*3.1. Hydrochemical characteristics of Groundwater*

The statistical results of water quality for groundwater samples are given in Table 3. pH is one of the most important parameters for evaluating the suitability of drinking water [60]. The Chinese national standard proposes that the pH value of groundwater suitable for drinking is 6.5–8.5 [59]. As Table 3 shows, pH values of the groundwater range from 7.27 to 7.85, with a mean value of 7.59. Therefore, the groundwater in the study area is weakly alkaline water that can be used for drinking.

TH represents dissolved Ca2+ and Mg2+ in groundwater. High TH in groundwater may affect the taste of drinking water and reduce the efficacy of detergents [34]. In addition, regarding human health, the long-term drinking of extremely hard water may increase the incidence of urolithiasis, anencephaly, prenatal mortality, and some cancer-related cardiovascular diseases [68]. In this study, TH varies between 167 and 869 mg/L with a mean of 426 mg/L. According to the national Chinese drinking water standards, samples S1, S9, and S10 are extremely hard water, with TH exceeding the acceptable limit of 450 mg/L for drinking. These samples are predominantly distributed in the southern part of the study area (Figure 2a). TH enrichment in groundwater may be due to the dissolution of soluble salts and minerals, as well as to human intervention [2].

TDS is one of the major water quality parameters, mainly representing the various minerals present in the water [6]. TDS varies in a wide range of 280–1312 mg/L, with a mean value of 689 mg/L (Table 3). Based on TDS content, Liu et al. [69] categorized waters as freshwater (TDS < 1000 mg/L) and brackish water (TDS > 1000 mg/L). Only sample S10 in Yicheng is brackish water (Figure 2b). Generally speaking, higher TDS usually indicates stronger water-rock interaction and may also be affected by domestic wastewater, irrigation return flow, and fertilization [1,70]. High TDS in groundwater is generally harmless in healthy people and may cause constipation or have a laxative effect, but it may have a greater impact on people with kidney and heart disease [6,33,71].

Cl<sup>−</sup> and SO<sup>4</sup> <sup>2</sup><sup>−</sup> in groundwater are mainly related to the regional lithological conditions and are also affected by anthropogenic sources [68]. The concentration of Cl− is between 7.93 and 88.1 mg/L and is lower than the Chinese national standard of 250 mg/L. The concentration of SO<sup>4</sup> <sup>2</sup><sup>−</sup> in the study area ranged from 68 to 536 mg/L, with a mean of 182.16 mg/L. Samples S5 and S10 exceeded the acceptable limit of SO<sup>4</sup> <sup>2</sup><sup>−</sup> for drinking. High SO<sup>4</sup> <sup>2</sup><sup>−</sup> concentration is observed in the Yaodu and Yicheng parts of the central and south of the study area (Figure 2c). The Ordovician karst aquifers widely distributed in the study area are affected by gypsum dissolution, and the hydrochemical type of groundwater is SO<sup>4</sup> <sup>2</sup>−·HCO<sup>3</sup> <sup>−</sup>-Ca·Mg. In addition, the oxidation of sulfur in coal-bearing strata (S + O<sup>2</sup> + 2H2O→SO<sup>4</sup> <sup>2</sup><sup>−</sup> + 4H<sup>+</sup> ) will also cause increased sulfate concentration in groundwater [72]. Therefore, the high mean value of SO<sup>4</sup> <sup>2</sup><sup>−</sup> in this study is probably due to the high natural background value rather than pollution.

F − in drinking water is essential for human health at low concentrations, such as protecting teeth from caries [2]. However, excessive fluoride intake can cause dental fluorosis, skeletal fluorosis, and thyroid disease in adults [17,73]. The Chinese national standard stipulates that F− concentration in drinking water should be less than 1.0 mg/L. In this study, F− is in the range of 0.25–1.71 mg/L, with an average value of 0.75 mg/L. Two groundwater samples in Yaodu did not meet the requirement of the national standard (Figure 2d). The high concentration of fluoride in groundwater may be mainly related to the lithology of the region, especially the dissolution of fluoride-bearing minerals [16,74].

Both cyanide and volatile phenol are toxic organics. The concentration of cyanide in all groundwater samples is less than 0.0004 mg/L. For volatile phenols, except for sample S10 in Yicheng, whose value is 0.002 mg/L, the other samples are 0.0003 mg/L. CODMn is an indicator that can indirectly reflect the organic pollution of groundwater [44,74]. The CODMn values for the samples are observed to be from 0.1 to 0.9 mg/L, with an average of 0.25 mg/L. Sample S10 in Yicheng has the highest volatile phenol and CODMn values. As shown in Table 3, the concentrations of cyanide, volatile phenol, and CODMn are all within the drinking water standard limit stipulated by the national standard, indicating that the groundwater is less affected by organic pollution.


**Table 3.** Statistical analysis results for hydrochemical parameters of groundwater.

<sup>a</sup> percentage of the sample exceeding the permissible limits. Units for all parameters are in mg/L, except for pH (non-dimensional).

**Figure 2.** Spatial distributions of mass concentrations of groundwater hydrochemical parameters: (**a**) TH, (**b**) TDS, (**c**) SO42<sup>−</sup>, (**d**) F<sup>−</sup>, (**e**) NO3-N, (**f**) NO2-N, (**g**) NH4-N, (**h**) Fe, (**i**) Mn, (**j**) Hg, (**k**) As, (**l**) Cd, (**m**) Cr6+, and (**n**) Pb. **Figure 2.** Spatial distributions of mass concentrations of groundwater hydrochemical parameters: (**a**) TH, (**b**) TDS, (**c**) SO<sup>4</sup> <sup>2</sup>−, (**d**) F−, (**e**) NO<sup>3</sup> -N, (**f**) NO<sup>2</sup> -N, (**g**) NH<sup>4</sup> -N, (**h**) Fe, (**i**) Mn, (**j**) Hg, (**k**) As, (**l**) Cd, (**m**) Cr6+, and (**n**) Pb.

F− in drinking water is essential for human health at low concentrations, such as protecting teeth from caries [2]. However, excessive fluoride intake can cause dental fluorosis, skeletal fluorosis, and thyroid disease in adults [17,73]. The Chinese national standard stipulates that F− concentration in drinking water should be less than 1.0 mg/L. In this study, F− is in the range of 0.25–1.71 mg/L, with an average value of 0.75 mg/L. Two In recent years, nitrogen pollution (NO3-N, NO2-N, and NH4-N) has become a hot issue for many researchers due to its adverse effects on groundwater quality and human health [2,12,14,49,74–76]. The extensive use of nitrogenous fertilizers in agricultural activities is one of the most common sources of nitrogen pollution in groundwater [1,63]. Measured values of NO3-N, NO2-N, and NH4-N are in the range of 0.002–11.3, 0.004–0.7 and 0.025–0.16 mg/L, respectively. Higher NO3-N and NO2-N concentrations are observed in the central and southwest parts of the area, while a high value of NH4-N is mainly distributed around Yicheng (Figure 2e–g). According to the Chinese standards, groundwater is unacceptable for drinking when the NO3-N, NO2-N, and NH4-N concentration in

groundwater is higher than 20, 1, and 0.5 mg/L, respectively. Therefore, the groundwater in the study area is less contaminated with nitrogen and is suitable for drinking.

PTEs content in groundwater is usually low. However, even in very low concentrations, -they can create biological toxicity and pose serious threats to aquatic ecosystems and human health [20,21,41]. As shown in Table 3, the Fe, Mn, Hg, and Cr6+ concentrations range from 0.03 to1.41, 0.01–0.139, 0.00001–0.00006, and 0.004–0.034 mg/L, respectively. The concentrations of As, Cd, and Pb are 0.0002, 0.002, and 0.011 mg/L, respectively. The mean concentration of metals is in the following order: Fe > Mn > Pb > Cr6+ > Cd > As >Hg. All metals, except for Fe, Mn and Pb, are within the permissible levels for drinking water. Samples with high concentrations of Fe and Mn are mainly found in the southeastern parts of the basin (Figure 2h,i). Fe and Mn have similar geochemical behavior. Their dissolution and migration to groundwater are affected by reduction conditions, residence time, well depth, and salinity [77]. The similarity in the spatial distribution of Cr6+ and NO<sup>3</sup> −-N concentrations may be related to the synergistic role of nitrogen (N)-bearing fertilizers to elevated Cr6+ concentration in groundwater. This may be due to the production of H<sup>+</sup> and soil acidification during the nitrification process of NH4+ oxidation to NO<sup>3</sup> −, favoring the increased dissolution of Cr3+ which is subsequently oxidized into Cr6+ by natural and/or anthropogenic factors [46,47].
