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Heavy metals are the most toxic and hazardous pollutants in the environment (1) whose occurrence in various ecosystems, especially aquatic ecosystems, due to properties for instance high stability, high toxicity, mutagenicity, carcinogen, etc. serves as a threat. Heavy metals come into the environment from different sources for example natural processes, human activities and atmospheric deposition. Though, given rapid population growth, the economic life improvement, and as a consequence of widespread human activities, the function of human resources in the creation and entry of these pollutants into aquatic ecosystems, especially sediments, is far more than other sources (1). After entering the aquatic ecosystems, these elements can be dangerous to living organisms, and they accumulate through various chemical derivatives, such as absorption or sedimentation and adsorption of suspended particles. Therefore, sediments can be measured as the final reservoir of heavy metals (2). But when changes in environmental circumstances such as pH, electrical conductivity, oxidation and reduction reactions, organic matter, salinity, chemical and biological oxygenation of water or sediments, these elements can be released from sediments in the aqueous environment and A-R-T-I-C-L-EI-N-F-O A-B-S-T-R-A-C-T Article Notes: Received: Jul 14, 2017 Received in revised form: Dec 16, 2017 Accepted: Dec 25, 2017 Available Online: Jan 1, 2018 Background & Aims of the Study: Heavy metals pollution in aquatic ecosystems threats living organisms' health and environment. Hence, the present study aims to assess concentration of lead, chromium, copper, zinc and their ecological risk in the surface sediments of the Bashar River during the summer and winter seasons. Materials & Methods: This is a field study; sampling of surface sediment was done in 8 stations in 2016. The samples were digested by the composition of ratio of 1:4 Percholoric acid and Nitric acid and then were analyzed by atomic absorption spectrophotometry. Statistical analysis of all data was also done using SPSS 21 and various criteria were used to fine the amount of contamination of heavy metals. Results: The results showed that the average total concentration of Zn, Cu, Pb and Cr in the summer were (48.16, 39.2, 14.45 and 10.35) and as for winter was (39.88, 26.93, 12.23 and 10.301) mg/kg respectively. In addition, evaluating geo-accumulation indexes, pollution load and pollution factor indicated the low pollution status of the area in both seasons. Conclusions: According to the results, pollution status and the RI level of heavy metals in the Bashar River is low and similar in summer and winter. Also, the sources and changes in elements concentration in different stations and seasons may be as a result of the institution of various urban and industrial contamination sources along the river, and its physical and geobiochemical changes in water flow volume entering the river, especially in winter.

Assessment of Ecological Hazard of Heavy Metals (Cr, Zn, Cu, Pb) in Surface Sediments of The Bashar River, Yasouj, Iran