## 2 Overview of the Study Area  
Mudanjiang City is situated in the southeastern region of Heilongjiang Province, China, within the Changbai Mountains and the Songhua River Basin. The Mudanjiang River, the second largest tributary of the Songhua River, originates in Mudanling, Dunhua City, Jilin Province. It flows northward, subsequently veering northeast into Jingpo Lake, Heilongjiang Province, before ultimately merging with the Songhua River from south to north through the sections of Mudanjiang, Hailin County, Linkou County, and Yilan County [17]. The Mudan River has a total length of 726 km and a basin area of 37,023 square kilometers. The upstream area encompasses 8480 km\({}^{2}\) in Jilin Province, while the middle and downstream areas cover 28,543 km\({}^{2}\) in Heilongjiang Province. The Mudan River is situated between 128\({}^{\circ}\) E and 131\({}^{\circ}\) E longitude and 43\({}^{\circ}\) N and 46\({}^{\circ}\) N latitude. The research area spans a total length of 62 km, has a water surface area of 11.026 km\({}^{2}\), and has a watershed area of 277.5 km\({}^{2}\). The gradient measures 1/3600, the average river width is 175 m, the maximum width exceeds 300 m, the average depth is 3 m, and the maximum depth reaches 5 m.  
The climate of the Mudan River Basin is characterized as a cold temperate continental monsoon climate, including a brief spring that warms rapidly, accompanied by strong winds and a propensity for drought. Summer is characterized by warmth and precipitation; fall is brief and experiences rapid cooling [18]; winter is prolonged and frigid, with an average yearly temperature of 6.1 \({}^{\circ}\)C. January is the coldest month, with an average temperature of \(-\)17 \({}^{\circ}\)C and a minimum recorded temperature of \(-\)35 \({}^{\circ}\)C. July is the warmest month, with an average temperature of 22 \({}^{\circ}\)C and a maximum temperature of 38 \({}^{\circ}\)C. The freezing period extends from mid-November to mid-April in the subsequent year [19]. The Mudan River Basin features arable terrain and ample water reserves, with an average annual runoff of 5.25 billion m\({}^{3}\), a peak runoff of 9.49 billion m\({}^{3}\) recorded in 1960, and a low runoff of 1.55 billion m\({}^{3}\) noted in 1978. The average annual discharge is 168 m\({}^{3}\)/s, accompanied by an average annual precipitation of approximately 540 mm. The topography of the research region is elevated in the southwest and depressed in the northeast. Numerous landforms exist, including mountains, hills, terraces, and plains. Mountains are predominantly composed of granite, gneiss, and several other rocks, characterized by their impressive elevation and steep inclines. Hilly regions are generally characterized by low hills and relatively level surfaces, and they play a significant role in agricultural output [20]. The plain is predominantly located along rivers and valleys, characterized by good soil conducive to agricultural cultivation. The Mudan River Basin is influenced by several fault zones, primarily the Dunhua-Mishan fault zone and the Yilan-Yitong fault zone. These fault zones govern the formation of topography, landforms, and hydrological systems within the basin, and may also precipitate geological calamities, including earthquakes. The basin has fold structures, such as the Taipingling anticline and Zhangguangcailing syncline, which significantly influence the stratigraphic distribution and the genesis of mineral resources [21]. The primary categories of groundwater in the study area include pore water, fissure water, and karst water. There are eight primary soil types, categorized into simple and highly active luvisols, thin layer soils, black soils, volcanic soils, gley soils, organic soils, anthropogenic soils, and rudimentary soils [22]. Figure 1 is the basic overview map of the study area, which mainly reflects the location and elevation of the study area, and Figure 2 is the basic lithologic characteristics map of the study area. Figure 1: The schematic diagrams depict the study area.