Exploring Nickel and Zinc contamination in cultivable lands of eastern Uttar Pradesh, India

In modern agricultural practices, soils are increasingly exposed to multiple anthropogenic and natural pollutants, with heavy metals playing a significant role. Despite their critical environment and health impact, substantial gaps remain in understanding the levels of these contaminants and their outcomes on soil and water systems. To address the lack of pertinent data, this study assessed the speciation and contamination levels of nickel (Ni) and zinc (Zn) in soil samples collected from Varanasi (n = 9) and Mirzapur (n = 6) districts of eastern Uttar Pradesh, India. The modified Tessier method was used for the sequential extraction procedure to understand heavy metals mobility and availability. Among the geochemical fractions (water soluble, exchangeable, organic matter bound, carbonate bound/specifically sorbed, bound to Fe-oxides (Fe-MnOB)), the residual fraction was predominant. For Ni, principal component analysis revealed the highest positive loading factor for soil pH (0.941), and the residual fraction also demonstrated a significant positive loading value (0.779), indicating their stabilizing influence. In the case of Zn, the highest positive loading factor was observed for diethylenetriaminepentaacetic acid-Cu (0.956), while the highest negative loading factor was associated with soil pH (-0.935). For Zn, negative loadings of pH and CEC contrasted with positive associations of DTPA-Cu and Ni, suggesting differential mobility and source behavior. For Zn speciation, the residual fraction showed the highest positive loading (0.944). The study concluded that the majority of contamination indices for Ni and Zn fell within the slightly to moderately contaminated zone, with geogenic sources playing a dominant role compared to anthropogenic inputs. These results offer a valuable reference point for supervising heavy metal contamination and understanding the potential migration of pollutants within soil, water, and human systems. This study may be the first detailed speciation and index-based assessment of Ni and Zn in eastern UP agricultural soils. Periodic assessment and management of these contaminants are recommended to mitigate their environmental and health impacts.