تغييرپذيري محتواي سزيم 731 و ارتباط آن با برخي ويژگيهاي خاك و پذيرفتاري - مغناطيسي در سايت مرجع شهرستان فريدن

Cesium-137, Variability, Reference site, Soil properties, Magnetic susceptibility, Fereydan area

به‌منظور بررسي تغييرپذيري محتواي سزيم-137 در سايت مرجع شهرستان فريدن و ارتباط آن با برخي ويژگي­هاي خاك، تعداد 96 نمونه از اعماق 15-0 و 30-15 سانتي­متري و هم‌چنين براي تعيين توزيع عمودي آن، 8 نمونه خاك از دو پروفيل مرجع از اعماق 5-0، 15-5، 25-15 و 35-25 سانتي­متري برداشته شد. دستگاه اسپكترومتري اشعه گاما براي آناليز سزيم-137 مورد استفاده قرار گرفت. مقادير پذيرفتاري مغناطيسي در دو فركانس 0/46 و 4/6 كيلوهرتز به وسيله دستگاه Bartington اندازه­گيري شد. ويژگي­هاي خاك، شامل نيتروژن كل، ماده ­آلي، ظرفيت تبادل كاتيوني، آهك، هدايت الكتريكي، pH، توزيع اندازه ذرات، سنگريزه و چگالي ظاهري در نمونه­ ها تعيين شد. سزيم-137 از خود تغييرپذيري نسبتاً كم آشكار كرد. نتايج نشان داد، چگالي ظاهري بيش‌ترين همبستگي مثبت را با سزيم-137 (0/01>r=0/46,p) دارد. بين رس و سزيم-137، هم‌بستگي منفي (0/01>r=0/52,p) مشاهده شد. كاني­ شناسي رس نشان داد كه كاني­ هاي ايليت، كائولينيت و كلريت در سايت مرجع مورد مطالعه وجود دارند. نتايج تجزيه و تحليل رگرسيون خطي چندگانه با روش گام به گام نشان داد كه چگالي ظاهري، درصد رس و پذيرفتاري مغناطيسي در فركانس پايين، 43 درصد از كل تغييرپذيري سزيم-137 را در سايت مورد مطالعه توجيه مي­كنند.

Introduction 137Cs is an artificial radionuclide with a half-life of 30.2 years and strong gamma radiation that, due to nuclear explosions in the 1950s, and 1960s entered the earth's surface by the rain. The radionuclide has been widely used for the evaluation rates and patterns of soil erosion and deposition. 137Cs was rapidly adsorbed to fine particles of soil surface horizon and its distribution is basically accompanied by the physical processes of soil, such as erosion and tillage. 137Cs is distributed exponentially as a function of depth to a maximum depth ranging from 5 to 15 cm in the undisturbed soil. The basis of calculating of erosion by 137Cs method is based on comparison between the amount of 137Cs in studied soil and the reference site (local, flat, un-eroded, undisturbed and stable site) and 137Cs loss or gain, relative to the reference inventory, indicates soil erosion and deposition, respectively. So, the reliable and accurate reference estimate of 137Cs is essential. No attempt has been made to explore the variability of 137Cs inventory in the reference sites in Iran. Therefore, This study was conducted to evaluate variability of 137Cs inventory and its relationship with some soil properties at the reference site located in Fereydan district in western of Isfahan province. Materials and Methods 96 soil samples were collected from 0-15 cm and 15-30 cm depths. Moreover, for evaluation of vertical distribution of 137Cs, two profiles were excavated and 8 soil samples were collected from 0-5, 5-15, 15-25 and 25-35 cm depths. Gamma-ray spectrometry device was used for 137Cs analysis. Magnetic susceptibility values were measured using Bartington susceptimeter at low (0.46 kHz) and high (4.6 kHz) frequencies and magnetic susceptibility dependent was calculated by low and high frequency values. Soil properties including total nitrogen, organic material, cation exchange capacity, calcium carbonate equivalent, EC, pH, texture, gravel and bulk density were determined. Descriptive statistics including mean, minimum, maximum, standard deviation (SD), coefficients of variation (CV), skewness, and kurtosis were calculated by SPSS software, v.16. The distribution of variables was evaluated using the Kolmogorov-Smirnov test. Pearson correlation coefficient and regression analysis were used to examine the relationships between 137Cs and soil properties. Results and Discussion The coefficient of variation of 137Cs inventory were 15.05 and 14.05% in 0-15 cm and 15-30 cm depth respectively. The result indicated that 137Cs has uniformly distributed in studied reference site. The results of the study showed that OM, OM/Clay, CEC/Clay, EC, Sand, BD, χlf and χhf were correlated with 137Cs. The results of multiple regression analysis indicated that bulk density, clay content and magnetic susceptibility at low frequency explained 43% of total variability of 137Cs in the studied reference site. Conclusions In the present study, variability of 137cs was investigated in a reference site in Ferydan district in Isfahan Province. The study showed that 137Cs had low variability. Among the soil physical and chemical properties, Soil pH, silt, gravel, nitrogen and calcium carbonate equivalent did not show significant correlation with 137Cs. The highest positive significant correlation was obtained for BD and 137Cs. Also, magnetic susceptibility at low and high frequencies showed significant correlations with 137Cs. In the multiple regression analysis, BD, clay content, and magnetic susceptibility were included in the model for explaining the 137Cs variability. It is suggested that similar study be done in the other reference sites in the western Iran.