**1. Introduction**

The hyporheic zone is an important transitional area for the dynamic interaction of surface water and groundwater. Its physical properties and biogeochemical environment jointly carry the changing process of water flow, material, and energy exchange.

The hyporheic zone (HZ) is an essential member of a river ecosystem which can be saturated with interstitial sediment beneath the streambed and close to the riverbed (Figure 1a,b [1–3]). It is the zone of fraternization, exchange, and interaction between groundwater and surface water. The surface water and groundwater interacted and combine to create a new interstitial or transitional zone of water with different properties.

**Citation:** Liu, R.; Liu, F.; Jiao, J.; Xu, Y.; Dong, Y.; R.M., E.-W.; Zhang, X.; Chen, H. Potential Toxic Impacts of Hg Migration in the Disjointed Hyporheic Zone in the Gold Mining Area Experiencing River Water Level Changes. *Water* **2022**, *14*, 2950. https://doi.org/ 10.3390/w14192950

Academic Editors: Peiyue Li, Jianhua Wu and Saurabh Shukla

Received: 31 August 2022 Accepted: 15 September 2022 Published: 21 September 2022

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The HZ can be divided into two zones, the "surface hyporheic" zone, and the "interactive hyporheic" zone. The new characteristics of the hyporheic zone are dynamic zone with hydraulic pressure at numerous scales (catchment scale and smaller scale) [4–8]. It has a higher concentrations of chemicals with low oxygen content, special hydrological processes, and ecological systems. Many hydrogeologists consider the hyporheic zone as part of the groundwater system, since it contains subsurface water in the saturated zone. The physiochemical characteristics of water flow in the subsurface are the main key to material and energy exchange between surface water and groundwater [9]. Due to the differences in physiochemical characteristics such as redox, pH, and temperature between river water and groundwater, drives variation in biological, chemical, and even temperature gradients between surface and groundwater.

Heavy metals are the extremely distributed deposits in the soil sediment and water nearby gold mining [10–12]. Heavy metals are classified into essential and nonessential metals. Heavy metals sources are categorized as natural sources such as ore deposits, bedrock weathering, geological weathering, atmospheric precipitation, storms, and wind bioturbation, whereas anthropogenic sources such as agrochemicals activities, mining processing, shipping, industrial products, manufacturing development and process (e.g., batteries and pharmaceutical products), smelting, fuel generation, electroplating, sludge discharge, energy transmission, dense urban areas, wastewater irrigation and [13–15]. Globally, the water discharges from the mining of ores, heaps leaching, milling and ore processes to liberate the metal and remove the waste are predominantly sources of metals in soil, surface, and groundwater pollution around these mine areas. Mine waters are typically rich in metals and elements such as iron, copper, manganese, arsenic, mercury, lead, and zinc [16–19]. By water-rock interaction, the heavy metals move to water in order to sediment and can accumulate in water or soil by bioaccumulation processes, which may enter the food chain and create a health risk. Consequently, detecting the processes of surface and groundwater interface is highly significant to evaluate and assume the behavior and consequences of mine pollution. The main four attributes which controlled the element behavior in certain heavy metals are the valence state, compound state, binding state, and structural state [20]. In addition to the stability of the element in the soil organized the occurrence form of this element.

Mercury is one of the most critical pollutions and ranked the third most toxic elements by the US Government Agency for Toxic Substances and Disease Registry [21–23]. As well as this, it is one of the main pollutant structures in the gold mining areas [24,25]. It can reach the human body through food and water which can act naturally by ore rock interaction or by human activities (by using dental amalgam, batteries, thermometers, barometers, and medical waste) in modern life. Mercury (inorganic or as organic) can bioaccumulate in soil and water and cause bad effects on human health [26–28].

When mercury enters the soil, various forms will be redistributed among the soil solid phases. MA et al. [29] revealed that there are significant differences in the mass concentrations of dissolved mercury (DHg) and total methyl mercury (TMeHg) in the stratum water samples in the water storage period (September to October), submerged period (November to December), water withdrawal period (February to March) and drying period (May to June). Changes in the surface water level will cause the content of heavy metals and the occurrence of environmental changes [30,31]. Wang [32] studied the change in mercury under laboratory conditions, he concluded that the content in different forms under dry and wet environments after flooding, shows the mercury Migration ability is: acid-soluble mercury > inert mercury > Alkali-soluble mercury > water-soluble mercury; and under the condition of drying, inert mercury > alkali-soluble mercury > acid-soluble mercury > water-soluble mercury. On the other hand, Tang [33] studied the speciation distribution of mercury in wetland sediments with different water level gradients, and he noticed that the change in groundwater level is directly affected by the river water level during high and low water seasons, and the fluctuation of surface water level causes different changes in the content of water and polluting elements in the soil layer [34]. The

river water and groundwater level in the hyporheic zone in the wet season and dry season directly affect the risk of groundwater pollution [35,36], and the disconnected hyporheic zone is the bottom line for an early warning of regional pollution risks, so the development of disconnected river reaches. It is very important to study the vertical migration of the mercury information profile and its ecological risk.

Analysis of water, sediments, and members of indigenous biota can be used to determine the relative pollution of aquatic habitats by mercury and other heavy and trace metals. This analysis includes different monitoring techniques, and scientific problem usually include soil and water quality, hydrological processes composition of vegetation, and animal population. Previous studies mainly focus on reports on the potential ecological hazard assessment of heavy metal pollution in static water and soil environments [37,38]. The migration and transformation rules and ecological risks of mercury occurrence forms in the subsurface under the conditions of river water level changes in wet and dry periods are not well studied. The research topic provides a scientific basis for the prevention and control of groundwater pollution along the river.

Xiaoqinling gold-mining area is considered the third largest gold-producing region in China, and gold production in the area began about 900 years ago. In Mayu alone, there are 72 abandoned ancient mines, and production from 1980 to 2003 reached 562,500 t [39]. Furthermore, the large-scale exploitation of gold and mining activities began in 1975 [40]. Currently, more than 29 large gold mining companies have many gold mills, which are mainly in the Shuangqiao river. Based on the geochemical characteristics landscape of the region, five areas are included (Figure 1a,b): the prockbase mountain area, piedmont alluvial-pluvial inclined tableland, loess ravine tableland, Yellow river, Weihe and Shuangqiao river alluvial terrace [40]. Both agricultural activities and gold-mining activities were commonly affected in the study area. The more complicated structures include faults and folds in the north and south, the main regional faults are the Taiyao, Guanyintang, Xiaohe, and Huanchiyu. The structure system influences the heavy mineral distribution in the study area. Several gold mills are scattered along numerous streams which are tributaries of the Yellow river. Since mining pactivities during the 1980s to 1990s, artisanal gold in which elemental Hg has been used widely for gold processing [39]. Up to the present time, wastewater is still discharged directly into rivers and streams due to more than eight million tons of tailings having been produced with high-risk pollution of heavy metals for the soil and water. The main sources of heavy and trace elements especially Hg in the polluted soil were identified through the correlation analysis and comparing the maps of these heavy metal concentrations distribution and land-use types [41]

The Shuangqiao River in the Xiaoqinling gold mining area is a typical area of mercury pollution [42,43]. Xu et al. [44] studied mercury pollution in the sediment. The soil content is significantly higher than that of the three geochemical landscapes: the piedmont alluvialproluvial slope, the loess gully residual plateau, and the Huangwei River alluvial plain; the accumulation or pollution of heavy metals caused by mining activities becomes the soil Hg, Pb, Cd, Cu, and Zn in the study area. Owing to the chemical factor affecting the enrichment of heavy metals in soil [45], the gold mining activities are closely related to Hg in the Hyporheic zone of the study area, the authors studied the disjointed continuous type in dry season. The migration law and influencing factors of Hg morphological characteristics in the subsurface zone in the vertical soil layer [42].

**Figure 1.** (**a**) view of a river shows the position of the hyporheic zone. (**b**) the directional flow of water through the hyporheic zone. (after [2,3]), (**c**) location map of the study area. (**d**) The map of the sample and its landscape distribution after [9]. (**e**) Distribution of water seepage test.
