**4. Results**

*4.1. The Influence of the Occurrence Form and Content of Soil Vertical Hg in the Hyporheic Zone of the Alluvial-Proluvial Area in the High and Low Water Season*

It can be seen from Figure 2 that the stable environment of mercury in the undercurrent zone with pH between 8.39–8.87. The mercury forms in the wet and dry periods are mainly in the residual state, indicating that the source of Hg in the undercurrent zone of the gold mining area mainly is the mineral lattice [51], which may be implicated in mining wastewater into the undercurrent zone. The total mercury content (25.22 mg/kg) in the main enriched soil layers in the subsurface in winter was higher than that in summer (11.61 mg/kg). Mercury content displays a negative correlation with pH values. The diagrammatic cross-section in Figure 1e shows that the total amount of mercury in the undisturbed soil and the characteristics of the residue state in the vertical direction of the silt-sand-cobble-tailings-silt-sand-cobble alluvial layer, whether it was the wet season (summer) or the dry season (winter). Furthermore, the high value appears in both 0–10 cm silt containing tailings slag and 30–40 cm silt, however, the highest value is in 30–40 cm silt. In summer, the total and residual contents of mercury in 0–10 cm silt containing tailings were 1.48 mg/kg and 1.45 mg/kg, respectively; the total and residual mercury contents in 30–40 cm silt were 11.64 mg/kg and 10.00 mg/kg, respectively. *K*g. Nevertheless, in winter, the total and residual mercury contents in 0–10 cm silt containing tailings are 1.447 mg/kg and 1.445 mg/kg, respectively; the total mercury and residual mercury contents in 30–40 cm

sand and pebbles are 19.49 mg/kg and 20.63 mg/kg, to the 40–80 cm sand and pebble layer, this means the total mercury content decreased. *Water* **2022**, *14*, x FOR PEER REVIEW 8 of 16

> **Figure 2.** Occurrence forms of mercury in the soil layer of the vadose zone before the water seepage test was performed in the dry and wet periods. **Figure 2.** Occurrence forms of mercury in the soil layer of the vadose zone before the water seepage test was performed in the dry and wet periods.

### *4.2. The Influence of the Occurrence Form and Content of Vertical Hg in the Hyporheic Zone under Different Hydrodynamic Excitation Conditions during High and Low Water Seasons 4.2. The Influence of the Occurrence Form and Content of Vertical Hg in the Hyporheic Zone under Different Hydrodynamic Excitation Conditions during High and Low Water Seasons*

The morphological characteristics of mercury occurrence of the vertical Hg in the subsurface under high and low water level seepage conditions in wet and dry periods (As shown in Figure 3): the mercury content is still mainly in the residual state, and the highest value is in 30–40 cm silt. Furthermore, the mercury content in the stratum in winter is still higher than that in summer, as well as the total and residual mercury content under high-water seepage conditions is higher than the corresponding mercury content. Whereas the total and residual mercury content in wet 30–40 cm silt sand under high water level seepage conditions is 10.21 mg/kg and 9.91 mg/kg, respectively, while the total amount of mercury in 30–40 cm silt sand under low water level seepage conditions in the wet season and residual state were 1.65 mg/kg and 1.14 mg/kg, respectively. On the other hand, the total amount and residual state of mercury in 30–40 cm silt were The morphological characteristics of mercury occurrence of the vertical Hg in the subsurface under high and low water level seepage conditions in wet and dry periods (As shown in Figure 3): the mercury content is still mainly in the residual state, and the highest value is in 30–40 cm silt. Furthermore, the mercury content in the stratum in winter is still higher than that in summer, as well as the total and residual mercury content under high-water seepage conditions is higher than the corresponding mercury content. Whereas the total and residual mercury content in wet 30–40 cm silt sand under high water level seepage conditions is 10.21 mg/kg and 9.91 mg/kg, respectively, while the total amount of mercury in 30–40 cm silt sand under low water level seepage conditions in the wet season and residual state were 1.65 mg/kg and 1.14 mg/kg, respectively. On the other hand, the total amount and residual state of mercury in 30–40 cm silt were 13.02 mg/kg and 12.00 mg/kg, respectively, however, the total amount and residual state of mercury under the condition of high and low water level seepage in the dry season in the 30–40 cm silt under the conditions were 3.01 mg/kg and 2.01 mg/kg (as shown in Figure 4).

13.02 mg/kg and 12.00 mg/kg, respectively, however, the total amount and residual state of mercury under the condition of high and low water level seepage in the dry season in the 30–40 cm silt under the conditions were 3.01 mg/kg and 2.01 mg/kg (as shown in

Figure 4).

**Figure 3.** Characteristics of mercury transport in the subsurface under different hydrodynamic excitation conditions in wet and dry periods.

**Figure 4.** Characteristics of pH transport in the subsurface under different hydrodynamic excitation conditions in wet and dry periods. **Figure 4.** Characteristics of pH transport in the subsurface under different hydrodynamic excitation conditions in wet and dry periods.

### **5. Discussion 5. Discussion**

release [42].

### *5.1. Factors Affecting Mercury Migration and Transformation 5.1. Factors Affecting Mercury Migration and Transformation*

(1) The form of mercury in wet and dry seasons is mainly in the residual state, and the mercury content in the main enriched soil layer in the Hyporheic zone in winter (20.64 mg/kg) and higher than in summer (11.61 mg/kg). The temperature in the wet season is 29–36.9 °C, and the temperature in the dry season is 2–11.5 °C. Moreover, affected by temperature, soil mercury release flux shows that it is restricted by the atmospheric temperature with a positive correlation. The higher the atmospheric temperature, the greater the soil mercury release flux. Even air temperature affects mercury volatilization, but other factors also control soil volatilization, such as humidity, topographical (1) The form of mercury in wet and dry seasons is mainly in the residual state, and the mercury content in the main enriched soil layer in the Hyporheic zone in winter (20.64 mg/kg) and higher than in summer (11.61 mg/kg). The temperature in the wet season is 29–36.9 ◦C, and the temperature in the dry season is 2–11.5 ◦C. Moreover, affected by temperature, soil mercury release flux shows that it is restricted by the atmospheric temperature with a positive correlation. The higher the atmospheric temperature, the greater the soil mercury release flux. Even air temperature affects mercury volatilization, but other factors also control soil volatilization, such as humidity, topographical conditions, and meteorology [52].

conditions, and meteorology [52]. (2) Under the condition of water seepage, the mercury form changes regardless of the dry or the wet season, the various occurrence forms of 0–30 cm mercury in the vertical direction are decreasing, and still the main one. Nonetheless, in the sand and pebble layer, the main forms of mercury are diversified, including residue state, strong organic state, and humic acid state (see Tables 3 and 4). Currently, the pH of the layer with the largest change in morphological content also tends to be acidified (as shown in Figure 5). Acidic conditions may increase the content of available mercury in the soil, which leads to an increase in its bioavailability and enhanced migration ability. Adsorption-desorption has a great influence on the migration and transformation process. Occasionally, the influence mechanism of soil pH on the complexation-chelation, oxidation-reduction and methylation reactions in the process of mercury migration and transformation is reflected in ion competitive adsorption, valence state change, and methylation reaction, respectively. and promote the synthesis of factors [53]. The change in humic acid will promote the continuous decrease in water-soluble mercury, ex-(2) Under the condition of water seepage, the mercury form changes regardless of the dry or the wet season, the various occurrence forms of 0–30 cm mercury in the vertical direction are decreasing, and still the main one. Nonetheless, in the sand and pebble layer, the main forms of mercury are diversified, including residue state, strong organic state, and humic acid state (see Tables 3 and 4). Currently, the pH of the layer with the largest change in morphological content also tends to be acidified (as shown in Figure 5). Acidic conditions may increase the content of available mercury in the soil, which leads to an increase in its bioavailability and enhanced migration ability. Adsorption-desorption has a great influence on the migration and transformation process. Occasionally, the influence mechanism of soil pH on the complexation-chelation, oxidation-reduction and methylation reactions in the process of mercury migration and transformation is reflected in ion competitive adsorption, valence state change, and methylation reaction, respectively. and promote the synthesis of factors [53]. The change in humic acid will promote the continuous decrease in water-soluble mercury, exchangeable mercury, and acid-soluble mercury in soil with the increase in alkali-soluble mercury, organically bound mercury and residual mercury [54]. But to the sand pebble layer, this is because in the disturbed system, almost all forms of mercury have mercury release [42].

changeable mercury, and acid-soluble mercury in soil with the increase in alkali-soluble mercury, organically bound mercury and residual mercury [54] . But to the sand pebble layer, this is because in the disturbed system, almost all forms of mercury have mercury

(3) The convective exchange law of mercury content caused by the pressure gradi-

amount of mercury leaching under the condition of high-water level seepage at 40 cm) is

*Water* **2022**, *14*, 2950

