ANALYSIS OF PREFERENTIAL FLOW AND DIFFUSION DISTANCE OF POLLUTANTS IN AIR-SPARGING

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In groundwater remediation such as soil vapor extraction, air-sparging etc., we usually observe "tailing," the phenomenon that the concentration of the recovered pollutants decreases very slowly. Understanding the tailing is important because it controls the time required for the clean-up of polluted sites. Researchers have ascribed this phenomenon to the slow elution of pollutants from the micro-pores of the soil material. However, we came up with another hypothesis that the tailing can be caused from the configuration of the air and water in these remediation systems.In this research, a stainless column of 30 cm in inner diameter and 50 cm in inner height was prepared. Sand from Tedori river (obtained from a local constructor) was air-dried and sieved, and then 0.5 - 2.0 mm fraction of it was packed into the column up to the height of 50 cm. The column was connected to the water reservoir to enable to control the water table in the sand layer. Gas injection port was set 10 cm above the bottom of the column. The gas injection port were connected to nitrogen, helium and carbon dioxide cylinders via mass flow controllers. Carbon dioxide was dissolved into the water in the column, and then, nitrogen was supplied and the concentration of carbon dioxide in the effluent gas was measured. As the results, we observed an apparent "tailing," the time-concentration relationship was practically linear in double-log plots. As our sand medium has little micro-pores, we concluded that the "tailing" was caused not by the micro-pore diffusion. With the help of recent theory of analyzing the tailing, at this point, we consider that the tailing was caused by the distribution of the "diffusion distance" in the water phase, a, and the distribution obeys the power law distribution: f(a)〜a^<1.15>.