**Table 3.** List of mercury speciation content and its percentage in subsurface flow zone in the dry season.




season.

**To tal** 

**/kg** 

**Water Solub le State** 

**mg/k**

**Different Hydrody namic Excitatio n** 

Low water level infiltratio n test

High water level infiltratio n test

**Sample** 

**No. pH Moisture** 

**Content** 

% **mg**

dd-1 8.56 8.67 1.0

dd-2 8.82 16.52 0.4

dd-3 8.49 11.39 4.1

dd-4 8.89 3.24 0.3

gd-1 8.68 8.3 0.9

gd-2 8.82 12.14 0.1

gd-3 8.35 17.26 25.

and the interface flow increases in this area.

not obvious. The original value (the mercury content after the water seepage test) is lower than the leaching amount of mercury under low water seepage conditions. The authors believe that the reason is the structure and hydrogeological characteristics of the flow sediments strongly influence the surface and groundwater exchange. Nevertheless, the silt and tailings sand are deposited on the riverbed; in addition, the silts bed has very low permeability and porosity, therefore it also reduces the exchange of undercurrent, and the low permeability rate promotes leaching and water-rocking, and a huge amount of mercury ions and their complexed forms are dissolved out. The inhomogeneous structure of the riverbed also creates effective anisotropy, and it causes restriction of the penetration of solutes, especially in the vertical direction [55]. Salehin et al. [56] conducted a series of experiments on the river-groundwater exchange, and constructed a two-dimensional inhomogeneous riverbed structure by artificial methods. Consequent-

**Figure 5.** Line chart of potential ecological risks of mercury in the subsurface under changing river water levels. (**a**)—The line chart of the ecological risk of mercury in the subsurface under the condition of changing river water level in winter; (**b**)—The line chart of the ecological risk of mercury in the subsurface under the condition of changing the river water level in winter. **Figure 5.** Line chart of potential ecological risks of mercury in the subsurface under changing river water levels. (**a**)—The line chart of the ecological risk of mercury in the subsurface under the condition of changing river water level in winter; (**b**)—The line chart of the ecological risk of mercury in the subsurface under the condition of changing the river water level in winter.

**Table 3.** List of mercury speciation content and its percentage in subsurface flow zone in the dry **The Hg Speciation Content The Percentage of Hg Speciation Content in the Total Amount Ion Exchan ge State Carbon ate State Humic Acid State Hum ic Acid State Iron-Ma nganese Oxidatio n State Stron g Organ ic State Resid ue State Water Solub le State Ion Exchan ge State Carbon ate State Humic Acid State Hum ic Acid State Iron Mangan ese Oxidatio n State Stron g Organ ic State Resid ue State g mg/kg mg/kg mg/k <sup>g</sup>mg/kg mg/kg mg/kg % % % % % % %**  <sup>38</sup>0.001 0.002 0.005 0.010 0.008 0.003 1.009 0.09 0.19 0.50 0.92 0.78 0.29 97.24 <sup>23</sup>0.001 0.001 0.002 0.005 0.004 0.003 0.407 0.20 0.25 0.56 1.24 0.98 0.60 96.17 <sup>06</sup>0.008 0.005 0.008 0.245 0.086 0.562 3.194 0.19 0.11 0.18 5.96 2.08 13.69 77.78 (3) The convective exchange law of mercury content caused by the pressure gradient in the same layer is reflected in the fact that 0–40 cm does not change significantly after leaching due to the low original content, while the leaching amount of mercury (the amount of mercury leaching under the condition of high-water level seepage at 40 cm) is not obvious. The original value (the mercury content after the water seepage test) is lower than the leaching amount of mercury under low water seepage conditions. The authors believe that the reason is the structure and hydrogeological characteristics of the flow sediments strongly influence the surface and groundwater exchange. Nevertheless, the silt and tailings sand are deposited on the riverbed; in addition, the silts bed has very low permeability and porosity, therefore it also reduces the exchange of undercurrent, and the low permeability rate promotes leaching and water-rocking, and a huge amount of mercury ions and their complexed forms are dissolved out. The inhomogeneous structure of the riverbed also creates effective anisotropy, and it causes restriction of the penetration of solutes, especially in the vertical direction [55]. Salehin et al. [56] conducted a series of experiments on the river-groundwater exchange, and constructed a two-dimensional inhomogeneous riverbed structure by artificial methods. Consequently, he discovered that the water flow prefers to pass through the high permeability area, and the interface flow increases in this area.

### <sup>55</sup>0.010 0.001 0.001 0.054 0.045 0.226 0.018 2.81 0.32 0.37 15.09 12.63 63.82 4.95 *5.2. Potential Ecological Risks*

<sup>23</sup>0.001 0.001 0.006 0.003 0.003 0.004 0.904 0.13 0.14 0.67 0.34 0.28 0.44 97.99 <sup>52</sup>0.001 0.001 0.002 0.002 0.002 0.003 0.141 0.39 0.67 1.18 1.23 1.28 2.22 93.02 <sup>22</sup> 0.014 0.020 0.018 0.024 0.143 0.376 24.626 0.05 0.08 0.07 0.10 0.57 1.49 97.64 During the history of gold mining areas, there was disorderly and random mining. Consequently, mercury rollers which is a beneficiation process liable to cause mercury pollution are used for illegal gold extraction in the mining valleys in the mountains from the 1990s to around 2010, thus the wastewater was directly discharged into the Shuangqiao River and other rivers with the mine water. Indisputably, it leads to serious ecological problems in rivers. Xu et. al., [57], comparatively investigated four types of soil and water pollution in the study area: atmospheric deposition, river irrigation, mining sewage irrigation, and tailings slag leaching. They summarized that the largest risk is river irrigation type, followed by tailings leaching type [58], and heavy metal elements in river sediments are homologous to those in tailings. As of Table 5 and Figure 5, it can be realized that the potential ecological risk of mercury in the disjointed Hyporheic zone is much higher than the extreme ecological hazard with a threshold (320), regardless of whether the hydraulic connection is in winter or summer, and the maximum risk of mercury in the original soil layer in winter and summer reaches 25,005.5 and 13,886.48, respectively, 78.14 and 43.40 times of the extreme ecological hazard threshold. Likewise, the pollution risk of 30–50 cm sand and pebble are the greatest, while the ecological risk of 0–30 cm silt layer

and waste residue layer is relatively small. After leaching, the potential ecological risk of mercury in most soil layers in the Hyporheic zone has been reduced, especially after leaching at a constant water level. On the other hand, the risk value of the sand and pebble layer in winter has dropped from the original 25,005.5 to 5133, and the risk value of the sand and pebble layer in winter has been reduced from the original. Moreover, 13,886.48 dropped to 1656.07, which means that mercury entered the groundwater body. When a flood occurs (within 30 min), the mercury content in the surface layer decreases rapidly, attributable to many influencing factors described above, which produce an increase in the sand and pebble layer below (changed to 31,525); in addition, the potential ecological risk of mercury in the soil layer under normal water level leaching decreased, increasing the ecological risk of groundwater.

**Table 5.** The potential ecological risk value of mercury in the subsurface zone under the condition of changing river water level.

