Lekmine Greg, Sookhak Lari Kaveh, Johnston Colin D, Bastow Trevor P, Rayner John L, Davis Greg B
CSIRO Land and Water, Private Bag No. 5, Wembley, WA 6913, Australia.
CSIRO Land and Water, Private Bag No. 5, Wembley, WA 6913, Australia; School of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA 6027, Australia.
J Contam Hydrol. 2017 Jan;196:30-42. doi: 10.1016/j.jconhyd.2016.12.003. Epub 2016 Dec 9.
Understanding dissolution dynamics of hazardous compounds from complex gasoline mixtures is a key to long-term predictions of groundwater risks. The aim of this study was to investigate if the local equilibrium assumption for BTEX and TMBs (trimethylbenzenes) dissolution was valid under variable saturation in two dimensional flow conditions and evaluate the impact of local heterogeneities when equilibrium is verified at the scale of investigation. An initial residual gasoline saturation was established over the upper two-thirds of a water saturated sand pack. A constant horizontal pore velocity was maintained and water samples were recovered across 38 sampling ports over 141days. Inside the residual NAPL zone, BTEX and TMBs dissolution curves were in agreement with the TMVOC model based on the local equilibrium assumption. Results compared to previous numerical studies suggest the presence of small scale dissolution fingering created perpendicular to the horizontal dissolution front, mainly triggered by heterogeneities in the medium structure and the local NAPL residual saturation. In the transition zone, TMVOC was able to represent a range of behaviours exhibited by the data, confirming equilibrium or near-equilibrium dissolution at the scale of investigation. The model locally showed discrepancies with the most soluble compounds, i.e. benzene and toluene, due to local heterogeneities exhibiting that at lower scale flow bypassing and channelling may have occurred. In these conditions mass transfer rates were still high enough to fall under the equilibrium assumption in TMVOC at the scale of investigation. Comparisons with other models involving upscaled mass transfer rates demonstrated that such approximations with TMVOC could lead to overestimate BTEX dissolution rates and underestimate the total remediation time.
了解复杂汽油混合物中有害化合物的溶解动力学是长期预测地下水风险的关键。本研究的目的是调查在二维流动条件下，可变饱和度下苯系物和三甲苯（TMBs）溶解的局部平衡假设是否有效，并评估在调查尺度上验证平衡时局部非均质性的影响。在水饱和砂柱的上三分之二区域建立了初始残余汽油饱和度。保持恒定的水平孔隙流速，并在141天内通过38个采样端口采集水样。在残余非水相液体（NAPL）区域内，苯系物和三甲苯的溶解曲线与基于局部平衡假设的TMVOC模型一致。与先前数值研究的结果相比，表明存在垂直于水平溶解前沿形成的小规模溶解指进，主要由介质结构和局部NAPL残余饱和度的非均质性引发。在过渡区，TMVOC能够代表数据表现出的一系列行为，证实了在调查尺度上的平衡或近平衡溶解。该模型在局部与最易溶的化合物，即苯和甲苯存在差异，这是由于局部非均质性表明在较小尺度上可能发生了流动绕过和窜流。在这些条件下，传质速率仍然足够高，在调查尺度上仍符合TMVOC中的平衡假设。与其他涉及放大传质速率的模型比较表明，TMVOC的这种近似可能导致高估苯系物的溶解速率并低估总修复时间。
