Natural background groundwater composition in the Azores archipelago (Portugal): a hydrogeochemical study and threshold value determination

Evaluating natural background levels of heavy metals in shallow groundwater of the Pearl River Delta via removal of contaminated groundwaters: Comparison of three preselection related methods

Assessing natural background levels (NBLs) in groundwater is a global concern. Knowledge on groundwater NBLs in urbanized areas is challenging due to the impact of complex human activities. Preselection related methods are common ones for assessing groundwater NBLs. The present study used three preselection related methods to assess groundwater heavy metals (lead, zinc, barium) NBLs in four groundwater units of the Pearl River Delta (PRD) where urbanization continues, and to identify the best one for assessing groundwater NBLs in urbanized areas. Here, methods include a preselection method (method-P), a preselection dominated method (method-PD), and a statistic dominated method (method-SD). Results showed that the method-PD was better than other two methods for assessing groundwater NBLs of heavy metals in the PRD. This is supported by the evidence that differences among heavy metals concentrations in various land-use types in residual datasets formed by the method-PD were insignificant. NBLs of lead in groundwater units I to IV assessed by the method-PD were 2.8 μg/L, 5.9 μg/L, 5.8 μg/L, and 2.6 μg/L, respectively. NBLs of zinc in groundwater units I to IV assessed by the method-PD were 30 μg/L, 180 μg/L, 160 μg/L, and 100 μg/L, respectively. NBLs of barium in groundwater units I to IV assessed by the method-PD were 120 μg/L, 120 μg/L, 90 μg/L, and 50 μg/L, respectively. Compared to the method-PD, the method-SD often underestimates groundwater NBLs of heavy metals because of using the experiential evaluation for residual datasets. The method-P also has an inaccurate evaluation of groundwater NBLs of heavy metals in comparison with the method-PD, owing to both of using the experiential evaluation and the absence of a function for outliers test. The method-P combining with an outliers test would be better than itself for assessing groundwater NBLs. Therefore, the method-PD is the first choice to be recommended for assessing groundwater NBLs in urbanized areas such the PRD. However, this method should not be taken into account for assessing groundwater NBLs in areas where groundwater Cl/Br mass ratios are invalid. Instead, the method-SD and the method-P combining with one outliers test may be choices, because no constraint for these two methods.

BRIDGE methodology-based quality standards to assess aquifer chemical status in the southwest Bengal Basin, Bangladesh

Assessment of natural background levels (NBLs) of compositional groundwater parameters helps to identify the potential threats to groundwater resources. This study is the first attempt to apply the pre-selection-based BRIDGE (Background cRiteria for the IDentification of Groundwater thrEshold) methodology to calculate the NBLs and threshold values (TVs) of major groundwater constituents in the southwest Bengal Basin, Bangladesh. A database consisting of 78 groundwater samples was used to assess the NBLs and associated TVs of the major groundwater parameters (EC, Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl^-, NO₃^-, SO₄^2-, PO₄^3-, Mn²⁺, and Fe²⁺). NBLs were derived based on 90th and 97.7th percentiles. The status of regional groundwater resources was assessed by applying 90th percentile NBL on a regional dataset (n = 196). Results revealed the "poor" chemical status of shallow aquifers denoting heavy deterioration of the groundwater quality due to anthropogenic interventions. Nitrate contamination and salinization were identified as the major threats to the deep groundwater of the southwest Bengal Basin. Finally, to verify the chemical status of groundwater in a heavily urbanized area, derived TVs were applied throughout the experimental site Khulna. Twenty-five deep groundwater samples were collected for this purpose. Though most of the parameters exhibited "good" chemical status, nitrate demonstrated anthropogenic groundwater contamination in Khulna City. Thus, the developed TVs would provide an early warning system of pollution. On a national scale, it is expected to facilitate the sustainable groundwater management of the country and contribute to achieving the Sustainable Development Goals (SDG) of the United Nations (UN) in Bangladesh.

Hydrogeochemical Characteristics of Bottled Waters Sourced from Bedrock Aquifers in South Korea: Evaluation of Water Type and Natural Background Levels

The hydrogeochemical properties of bottled waters (n = 37) were examined to evaluate the factors governing their quality and to suggest the natural background levels (NBLs) of groundwater. The bottled waters were sourced from bedrock aquifers of various geological types and analyzed for 14 physicochemical parameters and 48 trace elements. The bottled waters mainly consisted of the Ca-HCO3 type with low TDS (mean = 158.4 mg/L; n = 33) regardless of geological type, indicating low degrees of water–rock interaction. The results of principal component analysis (PCA) showed that these waters were characterized by the dissolution of calcite and Ca-plagioclase (PC1) and the weathering of Na-plagioclase and cation exchange (PC2). The PCA results with low concentrations of TDS and F (mean = 0.4 mg/L) revealed that the waters represent slightly mineralized groundwater, probably because the boreholes were installed in fractured aquifers, avoiding high F concentrations (>1.5 mg/L). The 90th percentiles for the Ca-HCO3 type bottled waters were proposed as the NBLs for Korean groundwater for 11 major elements and 20 trace elements. The NBLs of NO3 (7.9 mg/L) and F (0.9 mg/L) were similar to the 90th percentiles of EU bottled waters (n = 1785), implying the suggested NBLs are acceptable for groundwater quality management.

Source apportionment and natural background levels of major ions in shallow groundwater using multivariate statistical method: A case study in Huaibei Plain, China

Understanding the sources, natural background levels (NBLs), and threshold values (TVs) of the major ions in groundwater is essential for the effective protection of water resources. In this study, a total of 70 shallow groundwater samples were collected in Suzhou, Huaibei Plain, China. A variety of statistical methods and cumulative probability distribution techniques were performed to identify the sources, NBLs, and TVs of the major ions. The major ion concentrations found in decreasing order as follows: HCO₃^- > SO₄^2- > NO₃^- > Cl^- and Na⁺ > Ca²⁺ > Mg²⁺. Piper diagram for hydrochemical types shows that groundwater types were Mg-HCO₃ (36%), Ca-HCO₃ (34%), and Na-HCO₃ (30%). According to the factor and the Unmix model analysis, anthropogenic (agriculture-related) and geogenic source (water-rock interactions-related) were identified to be responsible for the chemical composition of the groundwater in the study area, and their mean contributions for the major ion concentrations are 47.9% and 52.1%, respectively. The NBLs for Na⁺, Ca²⁺, Mg²⁺, Cl^-, SO₄^2-, and NO₃^- were determined to be 29.5-44.2, 26.2-38.9, 18.9-39.5, 1.0-9.9, 12.9-19.4, and 2.1-16.5 mg/L, respectively, and the TVs were calculated as 122.1, 169.5, 39.5, 129.6, 134.7, and 18.3 mg/L, respectively. Moreover, this study shows the feasibility and reliability of using these multivariate statistical methods and natural background levels to evaluate the status of groundwater quality.

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

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

Two distinct mechanisms of fluoride enrichment and associated health risk in springs' water near an inactive volcano, southeast Iran

Groundwater fluoride contamination is a major issue of water pollution in the world with health hazards such as dental and skeletal fluorosis. This research focused on exposure to the high concentration of fluoride in the springs water in the Bazman volcanic area, southeast Iran. The combination of chemical/isotopic analysis, geochemical modeling, health risk assessment and multivariate statistical methods were applied to investigate the contamination and sources of fluoride in the samples. Groundwater samples were collected from cold and thermal springs. Major ions, fluoride, trace elements and stable isotopes δ¹⁸O and δD were measured in the samples using standard methods, ICP-MS and OA-ICOS, respectively. Fluoride content in springs varied from 0.5 to 3.75 mg/L with an average value of 1.66 mg/L. The highest fluoride concentrations were observed in the eastern cold springs while thermal springs showed the minimum fluoride contents. The majority of samples showed F contents higher than the calculated optimal concentration of fluoride (0.75 mg/L). Reaction of fluorite mineral with HCO₃ and replacement of F in clay minerals and metal oxy-hydroxides with OH^- in water were likely cause fluoride enrichment in the eastern springs. Whereas, in the western springs and thermal springs, origin of fluoride was related to weathering of muscovite, cryolite, apatite and fluoroapatite minerals. The δ¹⁸O and δ²H of the water samples displayed the impact on evaporation on fluoride enrichment in all spring water samples. The average value of contamination index (C_d) in the water samples was 1.94 categorizing medium risk level while springs S7, S8, S9 and S4 were above the threshold value of C_d index. The fluoride hazard quotient (HQ) showed that 25%, 44%, 56% and 0% of springs' water resources had high risk level for age group of adults, teenager, children and infants, respectively. Therefore, health risk of fluoride in drinking water resources were in the following order: children > teenager > adults > infants.

Groundwater nitrate contamination and agricultural land use: A grey water footprint perspective in Southern Apulia Region (Italy)

In this paper, we present a new approach based on the Grey Water Footprint (GWF) concept for the determination of groundwater nitrate contamination, with a focus on the agricultural impact in the Southern Apulia Region (Italy). The GWF assessment allows us to highlight wells where nitrate contamination is higher and for which a verification of the nature of contamination is necessary, potentially identifying certain contexts associated with risk factors present in the area. Data show higher nitrate GWF values for vineyards than for olive groves, particularly in areas used for the production of table grapes. Indeed, it is possible to observe that the Western Ionian-Tarantino arch is characterized by a high average level of the GWF indicator, area characterized by an agricultural land use (vineyards, orchards, simple arable land). Another important area is the Brindisi plain, predominantly characterized by an agricultural vocation and a related fertilizer use. The situation in the whole Salento peninsula is more heterogeneous. The results of the GWF show high values in equally distributed points with a clear frequency in the coastal areas of the Ionic arch (Eastern Tarantino and Salento). The Water Footprint methodology and in particular the GWF approach used in this study conceives a useful indicator for the agricultural policy planning processes, a criterion to establish land use management according to the status of hydrological basin and a tool for assessing the pollution monitoring programs.

Contribution of hydrochemical and geoelectrical approaches to investigate salinization process and seawater intrusion in the coastal aquifers of Chaouia, Morocco

This study aims to identify groundwater salinization origin and to determine seawater intrusion extension toward the inland in Chaouia, Morocco. To reach these objectives, firstly, 46 groundwater samples were analyzed for major chemical elements during January 2012 and, secondly, 10 electrical resistivity tomography (ERT) profiles were performed perpendicularly to the coastal fringe. Statistical analysis provided the distinction between three Clusters reflecting different hydrochemical processes. Cluster I and Cluster II-a showed a high water electrical conductivity (EC) (from 2.3 to 11.2mS/cm) with the dominance of Na⁺ (668mg/L on average) and Cl^- (1735mg/L on average) ions as a consequence of seawater intrusion. However, Cluster II-b presented low ECs (from 0.5 to 1.7mS/cm) and Ca²⁺ (99.6mg/L on average) and HCO₃^2- (235.2mg/L on average) ions dominance. Water chemistry in these wells was controlled by water-rock interaction, cation exchange, and anthropogenic activities. The Hydrochemical Facies Evolution Diagram highlighted the succession of different water facies developed between intrusion and freshening phases. The formation of Na-HCO₃ facies, which characterizes the last facies of freshening phase, followed the succession of Na-Cl, MixNa-MixCl, MixCa-MixCl, MixCa-MixHCO₃, and Na-HCO₃. In contrast, Na-Cl facies formation, which characterizes the last facies of intrusion phase, followed the evolution of Ca-HCO₃, Ca-MixHCO₃, Ca-MixCl, MixCa-MixCl, MixCa-Cl, and Na-Cl. Moreover, the obtained ERT results allowed determining the extent of different hydrochemical facies and provided more details about seawater intrusion extension. The conductive level assigned to seawater contamination showed a resistivity less than 36Ω.m, which remains limited to 3000m from the ocean, where Na-Cl water type dominates. The seawater intrusion depth varied between 5 and 40m from the surface. Overall, this original study in Chaouia region demonstrated the effectiveness of combining hydrochemical and ERT methods to investigate seawater intrusion, particularly in areas with restrictive water chemistry data.

Nitrate pollution of groundwater; all right…, but nothing else?

Contamination from agricultural sources and, in particular, nitrate pollution, is one of the main concerns in groundwater management. However, this type of pollution entails the entrance of other substances into the aquifer, as well as it may promote other processes. In this study, we deal with hydrochemical and isotopic analysis of groundwater samples from four distinct zones in Catalonia (NE Spain), which include 5 different aquifer types, to investigate the influence of fertilization on the overall hydrochemical composition of groundwater. Results indicate that intense fertilizer application, causing high nitrate pollution in aquifers, also homogenize the contents of the major dissolved ions (i.e.; Cl(-), SO4(2-), Ca(2+), Na(+), K(+), and Mg(2+)). Thus, when groundwater in igneous and sedimentary aquifers is compared, significant differences are observed under natural conditions for Cl(-), Na(+) and Ca(2+) (with p-values ranging from <0.001 to 0.038), and when high nitrate concentrations occur, these differences are reduced (most p-values ranged between 0.054 and 0.978). Moreover, positive linear relationships between nitrate and some ions are found indicating the magnitude of the fertilization impact on groundwater hydrochemistry (with R(2) values of 0.490, 0.609 and 0.470, for SO4(2-), Ca(2+) and Cl(-), respectively). Nevertheless, the increasing concentration of specific ions is not only attributed to agricultural pollution, but to their enhancing effect upon the biogeochemical processes that control water-rock interactions. Such results raise awareness that these processes should be evaluated in advance in order to assess an adequate groundwater resources management.