## 3 Results and Discussion
### Evolution of Cd Pollution Risk Sources  
Over the past two decades, the risk of Cd contamination in agricultural soils has progressively intensified, transitioning from predominantly geogenic influences to a complex interplay of agricultural, industrial, traffic-related, and natural sources. In the early years, soil Cd concentrations remained below natural background levels, primarily governed by inherent geochemical properties [47]. However, as time progressed, isolated instances of Cd exceedance emerged, particularly in mining-intensive regions, where mineral extraction and associated activities contributed to elevated Cd levels. Notably, in 2012, high Cd concentrations were observed near the Changjiang mining area, indicating localized enrichment due to mining activities. In recent years, agricultural production, industrial emissions, and transportation-related pollution have collectively become the primary contributors to Cd accumulation in agricultural soils. The geographical detector model was applied, and the q-statistic values derived from the factor detector indicate the explanatory power of each variable for the spatial differentiation of Cd concentrations (Figure 10).  
Agricultural sources play a critical role in Cd accumulation, with pesticide application intensity and phosphate fertilizer usage exhibiting the highest explanatory power. The substantial impact of pesticides may be attributed to the increased reliance on Cd-containing agrochemicals in response to the high-temperature and high-humidity conditions that exacerbate pest outbreaks. Similarly, the extensive application of phosphate fertilizers, which may contain trace amounts of Cd, further contributes to Cd enrichment in agricultural soils.  
Figure 9: Temporal variations in Cd exceedance rates in Hainan Island’s agricultural soils from 2005 to 2023.  
Industrial sources significantly influence Cd contamination, with manufacturing activities and PM\({}_{2.5}\) emissions identified as the primary risk factors. Industrial processes release Cd through atmospheric deposition, wastewater discharge, and solid waste disposal, all of which contribute to Cd accumulation in soil.  
Regarding transportation, highways have a more pronounced impact than provincial roads, national roads, or county roads. This can be attributed to the high traffic volume and associated emissions, including vehicle exhaust, tire wear, and road maintenance materials, particularly along Hainan Island's ring highway.  
Natural sources were found to influence Cd mobility and distribution, with precipitation identified as a key driver. The tropical marine climate of Hainan, characterized by high temperatures and humidity, accelerates organic matter decomposition and enhances Cd solubility. Increased rainfall contributes to soil acidification by reducing cation exchange capacity and base saturation, thereby increasing the bioavailability and ecological risk of Cd. Soil type and DEM exhibit lower explanatory power, suggesting that while geological characteristics play a role, anthropogenic sources dominate Cd distribution patterns. Similarly, soil pH and soil organic carbon contribute minimally, likely due to the overriding influence of external Cd inputs.  
These findings suggest that the spatial and temporal dynamics of Cd pollution in agricultural soils are governed by a combination of anthropogenic and natural factors. Given the increasing contribution of agricultural inputs, industrial emissions, and transportation activities to Cd accumulation, targeted mitigation strategies are essential. Future efforts should focus on optimizing agricultural management practices, strengthening industrial pollution controls, and implementing comprehensive monitoring systems to assess the impact of environmental factors on Cd mobility. These measures will be crucial for reducing Cd contamination risks and ensuring the long-term sustainability of agricultural production on Hainan Island.