Coupling fractionation and batch desorption to understand arsenic and fluoride co-contamination in the aquifer system

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.

Co-occurrence of arsenic and fluoride in groundwater of Guide basin in China: Genesis, mobility and enrichment mechanism

Endemic arsenic poisoning and fluorosis caused by primary high arsenic (As) and high fluoride (F-) groundwater have become one of the most serious environmental geological problems faced by the international society. High As and high F- groundwater exists in Neogene confined aquifers in Guide basin, with concentrations of 355 μg/L and 5.67 mg/L, respectively, and showing a co-occurrence phenomenon of As and F- in the groundwater. This poses a double threat to the health of tens of thousands of local residents. In this study, based on the systematic collection of groundwater and borehole sediment samples, analysis of hydrochemistry and isotope indexes, combined with laboratory tests, purpose of this study is to reveal the migration rule and co-enrichment mechanism of As and F- in aquifers, and finally establish a hydrogeochemical conceptual model of the enrichment process of As and F-. The main conclusions are as follows: hydrochemical type of unconfined and confined groundwater in Guide basin is Ca-Na-HCO3 and Na-Cl-HCO3 type, respectively. Main minerals in sediments are quartz and plagioclase. Concentrations of As and F- are lower in unconfined groundwater, but higher in confined groundwater, and which show a gradual increasing trend along the groundwater flow path. The mineralization of natural organic matter in confined aquifer causes iron and manganese oxide minerals containing As to dissolve gradually, which leads to the gradual release of As into groundwater. Large amount of HCO3- produced by mineralization of organic matter precipitate with Ca2+ in groundwater, resulting in reduction of Ca2+ content, promoting the dissolution of fluoride-containing minerals such as fluorite (CaF2), and continuously releasing F- into groundwater. Meanwhile, competitive adsorption reactions in confined aquifers causes more As and F- to be released from mineral surface into groundwater, which gradually migrate and accumulate along groundwater flow. Finally, it is established that a conceptual model for the formation of high As and F- groundwater in the confined aquifer of Guide basin. The research results not only help to improve our understanding of the formation and evolution of groundwater with high As and F- with similar geological background, but also provide scientific basis for rational development and utilization of groundwater, and prevention and control of chronic As and F- poisoning in local and similar areas.

Machine learning approach for assessment of arsenic levels using physicochemical properties of water, soil, elevation, and land cover

Groundwater is an essential resource; around 2.5 billion people depend on it for drinking and irrigation. Groundwater arsenic contamination is due to natural and anthropogenic sources. The World Health Organization (WHO) has proposed a guideline value for arsenic concentration in groundwater samples of 10[Formula: see text]g/L. Continuous consumption of arsenic-contaminated water causes various carcinogenic and non-carcinogenic health risks. In this paper, we introduce a geospatial-based machine learning method for classifying arsenic concentration levels as high (1) or low (0) using physicochemical properties of water, soil type, land use land cover, digital elevation, subsoil sand, silt, clay, and organic content of the region. The groundwater samples were collected from multiple sites along the river Ganga's banks of Varanasi district in Uttar Pradesh, India. The dataset was subjected to descriptive statistics and spatial analysis for all parameters. This study assesses the various contributing parameters responsible for the occurrence of arsenic in the study area based on the Pearson correlation feature selection method. The performance of machine learning models, i.e., Extreme Gradient Boosting (XGBoost), Gradient Boosting Machine (GBM), Decision Tree, Random Forest, Naïve Bayes, and Deep Neural Network (DNN), were compared to validate the parameters responsible for the dissolution of arsenic in groundwater aquifers. Among all the models, the DNN algorithm outclasses other classifiers as it has a high accuracy of 92.30%, a sensitivity of 100%, and a specificity of 75%. Policymakers can utilize the accuracy of the DNN model to approximate individuals prone to arsenic poisoning and formulate mitigation strategies based on spatial maps.

Elemental composition of rural household dust in Brahmaputra fluvial plain: insights from SEM-EDS, receptor model, and risk assessment

The study attempts to look into the morphological characteristics, elemental composition, contamination, source contributions, and associated health risks in household dust of Napaam, a rural region in the Brahmaputra flood plain in North East India. Morphological evidence suggests that most of the house dust particles were sourced from vehicle abrasion and soil. Three contamination indices-enrichment factor (EF), index of geo-accumulation (I_geo), and pollution load index (PLI) indicated that Cl and four trace elements (Cu, Zn, As, and Pb) are significantly enriched in house dust with extreme pollution load. Principal component analysis (PCA) and positive matrix factorization (PMF) revealed 3 potential major sources of elements in house dust-traffic + re-suspension of road dust (35.8%), soil dust (22.2%), and river sediment deposit (16.4%). Two minor sources-biomass burning (13.3%), and construction activities (12.3%) were also identified. Based on health risk assessment (HRA), both children and adult were found to be susceptible to non-carcinogenic and carcinogenic risks.

Potential arsenic-chromium-lead Co-contamination in the hilly terrain of Arunachal Pradesh, north-eastern India: Genesis and health perspective

In the recent times, multi-metal co-contamination in the groundwater of various parts of the globe has emerged as a challenging environmental health problems. While arsenic (As) has been reported with high fluoride and at times with uranium; and Cr & Pb are also found in aquifers under high anthropogenic impacts. The present work probably for the first time traces the As-Cr-Pb co-contamination in the pristine aquifers of a hilly terrain that are under relatively less stress from the anthropogenic activities. Based on the analyses of twenty-two (n = 22) groundwater (GW) samples and six (n = 6) sediment samples, it was found that Cr being leached from the natural sources as evident from 100% of samples with dissolve Cr exceeding the prescribed drinking water limit. Generic plots suggests rock-water interaction as the major hydrogeological processes with mixed Ca²⁺-Na⁺-HCO₃^- type water. Wide range of pH suggests localized human interferences, as well as indicative of both calcite and silicate weathering processes. In general water samples were found high only with Cr and Fe, however all sediment samples were found to contain As-Cr-Pb. This implies that the groundwater is under-risk of co-contamination of highly toxic trio of As-Cr-Pb. Multivariate analyses indicate that the changing pH as the causative factor for Cr leaching into the groundwater. This is a new finding for a pristine hilly aquifers, and we suspect such condition may also be present in other parts of globe, and thus precautionary investigations are needed to prevent this catastrophic situation to arise, and to alert the community in advance.

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.

Spatial distribution characteristics and prediction of fluorine concentration in groundwater based on driving factors analysis

Excess fluoride (F^-) in groundwater can be hazardous to human health. A total of 360 ground water samples was collected from northern Anhui, China, to study the levels, distribution, and source of F^-. And on this basis, predicting the spatial distribution of F^- in a wider scale space. The range of F^- was 0.1-5.8 mg/L, with a mean value of 1.2 mg/L, and 26.4 % of the samples exceeded the acceptable level of 1.5 mg/L. Moreover, the water-rock interaction (fluorite dissolution) and cation alternate adsorption were considered to be two main driving factors of high F^- in groundwater. To further illustrate the spatial effects, the BME-RF model was established by combining the main environmental factors. The spatial distribution of F^- was quantitatively predicted, and the response to environmental variables was analyzed. The R² of BME-RF model reached 0.93, the prediction results showed that the region with 1.0-1.5 mg/L of F^- accounts for 47.2 % of the total area. The predicted F^- content of nearly 70 % of groundwater in this area has exceeded 1.0 mg/L, which was dominated by Na⁺ and HCO₃^- type. The spatial variability of F^- in the study area was mainly affected by hydrogeological conditions, and the vertical distribution characteristics were related to the spatial variation of slope, distance from runoff, and hydrochemical types. The results of the study provide new insights into the F^- concentration prediction in underground environment, especially in the borehole gap area.

Arsenic in the groundwater of the Upper Brahmaputra floodplain: Variability, health risks and potential impacts

The upper Brahmaputra floodplain (UBF) is highly enriched with geogenic contaminants, mostly the Arsenic (As), owing to its sensitive geomorphology, aquifer groundwater (GW) dynamics, huge sediment deposition along with natural and anthropogenic disturbance significantly accelerate the As leaching rate into the groundwater, leading to an increase the health risk. Concerning the above active processes in the Lakhimpur district of UBF, current work aims to investigate seasonal dynamics of As contamination and associated health hazard for local community. To carry out this study, 51 GW samples analyzed which reveal the seasonal variation in As concentrations with the minimum average concentration in the monsoon season (4.7 μg.L^-1) and the maximum in the post-monsoon season (18.5 μg.L^-1) with 50% of the samples exceeding permissible limits. The differences in the local geological conditions and the GW flow may contribute to the spatial variations in mean As concentration in the study area. Also, the active GW recharge process identified in post-monsoon season accelerates the As leaching in the area's aquifers. Further, results indicate higher As levels associated with a pH range of 6-7 favours As desorption from minerals under reducing conditions. The hazard index indicates that the children population has high carcinogenic risk compared to adult populations. Furthermore, the study highlights the social risk potential arising from public health crises due to As exposure. Overall, results indicate high As concentrations in region with moderate seasonal variability and demand a dire attention for long-term monitoring to provide sustainable water resources to safeguard the people at risk.

Monitoring and prediction of high fluoride concentrations in groundwater in Pakistan

Concentrations of naturally occurring fluoride in groundwater exceeding the WHO guideline of 1.5 mg/L have been detected in many parts of Pakistan. This may lead to dental or skeletal fluorosis and thereby poses a potential threat to public health. Utilizing a total of 5483 fluoride concentrations, comprising 2160 new measurements as well as those from other sources, we have applied machine learning techniques to predict the probability of fluoride in groundwater in Pakistan exceeding 1.5 mg/L at a 250 m spatial resolution. Climate, soil, lithology, topography, and land cover parameters were identified as effective predictors of high fluoride concentrations in groundwater. Excellent model performance was observed in a random forest model that achieved an Area Under the Curve (AUC) of 0.92 on test data that were not used in modeling. The highest probabilities of high fluoride concentrations in groundwater are predicted in the Thar Desert, Sargodha Division, and scattered along the Sulaiman Mountains. Applying the model predictions to the population density and accounting for groundwater usage in both rural and urban areas, we estimate that about 13 million people may be at risk of fluorosis due to consuming groundwater with fluoride concentrations >1.5 mg/L in Pakistan, which corresponds to ~6% of the total population. Both the fluoride prediction map and the health risk map can be used as important decision-making tools for authorities and water resource managers in the identification and mitigation of groundwater fluoride contamination.

Water-Rock Interaction Processes: A Local Scale Study on Arsenic Sources and Release Mechanisms from a Volcanic Rock Matrix

Arsenic is a potentially toxic element (PTE) that is widely present in groundwater, with concentrations often exceeding the WHO drinking water guideline value (10.0 μg/L), entailing a prominent risk to human health due to long-term exposure. We investigated its origin in groundwater in a study area located north of Rome (Italy) in a volcanic-sedimentary aquifer. Some possible mineralogical sources and main mechanisms governing As mobilization from a representative volcanic tuff have been investigated via laboratory experiments, such as selective sequential extraction and dissolution tests mimicking different release conditions. Arsenic in groundwater ranges from 0.2 to 50.6 μg/L. It does not exhibit a defined spatial distribution, and it shows positive correlations with other PTEs typical of a volcanic environment, such as F, U, and V. Various potential As-bearing phases, such as zeolites, iron oxyhydroxides, calcite, and pyrite are present in the tuff samples. Arsenic in the rocks shows concentrations in the range of 17-41 mg/kg and is mostly associated with a minor fraction of the rock constituted by FeOOH, in particular, low crystalline, containing up to 70% of total As. Secondary fractions include specifically adsorbed As, As-coprecipitated or bound to calcite and linked to sulfides. Results show that As in groundwater mainly originates from water-rock interaction processes. The release of As into groundwater most likely occurs through desorption phenomena in the presence of specific exchangers and, although locally, via the reductive dissolution of Fe oxy-hydroxides.

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.

Anthropogenic dominance on geogenic arsenic problem of the groundwater in the Ganga-Brahmaputra floodplain: A paradox of origin and mobilization

The Ganga-Brahmaputra floodplains constitute a complex system that is vulnerable to arsenic recycling owing to its geomorphic sensitivity, aquifer profiles, high meandering scars and extreme sediment deposition, along with extreme monsoonal disturbances; and are subjected to significant alterations in arsenic recycling. We have put an effort to delineate the similarities and dissimilarities pertaining to the arsenic prevalence, origin and mobilization in the two hotspots, namely the Mid-Gangetic Floodplains (MGFP) and the Brahmaputra Floodplains (BFP). Pertaining to this, we collected 384 representative water samples for hydrogeochemical investigations, multivariate analyses, and saturation status based predictive modelling, with BFP having a maximum concentration of arsenic (As) reaching to almost 97.9 μgL^-1 and MGFP having a maximum concentration of 50.1 μgL^-1. Seasonality impelled changes and conforming riverine recharges are leading major ionic differentiations in both the floodplains across seasons. Meandering and aquifer dynamics control As prevalence in the MGFP and BFP, respectively. Non-interdependent HCO₃^- recharge mediated As-recycling was found in the BFP. Carbonate weathering is dominant in the MGFP, while both carbonate and silicate weathering take precedence in the BFP. Multivariate analysis hints at fertilizer influence on As mobilization in the MGFP. Reductive hydrolysis of Fe-OOH mediated As-release is more prominent in the BFP. Seasonal arsenic fluctuations are going to have more climatic dependency in near future owing to the increasing erratic rains, pumping and recharge events. Erratic precipitation will provoke immediate response in both floodplains in terms of As mobilization which urgently needs attention to counter increasing arsenic vulnerability.

Naturally occurring potentially toxic elements in groundwater from the volcanic landscape around Mount Meru, Arusha, Tanzania and their potential health hazard

The population of the semi-arid areas of the countries in the East African Rift Valley (EARV) is faced with serious problems associated with the availability and the quality of the drinking water. In these areas, the drinking water supply largely relies on groundwater characterised by elevated fluoride concentration (> 1.5 mg/L), resulting from interactions with the surrounding alkaline volcanic rocks. This geochemical anomaly is often associated with the presence of other naturally occurring potentially toxic elements (PTEs), such as As, Mo, U, V, which are known to cause adverse effects on human health. This study reports on the occurrence of such PTEs in the groundwater on the populated flanks of Mt. Meru, an active volcano situated in the EARV. Our results show that the majority of analysed PTEs (Al, As, Ba, Cd, Cr, Cu, Fe, Mn, Ni, Se, Sr, Pb, and Zn) are within the acceptable limits for drinking purpose in samples collected from wells, springs and tap systems, suggesting that there is no immediate health risk associated with these PTEs. However, some of the samples were found to exceed the WHO tolerance limit for U (> 30 μg/L) and Mo (> 70 μg/L). The sample analysis also revealed that in some of the collected samples, the concentrations of total dissolved solids, Na⁺ and K⁺ exceed the permissible limits. The concerning levels of major parameters and PTEs were found to be associated with areas covered with debris avalanche deposits on the northeast flank, and volcanic ash and alluvial deposits on the southwest flanks of the volcano. The study highlights the need to extend the range of elements monitored in the regional groundwater and make a more routine measurement of PTEs to ensure drinking water safety and effective water management measures.

Comprehensive and critical appraisal of plant-based defluoridation from environmental matrices

Fluoride is recognized as one of the global environmental threats because of its non-biodegradable nature and long-term persistence in the environment. This has created the dire need to explore various defluoridation techniques (membrane process, adsorption, precipitation, reverse osmosis, ion exchange, and electrocoagulation). Owing to their cost ineffectiveness and high operational costs, these technologies failed to find any practical utility in fluoride remediation. Comparatively, defluoridation techniques involving the use of low-cost plant-derived adsorbents and fluoride phytoremediators are considered better alternatives. Through this review, an attempt has been made to critically synthesize information about various plant-based bioadsorbents and hyperaccumulators from existing literature. Moreover, mechanisms underlying the fluoride adsorption and accumulation by plants have been thoroughly discussed that will invigorate the researchers to develop novel ideas about process/product modifications to further enhance the removal potential of the adsorbents and plants. Literature survey unravels that various low-cost plant-derived adsorbents have shown their efficacy in defluoridation, yet there is an urgent need to explore their pragmatic application on a commercial scale.

Hazardous elements in the soil of urban cemeteries; constructive solutions aimed at sustainability

Urban cemeteries on a global scale raise concerns due to their potential to concentrate differing levels of hazardous pollutants in their native soils due to the unnatural concentration of burials in a limited space. It is paramount for sustainability that designers of future cemeteries take this into account in order to minimize the deposition and movement of these contaminants within the soil profile. The objective of this manuscript is to identify the levels of certain hazardous element contamination, specifically heavy metals, in the soil of horizontal urban cemeteries that do not utilize herbicides for weed control. In this, solutions were sought for the construction of future urban cemeteries capable of mitigating further contamination of the environment by the increase in interments. The soils of three urban cemeteries (A, B and C) in the Brazilian city of Carazinho, in Rio Grande do Sul State, were sampled with 5 monitoring points in the internal area and 5 points in the external area of the cemeteries. At each point, 3 replications were performed at two depths (0-20 and 20-40 cm), totaling 180 samples in all, to determine the concentration of the following metals: copper (Cu), zinc (Zn), iron (Fe), manganese (Mn), lead (Pb), and chromium (Cr) (g kg^-1). In addition, online interviews with 15 architects who design cemeteries were conducted. Architectural design solutions to mitigate environmental contamination were modeled utilizing the Building Information Modeling system (BIM). The results showed an excess of Cu in the soil of cemeteries A, B and C, surpassing the standards allowed by Brazilian federal regulations. A total of 80% of the interviewed architects expressed their preference for the vertical cemetery, with gas and effluent treatment systems to mitigate environmental impacts.

Imprints of pandemic lockdown on subsurface water quality in the coastal industrial city of Tuticorin, South India: A revival perspective

Globally, the incidences of environmental improvements owing to seizing the anthropogenic activities during the lockdown have been reported through news articles and photographs, yet a formal scholarly study has been lacking to substantiate the imprints of lockdown. We hereby present the imprints of lockdown on water quality (both chemical and biological) parameters during the nationwide lockdown (COVID-19 epidemic) in India between 25th March to 30th May 2020. The present study describes the changes in chemical and biological water quality parameters based on twenty-two groundwater samples from the coastal industrial city of Tuticorin in Southern India, taken before (10 and 11th February 2020) and during the lockdown (19 and 20th April 2020) periods. The physico-chemical parameters compared are pH, total dissolved solids (TDS) and electrical conductivity (EC), nitrate (NO₃), fluoride (F), chromium (Cr), iron (Fe), copper (Cu), zinc (Zn), cadmium (Cd), lead (Pb), arsenic (As), and selenium (Se), and the bacterial parameters are total coliforms, fecal coliforms, E. coli, and fecal streptococci. Among the metals, the significant reductions in Se (42%), As (51%), Fe (60%) and Pb (50%) were noticed probably owing to no or very less wastewater discharges from metal-based industries, seafood-based industries and thermal power plants during the lockdown. Reduction in NO₃ (56%), total coliform (52%) and fecal coliforms (48%) indicated less organic sewage from the fishing industries. Contents of Cr, Cu, Zn and Cd, however, remained similar and fluoride did not show any change, probably as they were sourced from rock-water interactions. Similarly, we did not observe alterations in E. coli and fecal streptococci due to no significant change in domestic sewage production during the lockdown. The multivariate analyses aptly illustrated this and the principal component analyses helped to identify the sources that controlled water qualities of the lockdown compared to the pre-lockdown period. Our observation implies that groundwater is definitely under active interaction with surface waters and thus a quick revival could be observed following the seizing of anthropogenic activities.

Assessment of background levels and pollution sources for arsenic and fluoride in the phreatic and confined groundwater of Xi'an city, Shaanxi, China

The presence of arsenic and fluoride in groundwater and their impacts on human health have been reported in many countries worldwide, but little information is available on As or F^- contamination in Xi'an city. This study highlights the distribution and sources of As and F^- anomalies in different aquifers of Xi'an city, based on the assessment of natural background levels (NBLs) and threshold values (TVs). Groundwater samples collected from phreatic and confined aquifers were analyzed to evaluate NBLs and TVs, using median  + 2MAD, Tukey inner fence (TIF), and percentile-based methods. Results showed that NBLs and TVs of As and F^- in the phreatic aquifer were lower than those in the confined aquifer, indicating importance of the geological effects on the enrichment of arsenic and fluoride in the confined aquifer. Combined with hydrogeochemical methods, the distributions of As and F^- anomalies show that high concentrations of As in both aquifers and F^- in the confined aquifer can be attributed to the upward flow of geothermal water through faults and ground fissures, while high concentrations of F^- in the phreatic aquifer may be greatly influenced by contaminated rivers. Although geological structures such as faults and ground fissures contribute to the high concentrations of potentially toxic elements, anthropogenic activities cannot be ignored because over exploitation of groundwater accelerates the development of ground fissures and results in the upward flow and mixing of geothermal water with groundwater in the upper aquifers.

Microbes involved in arsenic mobilization and respiration: a review on isolation, identification, isolates and implications

Microorganisms play an important role in arsenic (As) cycling in the environment. Microbes mobilize As directly or indirectly, and natural/geochemical processes such as sulphate and iron reduction, oxidative sulphide mineral dissolution, arsenite (AsO₃^3-) oxidation and arsenate (AsO₄^3-) respiration further aid in As cycle in the environment. Arsenate serves as an electron donor for the microbes during anaerobic conditions in the sediment. The present work reviews the recent development in As contamination, various As-metabolizing microbes and their phylogenetic diversity, to understand the role of microbial communities in As respiration and mobilization. It also summarizes the contemporary understanding of the intricate biochemistry and molecular biology of natural As metabolisms. Some successful examples of engineered microbes by harnessing these natural mechanisms for effective remediation are also discussed. The study indicates that there is an exigent need to have a clear understanding of environmental aspects of As mobilization and subsequent oxidation-reduction by a suitable microbial consortium.

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.

Scenario, perspectives and mechanism of arsenic and fluoride Co-occurrence in the groundwater: A review

Arsenic (As) and fluoride (F^-) are the two most conspicuous contaminants, in terms of distribution and menace, in aquifers around the world. While the majority of studies focus on the individual accounts of their hydro-geochemistry, the current work is an effort to bring together the past and contemporary works on As and F^- co-occurrence. Co-occurrence in the context of As and F^- is a broad umbrella term and necessarily does not imply a positive correlation between the two contaminants. In arid oxidized aquifers, healthy relationships between As and F^- is reported owing desorption based release from the positively charged (hydr)oxides of metals like iron (Fe) under alkaline pH. In many instances, multiple pathways of release led to little or no correlation between the two, yet there were high concentrations of both at the same time. The key influencer of the strength of the co-occurrence is seasonality, environment, and climatic conditions. Besides, the existing primary ion and dissolved organic matter also affect the release and enrichment of As-F^- in the aquifer system. Anthropogenic forcing in the form of mining, irrigation return flow, extraction, recharge, and agrochemicals remains the most significant contributing factor in the co-occurrence. The epidemiological indicate that the interface of these two interacting elements concerning public health is considerably complicated and can be affected by some uncertain factors. The existing explanations of interactions between As-F are indecisive, especially their antagonistic interactions that need further investigation. "Multi-contamination perspectives of groundwater" is an essential consideration for the overarching question of freshwater sustainability.

Seasonal dynamics of metal phase distributions in the perennial tropical (Brahmaputra) river: Environmental fate and transport perspective

An evaluation of the seasonal dynamics of metal (Cd, Cr, Cu, Pb and Zn) phase distribution and speciation, along with different hydrogeochemical facies, was conducted along the entire stretch of a perennial tropical (Brahmaputra) River. Water samples were collected from twelve locations during the monsoon, post-monsoon and pre-monsoon seasons. A characteristic trend in bioavailability was observed in the sequence of Zn > Cu > Pb > Cr > Cd, reflecting non-significant seasonal changes in dissolved and particulate phase metal fractions at all twelvelocations. Partition (or distribution) coefficients (K_ds) for Cu (5.30-6.50), Zn (4.47-6.44), Pb (3.69-4.97) and Cr (3.25-5.15), reflect the influences of anionic inorganic constitutents (aqua ion complexes), environmental variables, and river hydrology. MINTEQA2 calculations showed that metal forms complexes with other inorganic ions such as Cl^-, SO₄^2-, HCO^3- and strong binding as organic complexes (for Cu and Pb) reduces their leaching potential to groundwater system. High log K_d values (≥5.3) and significant spatial variations at locations R3 to R8 of Pb, Cu, and Zn during pre-monsoon period may be explained by the formation of complexes with organic ligands in sewage and domestic wastes (humic and fulvic acids). Multivariate statistical analysis revealed multi-metal co-contamination, mainly of Zn, Cu, Cd and Pb and dissolved nutrients, loading in different clusters, emphasizing the importance of climatic, anthropogenic, terrigenous and lithological sources as controlling factors for seasonal metal dynamics in the river water.

Arsenic exposure and perception of health risk due to groundwater contamination in Majuli (river island), Assam, India

Island populations are rarely studied for risk of arsenic (As) poisoning. As poisoning, multimetal contamination and people's perceptions of health risks were assessed on India's Majuli Island, the largest inhabited river island in the world. This holistic approach illustrated the association of groundwater contamination status with consequent health risk by measuring levels of inorganic arsenic (iAs) in groundwater, borehole sediment and biological samples (hair, nails and urine). Piper and Gibbs's plots discerned the underlying hydrogeochemical processes in the aquifer. Demographic data and qualitative factors were evaluated to assess the risks and uncertainties of exposure. The results exhibited significant enrichment of groundwater with As, Mn and Fe along with significant body burden. Maximum Hazard Index values indicated severe non-carcinogenic health impacts as well as a significantly elevated risk of cancer for both adults and children. Most (99%) of the locally affected population did not know about the adverse health impacts of metal contamination, and only 15% understood bodily ailments and health issues. Various aspects of the island environment were used to elucidate the status of contamination and future risk of disease. A projection showed adverse health outcomes rising significantly, especially among the young population of Majuli, due to overexposure to not only As but also Ba, Mn and Fe.

Control of the mobilization of arsenic and other natural pollutants in groundwater by calcium carbonate concretions in the Pampean Aquifer, southeast of the Buenos Aires province, Argentina

The water supply for human consumption in the Chaco-Pampean region in Argentina is restricted by the low quality of groundwater due to elevated concentrations of arsenic and other trace elements. Previous studies indicated a complex concurrence of factors and processes that are believed responsible to control the distribution of arsenic in groundwater. For a better understanding of the origin of trace elements in the Pampean aquifer, flow-through experiments with loess and calcrete samples representative of the sediments that constitute the aquifer were carried out in continuous flow reactors. The aqueous solutions were collected and the concentrations of SiO₂(aq), Ca²⁺, SO₄^2-, Na⁺, Cl^-, F^- and trace elements (Ba, Sr, V, and As) were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES), high performance liquid chromatography (HPLC) and capillary electrophoresis. The experiments showed differences in the release rate of elements to the solution according to the type of sediment. The highest concentrations of V, Ba, and As were measured in experiments conducted with loess, and these elements were released quickly to the solution in the first minute of the test. In the case of loess, V and As are suggested to be adsorbed on the solid particles surface. Conversely, the experiments conducted with calcrete showed a lower but continuous release of those elements. This last result may indicate that the trace elements were coprecipitated in the calcite. In addition, it was demonstrated that F did not come from the dissolution of minerals such as fluorapatite, but both desorption from solid surface and dissolution from calcite minerals account for the release of F. This study support that both dissolution and adsorption-desorption processes can control the mobility of trace elements, with an emphasis on the role of calcrete in the retention and the mobilization of trace elements in the Pampean aquifer.

Multilayer arsenic mobilization and multimetal co-enrichment in the alluvium (Brahmaputra) plains of India: A tale of redox domination along the depth

The study attempts to understand arsenic (As) mobilization in a shallow aquifer with depth variation while focusing on the potential co-occurrence of As with priority metals (zinc and lead), using a pilot scale multilevel groundwater monitoring system (MGWS). Groundwater samples (n = 72) were collected bi-weekly (every 15 days) from the multilevel sampler (4.6, 9.2 and 13.8 m depths), installed at Tezpur, Sonitpur district of Brahmaputra floodplain (BFP), Assam, India, for a period of 1 year (August 2013-July 2014). Both geogenic and anthropogenic influences were found to affect the studied unconfined aquifer. At 4.6 m, weathering dominated due to interaction with CO₂ and infiltrating water. Prevalent high pH (7.9-8.6) at all three depths in association with strong oxidizing condition (at 4.6 m) during the drier months seem to play a crucial role in desorption based As release. Multivariate analyses revealed that redox potential (ORP) remains the primary controller of As release at all three depths. With depth, stronger anoxic conditions resulted in the dominance of reductive hydrolysis leading to a co-occurrence scenario of As (max 4.6 μgL^-1) with Zn (max 2514 μgL^-1) and Pb (max 740 μL^-1) with influences of anthropogenic modes of activities like agriculture and dry deposition from a brick kiln. Multi-element enrichment is an emerging concern but the bigger picture would be to understand the peculiarities of individual aquifers, as a generalization can lead to missing a ton of information. In this regard, long-term multilevel monitoring can help in the predictive understanding of the vertical stratification and co-occurrences of multi-metals that can subsequently be applied for water production at the safer depths.

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.

Provenance, prevalence and health perspective of co-occurrences of arsenic, fluoride and uranium in the aquifers of the Brahmaputra River floodplain

The present work focuses on understanding the provenance, prevalence and health perspective of As and F^- along with possible co-occurrence of uranium (U) in the aquifers of the Brahmaputra floodplains (BFP), India. Groundwater (n = 164) and sediment samples (n = 5) were obtained from the upper, middle and lower BFP. Energy dispersive spectroscopy (EDX) revealed the presence of As, U and Fe in the sediment matrix. Regression analysis showed a weaker relationship between As and F^- co-occurrence. Hierarchical cluster analysis (HCA) and principal component analysis (PCA) suggested reductive dissolution of Fe (hydr)oxides responsible for As release in the BFP, especially in the upper and lower BFP. Bicarbonate appeared to compete with As oxyanions for adsorption on positively charged surfaces leading to As release. Arsenic desorption in presence of PO₄^3-, F^- and HCO₃^- at elevated pH appeared greatest in the upper BFP, suggesting the highest potential for co-occurrence. Co-occurrence, were mainly in isolated aquifers of the upper BFP owing to desorption of adsorbed As and F^- from Fe (hydr)oxides at higher pH. Weathering and dissolution of clay minerals in the upper BFP, and competitive desorption in presence of HCO₃^- and PO₄^3- in the middle and lower BFP, respectively, explain variabilities in F^- release. Amorphous Fe (hydr)oxides like ferrihydrite act as sinks of U. Concentrations of As and F^- will likely increase in the future as projected from the saturated levels of goethite and ferrihydrite. Hazard indices (HI) revealed that children (3-8 years) were at greater health risk than adults.

Fluoride and iodine enrichment in groundwater of North China Plain: Evidences from speciation analysis and geochemical modeling

To better understand the enrichment of fluoride and iodine in groundwater at North China Plain (NCP), speciation analysis and geochemical modeling were conducted to identify the key hydrochemical processes controlling their mobilization in groundwater system. Groundwater fluoride and iodine concentrations ranged from 0.18 to 5.59mg/L and from 1.51 to 1106μg/L, respectively, and approximately 63% and 32.3% of groundwater fluoride and iodine were higher than the guidelines for drinking water (1.5mg/L and 150μg/L). High fluoride concentration (>1.5mg/L) can be detected in groundwater from the flow-through and discharge areas of NCP, and high iodine groundwater (>150μg/L) is mainly scattered in the coastal area. Na-HCO₃/Cl type water resulted from water-rock interaction and seawater intrusion favors fluoride and iodine enrichment in groundwater. Speciation analysis results indicate that (1) fluoride complexes in groundwater are dominated by free fluoride, the negative charge of which favors fluoride enrichment in groundwater under basic conditions, and (2) iodide, iodate and organic iodine co-occur in groundwater at NCP with iodide as the dominant species. The geochemical modeling results indicate that groundwater fluoride is mainly associated with the saturation states of fluorite and calcite, as well as the adsorption equilibrium onto goethite and gibbsite, including the competitive adsorption between fluoride and carbonate. Groundwater iodine is mainly controlled by redox potential and pH condition of groundwater system. Reducing condition favors the mobilization and enrichment of groundwater iodide, which has the highest mobility among iodine species. Under reducing condition, reductive dissolution of iron (oxy)hydroxides is a potential geochemical process responsible for iodine release from sediment into groundwater. Under (sub)oxidizing condition, as groundwater pH over the 'point of zero charge' of iron (oxy)hydroxides, the lowering adsorption capacity of groundwater iodide/iodate on minerals leads to the release of sediment iodine into groundwater.

Medical geology in the framework of the sustainable development goals

Exposure to geogenic contaminants (GCs) such as metal(loid)s, radioactive metals and isotopes as well as transuraniums occurring naturally in geogenic sources (rocks, minerals) can negatively impact on environmental and human health. The GCs are released into the environment by natural biogeochemical processes within the near-surface environments and/or by anthropogenic activities such as mining and hydrocarbon exploitation as well as exploitation of geothermal resources. They can contaminate soil, water, air and biota and subsequently enter the food chain with often serious health impacts which are mostly underestimated and poorly recognized. Global population explosion and economic growth and the associated increase in demand for water, energy, food, and mineral resources result in accelerated release of GCs globally. The emerging science of "medical geology" assesses the complex relationships between geo-environmental factors and their impacts on humans and environments and is related to the majority of the 17 Sustainable Development Goals in the 2030 Agenda of the United Nations for Sustainable Development. In this paper, we identify multiple lines of evidence for the role of GCs in the incidence of diseases with as yet unknown etiology (causation). Integrated medical geology promises a more holistic understanding of the occurrence, mobility, bioavailability, bio-accessibility, exposure and transfer mechanisms of GCs to the food-chain and humans, and the related ecotoxicological impacts and health effects. Scientific evidence based on this approach will support adaptive solutions for prevention, preparedness and response regarding human and environmental health impacts originating from exposure to GCs.