Assessment of Cu, Zn, Mn, and Fe enrichment in Mt. Kenya soils: evidence for atmospheric deposition and contamination

摘要

Mountains are the preferred sites for studying long-range atmospheric transportation and deposition of heavy metals, due to their isolation and steep temperature decrease that favors cold trapping and condensation of particulate forms of heavy metals. Any enrichment of heavy metals in mountains is presumed to primarily occur through atmospheric deposition. In this particular study, we assessed the status of 27 subsurface soils collected along two elevation gradients of Mt. Kenya using enrichment factors (EFs) as the ecological risk assessments. The collected soils were analyzed for total organic carbon, zinc (Zn), iron (Fe), manganese (Mn), and copper (Cu). The mean concentration of Mn, Fe, Zn, and Cu was 0.376 mg/kg, 47.6 mg/kg, 12.3 mg/kg, and 4.88 mg/kg in Chogoria and 0.560 mg/kg, 113 mg/kg, 12.7 mg/kg, and 2.70 mg/kg in Naro Moru respectively. These concentrations were below the US-EPA maximum permissible levels for soils, implying that the levels recorded had low toxicity. Meanwhile, the mean enrichment factors for Mn, Cu, and Zn were 0.447, 131, and 78.8 in Chogoria and 0.463, 38.9, and 53.0 in Naro Moru respectively. This implied that Zn and Cu in Chogoria sites were extremely enriched, while in Naro Moru, enrichment levels ranged from significant to extreme. However, Mn was found to have minimal enrichment in all the sites. Lower montane forest and bamboo zone recorded relatively high enrichment due to distance from source of pollution. Ericaceous zone also had high mean enrichment due to influence of wind which favors higher deposition at mid-elevations.