Assessing the Long-Term Leaching Behavior of Cadmium and Zinc in Soil Beneath a Smelter Site

In this study, we investigate the leaching behavior of heavy metals from soils beneath a smelter plant, with a focus on their migration to groundwater. Sequential extraction methods and traditional batch tests (e.g., SPLP, TCLP) often inadequately represent real-world leaching scenarios. We addressed this gap by conducting column experiments simulating long-term exposure to synthetic rainwater on soils heavily contaminated with cadmium (Cd) and zinc (Zn). Soil samples were collected from ten random points at each of the two distinct sites, showing varied contamination levels (Site A: Cd: 546 mg/kg, Zn: 28,597 mg/kg; Site B: Cd: 82.3 mg/kg, Zn: 5,582 mg/kg). We predicted 30-years cumulative elution (I_30yr) of Cd and Zn, extrapolating concentration and the rate parameters determined by column experiments with an accumulated liquid-solid ratio of 10 to a leaching model described in RIVM report (report no. 771402007). The results indicated a broad range of I_30yr values both within and between sites (Cd: 10 - 458,274 mg/m²; Zn: 170 - 2,225,537 mg/m²), suggesting heterogeneous leaching behavior. Statistical analyses revealed that I_30yr values did not correlate with the total metal concentration in soils, while exhibiting a weak correlation with the exchangeable fraction from sequential extractions. However, I_30yr were strongly correlated with SPLP leachate concentrations, and the degree of correlation was higher in the subsoil than in the topsoil (Cd: r=0.867, p=0.001; Zn: r=0.990, p<0.001). Our results confirm the high correlation between metal leaching behaviors determined by column experiments and batch SPLP extraction, and emphasize that the stabilizing Cd and Zn in soils under the smelter is crucial for preserving groundwater quality.