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Chronic exposure to geogenic contaminated groundwaters (GCGs) has caused serious health problems among billions of people worldwide. The hydrogeological features, main sources of geogenic hazardous substances, dominant hydrogeochemical processes and key factors controlling the occurrence of high arsenic (As ≥ 10 μg/L), high fluoride (F ≥ 1 mg/L), and high iodine (I ≥ 100 μg/L) groundwater are discussed in this review. Although the global and/or regional occurrence, and the environmental and geochemical behaviors of groundwater As, F and I have been well documented in the past three decades, there is still a lack of holistic theoretical analysis to demonstrate the hydrogeological patterns of their (co)occurrence. We here propose four basic genetic types of GCGs by summarizing the characteristics of the distribution and major hydrogeochemical processes: leaching-enrichment type, burial-dissolution type, compaction-release type, and evaporation-concentration type. The complex genetic mechanisms of GCGs are thus integrated into a new theoretical framework to improve our understanding of their genesis and our capability of predicting their spatial and temporal distribution. Moreover, new paradigms of interdisciplinary convergent research are required to find best solutions to ensure the sustainable safe supply of groundwater resources and the well-being of the groundwater-dependent ecosystems and the human society. [ABSTRACT FROM AUTHOR]

• Groundwater flowing models underestimated the concentrations of groundwater As. • Redox gradients related to Fe-S-As cycling controlled the variations of groundwater As. • Abiotic reduction of Fe(III) minerals contributes ∼ 35 % of groundwater As. A multi-level field site was built at the As-affected area of the Datong basin. The one-year monitoring results showed that the three aquifers exhibit different redox conditions and hydrogeochemical behaviors. In the shallow aquifer, groundwater As concentration is up to 220.5 μg/L, which is related to the dissimilatory reduction of poor-crystalline Fe minerals. In the middle aquifer, groundwater sulfate reduction shows two sides effects on As mobilization that the reduced sulfide on the one hand promotes the release of sediment As via abiotic reductive dissolution of Fe minerals and on the other hand immobilizes groundwater As via the adsorption on the newly formed Fe(II)-sulfide minerals. In the deep aquifer, the reductive dissolution of crystalline Fe minerals was the dominant process causing the release of sediment As into groundwater with the highest As concentration up to 341.1 μg/L. Based on the one-year monitored data, a coupled flow and reactive transport model was developed. The model results showed that only considering the groundwater flow underestimated the concentration of groundwater As. When adding the hydro-biogeochemical processes, the good agreement was achieved between the observed and simulated groundwater As. In the shallow and deep aquifers, microbially mediated reduction of poor-crystalline and crystalline Fe minerals contributed approximately 85 % of groundwater As. In the middle aquifer, abiotic reduction of Fe minerals driven is responsible for around 35 % of the release of sediment As, and around 17 % of released As was re-adsorbed or co-precipitated onto the neo-formed Fe(II) minerals. Under different redox conditions, the hydrogeochemical behavior of As was dominantly controlled by the Fe-S-As cycling in the groundwater system. [ABSTRACT FROM AUTHOR]

To understand the effects of large-scale sediment compaction on iodine enrichment in the groundwater system of the North China Plain (NCP), hydrogeochemical and isotope (2H, 18O and 87Sr/86Sr) geochemical features of groundwater, seawater and pore water of the deep aquifer sediments were characterized. Groundwater iodine concentration ranged from 5.8 to 1110 μg/L and approximately 80% of groundwater samples have iodine concentration more than 100 μg/L. High iodine groundwater (>100 μg/L) is mainly distributed in the Bohai bay area where serious land subsidence occurs. The sediments deposited during several events of marine transgression are considered as the main source of groundwater iodine. The pore solution compacted from the clayey sediments has a iodine concentration up to 830 μg/L, indicating that marine source iodine was stored in the clayey layers. The hydrogen and oxygen isotope signatures of groundwater show that the groundwater from NCP is mainly recharged by precipitation and the pore water from sediment compaction due to intense land subsidence. The Cl/Br molar ratio and 87Sr/86Sr signatures of groundwater and sediment pore water further suggest the effects of mixing with compaction-released pore solution on the hydrochemical evolution of high iodine groundwater. Based on groundwater δ2H, Cl concentration and 87Sr/86Sr, the results of two end-members mixing model calculation indicate that the compaction-released pore solution contributes approximately 49.1–68.9% iodine to high iodine groundwater, and the iodine from compaction-released pore solution ranges between 407 and 572 μg/L. The results of this study therefore suggest that the compaction of clayey sediments can be a major hydrogeological process controlling the genesis of high iodine groundwater in deep aquifers at NCP. • Elevated iodine was observed in deep aquifers and aquitards pore water. • Compaction of clayey sediment promotes the mobilization of marine-source iodine. • Groundwater over-exploited affects regional aquifer recharge and solutes transport. [ABSTRACT FROM AUTHOR]

To understand the characteristics of Ca isotope composition and fractionation in silicate-dominated Quaternary aquifer system, hydrochemical and isotope studies ( 87 Sr/ 86 Sr, 13 C DIC and 44/40 Ca) were conducted on groundwater, sediment and rock samples from the Datong basin, China. Along the groundwater flow path from the basin margin to the center, groundwater hydrochemical type evolves from Ca-HCO 3 to Na-HCO 3 /Na-Cl type, which results from aluminosilicate hydrolysis, vertical mixing, cation exchange between CaX 2 and NaX, and calcite/dolomite precipitation. These processes cause the decrease in groundwater Ca concentration and the associated modest fractionation of groundwater Ca isotopes along the flowpath. The groundwater δ 44/40 Ca value varies from −0.11 to 0.49‰. The elevated δ 44/40 Ca ratios in shallow groundwater are attributed to vertical mixing involving addition of irrigation water, which had the average δ 44/40 Ca ratio of 0.595‰. Chemical weathering of silicate minerals and carbonate generates depleted δ 44/40 Ca signatures in groundwater from Heng Mountain (east area) and Huanghua Uplift (west area), respectively. Along the groundwater flow path from Heng Mountain to central area of east area, cation exchange between CaX 2 and NaX on clay mineral results in the enrichment of heavier Ca isotope in groundwater. All groundwater samples are oversaturated with respect to calcite and dolomite. The groundwater environment rich in organic matter promotes the precipitation of carbonate minerals via the biodegradation of organic carbon, thereby further promoting the elevation of groundwater δ 44/40 Ca ratios. [ABSTRACT FROM AUTHOR]

Natural organic matter (NOM) plays an important role in the iodine mobilization in the groundwater system. In this study, the groundwater and sediments from iodine affected aquifers in the Datong Basin were collected to perform chemistry analysis and molecular characteristics of NOM by Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR-MS). Total iodine concentrations in groundwater and sediments ranged from 1.97 to 926.1 μg/L and 0.001–2.86 μg/g, respectively. A positive correlation was observed between DOC/NOM and groundwater/sediment iodine. FT-ICR-MS results showed that the DOM in the high-iodine groundwater system is characterized by less aliphatic and more aromatic compounds with higher NOSC, indicating the features of more unsaturated larger molecule structures and more bioavailability. Aromatic compounds could be the main carriers of sediment iodine and were easily absorbed on amorphous iron oxides to form the NOM-Fe-I complex. More aliphatic compounds, especially those containing N/S, experienced a higher degree of biodegradation, which further mediated the reductive dissolution of amorphous iron oxides and the transformation of iodine species, thereby causing the release of iodine into groundwater. The findings of this study provide some new insights into the mechanisms of high-iodine groundwater. [Display omitted] • NOM with aromatic structures preferentially adsorbed to the amorphous iron oxides. • Amorphous iron oxides-aromatic compounds complexes were the main carriers of iodine. • Biodegradation of N/S-containing aliphatic compounds results in iodine release. • Interaction between iron oxides and NOM affects iodine mobilization and species. [ABSTRACT FROM AUTHOR]

The geochemical behaviors of colloids in aquifers played an important role in determining the fate of iodine in groundwater system. To decipher the impact of colloids on iodine mobilization in aquifers, three successive pore-sized colloids filtration (0.45 μm, 30 kDa and 5 kDa) were conducted on in-situ during groundwater sampling. The results showed that the distribution ratios ( f ) of total iodine (I T ) and iron in the dissolved solution (i.e., 5 kDa ultrafiltered) were from 0.78 to 0.99 and from 0.56 to 0.94, respectively. Natural organic matter (NOM) in the colloidal fractions obtained the f values ranging from 0.14 to 1.00. The decreased Eh values from recharge area to discharge area indicated redox potential of groundwater system changed from oxidizing to reducing along groundwater flowpath, and interestingly, the corresponding iodine fractions in groundwater were decreasing in dissolved solution and increasing in colloidal fractions. Inverse correlation between Fe and DOC and positive correlation between iodine and DOC suggested the occurrence of reductive dissolution of iron oxyhydroxides and degradation of organic iodine in groundwater system. Iodine distribution increased in dissolved solution and decreased in colloids with pH increase. Moreover, as pH increased, f (Fe) and f (DOC) decreased in dissolved solution and increased in colloids. Relatively weak correlation of f (I T ) with f (Fe) and strong relationship between f (I T ) and f (DOC) in the large grain size fractions suggested the Fe-OM complexes controlled iodine distribution in groundwater, which depends on the presence of Fe bridges. Negligible association of iodine with Fe and NOM in the small grain size fractions might be attributed to the effects of abundant OH − content in groundwater. [ABSTRACT FROM AUTHOR]

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 5.59 mg/L and from 1.51 to 1106 μg/L, respectively, and approximately 63% and 32.3% of groundwater fluoride and iodine were higher than the guidelines for drinking water (1.5 mg/L and 150 μg/L). High fluoride concentration (> 1.5 mg/L) can be detected in groundwater from the flow-through and discharge areas of NCP, and high iodine groundwater (> 150 μg/L) is mainly scattered in the coastal area. Na-HCO 3 /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. [ABSTRACT FROM AUTHOR]

Characterizing the properties of main host of iodine in soil/sediment and the geochemical behaviors of iodine species are critical to understand the mechanisms of iodine mobilization in groundwater systems. Four surface soil and six subsurface sediment samples were collected from the iodine-affected area of Datong basin in northern China to conduct batch experiments and to evaluate the effects of NOM and/or organic-mineral complexes on iodide/iodate geochemical behaviors. The results showed that both iodine contents and k f -iodate values had positive correlations with solid TOC contents, implying the potential host of NOM for iodine in soil/sediment samples. The results of chemical removal of easily extracted NOM indicated that the NOM of surface soils is mainly composed of surface embedded organic matter, while sediment NOM mainly occurs in the form of organic-mineral complexes. After the removal of surface sorbed NOM, the decrease in k f -iodate value of treated surface soils indicates that surface sorbed NOM enhances iodate adsorption onto surface soil. By contrast, k f -iodate value increases in several H 2 O 2 -treated sediment samples, which was considered to result from exposed rod-like minerals rich in Fe/Al oxyhydroxide/oxides. After chemical removal of organic-mineral complexes, the lowest k f -iodate value for both treated surface soils and sediments suggests the dominant role of organic-mineral complexes on controlling the iodate geochemical behavior. In comparison with iodate, iodide exhibited lower affinities on all (un)treated soil/sediment samples. The understanding of different geochemical behaviors of iodine species helps to explain the occurrence of high iodine groundwater with iodate and iodide as the main species in shallow (oxidizing conditions) and deep (reducing conditions) groundwater. [ABSTRACT FROM AUTHOR]

High iodine concentrations in groundwater have caused serious health problems to the local residents in the Datong basin, northern China. To determine the impact of water-sediment interaction and irrigation practices on iodine mobilization in aquifers, isotope ( 2 H, 18 O and 87 Sr/ 86 Sr) and hydrogeochemical studies were conducted. The results show that groundwater iodine concentrations vary from 14.4 to 2180 μg/L, and high iodine groundwater (>150 μg/L) mainly occurs in the central area of the Datong basin. Sediment iodine content is between <0.01 and 1.81 mg/kg, and the co-occurrence of high iodine and high DOC/TOC concentrations of groundwater and sediment samples in the deeper aquifer indicates that the sediment enriched in iodine and organic matter acts as the main source of groundwater iodine. The 87 Sr/ 86 Sr values and groundwater chemistry suggest that aluminosilicate hydrolysis is the dominant process controlling hydrochemical evolution along groundwater flowpath, and the degradation of TOC/iodine-rich sediment mediated by microbes potentially triggers the iodine release from the sediment into groundwater in the discharge area. The vertical stratification of groundwater 18 O and 2 H isotope reflects the occurrence of a vertical mixing process driven by periodic surface irrigation. The vertical mixing could change the redox potential of shallow groundwater from sub-reducing to oxidizing condition, thereby affecting the iodine mobilization in shallow groundwater. It is postulated that the extra introduction of organic matter and O 2 /NO 3 /SO 4 could accelerate the microbial activity due to the supplement of high ranking electron acceptors and promote the iodine release from the sediment into shallow groundwater. [ABSTRACT FROM AUTHOR]

The South-to-North Water Diversion Project (SNWDP) has greatly improved the water shortage in the North China Plain. However, the impact of the SNWDP on the evolution of groundwater chemistry, especially fluoride and salinity, was still unknown. Cangzhou, one of the recipient cities of SNWDP and suffered from severe land subsidence, was selected to collect the deep confined groundwater samples before and after the SNWDP. The results showed that groundwater fluoride concentration decreased after the SNWDP, with a median value of 4.39 mg/L in 2017 and 3.00 mg/L in 2021, respectively. This trend was more clearly observed in the land subsidence area, which could be related to the reduction of land subsidence, due to the stopping pumping of deep groundwater. The pore water in clayey sediments contains fluoride up to 7.02 mg/L, which can be released into groundwater due to the sediment compaction before the SNWDP. The extensive exploitation during last decades changed the groundwater recharge patterns, resulting in over 60% deep groundwater being recharged by the pore water released from clayey sediment. The results of the Small Baseline Subset Interferometric Synthetic Aperture Radar (SBAS-InSAR) suggested that land subsidence has slowed down after the SNWDP, especially in the central area of land subsidence, thereby restricting the release of fluoride-rich pore water. However, groundwater salinity increased after the SNWDP, and the number of samples exceeding the drinking standard (TDS <1000 mg/L) increased by 26.4%. The rising groundwater level favors the water-rock interaction, promoting the dissolution of soluble minerals, for instance, halite, which was reflected by the results of inverse and forward modeling using the PHREEQC. Groundwater salinization after the SNWDP potentially induces the fluorite dissolution and causes the elevation of groundwater fluoride concentration. However, the calculated results indicate that the amount of F elevation related to groundwater salinization is significantly lower than the contribution of fluoride-rich pore water from clay sediment compaction. The findings of this study provide a comprehensive assessment of the influence of the SNWDP on the groundwater quality and some new insights for the management of groundwater resources. [Display omitted] • Groundwater F decreased and salinity increased in the Cangzhou after the SNWDP. • F concentration of pore water trapped in sediment was up to 7.02 mg/L. • The SNWDP restricts the release of pore-water preserved in the clayey sediment. • The elevation of groundwater F caused by salinization is less than caused by the release of F-rich pore water. [ABSTRACT FROM AUTHOR]

• High iodine groundwater occurs in discharge area of fluvial/lacustrine aquifer systems. • Local transformation of sediment Fe minerals explains the enrichment of pore-water iodine in the deep sediments. • Iodine-rich pore-water in the shallow sediments near the coastal area is related to the marine transgressions. The wide occurrence of high iodine groundwater is posing negative effects on human health. The land subsidence due to groundwater over-exploitation causes the release of iodine-rich pore-water trapped in clayey sediment into groundwater, however, the mechanisms of iodine enrichment in pore-water were still unclear. In this study, we selected the Datong Basin (DB) and North China Plain (NCP) with few and severe land subsidence, respectively, to understand the contributions of complex hydro-biogeochemical processes on the enrichment of iodine in groundwater and pore-water by using the inorganic/organic carbon isotope and batch experiments. The results showed that at the DB, the range of groundwater iodine was 4–2175 µg/L, and the enrichment of groundwater iodine is mainly related to the transformation of iron oxyhydroxides triggered by microbial degradation of organic matter, which was reflected by the more depleted δ13C DIC signatures of groundwater and gradual increasing trend of ratio of HCl-extractable Fe(II)/sediment Fe total and sediment δ13C DOC signatures with depth. In comparison, groundwater iodine at the NCP had the range of 2–1106 µg/L, and pore-water rich in iodine preserved in the clayey sediment serves as the main source. The depth profiles of sediment iron mineral fractions and δ13C DOC signatures suggested that the NCP sediments experienced lower degree of reductive transformation of iron minerals and microbial degradation of natural organic matter. At some specific depths higher than 300 m, the evident higher ratios of HCl-extractable Fe(II)/sediment Fe total indicate that the local transformation of iron minerals can explain the pore-water rich in iodine at deep sediments, which was supported by the results of microbial batch experiments. The pore-water with high iodine concentration preserved in the shallow sediment is more likely related to the marine transgressions. This study helps to improve our understanding of the mechanisms of high iodine groundwater, which may provide some new insights on further groundwater management. [ABSTRACT FROM AUTHOR]

Abstract: Characterizing the speciation of iodine in groundwater is essential for understanding its hydrogeochemical behavior in aquifer systems. To quantify the variations in iodine speciation and assess factors controlling the distribution and transformation of iodine, 82 groundwater samples and 1 rain water were collected from the Datong basin, northern China in this study. Factor analysis (FA) and excitation emission matrix with parallel factor analysis (EEM–PARAFAC) were used to clarify the potential relationships among iodine species and other hydrochemical parameters. The iodine concentrations of groundwater range from 6.23 to 1380μgL−1 with 47% of samples exceeding its drinking water level of 150μgL−1 as recommended by the Chinese government. 57% of samples have ratios of iodate to total iodine greater than 60%, while iodide as the major species in 22% of the samples. Significant amounts of organic iodine with concentrations higher than 100μgL−1 were observed in 9 groundwater samples. Redox conditions of groundwater system strongly affect iodine concentration and speciation of inorganic iodine in groundwater, and extremely reducing condition restricts the iodine release from sediments into groundwater. The results of FA show that iodine mobilization in groundwater is related to the nature of dissolved organic matter. EEM-PARAFAC model demonstrates the dominance of terrestrial DOM sources and the presence of microbial activities in groundwater system of the Datong basin. It is proposed that degradation of organic matter and reductive dissolution of iron oxyhydroxides are major hydrogeochemical processes responsible for the mobilization of iodine release and the genesis of organic iodine. [Copyright &y& Elsevier]

Abstract: To better understand the occurrence of high F and As in groundwater of the Datong basin, a total of 486 groundwater samples were collected for hierarchical cluster analysis (HCA) of eighteen hydrochemical parameters. Groundwater samples can be divided into thirty-six and nineteen groups for shallow and deep groundwater, respectively. Results show that high F samples in shallow groundwater contain F as high as 22mg/L and mainly occur in the discharge area in the basin center, and the highest F concentration of deep groundwater samples is 8.3mg/L which mainly occur in the western mountain front area. The groundwater with elevated HCO3 concentration favors F enrichment in the Datong basin. Nearly all of the high F samples are oversaturated with respect to calcite and undersaturated with respect to fluorite, indicating that fluorite solubility is a limit for F enrichment. Besides, evapotranspiration has a stronger effect on fluoride enrichment, especially for the shallow groundwater. For the both F and As enrichment samples in deep groundwater, the desorption of Fe-(hydr)oxides is suggested to be the major mechanism. High As samples of shallow and deep groundwater mainly occur between Senggan River and Huangshui River. The highest arsenic concentration reaches up to 469μg/L, and all samples of high arsenic groundwater have low concentrations of NO3 and SO4, indicating the prevailing reducing conditions in the aquifer system at Datong. The reductive dissolution of Fe-(hydr)oxides driven by sulfate reduction and biodegradation of organic matters is postulated to be the major process controlling arsenic enrichment in groundwater. [Copyright &y& Elsevier]

Geogenic iodine-contaminated groundwater represents a threat to public health in China. Identifying high-iodine areas is essential to guide the mitigation of this problem. Considering that traditional analytical techniques for iodine testing are generally time-consuming, laborious, and expensive, alternative methods are needed to supplement and enhance existing approaches. Therefore, we developed an artificial neural network (ANN) model and assessed its feasibility in terms of predicting high iodine levels in groundwater in China. A total of 22 indicators (including climate, topography, geology, and soil properties) and 3185 aggregated samples (measured groundwater iodine concentrations) were utilized to develop the ANN model. The results showed that the accuracy and area under the receiver operating characteristic curve of the model on the test dataset are 90.9% and 0.972, respectively, and climate and soil variables are the most effective predictors. Based on the prediction results, a high-resolution (1-km) nationwide prediction map of high-iodine groundwater was produced. The high-risk areas are mainly concentrated in the central provinces of Henan, Shaanxi, and Shanxi, the eastern provinces of Henan, Shandong, and Hebei, and the northeastern provinces of Liaoning, Jilin, and Heilongjiang. The total number of people estimated to potentially be at high-risk areas because they use untreated high-iodine groundwater as drinking water is approximately 30 million. Considering the growing demand for groundwater in China, this work can guide the prioritization of groundwater contamination mitigation efforts based on regional groundwater quality levels to enhance environmental management. [Display omitted] • An ANN was developed to predict areas with geogenic iodine-contaminated groundwater. • The model showed outstanding efficacy for predicting high-iodine groundwater areas. • A high-resolution nation-wide prediction map of high-iodine groundwater was constructed. • Climate and soil properties are essential factors contributing to iodine accumulation. [ABSTRACT FROM AUTHOR]

Groundwater fluoride is posing a health risk to humans, and analyzing groundwater quality is time-wasting and expensive. Statistical methods provide a valuable approach to study the spatial distribution of groundwater fluoride. Random Forest (RF), Artificial Neural Network (ANN), and Logistic Regression (LR) were used in this study for groundwater fluoride prediction in Datong Basin. The groundwater chemistry of 482 groundwater samples was collected and used to figure out the performance of three statistical technologies and extract the main factors controlling the enrichment of fluoride in groundwater. The data was separated into two parts for the statistical analysis, 80% for training and 20% for testing. The Chi-squared was applied to select the most relevant variables, and TDS, Cl−, NO 3 −, Na+, HCO 3 −, SO 4 2−, K+, Zn, Ca2+, and Mg2+ were selected as best inputs for the fluoride prediction. Models were evaluated using the confusion matrix and The receiver operating characteristic area under the curve ROC (AUC). The results suggest that within ten input variables, the accuracies of RF, ANN, and LR were 0.89, 0.85, and 0.76, respectively. The mean decrease in impurity (MDI) and permutation feature demonstrates that eight of ten parameters, including TDS, Cl−, NO 3 −, Na+, HCO 3 −, SO 4 2−, Ca2+ and Mg2+ are the variables influencing the groundwater fluoride in the study area. RF exhibited the best model with high conformity and confidence in predicting groundwater fluoride contamination in the study area. • Three algorithms were employed for the prediction of groundwater fluoride. • The variables influencing the fluoride in the study area were evaluated. • Python 3.7 was used for the analysis of the three models. [ABSTRACT FROM AUTHOR]

Long-term intake of groundwater with elevated iodine concentration can cause thyroid dysfunction in humans; however, little is known on the mechanisms controlling the fate of iodine in groundwater systems. In this study, the groundwater and aquifer sediment samples from the Datong basin, a geologic iodine-affected area, were collected to perform the batch incubation experiments to understand the release and enrichment of iodine in groundwater systems. The results showed that the groundwater from the deep confined aquifer had a total iodine concentration of 473 μg/L, higher than that of shallow groundwater, and iodide is the dominant species of iodine. The deep confined aquifer was characterized by the reducing conditions. Meanwhile, a higher ratio of Fe(II) to total Fe was observed in bulk deep aquifer sediments (59%) in comparison with that of shallow sediments (33%). The results of batch incubation experiments showed that during the reductive transformation of Fe minerals in shallow aquifer sediments, iodide concentration in solution was gradually increasing from 24.7 to 101.5 μg/L after 10 days. It suggests that the transformation of Fe minerals in aquifer sediments acts as a diver causing the release of iodine from sediment into groundwater, which was further supported by the features Fe K-edge EXAFS before and after the batch experiments. Moreover, the changes in iodine species from iodate or organic iodine into iodide during the release further promotes the release of sediment iodine, which was supported by the developed geochemical models. The prevalence of reducing condition in deep aquifer favors the enrichment of released iodide. This study provides new insights into the mechanisms of iodide enrichment observed in deep confined aquifer. Unlabelled Image • The enrichment of iodide was observed in deep confined aquifers. • Reducing conditions in deep aquifer lead to transformation of iron minerals. • Changes in iodine species promote iodide enrichment in groundwater. • Transformation of iron minerals leads to release of sediment iodine. [ABSTRACT FROM AUTHOR]

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-HCO 3 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. Unlabelled Image • High F groundwater is distributed in central plain and coastal area of the NCP. • Hydrochemical evolution along groundwater flow favors F mobilization in groundwater. • Pore water compacted from clayey sediments had a high F concentration. • Land subsidence causes the release of F trapped in strata into groundwater. [ABSTRACT FROM AUTHOR]

Local residents in the Datong Basin of northern China are exposed to groundwater with elevated iodine concentrations. Natural organic matter (NOM) has been linked to the heterogeneous distribution of elevated iodine in groundwater used for irrigation purposes, but little is known about the effects of hydrologic fluctuations and NOM characteristics on the transport and enrichment of iodine in the groundwater. Cl/Br molar ratios in Datong Basin groundwater range widely from 133 to 2099. A rapid increase in Cl/Br molar ratio with increasing Cl content indicates hydrologic fluctuations from the upper groundwater to the deeper aquifer due to large-scale irrigation activities in the Basin. A two end-member model of groundwater δ2H and δ18O values suggests the contribution of upper water recharging groundwater ranges from 20.7 to 49.5%. This vertical recharge process predominantly controls iodine enrichment and distribution in the groundwater. Additionally, the correlation between DOC concentration and δ18O signatures indicates considerable fresh organic matter is imported into the aquifer during the vertical recharge process. Iodine mobilization is likely promoted by young carbon transported to the deeper aquifer in the organo‑iodine form. Excitation-emission matrix (EEM) results indicate humic-like substances dominate NOM in the groundwater. Evidence from a PARAFAC model suggests organic matter in groundwater samples is associated with microbially-mediated degradation processes in an anaerobic environment. The drawdown migration of organic matter from the upper soil/sediments or surface could provide an extra energy source that promotes microbial activity. Buried sedimentary iodine coupled with anaerobic microbial respiration of subsurface organic carbon within the aquifer could lead to the release of iodine into the groundwater. These findings pave the way for a more comprehensive assessment of the susceptibility of drinking water aquifers, thereby supporting the management of groundwater resources. Unlabelled Image • Irrigation activities caused vertical recharge in aquifers. • Fresh organic matters were input into aquifers. • Iodine transportation enhanced by NOM. [ABSTRACT FROM AUTHOR]

Groundwater iodine has direct importance for human dietary iodine intake in areas where drinking water is of groundwater origin. Iodine affected aquifer system in Datong Basin of North China was studied with a focus on strontium isotopes and major ion chemistry, in order to estimate the controlling hydrological and geochemical processes. The data revealed a rather complicated mixing pattern of various groundwater source end-members that were subject to different water-rock interactions, such as silicate weathering, evaporite and carbonate dissolution. Groundwater from the recharge area of east margin was characteristic by 87Sr/86Sr ratios higher than 0.71643, which implied the silicate weathering process. Groundwater from the discharge area and west margin might be originated from evaporite and carbonate dissolution processes with low 87Sr/86Sr ratios. Mixing models of three end-members based on Sr isotope and contents reflected that the type of water-rock interactions shifted from silicate weathering towards evaporite dissolution along the groundwater flow path from east margin to basin center. Abundant iodine and natural organic matters (NOMs) were discovered in groundwater located at the discharge area with Sr isotopic values between 0.710 and 0.717, implying evaporite dissolution governing iodine and NOMs enrichment in groundwater. Silicate weathering process had negligible influence on iodine and NOMs enrichment in groundwater. While carbonate dissolution made negative contribution for iodine releasing to aqueous phases. The PHREEQC inverse modeling results illustrated that dissolution of halite, calcite, gypsum and kaolinite and cation exchange significantly changed chemical composition of groundwater along the groundwater paths from the two basin margins to central area. • Elevated iodine groundwater was found in Datong Basin. • Silicate weathering caused high 87Sr/86Sr values. • Evaporite and carbonate dissolution caused low 87Sr/86Sr values. • Evaporite dissolution governed iodine releasing to groundwater. • Iodine enrichment was related to NOMs by water-rock interaction. [ABSTRACT FROM AUTHOR]

Arsenic (As) contamination of groundwater in Bangladesh is a serious natural calamity and public health hazard. High As concentrations in groundwater (>10 μg l −1 ) generally occur in Holocene alluvial aquifers. In this study, hydrochemical characteristics and factors influencing As distribution and release were investigated in Prayagpur, Southwestern Bangladesh. Fifty groundwater samples from the tubewells to depths from 21 m to 81 m were collected for hydrochemical and isotopic analysis. The results showed that As concentration in groundwater varied from 6.05 to 590.7 μg l −1 (average 58.31 μg l −1 ), and about 82% samples contained As exceeding 10 μg l −1 . Elevated As concentration mostly occurred in shallow aquifers, which was associated with reducing conditions. High As groundwater was distinguished with weak alkaline pH value, moderate HCO 3 − and total dissolve solid (TDS), and low NO 3 − and SO 4 2- concentrations. The groundwater was dominated by Ca·Mg-HCO 3 type water that was different from other high As aquifers distributed in the arid area. Moderate alkalinity and the reductive condition, resulted from biochemical reactions were the key controlling factors of As mobilization in aquifers. Environmental isotopes (δ 18 O and δ 2 H) confirmed that evaporation did not play a vital role in the process of As enrichment in the groundwater. [ABSTRACT FROM AUTHOR]

Geogenic contaminated groundwater (GCG), characterized by elevated arsenic, fluoride, and iodine levels, present a significant challenge to public health and government management. Conventional survey-based approaches of collecting groundwater samples, conducting physicochemical tests, and performing spatial interpolation to obtain regional groundwater chemical component maps are inefficient and costly. More importantly, it does not take into account the actual hydrogeological conditions or the characteristics of pollutant transport and enrichment. To address this issue, we utilized Support Vector Machine (SVM), Random Forest (RF), Adaptive Boosting (AdaBoost), and Extreme Gradient Boosting (XGBoost) to analyze the likelihood of occurrence of arsenic, fluoride, and iodine as well as their spatial distribution in shallow groundwater from the Hetao Basin. Our study incorporated 20 indicators related to meteorology, soil physicochemical properties, and groundwater conditions, along with 1505 labeled samples consisting of groundwater arsenic, fluoride, and iodine concentrations and their corresponding coordinates. Subsequently, the study automatically analyzed the meteorological, soil physicochemical properties and groundwater conditions by constructing a machine learning model using the available data. In order to optimise and select the best prediction model, this paper presents a quantitative evaluation of the prediction performance of various machine learning models. The accuracy (AC), area under curve (AUC) and mean squared error (MSE) were calculated to predict the spatial distribution of CGC. Subsequently, the optimized model for predicting the spatial distribution of GCG was selected. The results showed that the XGBoost algorithm provided optimal predictions for groundwater with arsenic concentrations above 10 μg/L and fluoride concentrations exceeding 1.5 mg/L, whereas the RF model provided the best predictions for groundwater with arsenic concentrations surpassing 50 μg/L and iodine concentrations exceeding 100 μg/L. Subsequently, groundwater health risk zones were delineated based on an optimal prediction model, and demographic analysis was conducted in both the direct and potential groundwater risk zones. Model predictions indicated that hundreds of thousands of people in the Hetao Basin were facing a public health crisis caused by high concentrations of arsenic, fluoride and iodine in groundwater. These findings underscore the significant health challenge in the study area. Considering the agricultural development and increasing groundwater use in the area, our findings can guide local governments in managing the extent of groundwater development, establishing control zones, and enhancing protection measures for populations at risk from groundwater contamination. [Display omitted] • Machine learning methods are used to predict the spatial distribution of GCG. • Quantitatively assessing the predictive accuracy of machine learning models (SVM, RF, AdaBoost and XGBoost) • Statistics on the number of people affected by GCG based on the optimal forecasting model [ABSTRACT FROM AUTHOR]

Analyses of major elements, environmental isotope ratios (δ18O, δ2H), and PHREEQC inverse modeling investigations were conducted to understand the processes controlling the salinization of groundwater within the Datong Basin. The hydrochemical results showed that groundwater with high total dissolved solid (TDS) concentrations was dominated by sodium bicarbonate (Na-HCO3), sodium chlorite (Na-Cl), and sodium sulfate (Na-SO4) type waters, whereas low-TDS groundwater from near mountain areas was dominated by calcium bicarbonate (Ca-HCO3) and magnesium bicarbonate (Mg-HCO3) type waters. The characterization of the major components of groundwater and PHREEQC inverse modeling indicated that the aluminosilicate hydrolysis, cation exchange, and dissolution of evaporites (halite, mirabilite, and gypsum) governed the salinization of groundwater within the Datong Basin. The environmental isotope (δ18O, δ2H) and Cl−/Br− ratios revealed the impact of fast vertical recharge by irrigation returns and salt-flushing water on the groundwater salinization. According to the analyses of major hydrochemical components and PHREEQC inverse modeling, evaporite dissolution associated with irrigation and salt-flushing practice was probably the dominant controlling factor for the groundwater salinization, especially in the central part of the basin. Therefore, groundwater pumping for irrigation and salt-flushing should be controlled to protect groundwater quality in this area. [ABSTRACT FROM AUTHOR]

• The regional distribution of thioarsenate in fresh groundwater was studied. • pH, redox and sulfide content are the critical factors in the formation of thioarsenate. • Dithioarsenate (DTA) tends to be formed in a strong reduction environment with high sulfides. • The formation of monothioarsenate (MTA) may be related to elemental sulfur and can be transformed into DTA. Understanding the formation of thioarsenate, an important arsenic species, is of great significance with respect to explaining the mobilization of arsenic in fresh groundwater systems. Monothioarsenate (MTA), dithioarsenate (DTA), and trithioarsenate (TriTA) are three main forms of thioarsenate in fresh groundwater. A total of 30 groundwater samples were collected from the Datong basin to conduct hydrochemistry and arsenic speciation analysis. Geochemical simulations were established to verify the occurrence of thioarsenate. The results show thioarsenate was detected in 40% of the groundwater samples at concentrations ranging from 2.36 to 209.9 μg/L; the highest concentration accounted for 68% of total arsenic. Thioarsenate mainly occurs in the discharge area of the Datong basin, where the groundwater was characterized by strongly reducing conditions (Eh up to −175.3 mV) and higher concentrations of arsenic (up to 3450 μg/L) and sulfide (up to 8480 μg/L). A strong positive correlation (r = 0.81, p < 0.05) was observed between thioarsenate and sulfide. Strong reducibility and high sulfide concentration are beneficial for the formation of DTA vs. MTA. Geochemical modelling results confirm that suitable S/As mass ratio (<2.2) favors the formation of thioarsenate and that compared with MTA, DTA is easier to form under strong reduction environment and DTA is directly affected by the concentrate of sulfides, while MTA may be related to elemental sulfur. This understanding of the different formation conditions of thioarsenate species helps to explain the migration and enrichment of As in sulfide-rich groundwater. [ABSTRACT FROM AUTHOR]

Arsenic (As)-contaminated groundwater is a global concern with potential detrimental effects on the health of hundreds of millions of people worldwide. However, the extent of this problem may be more severe than anticipated, as many wells have not been tested and may contain unsafe-level of As. An optimized statistical regression model was developed to predict the probability of geogenic high As groundwater (As > 10 μg/L) in this study. Easily obtained hydrogeochemical and geological parameters that are significantly related to As geochemical behaviors were selected as explanatory variables in the model. The results indicate that pH, Cl − , HCO 3 − , SO 4 2− , and NO 3 − concentrations, stratigraphic information, and well depth are excellent predictors of As exposure in the Datong Basin, China. Predicted unsafe wells correspond well with the known distribution of high As groundwater in the Datong Basin. The successful application of a data set from Bangladesh also demonstrated the applicability and credibility of this proposed method. [ABSTRACT FROM AUTHOR]

Summary An in situ Fe-sulfide coating approach has been developed for As-contaminated groundwater remediation. Alternate injection of Fe(II), O 2 -free water and S 2 − can realize Fe-sulfide coating onto quartz sands with minor changes in porosity. As(III) uptake experiment indicated that the retardation factor for As(III) was 37 and dynamic retention capacity was 44.94 mg As(III)/g Fe, which was much higher than the maximum adsorption capacity for As(III) by FeS and FeS 2 . This result indicated that adsorption cannot be the only mechanism for As(III) uptake by Fe-sulfide coating layer. The SEM image and FTIR spectra results suggested that interaction between As(III) and Fe-sulfides and formation of As-sulfide precipitates could significantly contribute to As(III) uptake by Fe-sulfide coating layer. Alternate injection of Fe(II) + As(III) and S 2 − was conducted to simulate in situ As immobilization from real groundwater. The SEM image showed that the quartz sands were mainly covered by crystalline framboidal pyrite after such amendment. The breakthrough of As(III) was not observed during this experiment and the removal capacity for As(III) was 109.7 mg As/g Fe. The As(III) immobilization mechanism during alternate injection of Fe(II) + As(III) and S 2 − was significantly different from that of As(III) uptake by Fe-sulfide coating. The direct interaction between As(III) and S 2 − produced As-sulfides contributed to the high As(III) removal capacity during alternate injection of Fe(II) + As(III) and S 2 − . This result indicated that alternate injection of Fe(II) and S 2 − approach has an attractive application for As-contaminated groundwater remediation under strongly reducing environment. [ABSTRACT FROM AUTHOR]

An aquifer Fe-coating technology was evaluated for in-situ As remediation. The groundwater in the aimed aquifer has low dissolved Fe(II) concentration and high As(III) concentration, which has a low affinity toward Fe-oxides/hydroxides. To overcome these challenges, dissolved Fe(II) (5.0 mM) and NaClO (2.6 mM) were injected into the studied aquifer to promote the formation of Fe oxides/hydroxides and to oxidize As(III) into As(V), thus removing aqueous As via adsorption and/or co-precipitation. During field experiment, As concentration in groundwater from the pumping well significantly decreased. Fe and As speciation calculations indicate that incorporation of negatively charged As(V) into goethite was the probable mechanism for As removal. Both chemical sequential extraction results and spectroscopic data also support that alternating injection of Fe(II) and NaClO can achieve aquifer Fe coating and immobilize As via adsorption onto Fe oxides/hydroxides. Geochemical modelling further confirms that although competition for sorption sites between As and other dissolved species is expected in the natural groundwater system, high surface area of the Fe oxides/hydroxides can provide sufficient sites for As retention. The ability to effectively decrease As concentration of in-situ aquifer Fe-coating technology indicates that this approach should have extensive applicability to similar high As groundwater occurred worldwide. [ABSTRACT FROM AUTHOR]

In situ arsenic removal from groundwater by an aquifer iron coating method has great potential to be a cost effective and simple groundwater remediation technology, especially in rural and remote areas where groundwater is used as the main water source for drinking. The in situ arsenic removal technology was first optimized by simulating arsenic removal in various quartz sand columns under anoxic conditions. The effectiveness was then evaluated in an actual high-arsenic groundwater environment. The arsenic removal mechanism by the coated iron oxide/hydroxide was investigated under different conditions using scanning electron microscopy (SEM)/X-ray absorption spectroscopy, electron probe microanalysis, and Fourier transformation infrared spectroscopy. Aquifer iron coating method was developed via a 4-step alternating injection of oxidant, iron salt and oxygen-free water. A continuous injection of 5.0 mmol/L FeSO 4 and 2.5 mmol/L NaClO for 96 h can form a uniform goethite coating on the surface of quartz sand without causing clogging. At a flow rate of 7.2 mL/min of the injection reagents, arsenic (as Na 2 HAsO 4 ) and tracer fluorescein sodium to pass through the iron-coated quartz sand column were approximately at 126 and 7 column pore volumes, respectively. The retardation factor of arsenic was 23.0, and the adsorption capacity was 0.11 mol As per mol Fe. In situ arsenic removal from groundwater in an aquifer was achieved by simultaneous injections of As(V) and Fe(II) reagents. Arsenic fixation resulted from a process of adsorption/co-precipitation with fine goethite particles by way of bidentate binuclear complexes. Therefore, the study results indicate that the high arsenic removal efficiency of the in situ aquifer iron coating technology likely resulted from the expanded specific surface area of the small goethite particles, which enhanced arsenic sorption capability and/or from co-precipitation of arsenic on the surface of goethite particles. [ABSTRACT FROM AUTHOR]

Summary It is generally accepted that microbial processes play a key role in the mobilization and enrichment of arsenic (As) in groundwater. However, the detailed mechanism of the metabolic processes remain poorly understand. We apply isotopic measurements of iron (δ 56 Fe vs. IRMM-14), sulfur (δ 34 S SO4 vs. V-CDT) and carbon (δ 13 C DIC vs. V-PDB) to an experimental field plot in the Datong Basin, northern China. An array of monitoring wells was installed in a ≈1700-m 2 plot in which high concentrations of As, ranging from 4.76 to 469.5 μg/L, were detected in the groundwater. The measured range of δ 34 S SO4 values from 10.0‰ to 24.7‰ indicates the prevalence of microbial sulfate reduction within aquifers. The range of δ 56 Fe values measured in the groundwater suggests microbial Fe(III) reduction and the occurrence of isotopic exchange between Fe(II)aq and FeS precipitation. The low δ 13 C DIC values (up to −33.6‰) measured in groundwater are evidences for the microbial oxidation of organic matter, which is interpreted as the light carbon pool within the aquifer sediments. The high As (As > 50 μg/L) groundwater, which has higher δ 34 S SO4 and δ 56 Fe values and lower δ 13 C values, indicates the following: (1) microbial reduction of sulfate causes the mobilization of As through HS − abiotic reduction of Fe(III) minerals and/or formation of As-sulfur components; and (2) direct microbial reduction of Fe(III) oxides, hydroxides and oxyhydroxides cannot increase As concentrations to greater than 50 μg/L. Re-oxidation of Fe-sufide explains how sample C1-2 can have a high As concentration and low δ 34 S SO4 and high δ 56 Fe values. The results provide new insight into the mechanism of As enrichment in groundwater. [ABSTRACT FROM AUTHOR]

Highlights: [•] We present an iron isotope explication on the arsenic mobilization in the hyporheic zone. [•] The variation of δ56Fe values and the As concentration water are similar at each depth. [•] Microbial reduction of Fe(III) oxides/hydroxides is the dominant process that promotes As mobility in the hyporheic zones. [Copyright &y& Elsevier]

Hydrogeochemical investigations were carried out in the Datong (大同) Basin, northern China to understand the sources and mechanisms of fluoride enrichment in the groundwater. The results indicate that fluoride concentrations reached up to 7.2 mg/L, and the hydrochemical type changed from Ca-HCO 3, Ca-Mg-HCO 3 water in the recharge areas to Na-HCO 3, Na-Mg-HCO 3 in the flow-through area and ultimately to Na-Cl-HCO 3, Na-SO 4-Cl and Na-Cl in the discharge areas. The total fluorine contents in the soils ranged between 277.6 and 1 292.5 mg/kg, which can provide large amounts of fluorine from fluorine-bearing minerals, such as fluorite, biotite and amphibole. The concentration of water-soluble fluorine ranged from 3.9 to 49.9 mg/kg (mostly less than 1% of the total fluorine) in the soils and rocks that, may have released sufficient fluorine into the groundwater. The results of PHREEQC simulations indicate that most of the analyzed samples are under-saturated with respect to fluorite (CaF 2) from the recharge area to the discharge area. The fluctuation of F concentration in the 11 months indicates that the seasonal variation and the agricultural activity also affect the fluoride content in the groundwater. Fluoride enrichment in groundwater is predominantly controlled by fluorite solubility, residence time, evapotranspiration intensity and seasonal variation in this arid and semi-arid area. [ABSTRACT FROM AUTHOR]

Highlights: [•] Multiple (S, O and C) isotopic method is used to demonstrate the level of As in groundwater. [•] Wide ranges of δ34S and δ18O values and a positive correlation between δ34S and δ18O values can be found. [•] Wide ranges of δ13C values and minor variation of Fe concentration are observed. [ABSTRACT FROM AUTHOR]

Summary: Environment isotopes (δ18O and δ2H) and Cl/Br ratios in groundwater have been used to trace groundwater recharge and geochemical processes for arsenic contamination in Datong Basin. The arsenic concentrations of groundwater samples ranged from 0.4 to 434.9μg/L with the average of 51.2μg/L, which exceeded China’s drinking water standard (10μg/L). All the groundwater samples are plotted on or close to the meteoric water line of the δ18O vs. δ2H plot, indicating their meteoric origin. The relationship between δ18O values and Cl/Br ratios and Cl concentrations demonstrate that leaching and mixing are the dominant processes affecting the distribution of high arsenic groundwater in this area. The observed non-linearity in the trend between δ18O and arsenic concentration is due to combined effects of mixing and leaching. The similarity of the trend in Cl/Br ratios and δ18O values for high arsenic groundwater demonstrate that extensive leaching of irrigation return and salt flushing water flow could be the dominant process driving arsenic mobilization in the groundwater system. Moreover, the long term irrigation practice can cause the drastic change of the biogeochemical and redox condition of in the aquifer system, which in turn promotes the mobilization of arsenic. Therefore, groundwater pumping for irrigation in this area of waterborne endemic arsenic poisoning should be under strict control to protect groundwater quality in this area. [Copyright &y& Elsevier]

• The structures of microbial communities and DOM change in different redox zones. • Aerobic bacteria limit the reduction of Fe and As via depletion of O 2 and NO 3 −. • SRB/FeRB promote reduction of As, Fe and SO 4 2− by using protein-like DOM. • As-Fe-humic DOM complexation enhance the mobility of As and Fe in groundwater. • Microbial adaptative mechanisms on As stress are characterized by PLFAs. The phospholipid fatty acids (PLFA) and dissolved organic matter (DOM) 3D fluorescent signatures were appointed to elaborate arsenic (As) mobilization mechanism in groundwater from the Datong Basin. Groundwater samples were collected along the groundwater flow path from recharge zones to discharge zones according to redox sensitive parameters. Variations of Eh, NO 3 − and SO 4 2− manifested the redox condition shifted from a weakly oxidative environment to highly reductive environment. In recharge zones, some aerobic bacteria may serve as the dominant species, such as Thiothrix (S-oxidizing bacteria: SOB), Gallionella (Fe-oxidizing bacteria: FeOB) and Fluviicola , characterized by 16:1ω7, 16:0 and 18:1ω7 PLFAs. The biogeochemical processes were mainly governed by aerobic bacteria exploiting protein-like DOM as electron donors to maintain metabolism together with depleting of O 2 and NO 3 − (electron acceptors), which restricted the reduction of As. While in discharge zones, the anaerobic microbes played a predominant role, such as Desulfosporosinus (Sulfate reducing bacteria: SRB) and Clostridia (Fe reducing bacteria: FeRB), indicated by cy17:0, cy19:0 and 18:1ω9 PLFAs. SRB and FeRB mainly utilized protein-like DOM as energy sources for respiratory action. Simultaneously, biogenic reductive dissolution of Fe(III) (hydr)oxides and reduction of As(V), SO 4 2− resulted in geogenic immobilized As reductive desorption into aquifers. The positive linear correlations between humic substances and Fe, As suggested that As-Fe-DOM complexation can enhance the transport of As and Fe in aqueous environments. The negative relationships between SRB/FeRB and As/Fe concentrations demonstrated that the HS− could react with Fe(II) and As(III) to form secondary Fe(II) sulfides or As-bearing sulfides, which subsequently sequestered As from groundwater via sorption or coprecipitation. In conclusion, the utilization of biodegradable protein-like DOM by SRB/FeRB being associated with reductive dissolution of As-bearing Fe (hydr)oxides minerals and As-Fe-DOM complexation promoted As mobility in groundwater aquifers. [ABSTRACT FROM AUTHOR]

• Physical factor was incorporated into the study of Fe-S-As interactions. • Water flow eliminated block of surface sites caused by adsorbed arsenic. • The fast flow facilitated the formation of pyrrhotite. The sulfidation of Fe oxides/hydroxides has an important effect on the migration and transformation of As in groundwater. We examine the sulfidation of As (III)-bearing ferrihydrite and subsequent As repartitioning induced by sulfide in column experiments. Groundwater velocities has an obvious influence on sulfidation and the subsequent formation/transformation of secondary minerals. At a slower flow 1 pore volume·d-1), adsorbed As blocks the ferrihydrite surface, resulting in a slower sulfidation and thus slower generation of S0 and Fe2+. Mackinawite is subsequently generated by reaction of Fe2+ with excess sulfide and then reacts with S0 to form greigite. At a faster flow (5 pore volumes·d-1), stronger water elution results in desorption of much of the adsorbed As. Faster sulfidation produces considerable amounts of S0 and Fe2+ and restricts the formation of mackinawite. Fe2+ then adsorbs and catalyzes the conversion of residual ferrihydrite to hematite. The mackinawite also reacts with S0 to form pyrrhotite rather than greigite. Overall, heavy sulfidation does not effectively prevent the release of As caused by the reduction of ferrihydrite at either groundwater velocities. Sufficient As retention may be achieved by adjusting the hydrodynamic intensity of the system to determine the product of the sulfidation. [ABSTRACT FROM AUTHOR]

• The origin of the spring was investigated using hydrochemical and isotopic signals (18O, 2H and 3H). • The lake is a window into the groundwater table cut out of low-lying terrain in sand dunes. • Lowing of the regional groundwater level resulted in the lake' shrinking. • The paleo-channel has considerable effects on the genesis of the spring. • The artificial recharge accounts for 35% of lake resource, while 65% from natural origin. The Crescent Moon Spring is a unique scenic spot located in an aeolian sand environment of northwestern China. The water level of this spring has continuously declined in recent decades, causing both the government and many scholars great concern. To better understand its hydrologic structure, this study investigated the origin of the spring and its recharge sources based on the hydrochemical and isotopic (18O, 2H and 3H) composition of 56 water samples collected along the groundwater flow path. The results indicate the spring is a window into the groundwater table cut out of low-lying terrain in sand dunes, with the geographic and sedimentary conditions affecting its genesis and hydrochemistry. The spring and shallow sedimentary aquifers are recharged primarily by river water, while deep bedrock aquifers are recharged by deep lateral groundwater flow via fault zones in the piedmont of the Qilian Mountains. The results of recharge altitude calculations demonstrate a possible groundwater supply from deep bedrock aquifers to the south of spring. The recorded environmental 3H content varied from 11 to 40 T.U., indicating the groundwater is primarily recharged by glacier meltwater from modern precipitation, with only a small fraction from pre-modern times. End-member analysis using Cl- and δ18O shows recharge ratios of influent water from the Dang River and deep groundwater represent about 43 and 22% of the lake water, respectively, with artificial recharge accounting for the remaining 35%. Regional hydrogeological conditions, arid climate, and sand deposition have negative influences on the maintenance of lake water levels, but observed regional groundwater level declines resulting from the construction of the Dang River Reservoir, seepage from drainage canals, and excessive exploitation of groundwater are the most important factors contributing to the ecological crisis in the study area. These results provide insights into the origin and recharge sources of the Crescent Moon Spring that will guide efforts to preserve the water level. [ABSTRACT FROM AUTHOR]