Concentration, Sources, and Associated Risks of Trace Elements in the Surface Soil of Kathmandu Valley, Nepal

Trace elements occur naturally in rocks and soils, but increasingly higher quantities of elements are being released into the environment by anthropogenic activities. In particular, urban soils are marked with elevated levels of trace elements due to vehicular emissions, industrial disposals, urban waste, chemical fertilizers, and construction materials. This study reports the concentration of twenty elements including highly toxic heavy metals (Mg, Al, P, Ca, Fe, K, Si, Na, S, Ti, V, Se, Cr, Cu, Ni, Mn, As, Pb, Zn, and Cd) in the surface soil (0–20 cm) of Kathmandu Valley, Nepal. Three soil pollution indices (geo-accumulation index, single pollution index, and enrichment factor) were employed to determine the severity of trace element contamination. Our results suggested that the studied sites are under strong to moderate contamination in the order of Cd > As > Pb > Zn > Cr > Mn > Ni > Cu. We observed a high concentration of Pb, Zn, As, and Cd in the surface soil of the Kathmandu Valley mainly due to anthropogenic activities. The concentration of As and Cd was found several times higher than the global average background value and a deficient amount of P, S, and Mn was observed.