## 1 Introduction  
Many areas around the world still rely heavily on groundwater for daily water consumption. Thus, the maintenance of a suitable groundwater quality is crucial [1, 2, 3, 4, 5]. During the 2000s, however, various human activities significantly impacted groundwater quality and availability through various forms of pollution [4], with landfills, mines, and industrial plants being some of the main sources of groundwater pollutants [6, 7]. The most common pollutants include heavy metals, non-aqueous phase liquids (NAPLs), and volatile organic compounds (VOCs) [8, 9]. These pollutants are hazardous chemicals that can potentially negatively impact human health. Unfortunately, pollution from illegal waste dumping sites also occurs, causing serious health and environmental problems [10, 11]. Previous studies have demonstrated that the respiratory exposure of VOCs via the inhalation route is associated with the risk of specific diseases [12]. However, the characteristics of pollution caused by illegal waste dumping as well as ways of predicting and addressing the problem are still poorly understood. This limited understanding is due to the dynamic and highly complex nature of pollution caused by illegal waste dumping, and the complexity originates from multiple pollution sources and the artificially induced pollution-prevention changes (e.g., excavation and pumping) in hydrologic environments.  
At illegal waste dumping sites, pollutants leak from the waste to the groundwater over a long period of time. At the time of discovery, the distribution of contamination may be widespread, requiring countermeasures such as monitoring and remediation, which generally require considerable time and expense [13; 14; 15]. Consequently, it would be desirable to understand the characteristics of pollutant transport and to predict it in dynamic and complex hydrologic environments in illegal waste dumping sites [16]. For this purpose, simulations of pollutant transport in the presence of groundwater flow require consideration of the characteristic history of the illegal waste dumping site in addition to the conventionally considered subsurface flow and transport properties [17; 18]. However, such simulations have not been conducted yet.  
In the prefecture of the Tohoku region, Japan, an illegal dumping site was discovered in 1990, with various countermeasures having been implemented to date (2021) due to the existence of 1,4-dioxane groundwater pollution--a regulated substance (groundwater standard: \(\leq\)0.05 mg/L) [19; 20; 21; 22; 23]. At this site, multiple pollutant sources have been expected based on data from monitoring wells, and multiple countermeasures (e.g., the installation of impermeable walls) that potentially impact the hydrologic environment have been implemented. 1,4-Dioxane is highly miscible with water, and its biodegradation and adsorption to soil may be neglected [24; 25; 26], making this site suitable for fundamental studies on pollutant transport in the dynamic and complex hydrologic environments of illegal waste dumping sites. In this context, the objective of this study was to clarify the possibility of numerically predicting pollutant transport in the dynamic and complex hydrologic environments at this site in Japan, and to investigate the characteristics of pollutant transport under both naturally occurring and artificially induced groundwater flow using various countermeasures including remediation with pumping.