Mapping the overall soil pollution by heavy metals using limitation scores

Background and objectives: Considering progressive growth of industry and technology, the accumulation of environmental contaminants, especially heavy metals (HMs) in the soil is of increasing worldwide concern about the probable soil pollution risks. Due to the multiplicity of the HMs’ sources, these serious elements are usually found along with each other in the soil. Therefore, the precise evaluation of the environmental pollution risks needs all of the existing pollutants to be concomitantly assessed. Representing overall soil pollution in the Zanjan Zinc Specialized Industrial Town (briefly Zinc Town) in this study, single-element maps of heavy metals were incorporated using cumulative limitation scores (LS).

Materials and methods: In the present study, 85 topsoil samples (0-20 cm) using a regular grid sampling pattern were collected and total contents of Zn, Pb and Cd were measured for each soil sample using atomic absorption spectroscopy. Assessing the spatial structure of studied elements, their contents were interpolated using ordinary kriging method in ArcGIS 10.1 software. Assessing the pollution risk of studied elements, the total concentration of heavy metals is first transformed to LSs using the exponential transfer function determined by using two threshold values: permissible concentration and seriously polluted soil. Calculated cumulative LSs for studied elements were considered as an environmental variable and its values were interpolated and classified.

Results: Mapping the total concentration of Zn, Pb and Cd revealed that elevated soil HMs concentrations were mostly found in the northwest of studied area, i.e. lands located surrounding the Zinc Town. Increasing the distance from the industrial source of pollution, pollution intensity decreased gradually. It seems that Zinc Town is the main factor which controls the spatial distribution of Zn, Pb and Cd in the studied soils. Although the observed and estimated values for the total concentration of Zn were higher than Pb and Cd, its values did not exceed its maximum permissible value in Iranian soils; whereas the total concentration of Pb and Cd in sizeable parts of the studied area exceeded their national maximum permissible concentration and in some parts of the area located around the Zinc Town, exceeded than the seriously pollution threshold value. Mapping the spatial distribution of LSs showed that elevated soil LSs were mostly found in the lands located surrounding the Zinc Town. Therefore, it can be concluded that Zinc Town not only controls the spatial distribution of pollutant elements in the area, but also is the main factor which causes the HMs concentration to be exceeded than their maximum permissible or even seriously pollution threshold values in some parts of the studied area and finally, lower the land quality in these areas.

Conclusion: Totally, industrial activities related to Zn production caused to simultaneous entrance of several heavy metals to the adjacent soils and lead to degradation of the lands in studied area. The interpolated heavy metal values were observed in the very different range for each specific element, as different threshold values. These caused some difficulties to perceive the soil pollution risk just by single-element maps of heavy metals. Since the limitation scores are standardized and can be summed for different variables, this technique is capable to represent the contemporaneous effects of several pollutants on the soil quality.