Heavy metal accumulation in soil and trees of Bhairahawa, a tropical urban city of Nepal: implications for pollution monitoring

Heavy metals (HMs), including copper (Cu), zinc (Zn), nickel (Ni), lead (Pb), and cadmium (Cd), accumulation in the soil and trees of less polluted (LP) and heavily polluted (HP) sites in the tropical urban area of Nepal was carried out. The metal accumulation index (MAI), bioaccumulation factor (BAF), enrichment factor (EF), pollution load index (PLI), geo-accumulation index (Igeo), and contamination factor (CF) were also measured. Soil of HP site exhibited higher concentration of HM. PLI, Igeo, and CF indicated that Cu contamination was severe at HP site. EF suggested that Cu, Zn, and Ni originated from industrial and vehicle sources. Trees of HP site accumulated higher concentration of HM than LP site. Syzygium cumini accumulated Cu (53.00 mg/kg), Ficus benghalensis accumulated Zn (143.33 mg/kg), Alstonia scholaris, Artocarpus heterophyllus, Ficus religiosa, and Neolamarckia cadamba accumulated Ni (13.00, 13.67, 13.67, and 13.00 mg/kg, respectively), A. scholaris, Litchi chinensis, and S. cumini accumulated Pb (20.67, 20.33, and 20.33 mg/kg, respectively), and A. heterophyllus accumulated Cd (5.33 mg/kg). The highest MAI was observed in bark of A. heterophyllus (17.28) and leaves and bark of A. scholaris (11.10 and 13.28, respectively). A. heterophyllus and F. benghalensis exhibited high BAF. These trees could be utilized as bioaccumulators to detect and mitigate pollutants.