آلودگي زيست محيطي آب و رسوبات رودخانه گرگانرود در محدوده شهر گنبد

Environmental contamination of Gorganrood Water and Sediment in district of Gonbad-Kavoos City

رودخانه گرگانرود يكي از مهم‌ترين رودخانه هاي استان گلستان است كه نقش مهمي در تامين آب اين استان دارد. هدف از اين مطالعه ارزيابي آلودگي رودخانه گرگانرود در محدوده شهر گنبد مي‌باشد. به اين منظور ويژگي‌هاي فيزيكي و شيميايي 10 نمونه آب و رسوب و نيز غلظت عناصر بالقوه سمي در اين نمونه‌ها با استفاده از روش‌هاي استاندارد اندازه گيري شد. با توجه به مقادير جامدات حل‌شده كل (844 تا 3532 ميلي‌گرم بر ليتر) و هدايت الكتريكي (1535 تا 5970 ميكروزيمنس بر سانتي‌متر)، آب رودخانه گرگانرود براي مصارف آبياري نامناسب است. داده‌هاي هيدروشيميايي نشان مي‌دهد كه تيپ نمونه هاي آب در نمونه‌هاي محدوده شهر كلروره سديك و در نمونه‌هاي خارج از محدوده شهر سولفاته سديك مي باشد. اين امر احتمالاً نشانگر تاثير ورود فاضلاب‌هاي شهري بر تغيير تيپ آب است. غلظت عناصر بالقوه سمي به‌ويژه آرسنيك، كادميم، كروم و سرب در تمامي نمونه‌هاي آب بالاتر از مقادير استاندارد در رودخانه‌هاي جهان است. شاخص هاي ژيوشيميايي (ضريب غني‌شدگي، ضريب آلودگي و ضريب بار آلودگي) و روابط همبستگي بين عناصر بالقوه سمّي و عناصر اصلي در نمونه‌هاي رسوب نشان‌دهنده تاثير فعاليت‌هاي انسان بر افزايش غلظت عناصر به‌ويژه در ايستگاه‌هاي محدوده شهر مي باشد. همچنين، نتايج به دست آمده نشانگر تاثير ويژگي‌هاي فيزيكي وشيميايي رسوبات، به‌ويژه ميزان مواد آلي، ظرفيت تبادل كاتيوني، pH و ميزان كربنات بر جذب عناصر بالقوه سمّي مي‌باشد.

Introduction Rivers are the key resources for drinking and agricultural purposes and their quality assessment is very important. The chemical quality of surface waters is influenced by natural processes and anthropogenic activities (e.g. discharge of urban, agricultural and industrial wastewaters). Pollutants discharging into a river from both natural and anthropogenic sources are distributed between sediment and water. Thus, in evaluating the pollution condition of a water body, both sediment and water should be considered. Sediments are generally regarded as an important component of the total river systems, since they provide a bank of environmental information for both natural and anthropogenic pollution. Of the various pollutants, potentially toxic elements (PTEs) are of environmental concern, because they are the most toxic, persistent and abundant pollutants that cab accumulate in aquatic habitats and their concentration increases through biomagnification. Regarding the importance of rivers in supply of water, the assessment of hydrochemical properties and PTEs concentration in water and sediment is very important. Gorganrood is an important river in Golestan province (NE of Iran), which plays an important role in providing water supply. This river recharges from Aladagh Mountains and discharges into Caspian Sea. The Gorganrood River is about 350 Km long and its drainage area is about 1025 Km2. This river trends E-W across the study area and is supplied by many tributaries. The average water discharge of Gorganrood in autumn and spring is 4.6 and 12.3 m3/sec, respectively. This river flows through the recent alluviums (silt, sand and clay). During the last years, various domestic, agricultural and industrial wastewaters in Gonbad-e-Kavoos district discharge into the Gorganrood. The wastewaters are constant polluting source for rivers. This study aimed to assess the quality of Gorganrood River water as well as the pollution of bed sediments at Gonbad-e-Kavoos district. Material and Methods To evaluate the quality of water and sediments at the study area, 10 water and sediment samples were collected in the month of November in year 2012 by standard methods. The physicochemical parameters of water samples including pH and electrical conductivity (EC) were determined in situ using portable conductivity and pH meter. At each sampling station, water samples were collected in 1-litre prewashed polyethylene bottles. Water samples were filtered using a vacuum pump and 0.45?m pore-size filter papers in order to separate particulate matter. The filtered samples were then split into two portions: one portion was acidified with HNO3 for dissolved element measurement and the second unacidified portion was used for determination of dissolved anions. The samples were kept at 4°C prior to analysis.The concentration of major ions in water samples was measure using the standard methods: Ca and Mg were determined using titration with EDTA, Cl was measured using titration with AgNO3 and bicarbonate was determined using titration with HCl. Flame photometry was applied for determining Na and K concentration. Nitrate and phosphate values were measured using spectrophotometer. Cationic and anionic charge balance was calculated as a proof for the precision of the data. Total dissolved solid (TDS) was measured using evaporation method. The piper diagram was used to assess the water type. Sediment samples were air dried, and then, the < 63?m size fraction was recovered by sieving. Total elemental concentrations of sediments were analyzed after digestion with HF-HNO3-HClO4. PTEs concentration in water and sediment samples was measured using an ICP-OES instrument. Hydrometery and sieving were used to determine the sediment texture. pH of sediment samples was measured in deionized water with a 1:2 soil/solution ratio after equilibration for 15 min. The organic matter content was measure by measuring the amount of organic carbon in the samples (oxidation with potassium dichromate in a strong acid medium). Back-titration method was applied to measure the carbonate content. The cation exchange capacity was determined by saturation with 1 M ammonium acetate solution, in a buffered medium at pH 7. Results and Discussion Based on the values of total dissolved solids (844-3532 mg/l) and electrical conductivity (1535-5970 µs/cm), the Gorganrood water is not suitable for irrigation purposes. The average major ion concentration (meq/l) in the samples decreases in the order: Na+ (15.3) > Mg2+ (10.4) > Ca2+ (7.06) > K+ (0.1) and SO42- (13.9) > Cl- (13.2) > HCO3- (6). The piper diagram shows that the water type in the city district samples is Na-Cl, but in other samples is Na-SO42-, perhaps due to the effect of municipal wastewaters discharge on water type. Despite the extensive agricultural activities in the Gorganrood basin, the nitrate concentration of the samples are below the maximum allowable concentration for drinking usage (50 mg/l, WHO 2011), perhaps due to the denitrification process. Phosphate concentration in unpolluted waters typically ranges from 0.01-.0.1 mg/l. The mean phosphate content of the studied water samples (3.3 mg/l) exceeds the standard phosphate concentration in unpolluted waters, which is due to the phosphatic fertilizers used in agricultural lands around the river. Compared with standard values in world rivers, the concentrations of potentially toxic elements especially As, Cd, Cr and Pb in Gorganrood water samples are high. Metal index values were calculated as follows: Ci in the above formulas is the concentration of the examined element in the water sample and C0 is the maximum allowable concentration of that element. On the base of MI values, the studied water samples are polluted with PTEs. The texture of samples based on size fractionation is sandy-mud, sandy-silt and mudy-silt. Sediment pH values ranged from 7.5 to 8. The organic matter content varied between 1.8 to 9.2 %. The carbonate contents varied from 14 to 24 %. The range of CEC varied from 2.2 to 5.1 meq in 100 g. Compared with world average sediment composition, the concentrations of PTEs in studied sediments (except for Cd) are lower, perhaps due to the sandy texture of the sediments and/or high EC of river water. The geochemical indices (enrichment factor, contamination factor and pollution level index) and correlation analysis between elements confirm the effect of anthropogenic activities on the increase of elemental content in sediments, especially within the city district. The results also show the effect of sediment properties, especially organic matter content, CEC, pH and carbonate content on adsorption of potentially toxic elements.