پایش غلظت فلزات سنگین ناشی از طوفان گردوغبار در خاک اکوسیستم‏ های زراعی گندم در جنوب استان فارس

Monitoring of heavy metal caused by dust storms in wheat agronomic ecosystem soils in south of Fars Province

Introduction
Heavy metals in dust particles play an important role in the contamination of agricultural soils. Heavy metals are one of the most dangerous pollutants in the environment due to their bioaccumulation power. That is, they are able to accumulate in the body system of living organisms and increase their concentration with greater exposure to pollutants. Heavy metals are generally referred to as a group of elements that having a specific greater than 6 g/cm3 gravity and greater than 50 g an atomic weight. The most important heavy metals that are important for environmental protection include cadmium, arsenic, cobalt, vanadium, zinc, mercury, iron, manganese, nickel, lead, chromium and copper which for non-biodegradability in nature and their long life span, they are important pollutants of the environment. Also pollution of agricultural lands with metals by chemical action such as sewage sludge, chemical fertilizers and industrial wastewater application, but deposition of dry and wet dust particles caused by the dust storm phenomenon is one of the most important sources of pollutants for wheat agronomic ecosystems to heavy metals. Pollutant of dust storm with heavy metals is considered a serious problem due to toxicity, degradability and cumulatively. Deposition of dust particles contaminated with heavy metals on crops land leads to combine with soil solution, and plants absorb the heavy metals, paving the main route of transfer of metals to the food chain. Deposition of dust particles with heavy metals in the soil of wheat agro ecosystems can endanger human health; then identification of heavy metals content in wheat fields is the first step in reducing the health risk that this research is trying to this goal.

Materials and methods
In this study, the dust storm in the 16 year period (2000-2015) related to stations of Larestan, Jahrom, Darab and Fasa was obtained from the Meteorological Organization and analyzed frequency of dust storm in the seasonal and annual scales. Next, the dust particle entry pathway to the study area in the May 12, 2018 was performed using by HYSPLIT model. Subsequently, soil samples were taken from wheat fields of Larestan, Fasa, Darab and Jahrom. Four wheat ecosystems were identified in each study area and soil sampling was performed from a depth of 1 cm at 400 cm2 area in each wheat ecosystem. The soil sampling was divided two stages before and after the dust storm. Sampling was done before the dust storm in the months of December and March and after the dust storm in May and June. Soil samples were transferred to the laboratory and concentrations of lead, cadmium, and nickel were measured by flame atomic absorption spectrometry. Finally, the amounts of heavy metals in the soil samples were evaluated by the Igeo index.

Results and discussion
The maximum frequency of dust storms in the southern part of Fars province is in the warm months, especially in spring, with an average of 18 dust storms per season. The minimum of this environmental phenomenon is in the cold months, especially in the autumn, which occurs less than once in the season. Also among the stations in the study area, Fasa station has the maximum of annual dust storm with 42 occurrences. Next, Jahrom, Darab and Lar stations are in the next rank with 39, 25 and 19 storms per year respectively. The path of dust particles entering the study area on 12 May 2018 was tracked using by HYSPLIT model backward method and results represented that deployment of thermal low-pressure in the Persian Gulf has led to instable of atmosphere and the shamal winds moved through the deserts of Iraq and Saudi Arabia to the Persian Gulf and it has caused a massive dust particles to be transported to the southern provinces of Iran. Also results showed that concentrations of lead and cadmium in the soil of wheat agronomic ecosystems of Fars province increased under the influence of dust storms; but the concentration of nickel remained unchanged. The highest concentrations of heavy metals were in the Darab and Lar ecosystems and the lowest were in the Jahrom and Fasa. The mean comparison test showed a significant difference between the concentrations of heavy metals in agricultural ecosystems in south of Fars province before and after the dust storm at 0.05 and 0.01 levels and dust storm led to increase the heavy metals in the soil of wheat ecosystem. Also, the land pollution standard showed that in the agricultural wheat ecosystems of Darab, the concentration of all metals was higher than the global limit and was in the moderate to severe pollution category. In this study, the effect of desert dust on heavy metal concentrations in soil ecosystems was investigated and their concentrations were measured before and after the occurrence of dust and their differences were compared with statistical tests. It showed that the concentration of lead and cadmium in the soil of wheat agronomic ecosystems increased under the influence of dust storms and nickel concentration remained unchanged.

Conclusion
In addition to combustion sources, industries and factories, traffic and the use of fertilizers and municipal wastewater, desert dust particles are also a contributor to air pollution and an increase in heavy metal concentrations in agricultural soil that can affect health. Damage the environmental ecosystems and organisms, especially humans, which are most consumed by agricultural soils and plants. Deposition of dust particles caused by dust storms in wheat cultivated soils results in increased concentration of heavy metals in the soil and its uptake by the roots of plants and its transfer to the crop which endangers for human health. Because wheat is one of the most consumed foods in the human body and the transfer of toxic and heavy metals through soil, roots and plants causes to accumulate in wheat. Therefore, in order to achieve world-class quality and health products, solutions must be provided to reduce the concentration of heavy metals in consumer products. The results of this study can be made available to planners and agricultural and health expert.

Keywords: Atomic absorption, Fars Province, HYSPLIT models, Igeo index, Lead.