Pedo-geological assessment of nickel distribution in extensively cultivated soils by total and residual fraction concentrations: (Northwest Türkiye)

Nickel (Ni) contamination in agricultural soils southeast of İznik Lake, northwest Türkiye, was examined for its extent and origins. Two analytical methods were employed to determine the relationship between Ni contents in these long-cultivated soils and their geological surroundings. For this, 33 soil samples were collected from various parent materials. Statistical analysis was performed on the relationship between Ni concentrations, x-ray fluorescence (XRF) for total metal concentration, and inductively coupled plasma optical emission spectroscopy (ICP-OES) for European Community Bureau of Reference (BCR) residual fraction (F4). A pedo-geological context was used to construct and analyze contamination factor, geoaccumulation index, and enrichment factor for Ni’s geochemical risk assessment. Ni source rocks were identified with x-ray diffraction (XRD). XRF showed a mean Ni concentration of 63.97 mg kg⁻¹ and a lower limit Ni concentration of 45.22 mg kg⁻¹ for the BCR residual fraction F4, which is above normal upper continental crust background values of 34 mg kg⁻¹, indicating enrichment relative to natural baselines. Serpentinized peridotite blocks in these units most probably transfer nickel (Ni) to the basin, creating geogenic pollution. Ni enrichment is limited and linked to geological characteristics, while basin-level geogenic contamination is minimal. Nonetheless, agrochemical enhancements may be possible in Ni-rich locations. Ni contamination in İznik is mostly caused by the excess and uncontrolled use of agrochemicals in agriculture, which requires immediate ameliorative measures. Along with this, the methods used in this study can be applied to other regions with potential Ni contamination risks.