Sale Tom C, Zimbron Julio A, Dandy David S
Department of Civil & Environmental Engineering, Colorado State University, Fort Collins, CO 80523-1320, USA.
J Contam Hydrol. 2008 Nov 14;102(1-2):72-85. doi: 10.1016/j.jconhyd.2008.08.002. Epub 2008 Sep 7.
The following explores the issue of how reductions in contaminant loading to plumes will effect downgradient water quality. An idealized scenario of two adjacent layers of uniform geologic media, one transmissive and the other low permeability, is considered. A high concentration source, similar to a thin DNAPL pool, is introduced in the transmissive layer immediately above the low permeability layer. While the source is active, dissolved constituents are driven along the contact by advection and into the low permeability layer by transverse diffusion. Removing the source reverses the concentration gradient between the layers, driving back diffusion of contaminants from the low permeability layer. Laboratory studies involving four contaminants demonstrate that 15 to 44% of the introduced contaminant moves into the low permeability zone (along a distance of 87 cm in a sand tank) over a period of 25 days. The greatest movement of contaminants into the low permeability zone is seen with the contaminants with the greatest sorption coefficients. A unique two-dimensional analytical solution is developed for the two-layer scenario. Processes addressed include advection; transverse dispersion; adsorption and degradation in the transmissive zones; and diffusion, adsorption, and degradation in the low permeability layer. Laboratory data agree favorably with the analytical solutions. Collectively, the laboratory results and analytical solutions provide a basis for testing other modeling approaches that can be applied to more complex problems. A set of field-scale scenarios are considered using the analytical solutions. Results indicate that improvement in water quality associated with source removal diminish with distance downgradient of the source. Furthermore, contaminant degradation and contaminant adsorption in the stagnant zone are shown to be critical factors governing the timing and magnitude of downgradient improvements in water quality. For five of six scenarios considered, observed improvements in water quality 100 m downgradient of the source fall in the range of 1 to 2 orders of magnitude 15 years after complete source removal. The sixth scenario, involving a contaminant half-life of three years and no adsorption, shows greater than three order of magnitude improvements in downgradient water quality within one year of source removal.
以下探讨了减少进入羽流的污染物负荷将如何影响下游水质这一问题。考虑了一个理想化的场景，即两个相邻的均匀地质介质层，一层具有透水性，另一层具有低渗透性。在低渗透层正上方的透水层中引入一个高浓度源，类似于一个薄的重质非水相液体池。当源处于活跃状态时，溶解成分通过平流沿着接触面移动，并通过横向扩散进入低渗透层。去除源会使两层之间的浓度梯度反转，驱使污染物从低渗透层反向扩散。涉及四种污染物的实验室研究表明，在25天的时间里，15%至44%的引入污染物会进入低渗透区（在一个砂槽中沿87厘米的距离）。进入低渗透区的污染物移动量最大的是吸附系数最大的污染物。针对两层场景开发了一种独特的二维解析解。所涉及的过程包括平流；横向弥散；透水区域中的吸附和降解；以及低渗透层中的扩散、吸附和降解。实验室数据与解析解吻合良好。总体而言，实验室结果和解析解为测试可应用于更复杂问题的其他建模方法提供了基础。使用解析解考虑了一组现场尺度的场景。结果表明，与源去除相关的水质改善随着源下游距离的增加而减弱。此外，停滞区中的污染物降解和污染物吸附被证明是控制下游水质改善时间和程度的关键因素。在所考虑的六个场景中的五个场景中，在完全去除源15年后，在源下游100米处观察到的水质改善幅度在1至2个数量级范围内。第六个场景，涉及一种半衰期为三年且无吸附的污染物，在去除源后的一年内，下游水质改善幅度超过三个数量级。
