**1. Introduction**

As an essential water source, groundwater plays a significant role in supplying urban and rural residents with water. The water table fluctuates under natural and human factors, such as rainfall, evaporation, exploitation [1], and recharge [2]. The area between the highest and lowest groundwater table is referred to as the groundwater table fluctuation zone [3,4]. Water table fluctuations allow both saturated and unsaturated soil to alternate within the environment. These fluctuations cause significant changes in the biochemical characteristics of that zone [5], including the adsorption, desorption, and dilution of pollutants, which correspondingly alter the groundwater environment. The influence of water table fluctuations on pollutant migration and transformation has attracted the attention of scholars.

Current studies mainly focus on solute migration and transformation, such as nitrogen [6], organic matter [7], heavy metals [8–10], and groundwater quality [11] under water table fluctuations. Liu et al. studied the nitrate change law under water table rise

**Citation:** Qu, J.; Yan, T.; Zhang, Y.; Li, Y.; Tian, R.; Guo, W.; Jiang, J. An Experimental Study on the Migration of Pb in the Groundwater Table Fluctuation Zone. *Appl. Sci.* **2022**, *12*, 3870. https://doi.org/10.3390/ app12083870

Academic Editor: Bing Bai

Received: 1 March 2022 Accepted: 10 April 2022 Published: 12 April 2022

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using a sandbox model experiment, finding that the water environment gradually changed into a state of relative reduction, and the lateral flow of the experiment was conducive to the migration of nitrate [12]. Liu et al. studied the variation in nitrate concentration for two water table fluctuation conditions (the water table stayed constant and varied by 20 cm/10 days) [13]. Tian et al. adopted an experimental simulation involving three kinds of groundwater tables and different surface runoff velocities to study nitrate change laws on solute migrates to the soil surface. Their results showed that the solute transport process shares an essential relationship with surface runoff velocity and the groundwater table [14]. Wang et al. simulated and verified the regularity of soil salt migration under water table fluctuations at different groundwater depths using a laboratory column experiment with homogenic medium. They assumed that the capillary pressure and temperature field variation caused by water table fluctuations significantly influence the migration of organic pollutants [15]. Wang et al. used the TMVOC model (a numerical simulator for three-phase non-isothermal flow of water, soil gas, and a multi-component mixture of volatile organic chemicals) to simulate how benzene, toluene, ethylbenzene, and o-xylene (BTEX) migrate in areas caused by steam extraction under natural attenuation and groundwater table fluctuations [16]. Oostorm et al. used a two-dimensional sandbox to study the distribution of pollutants migrating from their source into soil and groundwater under water table fluctuations. They observed that pollutants migrate with the infiltrated water flow and dissolve when the water table rises, increasing the pollutant concentration in the sample solution [17]. Bustos Medina et al. studied iron hydroxide blockage in wells and its effect on groundwater table fluctuations. This blockage affects indexes such as the water table, pH, EC (electrical conductivity), and DO (dissolved oxygen). By adopting hydrogeochemical simulations, Medina et al. determined the minerals' reaction mode, such as iron ions and manganese ions in the aquifer [18]. Li et al. explored the nitrogen transport law for fluctuations in different water tables using laboratory-based column experiments and numerical simulation. Their results showed that groundwater table fluctuations influence the variation of dissolved oxygen in the solution and decrease the nitrate and ammonium concentrations, which is conducive to removing ammonium [19]. Liu et al. employed a numerical model to predict changes in Beijing's groundwater table when the South–North Water Diversion Project was open. They also analyzed changes to the vadose zone and the impact of solid waste on the groundwater environment [20]. On account of the South–North Water Diversion Project, Cao et al. analyzed chemical quality predictions in the Baoding Plain for when the groundwater table rises using hydrogeochemical simulations [21].

Heavy metals (Pb) pose potential risks to the soil ecosystem and human health [22–24]. China and other nations have focused on soil pollution prevention and control plans and the study of Pb migration characteristics [25,26]. Domestic and foreign scholars have studied the adsorption, desorption, and migration of heavy metal pollutants such as Cu, Cr, and Zn in soil [27–30]. Nonetheless, there are few reports on the migration laws of heavy metals under groundwater table fluctuations. Therefore, in this article, technical methods, including adsorption and desorption tests, water table fluctuation experiments, etc., were used to analyze the distribution characteristics of Pb in the vadose and saturated zones under different typical media. Our methods help research the migration of Pb in the groundwater table fluctuation zone and provide theoretical support for heavy metal pollution treatments for soil and groundwater, remediation, and protection.
