Groundwater fluoride enrichment in an active rift setting: Central Kenya Rift case study

Isotopic and trace element record of changing metal source contributions to tropical freshwater Lake Naivasha (Kenya)

Lake Naivasha, Kenya's second-largest freshwater body is a wetland of international ecological importance and currently subjected to unprecedented anthropogenic influence. The study aims to chronologically reconstruct the main human activities and background weathering reactions that govern metal mobilizations into the lake and their potentially adverse effects on its ecological status. We combine extensive geochemical analyses (major, trace elements, Zn-Pb isotope ratios) in a dated lake sediment record and catchment rocks with remote sensing techniques. Downcore geochemical variations reflect natural ecosystem destabilizations occurring as early as the first half of the 20th century. These coincide with changes towards less radiogenic Pb-isotope values which persist towards the top of the core (206Pb/207Pb = 1.243 at core base ∼1843, to 206Pb/207Pb = 1.225 at ∼1978). We interpret the land-clearance for agricultural purposes on the Aberdare Range and documented early aviation activities as possible vectors of this early Pb-isotope excursion. The overlapping Pb-isotope signatures between sediment sources and anthropogenic contributions challenges a straightforward deconvolution of the two. Our conservative model calculations suggest, nevertheless, that an addition of up to ∼1.8 % Pb-gasoline influx to the total Pb flux, peaking in the 1980s is able to explain the Pb distribution trend. Homogeneous Zn-isotope compositions in sediments deposited until ∼1970s (δ66/64Zn = 0.216-0.225 ‰) do not follow major hydro-climatic events or anthropogenic forcing but likely reflect lake-specific natural cycling. Subsequent higher variations to both heavier (up to δ66/64Zn = 0.242 ± 0.005 ‰) and lighter (down to δ66/64Zn = 0.184 ± 0.003 ‰) Zn-isotope values are contemporaneous with intensification of large-scale horticultural industry in the catchment. Together with supporting indicators, the lighter Zn-isotope compositions in youngest analysed sediments (21st century) are attributable to increased biological productivity (algal blooms) and ongoing lake eutrophication. Our study demonstrates the applicability of the heavy metal isotope tool to reconstruct human influences on lake environments with complex geological settings such as the East African Rift System.

The influence of geology on the quality of groundwater for domestic use: a Kenyan review

Kenya's population, akin to other Sub-Saharan countries, is rapidly growing. With the increasing unreliability of surface water, groundwater resources are becoming highly relied on for domestic and industrial use. Despite several known contaminants reported in different parts of the country, no study has attempted to correlate groundwater quality in the different geological provinces. This review critically synthesizes the influence of Kenya's diverse geology on groundwater quality for human consumption. This was achieved through a review of published journal articles and other research material through research and government databases. Groundwater was categorised based on the major geological provinces including the Archaean volcanic Nyanzian Craton, the Proterozoic metamorphic Mozambique Mobile Belt (MMB) and volcanic Kisii Group, the Palaeozoic and Mesozoic sediments, and Tertiary volcanic Rift Valley. Groundwater quality in these regions showed a characteristic high concentration of fluoride (F-) in volcanic aquifers of the Rift Valley and Nyazian Craton and metamorphic aquifers of the MMB, where mineral dissolution was the main process of F- release. High salinity was common in metamorphic aquifers in the MMB and the Palaeozoic and Mesozoic sedimentary aquifers where mineral dissolution and seawater intrusion were the common contributors to salinity. Other contaminants such as lead and iron were reported in localised areas in the sedimentary and metamorphic aquifers, respectively. Anthropogenic contaminants such as Escherichia coli (E. coli), NO3 -, and NO2 - were common in shallow groundwater resources in most informal settlements in urban areas. Due to the presence of health implications, of the highlighted contaminants, such as fluorosis, high blood pressure and diarrhoea (due to high F- and salinity) in affected regions, this review highlights the need for an active water resource management program in any country relying on groundwater resources to determine the presence of all region-specific potentially harmful chemical elements and mitigation measures in all its water resources.

Spatial distribution, geochemical processes of high-content fluoride and nitrate groundwater, and an associated probabilistic human health risk appraisal in the Republic of Djibouti

In the northern East African Rift System, the Republic of Djibouti relies exclusively on groundwater, with levels of fluoride (up to 14 mg/L) and nitrate (up to 256 mg/L) posing potential health risks. To address this, 362 samples were considered, including 133 shallow groundwater samples, along with new and previously published data dating back to 2012 on deep (88) and thermal (141) groundwater samples. To understand the enrichment mechanisms, dissolved anion and cation constituents, geochemical and thermodynamic tools, and stable isotope ratios, such as δ2H(H2O), δ18O(H2O), δ15N(NO3-), and δ18O(NO3-), were used. In particular, two activity diagrams (Mg2+ vs. Ca2+ and Na+ vs. Ca2+), focused on aqueous and solid fluoride species in an updated thermodynamic dataset of 15 fluoride-bearing minerals, are shown for the first time. The dataset offers new and valuable insights into fluoride geochemistry (classic thermodynamic datasets combined with geochemical codes rely solely on fluorapatite and fluorite F-bearing minerals). Activity diagrams and geochemical modeling indicate that mineral dissolution primarily drives groundwater fluoride enrichment in all water types, whereas the elevated nitrate levels may stem from organic fertilizers like animal manure, as indicated by nitrate isotopes and NO3-/Cl- vs Cl- diagrams. Despite the arid climate and 2H18O enrichment in shallow waters, evaporation seems to play a minor role. Monte Carlo simulations and sensitivity analysis were used to assess the health risks associated with elevated F- and NO3- concentrations. Mapping-related spatial distribution analysis identified regional contamination hotspots using a global Moran's I and GIS tools. One fluoride and three nitrate contamination hotspots were identified at a p-value of 0.05. Groundwater chemistry revealed that 88 % of groundwater being consumed exceeded the permissible levels for fluoride and nitrate, posing potential health risks, particularly for teenagers and children. This study pinpoints specific areas with excessive nitrate and fluoride contamination, highlighting a high non-carcinogenic risk.

Poor oral health and the risk of esophageal squamous cell carcinoma in Malawi

Esophageal squamous cell carcinoma (ESCC) is the second most common cancer in Malawi. Risk factors for this cancer in Malawi are poorly understood. Poor oral health has previously been linked to increased ESCC risk in other high-incidence regions, including parts of Eastern and Southern Africa. We assessed the relationship between oral health and ESCC risk in a sex, age and location frequency-matched case-control study based at two hospitals in Lilongwe, Malawi from 2017 to 2020. Trained interviewers used a structured questionnaire and direct observation to collect data on demographics; behaviors; oral hygiene habits; the sum of decayed, missing or filled teeth (DMFT score); oral mucosa status; lip depigmentation and dental fluorosis via a visual scale. Logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (95% CI), adjusted for known and suspected ESCC risk factors. During the study period, 300 cases and 300 controls were enrolled. Subjects in the highest tertile of DMFT score (≥7) had an increased risk of ESCC with an adjusted OR of 1.96 (95% CI: 1.16-3.36) compared to those with a DMFT score of 0. Severe dental fluorosis was associated with a statistically nonsignificant increased risk of ESCC (adjusted OR = 2.24, 95% CI: 0.97-5.49) compared to individuals with no fluorosis. Associations with oral mucosa status, lip depigmentation and toothbrushing method and frequency were mostly null or uncertain. Poor oral health, indicated by a higher DMFT score, was associated with increased ESCC risk in Malawi. Dental fluorosis is another possible risk factor in this population, but further evaluation is necessary to clarify any effects of fluorosis on ESCC risk.

Variability of groundwater fluoride and its proportionate risk quantification via Monte Carlo simulation in rural and urban areas of Agra district, India

This study quantifies the groundwater fluoride contamination and assesses associated health risks in fluoride-prone areas of the city of Taj Mahal, Agra, India. The United States Environmental Protection Agency (USEPA) risk model and Monte Carlo Simulations were employed for the assessment. Result revealed that, among various rural and urban areas Pachgain Kheda exhibited the highest average fluoride concentration (5.20 mg/L), while Bagda showed the lowest (0.33 mg/L). Similarly, K.K. Nagar recorded 4.38 mg/L, and Dayalbagh had 1.35 mg/L. Both urban and rural areas exceeded the WHO-recommended limit of 1.5 mg/L, signifying significant public health implications. Health risk assessment indicated a notably elevated probability of non-carcinogenic risk from oral groundwater fluoride exposure in the rural Baroli Ahir block. Risk simulations highlighted that children faced the highest health risks, followed by teenagers and adults. Further, Monte Carlo simulation addressed uncertainties, emphasizing escalated risks for for children and teenagers. The Hazard Quotient (HQ) values for the 5th and 95th percentile in rural areas ranged from was 0.28-5.58 for children, 0.15-2.58 for teenager, and 0.05-0.58 for adults. In urban areas, from the range was 0.53 to 5.26 for children, 0.27 to 2.41 for teenagers, and 0.1 to 0.53 for adults. Physiological and exposure variations rendered children and teenagers more susceptible. According to the mathematical model, calculations for the non-cancerous risk of drinking water (HQ-ing), the most significant parameters in all the targeted groups of rural areas were concentration (CW) and Ingestion rate (IR). These findings hold relevance for policymakers and regulatory boards in understanding the actual impact and setting pre-remediation goals.

Deciphering spatio-seasonal patterns, driving forces, and human health risks of nitrate and fluoride enriched water bodies in the Inner Mongolia Reaches of the Yellow River Basin, China

The ecology and environment of the Yellow River Basin is threatened by fluoride and nitrate contamination induced by anthropogenic activity and geogenic factors. As a result, deciphering the spatio-temporal variability of fluoride and nitrate contamination in this area remains a challenge. Three hundred eighty-six samples of surface water and groundwater from the Inner Mongolia Reaches of the Yellow River Basin were taken for this investigation. According to the results of the multivariate statistical and geostatistical analyses, the fluoride pollution was primarily discovered in the middle and lower reaches of the study area and was determined to be more severe during the dry season. In contrast, nitrate contamination was found to be more severe during the wet season while being widely distributed in groundwater and concentrated in areas with intensive agricultural activities. The primary mechanisms governing the spatial-seasonal patterns of NO3- and F- pollution were shown by the principal component analysis, isotopic, and hydrochemical diagrams. The water-rock interaction or evaporation was crucial in the enrichment of F-. The human inputs (e.g., fertilizer or sewage) dominated fluoride and nitrate contamination. Additionally, the alkaline environment played a role in the generation of NO3- and F-. The health risk assessment concluded that the threat of fluoride contamination was greater than that of nitrate contamination. Children faced the greatest health risks, followed by females and males. These findings would serve as a guide for water management and pollution control in the Yellow River Basin.

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.

Fluoride occurrence in geothermal water of fault zone area, Southeast China

Fluoride (F^-) is a common trace component in groundwater, and long-term exposure to high-F^- groundwater is harmful to human health. Fluoride concentrations that exceed the World Health Organization guideline for drinking water (1.5 mg/L) have been detected in thermal and non-thermal groundwater from Southeast China, where the causes of the high fluoride occurrence are lack of study. To study the formation and migration mechanism of high fluoride groundwater from Southeast China, we carried out a systemic sampling of water samples in the surrounding area of Heyuan deep fault zone and Zijin-Boluo fault zone, then a comprehensive discussion including water hydrogeochemical, stable isotope composition, as well as hydrogeology analysis was conducted. Fluoride concentrations in geothermal and non-thermal water samples range from 1.11 to 22.76 mg/L and 0.04-8.3 mg/L, respectively. High temperature, alkaline conditions, and the depleted Ca²⁺ by reverse cation exchange and calcite precipitation would promote the release of fluoride from host rock to geothermal water. The availability of Ca-bearing and F-bearing minerals in host rock causes significant differences in fluoride concentrations between carbonate reservoir and granite reservoir. Hydrochemical diagrams reveal that the composition of groundwater is affected by mixing and that fractures act as the mixing channels in our study area. The addition of cold groundwater limits the fluoride concentrations by lowing temperature and increasing Ca²⁺ levels of geothermal water. Additionally, the relationship between F^- and SiO₂ indicates that geothermal water promotes the fluoride enrichment in cold groundwater, especially in confined aquifers which are more susceptible to geothermal water. The direct input of geothermal fluoride and secondary enrichment caused by alkaline condition contribute to the formation of high F^- concentrations (7.2-8.3 mg/L) in confined groundwater. Our findings highlight that the natural evolution of geothermal systems in fault zone can result in the formation of geogenic contaminated groundwater.

Iron functionalized silica particles as an ingenious sorbent for removal of fluoride from water

The paucity of safe drinking water remains a global concern. Fluoride is a pollutant prevalent in groundwater that has adverse health effects. To resolve this concern, we devised a silica-based defluoridation sorbent from pumice rock obtained from the Paka volcano in Baringo County, Kenya. The alkaline leaching technique was used to extract silica particles from pumice rock, which were subsequently modified with iron to enhance their affinity for fluoride. To assess its efficacy, selected borehole water samples were used. Scanning electron microscopy, X-ray diffraction, Fourier transform infrared and X-ray fluorescence spectroscopy was used to characterize the sorbent. The extracted silica particles were 96.71% pure and amorphous, whereas the iron-functionalized silica particles contained 93.67% SiO₂ and 2.93% Fe₂O₃. The optimal pH, sorbent dose and contact time for defluoridation of a 20 mg/L initial fluoride solution were 6, 1 g and 45 min, respectively. Defluoridation followed pseudo-second-order kinetics and fitted Freundlich's isotherm. Fluoride levels in borehole water decreased dramatically; Intex 4.57-1.13, Kadokoi 2.46-0.54 and Naudo 5.39-1.2 mg/L, indicating that the silica-based sorbent developed from low-cost, abundant and locally available pumice rock is efficient for defluoridation.

Accumulation of high concentration fluoride in the Ulungur Lake water through weathering of fluoride containing rocks in Xinjiang, China

Fluoride is a potential contaminant at high concentrations when used for drinking due to its adverse human health effects. The Ulungur Lake in Xinjiang, China has a long history of high fluoride concentration in lake water, but the mechanism leading to such high concentrations of fluoride is still unclear. In this study we evaluate the fluoride concentration in different water bodies and upstream rock formations in the Ulungur watershed. The result show that fluoride concentration in the Ulungur Lake water fluctuates around 3.0 mg L^-1, although the fluoride concentrations in the feeding rivers and groundwater are all lower than 0.5 mg L^-1. A mass balance model is developed for water, fluoride, and total dissolved solid in the lake, and the model explains why the concentration of fluoride in the lake water is higher than those in river and ground water. Bedrock compositions are measured from nearby formations which confirm the potential of these rocks to release fluoride into water bodies through water-rock interactions. The whole-rock concentrations of fluoride are in the range of 0.4-2.4 g kg^-1 and the water-soluble concentrations of fluoride in the upstream rocks are 0.26-3.13 mg L^-1. Biotite and hornblende are identified as the fluorine containing minerals in the Ulungur watershed. The concentration of fluoride in the Ulungur has been declining slowly in recent years due to increased water inflow fluxes, and our mass balance model predicts that the fluoride concentration will eventually reach 1.70 mg L^-1 under a new steady state, but it requires about 25-50 years to reach the new steady state. The yearly fluctuation of fluoride concentration in the Ulungur Lake is likely due to changes in water-sediment interactions reflected in changes in lake water pH.

Prediction of elevated groundwater fluoride across India using multi-model approach: insights on the influence of geologic and environmental factors

Elevated fluoride in groundwater is a severe problem in India due to its extensive occurrence and detrimental health impacts on the large population that thrives on groundwater. Although fluoride is primarily a geogenic pollutant, existing model-based studies lack the amalgamation of the influence of geologic factors, specifically tectonics, for identifying groundwater fluoride distribution. This drawback encourages the present study to investigate the association of the tectonic framework with fluoride in a multi-model approach. We have applied three machine learning models (random forest, boosted regression tree, and logistic regression) to predict elevated groundwater fluoride based on fluoride measurements across India. The random forest model outperformed other models with an accuracy of 93%. Tectonics was found to be one of the most important predictors alongside "depth to water table." Two major areas of high risk identified were the northwest parts and the south-southeast cratonic peninsular region. The random forest model also performed significantly well over the validation dataset. We estimate that nearly 257 million people are exposed to elevated fluoride risk in India. We endeavor that the findings of our study would be an effective tool for identifying the areas at risk of elevated fluoride and also assist in undertaking effective groundwater management strategies.

Geochemical fingerprint and spatial pattern of mine water quality in the Shaanxi-Inner Mongolia Coal Mine Base, Northwest China

The spatial distribution of mine water quality and geochemical controls must be investigated for water safety and ecosystem protection in Shaanxi-Inner Mongolian Coal Mine Base (SICMB). Based on 122 mine water samples collected from 14 mining areas, self-organizing maps (SOM) combining with principal component analysis (PCA) derived that the mine water samples were classified into seven clusters. Clusters 1 and 3 (C1 and C3) samples were dominant by HCO3-Ca and mixed types, which were distributed in the recharge area of the middle SICMB. In this area, the active groundwater circulation contributed to the good water quality. Cluster 2 (C2) samples were characterized by HCO3-Na type, mainly distributed in the discharge area of the middle SICMB. These samples were threatened by heavy fluorine contamination and high residual sodium carbonate (RSC) because of slow groundwater flow in this area. Clusters 4 and 5 (C4 and C5) samples, distributed in the northeast and middle SICMB, were characterized by high Cl- concentration and light fluorine contamination. They were influenced by anthropogenic input through faults or underground mining. In contrast, Clusters 6 and 7 (C6 and C7) samples with high salinity and sulfate were distributed in the southwest SICMB. The deep groundwater circulation enhanced water-rock interaction and contributed to poor water quality. These findings are beneficial to the management of mine water resources in the SICMB and provide an insight to investigate the mine water quality in large spatial scale.

Application of water quality indices and health risk models in the arid coastal aquifer, Southern Saudi Arabia

A systematic study was performed in the arid coastal aquifer to evaluate groundwater quality using drinking water quality indices (DWQI) and health risk assessment models in southern Saudi Arabia. Groundwater samples were collected (n = 80) and analysed for major and minor ions. Results suggest that 85% of wells are unsuitable for drinking due to high salinity and hardness. Likewise, high NO₃^- and F^- are encountered in 51% and 46% of wells, respectively. High salinity, Cl^- and SO₄^2- are noticed in the coastal wells, which are derived from saline sources, evaporation and anthropogenic activities. High NO₃^- is originated from anthropogenic sources and the nitrification process. Recharge of wastewater with high NO₃^- is mixed with high salinity groundwater in this shallow aquifer. DWQI indicates that 66% of samples are poor to unsuitable classes. Wells with poor quality groundwater existed in the coastal belt and water quality is degraded while moving from upstream to downstream. The average values of hazard quotient (HQoral, NO₃^-, F^-) and total hazard index (THI) ensured that highly vulnerable groups are in the order of infants > children > adults. In the study region, the THI are > 1 in 75% (adults), 89% (children) and 94% (infants) of samples, respectively. The spatial distribution of HQoral reveals that groundwater in the coastal and southern regions is not advisable for direct oral ingestion, which causes serious non-carcinogenic health risk to inhabitants. Groundwater in these regions needs proper treatment to remove the contaminants before use.

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.

The effect of mining development in karst areas on water acidification and fluorine enrichment in surface watersheds

High fluoride water is a crucial driving factor for endemic fluorosis. As an important research content of hydrogeochemistry, the enrichment of fluorine in alkaline water has received a fair amount of scholarly attention, but the understanding of the migration and enrichment of fluorine in acid mine drainage (AMD) in karst area remains very limited. An analysis of 13 consecutive periods of hydrochemical samples (312 samples in total) revealed that the weathering of carbonates and sulfide-rich coal measures induced a pH as low as 2.29 in the Chetian River in Jinsha, Southwest China. The highest content of fluorine in AMD was 23.8 mg/L, and the average content in the basin was 1.4 mg/L. In terms of the seasonal variation in the whole basin, the fluorine content were higher in the rainy season than in the dry season. The mineral saturation index shows that the dissolution of fluorapatite and fluorite is an important source of fluorine. The chloro-alkaline indices displayed a strong ion exchange process in the basin, promoting the release of fluorine in silicate minerals. In comparison, the contribution of external inputs, such as atmospheric deposition, was less. Additionally, evaporation was shown to have a limited influence on fluorine enrichment. Meanwhile, pH was an essential factor driving the dynamic transformation of the mode of occurrence of fluorine in water. In the upstream alkaline water, the main occurrence form of fluorine was free F^-, while the F/Al ratio for most of the acidic samples was ≤ 1.0, indicating the main occurrence form of fluorine was likely AlF²⁺. The conclusion of this study provides a new understanding for deepening the geochemical characteristics of fluorine in karst surface water.

Holocene bidirectional river system along the Kenya Rift and its influence on East African faunal exchange and diversity gradients

Although biodiversity in East Africa is overall extremely high, species richness is not geographically uniform for fishes and mammals. We investigated the biogeographic relevance of past river activity in the Kenya Rift. We show that during a humid period 12,000 to 8,000 years ago, a river system connected currently isolated rift lakes and was partly connected to the Nile. While this river system formed pathways for the dispersal of fishes between lakes, it also acted as a barrier to the range expansion of forest mammals. This fairly recent hydrological connectivity between lakes has been a key driver of modern biodiversity patterns in East Africa. Climate-driven changes in drainage networks on multimillennial timescales are an important hypothesis in biodiversity research.

East Africa is a global biodiversity hotspot and exhibits distinct longitudinal diversity gradients from west to east in freshwater fishes and forest mammals. The assembly of this exceptional biodiversity and the drivers behind diversity gradients remain poorly understood, with diversification often studied at local scales and less attention paid to biotic exchange between Afrotropical regions. Here, we reconstruct a river system that existed for several millennia along the now semiarid Kenya Rift Valley during the humid early Holocene and show how this river system influenced postglacial dispersal of fishes and mammals due to its dual role as a dispersal corridor and barrier. Using geomorphological, geochronological, isotopic, and fossil analyses and a synthesis of radiocarbon dates, we find that the overflow of Kenyan rift lakes between 12 and 8 ka before present formed a bidirectional river system consisting of a “Northern River” connected to the Nile Basin and a “Southern River,” a closed basin. The drainage divide between these rivers represented the only viable terrestrial dispersal corridor across the rift. The degree and duration of past hydrological connectivity between adjacent river basins determined spatial diversity gradients for East African fishes. Our reconstruction explains the isolated distribution of Nilotic fish species in modern Kenyan rift lakes, Guineo-Congolian mammal species in forests east of the Kenya Rift, and recent incipient vertebrate speciation and local endemism in this region. Climate-driven rearrangements of drainage networks unrelated to tectonic activity contributed significantly to the assembly of species diversity and modern faunas in the East African biodiversity hotspot.

Hydro-geochemical analysis based on entropy and geostatistics model for delineation of anthropogenic ground water pollution for health risks assessment of Dhenkanal district, India

Consumption of poor quality water causes serious human health hazards. Therefore, it is very crucial to investigate factors influencing the quality of groundwater and its suitability for drinking purpose. In the present study, groundwater quality of the Dhenkanal district of Odisha, India was characterized and the spatial distribution of different water quality parameters were analyzed using the multivariate statistics, entropy theory, and geostatistics techniques. In the present study 112 number of groundwater tube well samples were collected from the study area. The entropy theory revealed that SO₄^2-, Mg⁺² and Cl^- were the most influencing parameters. A similar observation was also observed based on the correlation coefficient analysis. Groundwater quality index (GWQI) and entropy-weighted water quality index (EWQI) classifications indicated that 78.57 and 43.75% of the collected groundwater samples were categorized under excellent water quality, whereas, the rest of the samples were varying from good to medium drinking water quality. In addition, the result of EWQI classification offers more realistic assessment than that of GWQIs owing to its high precision, simplicity and without application of artificial weight. The correlation coefficient between Ca⁺² and HCO₃^-, Mg⁺² and PO₄^- were significantly high which might be due the presence of CaHCO₃ and MgPO₄ in the groundwater samples. The GWQI revealed a weak spatial dependence of groundwater quality.

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.

Groundwater fluoride across the Punjab plains of Pakistan and India: Distribution and underlying mechanisms

Chronic exposure from drinking well-water with naturally high concentrations of fluoride (F^-) has serious health consequences in several regions across the world including South Asia, where the rural population is particularly dependent on untreated groundwater pumped from private wells. An extensive campaign to test 28,648 wells was conducted across the Punjab plains of Pakistan and India by relying primarily on field kits to document the scale of the problem and shed light on the underlying mechanisms. Groundwater samples were collected from a subset of 712 wells for laboratory analysis of F^- and other constituents. A handful of sites showing contrasting levels of F^- in groundwater were also drilled to determine if the composition of aquifer sediment differed between these sites. The laboratory data show that the field kits correctly classified 91% of the samples relative to the World Health Organization guideline for drinking water of 1.5 mg/L F^-. The kit data indicate that 9% of wells across a region extending from the Indus to the Sutlej rivers were elevated in F^- relative to this guideline. Field data indicate an association between the proportion of well-water samples with F^- > 1.5 mg/L and electric conductivity (EC) > 1.5 mS/cm across six floodplains and six intervening doabs. Low Ca²⁺ concentrations and elevated bicarbonate (HCO₃^- > 500 mg/L) and sodium (Na⁺ > 200 mg/L) in high F^- groundwater suggest regulation by fluorite. This could be through either the lack of precipitation or the dissolution of fluorite regulated by the loss of Ca²⁺ from groundwater due to precipitation of calcite and/or ion exchange with clay minerals. Widespread salinization of Punjab aquifers attributed to irrigation may have contributed to higher F^- levels in groundwater of the region. Historical conductivity data suggest salinization has yet to be reversed in spite of changes in water resources management.

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.

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

A hydrochemical analysis of groundwater (GW) was conducted to investigate the factors controlling GW fluoride (F) in a large irrigation plain in the Yellow River Basin, Guanzhong Plain, China. Area-dependent variations in F were observed in the study region. The F concentrations of 93% of samples on the south bank of the Weihe River and the western part of the Qishui River were <1 mg L^-1, whereas those of 73% of GW samples for the eastern part of the Qishui River exceeded the national limit. A forward model based on mass budget equations identified carbonate weathering as the dominant factor regulating hydrochemistry in low-F GW, whereas the factors in the high-F zone were evaporate dissolution and evaporation. The high-F GW displayed a distinctive major ion chemistry, which could be attributed to a high pH, low Ca²⁺, and high HCO₃^- and Na⁺ concentrations. An analysis of the correlation between F/Cl and F concentrations and fluid-mineral equilibria indicated distinct forces driving the behavior of F in the subparts of the high-F GW zone, including irrigation-induced F dilution, F enrichment through Na-Ca exchange, and adsorption of F on clay minerals. The order of vulnerable segments of the population in terms of risk posed by F in GW was: infants > children > adults. These results can enhance the understanding of F behaviors in GW and provide insights into the effect of irrigation practices on GW F concentration.

Application of water quality index and chemometric methods on contamination assessment in the shallow aquifer, Ganges River basin, India

Water quality index and chemometric methods were employed to assess the groundwater quality and contamination sources in the upper Ganges basin (UGB) and lower Ganges basin (LGB) as groundwater is a sole source for drinking, domestic and agricultural uses. Groundwater samples were collected from UGB (n = 44) and LGB (n = 26) and analysed for physicochemical parameters. Groundwater in this basin is desirable (51%) to permissible (TDS < 1000 mg/l, 96%) classes for drinking. Chemical constituents in the groundwater are lower than the maximum allowable limit recommended by the WHO for drinking except K. Drinking water quality index (DWQI) values reveal that groundwater belongs to excellent (89%) and good (10%) classes. However, the high concentrations of Fe and Mn in 61 and 77% of samples, respectively, restrict the usage for drinking according to USEPA recommendations. Both LGB and UGB groundwater in shallow wells have elevated concentration of TDS, EC and other ions (Ca²⁺, Cl^- and SO₄^2- in LGB; major ions, NO₃^-, PO₄^3-, F^-, Fe and Mn in UGB) and imply the influences of anthropogenic activities. Principal component analysis and hierarchical cluster analysis reiterated that groundwater quality is affected by the anthropogenic activities as well as mineral dissolutions (carbonate and silicate minerals). This study highlighted that the infiltration of wastewater from various contamination sources likely triggered the dissolution of the minerals in the vadose zone that resulted in the accumulation of ions in the shallow aquifer. An effective management plan is essential to protect this shallow aquifer.

A deep insight into the transcriptome of midgut and fat body reveals the toxic mechanism of fluoride exposure in silkworm

Fluoride generally exists in the natural environment, and has been reported to induce serious environmental hazard to animals, plants, and even humans via ecological cycle. Silkworm, Bombyx mori, which showed significant growth and reproductivity reduction when exposed to fluoride, has become a model to evaluate the toxicity of fluoride. However, the detailed mechanism underlying fluoride toxicity and corresponding transport proteins remain unclear. In this study, we performed RNA-seq of the larval midgut and fat body with fluoride exposure and normal treatment. Differential analysis showed that there were 4405 differentially expressed genes in fat body and 4430 DEGs in midgut with fluoride stress. By Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analyses, we identified several key pathways involved in the fluoride exposure and poisoning. We focused on the oxidative phosphorylation and MAPK signal pathway. QRT-PCR confirmed that oxidative phosphorylation process was remarkably inhibited by fluoride exposure and resulted in the blocking of ATP synthesis. The MAPK signal pathway was stimulated via phosphorylation signal transduction. Moreover, by protein structure analysis combined with the DEGs, we screen 36 potential membrane proteins which might take part in transporting fluoride. Taken together, the results of our study expanded the underlying mechanisms of fluoride poisoning on silkworm larval growth and development, and implied potential fluoride transport proteins in silkworm.

Dissolved Organic Matter in Continental Hydro-Geothermal Systems: Insights from Two Hot Springs of the East African Rift Valley

Little is known about the quantity and quality of dissolved organic matter (DOM) in waters from continental geothermal systems, with only a few reports available from the Yellowstone US National Park. In this study, we explored the chemodiversity of DOM in water samples collected from two geothermal hot springs from the Kenyan East African Rift Valley, a region extremely rich in fumaroles, geysers, and spouting springs, located in close proximity to volcanic lakes. The DOM characterization included in-depth assessments performed by negative electrospray ionization Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Reduced, saturated and little aromatic DOM compounds were dominant in the hot spring waters collected from either the Ol Njorowa gorge (ON) or the south shore of the soda-saline Lake Elementaita (ELM). Oxygen-poor and sulfur-bearing DOM molecules prevailed in ON, probably reflecting abiotic sulfurization from sulfide-rich geofluids. Nitrogen-bearing aliphatic and protein-like molecules were abundant in ELM, possibly perfusing through the organic-rich sediments of the adjacent Lake Elementaita. Notably, the heat-altered DOM of ancient autochthonous derivation could represent an overlooked source of aliphatic organic carbon for connected lentic environments, with a potential direct impact on nutrient cycling in lakes that receive geothermal water inputs.

Application of Octacalcium Phosphate with an Innovative Household-scale Defluoridator Prototype and Behavioral Determinants of its Adoption in Rural Communities of the East African Rift Valley

Natural fluoride contamination of drinking water is a serious issue that affects several countries of the world. Its negative health impact is well documented in the East African Rift Valley, where water consumption with fluoride ( F - ) concentration greater than 1.5 mg/L can cause fluorosis to people. Within the framework of the European Union (EU) Horizon 2020 FLOWERED project, we first designed an effective defluoridation device based on innovative application of octacalcium phosphate (OCP) and then explored its acceptance within rural communities. The prototype (FLOWERED Defluoridator Device [FDD]) essentially is composed of a 20-L tank and a recirculating pump that guarantees the interaction between water and OCP. The device is powered by a car battery for a fixed pumping working time using a fixed amount of OCP for every defluoridation cycle. The results of tests performed in the rural areas of Tanzania show that a standardized use of the prototype can lower the dissolved F - from an initial concentration of 21 mg/L to below the World Health Organization (WHO) drinkable limit of 1.5 mg/L in 2 h without secondary negative effects on water quality. The approximate cost of this device is around US$220, whereas that of OCP is about $0.03/L of treated water. As with any device, acceptance requires a behavioral change on behalf of rural communities that needed to be investigated. To this end, we piloted a survey to explore how psychological and socioeconomic factors influence the consumption of fluoride-free water. Results show that the adoption of FDD and OCP is more appealing to members of the rural communities who are willing to pay more and have a high consumption of water. Moreover, we suggest that given the low level of knowledge about fluorosis diseases, the government should introduce educational programs to make rural communities aware of the negative health consequences. Integr Environ Assess Manag 2020;16:856-870. © 2020 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

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.

Hydrogeochemical processes controlling the mobilization and enrichment of fluoride in groundwater of the North China Plain

Fluoride enrichment in groundwater at the North China Plain (NCP) is posing a potential health risk to human being. To better understand the enrichment mechanism of fluoride in the groundwater systems, the groundwater, sediments samples and pore water compacted from clayey sediments were collected to perform the chemistry analysis and geochemical inverse modeling. The results showed that fluoride concentration in groundwater from the NCP has a range of 0.38-7.35 mg/L, and 67.8% of groundwater samples have the fluoride concentration higher than 1.5 mg/L. High fluoride groundwater was mainly distributed in the central plain and coastal area of the NCP, and characterized by the Na-HCO₃ or Na-Cl type water, lower Ca and higher TDS concentrations. Along groundwater flow-path from the mountainous to coastal areas, several hydrogeochemical processes control the mobilization and enrichment of fluoride in groundwater, including cation exchange between Ca and Na on the surface of clay minerals, precipitation/dissolution of carbonates, dissolution of fluorite, marine transgressions, and release of pore water trapped in clayey sediments caused by land subsidence. The fluoride concentrations in the pore water compacted from the sediments ranged from 2.92 to 4.48 mg/L. At the central plain and coastal area, the wide occurrence of land subsidence resulted from the groundwater over-exploration leads to the release of pore water rich in fluoride into surrounding aquifers, thereby elevating the concentration of groundwater fluoride. The resulted groundwater environment with high salinity and some newly-introduced ions, such as Mg, promote the dissolution of fluorite, which was under-saturated in the groundwater samples from the NCP, further enhancing the fluoride enrichment in the groundwater at the coastal area. The findings of this study will provide new insights on the generation of high fluoride groundwater.

Identifying Groundwater Fluoride Source in a Weathered Basement Aquifer in Central Malawi: Human Health and Policy Implications

Consumption of groundwater containing fluoride exceeding World Health Organization (WHO) 1.5 mg/L standard leaves people vulnerable to fluorosis: a vulnerability not well characterised in Malawi. To evaluate geogenic fluoride source and concentration, groundwater fluoride and geology was documented in central Malawi where groundwater supplies are mainly sourced from the weathered basement aquifer. Lithological composition was shown as the main control on fluoride occurrence. Augen gneiss of granitic composition posed the greatest geological fluoride risk. The weathered basement aquifer profile was the main factor controlling fluoride distributions. These results and fluoride-lithology statistical analysis allowed the development of a graded map of geological fluoride risk. A direct link to human health risk (dental fluorosis) from geological fluoride was quantified to support science-led policy change for fluoride in rural drinking water in Malawi. Hazard quotient (HQ) values were calculated and assigned to specific water points, depending on user age group; in this case, 74% of children under six were shown to be vulnerable to dental fluorosis. Results are contrary to current standard for fluoride in Malawi groundwater of 6 mg/L, highlighting the need for policy change. Detailed policy recommendations are presented based on the results of this study.

A review on the potential sources and health implications of fluoride in groundwater of Sub-Saharan Africa

Groundwater is a major source of drinking water for millions of people around the world. Over 400 million people in Africa depend solely on it as their main source of water supply. Fluoride is a common contaminant in groundwater. In low concentration (0.5-1.0 mg/L), fluoride is needed by humans for healthy development of bones and teeth, however, a concentration >1.5 mg/L has been linked with several fluorosis and non-fluorosis diseases. Dental and skeletal fluorosis are the major fluorosis diseases commonly reported with the consumption of fluoride-rich water. Although fluoride intake through other pathways such as the drinking of tea and eating of vegetables have been reported, the drinking of fluoride-rich water remains the major pathway of fluoride into humans. Cases of high fluoride levels in groundwater have been reported in almost all the sub-Saharan Africa region but it is more prevalent in East African countries, Sudan and South Africa. Although fluoride is present in surface water mostly in the East African Rift Valley across different countries in East Africa, its significant or high levels are usually associated with groundwater. Geogenic sources such as fluorite, apatite, biotite, amphibole, micas, topaz, cryolite, muscovite and fluorspar have been identified as the major sources of fluoride in groundwater. High fluoride levels have been reported across sub Saharan Africa, with generally higher levels in East Africa resulting from the volcanic activities in the rift system. Dental fluorosis has been reported in many sub-Saharan African countries including South Africa, Tanzania, Uganda, Ethiopia, Kenya, Sudan, Niger, Nigeria, Benin, Ghana and Malawi. Geothermal temperature has been regarded as one of the driving forces for high fluoride levels recorded in groundwater from deep aquifers and geothermal springs. The most affected people with the consumption of fluoride-rich water are the poor with low socioeconomic status who live in rural areas. Some of the proposed alternative sources include rainwater and fog water harvesting and blending of water from various sources. Low-cost and sustainable deflouridation technique remains one of the best ways to treat fluoride contaminated water either at communal level or at the point-of-use.

Evaluation of high fluoride contaminated fractured rock aquifer in South Africa - Geochemical and chemometric approaches

The first systematic study on high fluoride contaminated fractured rock aquifer in South Africa using geochemical and chemometric approach is presented. Groundwater samples were collected from 49 boreholes and analysed for EC, pH, major and minor ions. The groundwater with high fluoride is associated with NaCl water types. The groundwater chemistry is governed by mineral weathering, evaporation and wastewater infiltration. PHREEQC modelling indicates that carbonate mineral saturation is decreasing with increasing salinity due to evaporation whereas fluorite saturation increases with increasing F being one of the prime controlling factors. Silicate minerals namely chalcedony, quartz and talc express saturation and over-saturation while chrysotile and sepiolite are undersaturated in most of the samples. PCA provided four factors and justified the role of mineral weathering, evaporation, ion exchange, longer residence time and anthropogenic impacts on water chemistry. R-mode and Q-mode cluster analysis resulted in four clusters. In cluster 1, (29%) of groundwater is less mineralized due to recent recharge. In cluster 2 (45%), groundwater chemistry is governed by weathering of silicates and fluorite minerals. Cluster 3 (20%) reveals the impact of anthropogenic activities and induced mineral weathering. Cluster 4 shows high EC, TDS, major ions, F and low HCO₃ implying that groundwater is affected by evaporation and longer residence time promoting mineral-water interaction and precipitation of carbonate minerals. More than 50% of the study area is degraded due to fluoride contamination which requires proper remedial actions and further investigations on human health risk due to impact of fluoride contamination in groundwater is recommended.

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.

Surface and Groundwater Hydrochemistry of the Menengai Caldera Geothermal Field and Surrounding Nakuru County, Kenya

In order to assess the sustainability and impact of production from geothermal reservoirs on hydrological systems, a thorough understanding of local and regional hydrogeological systematics is a prerequisite. The Menengai Caldera in the Kenya Great Rift Valley is one of the largest explored geothermal fields in the country. This paper presents a hydrochemical investigation of the Menengai Caldera geothermal field and the ground and surface waters of the surrounding Nakuru County. Our results demonstrated a similar, sodium-alkaline dominated, ionic composition across all water types. Geothermal wells return the highest cation/anion concentrations and largely demonstrate a meteoric source from their δ18O and δ2H signature. Wells MW-09 (central part of the caldera), MW-18 (eastern part) and MW-20 (central part) showed a more evaporitic signature, closely matching with our own calculated Lake Evaporation Line, suggesting an increased mixing influence of Lake Nakuru waters. MW-09 also showed evidence of high-temperature oxygen isotopic exchange and significant water-rock interaction. Lake samples largely demonstrated seasonal shifts in ionic and isotopic values. Lake Nakuru ionic composition and isotopic values increased throughout the 12-month wet–dry–wet season sampling period. This correlated with a decrease in area which suggests a lessening of water inflow and facilitates increased evaporation. Groundwaters demonstrated clear evidence of mixing between meteoric, irrigation and lake waters. These observations enhanced the understanding of the hydrological system surrounding the Menengai Caldera and, when combined with future studies, will provide a powerful tool to assess the sustainability and impact of soon-to-be completed geothermal power production operations.

Defluoridation of water through the transformation of octacalcium phosphate into fluorapatite

The consumption of water with fluoride concentration higher than 1.5 mg/L (WHO recommended limit) is recognized to cause serious diseases, and fluoride removal from natural contaminated waters is a health priority for more than 260 million people worldwide. The octacalcium phosphate (OCP), a mineralogical precursor of bio-apatite, is here tested as a fluoride remover. A new two-step method for the synthesis of OCP is proposed: 1) synthesis of brushite from calcium carbonate and phosphoric acid; 2) subsequent hydrolysis of brushite. Fluoride removal experiments are performed in batch-mode using different initial concentrations of fluoride (from 40 to 140 mg/L) and reaction times. Most of fluoride is removed within the first 2 h of all experiments, and the drinkable limit of 1.5 mg/L is reached within a minimum of 3 h for an initial fluoride concentration of 40 mg/L. The experimental fluoride removal capacity of OCP is 25.7 mg/g, and 4 g of OCP can effectively treat 1 L of water with fluoride concentration up to 50 times higher than the drinking limit of 1.5 mg/L. XRD and chemical characterization of the solid phases, before and after the removal experiments, indicate that OCP transforms into fluorapatite (FAP) uptaking fluoride from solution.

Elucidating various geochemical mechanisms drive fluoride contamination in unconfined aquifers along the major rivers in Sindh and Punjab, Pakistan

The present study aims to investigate the spatial distribution and associated various geochemical mechanisms responsible for fluoride (F⁻) contamination in groundwater of unconfined aquifer system along major rivers in Sindh and Punjab, Pakistan. The concentration of F⁻ in groundwater samples ranged from 0.1 to 3.9 mg/L (mean = 1.0 mg/L) in Sindh and 0.1-10.3 mg/L (mean = 1.0 mg/L) in Punjab, respectively with 28.9% and 26.6% of samples exhibited F⁻ contamination beyond WHO permissible limit value (1.5 mg/L). The geochemical processes regulated F⁻ concentration in unconfined aquifer mainly in Sindh and Punjab were categorized as follows: 1) minerals weathering that observed as the key process to control groundwater chemistry in the study areas, 2) the strong correlation between F⁻ and alkaline pH, which provided favorable environmental conditions to promote F⁻ leaching through desperation or by ion exchange process, 3) the 72.6% of samples from Sindh and Punjab were dominated by Na⁺- Cl⁻ type of water, confirmed that the halite dissolution process was the major contributor for F⁻ enrichment in groundwater, 4) dolomite dissolution was main process frequently observed in Sindh, compared with Punjab, 5) the arid climatic conditions promote evaporation process or dissolution of evaporites or both were contributing to the formation of saline groundwater in the study area, 6) the positive correlation observed between elevated F⁻ and fluorite also suggested that the fluorite dissolution also played significant role for leaching of F⁻ in groundwater from sediments, and 7) calcite controlled Ca²⁺ level and enhanced the dissolution of F-bearing minerals and drive F⁻ concentration in groundwater. In a nut shell, this study revealed the worst scenarios of F⁻ contamination via various possible geochemical mechanisms in groundwater along major rivers in Sindh and Punjab, Pakistan, which need immediate attention of regulatory authorities to avoid future hazardous implications.

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

The objective of this study is to develop a Fluoride Index for mitigation of geogenic contamination by Managed Aquifer Recharge (FIMAR). This index was tested by applying to the Pambar river basin in southern India. About 40% of the study area had fluoride >1.5 mg/L, contributed from the dissolution of fluorite, fluorapatite, biotite and hornblende. The relationship between groundwater level and fluoride concentration exhibited two types of relationship. In shallow wells, groundwater recharge during monsoon increases the groundwater level and dilutes the fluoride concentration. During summer, evaporation decreases the groundwater levels and increases the fluoride content. In deep wells, the fluoride-rich salts that is deposited in the unsaturated zone due to evaporation in the pre-monsoon season is flushed during groundwater recharge and thus, the fluoride concentration increases with increase in groundwater level. This relationship was used as a key layer in FIMAR. Overlay analysis including the well type derived from the groundwater level and fluoride fluctuation, average fluoride concentration, geology, geomorphology, soil, drainage density, land use, lineament density and thickness of the weathered zone indicated 30% of the area was suitable for MAR to mitigate fluoride contamination. Since the construction of new MAR structures is cost-intensive, it is proposed to rejuvenate the existing ponds in the areas identified by FIMAR to induce recharge and dilute fluoride concentration. The positive impact was confirmed using numerical and geochemical modelling. This proves that FIMAR can be used as a planning tool in fluoride endemic areas and that the future MAR methods will be beneficial to mitigate geogenic fluoride contamination.

Geochemical Evolution of Fluoride and Implication for F− Enrichment in Groundwater: Example from the Bilate River Basin of Southern Main Ethiopian Rift

Groundwater is the most important source of drinking water. Fluoride was found in high concentrations in the groundwater from deep wells of the water supply in the southern main Ethiopian rift. The high concentration of fluoride is dominantly geogenic rather than anthropogenic in origin, as the agricultural area was not found to be contaminated with NO3−. Knowledge of fluoride enrichment will help to provide management plans for developing deep groundwater and minimizing the health risks of exposure to fluoride. The chemical processes of fluoride were investigated in the waters in the Bilate River basin using hydrochemical and isotopic tools. The F− concentration ranged from 0.5 to 1.29 mg/L in water from shallow wells and from 0.48 to 5.61 mg/L in water from deep wells. Seventy percent of deep well samples had F− > 1.5 mg/L higher than the World Health Organization potable guideline. The high fluoride concentration in the groundwater was mainly situated in the rift valley of the Bilate River basin, in contrast with low F− groundwater in the highland. The concentration of fluoride was lowest in Ca-Mg-HCO3 type groundwater and highest in Na-HCO3 type groundwater. Moreover, F− was positively correlated with HCO3−, Na+, Na+/Ca2+ and pH in groundwater and Na+/Ca2+ ratios were increased along the flow path. Hydrogeological, hydrodynamic and hydrochemical conditions are responsible for fluoride accumulation in the deep aquifers. Strong dynamic flow in highland areas flush away weathered chemical components (e.g., F−). Thus, surficial weathering is not a major controlling factor for high concentrations of Fluoride in deep groundwater but the combination of silicate hydrolysis and ion exchange mainly control fluoride enrichment in stagnant flow environments.

Seasonal variation of organic matter characteristics and fluoride concentration in the Maji ya Chai River (Tanzania): Impact on treatability by nanofiltration/reverse osmosis

The Maji ya Chai River in Northern Tanzania, a fluoride-rich tropical area, shows a seasonal variation of natural organic matter (NOM) and fluoride concentration. Water samples collected monthly during one year from two locations of the River were characterized. High levels of precipitation in the rainy seasons increased the total organic carbon (TOC) concentration to as high as 36 mgC L^-1 and diluted the fluoride concentration from a dry season high of 24 mg L^-1 to <4 mg L^-1. A black water swamp in the Maji ya Chai River catchment was confirmed as the main source of NOM, fluoride, salinity, and inorganic carbon entering the River in the rainy season. The water samples were filtered by a number of nanofiltration/reverse osmosis (NF/RO) membranes to identify the retention mechanisms and the impact of varying water quality on treatability. While the denser membranes removed fluoride due to size exclusion, for the membranes with bigger pore radius charge repulsion was the dominant mechanism of fluoride retention. Regardless of the seasonal conditions a TOC concentration <2 mgC L^-1 was achieved by all membranes at 50% recovery, as NF/RO membranes remove TOC mainly by size exclusion. Two swamp water samples, containing high TOC (79 and 183 mgC L^-1), were filtered to determine the characteristics of NOM which permeate the NF/RO membranes. Liquid chromatography organic carbon detection (LC-OCD) was used to characterize the fractions in the permeates, consisting of about 1% of the original NOM. The average molecular weight of the permeate humic substances (HS) was more than four times larger than the membrane molecular weight cut-off. This suggests that large HS can permeate the NF/RO membranes through diffusion. Moreover, the relatively high aromaticity of the permeate HS (1.7-5.2 L mg^-1 m^-1) indicated the high content of hydrophobic-aromatic fractions.

Fluoride prevalence in groundwater around a fluorite mining area in the flood plain of the River Swat, Pakistan

This study investigated the fluoride (F^-) concentrations and physicochemical parameters of the groundwater in a fluorite mining area of the flood plain region of the River Swat, with particular emphasis on the fate and distribution of F^- and the hydrogeochemistry. To better understand the groundwater hydrochemical profile and F^- enrichment, groundwater samples (n=53) were collected from shallow (24-40m), mid-depth (48-65m) and deep (85-120m) aquifers, and then analysed using an ion-selective electrode. The lowest F^- concentration (0.7mg/L) was recorded in the deep-aquifer groundwater, while the highest (6.4mg/L) was recorded in shallow groundwater. Most groundwater samples (62.2%) exceeded the guideline (1.5mg/L) set by the World Health Organization (WHO); while for individual sources, 73% of shallow-groundwater samples (F^- concentration up to 6.4mg/L), 42% of mid-depth-groundwater samples, and 17% of deep-groundwater samples had F^- concentrations that exceeded this permissible limit. Assessment of the overall quality of the groundwater revealed influences of the weathering of granite and gneisses rocks, along with silicate minerals and ion exchange processes. Hydrogeochemical analysis of the groundwater showed that Na⁺ is the dominant cation and HCO₃^- the major anion. The anionic and cationic concentrations across the entire study area increased in the following order: HCO₃>SO₄>Cl>NO₃>F>PO₄ and Na>Ca>Mg>K, respectively. Relatively higher F^- toxicity levels were associated with the NaHCO₃ water type, and the chemical facies were found to change from the CaHCO₃ to (NaHCO₃) type in calcium-poor aquifers. Thermodynamic considerations of saturation indices indicated that fluorite minerals play a vital role in the prevalence of fluorosis, while under-saturation revealed that - besides fluorite minerals - other F^- minerals that are also present in the region further increase the F^- concentrations in the groundwater. Finally, a health risk assessment via Dean's classification method identified that the groundwater with relatively higher F^- concentrations is unfit for drinking purposes.

Geochemical signatures and isotopic systematics constraining dynamics of fluoride contamination in groundwater across Jamui district, Indo-Gangetic alluvial plains, India

A data set of 76 water samples are obtained from surface and sub-surface water bodies to investigate chemical parameters and stable isotopic signatures in order to drive factors leading to fluoride (F^-) contamination in groundwater of parts of Jamui district, India. Hydrochemical facies reveals that F^- concentration is lower in Ca²⁺-HCO₃^- facies representative of recharge area, while discharge area has a tendency towards Na⁺-HCO₃^- facies with elevated F^- concentration. The ionic ratios Na⁺/Ca²⁺>1, Na⁺/Cl^->1, (Ca²⁺+Mg²⁺)/HCO₃^-<1, Na⁺+K⁺ = 0.5TZ⁺ and Ca²⁺+Mg²⁺ = TZ⁺ witness silicate weathering by water-rock interaction coupled with ion exchange and prolonged residence time, are the principle factors for fluoride contamination (3.6 mg/L to 5.8 mg/L) in 67% of deeper bore wells. Geochemical modelling testifies excess of alkalinity due to the dominance of bicarbonate ion leading to calcite precipitation and dissolution of fluoride in solution contributing to fluoride contamination. The chemometric analysis reveals that the water chemistry of the study area is controlled by both anthropogenic and natural sources, and enrichment of fluoride in groundwater is possibly from geogenic source (fractured granite gneiss). The stable isotope plot shows that most of the samples fall along local meteoric water line indicating that the groundwater is originated from local precipitation with a possibility of evaporative enrichment. Groundwater enriched in δ¹⁸O is positively correlated with F^- suggesting evaporation and longer residence time of water. Spatially elevated F^- prevails in the eastern bank of Kiul River and along the groundwater flow direction, which is attributed to control of dynamics of hydrogeological conditions.

Geochemical processes controlling the groundwater chemistry and fluoride contamination in the Yuncheng Basin, China-An area with complex hydrogeochemical conditions

Hydrogeochemical and stable isotope analyses and geochemical modeling were carried out to identify the major geochemical processes controlling the groundwater chemistry and fluoride contamination in the aquifers of the Yuncheng Basin, China, an area with complex hydrogeochemical conditions and severe fluoride contamination of the groundwater. The major findings of this case study include the following: 1) Cation exchange and salt effects are vital controls on the enrichment of fluoride in groundwater in the area by reducing the activity of Ca²⁺/F^- in groundwater via ion complexation. Cation exchange increased the fluoride concentration by 2.7 mg/L when the Na/Ca molar ratio increased from 0.24 to 9.0, while the salt effect led to a ca. 5–10% increase in complex F^- in groundwater due to the further dissolution of fluoride-bearing minerals in the aquifers, as suggested by a model calculation. 2) Anthropogenic contamination from pesticide and fertilizer use and industrial waste discharge is also a main source of fluoride in the groundwater. 3) Evaporation and ion effects favor the enrichment of fluoride in groundwater by encouraging the removal of Ca via precipitation. 4) The desorption of fluoride from mineral/organic matter surfaces is enhanced under alkaline conditions and a high HCO₃ content in groundwater.

Diverse mechanisms drive fluoride enrichment in groundwater in two neighboring sites in northern China

Excessive amounts of fluoride in drinking groundwater are harmful to human health, but the mechanisms responsible for fluoride enrichment in groundwater are not fully understood. Samples from two neighboring areas with endemic fluorosis were collected to test the hypothesis that there are distinctly different mechanisms responsible for the enrichment of fluoride in these groundwater. Hydrochemistry, stable isotopes and geochemical simulation were conducted together to investigate the fluoride spatial distribution and the diversity of responsible mechanisms. Our results showed that the spatial distributions of fluoride are different: I) high [F] in fresh shallow groundwater (SG_QJ) and II) medium [F] in fresh to brackish deep groundwater (DG_QJ) in the Qiji area; and III) medium [F] in brackish shallow groundwater (SG_YH) and IV) low [F] in fresh deep groundwater (DG_YH) in the Yanhu area. We also found that the fluoride concentration in groundwater is primarily controlled by the dissolution equilibrium of fluorite, as suggested by the correlation between [F] and [Ca]. However, there are other significant mechanisms: 1) for SG_QJ, fluoride-bearing minerals (such as fluorite) dissolution, along with moderate evaporation, cation exchange and the more alkaline conditions are the driving factors; 2) for SG_YH, the contributing factors are strong evaporation, the salt effect, dissolution of evaporites, gypsum and dolomite, bicarbonate-fluoride competition and anthropogenic activity; 3) for DG_QJ, cation exchange, alkaline conditions and competitive adsorption are major factors; and 4) dolomite dissolution promotes the [F] increase in DG_YH. Our findings suggest that the hydrogeochemical conditions play key roles in the enrichment of fluoride and that caution should be taken in the future when evaluating fluoride occurrence in groundwater, even in nearby areas.

Urban groundwater quality in sub-Saharan Africa: current status and implications for water security and public health

Groundwater resources are important sources of drinking water in Africa, and they are hugely important in sustaining urban livelihoods and supporting a diverse range of commercial and agricultural activities. Groundwater has an important role in improving health in sub-Saharan Africa (SSA). An estimated 250 million people (40% of the total) live in urban centres across SSA. SSA has experienced a rapid expansion in urban populations since the 1950s, with increased population densities as well as expanding geographical coverage. Estimates suggest that the urban population in SSA will double between 2000 and 2030. The quality status of shallow urban groundwater resources is often very poor due to inadequate waste management and source protection, and poses a significant health risk to users, while deeper borehole sources often provide an important source of good quality drinking water. Given the growth in future demand from this finite resource, as well as potential changes in future climate in this region, a detailed understanding of both water quantity and quality is required to use this resource sustainably. This paper provides a comprehensive assessment of the water quality status, both microbial and chemical, of urban groundwater in SSA across a range of hydrogeological terrains and different groundwater point types. Lower storage basement terrains, which underlie a significant proportion of urban centres in SSA, are particularly vulnerable to contamination. The relationship between mean nitrate concentration and intrinsic aquifer pollution risk is assessed for urban centres across SSA. Current knowledge gaps are identified and future research needs highlighted.

Fluoride abundance and controls in fresh groundwater in Quaternary deposits and bedrock fractures in an area with fluorine-rich granitoid rocks

This study focuses on fluoride (F(-)) concentrations in groundwater in an area in northern Europe (Laxemar, southeast Sweden) where high F(-) concentrations have previously been found in surface waters such as streams and quarries. Fluoride concentrations were determined over time in groundwater in the Quaternary deposits ("regolith groundwater"), and with different sampling techniques from just beneath the ground surface to nearly -700m in the bedrock (fracture) groundwater. A number of potential controls of dissolved F(-) were studied, including geological variables, mineralogy, mineral chemistry and hydrology. In the regolith groundwater the F(-) concentrations (0.3-4.2mg/L) were relatively stable over time at each sampling site but varied widely among the sampling sites. In these groundwaters, the F(-) concentrations were uncorrelated with sample (filter) depth and the water table in meters above sea level (masl), with the thicknesses of the groundwater column and the regolith, and with the distribution of soil types at the sampling sites. Fluoride concentrations were, however, correlated with the anticipated spatial distribution of erosional material (till) derived from a F-rich circular granite intrusion. Abundant release of F(-) from such material is thus suggested, primarily via dissolution of fluorite and weathering of biotite. In the fresh fracture groundwater, the F(-) concentrations (1.2-7.4mg/L) were generally higher than in the regolith groundwater, and were uncorrelated with depth and with location relative to the granite intrusion. Two mechanisms explaining the overall high F(-) levels in the fracture groundwater were addressed. First, weathering/dissolution of fluorite, bastnäsite and apophyllite, which are secondary minerals formed in the fractures during past hydrothermal events, and biotite which is a primary mineral exposed on fracture walls. Second, long water-residence times, favoring water-rock interaction and build-up of high dissolved F(-) concentrations. The findings are relevant in contexts of extraction of groundwater for drinking-water purposes.