ارزیابی زیست محیطی آلودگی فلزات سنگین درمنابع خاک محدوده شهرک صنعتی چرم میامی (شرق مشهد)

Introduction
The increasing development of industries has led to tge production of wastewater the discharge of discharge of which into agriculatural water and soil has created complex environmental problems. Amond the pollutants in industrial wastewater are heavy metals that pose a threat to human health and food hygiene , even in low concentrations. Heavy metals are among the pollutants that are important due to toxicity and stability in the environment and public health concerns.These metals are very important because of the threats that can affect human health. Enviromental pollution with heavy metals due to toxicity and nondegradability is considered aserious problem and due to its accumulating and carcinogenic properties can cause numerous health and environmental problems.
Various human activities such as agrilture, industry and mining along with the characteristics of mineralization, lithology, geological processes and weathering in any region can cause the release of heavy metals in water and soil resources. The soil provides an environment for geochemical deposition of pollutants .Human activities and pollution created as a result of industrialization and technological development have led to soil pollution and the gradual extinction of plants and animals in the environment.
Maiamei leather industrial town is located in the northeast of the country(Mashhad-Sarakhs road), in khorasan Razavi province. Due to the high consumption of water in tanning operations, the leather industry always releases a lot of effluent in to the surface water sources, groundwater and soil. This industry a uses a lot of damage to the environment by importing chemical such as lime, sodium, sulfide, sulfat, ammonium, sodium chloride through wastewater. The leather industry is one of the most polluted light industries in the country, which producses several hundred tons of industrial wastewater annually. It is a large volume of heavy metals that cause several damage to the soil. In iran,t he cost of supplying a treatment plant for leather production is high, so wastewater treatment systems are not enough.
Methodology
The purpose of this study is to investigate the concentration of heavy metals and soil pollution in the study area. For this purpose, after library studies and geological maps and field studies, soil sampling was performed.20 soil samples were taken from a depth of 10 to 20 cm in the area around the industrial town. The samples were transferred to the environmental laboratory of Islamic Azad University of Mashhad and the values of PH and EC soil parameters were measured. To determine PH, 10 gr of soil that was passed through a 2mm sieve after drying in the open air, was used. Soil samples were poured into 100 cc and 25ml of ionized water was added to it. The samples remained in place for 16 hours to achieve equilibrium. After 16 hours. the samples were read. To measure soil solutes, certain weight ration of sediment and water (1 to 5) were used, so that 10 gr of sediment, which was dried in the open air and passed 2mm of sludge, was poured into a 250cc container and 150ml of distilled water was added. It was placed on the stereo for 5 minutes and after passing through paper. the reading was done using. To separate the sand and mud, a 4mesh sieve was used and separate the silt and clay hydrometric method. The classify the soil, asoil texture triangle was used according to the American Agricultural Soil Standard, taking into account the weight percentage of clay, silt and sand.
After drying, the samples were transferred to Zarazma laboratory for analysis with the device ICP-OES. The results were processed and evaluated by SPSS and Excel software.
Conclusion
The amount of PH in the soil of the region dose not change much. The lowest value is 7.93 and the maximum value is 8.8. with increasing PH concentration OH and the negative charge of colloids increases and causes a decreases in the activity of heavy metals and their accessibility in the soil. The change EC it is high in the region. Its minimum amount is 253 and its maximum amount is2037.The presence of calcium and magnesium in some samples has caused an increase. The soil of the region is in the sub-alkaline , range with average PH 8.36. This soil has a greater ability to absorb most heavy metals and their exite from the soil solution. The soil tecture is mudsandy and average EC soil is 810.95.
To evaluate the concentration of heavy metals contamination in the soils of the region, it is usually used to compare the concentration of elements in the region with the presented standards and the enrichment coefficient, pollution coefficient and accumulation index. Heavy metal concentrations in regional soil samples were compared with global shale. The average concentrations of calcium, thorium and strontium are higher than the global shale and the average of the other elements is lower than the global shale.
Among the elements, only calcium with an enrichment index coefficient of more than 5, shows moderate to severe enrichment. The rest of the elements with an index enrichment coefficient of less than 2 have low enrichment. Enrichment maps showed that the concentration of calcium in the northwest and southwest of the region was high and decreased in the center. According to the results of the contamination index, calcium contamination with and average (8.13),has average high pollution in the soils of the region. Thorium with an average (3.4) and stronsium with an average (2.62), show moderate pollution. The results of the soil accumulation index of the region showed that only the calcium with an average of (1.3) has the highest pollution. Thorium has an average of (0.02) non polluted to moderate pollution. Stronsium and arsenic with an average of less than zero are unpolluted.
Magnesium in the southern and centeral parts shows a higher enrichment coefficient and in the northern parts its concentration decreases. Groundwater abstraction and the use of agricultural fertilizers can cause increased magnesium in the soil of the region. The presence of igneous rocks in the center and south of the region has increased the concentration of strontium in these parts.
The use of large amounts calcium hydroxide in the liming stage of the tanning process and the transfer of this lime to the tanning effluent and the addition of lime to the effluent to neutralize the acidity of the effluent, has increased and concentrated calcium especially in the northwest and southwest. As we approach the centeral part of the region, the amount of calcium decreases and the amount of magnesium increases.