Center for Applied Geoscience, University of Tübingen, Sigwartstrasse 10, D-72076 Tübingen, Germany.
J Contam Hydrol. 2011 Mar 25;122(1-4):26-39. doi: 10.1016/j.jconhyd.2010.10.006. Epub 2010 Nov 9.
Oxygen transport across the capillary fringe is relevant for many biogeochemical processes. We present a non-invasive technique, based on optode technology, to measure high-resolution concentration profiles of oxygen across the unsaturated/saturated interface. By conducting a series of quasi two-dimensional flow-through laboratory experiments, we show that vertical hydrodynamic dispersion in the water-saturated part of the capillary fringe is the process limiting the mass transfer of oxygen. A number of experimental conditions were tested in order to investigate the influence of grain size and horizontal flow velocity on transverse vertical dispersion in the capillary fringe. In the same setup, analogous experiments were simultaneously carried out in the fully water-saturated zone, therefore allowing a direct comparison with oxygen transfer across the capillary fringe. The outcomes of the experiments under various conditions show that oxygen transport in the two zones of interest (i.e., the unsaturated/saturated interface and the saturated zone) is characterized by very similar transverse dispersion coefficients. An influence of the capillary fringe morphology on oxygen transport has not been observed. These results may be explained by the narrow grain size distribution used in the experiments, leading to a steep decline in water saturation at the unsaturated/saturated interface and to the absence of trapped gas in this transition zone. We also modeled flow (applying the van Genuchten and the Brooks-Corey relationships) and two-dimensional transport across the capillary fringe, obtaining simulated profiles of equivalent aqueous oxygen concentration that were in good agreement with the observations.
氧在毛管边缘的传输对于许多生物地球化学过程都很重要。我们提出了一种基于光纤探头技术的非侵入式技术，可测量不饱和/饱和界面处氧的高分辨率浓度分布。通过进行一系列准二维流动实验室实验，我们表明，在毛管边缘饱和部分中的垂直水动力弥散是限制氧传质的过程。为了研究粒径和水平流速对毛管边缘横向垂直弥散的影响，测试了许多实验条件。在相同的设置中，同时在完全饱和的区域中进行类似的实验，因此可以直接比较通过毛管边缘的氧传递。在各种条件下进行的实验结果表明，两个感兴趣区域（即不饱和/饱和界面和饱和区）中的氧传输特征具有非常相似的横向弥散系数。没有观察到毛管边缘形态对氧传输的影响。这些结果可以用实验中使用的狭窄粒径分布来解释，这导致在不饱和/饱和界面处水饱和度急剧下降，并且在这个过渡区域中没有被困气体。我们还对流动（应用 van Genuchten 和 Brooks-Corey 关系）和通过毛管边缘的二维传输进行了建模，获得了等效水相氧浓度的模拟分布，这些模拟分布与观测结果非常吻合。
