بررسي آزمايشگاهي وابستگي مكاني پراكنش در دو نوع خاك شني همگن اشباع

Laboratory Investigation of Spatial-Dependency of the Dispersivity in Two Kinds of Homogeneous Saturated Sandy Soils

پراكنش يكي از پارامترهاي مهم براي شبيه سازي انتقال آلاينده در محيط هاي متخلخل مي باشد. در اين تحقيق، به ﻣﻨﻈﻮر ﺑﺮرﺳﯽ ﺗﻐﯿﯿﺮات ﭘﺮاﮐﻨﺶ ﻃﻮﻟﯽ ﺑﺎ ﻣﻘﯿﺎس، آزﻣﺎﯾﺶﻫﺎي ردﯾﺎﺑﯽ در آزﻣﺎﯾﺸﮕﺎه اﻧﺠﺎم ﺷﺪ. آزﻣﺎﯾﺶﻫﺎ ﺑﻪ ﺗﺮﺗﯿﺐ در دو ﻧﻮع ﺧﺎك ﺷﻨﯽ ﻫﻤﮕﻦ اﺷﺒﺎع، ﺷﻦ درﺷﺖ و ﺷﻦ ﻣﺘﻮﺳﻂ و در ﯾﮏ ﺗﺎﻧﮏ ﺷﻦ ﺑﻪ اﺑﻌﺎد 250×10× 60 ﺳﺎﻧﺘﯽﻣﺘﺮﻣﮑﻌﺐ اﻧﺠﺎم ﮔﺮﻓﺖ. ﻓﺮآﯾﻨﺪ اﻧﺘﻘﺎل آﻻﯾﻨﺪه در ﺳﻪ ﮔﺮادﯾﺎن ﻫﯿﺪروﻟﯿﮑﯽ 0/017، 0/025 و 0/034 ﺑﺮرﺳﯽ ﮔﺮدﯾﺪ. ﭘﺎراﻣﺘﺮﻫﺎي اﻧﺘﻘﺎل آﻻﯾﻨﺪه ﺑﺎ اﺳﺘﻔﺎده از ﻧﺮماﻓﺮاز CXTFIT2.1 ﺗﺨﻤﯿﻦ زده ﺷﺪﻧﺪ. ﺑﺮاﺳﺎس ﻧﺘﺎﯾﺞ ﺑﻪدﺳﺖ آﻣﺪه، ﻣﻘﺎدﯾﺮ ﭘﺮاﮐﻨﺶ ﺧﺎك ﺷﻨﯽ ﻣﺘﻮﺳﻂ و داﻣﻨﻪ ﺗﻐﯿﯿﺮات آن ﺑﯿﺸﺘﺮ از ﺧﺎك ﺷﻨﯽ درﺷﺖ ﺑﻮد. اﯾﻦ اﻣﺮ ﺑﯿﺎنﮔﺮ ﻓﺮاﯾﻨﺪ ﻏﺎﻟﺐ ﭘﺨﺶ در اﻧﺘﻘﺎل آﻻﯾﻨﺪه در ﺧﺎك ﺷﻨﯽ ﻣﺘﻮﺳﻂ ﺑﻮد. ﺗﺤﺖ ﮔﺮادﯾﺎنﻫﺎي ﻫﯿﺪروﻟﯿﮑﯽ ﻣﺨﺘﻠﻒ، ﭘﺮاﮐﻨﺶ ﺑﺎ ﻓﺎﺻﻠﻪ از ﻣﻨﺒﻊ آﻟﻮدﮔﯽ اﻓﺰاﯾﺶ ﯾﺎﻓﺖ. در ﮔﺮادﯾﺎن ﻫﯿﺪروﻟﯿﮑﯽ 0/017، اﻓﺰاﯾﺶ ﭘﺮاﮐﻨﺶ ﺑﺎ ﻓﺎﺻﻠﻪ ﺑﻪﺻﻮرت ﻏﯿﺮﺧﻄﯽ ﺑﻮد؛ در ﺣﺎﻟﯽ ﮐﻪ در ﮔﺮادﯾﺎنﻫﺎي ﻫﯿﺪروﻟﯿﮑﯽ 0/025 و 0/034 ﭘﺮاﮐﻨﺶ ﺑﺎ ﻓﺎﺻﻠﻪ ﺑﻪﺻﻮرت ﺧﻄﯽ اﻓﺰاﯾﺶ ﯾﺎﻓﺖ. در ﮐُﻞ، ﭘﺮاﮐﻨﺶ ﺧﺎك ﺷﻨﯽ ﻣﺘﻮﺳﻂ، ﺑﻪوﯾﮋه ﺗﺤﺖ ﯾﮏ ﮔﺮادﯾﺎن ﻫﯿﺪروﻟﯿﮑﯽ ﮐﻮﭼﮏ، ﺑﻪﺷﺪت واﺑﺴﺘﻪ ﺑﻪ ﻣﻘﯿﺎس ﺑﻮد و ﺗﻐﯿﯿﺮات آن از ﯾﮏ ﺗﺎﺑﻊ ﻧﻤﺎﯾﯽ ﭘﯿﺮوي ﻣﯽﮐﺮد.

Dispersivity is one of the important parameters for simulating solute transport in porous media. In this study, laboratory tracer tests were conducted to investigate the spatial variation of longitudinal dispersivity. The tests were carried out in a sand tank with dimensions of 250´10´ 60 cm3 with two kinds of saturated homogeneous coarse and medium sands, respectively. The solute transport process was studied at three hydraulic gradients of 0.017, 0.025 and 0.034. The solute transport parameters were estimated using CXTFIT2.1 software. Based on the obtained results, the average dispersivity values of the medium sand and their range were higher than those for the coarse sand. This pointed out that the dispersion was the dominant process in solute transport in the medium sand. Under the various hydraulic gradients, the dispersivity increased with increasing distance from the contaminant source. The dispersivity increasing followed a non-linear function of travel distance at hydraulic gradient of 0.017, while it followed a linear function of travel distance at hydraulic gradients of 0.025 and 0.034. In general, the dispersivity of the medium sand, especially under a small hydraulic gradient, was strongly space-dependent and its variations followed an exponential function.