Assessing the total concentration, pollution level and speciation of potentially toxic metals in agricultural soils of Jovin Plain (Sabzevar area, Razavi Khorasan Province)

Soil contamination with potentially toxic metals is one of the most critical challenges in terms of food and human health safety. The majority of the studies on metal pollution of agricultural soils have been carried out by analyzing the total concentration of metals in the soils. However, metals are present in different chemical forms or species in soils, so they should be considered in a full-fledged environmental assessment. The purpose of this research is to assess the total concentrations, pollution level, source and speciation of potentially toxic metals in agricultural soils of Jovin Plain in Razavi Khorasan Province.

Twenty-seven soil samples were randomly collected from the entire plain at a depth of about 50 cm. Soil physicochemical parameters (organic matter, pH, carbonate content, soil texture) along with the total concentrations of potentially toxic metals (zinc, cadmium, arsenic, chromium and nickel) were measured using standard methods. The level of soil metal pollution was assessed by calculating pollution indices (enrichment coefficient, land accumulation, pollution load index) and their possible origins were apportioned by multivariate statistical methods (correlation and principal component analysis). The modified four-step BCR method was used to determine the species or chemical phases of metals in soil samples.         

It was found that the cultivated soils were alkaline in nature, clay loam in texture and moderate in terms of organic matter content. In majority of the soil samples, the concentration of all studied metals was higher than the mean crust and the global mean soils. Evaluating the level of soil metal pollution also showed that the soils were moderately to heavily contaminated in terms of chromium, nickel and copper and heavily contaminated in terms of arsenic and zinc. The results of the multivariate statistical analysis revealed that chromium and nickel are mainly of geogenic sources attributable to the ophiolite rocks in the region, while zinc, cadmium, copper and arsenic were mainly derived from anthropogenic sources e.g. agricultural activities in the study area. Based on speciation analysis studies, chromium and nickel were mainly extracted from the residual phase (F4), zinc and copper from the oxidizable phase (F3), arsenic from the reducible phase (F2) and cadmium was mainly retrieved from the soluble/exchangeable phase (F1). The high extraction percentage of chromium and nickel from the residual (stable) phase indicates that the source of these metals in the soils was mostly geogenic with very limited mobility and bioavailability. On the other hand, the high percentage of metals retrieved from the first three phases (F1+F2+F3) revealed that they were mainly from anthropogenic sources having high mobility and bioavailability in soil samples.

Based on the assessment made in this research, it was found that the metals in the agricultural soils of Jovin Plain have surpassed the standard guideline limits and apart from chromium and nickel, the other studied metals have been derived from anthropogenic sources (agricultural activities) in the study area. The chemical speciation analysis also showed that the metals in the soils occurred in different chemical forms or phases depending on their different origin and pollution level, which in turn lead to the different mobility and bioavailability of these metals in the soil samples.