عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Different human activities depend on water availability so most of the civilizations have been emerged and developed in the proximities of rivers. However, such developments lead to polluting water bodies (Alizade, 2004). For sustainable agricultural and industrial developments monitoring the quality of water resources is inevitable. Heavy metals negatively impact water quality as they form complex and poisonous compounds when entering water bodies (Ostan, 2005). Heavy metals exchange between sediments and water and the equilibrium exists between these phases in rivers. Such equilibrium changes by any variation in media reaction, oxidation and reduction condition, the amount of organic matter, microorganisms' activities, concentration of soluble elements and cation exchange capacity of suspended particles. The sources of heavy metals in water resources are anthropogenic (from human activities) or naturally released from mineral weathering (Khosheqbal et al. 2012). Hoba et al. (1983) reported that severe erosion of stones in the Vender river watershed and releasing a lot of Ca2+ led to formation of carbonates of calcium and some heavy metals and also Fe and Mn hydroxides in sediments of the river. Zhao et al. (2012) analyzed heavy metal concentration of fish and crabs' tissues and found a significant and positive correlation between the concentration in legs of crabs, skins and heads of fish and the concentrations in water. Sediments are considered pollutants as they absorb and transport heavy metals. Akhondi et al. (1989) studied heavy metal concentration of the Qomroud River and reported that the concentrations of Cd, Zn and Pb in water are below the permissible threshold for drinking. They expressed that sources of heavy metals are petroleum refineries, metal mills, electrical and textile industries. The Zayanderoud River is one of the main rivers of interior plateau of Iran also a water resource for drinking, industries and croplands in Chaharmahal & Bakhtiari and Esfahan provinces. The river origins from the Zagros Mountains and passes through cultivated lands, orchards, fish farms and proximity of steel mill industries and pours into Gavkhoni wetland in the southeast of Esfahan City. Due to the importance of the river, the objectives of this study were monitoring spatial and temporal variations of heavy metal concentrations in water and sediments of the Zayanderoud River.
Mean annual rainfall of the Zayanderoud watershed is 450mm and the surface area of the watershed is 27100 km2 (Johari, 2010). To fulfill the objective 10 points were selected between the Zayanderoud Dam in the west and Zarrinshahr in the east based on overlook land uses and the accessibility of the river for sediment and water samplings. Water and sediment samples were collected bimonthly according to the Geological Survey of US (1978) in 2012. Sediment samples were collected from the surficial (0-10 cm) of river bed, air dried, grinded and passed through 2 mm sieve. Total heavy metals Pb, Zn, Cu and Cd of sediments were extracted by acid (Sposito et al., 1982) and the extracted heavy metals and the concentration of water samples were measured using Atomic Absorption Spectrometer. Landsat ETM+ data of 2012 were used and the land use map of the study area was delineated.
According to the land use map, the overlooked areas to the river were rangelands, stone outcrops, settlements, rice cultivated areas, orchards and industrial zones. By the temporal and spatial variations of mean Pb concentration in water and sediments, the highest mean Pb concentration belonged to point 10 which was significantly different from of all points except point 9. The results also indicated that the maximum mean Pb concentration of sediments was 20.63 ppm and belongs to point 9. Wang et al. (2012) studied the water quality of the Lancang River and expressed that the exhausted smoke of cars was the source of Pb. The main sources of Pb in the study area could be steel mill industries as point 9 locates in the vicinity of these industries. A significant and positive correlation existed between Pb concentration of water and of sediments in all points. Also, the Pb concentration of water and sediments increased in rainy months which showed possible transportation of Pb by runoff from polluted areas to the river. Significant intercorrelation coefficients among Pb, Zn and Cu indicated possible common sources of these elements which are different from Cd resources. Spatial and temporal variations of Zn mean concentration in water were significant. The maximum mean concentration of Zn in water was 6.2 ppb which was below the permitted threshold for drinking according to the Institute of Standards and Industrial Researches of Iran (2010). Maximum concentration of Zn in sediments was 34.36 ppm. Zn used as a plant micronutrient and a component of insecticides and herbicides in croplands and orchards in the area. On the other hand, maximum Zn mean concentration observed in point 9 (5.6 ppb) and in sediments of point 2. The land use map showed that the overlooked land use of point 2 were orchards while of point 9 were steel mill industries. Therefore, the source of Zn could be fertilizers and insecticides which applied in orchards. Likewise, Wang et al. (2012) reported that Zn application as fertilizer and insecticide in a part of China led to river pollution. Namazi et al. (2014) analyzed the dust in the proximities of steel mill industries and reported that they contain heavy metals consist of Zn. Mean Cd concentration in water increased from the west to the east of studied points. The highest and lowest Cd mean concentration sediments were in points 10th and 1st, respectively. Maximum mean Cu concentration was observed in the sediments of the 6th point.
Regarding the permissible thresholds of studied heavy metals for drinking water (Institute of Standards and Industrial Researches of Iran, 1388), the concentrations of the studied heavy metals were below the threshold and the water in all points was safe for drinking. The results also indicated that industrial areas, croplands and orchards were the main non-point resources of heavy metal for the river and runoff was one of the main ways of transferring pollutants to the river.