Determination of baseline concentration of some heavy metals in surface soils of Khuzestan province by fractal concentration-area method

In many countries, baseline concentrations of heavy metals in soil have been studied; this information is used as a reference to assess soil quality status (Karim et al, 2015). Soil concentrations of elements in the soil are a function of the mineralogical composition of the parent material and weathering processes affecting soil formation as well as properties such as particle size, clay content and soil organic matter (Azimzadeh and Khademi, 2013). As a result, the natural concentration of elements in soils is widely variable and the use of baseline (threshold concentration of heavy metals) of other countries and the global average to identify the extent and risks of heavy metal pollution in soils of areas where environmental boundaries are not defined is incorrect (Ghanavati et al, 2019; Nazarpour, 2018). Fractal geometry studies natural phenomena and complex and irregular objects with mathematical relations. One of the major applications of fractal geometry is in estimating the thresholds and thus separating the abnormal community from the field based on their fractal dimension differences (Nazarpour et al, 2015). In this research, this model has been used to determine the baseline concentration of heavy metals.
In this study, in order to evaluate the background concentration of some heavy metals in surface soils of Khuzestan province. The sampling method was that first, using random sampling distribution in GIS, the proposed neighborhoods in the study area were determined. The points were selected to cover the entire study area. Samples were taken in combination (mixture of 3 samples together, with a distance of 50 to 100 m) from a depth of zero to 20 cm from the soil surface and with an approximate weight of 500 g and a total of 87 samples were prepared. The samples were dried in room air for 48 hours, then crushed and passed through a 200 mesh polyethylene sieve. After preparing the samples, heavy metals were measured by induction coupled plasma spectroscopy (ICP-OES) of the model device (Model Varian735).
According to the results, the studied heavy metals show a wide range of concentrations. The concentrations of Ni, Co and Pb were in the range between: 38-320, 5-16 and 5-41 mg/kg. The lowest and highest mean concentrations of the studied metals were related to Pb and Ni, 9.09 and 66.34 mg/kg, respectively. High concentrations of heavy metals indicate the impact of human activities in the region (Sadeghdoust et al, 2020). In this study, after sorting the data from high to low and determining the frequency of each concentration, logarithmic plots of cumulative frequency of metal concentration versus area were drawn. By obtaining the breaking points, the threshold of each metal was determined for the concentration-area method. Baseline values for Co, Ni and Pb were 6.93, 47.59 and 6.26 mg/kg, respectively. The results showed that the application of fractal methods in separating the amount of baseline from other geochemical populations is very appropriate.
Geochemical maps prepared by fractal method of concentration-area have a very good adaptation to the conditions of the region in terms of natural conditions of the region, land use and especially the effect of industrial units on the concentration and abundance of heavy metals and heavy metal pollution in the region. The results show the effect of anthropogenic factors on the concentration of metals. In general, the results showed that different factors, including human and natural factors together, are always effective in the distribution and concentration of heavy metals. Therefore, in order to maintain the balance of the ecosystem, human health, identify adverse effects on the environment and its proper management, it is necessary to determine the concentration of the field or the limits of environmental safety according to climatic conditions, region and soil properties.