Pollution indices of heavy metals in agricultural soils irrigated with raw sewage (Meshginshahr, Ardabil)

Introduction
As a great reservoir of nutrients and pollutants, soil plays an important role in health and socio-ecological sustainability. Soil pollution increases as a result of the entry of heavy metals from operations such as agriculture, urbanization, and industrialization. Unlike organic pollutants, heavy metals cannot be decomposed and remain in the soil for more than 150 years. The continuous increase in the concentration of heavy metals in the soil due to wrong agricultural operations has had a serious effect on human health. Long-term use of wastewater in land irrigation often increases the amount of heavy metals in the soil. The present research aims to investigate the amount of heavy metals and pollution indicators.
 
Materails and Methods
The study area is located in agricultural soils irrigated with raw sewage in Barzil village of Meshginshahr city (38° 23′ 34″ N and 47° 1′ 7″ E). To perform this research, a regular griding method with a 250 m dimension was done and 97 surface soil samples (0 to 30 cm) were taken. After transferring to the laboratory, the samples were dried and passed through a 2 mm sieve. The physical and chemical characteristics of the soil including pH, EC, texture, organic carbon, and Calcium Carbonate Equivalent (CCE) are measured. The concentration of heavy metals Copper (Cu), Zinc (Zn), Cadmium (Cd), Nickel (Ni), Chromium (Cr), Lead (Pb), Iron (Fe), and Manganese (Mn) was measured by Aqua Regia digestion method and using Atomic Absorption Spectrometry. The spatial distribution of heavy metals was displayed using the Kriging interpolation method. Pollution indices of Enrichment Factor (EF), Geo-accumulation Index (I_geo), Contamination Factor (CF), and Pollution Load Index (PLI) were calculated.
 
Results and Discussion
The maximum values ​​of pH and electrical conductivity of the soil in some places irrigated with wastewater have reached 7.70 and 4.35, respectively, and their average values ​​have reached 6.69 and 1.45, respectively. The organic carbon of the studied soil samples varies from at least 0.59% to 3.50% an average of 2.14%. The relatively high amount of organic carbon can be attributed to the land use type of garden. Four texture classes of sandy loam (65%), loamy sand (23%), loam (10%), and sand (2%) have been observed. The average concentration of the three metals Zn (85.41 mg Kg^-1), Cd (2.42 mg Kg^-1), and Pb (17.38 mg Kg^-1) was higher than the average of their continental reference values (0.7, 0.2 and12.50 mg Kg^-1, respectively). The higher values rather than continental reference values indicate human intervention and its effect on increasing the concentration of these element contents. It means that irritating sewage caused increasing heavy metal concentration in the study area. The averages of Cu, Ni, Cr, Fe, and Mn were lower than continental references. Pollution indices indicate the state of accumulation of polluting elements in a place compared to the initial values ​​in the parent materials. The EF index of Cd (75.85) is the highest value among the eight metals and 99% of the the study area is classified as a very high enrichment class. The EF of Pb (8.68), Zn (7.42), and Cu (6.14) are in lower ranks. 56.7 % of study area classified as considerable enrichment by Cu and 46.4 % by Zn. The EF clearly indicates the involvement of human activities in the accumulation of four elements Cd, Pb, Zn, and Cu in the study area. Also, moderate enrichment class is caused by Mn, Cu, Zn, and Cr in 63.9, 42.3, 42.3, and 25.8% of the study area, respectively. The lowest and highest amount of the I_geo index is related to Ni (-6.90) and Zn (3.72), respectively. The average of I_geo varies as I_geoCd> I_geopb> I_geoCU> I_geoZn> IgeoMn> IgeoCr> IgeoFe> IgeoNi that introduces Cd as the most pollutant metal. The negative values ​​of I_geo indicate the absence of heavy metal pollution and so absence of pollution. The entire study area grouped as non-polluted or clean class according to Ni, Cr, Fe, and Mn but 86.6% of the area grouped as clean considering Cu and Zn. Cd placed 38.1% of the area in the medium pollution class and 59.8% of the area in the severe pollution class. 69.1% of the area was found to be clean and only 28.9% of the area was moderately polluted with Pb. According to this index, Cd is in the extremely polluted class in the whole study area. The lowest (0.01) and highest (19.72) value of CF belongs to Ni and Zn, respectively. The average of this index varies from 0.13 for Ni to 12.09 for Cd. Except for Cd, which placed 98% of the area in a very high pollution class, the rest of the metals had low or moderate pollution classes. Meanwhile, the low pollution classes had higher contributions than the medium pollution classes. 100% of the area was grouped in the low pollution class considering Ni, Cr, Fe, and Mn but according to Zn and Cu 71.1 and 60.8% of the area was placed in the low pollution class, respectively. Medium pollution class was observed only by three metals Pb (73.2%), Cu (39.2%), and Zn (24.7%). PLI values ​​less than 1 indicate ideal conditions where no pollution has occurred. The values of the calculated PLI index were less than 1 in the whole study area indicating the absence of pollution.
 
Conclusion
Among the four indices, the I_geo index has classified a larger extent of ​​the studied area in extremely polluted classes, while the PLI index does not show any pollution in the study area. Because I_geo, like the other two indices (EF and CF), is an individual index and considers the concentration of each metal separately, the PLI index is a cumulative index and shows the cumulative effects of all metals. In other words, high concentrations of metals disappear among low concentrations and individual effects of metals are not visible. This may mislead decision makers in dealing with the type and origin of pollution and cause negligent actions. Therefore, it is recommended that considering the harmful effects of each of the metals, individual indicators should be taken seriously.