4.1 Major ions and hydrochemistry
Fig. 1 shows the results of sampling and analysis in May and September 2019. According to the results of the hydrochemical analysis, theTDS concentration of the shallow groundwater was 719±310 mg L-1. Most of the samples were low-salinity fresh water (< 1000 mg L-1) and 26% were in the range of brackish water (1000–3000 mg L-1). The mean annual temperature of the groundwater samples is approximately 17.1°C, with a range of from 16.6°C–17.3°C. The average pH value is 7.31, with a range of from 7.01–8.19, which shows little variation across the study area. The chemical composition of groundwater was mainly controlled by ions (SO42-, Cl-, HCO3-, Na+, Ca2+, Mg2+). The anion components were mainly HCO3-, followed by SO42- and Cl-, with concentrations of 617±220, 83.7±73.1, and 54.0±58.8mg L-1, respectively. The dominant cation was Na+, followed by Ca2+ and Mg2+, with concentrations of 186±120, 46.2±27.9, and 39.5±12.4 mg L-1, respectively. According to the drinking water quality standards recommended by the WHO (2011), the main factors affecting the groundwater quality in the study area were the concentrations of As and F. The As concentration of groundwater in the study area was 5.75±5.42 μg L-1, showing clear spatial variability. The proportion of high-arsenic groundwater samples above >1 μg L-1 reached 74%, nearly 1/4 of which exceeded the threshold of > 10 μg L-1 recommended by the WHO (Fig. 1). The F- concentration of groundwater was 1.29±0.40 mg L-1, and 31% of the considered samples exceeded the WHO-recommended limit of 1.50 mg L-1.
The dominant ions determine the groundwater types. According to the Piper diagram (Fig. 2), the groundwater types in the study area are dominated by HCO3-Na, followed by HCO3-Na•Mg, HCO3-Na•Ca, and HCO3-Na•Ca•Mg. The high-arsenic groundwater types are dominated by HCO3-Na (Fig. 2).
4.2 Hydrogeochemical processes
4.2.1 Evaporation and dissolution processes:Water-rock interaction is a critical process in determining the chemical composition of groundwater. Hydrogeochemical processes such as dissolution, ion exchange, oxidation, and reduction occurring in the process of water-rock interaction are the main factors controlling the chemical characteristics of groundwater. The hydrogeochemical processes were identified based on distinct hydrochemicals and their concentrations (Gibbs, 1970; Liu et al., 2018; Taheri et al., 2017; Xing et al., 2013; Zhu et al., 2011). The Gibbs diagram is an analytical method that uses the relationship between the ratios of Na/(Na+ Ca) and Cl/(Cl+HCO3), and TDS to reflect the controlling factors of main ions in the water. According to the Gibbs diagram (Fig. 3), TDS in the study area was 722±296 mg L-1, Cl/(Cl+HCO3) ranged from 0.01 to 0.03, and Na/(Na+Ca) ranged from 0.23 to 0.94. Most of the analytical samples were located in the areas of water-rock interaction and evaporation crystallization (Fig. 3), confirming that the water-rock interaction and evaporation processes have an impact on the formation and evolution of groundwater in the study area.
In this study, PHREEQC 3.40 was used to calculate the mineral saturation indices, indicating that the SI values of near-saturated minerals, calcite (0.41), aragonite (0.26), and magnesite (0.04), were close to 0 and in a quasi-equilibrium state. The SI values of the unsaturated minerals, halite (-6.52), gypsum (-1.99), anhydrite (-2.23), and fluorite (-1.018), were less than -0.5, indicating a dissolution tendency. The SI of dolomite (0.70) was greater than 0.5, indicating a chemical precipitation tendency (Table 1). Cl-, F- and SO42- in groundwater were partly derived from the dissolution and release of halite, fluorite, gypsum, and anhydrite minerals.
Table 1. Saturation indices in groundwater from Taihe, Anhui Province, in Huaihe River Basin, China
Sample grouping ID
|
SI(h)
|
SI(g)
|
SI(an)
|
SI(d)
|
SI(c)
|
SI(ar)
|
SI(f)
|
SI(m)
|
Group 1: As< 3 μg L-1;
|
-6.80
|
-1.94
|
-2.18
|
1.02
|
0.47
|
0.32
|
-0.87
|
0
|
Group 2: 5>As ≥ 3 μg L-1;
|
-6.32
|
-1.94
|
-2.19
|
0.50
|
0.42
|
0.27
|
-1.02
|
0.12
|
Group 3: 10>As ≥ 5 μg L-1;
|
-6.43
|
-2.06
|
-2.31
|
0.94
|
0.36
|
0.21
|
-1.12
|
0.02
|
Group 4: 5 As ≥ 10 μg L-1
|
-6.52
|
-2.00
|
-2.25
|
0.35
|
0.38
|
0.23
|
-1.06
|
0.02
|
SI(h): halite, SI(g): gypsum, SI(d): dolomite, SI(c): calcite,
SI(f): Fluorite, SI(an): anhydrite, SI(m): Magnesite, SI(ar): Aragonite
4.2.2 Evaporation and concentration processes:Solutes also commonly found in groundwater are Cl and Br. Due to the conservative behavior and high solubility of Cl and Br in natural water, ion exchange reaction and mineral surface adsorption cannot significantly change the concentrations of Cl and Br. With the increase of chloride ion concentration, the dissolution of halite (NaCl) will produce a rapid increase in the Cl/Br ratio. In contrast, the evaporation process of groundwater can change the absolute concentrations of Cl and Br in groundwater, but will not change the Cl/Br ratio before the groundwater is saturated with halite. Therefore, Cl, Br, and Cl/Br ratios can be used to identify and distinguish the dissolution, evaporation, and other evolution processes of groundwater (Cartwright et al., 2006; Deng et al., 2009; Han et al., 2014; Taheri et al., 2017; Xie et al., 2012; Xing et al., 2013). The Cl- concentration range of the test samples was 0.70–210 mg L-1, the mean value was 54.0±58.8mg L-1, the Br- concentration range was 10.7–324 μg L-1, and the mean value was 104 ±88μg L-1. There was a significant positive correlation between the Cl- and Br concentrations, with a correlation coefficient of 0.75 (P≤0.01). As the Cl and Br concentrations of the test samples were relatively low, the mean value of Cl/Br (mol) was 1097 ±1044 and the ratio varied from 51.0 to 4603. A majority of Cl/Br ratios of the test samples exceeded 600, showing significant spatial variability. The Cl/Br ratios of water samples above the WHO limit (>10 μg L-1) ranged from 544 to 3093, with an average of 993. The mineral structure of halite (NaCl) does not contain large Br, and its Cl/Br ratios are generally 104-105. The dissolution of halite will result in the rapid increase of Cl/Br ratio with the increase of Cl- concentration. The highest value of Cl/Br ratios of the test samples exceeded 4600, and the concentration of Cl- in groundwater did not exceed 6 mmol L-1. The dissolution of a small amount of halite in groundwater was the most likely mechanism for the rapid increase of Cl/Br ratios. The large variation range of Cl/Br ratios reflected the different dissolved amount of halite in each test sample. As shown by the relationship between Cl/Br ratios and Cl concentrations (Fig. 4), evaporation and halite dissolution are the dominant processes controlling the distribution of shallow groundwater. The Cl/Br ratios of high-arsenic groundwater are relatively unchanged with the increase of Cl- concentrations, indicating that the high-arsenic groundwater is more affected by evaporation.
4.2.3 Ion exchange processes:The Na/Cl ratio (mol) is a hydrogeochemical parameter characterizing the degree of Na+ enrichment in groundwater that can be used to reflect the degree of ion exchange (Han et al., 2014; Taheri et al., 2017; Xing et al., 2013; Yang et al., 2016). Huaihe River Basin is an arid- semi-arid region with strong evaporation, which leads to the accumulation of halite in the sedimentary layer. The dissolution of halite is one of the sources of Na+ and Cl- in groundwater in basin regions. If the dissolution of halite is the main source of Na+ and Cl-, the ratio of Na/Cl (mol)- should be 1:1, and Na+ above this ratio may have other sources. In this study, the Na/Cl ratios of groundwater samples collected in the entire region were 9.63±57.4 and those of most samples were substantially larger than 1:1, showing significant spatial variability. The Na/Cl ratio decreased with the increase of Cl concentration. The Na/Cl ratios of contaminated groundwater (As≥10 μg L-1) were 15.7±16.0, above the dissolution line of halite (Fig. 5). It can therefore be inferred that the Na+ of groundwater in the study area is not derived only from the dissolution of halite. The groundwater may experience overall strong cation exchange and the ion exchange of high-arsenic groundwater is more significant.
4.4 Source of arsenic and its mobilization
Under the pH and Eh conditions of the natural environment, arsenic exists mainly as As (Ⅴ) in an inorganic oxidation state or As (Ⅲ) in a reduction state. Arsenic minerals in sediments usually exist in mineral phases such as arsenate, arsenite, and sulfide. There are many possible hydrogeochemical factors that trigger the release of arsenic from the solid phase into the groundwater. Changes in groundwater regime, redox potential (Eh), acidity, and alkalinity (pH) exert an influence on arsenic in sediments, through the adsorption and resolution process and then affect the concentration of As in water (Chen et al, 2017; Wang et al., 2015; Duan et al., 2017; Gao et al., 2020b;Gao et al., 2010; Elizabeth et al., 2019; Huang et al., 2012; Taheri et al., 2017; Zhang et al., 2017).
Alkalinity is a chemical measurement of a water’s ability to neutralize acids. Alkalinity is also a measure of a water’s buffering capacity or its ability to resist changes in pH upon the addition of acids or bases (Rice et al., 2012). The alkalinity in natural water mainly depends on the presence of bicarbonate (HCO3-), carbonate (CO3-) and hydroxide (OH-). The total alkalinity of the test samples was 515±169 mg L-1, the total acidity was 20.0±4.63 mg L-1, and the groundwater was alkaline. According to the law of carbonate balance, when the pH value is 4.5–10, the HCO3- alkalinity occurs. When the pH value is ≤8.32, all CO32- is converted to HCO3- (Chen et al., 2009; Yang et al., 2016; Zhu et al., 2011). The total alkalinity of the test samples had a highly significant positive correlation with the concentration of HCO3-, with a correlation coefficient R=0.997 (P ≤ 0.01). Therefore, the total alkalinity in the water samples is HCO3-alkalinity and it generally reflects the content of HCO3-. The total alkalinity of high-arsenic groundwater was mainly ranged between 400–700 mg L-1. The weathering of carbonate mineral and ion exchange reactions in the study area increased the alkalinity of the groundwater.
The SO42- in the groundwater could be derived from both gypsum dissolution and sulfide oxidation and there was a positive correlation between As and SO42- contents in the test samples (correlation coefficient R=0.58) (Fig. 6). The mean concentrations of SO42- in the groundwater with As < 3, 3 ≤ As < 5, 5 ≤ As < 10 and As ≥ 10 μg L-1 in the analytical samples were 0.74, 1.09, 0.92, and 0.93 mmol L-1, respectively, and the high-arsenic groundwater showed a relatively high SO42- concentration. The SO42-/Ca2+ (mol) ratio of groundwater in the entire region was 0.76. SO42- in the groundwater originated not only from the dissolution of gypsum minerals, but also from the oxidation of sulfide.
According to the phase analysis by X-ray diffraction, the main mineral components of the sediments in Huaihe River Basin are quartz, potash feldspar, calcite and clay minerals, with the contents of 47.1%, 3.79%, 8.27% and 33.4%, respectively. There are a small amount of pyrite and siderite in some samples, the contents are 2.5% and 47.1% respectively. No hematite is detected. Under reduction conditions, Arsenic sulfide is a stable host of arsenic, and its associated arsenic is highly correlated with the occurrence of groundwater arsenic (Duan et al., 2017; Elizabeth et al., 2019; Hu et al., 2015; Shahid et al., 2018; Taheri et al., 2017; Zhang et al., 2017). Therefore, it is speculated that arsenic in sediments from Huaihe River Basin may exist as arsenic-bearing sulfides phase under reduction conditions.
Due to the long-term exploitation of groundwater in large quantities, the environment of the groundwater flow system has changed, breaking the equilibrium of dynamic exchange between the solid and liquid phases of the aquifers, and triggering the release of arsenic from the solid phase into the groundwater. The dissolution of carbonate minerals usually increases the alkalinity (pH). Under high pH conditions, the oxidation of arsenic-containing sulfide leads to the release of arsenic and sulfur into the groundwater, promoting concentration of arsenic and SO42-. The increase in pH also promotes the desorption of arsenic from iron manganese oxides, thereby increasing the concentration of As in groundwater (Duan et al., 2017; Elizabeth et al., 2019; Taheri et al., 2017; Zhang et al., 2017).