Rahman Md Arifur, Jose Surabhin C, Nowak Wolfgang, Cirpka Olaf A
Universität Stuttgart, Institut für Wasserbau, Pfaffenwaldring 61, 70550 Stuttgart, Germany.
J Contam Hydrol. 2005 Nov 15;80(3-4):130-48. doi: 10.1016/j.jconhyd.2005.06.010. Epub 2005 Aug 22.
Vertical transverse mixing is known to be a controlling factor in natural attenuation of extended biodegradable plumes originating from continuously emitting sources. We perform conservative and reactive tracer tests in a quasi two-dimensional 14 m long sand box in order to quantify vertical mixing in heterogeneous media. The filling mimics natural sediments including a distribution of different hydro-facies, made of different sand mixtures, and micro-structures within the sand lenses. We quantify the concentration distribution of the conservative tracer by the analysis of digital images taken at steady state during the tracer-dye experiment. Heterogeneity causes plume meandering, leading to distorted concentration profiles. Without knowledge about the velocity distribution, it is not possible to determine meaningful vertical dispersion coefficients from the concentration profiles. Using the stream-line pattern resulting from an inverse model of previous experiments in the sand box, we can correct for the plume meandering. The resulting vertical dispersion coefficient is approximately approximately 4 x 10(-)(9) m(2)/s. We observe no distinct increase in the vertical dispersion coefficient with increasing travel distance, indicating that heterogeneity has hardly any impact on vertical transverse mixing. In the reactive tracer test, we continuously inject an alkaline solution over a certain height into the domain that is occupied otherwise by an acidic solution. The outline of the alkaline plume is visualized by adding a pH indicator into both solutions. From the height and length of the reactive plume, we estimate a transverse dispersion coefficient of approximately 3 x 10(-)(9) m(2)/s. Overall, the vertical transverse dispersion coefficients are less than an order of magnitude larger than pore diffusion coefficients and hardly increase due to heterogeneity. Thus, we conclude for the assessment of natural attenuation that reactive plumes might become very large if they are controlled by vertical dispersive mixing.
垂直横向混合是源自连续排放源的可生物降解羽流自然衰减的一个控制因素。我们在一个14米长的准二维砂箱中进行了保守性和反应性示踪剂试验，以量化非均质介质中的垂直混合。填充物模拟天然沉积物，包括由不同砂混合物组成的不同水相分布以及砂透镜内的微观结构。我们通过分析示踪染料实验稳态下拍摄的数字图像来量化保守示踪剂的浓度分布。非均质性导致羽流蜿蜒，从而使浓度剖面失真。如果不知道速度分布，就无法从浓度剖面确定有意义的垂直扩散系数。利用砂箱中先前实验反演模型得到的流线模式，我们可以校正羽流的蜿蜒。得到的垂直扩散系数约为4×10⁻⁹平方米/秒。我们观察到垂直扩散系数不会随着传播距离的增加而显著增加，这表明非均质性对垂直横向混合几乎没有影响。在反应性示踪剂试验中，我们在一定高度上持续向原本被酸性溶液占据的区域注入碱性溶液。通过向两种溶液中添加pH指示剂来显示碱性羽流的轮廓。根据反应性羽流的高度和长度，我们估计横向扩散系数约为3×10⁻⁹平方米/秒。总体而言，垂直横向扩散系数比孔隙扩散系数大不到一个数量级，并且几乎不会因非均质性而增加。因此，我们得出结论，对于自然衰减的评估，如果反应性羽流受垂直扩散混合控制，它们可能会变得非常大。
