Geochemical Baselines and Contamination Sources of Heavy Metals in Urban Soils of the Kathmandu Valley

Geochemical Baseline Values (GBVs) provide a critical reference for distinguishing between inherent elemental concentrations and anthropogenic accumulation. This study establishes GBVs of twenty-two elements, in the soils of the Kathmandu Valley. Elemental analysis of 106 soil samples was carried out using DELTA X-ray Fluorescent Spectrometer. Local GBVs are further applied to estimate the pollution status of heavy metals, with results compared against standard global background values. Soil contamination was quantified using pollution indices and the Enrichment Factor (EF). Soil fertility was assessed via soil organic matter (SOM) content, and its relationship with heavy metals was investigated using Pearson coefficient matrix. Pollution source identification was performed using Principal Component Analysis (PCA). Among three methodological approaches, the kneedle algorithm, proved most suitable for defining GBVs in our study area. The established baselines for Tungsten (15.33 mg/kg), Rubidium (170.59 mg/kg), Thorium (22.88 mg/kg), Uranium (5.18 mg/kg), Cadmium (10.96 mg/kg), and Zinc (112.17 mg/kg) exceeded global averages. Local GBVs revealed significant contamination and increased enrichment by Copper (Cu), Lead (Pb), and Manganese (Mn), whereas global references showed the high contamination of Cadmium (Cd) and Arsenic (As). Kathmandu valley soils exhibited moderate SOM content (3.03%). PCA analysis delineated three distinct sources; a lithogenic factor (Mn, Ni, As, Cr), an anthropogenic factor (Zn, Cu, Pb) linked to SOM, and an isolated Cd dominated factor. This research demonstrates the necessity of region-specific baselines for reliable environmental assessment and provides a clear, multi method delineation of contaminant sources.