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

在土壤蒸汽抽提、空气喷淋等地下水修复中，我们通常会观察到“尾矿”现象，即回收的污染物浓度下降非常缓慢。了解尾矿很重要，因为它控制着清理污染场地所需的时间。研究人员将这种现象归因于污染物从土壤材料的微孔中缓慢洗脱。然而，我们提出了另一个假设，即这些尾矿可能是由这些修复系统中的空气和水的配置引起的。本研究制备了内径30 cm，内高50 cm的不锈钢柱。来自Tedori河的沙子（从当地的建筑商那里获得）被风干并过筛，然后将0.5 - 2.0毫米的沙子装进50厘米高的柱中。该柱与水库相连，能够控制砂层的地下水位。注气口设置在柱底上方10cm处。注气口通过质量流量控制器与氮气、氦气和二氧化碳气瓶相连。将二氧化碳溶解在柱内的水中，然后向柱内供氮，测量出气中二氧化碳的浓度。结果，我们观察到明显的“尾迹”，时间-浓度关系在双对数图中几乎是线性的。由于砂介质中微孔较少，因此我们认为“尾砂”不是由微孔扩散引起的。借助最新的尾砂分析理论，此时我们认为尾砂是由水相中“扩散距离”a的分布引起的，其分布服从幂律分布：f(a) ~ a^<1.15>。

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>.