The influence of soil pollution by heavy metals on the land suitability for irrigated wheat farming in Zanjan region, northwest Iran

The main objective of this study was to assess the land suitability for irrigated wheat farming, surrounding the Zinc Specialized Industrial Town (briefly, Zinc Town), northwestern Iran, with soils polluted by zinc (Zn), lead (Pb), and cadmium (Cd). The concentrations of heavy metals (HMs) were measured in 110 topsoil samples (0–20 cm), as well as in 65 samples of wheat grains. The limitation scores for the climatic factors, soil properties, and HMs were calculated based on their limitation for wheat cropping. Afterwards, the accumulated limitation scores were used for calculation of the land suitability membership scores using an exponential function, and then the maps were produced. The results showed that there were optimal climatic conditions for irrigated wheat, whereas due to the youthfulness of studied soils, the content of coarse fragments (CF), as the most limiting soil property, made moderate to very severe limitations in more than half of sampling points. Regarding the Iranian threshold values of 500, 75, and 5 mg kg−1 as the MPC for Zn, Pb, and Cd, respectively, 7.27, 30.0, and 20.0% of soil samples were classified as the polluted soil to these pollutants. On the other hand, Pb and Cd concentrations in 24.6 (16 samples) and 29.2 (19 samples) % of cultivated wheat in the studied area exceeded the threshold value of 0.2 mg kg−1, respectively, whereas Zn concentration in none of the sampled plants was higher than the threshold value of 50.0 mg kg−1. Land suitability for wheat farming significantly decreased near the Zinc Town due to the HMs transferring from polluted soils to wheat grains. Therefore, potential threat of metallic pollutants for secure and safe food production should be considered as an important environmental factor to accurate assessment of land suitability for a given crop.