**6. Conclusions**

The form of mercury in the undercurrent zone where the river water-groundwater hydraulic connection is disconnected from the gold mining area is mainly in the residual state. Nonetheless, after the water content increases or the water level changes, creates variation in the occurrence forms of mercury are all decreasing, and the residual form is still the main form, whereas the main forms of mercury in the sand and pebble layer are diversified, but also including residual form, strong organic form, and humic acid form. This facilitates the entry of mercury into water bodies. The mercury content in the subsurface in winter is higher than that in summer. Relevant departments should monitor heavy metal pollution in groundwater and Hyporheic zones in different seasons.

Although the mercury content in groundwater has not been detected, the potential ecological risk of mercury in the disjointed Hyporheic zone near the river in the study area has reached an extreme ecological hazard. In a short 30-min constant water level leaching test, the potential risk of mercury to groundwater is higher. Furthermore, during the flood period, the long-term runoff of rivers at a constant water level is bound to pose a greater risk of groundwater pollution. Consequently, the following measures can be taken: (1) control the sources of heavy metals at source, (2) reduce the ecological risks of heavy metals to groundwater by physical chemistry, bioremediation and pesticide control in soil as soon as possible on the basis of river regulation.

**Author Contributions:** R.L., F.L. and J.J. wrote this manuscript. R.L. conceives the original idea of this study. E.-W.R.M. polished this manuscript. All authors (R.L., F.L., J.J., Y.X., Y.D., E.-W.R.M., X.Z. and H.C.) modified the manuscript. Authorship must be limited to those who have contributed substantially to the work reported. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the survey projects initiated by the Ministry of Natural and Resources of the People's Republic of China (DD20211317, DD20189220, 1212010741003,1212011220224, and 121201011000150022), the public welfare scientific research project launched by the Ministry of Natural and Resources of the People's Republic of China (201111020), the project of 2015 Natural Science Basic Research Program of Shaanxi (2015JM4129), and the project of 2016 Fundamental Research Funds for the Central Universities (open fund) (310829161128). Formation of extensional crenulation cleavage in the ductile of Xiaoqinlin Metamorphic Core Complex (2021KJXX-73), A research on intelligent sensing, coupling evolution and driving mechanism of mine geological environment The authors 'thanks also go to the experiment and testing lab of Xi'an Center, China Geological Survey for the statistical analysis of the data conducted.

**Conflicts of Interest:** The authors declare no conflict of interest.
