اثر تحكيم روي ضريب انتشار مولكولي يون كلر در يك خاك رسي

Effect of Consolidation on Chloride Diffusion Coefficient in a Clayey Soil

به ­علت كم­بودن تخلخل و آهسته بودن سرعت جريان آب در خاك­ هاي رسي، مهمترين مكانيزم انتقال آلودگي اﻧﺘﺸﺎر ﻣﻮﻟﮑﻮﻟﯽ اﺳﺖ. در اﯾﻦ ﺗﺤﻘﯿﻖ اﺛﺮ ﺗﺤﮑﯿﻢ ﺧﺎك رس روي ﺿﺮﯾﺐ اﻧﺘﺸﺎر ﻣﻮﻟﮑﻮﻟﯽ ﯾﻮن ﮐﻠﺮ ﺑﺮرﺳﯽ ﮔﺮدﯾﺪ. ﻧﻤﻮﻧﻪ ﺧﺎك از ﻣﻨﻄﻘﻪ ﻧﺎزﻟﻮ واﻗﻊ در ﺷﻤﺎلﻏﺮب ﺷﻬﺮﺳﺘﺎن اروﻣﯿﻪ ﺗﻬﯿﻪ ﺷﺪه و ﭘﺲ از ﺗﻌﯿﯿﻦ ﻣﺸﺨﺼﺎت ﻓﯿﺰﯾﮑﯽ، ﭘﺎراﻣﺘﺮﻫﺎي ﺗﺤﮑﯿﻢ ﺧﺎك ﺗﻌﯿﯿﻦ ﮔﺮدﯾﺪ. ﺑﺮاي ﺗﻌﯿﯿﻦ ﺿﺮﯾﺐ اﻧﺘﺸﺎر ﻣﻮﻟﮑﻮﻟﯽ ﯾﻮن ﮐﻠﺮ در درﺟﺎت ﺗﺤﮑﯿﻢ ﻣﺘﻔﺎوت در ﯾﮏ ﺑﺎزه ﺗﻨﺸﯽ اﻓﺰاﯾﺸﯽ و ﺳﭙﺲ ﮐﺎﻫﺸﯽ، ﻧﻤﻮﻧﻪ اﺳﺘﻮاﻧﻪاي ﺷﮑﻞ ﺧﺎك ﭘﺲ از رﺳﯿﺪن ﺑﻪ ﻣﯿﺰان ﺗﺤﮑﯿﻢ ﻣﺸﺨﺺ در دﺳﺘﮕﺎه ﺗﺤﮑﯿﻢ، ﺗﺤﺖ آزﻣﺎﯾﺶ اﻧﺘﺸﺎر ﻣﻮﻟﮑﻮﻟﯽ ﻗﺮار ﮔﺮﻓﺘﻪ و ﺿﺮﯾﺐ اﻧﺘﺸﺎر ﻣﻮﻟﮑﻮﻟﯽ آن اﻧﺪازهﮔﯿﺮي ﺷﺪ. اﯾﻦ ﮐﺎر ﺑﺎ اﺳﺘﻔﺎده از ﮐﺪ ﮐﺎﻣﭙﯿﻮﺗﺮي POLLUTE و ﺑﺎ ﺑﺮازش دادهﻫﺎي ﻧﻈﺮي روي دادهﻫﺎي ﻣﺸﺎﻫﺪهاي ﻏﻠﻈﺖ- زﻣﺎن اﻧﺠﺎم ﮔﺮدﯾﺪ. ﻧﺘﺎﯾﺞ ﻧﺸﺎن داد ﮐﻪ ﺑﺎ اﻓﺰاﯾﺶ ﻣﯿﺰان ﺗﺤﮑﯿﻢ ﺧﺎك و ﮐﺎﻫﺶ ﻧﺴﺒﺖ ﺗﺨﻠﺨﻞ آن از 0/4 ﺑﻪ 0/35 در ﺑﺎزه ﺗﻨﺸﯽ kPa 25 ﺗﺎ kPa 400، ﺿﺮﯾﺐ اﻧﺘﺸﺎر ﻣﻮﻟﮑﻮﻟﯽ قرار گرفته و ضريب انتشار مولكولي آن اندازه­گيري شد. اين كار با استفاده از كد كامپيوتري POLLUTE و با برازش داده­هاي نظري روي داده­هاي مشاهده­اي غلظت- زمان انجام گرديد. نتايج نشان داد كه با افزايش ميزان تحكيم خاك و كاهش نسبت تخلخل آن از 4/0 به 35/0 در بازه تنشي kPa 25 تا kPa 400، ضريب انتشار مولكولي از5.94×10-10 m2/s to 5.36×10-10 m2/s كاهش يافت. در مرحله باربرداري تحكيم از kPa 400 تا kPa 50 ضريب انتشار مولكولي خاك از5.36×10-10 m2/s to 5.42×10-10 m2/s.افزايش يافت. نتايج اين مطالعه نشان داد كه روند اﻧﺘﺸﺎر آﻟﻮدﮔﯽ ﺑﻪ ﻃﺮﯾﻘﻪ اﻧﺘﺸﺎر ﻣﻮﻟﮑﻮﻟﯽ در ﺧﺎك رﺳﯽ ﻧﺎزﻟﻮ، ﺗﺎﺑﻌﯽ از ﻣﯿﺰان ﺗﺤﮑﯿﻢ اﯾﻦ ﺧﺎك اﺳﺖ ﻟﯿﮑﻦ اﺛﺮ ﺗﺤﮑﯿﻢ در ﮐﺎﻫﺶ ﺿﺮﯾﺐ اﻧﺘﺸﺎر ﻣﻮﻟﮑﻮﻟﯽ ﺧﺎك ﻗﺎﺑﻞ ﻣﻼﺣﻈﻪ ﻧﯿﺴﺖ. ﺗﻄﺎﺑﻖ ﺧﻮب دادهﻫﺎي ﻣﺸﺎﻫﺪهاي و ﻧﻈﺮي ﻧﺸﺎن داد ﮐﻪ ﻣﺪل ﻓﯿﺰﯾﮑﯽ ﻃﺮاﺣﯽ ﺷﺪه، روشﻫﺎي آزﻣﺎﯾﺸﮕﺎﻫﯽ ﺑﻪﮐﺎر ﮔﺮﻓﺘﻪ ﺷﺪه و ﻣﺪل ﻧﻈﺮي اﺳﺘﻔﺎدهﺷﺪه از دﻗﺖ ﺧﻮﺑﯽ ﺑﺮﺧﻮردار ﻫﺴﺘﻨﺪ.

Due to low porosity and slow water flow in clayey soils, the dominant pollutant transport mechanism may be molecular diffusion. The effect of consolidation of clayey soil on the chloride diffusion coefficient was investigated. The soil sample was collected from Nazloo region on North West of Urmia. After determination of soil physical parameters, the consolidation parameters were obtained. For determination of the chloride diffusion coefficient at different degrees of consolidation, when the cylindrical soil sample reached a certain degree of consolidation in the consolidation apparatus, the diffusion test was conducted and the diffusion coefficient was measured. This was conducted by the computer code POLLUTE by fitting the computed data on the observed concentration-time data. The results showed that by increasing the degree of consolidation and decreasing the soil void ratio from 0.4 to 0.35, in the range of stresses from 25 kPa to 400 kPa, the diffusion coefficient decreased from 5.94×10-10 m2/s to 5.36×10-10 m2/s. In the unloading process of consolidation from 400 kPa to 50 kPa the diffusion coefficient increased from 5.36×10-10 m2/s to 5.42×10-10 m2/s. The results of this study showed that the process of pollutant migration by molecular diffusion in the fine grained clayey soil of Nazloo was dependent on the degree of consolidation of the soil but the effect of consolidation on the decrease of soil diffusion coefficient is not significant. The good agreement between the observed and predicted data showed that the designed physical model, the adopted laboratory procedures and the theoretical model had good accuracy.