected by the contaminated surface water. The multiple interaction relationship between LULC and hydrochemistry was further confirmed by the combination of principal component scores.

Characterization of drinking groundwater quality in rural areas of Inner Mongolia and assessment of human health risks

Groundwater is an important natural resource of drinking water in rural areas in Inner Mongolia, China. In this study, 4438 drinking groundwater samples were collected from the rural areas of 81 counties in Inner Mongolia, and were analyzed for 16 parameters, including pH, total hardness (TH), chemical oxygen demand (COD), total dissolved solids (TDS), sulfate (SO42-), chloride (Cl-), fluoride (F-), iron (Fe), manganese (Mn), arsenic (As), cadmium (Cd), hexavalent chromium (Cr), lead (Pb), aluminum (Al), cuprum (Cu), zinc (Zn). The groundwater quality was evaluated with water quality index (WQI) and human health risk assessment (HRA). Monte Carlo simulation were applied for the uncertainty and sensitivity analysis in the health risk assessment. The spatial map was employed based on the inverse distance weighted (IDW) interpolation technique. The results reveal that while the hazard quotient (HQ) suggests that the risk of single element contamination is feeble, the hazard index (HI) indicates a potential health risk for the local population. The observed cumulative carcinogenic risk (CCR) indicates a probable risks of carcinogenic health hazards in the study area. The sensitivity analysis revealed that daily ingestion rate (IR), exposure frequency (EF), and the concentrations of As, Mn, F-, and Cr are the most influential parameters for health hazards. The highly polluted areas are mainly distributed in the central and western regions of Inner Mongolia, including Xianghuangqi, New Barag Zuoqi, and Togtoh. It is observed that the groundwater may cause a potential health risk after long-term ingestion. The results of this study will contribute to groundwater management and protection in Inner Mongolia.

Risk factors for endemic chronic kidney disease of unknown etiology in Sri Lanka: Retrospect of water security in the dry zone

The prevalence of chronic kidney disease of unknown etiology (CKDu) is receiving considerable attention due to the serious threat to human health throughout the world. However, the roles of geo-socio-environmental factors in the prevalence of the CKDu endemic areas are still unknown. Sri Lanka is one of the countries most seriously affected by CKDu, where 10 out of 25 districts have been identified as the areas with the high prevalence of CKDu (10-20%). This review summarizes the geographical distribution of CKDu and its probable geochemical, behavioral, sociological, and environmental risk factors based on research related to hydrogeochemical influences on CKDu in Sri Lanka. More than 98% of CKDu patients have consumed groundwater as their primary water source in daily life, indicating the interactions of geogenic contaminants (such as F^-, total dissolved solids, Hofmeister ions) in groundwater is responsible for the disease. Apart from the hydrogeochemical factors, mycotoxins, cyanotoxins, use of some herbal medicines, dehydration, and exposure to agrochemicals were alleged as risk factors. Sociological factors, including poverty, living habits and anthropogenic activities, may also provoke the emergence of CKDu. Therefore, the interaction of geo-socio environmental risk factors should be sociologically and scientifically considered to prevent the prevalence of CKDu. Future in-depth studies are required to reveal the individual role of each of the postulated etiological factors, possibly using machine learning and advanced statistics.

Groundwater fluoride concentrations in the watershed sedimentary basin of Quetta Valley, Pakistan

Litho-geochemical characteristics of low and high fluoride (F^-) groundwater along with hydrological processes were investigated to delineate its genesis and enrichment mechanism in a watershed sedimentary basin. In this study, groundwater F^- concentration ranged from 0 to 20 mg/L with a mean and standard deviation of 2.8 and ± 3.7 mg/L, respectively. Out of N = 87, 63% of samples exceeded the World Health Organization (WHO) limit of 1.5 mg/L. The order of cationic and anionic dominance in groundwater samples with mean was found in decreasing order as Na⁺  > Mg²⁺  > Ca²⁺  > K⁺ and HCO₃^-  > SO₄^2-  > Cl^-  > PO₄^3-  > NO₃^- measured in milligrams per liter. Groundwater chemistry changed from Ca-HCO₃ to Na-HCO₃ type and low to high fluoride as we moved from mountain foot towards the synclinal basin. Low fluoride groundwater reflected weathering, recharge, and reverse ion exchange processes with Ca-HCO₃- and Ca-Mg-Cl-type water while high fluoride groundwater revealed base ion exchange, mixing, and desorption as dominant hydrological processes with Na-HCO₃ and Na-Cl types of water. Gibb's diagram showed rock weathering and mineral dissolution as the major geochemical processes controlling water chemistry with an insignificant role of evaporation in the semi-arid area. Fluoride was undersaturated with mineral fluorite, indicating fluoride in groundwater is released by secondary minerals. However, due to complex geological features, groundwater fluoride enrichment was affected by a broad-scale process across a wide area such as depth, residence time, and most important geomorphological units hosting the aquifer.

Groundwater fluoride chemistry and health risk assessment of multi-aquifers in Jilin Qianan, Northeastern China

Groundwater from deep confined aquifers is often recommended for use because of it's low fluoride health risk. Thus, this study appraised groundwater fluoride hydrochemistry in a multi-aquifer system in Jilin Qianan to determine the non-carcinogenic health risk liable from exploiting the respective aquifers. 124 samples collected from the tertiary confined aquifer (N), quaternary confined aquifer (Q₁), and quaternary phreatic aquifer (Q₃) during surveys in 2001 and 2017 was analyzed using hydrochemical, statistical, spatial, and health risk assessment methods. Results show that the dominant water facies in the respective aquifer layers was Na+K-HCO₃+CO₃ except in Q_1, where Ca+Mg - HCO₃ + CO₃ was marginally dominant. Fluoride concentrations outside the recommended guideline occurred in all the aquifers except N, where concentrations were optimum. The mean fluoride concentration of groundwater in the aquifers was of the order Q₃ (2017) > Q₃ (2001) > Q₁ > N ( mean 2.09, 2.03, 1.41 and 0.75 mg/L with 51.85%, 57.44%, 36.36% and 0% occurring beyond recommended guideline values respectively). Silicate weathering, cation exchange, and fluorite dissolution in an alkaline environment were the significant fluoride contributing processes. Evaporation and MgF⁺ complex additionally influenced Q₁ and Q₃ (2017). The total hazard quotient (THQ) from oral and dermal pathways shows fluoride health risks in the order: infant > children > adult. The associated risks likely from using water in the respective aquifer layers is of the order Q₃ (2017) > Q₃ (2001) > Q₁ > N. The mean groundwater fluoride in 2017 was marginally higher than that of 2001 ( 2.09>2.03 mg/L respectively) although the percentage of age group members disposed to fluoride risk from using water from Q₃ decreased from 2001 to 2017. Knowledge of local hydrogeology in exploiting deep groundwater free of fluoride pollution and on-site defluoridation treatment of groundwater was recommended in the study area and other areas with similar characteristics.

Assessment of potential health risk of major contaminants of groundwater in a densely populated agricultural area

As a key part of Bohai New Area development, Haixing County has been undergoing rapid development. In order to estimate potential risks of chemical parameters to human health of local residents, carcinogenic and non-carcinogenic risks via direct ingestion of drinking water were calculated using human health risk assessment (HHRS) based on triangular fuzzy number. The levels of pH, total dissolved solids, total harness, SO₄ ^2-, Na⁺, Cl^-, SO₄ ^2-, F^-, Fe (total iron), NO₃ ^-, and NO₂ ^- were more or less higher than the permissible limits except parameters As and Mn. The analysis results show that risk level for different crowds in the study area demonstrated an obvious variation, generally in the order of infants > children > adult males > adult females for non-carcinogenic risk values (R ⁿ), while the sequence of the carcinogenic risk values (R ^c) are adult males > adult females > children > infants. When the confidence level was 0.8, the non-carcinogenic risk values (R ⁿ) through drinking water intake were higher than 1, and this implied that potential health impacts on human health for local residents. However, the risks of carcinogenic risk values (R ^c) were lower than 1.0E-4, demonstrating minimal and acceptable health risk. Furthermore, according to the middle values (α = 1) of R ⁿ, the total non-carcinogenic risks for local residents were obtained in the following order: GW (Gaowan Town) > XJ (Xinji-Xiangfang County) > ZM (Zhaomaotao County) > HX (Haixing-Suji Town) > ZH (Zhanghuiting County) > XS (Xiaoshan County), and ZM > XJ > GW > HX > XS > ZH for R ^c. It was also found that the spatial distribution of fluoride level in drinking water is urgently needed to be identified. In conclusion, the potential health risks to residents should cause enough attention both from society and the academic community.

Assessment of water quality of best water management practices in lake adjacent to the high-latitude agricultural areas, China

A major inland alkalinity lake in Northeast China, the Chagan Lake, was studied for the changes of its water qualities over the past three decades. Water quality data, including total nitrogen (TN), total phosphorus (TP), pH, dissolved oxygen (DO), and fluoride (F^-), were analyzed to derive key indices for guiding water quality management. Our study found that the Chagan Lake had an average trophic state index (TSI) ranging 50 to 70; the average TSI for TP ranging between 70 and 80, and the average TSI for TN being 50. Over the past three decades, the TSI values generally trended lower, but there was a slight uptrend from 2012 onwards. Seasonal variations in the concentrations of TN and TP were identified. The TSI values in September were higher than those in May, while the values of un-ionized ammonia (UIA) during rainy seasons were higher than those during dry seasons. The average values of alkalinity and F^- in the lake water exceeded the upper limits set in the Chinese water quality standards, i.e., 20 mg/L and 1 mg/L, respectively. It was defined that the evolution of lake water quality proceeded in four consecutive periods, namely natural, deterioration, improvement, and risk period; the improvement period benefitted from a historical water conservation project. Our study concluded that the amount of irrigation discharge into the Chagan must be monitored, and controlled, in order to sustain the critical ecological functions currently provided by the Chagan Lake.

Assessment, formation mechanism, and different source contributions of dissolved salt pollution in the shallow groundwater of Hutuo River alluvial-pluvial fan in the North China Plain

With rapid urbanization and industrialization processes, the problem of groundwater pollution under the influence of various human activities has become increasingly severe in most developed areas of China. However, the problem of dissolved salt pollution caused by increasing concentrations of conventional ions is often overlooked and easier to be seen as a high background of natural formation rather than pollution. The Hutuo River alluvial fan in North China was selected as the study area; dissolved salt pollution is evaluated based on the factor analysis method (FA); groundwater exploitation, pollutant input, and the attenuation of the vadose zone were discussed to explain the salt pollution; the formation mechanism and different source contributions were also explored. The results show that the total hardness (TH) and nitrate are the main contributing indicators of salt pollution in the Hutuo River alluvial fan. The long-term overexploitation of groundwater promoted the leaching and nitrification reactions, resulting in a large area of moderate to strong salt pollution in the top unit of the alluvial fan, which accounted for 51.6% of the salt pollution according to the multivariate regression model. In addition, the input pollution generated by various types of pollution sources along with rainfall infiltration is also an important driving factor. The surface pollution load and hydrogeological conditions affected the cation exchange and leaching, resulting in a point distribution of strong salt pollution, with a contribution rate of 37.6%. The analysis of the factors that affect salt pollution and the specific contributions in different regions cannot only help decision-makers understand the causes of water quality deterioration but also propose solutions in a targeted manner.

High levels of fluoride contamination in groundwater of the semi-arid alluvial aquifers, Pakistan: evaluating the recharge sources and geochemical identification via stable isotopes and other major elemental data

Hydrogeochemical methods were integrated to delineate the geochemical factors controlling fluoride (F^-) contamination in groundwater at four sites in the districts of Lahore (Samada) and Kasur (Sari Chimba, Kot Maiga, and Chah Fatehwala) in Panjab province of Pakistan. Hydrochemical data and stoichiometric ratios indicate Na-Cl and Na-HCO₃ as the dominant water types with silicate weathering influencing overall hydrogeochemistry of the study area. The groundwater F^- concentrations ranged between 0.54 mg/L and 17.5 mg/L, with more than 70% samples having F^- concentrations above the World Health Organization (WHO) provisional drinking water guideline (1.5 mg/L). Saturation indices determined that 100% samples were saturated with respect to calcite and 96% samples were undersaturated with respect to fluorite, indicating the influence of calcite precipitation on fluoride enrichment. A positive correlation was observed between fluoride with pH, Na⁺, and HCO₃^-, confirming that high fluoride concentrations were the result of weathering of silicate minerals and the exchange of OH^- on clay surface under the alkaline pH conditions. The isotopic values of δ¹⁸O and δ²H in groundwater ranged from 9.14 to - 5.51‰ and 56.57 to - 39.5‰, respectively. The stable isotope data indicated the meteoric origin of groundwater with some evaporative effect, which is partly influencing groundwater quality such as high pH and salinity, as a result facilitating anion exchange (OH^- for F^-) on clays surface. The research indicates that the groundwater quality of the study area is not recommendable for drinking due to its high total dissolved solids (TDS) and elevated fluoride concentrations.

Fluoride contamination in groundwater and associated health risks in Karbi Anglong District, Assam, Northeast India

Health hazards due to excess intake of fluoride via groundwater contamination are a major concern worldwide. This study provides a comprehensive report on the human health risks associated with the consumption of groundwater contaminated by fluoride. Several groundwater samples were collected across 8 blocks of Karbi Anglong district of Assam, India. The concentration of fluoride was observed in the range of 0.15-17.13 mg/L. In 4 out of 8 studied blocks, the mean fluoride level exceeded the permissible limit (1.5 mg/L) as prescribed by the World Health Organization. Elevated fluoride levels in some parts of the district may be attributed mainly to dissolution from fluoride-containing minerals in the granitic rocks and regional geological settings. The health risk of fluoride was assessed in terms of hazard quotient (HQ). The HQ was observed in the ranges of 0.06-10.7 (adult) and 0.2-35 (children). Mean HQ values exceeded the safe level (HQ > 1) for children in all blocks, except B-6 and B-8. For adult population, the HQ value was above the safe limits in 13-40% of the sampled locations in different blocks and HQ values were within safe limits in B-6 and B-8. These findings suggest that some sites in the district need serious attention in order to ensure the health safety of local residents.

Fluoride contamination, health problems and remediation methods in Asian groundwater: A comprehensive review

In low concentration, fluoride is considered a necessary compound for human health. Exposure to high concentrations of fluoride is the reason for a serious disease called fluorosis. Fluorosis is categorized as Skeletal and Dental fluorosis. Several Asian countries, such as India, face contamination of water resources with fluoride. In this study, a comprehensive overview on fluoride contamination in Asian water resources has been presented. Since water contamination with fluoride in India is higher than other Asian countries, a separate section was dedicated to review published articles on fluoride contamination in this country. The status of health effects in Asian countries was another topic that was reviewed in this study. The effects of fluoride on human organs/systems such as urinary, renal, endocrine, gastrointestinal, cardiovascular, brain, and reproductive systems were another topic that was reviewed in this study. Different methods to remove fluoride from water such as reverse osmosis, electrocoagulation, nanofiltration, adsorption, ion-exchange and precipitation/coagulation were introduced in this study. Although several studies have been carried out on contamination of water resources with fluoride, the situation of water contamination with fluoride and newly developed technology to remove fluoride from water in Asian countries has not been reviewed. Therefore, this review is focused on these issues: 1) The status of fluoride contamination in Asian countries, 2) health effects of fluoride contamination in drinking water in Asia, and 3) the existing current technologies for defluoridation in Asia.

Distribution, formation and human-induced evolution of geogenic contaminated groundwater in China: A review

The sustainability of groundwater usage faces quality problem caused by anthropogenic activity as well as geogenic contamination. With varied climate zones, geomorphology and geological background, China faces a variety of geogenic contaminated groundwater (GCG) reported known as high TDS, Fe, Mn, As, F, I, NH₄⁺, U, Cr and low I, Se, etc., may still exist some others not fully known yet. The problem of GCG is more significant in northern China due to extensive groundwater usage, arid climate and widespread Holocene strata. High salinity groundwater is mainly distributed in semi-arid/arid northwestern inland basins and coastal areas. Elevated Fe and Mn are frequently concomitant and controlled by redox potential, prevailing in the Sanjiang Plain, Yellow River Basin, and middle and lower reaches of the Yangtze River Basin. High As groundwater occurs in reducing aquifer is mainly distributed in the Yellow River, Yangtze River and Huai River Basins as well as the Songnen Plain and Xinjiang. Fluoride is characterized by its areal distribution in northern China in comparison with scatter occurrence in the south. The dissolution of F-bearing minerals as well as evaporation effect both contribute to elevated F. High iodine groundwater mainly distributed in the Yellow-Huai-Hai River Basin and low iodine prevailing in piedmont areas both pose health issues. Iodine is related to decomposition of organic matter (OC) as well as marine origin. Contributed by OC mineralization naturally-occurring NH₄⁺ was found in reducing aquifers. The GCG triggers endemic disease in addition to reduce groundwater resource. The co-occurrence like high TDS and F, As and F are frequently observed posing major challenges for mitigation. Anthropogenic influence like abstraction and pollutant infiltration would alter groundwater flow and the redox condition causing the further evolution of GCG. Identification of GCG should be made in rural areas where private wells prevail to ensure resident's health.

Probabilistic risk assessment of Chinese residents' exposure to fluoride in improved drinking water in endemic fluorosis areas

Studies have yet to evaluate the effects of water improvement on fluoride concentrations in drinking water and the corresponding health risks to Chinese residents in endemic fluorosis areas (EFAs) at a national level. This paper summarized available data in the published literature (2008-2016) on water fluoride from the EFAs in China before and after water quality was improved. Based on these obtained data, health risk assessment of Chinese residents' exposure to fluoride in improved drinking water was performed by means of a probabilistic approach. The uncertainties in the risk estimates were quantified using Monte Carlo simulation and sensitivity analysis. Our results showed that in general, the average fluoride levels (0.10-2.24 mg/L) in the improved drinking water in the EFAs of China were lower than the pre-intervention levels (0.30-15.24 mg/L). The highest fluoride levels were detected in North and Southwest China. The mean non-carcinogenic risks associated with consumption of the improved drinking water for Chinese residents were mostly accepted (hazard quotient < 1), but the non-carcinogenic risk of children in most of the EFAs at the 95th percentile exceeded the safe level of 1, indicating the potential non-cancer-causing health effects on this fluoride-exposed population. Sensitivity analyses indicated that fluoride concentration in drinking water, ingestion rate of water, and the exposure time in the shower were the most relevant variables in the model, therefore, efforts should focus mainly on the definition of their probability distributions for a more accurate risk assessment.