## 3 Results and Discussion
### Spatial Distribution of Cd in Hainan Island  
Figure 7 and Table 3 illustrate the spatial variation of Cd concentrations across agricultural soils in the 18 administrative regions of Hainan Island. Cd concentrations exhibit a distinct spatial pattern, with lower levels in the central Hainan and increased concentrations in the northwest and southeast. The highest Cd concentrations are predominantly found in Haikou and Danzhou City in the northwest, as well as Sanya City in the south, with levels significantly exceeding background values. Prior studies, including those by [PERSON], align with the observed trend of significant Cd accumulation in Haikou City, reinforcing the need for ongoing surveillance in this region [43].  
Spatial differences in Cd concentration arise from a combination of natural processes and human activities. In certain northern areas of Hainan, geological conditions contribute to higher natural background Cd levels, which partially explains the elevated Cd concentrations in these regions. However, human activities are the dominant contributors to Cd accumulation in agricultural soils. Urban expansion, industrial growth, and tourism development in Haikou and Sanya have significantly contributed to increased anthropogenic emissions. These emissions include waste disposal, construction activities, and industrial discharges, all of which contribute to soil contamination. Moreover, Hainan's tropical climatic, with its high temperatures and frequent rainfall, enhances soil leaching, which in turn affects Cd mobility and redistribution. Additionally, long-term use of phosphate fertilizers and farm chemicals with varying Cd content is a major contributor to Cd buildup in agricultural soils [44; 45]. Regions with intensive farming activities, such as Haikou and Danzhou, are more susceptible to elevated Cd levels due to continuous fertilizer application.  
Conversely, central Hainan, including Baisha County, Tunchen County, and Qiongzhong County, reports Cd concentrations that are significantly lower than the provincial background value, indicating relatively clean conditions. This may be due to a combination of lower anthropogenic input, less intensive agricultural activity, and natural soil characteristics that limit Cd accumulation.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Reference** & **Sample Size** & **Mean (mg/kg)** & \
\begin{tabular}{c} **Minimum** \\ **(mg/kg)** \\ \end{tabular} &
\begin{tabular}{c} **Maximum** \\ **(mg/kg)** \\ \end{tabular} \\ \hline This study & 4110 & 0.12 & 0.01 & 3.11 \\ Hainan Province & 167 & 0.056 & 0.004 & 2.286 \\ background value [41] & & & & \\ Risk screening value & - & 0.30 & - & - \\ (GB15618-2018) [37] & 360 & 0.156 & 0 & 2.30 \\ [35] & 83 & 0.15 & 0 & 1.63 \\ China [42] & 121 regions & 0.160 & 0.030 & 1.350 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Synthesis of sample-number-weighted Cd concentration in agricultural soil of Hainan Island.  
Despite overall safe Cd levels in most agricultural soils of the elevated concentrations in urban and intensively cultivated areas of the northwest and southeast call for targeted pollution mitigation strategies, such as enhanced waste management, optimized fertilizer and farm chemicals use, and sustained environmental monitoring.