تالاب میقان در بخش مرکزی حوزۀ آبخیز اراک، عناصر سنگین ناشی از فعالیتهای شهری، صنعتی و کشاورزی را دریافت میکند. در راستای ارزیابی آلایندههای تالاب میقان، 29 نمونه رسوب و خاک از تالاب برداشت، غلظت عناصر سنگین تعیین و با مرجع محلی و مرجع پوستۀ بالایی مقایسه شد. میانگین غلظت آرسنیک 03/4، سرب 13، روی 50، کروم 53، مس 33 ، نیکل34 و کادمیوم 21/0 میلیگرمدر کیلوگرم و در بیشتر نمونهها کمتر از مرجع بود. ضریب آلودگی و درجۀ آلودگی در تمام نمونهها برای مس و کروم متوسط و برای سایر عناصر کم بود؛ همچنین شاخصهایی مانند ضریب ریسک اکولوژیک، ضریب غنیشدگی و ضریب زمینانباشت برای تمام نمونهها درجۀ کم را نشان دادند. شاخصهای مرکب نظیر مجموع درجۀ آلودگی، پتانسیل ریسک اکولوژیک و شاخص نمرو برای مجموع عناصر گویای درجۀ کم آلودگی بود. فاضلاب شهری، پساب صنایع و کشاورزی مهمترین منشأ ورود فلزات سنگین به تالاب میقان محسوب میشوند؛ بنابراین، بایستی اقدامات لازم برای کاهش عناصر سنگین در منابع آلاینده بهویژه فاضلاب شهری در همجوار با تالاب میقان انجام شوند.
عنوان مقاله [English]
Assessing the heavy metal contamination in sediments of Mighan playa using pollution indices
Mighan playa in the central part of Arak watershed receives heavy elements from urban, industrial and agricultural activities. In order to evaluate the pollutants of Mighan playa, 29 sediment and soil samples were collected from playa and heavy metal concentrations were determined and compared with local and upper crust reference. The concentration of heavy metals in most samples is lower than their concentration in the crust reference. Individual contamination index, such as contamination factor, determined the degree of contamination in all samples medium for Cu and Cr and low grade for other elements. However, indices such as ecological risk, enrichment factor, and geo-accumulation index were low for all samples. Moreover, integrated indices such as sum of degree of pollution, ecological risk potential and Nemerow index for sum of elements indicate low degree of contamination. Municipal wastewater, industrial and agricultural wastewater are the most important source of heavy metals in the Mighan playa. Therefore, efforts should be taken to reduce heavy elements from pollutants, especially municipal sewage in the vicinity of the Mighan playa.
Keywords: Heavy Metals, Sediment Pollution, Pollution Indices, Mighan Playa, Arak
The role of human activities in the production of heavy metals can be traced to industrial activities, agricultural effluents, mining activities, urban and rural wastewater, industrial and domestic solid waste. Long-term use of heavy metals (such as Hg, Cd, As, and Pb) in an ecosystem will increase the heavy metals in organisms and cause morbidity and mortality. Arak Plain in the center of the Mighan playa is at the center of major urban, agricultural and industrial activities. Industrial, agricultural, municipal wastewaters and leachate, landfill and atmospheric depletion in Arak were the most important heavy metal production potentials that may reach the Mighan playa. In this study, the role of heavy metals will be determined by examining the concentration of heavy metals in soil and sediments in and around Mighan playa with local and international reference. On the other hand, pollution were evaluated using different indices of pollution and the degree of contamination and the source of heavy metals were determined.
Material & Methods
Arak basin with an area of 5500 km2 was a closed area of which 62% of its mountains, 36% of plain and 2% of Mighan playa. The playa in the center of the basin consists of two parts of island and lake. Twenty-nine samples were collected from soil and sediment from different land uses at a depth of 10 cm and measured heavy metals such as As, Cr, Cu, Ni, Pb, Zn and Cd using ICP-MS at Zarazma Lab. in Tehran.
Discussion of Results & Conclusion
Investigation of 29 soil and sediment samples from Mighan playa showed that the mean concentrations of As, Cr, Cu, Ni, Pb, Zn and Cd were 4.03, 53, 23, 34, 13, 50 and 0.21 mg / kg, respectively. The heavy metal values were compared with the local (As=5.90, Cr=73, Cu=23, Ni=45, Pb=13, Zn=66, Cd=0.25 mg/kg) and upper crust reference values (As=4.80, Cr=92, Cu=28, Ni=47, Pb=17, Zn=67, Cd=0.09 mg / kg). The study showed that 17.5% of the samples have arsenic above local reference and 35% above upper crust reference. 28% of Cr is above the local reference and 3.5% above the upper crust reference. The values for Cu are 43% and 21%, for Ni, 24% and 17%, for Pb, 59% and 14%, for Zn, 24% and 24%, and for Cd, 62% and 72%, respectively. Therefore, the role of human activities is significant in increasing metals in the sediments of Mighan playa. Cd and Pb are two heavy metals that accounted for the largest proportion of human influences in the Mighan playa. Most of the soils around Mighan playa are rich in elements such as As, Cr, Cu, Ni, Pb, Zn and Cd. Arak's municipal wastewater treatment is also rich in Pb and Zn.
In all samples, the total contamination index is less than seven and falls within the low contamination level. In other words, soils and sediment of different land uses are not contaminated with heavy metals such as As, Cr, Cu, Ni, Pb, Zn and Cd. Ecological risk potential index was also low in all samples with less than 150. The Nemerow index fall within the safety domain in all samples (except for rangelands). The Nemerow index is within the precaution domain in the rangelands. Contamination indices in soil and sediment samples of different land use indicate no contamination of As, Cr, Ni and Zn (low pollution grade). Cu and especially Pb vary from low to medium pollution degree in most samples.
Therefore, comparing the concentrations of heavy metals with local reference and upper crustal reference showed that the Mighan playa was affected by pollutants such as municipal wastewater, industrial effluent and agricultural activities. The contamination indices indicate heavy metal have low contamination and safety domain in soil and sediments of Mighan playa. If it is not controlled heavy metal pollution in wastewaters, the degree of contamination tend to intermediate to advanced degree in future. Therefore, birds and aquatic animals will be exposed to heavy metals. Since the most important source of heavy metals is urban wastewater, therefore, measures should be taken to reduce heavy metals from municipal wastewater.