3.3.1. Analysis Based on Gibbs Diagram

3.3.1. Analysis Based on Gibbs Diagram The Gibbs diagram can macroscopically show the main ions and their changing trend in groundwater, judge the hydrochemical formation mechanism, and divide the controlling factors into atmospheric precipitation, evaporation concentration, and rock weathering [37]. The Gibbs diagram of the groundwater and Yellow River water in the study area is shown in Figure 12. It can be seen from the figure that the groundwater TDS in the study area is medium, and the γ(Na+)/γ(Na+ + Ca2+) values of groundwater are mostly between 0.4 and 0.8, which are jointly affected by water–rock interactions and evaporation concentrations. Among them, the Na+/(Na+ + Ca2+) ratio is more dispersed, indicating that the proportion of Na+ in different spatial positions is different. The sampling points on the left side are far away from the Yellow River and are mainly affected by rock weathering. The Na+ ratio is only 0.2. The groundwater sampling points on the right have an Na+ ratio ranging from 0.4 to 0.8. These sampling points are close to the Yellow River and are close to the γ(Na+)/γ(Na+ + Ca2+) value of the Yellow River water, indicating that the closer to the Yellow River, the shallower the groundwater depth, the weaker the rock weathering, and the stronger the evaporation. The Gibbs diagram can macroscopically show the main ions and their changing trend in groundwater, judge the hydrochemical formation mechanism, and divide the controlling factors into atmospheric precipitation, evaporation concentration, and rock weathering [37]. The Gibbs diagram of the groundwater and Yellow River water in the study area is shown in Figure 12. It can be seen from the figure that the groundwater TDS in the study area is medium, and the γ(Na<sup>+</sup> )/γ(Na<sup>+</sup> + Ca2+) values of groundwater are mostly between 0.4 and 0.8, which are jointly affected by water–rock interactions and evaporation concentrations. Among them, the Na+/(Na<sup>+</sup> + Ca2+) ratio is more dispersed, indicating that the proportion of Na<sup>+</sup> in different spatial positions is different. The sampling points on the left side are far away from the Yellow River and are mainly affected by rock weathering. The Na<sup>+</sup> ratio is only 0.2. The groundwater sampling points on the right have an Na<sup>+</sup> ratio ranging from 0.4 to 0.8. These sampling points are close to the Yellow River and are close to the γ(Na<sup>+</sup> )/γ(Na<sup>+</sup> + Ca2+) value of the Yellow River water, indicating that the closer to the Yellow River, the shallower the groundwater depth, the weaker the rock weathering, and the stronger the evaporation.

*Water* **2022**, *14*, x FOR PEER REVIEW 13 of 17

**Figure 12.** Gibbs map of groundwater in the study area. **Figure 12.** Gibbs map of groundwater in the study area.

#### 3.3.2. Analysis Based on the Ion Proportion Coefficient 3.3.2. Analysis Based on the Ion Proportion Coefficient 3.3.2. Analysis Based on the Ion Proportion Coefficient

The ion proportional coefficient method can be used to determine the origin of groundwater and the source or formation process of groundwater chemical composition by the milligram equivalent proportional coefficient of different ions [38]. The ion proportional coefficient method can be used to determine the origin of groundwater and the source or formation process of groundwater chemical composition by the milligram equivalent proportional coefficient of different ions [38]. The ion proportional coefficient method can be used to determine the origin of groundwater and the source or formation process of groundwater chemical composition by the milligram equivalent proportional coefficient of different ions [38]. The coefficient of γ(Na+)/γ(Cl−) can characterize the enrichment degree of Na+ in

The coefficient of γ(Na+)/γ(Cl−) can characterize the enrichment degree of Na+ in groundwater, which is a sign of salt leaching and the accumulation intensity of groundwater. Cl- in natural groundwater often comes from salt rock. The γ(Na+)/γ(Cl−) values of groundwater in the study area are almost all greater than one (Figure 13), indicating that Na+ has other major sources in addition to salt rock. Different from the groundwater affected only by evaporation, the γ(Na+)/γ(Cl−) value of groundwater in the study area does not remain constant but increases with an increasing Cl− concentration, indicating that the groundwater in the study area is jointly controlled by the evaporation concentration and the water–rock interaction. The coefficient of γ(HCO3−)/γ(Cl−) can reflect the hydrogeochemical process of anions in groundwater along the runoff path (Figure 13). The ratio of the two is above the 1:1 isoline, indicating that calcite, dolomite, and other minerals are dissolved in the study area. The coefficient of γ(Na<sup>+</sup> )/γ(Cl−) can characterize the enrichment degree of Na<sup>+</sup> in groundwater, which is a sign of salt leaching and the accumulation intensity of groundwater. Clin natural groundwater often comes from salt rock. The γ(Na<sup>+</sup> )/γ(Cl−) values of groundwater in the study area are almost all greater than one (Figure 13), indicating that Na+ has other major sources in addition to salt rock. Different from the groundwater affected only by evaporation, the γ(Na<sup>+</sup> )/γ(Cl−) value of groundwater in the study area does not remain constant but increases with an increasing Cl− concentration, indicating that the groundwater in the study area is jointly controlled by the evaporation concentration and the water–rock interaction. The coefficient of γ(HCO<sup>3</sup> −)/γ(Cl−) can reflect the hydrogeochemical process of anions in groundwater along the runoff path (Figure 13). The ratio of the two is above the 1:1 isoline, indicating that calcite, dolomite, and other minerals are dissolved in the study area. groundwater, which is a sign of salt leaching and the accumulation intensity of groundwater. Cl- in natural groundwater often comes from salt rock. The γ(Na+)/γ(Cl−) values of groundwater in the study area are almost all greater than one (Figure 13), indicating that Na+ has other major sources in addition to salt rock. Different from the groundwater affected only by evaporation, the γ(Na+)/γ(Cl−) value of groundwater in the study area does not remain constant but increases with an increasing Cl− concentration, indicating that the groundwater in the study area is jointly controlled by the evaporation concentration and the water–rock interaction. The coefficient of γ(HCO3−)/γ(Cl−) can reflect the hydrogeochemical process of anions in groundwater along the runoff path (Figure 13). The ratio of the two is above the 1:1 isoline, indicating that calcite, dolomite, and other minerals are dissolved in the study area.

**Figure 13.** γ(Na+)/γ(Cl−) and γ(HCO3−)/γ(Cl−). **Figure 13.** γ(Na+)/γ(Cl−) and γ(HCO3−)/γ(Cl− **Figure 13.** γ(Na ). <sup>+</sup> )/γ(Cl−) and γ(HCO<sup>3</sup> −)/γ(Cl−).

The γ(Ca2+ + Mg2+)/γ(HCO<sup>3</sup> −) ratio can reflect the dissolution characteristics of carbonate rocks in groundwater (Figure 14). Most of the samples are located below the 1:1 isoline, indicating that the ratio of Ca2+ to Mg2+ to HCO<sup>3</sup> − is slightly lower than one based on the dissolution of carbonate rocks, which may result in cation exchange reactions with Na<sup>+</sup> . Sample points are distributed in the carbonate dissolution area, indicating that carbonate in groundwater is widely involved in the dissolution of carbonate minerals (Figure 15). The γ(Ca2+ + Mg2+)/γ(HCO3−) ratio can reflect the dissolution characteristics of carbonate rocks in groundwater (Figure 14). Most of the samples are located below the 1:1 isoline, indicating that the ratio of Ca2+ to Mg2+ to HCO3− is slightly lower than one based on the dissolution of carbonate rocks, which may result in cation exchange reactions with Na+. Sample points are distributed in the carbonate dissolution area, indicating that carbonate in groundwater is widely involved in the dissolution of carbonate minerals (Figure 15). bonate rocks in groundwater (Figure 14). Most of the samples are located below the 1:1 isoline, indicating that the ratio of Ca2+ to Mg2+ to HCO3− is slightly lower than one based on the dissolution of carbonate rocks, which may result in cation exchange reactions with Na+. Sample points are distributed in the carbonate dissolution area, indicating that carbonate in groundwater is widely involved in the dissolution of carbonate minerals (Figure 15).

The γ(Ca2+ + Mg2+)/γ(HCO3−) ratio can reflect the dissolution characteristics of car-

*Water* **2022**, *14*, x FOR PEER REVIEW 14 of 17

*Water* **2022**, *14*, x FOR PEER REVIEW 14 of 17

**Figure 14.** γ(Ca2+ + Mg2+)/γ(HCO3−). **Figure 14.** γ(Ca2+ + Mg2+)/γ(HCO<sup>3</sup> −). **Figure 14.** γ(Ca2+ + Mg2+)/γ(HCO3−).

**Figure 15.** γ(Ca2+ + Mg2+)/γ(HCO3−) and γ(SO42−)/γ (HCO3−). **Figure 15.** γ(Ca2+ + Mg2+)/γ(HCO<sup>3</sup> −) and γ(SO<sup>4</sup> <sup>2</sup>−)/γ (HCO<sup>3</sup> −).

**Figure 15.** γ(Ca2+ + Mg2+)/γ(HCO3−) and γ(SO42−)/γ (HCO3−). 3.3.3. Analysis of Excessive Arsenic in Groundwater 3.3.3. Analysis of Excessive Arsenic in Groundwater

3.3.3. Analysis of Excessive Arsenic in Groundwater High-arsenic groundwater is generally distributed in alluvial plains and closed basins rich in organic matter. According to the data, the average As content in the Yellow River water is 0.00275 mg/L, which is not enough to cause excessive As in groundwater High-arsenic groundwater is generally distributed in alluvial plains and closed basins rich in organic matter. According to the data, the average As content in the Yellow River water is 0.00275 mg/L, which is not enough to cause excessive As in groundwater High-arsenic groundwater is generally distributed in alluvial plains and closed basins rich in organic matter. According to the data, the average As content in the Yellow River water is 0.00275 mg/L, which is not enough to cause excessive As in groundwater (content >0.01 mg/L). There are many fish ponds in the study area, but the migration ability of arsenic in the sediment and surface soil of fish ponds is weak. The water source

protection area is along the Yellow River, and there are no other pollution sources in the area. The possibility of groundwater arsenic exceeding the standard is very small. The sedimentary environment of sediment interbedded structures in the Yellow River alluvial plain has strong reducibility, and arsenic-rich ferromanganese oxides or hydroxides in primary sedimentary strata are released into groundwater due to reducibility dissolution. At the same time, the groundwater runoff is not smooth, and strong cation exchange occurs. The evaporation and concentration of groundwater aggravates the enrichment of arsenic. In addition, in recent years, water level changes caused by agricultural irrigation and water source replacement in the Yellow River region are potential factors that cause arsenic concentration changes.

#### 3.3.4. Analysis of Excessive Three Nitrogen in Groundwater

There are three main sources of "three nitrogen" pollution in groundwater: agriculture and human activities, including human and livestock manure, fertilizers, pesticide use, and sewage irrigation; urban industry, including wastewater, waste gas, and solid waste caused by chemical fuel combustion; and the random emission of "three wastes" of life. Under suitable natural geological conditions, these nitrogen-containing substances infiltrate into the groundwater through the soil and the vadose zone and accumulate continuously, resulting in an increasingly serious groundwater "three nitrogen" pollution. There is less development of industry in the right bank of the middle and lower reaches of the Yellow River and more development of fish pond aquaculture and other industries. The excess of "three nitrogen" in groundwater may be related to aquaculture and human domestic wastewater discharge.

#### **4. Conclusions**


water. At present, domestic pond aquaculture basically discharges wastewater without treatment. Nitrite, ammonia nitrogen, and organic nitrogen in domestic sewage, domestic garbage and other discharges enter the groundwater through discharge, leaching, and other channels, resulting in groundwater "three nitrogen" exceeding the standard.

**Author Contributions:** Conceptualization, X.T., H.T. and X.H.; Formal analysis, X.T.; Funding acquisition, R.G.; Investigation, H.T.; Methodology, X.T., H.T. and X.H.; Resources, H.T.; Software, X.T.; Supervision, R.G.; Validation, X.H.; Writing—original draft, X.T.; Writing—review and editing, R.G., Z.L. and S.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was financially supported by the National Natural Science Foundation of China (NSFC grant NO. 41402221), China Postdoctoral Science Foundation (NO. 2013M540999) and the Science and Technology Project on Water Conservancy of Henan Province (GG202026).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** We would like to extend special thanks to the editor and reviewers for insightful advice and comments on the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had role in the writing of the manuscript, or in the decision to publish the results.

#### **References**

