Evaluation of groundwater for nitrate and fluoride in Alappuzha region from the southwestern coast of India and associated health risks

Nitrate and fluoride are two of the most prevalent pollutants in drinking water and exposure to their high concentrations could cause methemoglobinemia and fluorosis. This study attempted to evaluate the groundwater quality (pH: 4.4-9) from a relatively understudied part of the southwestern coast in India (i.e., Alappuzha, Kerala state) and assessed the associated health risks from exposures to nitrate (0.2-5.8 mg/l) and fluoride (0.2-1.9 mg/l) present in the groundwater. Pollution index (PIG: 0.35-5.43) grouped about 21% samples in high pollution and very high pollution categories because of fluoride content above the WHO guidelines. The total hazard index (THI) for adult male (0.17-1.70; average: 0.75), adult female (0.19-1.85; average: 0.81) and children (0.35-3.40; average: 1.50) suggested more non-carcinogenic risks for children from 41.6% samples compared to adult male and female from 33.3% samples in the absence of any mitigation measure. These results provide additional data from the country with highest population and the largest groundwater use in the context of sustainability in availability and supply of groundwater under the increasing risks of population growth, climate change and industrial development.

Relative extents, mechanisms, and kinetics of fluoride removal from synthetic groundwater by prevalent sorbents

Fluoride (F) contamination in groundwater affects millions of people across the world. Although several sorbents have been identified for low-cost F removal, the choice of the optimal sorbent is dictated by the specific chemistry of contaminated groundwater. In this contribution, eight prevalent sorbents-activated alumina (AA), calcite, hydroxyapatite-coated calcite (HCC), natural chitosan, chalk, Mg-Al-CO3 layered double hydroxide (LDH), calcined Mg-Al-CO3 LDH (cLDH), and hydrous ferric oxide (HFO)-were categorized on their relative F removal mechanisms, extents, and kinetics from a typical synthetic groundwater, representative of contaminated aquifers of North India. Initially, batch experiments containing sorbents at 4 g·L-1 were conducted at a high F concentration (2.9 mM). The dominant F removal processes were identified by aqueous- and solid-phase characterization of reaction by-products. While chalk and calcite removed F by secondary precipitation of fluorite, HCC removed F by fluorapatite precipitation, and other sorbents removed F by sorption. Depending on the immobilization mechanism identified, the F uptake kinetics on each sorbent was modeled with either pseudo-second order or generalized rate equations. Among sorptive F uptake, cLDH exhibited the highest (10-2.15 mg·g-1·h-1) and HFO showed the lowest (10-4.15 mg·g-1·h-1) rates. Fluoride removal by precipitation was the fastest with chalk at 10-1.3 (h-1) (0.16). Subsequent experiments with AA and HCC at lower initial F concentration (0.42 mM) suggested increased uptake by ∼30x and ∼7x, respectively, relative to uptake in 2.9 mM initial F systems. For AA, apart from the widely-accepted mechanism of adsorption, an unidentified F-containing surface precipitate was formed. HCC was identified as the most promising sorbent with no sludge generation.

Application of classification machine learning algorithms for characterizing nutrient transport in a clay plain agricultural watershed

Excess nutrients in surface water and groundwater can lead to water quality deterioration in available water resources. Thus, the classification of nutrient concentrations in water resources has gained significant attention during recent decades. Machine learning (ML) algorithms are considered an efficient tool to describe nutrient loss from agricultural land to surface water and groundwater. Previous studies have applied regression and classification ML algorithms to predict nutrient concentrations in surface water and/or groundwater, or to categorize an output variable using a limited number of input variables. However, there have been no studies that examined the application of different ML classification algorithms in agricultural settings to classify various output variables using a wide range of input variables. In this study, twenty-four ML classification algorithms were implemented on a dataset from three locations within the Upper Parkhill watershed, an agricultural watershed in southern Ontario, Canada. Nutrient concentrations in surface water were classified using geochemical and physical water parameters of surface water and groundwater (e.g., pH), climate and field conditions as the input variables. The performance of these algorithms was evaluated using four evaluation metrics (e.g., classification accuracy) to identify the optimal algorithm for classifying the output variables. Ensemble bagged trees was found to be the optimal ML algorithm for classifying nitrate concentration in surface water (accuracy of 90.9%), while the weighted KNN was the most appropriate algorithm for categorizing the total phosphorus concentration (accuracy of 87%). The ensemble subspace discriminant algorithm gave the highest overall classification accuracy for the concentration of soluble reactive phosphorus and total dissolved phosphorus in surface water with an accuracy of 79.2% and 77.9%, respectively. This study exemplifies that ML algorithms can be used to signify exceedance of recommended concentrations of nutrients in surface waters in agricultural watersheds. Results are useful for decision makers to develop nutrient management strategies.

Hydrochemical characteristics and the impact of human activities on groundwater in a semi-arid plain: a case study of western Jilin Province, Northeast China

Groundwater is important for human survival and development, particularly in arid and semi-arid regions. This study aimed to analyze the hydrochemical characteristics, influencing factors, and the impact of human activities on groundwater in the semi-arid plains of western Jilin Province, northwest China. The study collected 88 and 151 phreatic and confined water samples, respectively, which were analyzed for 13 water quality indicators using statistical and graphical methods. In order to investigate the impact of anthropogenic activities on water quality and health risks, the improved combined weighted water quality index (ICWQI) based on the entropy weight, criteria importance though inter-criteria correlation (CRITIC), the coefficient of difference method, subjective weight based on quality grading criteria, and the water quality index (WQI) were proposed to evaluate the water quality of the study area. Meanwhile, the human health risk assessment (HHRA) model was used to assess the risks of nitrate to the health of humans in different ages and sex categories. The results indicated that the groundwater in the study area was weakly alkaline and the main hydrochemical types in the phreatic and confined water were HCO3-·Ca-Mg and HCO3--Na. Rock weathering was the dominant process responsible for the generation of groundwater ions, the ions in groundwater primarily originate from the dissolution of halite, gypsum, and feldspar, while dolomitization promotes an increase in Mg2+. Human activities lead to an increase in NO3- in groundwater and have an impact on water quality and human health risks. The ICWQI method was found to yield more precise and rational assessments of water quality. Groundwater quality is primarily affected by nitrate ions. The areas in which groundwater nitrate posed a higher risk to human health were found to be mainly in the saline-alkali lands of Qian'an, Tongyu, and Zhenlai. Fertilizers, pesticides, and livestock farming activities contribute to the pollution of surface water. This surface contamination then infiltrates abandoned confined wells, leading to contamination of the confined aquifers. This study can improve the understanding of groundwater hydrochemical characteristics and the impact of human activities on groundwater in the study area. This study can also contribute to the study of groundwater in semi-arid regions.

Effects of dumpsite leachate plumes on surface and groundwater and the possible public health risks

Groundwater is a sacred natural resource and its security is not negotiable. However, the high level of environmental indiscipline and the shambled state of waste management in Nigeria poses a significant risk to environmental health. In this study, we employed subsurface resistivity and hydrochemical data along with diagnostic surveyed questionnaires (DSQ) and physical surveys, to critically assess the impacts of dumpsites on the environmental matrices (soil, surface and groundwater) and the possible public health risk associated with it. The resistivity data indicates that the overburdened soil layer overlaying the groundwater systems is heavily burdened, exhibiting consistently low values ranging from 4.1Ωm to 131Ωm. The low values were attributed to the presence of high content of heavy metals (HMs) due to leachate plume infiltration, which significantly impacted the groundwater and the upper layer of the Earth's crust, causing a land cave-in, consequently slope and structural failures. The hydrochemical data indicate a high content of TDS (400-1612) mg/l, BOD (371-611) mg/l, COD (697-1117) mg/l and HMs (0.012-1.787) mg/l, which indicates high organic potency deposited in the leachate contaminants. The electrical conductivity (EC) values vary between 0.211 and 1.502 mg/l as against WHO permissible limits for potable drinking water. The high values of EC reflect a high level of dissolved materials, affirming the conductive nature of the dumpsites revealed by the resistivity data. Natural waste decomposition and burning are prime sources of pollutants with adverse effects on the environmental matrices and public health. The leachate plumes seeping the ground could metamorphose into a power force annihilating the load-bearing capacity of the topsoil, resulting in slope and structural failures. This study will aid policymakers in making informed decisions and implementing targeted strategies to mitigate dumpsite's adverse effects.

Characterization of molecular and apical effects of legacy-contaminated groundwater on early life stages of fathead minnows

Mechanistic toxicology approaches represent a promising alternative to traditional live animal testing; however, the often-noted uncertainties concerning the linkages between effects observed at molecular and apical levels curtails the adoption of such approaches. The objective of this study was to apply a novel transcriptomics tool, EcoToxChips, to characterize the effects of complex mixtures of contaminants in fish and to compare molecular response patterns to higher-level biological responses including swimming behavior, deformities, and mortality. Fathead minnow (FHM) embryos were exposed for seven days to increasing concentrations of groundwater collected from moderate (MIAZ) and high (HIAZ) industrial activity zones of a legacy contaminated site. There was a concentration-dependent disruption of photo-dependent swimming responses associated with avoidance behavior patterns and spinal deformities (HIAZ and MIAZ), and an induction of pericardial edema and mortality (HIAZ-10%). Parallel EcoToxChip analyses showed a shift from a majority of upregulated genes at lower concentrations to a majority of downregulated genes at higher concentrations for both treatment conditions. Many of the significantly differentially regulated genes were involved in biological pathways including induction of oxidative stress, activating of several metabolic processes and growth, cell death, and inhibition of signal transduction signaling processes. Several contaminants present in the groundwater mixtures could have contributed to an exceedance of antioxidant system capacities that possibly led to the deformities, altered swimming behaviours, and mortality observed in FHMs. Therefore, molecular response patterns could be linked to apical outcomes observed in this study. Overall, the results observed in this study demonstrate that transcriptomics approaches such as the EcoToxChip system could be supportive of risk assessment of complex contaminated sites.

Isotope hydrology tools in the assessment of arsenic contamination in groundwater: An overview

Groundwater is important for the survival of humanity and the demand for the same is drastically increasing globally. The precious water resources are under constant threat, either as a result of natural processes or due to the influence of the anthropogenic activities. Arsenic contamination of groundwater is one of those threats that have affected approximately over 500 million people in 107 countries globally. Although, many studies (∼1000 Nos.) have been carried out on arsenic hydrogeobiochemistry, only a few have reported, on the use of different isotopes in understanding the arsenic hydrochemistry, and its release mechanism and mobilization. Determination of the isotopic composition of a groundwater sample and its dissolved compounds enable a better insight into the hydrological processes that control the distribution and migration of arsenic in the subsurface hydrological system. The environmental isotopes of water molecules (δ18O and δ2H) have been widely used to assess the groundwater origin, its recharge mechanisms, the rock-water interactions and quality. The stable isotopes of dissolved compounds of water (δ34S, δ15N, δ13C, δ56Fe etc.) give better information on the reaction processes within these elements and thus act as a tracer for contaminants, while the radioactive isotopes, such as 14C, 3H, 81Kr, 36Cl, 39Ar etc., can be used to assess the residence time of groundwater and its renewability. This article reviews the different uses of environmental isotopes as tools for providing critical information on various hydrological processes in the arsenic contaminated regions that can't be obtained through conventional tools for better management of the groundwater resources.

Study of the presence and environmental risk of organic contaminants policed by the European Union and other organic compounds in the water resources of a region overlapping protected areas: The Guadiaro River basin (southern Spain)

The study presented here is a first qualitative assessment of the occurrence of organic contaminants contemplated and not yet contemplated in European Union environmental legislation in water resources in the little anthropized Guadiaro River basin (70% of its area is covered by natural vegetation), in southern Spain. Water samples were collected from four carbonate aquifers, two detrital aquifers and four surface water courses and were analyzed for (i) 171 organic contaminants, (ii) major ions and (iii) stable isotopes (δ18OH2O, δ2HH2O, δ13CDIC). An environmental risk assessment was conducted through calculation of risk quotients comparing measured concentrations with ecotoxicological data found in the literature. Twenty-five organic contaminants were detected, at least once, including pesticides, pharmaceuticals, drugs of abuse and polycyclic aromatic hydrocarbons (PAHs). Cocaine and its main metabolite were detected in 85% and 95% of water samples, respectively (0.001-0.18 μg/L and 0.004-0.6 μg/L, respectively). Pyrene (PAH) was found in all water samples (0.001-0.015 μg/L) and forest fires were pointed out as a potential diffuse source. Relationship between rivers and aquifers is reflected by the distribution of organic contaminants, essentially the drugs of abuse. Concentration of contaminants were generally higher in groundwater samples, especially from detrital aquifers, potentially due to an accumulation process promoted by irrigation-return flows and by its slow dynamic compared to that of karstic systems. Pyrene concentration was also higher in some springs from karstic aquifers. Hence, calculated risk quotients were in general higher in groundwater, meaning that the threat to surface aquatic systems can grow as aquifers increase their influence on the water courses as the dry season progresses. The relationship between δ13CDIC and most organic contaminants (especially pyrene) reveal the role of the soil as storage media.

Isotope forensics of polycyclic aromatic compounds (PACs) in a contaminated shallow aquifer

Diesel was accidently released into the shallow subsurface at an industrial site in the province of Québec, Canada, in the late 1980s. Subsequent remediation efforts removed much of the contamination; however, traces of petroleum hydrocarbons continue to impact the local aquifer. In addition to the historical diesel spill, more recent yet unconfirmed accidental releases from ongoing on-site and neighbouring industrial activities may have potentially contributed to elevated levels of polycyclic aromatic compounds (PACs) in groundwater. To identify the main source(s) of contamination, compound-specific stable carbon isotope ratios (δ13C) of PACs in groundwater monitoring wells were compared to those in asphalt produced from a nearby plant and in fuel oil #6 oil being used by local industry. The δ13C values of five individual compounds (biphenyl, C2-naphthalene, C1-fluorene, dibenzothiophene and phenanthrene) and two groups of combined C1-phenanthrenes/anthracenes in all groundwater samples were within analytical uncertainty (±0.5‰). Moreover, the δ13CPAC values in groundwater samples were distinct from those in asphalt and fuel oil #6, indicating negligible contributions from these sources. The similarity in δ13CPAC values across monitoring wells, including one situated in the former source zone containing a floating hydrocarbon phase, pointed to a common source of subsurface contamination that was attributed to the historical diesel spill. These results thus demonstrate that δ13CPAC values can be used for source apportionment in shallow aquifers decades after the original spill event.

Minimizing the exposure risk from groundwater pollution by optimizing the temporal extraction patterns

Optimization models for minimizing pollutant exposure from groundwater resources require time and resources that many communities might not have ready access to due to their economic conditions. In such cases, it might be useful to develop a "rule of thumb" approach for suggestions in case of uncertainties and inadequate means to address these uncertainties. Monte Carlo analysis was performed for a simplified groundwater system, and the effects of temporal extraction patterns, distance to the pollution source, dispersivity, pollutant pulse period, pore water velocity, and decay were examined for minimizing the high pollutant exposure risk from the extracted groundwater. Results indicate that, in a high uncertainty scenario, the best bet for minimizing the risk of high pollutant exposure would be to adopt a frequent temporal extraction pattern and supply the water as a mixture of extractions from multiple wells spread over an area. These findings can be used as a "rule of thumb" wherever time and resources might be the limiting factors.

Douglas-fir plantations impact stream and groundwater chemistry in western Europe: Insights from three case studies in France and Luxembourg

In rural areas, nitrate concentrations in surface waters most often originate from the leaching of excess N fertilizer in agricultural lands, whereas forested catchments often have good water quality. However, Douglas-fir plantations may induce nitrogen cycle unbalances which may lead to an excess of nitrate production in the soil. We hypothesize that the excess of production of nitrate in the soil and nitrate leaching to streamwater is greater in catchments planted with Douglas fir. We used paired catchments in both France and Luxembourg with different land covers (Douglas-fir, Spruce, Deciduous, Grassland and clearcut) which were monitored over a 3-5 year period in order to assess the effect of Douglas-fir plantations on the chemical composition of surface water. Nitrate concentration in the soil and groundwater were also monitored. The results show that nitrate concentrations in streams draining Douglas-fir catchments were two to ten times higher than in streams draining other land covers, but were similar to the clearcut catchment. Nitrate concentrations under Douglas-fir in groundwater (up to 50 mg L-1) and in the soil were also higher than under all other land covers. Soil nitrate concentration was related to stream nitrate concentration. This suggests that soil processes, through excessive nitrate production under Douglas-fir, are driving the nitrate concentration in the stream water and our hypothesis of a transfer of a fairly large proportion of this excessive production from the soil to the stream is supported. This study also shows that nitrate concentrations in surface and ground waters in rural areas could also originate from Douglas fir forested catchments. The impact of Douglas-fir is nevertheless reduced downstream through a dilution effect: mixing tree species at the catchment scale could thus be a solution to mitigate the effect of Douglas-fir on nitrate concentration in surface waters.

Improving groundwater nitrate concentration prediction using local ensemble of machine learning models

Groundwater is one of the most important water resources around the world, which is increasingly exposed to contamination. As nitrate is a common pollutant of groundwater and has negative effects on human health, predicting its concentration is of particular importance. Ensemble machine learning (ML) algorithms have been widely employed for nitrate concentration prediction in groundwater. However, existing ensemble models often overlook spatial variation by combining ML models with conventional methods like averaging. The objective of this study is to enhance the spatial accuracy of groundwater nitrate concentration prediction by integrating the outputs of ML models using a local approach that accounts for spatial variation. Initially, three widely used ML models including random forest regression (RFR), k-nearest neighbor (KNN), and support vector regression (SVR) were employed to predict groundwater nitrate concentration of Qom aquifer in Iran. Subsequently, the output of these models were integrated using geographically weighted regression (GWR) as a local model. The findings demonstrated that the ensemble of ML models using GWR resulted in the highest performance (R2 = 0.75 and RMSE = 9.38 mg/l) compared to an ensemble model using averaging (R2 = 0.68 and RMSE = 10.56 mg/l), as well as individual models such as RFR (R2 = 0.70 and RMSE = 10.16 mg/l), SVR (R2 = 0.59 and RMSE = 11.95 mg/l), and KNN (R2 = 0.57 and RMSE = 12.19 mg/l). The resulting prediction map revealed that groundwater nitrate contamination is predominantly concentrated in urban areas located in the northwestern regions of the study area. The insights gained from this study have practical implications for managers, assisting them in preventing nitrate pollution in groundwater and formulating strategies to improve water quality.

Hydrochemical characterizations and groundwater quality assessment in the coastal region of the Jiaodong peninsula, North China

Groundwater resources constitute a primary water source in the coastal region of Jiaodong Peninsula (CRJP), serving as an essential foundation for socio-economic development and municipal water supply. This study sought to evaluate the hydrogeochemical properties of the CRJP's groundwater using 73 samples collected in 2017, comprehensively analyzing the chemical composition and environmental factors using mathematical statistics and hydrochemical techniques. The results demonstrate that groundwater in the CRJP possesses TDS values ranging from 262 to 28,160 mg/L, with a pH ranging between 6.4 and 8.5, characterizing a weakly alkaline water system. The cation order in groundwater is Na+ > Ca2+ > Mg2+ > K+, while the anionic sequence comprises Cl- > HCO3- > SO42- > NO3-. According to the Piper diagram, groundwater samples predominantly clustered into SO4·Cl-Ca·Mg and SO4·Cl-Na types. Additionally, significant spatial variations exist in the primary chemical components of groundwater. Hydrogeochemical characteristics within the region are influenced both by natural and human activities; natural elements include weathering of silicate rocks, gypsum and carbonate minerals dissolution, while human practices comprise industrial and mining activities, agricultural practices, and domestic waste discharge. The results from a health risk assessment show that non-carcinogenic risks posed by nitrate intake via drinking water are considerably high for infants in comparison to adults, teenagers, and children. Furthermore, certain regions within the CRJP show notable seawater intrusion effects on groundwater studied.

Contaminants of emerging concern in the urban aquifers of Barcelona: Do they hamper the use of groundwater?

Urban aquifers are an alternative to obtain freshwater, but they are frequently polluted by contaminants of emerging concern (CECs). Therefore, there is a need to ascertain whether CECs are a water management challenge as they might limit the use of groundwater as safe drinking water even at ng L-1 concentration levels. To answer this question, it is required to evaluate human health-risk effects of measured CECs in the groundwater and to understand their behaviour at a field-scale. This study compiles data about the presence of CECs in the aquifers of Barcelona and its metropolitan area, evaluates health risk effects of measured CECs in the groundwater and presents approaches implemented to identify and quantify the coupled hydro-thermo-chemical processes that govern their fate in the subsurface. Some CECs might be harmful to humans, such as 5-methyl-1H-benzotriazole and the pharmaceuticals azithromycin valsartan, valsartan acid, lamotrigine, gabapentin, venlafaxine and lidocaine, which show very high to intermediate health risk effects. The number of harmful CECs and the level of their hazard increase from the groups of adults and 14-18 years old teens to the groups of 4-8 years old and 1-2 years old children. Thus, some CECs can limit the use of groundwater in Barcelona as potential drinking water source. Finally, knowledge gaps in understanding the integration of these processes into urban water resources management plans are identified, which will help to define groundwater potential uses and to assure the adequate protection of the human health and the environment.

Hydrogeochemical evolution of groundwater impacted by acid mine drainage (AMD) from polymetallic mining areas (South China)

Mining activities have long-term impacts on the groundwater of surrounding areas and deserve in-depth analysis and study. Herein, the geochemical mechanisms of acid mine drainage (AMD)-affected groundwaters were examined, and groundwater quality was assessed through water quality indices. 15 water samples from 7 domestic and 4 groundwater monitoring wells were tested for physical and chemical parameters in 2022, and multivariate statistical analysis was carried out with monitoring data from 21 domestic wells in 2010. The groundwater chemical composition varied from a predominantly Ca-HCO3 type in 2010 to a Ca-SO4 type in 2022. The isotopic values of δ18O and δD indicate that groundwater has not been significantly affected by evaporation. Changes in groundwater sulfate and total dissolved solids (TDS) levels over the twelve-year period confirmed the AMD infiltration impact on groundwater quality. The groundwater chemical properties changed more slowly than those of surface waters affected by AMD based on a cumulative increase in sulfate concentration of 29.94 mg/L. Changes in groundwater quality were investigated, namely, the spatiotemporal distribution of potentially toxic elements (PTEs), including Fe, Mn, Cd, Pb, and As. Mn concentrations in upstream groundwater areas near the mine decreased by 61.8% between 2010 and 2022. Conversely, groundwater in midstream areas had Mn concentrations of 2.25 mg/L and arsenic concentrations of 11.8 μg/L, both exceeding the WHO, 2022 standard. According to multivariate statistical analysis, Mn, Cd, and Pb originated from polymetallic minerals, whereas As was likely derived from the reduction of Fe/Mn hydroxyl oxides. AMD remediation improved contaminated upstream groundwater quality over 12 years, with a 36.8% improvement in WQI values. PTE distribution determined water quality changes; therefore, PTE contamination should be treated in mid- and downstream regions while contaminated groundwater should be treated upstream.

Groundwater antibiotics contamination in an alluvial-pluvial fan, North China Plain: Occurrence, sources, and risk assessment

Antibiotics in groundwater have received widespread concern because high levels of them harm aquatic ecosystems and human health. This study aims to investigate the concentration, distribution, ecological and human health risks as well as potential sources of antibiotics in groundwater in the Hutuo River alluvial-pluvial fan, North China Plain. A total of 84 groundwater samples and nine surface water samples were collected, and 35 antibiotics were analyzed using ultra-performance liquid chromatography-tandem mass spectrometry. The results indicated that 12 antibiotics were detected in surface water with the total concentrations ranging from 5.33 ng/L to 64.73 ng/L. Macrolides were the primary category of antibiotics with a detection frequency of 77.8% (mean concentration: 9.14 ng/L). By contrast, in shallow granular aquifers (<150 m), 23 antibiotics were detected and the total concentrations of them ranged from below the method detection limit to 465.26 ng/L (detection frequency: 39.7%). Quinolones were the largest contributor of antibiotics with detection frequency and mean concentration of 32.1% and 12.66 ng/L, respectively. And ciprofloxacin and ofloxacin were the two preponderant individual antibiotics. The mean concentration of groundwater antibiotics in peri-urban areas was approximately 1.7-4.9 times that in other land use types. Livestock manure was the predominant source of antibiotics in groundwater. Erythromycin, sulfametoxydiazine, ofloxacin, and cinoxacin exhibited medium ecological risks to aquatic organisms. All antibiotics posed no risks to human health. The findings of this study provide valuable insights into the occurrence and management of antibiotic contamination in the groundwater in the Hutuo River alluvial-pluvial fan.

Microplastic contamination in groundwater of rural area, eastern part of Korea

Microplastics (MPs) are emerging contaminants in agricultural regions owing to the widespread use of plastics in farming activities. Groundwater has a vital role in farming activities, and it can be contaminated by MPs, fragmented from plastic products used in agricultural activities. Following an appropriate sampling protocol, this study investigated the distribution of MPs in shallow to deep aquifers (well depths: 3-120 m) and cave water of an agricultural region in Korea. Our investigation found that MPs contamination can penetrate up to the deep bedrock aquifer. The abundance of MPs was lower during the wet season (0.014-0.554 particles/L) than during the dry season (0.042-1.026 particles/L), which may be attributed to the dilution effect of precipitation in the groundwater. The abundance of MPs increased as the MPs size decreased at all sampling points, and size ranges were 20.3-869.6 and 20.3-673.0 μm in the dry and wet seasons, respectively. Our findings showed lower MPs abundance compared to previous studies, and we inferred that it might be due to differences in groundwater sampling volume, low agricultural intensity, and the non-application of sludge fertilizers. Our findings suggest that repeated and long-term investigations are needed to identify better the factors that influence the results of MPs distribution in groundwater, including sampling methods and hydrogeological and hydrological conditions.

Spatiotemporal variability and controlling factors of groundwater depletion in endorheic basins of Northwest China

Recent global groundwater overpumping is threatening ecosystem stability and food security, particularly in arid basins. A solid investigation regarding the drivers of groundwater depletion is vital for groundwater restoration, hitherto, yet it remains largely unquantified. Here, a framework to quantify the contribution of natural forcing (NF) and anthropogenic perturbations (AP) to groundwater storage anomalies (GWSA) variability by separating the GWSA estimated by the Gravity Recovery and Climate Experiment (GRACE) satellite into natural- and human-induced GWSA was proposed in the northwest endorheic basin (NWEB) of China. Further, a multiple linear regression model was established for GWSA change prediction. Our results showed that, during the period 2003-2020, the GWSA depleted at a rate of 0.25 cm yr-1 in the entire NWEB. In addition, GWSA was found to decrease significantly (exceeding 1 cm yr-1) in the west of NWEB where there are heavily irrigated areas, and has become one of the regions with the most serious groundwater depletion in China. Whereas a significantly increasing trend (greater than 0.5 cm yr-1) was observed in the Qaidam basin and south part of the Tarim River basin, becoming a groundwater enrichment reservoir in NWEB. The negative contribution of AP to groundwater depletion has increased from 3% to 95% in the last decade, as determined by separating the effects of NF and AP on GWSA. The rapid expansion of the cropland area and the increase in water use due to population growth are investigated to be the main reasons for GWSA depletion, particularly in the North Tianshan Rivers, Turpan-Hami, and Tarim River basins. Therefore, we conclude that AP are dominating and accelerating groundwater depletion in the NWEB. The increase of GWSA in the Qaidam basin has been attributed to the increase in solid water melt and regional precipitation. The western route project of China's south-north water diversion and water-saving irrigation are important ways to solve the problem of groundwater depletion in NWEB. Our results emphasize that a more feasible framework capable of reliably identifying the driving factors of groundwater storage change is a necessary tool for promoting the sustainable management of groundwater resources under both NF and AP in arid endorheic basins.

Interaction between the hydrochemical environment, dissolved organic matter, and microbial communities in groundwater: A case study of a vegetable cultivation area in Huaibei Plain, China

Intensive vegetable planting has a profound impact on the surrounding aquatic environment. The self-purification ability of groundwater is poor, and it is difficult to return groundwater to its original state once polluted. Therefore, it is necessary to clarify the impact of intensive vegetable planting on groundwater. This study selected the groundwater of a typical intensive vegetable planting base in the Huaibei Plain of China as the research object. This work analyzed the content of major ions, the dissolved organic matter (DOM) composition, and the bacterial community structure in groundwater. Redundancy analysis was used to explore the interactions between the major ions, the DOM composition, and the microbial community. The results showed that under the influence of intensive vegetable planting, the F- and NO3--N contents in groundwater were significantly increased; the excitation-emission matrix combined with parallel factor analysis identified four fluorescent components (C1 and C2 were humus-like components, while C3 and C4 were protein-like components), which mainly consisted of protein-like components. Proteobacteria was the dominant phylum (mean = 69.27 %), followed by Actinobacteriota (mean = 7.25 %) and Firmicutes (mean = 4.02 %), which together explained over 80 % of the total abundance; and TDS, pH, K+, and C3 were the main influencing factors affecting the microbial community structure. This study provides a better understanding of the impact of intensive vegetable cultivation on groundwater.

Supercritical water oxidation for the destruction of spent media wastes generated from PFAS treatment

Granular activated carbon (GAC) and anion exchange resin (AIX) have been successfully demonstrated to remove per- and polyfluoroalkyl substances (PFAS) from contaminated water and wastewater. These treatment technologies, when applied for PFAS removal, generate spent media loaded with a high mass of PFAS requiring further treatment and disposal. This project is the first study on the use of supercritical water oxidation (SCWO) to destroy both the spent media and the PFAS adsorbed onto it. One sample of spent GAC and one sample of spent AIX were collected from full-scale groundwater remediation systems treating PFAS. A second spent AIX sample was collected from a mobile PFAS treatment unit. The total PFAS concentrations reported in the GAC, AIX and second AIX feedstock slurries were 0.21 mg/kg, 1.3 mg/kg and 0.9 mg/kg, respectively. Each feedstock was processed separately in a one (1) wet metric ton per day tubular reactor SCWO system. The study demonstrated that SCWO is a very effective PFAS destruction technology for spent GAC and AIX, derived from water remediation systems treating PFAS. The spent media were completely mineralized to water, carbon dioxide (CO2) and a negligible amount of residual minerals. Total target PFAS compound concentrations in the SCWO system effluents after treating spent GAC, AIX and second AIX feedstocks were 548, 77 and 796 ng/L, respectively. The results indicated that the percentage elimination of perfluorocarboxylic acids (PFCAs) was better than that of perfluosulfonic acids (PFSAs) and long-chain PFAS elimination was better than short-chain PFAS.

Exploring the potential for groundwater-related ground deformation in Southern New South Wales, Australia

Unsustainable groundwater extraction can lead to aquifer compaction, damages to infrastructure, changes of water accumulation in rivers and lakes and to a decrease of the aquifer's ability to store water for future generations. While this phenomenon is well identified across the globe, the potential for groundwater-related ground deformation is still largely unknown for most of the heavily exploited aquifers of Australia. This study fills that science gap by exploring signs of this phenomenon across a large region comprising seven of Australia's most intensively exploited aquifers, in the New South Wales Riverina region. To detect ground deformation, we processed 396 Sentinel-1 swaths acquired during 2015-2020 using a multitemporal spaceborne radar interferometry (InSAR), leading to the production of a near-continuous ground deformation maps covering ~280,000 km2. To explore potential groundwater-induced deformation hotspots, four criteria are used in a multiple-line of evidence approach: (1) the amplitude, shape, and extent of the InSAR ground displacement anomaly, (2) the spatial correspondence with groundwater extraction hotspots. (3) The correlations between InSAR deformation time series and change in head levels in 975 wells. Four areas are identified as potentially prone to inelastic, groundwater-related deformations, with average deformation rates ranging from -10 to -30 mm/yr, high rates of groundwater extraction, and ample critical head drops. Comparison of ground deformation and groundwater level time series also suggests potential for elastic deformation in some of these aquifers. This study will help water managers mitigating the groundwater-related ground deformation risk.

Review of Endocrine Disrupting Compounds (EDCs) in China's water environments: Implications for environmental fate, transport and health risks

Endocrine Disrupting Compounds (EDCs) are ubiquitous in soil and water system and have become a great issue of environmental and public health concern since the 1990s. However, the occurrence and mechanism(s) of EDCs' migration and transformation at the watershed scale are poorly understood. A review of EDCs pollution in China's major watersheds (and comparison to other countries) has been carried out to better assess these issues and associated ecological risks, compiling a large amount of data. Comparing the distribution characteristics of EDCs in water environments around the world and analyzing various measures and systems for managing EDCs internationally, the significant insights of the review are: 1) There are significant spatial differences and concentration variations of EDCs in surface water and groundwater in China, yet all regions present non-negligible ecological risks. 2) The hyporheic zone, as a transitional zone of surface water and groundwater interaction, can effectively adsorb and degrade EDCs and prevent the migration of high concentrations of EDCs from surface water to groundwater. This suggests that more attention needs to be paid to the role played by critical zones in water environments, when considering the removal of EDCs in water environments. 3) In China, there is a lack of comprehensive and effective regulations to limit and reduce EDCs generated during human activities and their discharge into the water environment. 4) To prevent the deterioration of surface water and groundwater quality, the monitoring and management of EDCs in water environments should be strengthened in China. This review provides a thorough survey of scientifically valid data and recommendations for the development of policies for the management of EDCs in China's water environment.

Contribution of groundwater to dietary requirements of essential metals in Lagos and Ogun States, Nigeria

Knowledge of dietary intakes of essential elements in groundwater is important for proper assessment of the actual contribution of drinking water to daily nutrient requirements. This study assessed the daily intake of nutritional elements in drinking groundwater of Lagos and Ogun States of Nigeria. One hundred and seventy (170) water samples were collected and analyzed for thirteen (13) nutritional elements (K, Mg, Ca, Co, Fe, Cu, Zn, Mo, Se, Na, Mn, Cr, and I) by Inductively Coupled Plasma-Mass Spectrometry (ICP/MS). Chronic daily intake CDI of the nutritional elements was higher for children and infants than adults in all cases. Percentages of tolerable daily intake reference value (TIRV) obtained for Lagos State groundwater were Mg 0.425%, Ca 0.309%, Fe 14.0%, Cu 1.84%, Zn 6.25%, Mo 0.057%, and Mn 1.08%; for Ogun State groundwater, % TIRV were Mg 1.99%, Ca 0.586%, Fe 81.1%, Cu 2.12%, Zn 9.21%, Mo 1.16%, and Mn 0.023%. The order of dermal absorption by an adult in Lagos when the water is used for bathing daily was Cr > Mn > Fe > Zn > Cu while the order in Ogun was Fe > Mn > Zn > Cu > Cr. The research concluded that groundwater alone cannot provide enough essential elements for human dietary needs in the study area.

Construction of arsenic selective chelating resin with iron precursor for removal of low-concentration arsenic: Breakthrough modeling and field deployment

The presence of exorbitant arsenic contamination in the aquatic environment causes astronomically immense health quandaries affecting millions of people, which may lead to death in the case of prolonged indigestion of arsenic-containing drinking water. Herein, we are reporting porous chelating resin with an iron precursor for the removal of arsenic ions from water. Weak acid cation resin was functionalized under varying experimental conditions to get a suitable resin with high arsenic uptake. The theoretical results revealed that the maximum Langmuir adsorption capacities of 3.27 mg g-1 and 1.13 mg g-1 were achieved for As(V) and As(III), respectively. The kinetics of adsorption followed the pseudo-second-order (PSO) model with a high determination coefficient (R2) of 0.9963 and 0.9895 for As(V) and As(III), respectively. The Adams-Bohart, Thomas, Yoon-Nelson, and Pore diffusion models were used to identify the breakthrough curve in the fixed bed adsorption column. The column performance improved with a larger bed height (55 cm), low concentration of influent (0.25 mg L-1), and low flow rate of influent (80 mL min-1). Under this condition, the breakthrough time and exhaustion time were 314 min and 408 min for As(V) and 124 min and 185 min for As(III), respectively.

Landscape of antibiotic resistance genes and bacterial communities in groundwater on the Tibetan Plateau, and distinguishing their difference with low-altitude counterparts

Groundwater is a vital source of drinking water for Tibetans. Antibiotic resistance genes (ARGs) and bacterial communities in groundwater on the Tibetan Plateau remain unclear. Furthermore, the characterization of their differences between high-altitude and low-altitude groundwater is still unrevealed. Herein, 32 groundwater samples were collected on the plateau, and intra- and extracellular ARGs (iARGs and eARGs), and bacterial communities were characterised through qPCR assays to 19 ARGs and 16S rRNA sequencing. It showed top four abundant intra- and extracellular last-resort ARGs (LARGs) were blaOXA-48, mcr-1, vanA, and vanB, whereas dominant common ARGs (CARGs) were tetA and ermB, respectively. CARGs had higher abundances than LARGs, and iARGs were more frequently detected than eARGs. Proteobacteria, an invasive resident phylum, and Firmicutes dominated eDNA release. Network analysis revealed all observed LARGs co-occurred with pathogenic and non-pathogenic bacteria. Community diversity was significantly associated with longitude and elevation, while nitrate correlated with ARGs. Comparative analysis demonstrated eARG frequencies and abundances were higher at high altitudes than at low altitudes. Additionally, Acinetobacter and Pseudomonas specifically dominated at high altitudes. This study reveals the widespread prevalence of ARGs, particularly LARGs, in groundwater on the less-disturbed Tibetan Plateau and underlines the potential risks associated with the LARG-carrying bacteria. ENVIRONMENTAL IMPLICATION: Antibiotic resistance genes (ARGs), which are defined as emerging environmental contaminants, are becoming a global concern due to their ability to confer antibiotic resistance to pathogens. Our findings highlight the prevalence of ARGs, particularly LARGs, in groundwater on the Tibetan Plateau, and the possibility that naturally-occurring pathogenic and non-pathogenic bacteria carry multiple LARGs. In addition, we further reveal differences in the distribution of ARGs and bacterial community between high-altitude and low-altitude groundwater. Collectively, our findings offer an important insight into the potential public risks related to groundwater on the Tibetan Plateau.

Evaluation of contaminants of emerging concern attenuation through a vegetation filter managed using different operating conditions

In wastewater treatment using Vegetation Filters (VFs), natural processes reduce contaminants present in water although some of them can reach the environment. In this study, 39 contaminants of emerging concern (CECs) are evaluated in a pilot VF under different operating conditions during almost four years. The use of woodchip amendments and the change from surface irrigation through furrows to drip irrigation (and from weekly to daily water application) provide CEC concentration reductions in the water infiltrating through the vadose zone. Biodegradation is the main process taking place and has been favoured mainly by woodchip soil amendments and the increased residence. Median attenuation percentages of the CECs most frequently detected with highest concentrations in applied wastewater vary between 52% and 100% at the end of the study (at 45 cm depth). Among targeted CECs, caffeine, and its transformation product paraxanthine are the most attenuated. Flecainide and venlafaxine show a persistent behaviour. However, their leaching concentrations are very low (< 31 ng/L). Concerning the underlying aquifer, the groundwater quality in terms of CEC concentrations is conditioned by the surrounding area rather than the operation of the VF. Levels in groundwater are always below those in wastewater and infiltrating water.

Novel insights into the mechanisms for Sb mobilization in groundwater in a mining area: A colloid field study

Colloids may play an important role in the geochemical cycle of antimony (Sb). However, the controlling behaviors of colloids on Sb fate in contaminated groundwater are not available. To investigate the effects of colloids on Sb mobility, groundwater samples from Xikuangshan Sb Mine's two main aquifers (the D3s2 aquifer and the D3x4 aquifer) were successively (ultra)filtered through progressively decreasing pore sizes (0.45 µm, 100 kDa, 50 kDa and 5 kDa). The results showed that 0.1-84.1% of Sb was adsorbed or carried by colloids, which corresponded to Sb concentration ranging between 0 and 2973 μg/L in the colloids (0.45 µm - 5 kDa). In both aquifers, Sb was closely associated with organic colloids (r = 0.72 p < 0.05 for the D3x4 aquifer, r = 0.94 p < 0.01 for the D3s2 aquifer). Parallel factor analysis of the three-dimensional fluorescence spectra determined that the protein-like substances in the D3x4 aquifer and the humus-like substances in the D3s2 aquifer controlled Sb behavior. X-ray absorption spectroscopy confirmed Sb complexing with organic substances. Competitive adsorption of As and Sb suppressed the complexation of colloids with Sb, particularly in the D3x4 aquifer (r = -0.71, p < 0.05). Sb mobility was also influenced by the redox of the groundwater system. As the oxidation-reduction potential and dissolved oxygen increased, Sb in the colloidal fractions decreased. These findings provide new insights into the mechanisms involved in Sb fate affected by colloids, establishing the theoretical basis for developing effective Sb and even metalloid pollution remediation strategies.

Voltammetric determination of inorganic arsenic in mildly acidified (pH 4.7) groundwaters from Mexico and India

Routine monitoring of inorganic arsenic in groundwater using sensitive, reliable, easy-to-use and affordable analytical methods is integral to identifying sources, and delivering appropriate remediation solutions, to the widespread global issue of arsenic pollution. Voltammetry has many advantages over other analytical techniques, but the low electroactivity of arsenic(V) requires the use of either reducing agents or relatively strong acidic conditions, which both complicate the analytical procedures, and require more complex material handling by skilled operators. Here, we present the voltammetric determination of total inorganic arsenic in conditions of near-neutral pH using a new commercially available 25 μm diameter gold microwire (called the Gold Wirebond), which is described here for the first time. The method is based on the addition of low concentrations of permanganate (10 μM MnO4-) which fulfils two roles: (1) to ensure that all inorganic arsenic is present as arsenate by chemically oxidising arsenite to arsenate and, (2) to provide a source of manganese allowing the sensitive detection of arsenate by anodic stripping voltammetry at a gold electrode. Tests were carried out in synthetic solutions of various pH (ranging from 4.7 to 9) in presence/absence of chloride. The best response was obtained in 0.25 M chloride-containing acetate buffer resulting in analytical parameters (limit of detection of 0.28 μg L-1 for 10 s deposition time, linear range up to 20 μg L-1 and a sensitivity of 63.5 nA ppb-1. s-1) better than those obtained in acidic conditions. We used this new method to measure arsenic concentrations in contrasting groundwaters: the reducing, arsenite-rich groundwaters of India (West Bengal and Bihar regions) and the oxidising, arsenate-rich groundwaters of Mexico (Guanajuato region). Very good agreement was obtained in all groundwaters with arsenic concentrations measured by inductively coupled plasma-mass spectrometry (slope = +1.029, R2 = 0.99). The voltammetric method is sensitive, faster than other voltammetric techniques for detection of arsenic (typically 10 min per sample including triplicate measurements and 2 standard additions), easier to implement than previous methods (no acidic conditions, no chemical reduction required, reproducible sensor, can be used by non-voltammetric experts) and could enable cheaper groundwater surveying campaigns with in-the-field analysis for quick data reporting, even in remote communities.

Human health risk assessment based on direct and indirect exposure to endocrine disrupting herbicides in drinking, ground, and surface water in Croatia

The recognition of certain herbicides as endocrine disrupting compounds has raised concerns due to their ability to interfere with the normal functioning of the endocrine system, which regulates various physiological processes in organisms. The objective of this study was to assess the possible human health risks associated with terbuthylazine and endocrine-disrupting herbicides atrazine, acetochlor, and metolachlor in the drinking, surface, and groundwater of the Zagreb city region, Croatia. We relied on advanced statistical methods and principal component analysis (PCA), which revealed higher levels of atrazine and acetochlor in drinking and groundwater samples and higher presence of metolachlor and terbuthylazine in surface waters. To evaluate the danger to human health, various exposure scenarios have been assessed. The risk of direct human exposure to analyzed herbicides through drinking or bathing with drinking (tap) or groundwater, as well as from recreational activities like swimming in rivers, streams, and lakes, has been quantified. In addition to these direct exposure scenarios, indirect ones based on consumer goods, fruits, and vegetables, treated with surface and groundwater for irrigation, were assessed to investigate the danger to human health. Judging by the reported herbicide levels there was no significant risk of carcinogenic (CR ≤ 1 × 10-6) or non-carcinogenic (HI < 1) diseases, not even when we assessed the so-called "cocktail effect" of combined the herbicide exposure in different waters.

A systematic study of the competitive sorption of per- and polyfluoroalkyl substances (PFAS) on colloidal activated carbon

Treatment of environmental media contaminated with per- and polyfluoroalkyl substances (PFAS) is crucial to mitigate mounting health risks associated with exposure. Colloidal activated carbon (CAC) has shown promise in treating contaminated soils, but understanding the interaction among PFAS during sorption is necessary for optimal remediation. This study investigated the extent to which PFAS of varying chain lengths and functional groups compete for sorption to CAC. Batch tests were conducted with natural soil and spiked water, using CAC at 0.2% w/w to remove seven PFAS with individual starting concentrations up to 0.05 mmol L-1. PFAS sorption to CAC was evaluated in three systems: a composite mixture of all studied compounds, a binary-solute system, and a single-solute system. The sorption experiments exhibited strong PFAS affinity to CAC, with removal rates between 41% and 100%, and solid/liquid partition coefficients (Kd) between 10 and 104 L kg-1. Differences were noticed among the various spiking mixtures, based on perfluorocarbon chain length, functional group, and the starting PFAS concentrations. Competition effects were detected when PFAS were in a multi-solute system, with an average 10% drop in removal, which can evidently become more relevant at higher concentrations, due to the observed non-linearity of the sorption process. The PFAS most vulnerable to competition effects in multi-solute systems were the short-chain perfluoropentanoic acid (PFPeA) and perfluorobutane sulfonic acid (PFBS), with an up to 25% reduction in removal. In bi-solute systems, perfluorooctane sulfonamide (FOSA) dominated over its ionisable counterparts, i.e. perfluorooctane sulfonic acid (PFOS) and perfluorononanoic acid (PFNA), indicating the importance of hydrophobic effects or layer formation in the sorption process. These results underscore the importance of considering competition in PFAS sorption processes when designing and implementing remediation techniques for PFAS-contaminated media.

The first evidence of microplastic occurrence in mine water: The largest black coal mining area in the Czech Republic

Climate change is creating new challenges for water supply worldwide, making the search for new sources of water vital. As mine water could serve as a potential source, this study investigated the presence of microplastics in water from terminated deep mines in the largest coal basin in the Czech Republic, as well as in water from nearby shallow wells. The particles found were analyzed for size, polymer composition, color and morphology using the ImageJ tool, infrared spectroscopy with Fourier transform (FTIR) and an optical stereomicroscope with a digital camera. Microplastics were detected in all tested sites. Their range accounted for 2.5-17.5 items/L for mine water samples and 2.5-20 items/L for well samples, with fibers being the dominant type. The average width of particles from mine water and wells amounted to 58 µm; 71 µm, length to 655 µm; 501 µm and area to 22,067 µm2; 28,613 µm2, respectively. Blue color was prevalent, among materials, in both cases, plastic coated paper was found dominant to Polyethylene terephthalate (PET), Polyester (PES), Tetrafluoroethylene-perfluoro (Propyl Vinyl Ether) - Copolymer (TFE-PPVE), and polypropylene (PP). The research provides the first evidence of microplastics' presence in underground waters from deep mines and shallow wells in the same area. The data suggest that it is almost impossible to find underground water sources free of microplastic contamination. In this context, atmospheric contamination from mine ventilation and infiltration through terminated mines were identified as potential sources, while infiltration through soil and rock formations is unlikely given the geological composition. The results of this study can serve as a relevant basis for further research on microplastics in mine waters. Additionally, the conclusions can advance the development in remediation technologies of microplastics from deep underground waters and their implementation in practice, particularly in light of upcoming legislation.

Assessment of groundwater vulnerability for seawater intrusion using DRASTIC model in coastal area at Patuakhali District, Bangladesh

Seawater intrusion is a growing concern for coastal regions in Bangladesh. Overexploitation of groundwater, low rainfall, a high rate of evaporation, rising sea level, cyclones, storm surges, and shrimp cultivation are the causes of seawater intrusion that polluted the groundwater near the coastal area. High values of EC, TDS, Na+, and Cl- ions in groundwater indicate the seawater intrusion. Piper's Trilinear and Durov diagrams of groundwater indicated that most gathered groundwater show predominantly Na-Cl-type of water. Based on the USSL diagram, it is observed that the salinity of the study area is medium to high and the alkalinity is medium to very high. Also, in view of the Wilcox outline, most of the samples in the area of interest are permissible to doubtful, and doubtful to unsuitable. Chloride is moderately correlated with Na+ (R2 = 0.590) and poorly correlated with (HCO3-  + CO32-) (R2 = 0.128), which indicates seawater intrusion. According to a comparison of the levels of Fe and Mn in the groundwater of the study area with those recommended by the World Health Organization (WHO) and Department of the Environment (DOE), twenty-two out of thirty-one samples exceed the acceptable limit for Fe and thirty out of thirty-one samples fall within the acceptable limit for Mn. The calculated water quality index reveals that most samples are of low drinking water quality. The research area's poor water type is found in the north-central and southern parts. The WQI geographic variation map reveals that the study's water has a WQI of more than 100, making it unsuitable for consumption. The final seawater intrusion vulnerability map clearly identifies the moderate to highly vulnerable zone of the study area using DRASTIC index parameters. DRASTIC index parameters revealed that some portions of the northwestern part of the sample area show a moderately vulnerable zone. And the rest of the sampling area indicates the highly vulnerable zone.

Linking groundwater microbiome and functional ecological clusters to geogenic high hexavalent chromium from deep aquifers in a loess plateau

Geogenic high hexavalent chromium [Cr(Ⅵ)] in groundwater is a global environmental problem. However, the groundwater microbiome and its linkage to geogenic high Cr(Ⅵ) from deep aquifers still need to be elucidated. Here, we evaluated geogenic Cr(Ⅵ), groundwater microbiome with featured functional ecological clusters and their interactive responses in groundwater from a deep aquifer in a loess plateau of Northern Shaanxi, China. We found that the compositions and structures of microbial communities in groundwater from the deep aquifer were significantly different between low Cr(Ⅵ) (LCG, < 50 μg/L) and high Cr(Ⅵ) groundwater (HCG, > 50 μg/L), with higher microbial diversity and richness in HCG (p < 0.05). Functional "specialists" related to Cr biotransformation, including Cr(Ⅵ) reducing bacteria (CRB) Rhodococcus, Nocardioides, Novosphingobium, and Acidovorax and Mn-oxidizing bacteria (MnOB) Sphingobium, and Ralstonia were positively correlated to total Cr and Cr(VI) concentrations in groundwater. Moreover, these CRB and MnOB were dominant in high Cr(VI) groundwater and associated by strong interspecific relation in an ecological cluster (p < 0.05), suggesting their indicator roles for high Cr(Ⅵ) and the contribution of MnOB mediated Cr(III) oxidation to Cr(VI) enrichment. RDA and path analysis further revealed that the geogenic Cr(Ⅵ) directly promoted the key Cr-related functional cluster with the groundwater depth, dissolved oxygen, and total dissolved solids as the cofactors indirectly influencing Cr(Ⅵ) and the functional clusters (p < 0.05). Collectively, our results highlight the significant roles of microbial ecological clusters especially functional "specialists" MnOB and CRB in groundwater Cr(Ⅵ) from deep aquifers in the loess plateau and provide a basis for sustainable management of high Cr(Ⅵ) groundwater.

Novel composites of activated carbon and layered double oxide for the removal of sulfate from synthetic and brackish groundwater

Sulfate (SO42-) is a major water and environmental concern that causes severe diarrhea, death of invertebrates and plant species, and clogging of industrial pipes. In the current work, treatment of SO42- from synthetic and real groundwater having 3901 mg(SO42-)/L was investigated for the first time using Zn-Al and Mg-Al layered double oxides doped granular activated carbon (GAC/Mg-Al LDO and GAC/Zn-Al LDO). The co-precipitation method was followed to synthesize the GAC/LDO composites using an Mg or Zn to Al molar ratio of 3:1. The GAC/Mg-Al LDO possessed a higher specific surface area (323.9 m2/g) compared to GAC/Zn-Al LDO (195.1 m2/g). The GAC/Mg-Al LDO demonstrated more than 99% removal of SO42- from synthetic water, while it was 50.9% for GAC/Zn-Al LDO and less than 1% for raw GAC at an initial concentration of 50 mg/L. The GAC/Mg-Al LDO was selected for further batch experiments and modeling investigation. The equilibrium data followed the Redlich-Peterson and Langmuir models with determination coefficients of 0.943 and 0.935, respectively. The maximum Langmuir adsorption capacity was 143.5 mg/g. In the real groundwater adsorption study, the screening experiment revealed high selectivity towards SO42- with 62% removal efficiency. The optimum dosage was found to be 50 g/L with an uptake capacity of 61.5 mg/g. The kinetic data of SO42- removal from synthetic and brackish water were in excellent agreement with the pseudo-second order model, and the equilibrium was attained in 5 h. Accordingly, it can be concluded that the GAC/Mg-Al LDO is an efficient material for treating SO42- from real groundwater and can be utilized as a pretreatment unit for high sulfate water resources.

Combining hydrochemistry and 13C analysis to reveal the sources and contributions of dissolved inorganic carbon in the groundwater of coal mining areas, in East China

East China is a highly aggregated coal-grain composite area where coal mining and agricultural production activities are both flourishing. At present, the geochemical characteristics of dissolved inorganic carbon (DIC) in groundwater in coal mining areas are still unclear. This study combined hydrochemical and carbon isotope methods to explore the sources and factors influencing DIC in the groundwater of different active areas in coal mining areas. Moreover, the 13C isotope method was used to calculate the contribution rates of various sources to DIC in groundwater. The results showed that the hydrochemical types of groundwater were HCO3-Ca·Na and HCO3-Na. The main water‒rock interactions were silicate and carbonate rock weathering. Agricultural areas were mainly affected by the participation of HNO3 produced by chemical fertilizer in the weathering of carbonate rocks. Soil CO2 and carbonate rock weathering were the major sources of DIC in the groundwater. Groundwater in residential areas was primarily affected by CO2 from the degradation of organic matter from anthropogenic inputs. Sulfate produced by gypsum dissolution, coal gangue accumulation leaching and mine drainage participated in carbonate weathering under acidic conditions, which was an important factor controlling the DIC and isotopic composition of groundwater in coal production areas. The contribution rates of groundwater carbonate weathering to groundwater DIC in agricultural areas and coal production areas ranged from 57.46 to 66.18% and from 54.29 to 62.16%, respectively. In residential areas, the contribution rates of soil CO2 to groundwater DIC ranged from 51.48 to 61.84%. The results will help clarify the sources and circulation of DIC in groundwater under the influence of anthropogenic activities and provide a theoretical reference for water resource management.

Characterization of trace and heavy metal concentration in groundwater: A case study from a tropical river basin of southern India

This study investigates the hydrogeochemistry of groundwater samples collected from the Shiriya River Basin (SRB), a tropical watershed located in Kasaragod, Kerala, southern India, with a special focus on trace elements. Fifty-four groundwater samples were collected from deep aquifers, which constitute weathered and fractured granitoids and mafic rocks, and the groundwater is tapped by bore wells from a fractured zone at a depth range of 60-100 m. Concentrations of Sr, Li, Ba, Al, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Cd, Ag, Au, Te, Pb, Re, and PGEs in groundwater were determined by using Q-ICPMS. Out of the 25 analysed trace elements in groundwater, only Sr (489.6 μg/L), Ba (226 μg/L), Li (11.76 μg/L) Mn (396.8 μg/L), Ni (68 μg/L) and Fe (2438.5 μg/L) show anomalous values. The PGEs and the majority of trace elements show values within the permissible limit. Raman spectral studies reveal the presence of celsian in aquifer rocks and are the source of Ba in groundwater. Further, XRF data of the rocks show a high enrichment of Fe and Mn in mafic dyke, basalt, and syenite, and Ba and Sr in granite, pegmatite, and granitic gneiss. Therefore, this study proved that the source of these elements is geogenic, i.e., they are released from the crystalline aquifer through rock-water interaction under alkaline conditions. The results of this study show that the groundwater of the basin has enough metals such as Na, K, Mg, Ca, Mn, Fe, Co, Cu, and Zn, which are good for health. Nevertheless, a few metals (Fe, Mn, Ba, Sr, Li, Ni) that may exert toxic effects on humans are also present in the groundwater of the SRB. As groundwater is found to be a dependable source of drinking water in such watersheds, a comprehensive study on the hydrogeochemistry of all watersheds in tropical regions is recommended.

Health risk assessment of toxic elements in groundwater in a major industrial and agricultural basin, (East of Ergene Basin, Turkey)

Ergene Basin is an important agricultural and industrial region and an important water resource. In this current research, groundwater quality of east of Ergene Basin was evaluated using water quality index (WQI), and health risk due to groundwater consumption was assessed with HQ, HI, and CR. Multivariate statistical analyses were performed to evaluate the multiple effects of pollutants on groundwater. Uncertainty and sensitivity analyses were also performed. All samples were in the "excellent" WQI category. Health risk values for adults and children were below the safe limit, both from digestion and dermal exposure. CR values of As in some samples exceeded the threshold value, both for adults and children. Sensitivity analysis revealed that ingestion rate and exposure frequency for adults and exposure duration and concentration of toxic metal for children were the most sensitive variables affecting probabilistic health risk.

Water quality index, ecotoxicology and human health risk modelling of surface and groundwater along illegal crude oil refining sites in a developing economy

Water quality index, ecotoxicology and human health risk models were applied to surface and groundwater samples along illegal crude oil refining sites in Rivers State, Nigeria. Eight (8) surface water and four (4) groundwater sampling points were identified along illegal refining sites. Thirty-six (36) samples in triplicates were collected monthly from each of the twelve (12) sampling points over a three (3) month period. Water samples were collected and analyzed using standard methods as prescribed by the American Public Health Association. The mean pH for surface and groundwater ranged from 5.61 ± 0.15 to 7.34 ± 0.10 and 5.80 ± 0.10 to 6.39 ± 0.13, respectively. Turbidity, TDS, and BOD data for surface water samples exceeded the WHO guideline values. The ionic dominance pattern of anions for both surface and groundwater water samples were the same and in the order Cl- > SO42- > NO3- > PO42-. Mean heavy metal concentration was in the order Pb > Ni > Fe > Cd > Mn > Cu for surface water and Pb > Cd > Fe > Mn > Ni > Cu for groundwater. Cd and Pb concentrations in both sources were generally high, with Cd exceeding the WHO guideline value (GV). The CCME water quality index model ranked 62.5% of surface water as marginal, 12.5% as good, 12.5% as poor, and 12.5% as fair. The impact of heavy metals on public health was in the order Pb > Cd > Ni > Fe > Mn, with 83% of samples seriously affected by Pb pollution. The potential ecological risk index ranged from 1.61 × 103 to 2.64 × 103 for surface water and 8.10 × 102 to 2.21 × 103 for groundwater. Heavy metal contamination was very high, and the ecological risk effect was extremely high. The health risk through oral ingestion was in the order of adults > infants > children. Two principal components, PC1 and PC2, explained 50.51% and 16.00% of the variations in surface water quality, respectively. For groundwater quality data, three principal components explained the observed variations in water quality data, of which 51.39% is attributed to PC1, 26.29% to PC2, and 16.58% to PC3.

Metabolic versatility enables sulfur-oxidizers to dominate primary production in groundwater

High rates of CO2 fixation and the genetic potential of various groundwater microbes for autotrophic activity have shown that primary production is an important source of organic C in groundwater ecosystems. However, the contribution of specific chemolithoautotrophic groups such as S-oxidizing bacteria (SOB) to groundwater primary production and their adaptation strategies remain largely unknown. Here, we stimulated anoxic groundwater microcosms with reduced S and sampled the microbial community after 1, 3 and 6 weeks. Genome-resolved metaproteomics was combined with 50at-% 13CO2 stable isotope probing to follow the C flux through the microbial food web and infer traits expressed by active SOB in the groundwater microcosms. Already after 7 days, 90% of the total microbial biomass C in the microcosms was replaced by CO2-derived C, increasing to 97% at the end of incubation. Stable Isotope Cluster Analysis revealed active autotrophs, characterized by a uniform 13C-incorporation of 45% in their peptides, to dominate the microbial community throughout incubation. Mixo- and heterotrophs, characterized by 10 to 40% 13C-incorporation, utilized the primarily produced organic C. Interestingly, obligate autotrophs affiliated with Sulfuricella and Sulfuritalea contained traits enabling the storage of elemental S in globules to maintain primary production under energy limitation. Others related to Sulfurimonas seemed to rapidly utilize substrates for fast proliferation, and most autotrophs further maximized their energy yield via efficient denitrification and the potential for H2 oxidation. Mixotrophic SOB, belonging to Curvibacter or Polaromonas, enhanced metabolic flexibility by using organic compounds to satisfy their C requirements. Time series data spanning eight years further revealed that key taxa of our microcosms composed up to 15% of the microbial groundwater community, demonstrating their in-situ importance. This showed that SOB, by using different metabolic strategies, are able to account for high rates of primary production in groundwater, especially at sites limited to geogenic nutrient sources. The widespread presence of SOB with traits such as S storage, H2 oxidation, and organic C utilization in many aquatic habitats further suggested that metabolic versatility governs S-fueled primary production in the environment.

Assessing antimicrobial and metal resistance genes in Escherichia coli from domestic groundwater supplies in rural Ireland

Natural ecosystems can become significant reservoirs and/or pathways for antimicrobial resistance (AMR) dissemination, with the potential to affect nearby microbiological, animal, and ultimately human communities. This is further accentuated in environments that provide direct human exposure, such as drinking water. To date, however, few studies have investigated AMR dissemination potential and the presence of co-selective stressors (e.g., metals/metalloids) in groundwater environments of human health significance. Accordingly, the present study analysed samples from rural (drinking) groundwater supplies (i.e., private wells) in the Republic of Ireland, where land use is dominated by livestock grazing activities. In total, 48 Escherichia coli isolates tested phenotypically for antimicrobial susceptibility in an earlier study were further subject to whole genome sequencing (WGS) and corresponding water samples were further analysed for trace metal/metalloid concentrations. Eight isolates (i.e., 16.7%) were genotypically resistant to antimicrobials, confirming prior phenotypic results through the identification of ten antimicrobial resistance genes (ARGs); namely: aph(3″)-lb (strA; n=7), aph(6)-Id (strA; n = 6), blaTEM (n = 6), sul2 (n = 6), tetA (n = 4), floR (n = 2), dfrA5 (n = 1), tetB (n = 1), and tetY (n = 1). Additional bioinformatic analysis revealed that all ARGs were plasmid-borne, except for two of the six sul2 genes, and that 31.2% of all tested isolates (n = 15) and 37.5% of resistant ones (n = 3) carried virulence genes. Study results also found no significant relationships between metal concentrations and ARG abundance. Additionally, just one genetic linkage was identified between ARGs and a metal resistance gene (MRG), namely merA, a mercury-resistant gene found on the same plasmid as blaTEM, dfrA5, strA, strB, and sul2 in the only isolate of inferred porcine (as opposed to bovine) origin. Overall, findings suggest that ARG (and MRG) acquisition may be occurring prior to groundwater ingress, and are likely a legacy issue arising from agricultural practices.

Ca/Na concentration-constrained variations of dissolved organic matter leaching from groundwater-irrigation area soil in North China Plain

This study investigates the quantity and quality variations of dissolved organic matter (DOM) leaching from the soil in groundwater irrigation area of the North China Plain, constrained by the concentration of Ca/Na. Soil samples with dominant humic-like (HLC) and protein-like (PLC) components were paired with parallel concentration gradients of Ca/Na extractants for equilibrium experiments. Fluorescence-PARAFAC, UV-visible spectroscopy, and multiple statistical analyses were combined for data analysis and interpretation. The results reveal that the primary DOM components remained dominant for specific soil sample, with a higher relative abundance of PLC (HLC) in Ca (Na) extract. HLC preferentially binds to soil phase in all extractions, while PLC is readily released into the solution. However, Ca inhibits HLC desorption and promotes PLC release more significantly than Na, as indicated by stronger ion/proton reaction (IPR) and electrostatic effect (ESE). The strongest IPR and ESE are seen in the HLC-dominated DOM extracted with Ca, suggesting a condition where Ca bridges to HLC and forms total dissolved organic carbon (DOC) that decreases. In contrast, Na extraction exhibits only a weaker ESE that is offset by soil-contained HLC and exchangeable Ca, resulting in subtle DOC decrease. The trends in leaching of HLC and PLC are self-dependent, and the level of variation in either component correlates with the increasing concentration of specific cations present. These findings underscore the crucial role of soil organic matter (SOM) composition and its interaction with leaching cations in soil management in large-scale groundwater irrigation areas, where SOM quality and groundwater chemistry vary.

Land use dynamics and its influences on groundwater depth levels in South region of National Capital Territory (NCT) of Delhi, India

The present study envisions the influences of land use dynamics on the spatial trend of groundwater depth levels over a period of two decades in the south region of NCT Delhi, India. The findings have inferred that among five major LULC categories, built-up has shown a sprawling trend (+2.17 km2/year) from the north, northeastern, and central portions to the confined patches observed in the south and southwest fringes of South Delhi from 2001 to 2021. Likewise, vegetation class has also witnessed significant increment (+1.91 km2/year) to the peripheral boundary, i.e., southern, southwest/eastern fringes, and central portions under recent initiatives of city forests, plantation drives, and urban green spaces programs. On the contrary, a negative rate of change has been observed in fallow land (-2.78 km2/year), agriculture (-1.22 km2/year), and water bodies (-0.07 km2/year). LULC transition matrix has also showed prominent conversion of fallow land into vegetation and built-up class, and change of vegetation into built-up, and fallow land category. Subsequently, corresponding LULC maps have been superimposed with long-term trends of groundwater depth levels (as spatial contours). For entire South Delhi region, depth to water levels ranged from 2.02 to 66.45 meters below ground level (mbgl) where shallower depths followed a negative trend and remain persistent throughout the time period in north and northeastern fringes. Higher fluctuations in groundwater depletion with positive trends directly get influenced with allied land use transitions such as a steady increase in built-up area and steeper depth levels (> 40 mbgl) as observed in central, southwestern, and southern parts. Moreover, buffer peripheries in the proximity of groundwater monitoring stations viz., Hauz Khas, Pushp Vihar, Jamali, Gadaipur, and Bhatti Kalan have observed deeper groundwater levels allied to built-up expansion. Thus, groundwater depletion trends can be ascribed to the incessant conversion of recharging areas into impervious zones along with uneven distribution of groundwater usage and supply. Conversely, expanding vegetative land has also shown improved groundwater depth levels. Therefore, land use influences must be managed in the long run for ensuring sustainable management of groundwater resources.

Experimental upscaling analyses for a surfactant-enhanced in-situ chemical oxidation (S-ISCO) remediation design

Surfactant-enhanced in-situ chemical oxidation (S-ISCO) is an emerging innovative remediation technology for the treatment of dense non-aqueous phase liquids (DNAPLs). S-ISCO combines the solubilization of contaminants by means of surfactants with the chemical oxidation by an oxidizing agent, thus, potentially increasing the efficiency of the state-of-the-art ISCO technique. Scientific investigations are needed to enable the technology transfer for potential field applications based on the development of a remediation design under well-defined boundary conditions. For this purpose, experimental upscaling analyses were performed using the special infrastructure of the research facility for subsurface remediation (VEGAS). Batch tests showed that oxidation of the selected surfactant E-Mulse 3® (EM3) by activated persulfate (Na-PS) reduced the solubilization of the model contaminants 1,4-DCB, naphthalene, and PCE. As a consequence, the processes of contaminant solubilization and degradation were temporally and spatially separated in the developed remediation design. A proof of concept was provided by performing an S-ISCO medium-scale experiment (100 cm length, 70 cm height, 12.5 cm width), with 1,2-DCB as model DNAPL contaminant to be treated. A groundwater circulation well (GCW) was used to inject a 60 g/L Na-PS solution and to effectively mix the reagents. Sampling of the experiment's outflow and the soil material after treatment showed that neither rebound effects nor residual mass loadings on the soil material could be detected after termination of the S-ISCO treatment. To further evaluate the S-ISCO remediation design under field-like conditions, a large-scale S-ISCO experiment was conducted (6 m length, 3 m height, 1 m width), allowing for an extensive sampling campaign to monitor relevant processes. An efficient contaminant removal from the former source zone could be reached by surfactant solubilization, decreasing contaminant levels from initially over 2000 mg/L 1,2-DCB to final concentrations below 5 mg/L 1,2-DCB. The heterogeneously distributed contaminant degradation, implemented by a three-filter GCW, was attributed to density-induced migration processes that impeded an optimal reaction zone. A density-dependent numerical transport could qualitatively match the observations. By comparing different simulation scenarios, an adapted operation of the GCW was established that provides for a more efficient distribution of the density-influenced oxidant injection.

Fluoride Binding Potential of Selected Phytochemicals: A Pilot Study

Fluoride (F-) contamination in drinking water is a major global concern. According to several studies, India and China are the most affected by the presence of excess F-. Long-time exposure to F- concentrations above 1.5 ppm can lead to hard and soft tissue fluorosis (F- toxicity). There are no effective cure or treatment for fluorosis and the condition is almost irreversible. Considering water to be the prime media through which F- reaches humans, maintaining optimal F- levels in water remains the only possible remedy. F- endemic areas have adapted several conventional defluoridation techniques to resolve the issue. Among these, adsorption with plant compounds is widely used for F- removal. Studies have shown that plant metabolites can ameliorate the toxic effects of F-. Based on this, we attempt to elucidate the potential binding and electrochemical bio-sensing properties of selected phytochemicals towards F-. The focus of the present work is to evaluate the interactions of phytochemicals with F-; for which, the binding studies of phytochemicals with F- have been elaborated by UV-visible spectroscopy and emission techniques. Benesi-Hildebrand's (BH) plot was used to calculate the binding constant (CUR - 34.9 × 103 (M-1), QUER - 13 × 103 (M-1), ESC -6.3 × 103 (M-1), FIS - 5.36 × 103 (M-1) and PCA -1.5 × 103 (M-1), and detection limit (CUR - 1.54 × 10-7 M, QUER - 0.156 × 10-6 M, ESC - 0.221 × 10-6 M, FIS - 0.175 × 10-6 M, and PCA - 5.8 × 10-6 M) for the F-:phytochemical mixtures. Further, the binding characteristics were confirmed using 1H-NMR titration experiments. Our findings highlight the potential of phytochemicals as effective binding agents for F-, thereby reducing its bioavailability.

Nitrates in Turkish waters: sources, mechanisms, impacts, and mitigation

Intensive technological developments, rapid population growth and urbanization, and excessive use of nitrogen fertilizers have caused water resources to be contaminated substantially by nitrates in Turkey. The accumulated information should be evaluated to draw a nationwide attention to the problem. The aim of this review article was to highlight the importance of nitrate (NO3) contamination and to discuss the measures to be taken to mitigate the contamination across the nation. Agriculture, especially chemical fertilizers used in irrigated agriculture, was the most important source of NO3 in groundwater. Also, the industrial and domestic discharges substantially contributed to NO3 in both groundwater and surface waters in many cases. The most severe and widespread groundwater (e.g., 344 mg NO3 L-1 in İzmir, 476 mg L-1 in Afyon, 477 mg L-1 in Antalya, and 948.0 mg L-1 in Konya) and surface water contaminations (e.g., 293.8 mg NO3 L-1 in İzmir, 63.3 mg L-1 in Eskişehir, 89.8 mg L-1 in Edirne, and 90.6 mg L-1 in Sakarya) occurred in the regions where intensive agriculture, industrial development, and rapid urbanization were clustered. Well-established irrigation and fertilizer management plans are critical for reducing fertilizer-related NO3 contaminations in the irrigated agriculture. Special attention should be given to the regions where industrially and domestically contaminated running water bodies are in contact with groundwater. Discharge of wastewaters to the streams, creeks, rivers, and lakes should be prevented. Well-designed studies are needed to evaluate potential health effects, including the risk of cancer, of NO3 in drinking water.

Effects of recharge process on groundwater nitrate concentration in an oasis of Tengger Desert hinterland, China

Groundwater nitrate concentrations cannot be effectively diluted in an oasis of desert hinterland without direct recharge from external rivers. Therefore, there is an urgent need to understand the relationship between groundwater nitrate concentration and the groundwater recharge process. Using hydrochemicals, stable isotopes, LUCC, and combining these with the MixSIAR model, the distributions of groundwater nitrate concentration in the Dengmaying Basin (DMYB) in 2006 and 2020 were obtained. The contributions of groundwater recharge and nitrate sources were also quantified. With the development of agriculture in the DMYB, groundwater irrigation leakage has gradually become a crucial recharge source of groundwater, with a recharge proportion reaching 30.3%. From 2006 to 2020, under the influence of well irrigation and agricultural fertilization, the groundwater nitrate concentration in the DMYB increased significantly, with an increased range of 1.3 ~ 14.3 mg L-1. Moreover, the δ15N-NO3- and δ18O-NO3- values of nitrate in cultivated soil water were similar to those in groundwater, which also proves the process of carrying nitrate from the vadose zone into groundwater by irrigation water. The contribution of anthropogenic sources (54.9%) to groundwater nitrate exceeded that of natural sources (45.1%) to groundwater nitrate in the DMYB. These results indicate that the potential for nitrate pollution in groundwater must be considered, even in desert oases.

A new probabilistic assessment process for human health risk (HHR) in groundwater with extensive fluoride and nitrate optimized by non parametric estimation method

Excessive amounts of fluoride (F-) and nitrate (NO3-) in groundwater pose a significant threat to human health. However, a quantitative approach to assessing the human health risks caused by these harmful substances is lacking. To optimize the probabilistic assessment process for human health risk (HHR), this study introduced kernel density estimation (KDE) into the stochastic simulation of F- and NO3- content in groundwater samples. The potential HHRs caused by F- and NO3- in Songyuan City were summarized by combining the probabilistic and deterministic assessments. This study found that the concentrations of F- and NO3- did not follow common probability density functions (PDFs), but the KDE method passed the Kolmogorov-Smirnov test with the critical value of 0.067 and 0.062, showing high fitting accuracy. Monte Carlo simulation indicated that the probability of NO3- for children and adult exceeding the standard is 21.95% and 15.14%, respectively, which is comparable with the results of the deterministic assessment, but the probabilistic assessment emphasized lower probability of HHRs in children caused by excess F-(4.14%). Global sensitivity analysis revealed that excessive NO3- in groundwater has the highest sensitivity of the HHR (>0.1), followed by other factors representing water use habits (>0.01). This study presents a refined probabilistic assessment method for HHR and provides a scientific reference for understanding the state of groundwater environments.

The seasonal distribution and pollution potential of dissolved heavy metals and nutrients in subterranean estuaries in southern India

In order to better understand the distribution pattern, pollution degree and the submarine groundwater discharge (SGD) of dissolved heavy metals, 15 subterranean estuaries (STEs) along southwest Indian coast were sampled over three contrasting seasons. The average concentration of metals were ranked as, pre-monsoon > monsoon > post-monsoon with 3 to 12-fold higher groundwater metal concentrations than the adjacent seawater. Average SGD derived essential metal fluxes were five times higher than the toxic metal fluxes of which Fe and Zn together contributed >90 %. Using the Single Factor Contamination Index, the majority of sites were minimally contaminated with only two sites indicating moderate ecological risk due to As. Higher fluxes of Fe, Cu and Zn were likely a result of rising anthropogenic activities. The SGD derived nutrient fluxes were an important source of DIP for primary production in coastal waters and represented 30 % and 44 % of the DIN and DIP inputs respectively.

Effect and genesis of soil nitrogen loading and hydrogeological conditions on the distribution of shallow groundwater nitrogen pollution in the North China Plain

The North China Plain (NCP) has experienced increasingly severe groundwater nitrogen (TN) pollution. However, the factors influencing TN distribution are still poorly understood. Previous studies have identified surface soil nitrogen (TSN) loading and intrinsic groundwater vulnerability (Inv) as the main factors controlling groundwater TN pollution. However, in this study, based on 3245 shallow groundwater samples in the NCP, the multiple regression analysis results(R2=0.105, p<0.001) revealed that the TN was not mainly controlled by TSN and Inv. The lower prediction accuracy indicated the large data dispersion of TN, which might be affected by nitrogen attenuation or accumulation. Thus, the NCP was divided into balance, attenuation, and accumulation zones according to the regression equation. The attenuation zone was mainly distributed in the inter-fan and fan edge parts of the pre-mountain alluvial floodplain, as well as the west and south of the runoff area, while the accumulation zone was mainly distributed in the top part of the pre-mountain alluvial floodplain and the east of discharge area. Multi-indicators comparative analysis showed that compared to the balance (Eh= 76.2 mV) and accumulation (Eh=126.7 mV) zones, the attenuation zone has a stronger reducing environment (Eh=30.8 mV) favorable to denitrification, which can reduce the TN pollution (0.49 mg/L) caused by surface nitrogen input and consume more electron donors. Conversely, the stronger oxidizing environment in the accumulation zone limited denitrification, resulting in higher TN concentrations (19.14 mg/L) in the aquifers under the same TSN and Inv conditions as the other two zones. The standardized effects and significance on each path of the structural equation model (SEMs) fully confirmed the reliability of the above zonal analysis. Importantly, the feature importance (23.6%) of random forest and standardized effects (0.455, p<0.001) of SEMs showed that the Eh had the strongest influence on TN. Thus, the redox conditions of the aquifer, in addition to TSN and Inv, played a crucial role in controlling the TN pollution in the groundwater of a large region. The zoning work and the analysis of influencing factors are important to guide scientific prevention and control of groundwater nitrogen pollution.

Assessment of drought conditions and prediction by machine learning algorithms using Standardized Precipitation Index and Standardized Water-Level Index (case study: Yazd province, Iran)

Drought as a natural phenomenon has always been a serious threat to regions with hot and dry climates. One of the major effects of drought is the drop in groundwater level. This paper focused on the SPI (Standardized Precipitation Index) and SWI (Standardized Water-Level Index) to assess meteorological and hydrological drought, respectively. In the first part, we used different time frames of SPI (3, 6, 12, and 24 months) to investigate drought in Yazd, a dry province in the center of Iran for 29 years (1990-2018). Then, in the second part, the relationship between SPI and SWI was investigated in the three aquifers of Yazd by some rain gauge stations and the closest observation wells to them. In addition to using SPI and SWI, we also used different machine learning (ML) algorithms to predict drought conditions including linear model and six non-linear models of K_Nearest_Neighbors, Gradient_Boosting, Decision_Tree, XGBoost, Random_Forest, and Neural_Net. To evaluate the accuracy of the mentioned models, three statistical indicators including Score, RMSE, and MAE were used. Based on the results of the first part, Yazd province has changed from mild wet to mild drought in terms of meteorological drought (the amount of rainfall according to SPI), and this condition can worsen due to climate change. The models used in ML showed that SPI-6 (score ave = 0.977), SPI-3 (score ave = 0.936), SPI-24 (score ave = 0.571), and SPI-12 (score ave = 0.413) indices had the highest accuracy, respectively. The models of Neural_Net (score ave = 0.964-RMSE ave = 0.020-MAE ave = 0.077) and Gradient_Boosting (score ave = 0.551-RMSE ave = 0.124-MAE ave = 0.248) had the highest and lowest accuracy in prediction of the SPI in all four-time scales. Based on the results of the second part, about the SWI, Random_Forest model (score = 0.929-RMSE = 0.052-MAE = 0.150) and model of Neural_Net (score = 0.755-RMSE = 0.235-MAE = 0.456) had the highest and lowest accuracy, respectively. Also, hydrological drought (reduction of the groundwater level) of the region has been much more severe, and according to the low correlation coefficient of average SPI and SWI (R2 = 0.14), we found that the uncontrolled pumping wells, as a main factor than a shortage of rainfall, have aggravated the hydrological drought, and this region is at risk of becoming a more arid region in the future.