Investigating the spatial pattern of total selenium concentration in soil surface in Central Iran (case study: Isfahan Province)

Introduction:
The study of the spatial distribution of heavy metals is very important in land management and planning. The geostatistics theory is used to estimate spatial variables in unmeasured points. By using the CoKriging estimator and using information about the correlation between variables, a more accurate estimation of a variable (main variable) can be obtained using auxiliary information (secondary variables). Due to the importance of Isfahan Province because of the large population living in this province and the extent of agricultural and industrial activities, there is a need for sufficient information on the distribution of selenium in this area. The present study investigated the distribution of selenium and provided maps of its distribution in the surface soils of this province.
Material and methods:
The study region has an area of about 6800 km2 and covers a large part of the agricultural lands and the major industrial centers of Isfahan. Samples were taken from depths of 0 to 20 cm from the soil surface and 255 samples from the whole area in 4 × 4 km networks by using a stratified random sampling design. All soil samples were analyzed for pH, electrical conductivity (EC), sand, clay, silt content, and organic matter percentage. Total selenium concentrations were measured by XRF in only 72 samples. Statistical and geostatistical calculations were performed by SPSS and WinGslib, respectively, and maps were drawn by ILWIS and Surfer software.
Results and discussion:
The mean concentration of selenium in the study area was 0.63 mg/kg with a min and max amount of 0.5 and 1.5 mg/kg, respectively. There was a positive and significant correlation between the selenium concentration and soil’s EC. Therefore, EC was used to reduce the selenium estimation error by the CoKriging method. While toxicity level of selenium starts at 5 (mg/kg) for humans and animals, in this study, deficiency threshold for total soil selenium was 0.6 mg/kg and, therefore, the study area can be identified as selenium-deficient because more than 50% of the area had a concentration of less than 0.6 mg/kg and about 12% of the area has a high concentration of selenium in unpolluted soils. No toxicity of this element to humans and livestock was observed. Large parts of Isfahan Province have an arid climate with alkaline pH soils. Given the low levels of selenium in both agricultural and non-agricultural lands, it seems that the native material of this area, which is often the alluvial constituent of the Zayandehrood River, is poor in selenium. On the other hand, the deficiency of this element in agricultural lands indicates that farmers in these areas do not use selenium-containing fertilizers. The highest concentration of selenium was located in the city of Mobarake. There is a huge steel industry in this area. The dominant wind direction of the region is introduced as a threat factor in the transfer of selenium-containing atmospheric emissions from the industrial areas of Mobarka to more populated areas of the province
Conclusion:
Industries are more concentrated in the city of Isfahan and its surrounding areas and thus selenium concentration is higher in these areas than the neighboring areas. The steel industry in Mobarakeh is an agent for the introduction of selenium into the atmosphere and finally the soil. Winds probably play a role in transmitting selenium to the northeastern part of the region. However, no toxicity of this element was observed in all the studied areas but even selenium deficiency was observed. Therefore, using selenium-rich fertilizers in the region is recommended.