Assessment of Heavy Metal Contamination in Soil and Plants Around Bhutan’s Main Landfill

Abstract

Heavy metal contamination in landfill environments poses significant risks to public health and the environment. This study assessed heavy metal concentrations of iron (Fe), zinc (Zn), copper (Cu), and manganese (Mn) in soil and selected plants across three sites in Bhutan’s main landfill using Flame Atomic Absorption Spectrophotometry. Results showed heavy metal concentrations in soil followed the order Fe > Mn > Zn > Cu, with Fe levels in some samples exceeding the WHO/FAO limit of 10 mg/kg set in 2001. This exceedance, observed in soil collected directly from the landfill environment, indicates contamination likely caused by leachate migration. At a depth of 15–30 cm, a significant difference (P = 0.05) in heavy metal concentrations was detected between areas above and below the landfill. Correlation analysis revealed the intricate interplay between heavy metal concentrations and soil attributes, with soil organic matter playing a pivotal role in binding heavy metals. At 0-15 cm depth, Zn showed strong negative correlations (r = -0.940, p < .01) with soil organic carbon (SOC percent) and soil organic matter (SOM percentage) and a positive correlation (r = 0.940, p < .01) with electrical conductivity (EC). At 15–30 cm depth, Mn negatively correlated (r = –0.836, p < .05) with pH and positively correlated (r = 0.878, p < .01) with SOC percentage and SOM percentage. Heavy metal concentration varied among plant species, with Fe being the most abundant metal at the two sites. The bioaccumulation outcome indicated that Zn was the most accumulated in the selected plant. Bidens pilosa L. and Poa annua L. showed higher heavy metal accumulation in roots, suggesting their potential for phytoremediation. The Zn, Cu, and Mn levels exceeded the permissible limits set by WHO/FAO in the specified years, whereas Fe levels in plants remained within the WHO/FAO standards. This study highlights heavy metal distribution, soil-plant interactions, and environmental risks in landfill areas, offering strategies for contamination management. Longitudinal studies are recommended to monitor temporal trends in heavy metal distribution, assess the cumulative impact of leachate migration over time, and refine mitigation strategies. In addition, leachate dilution rate assessments are necessary to better understand contaminant dynamics and environmental impacts.