Decadal Changes in Heavy Metal Concentrations of Crops Following Soil-Covering Remediation in Farmland Surrounding an Abandoned Ni-Mo Mine

Long-term field evidence remains limited regarding whether remediation of abandoned mining areas can sustainably reduce heavy metal accumulation in surrounding crops and associated dietary health risks. In this study, an abandoned Ni-Mo mining area in Zunyi, Guizhou Province, China was investigated using leafy vegetable samples collected from the same area and sampling locations in 2015 and 2026. The concentrations of Ni, Mo, Cd, Cr, Pb, Cu, Tl, V, and Zn were determined, and remediation performance was evaluated based on pollution indices, a human health risk assessment, and Monte Carlo simulation. Most metals showed substantial declines over the study period, with Pb, Tl, V, Cr, and Cd decreasing by 92.91%, 79.20%, 70.31%, 68.58%, and 65.59%, respectively, whereas Mo increased from 15.01 to 30.95 mg·kg−1. The Nemerow comprehensive pollution index decreased from 57.01 to 16.88, indicating a marked reduction in combined contamination, although considerable residual pollution persisted. The non-carcinogenic hazard index declined from 1025.78 to 186.33 for children and from 324.64 to 58.97 for adults, while the total carcinogenic risk decreased from 0.70 and 0.66 to 0.13, respectively. Despite these reductions, both non-carcinogenic and carcinogenic risks remained far above acceptable levels. Monte Carlo simulations further showed a systematic shift in risk distributions toward lower values, while the dominant risk structure changed from Pb-dominated non-carcinogenic risk to multi-element contributions and from Pb- to Ni-dominated carcinogenic risk. Overall, long-term soil-covering remediation substantially reduced crop heavy metal accumulation and dietary exposure risks, but did not restore the area to conditions suitable for safe agricultural use. Continued monitoring and targeted control of residual high-contribution elements, together with the anomalous increase in Mo, are therefore warranted.