*3.1. In-Situ Test and Sample Analysis*

The principal three sampling techniques with a variety of methods that can be carried out for the HZ are in situ, coring, and tracing. The methods will be chosen according to the purpose of research.

In this paper, in-situ seepage tests were carried out. After the seepage tests, we collected soil samples and tested and analyzed the contents of soil (according to Figure 1e), groundwater, and surface water in HZ. Mercury is volatile and therefore has been tested in winter and summer. Based on the collecting data of moisture content, temperature and content, the law of migration and transformation was analyzed. Among them, the monitoring data of soil profiles were analyzed, and 7 forms were analyzed. The stability of each form in natural soil was different, and the residue state was the most stable, the effect of leaching on morphology has not been studied. The ecological risk was analyzed based on the soil monitoring data.

During the wet season (June) and the dry season (December) in 2021, the survey and sampling were conducted by the Shuangqiao River in the alluvial-proluvial landform area. The sampling point is Sidi Village, where the groundwater and surface water are disconnected hydraulically. In winter and summer, 24 soil samples, 1 groundwater sample and 1 surface water sample were collected, respectively. The test lasted 30 min. Regarding different river water level settings, when the flood occurs, the flood level is 30 cm (the difference between the floodplain and the river bed), and the low water level is the average depth of the current river water level of 10 cm. The test includes occurrence form, total amount, water content, pH, and hydrogeological conditions of mercury and sees the sampling, analysis and testing methods for details [42].

The samples of water collected during the wet season (June) and the dry season (December) in 2021 were tested and analyzed using an atomic fluorescence spectrometer (AFS) in the test center of the Xi'an Institute of Geology and Mineral Resources. Instead, the mercury distinguishes into 48 sets of soil samples which were determined using AFS, as well as the ion-selective electrode analysis in the Hefei Mineral Resources Supervision and Testing Center, Ministry of Land and Resources. All of the tests were analyzed according to the standards DZ/T0279-2016, DD2005-03, and DZ/T0130.2-2006, specifying the detection limit and determination range of the method, which are 0.005 mg/kg and 0.02–6 mg/kg, respectively. The standard DD2005-03 divides the elements in soil into seven forms, namely the water-soluble form extracted by the extractant of deionized water, the ion-exchange form extracted by the extractant of magnesium chloride, the carbonate bound form extracted by the extractant of acetic acid-sodium acetate, the weak organic (humic acid) bound form extracted by the extractant of sodium pyrophosphate, the Fe-Mn bound form extracted by the extractant of hydroxylamine hydrochloride, the strong organic bound form extracted by the extractant of hydrogen peroxide, and the residual form extracted by hydrofluoric acid.
