Institute of Applied Geosciences (AGW), Karlsruhe Institute of Technology (KIT), Kaiserstr. 12, 76131, Karlsruhe, Germany.
Institute of Applied Geosciences (AGW), Karlsruhe Institute of Technology (KIT), Kaiserstr. 12, 76131, Karlsruhe, Germany.
Water Res. 2020 Jul 1;178:115755. doi: 10.1016/j.watres.2020.115755. Epub 2020 Apr 17.
Karst aquifers are important drinking water resources, but highly vulnerable to contamination. Contaminants can be transported rapidly through a network of fractures and conduits, with only limited sorption or degradation, which usually leads to a fast and strong response at karst springs. During migration, contaminants can also enter less mobile zones, such as pools or water in intra-karstic sediments, or advance from conduits into the adjacent fractured rock matrix. As contaminant concentrations in the main flow path(s) decrease, contaminants may migrate back into the main flow path and reach the karst springs at low (but significant) concentrations over a long time span. This is the conventional interpretation for the oft-observed steep rising limb and the long-tailed falling limb of tracer breakthrough curves in karst systems. Here, field measurements are examined from an alpine karst system in Austria where a series of distinctive, long-tailed breakthrough curves (BTCs) of conservative tracers were observed over distances up to 7400 m. Recognizing that the conventional advection-dispersion equation (ADE) cannot usually quantify such behavior, two other modeling approaches are considered, namely the two-region non-equilibrium (2RNE) model, which explicitly includes mobile and immobile zones, and a continuous time random walk (CTRW) model, which is based on a physically-based, probabilistic approach that describes anomalous (or non-Fickian) transport behavior characteristic of heterogeneous systems such as karst. In most cases, the ADE and 2RNE models do not quantify the low concentrations at longer travel times. The CTRW, in contrast, accounts for the long-tailed breakthrough behavior found in this karst system.
喀斯特含水层是重要的饮用水资源，但极易受到污染。污染物可以通过裂缝和管道网络快速传输，只有有限的吸附或降解，这通常会导致喀斯特泉的快速和强烈响应。在迁移过程中，污染物也可以进入流动性较差的区域，如水池或喀斯特沉积物中的水，或者从管道进入相邻的裂缝岩石基质。随着主流路径中的污染物浓度降低，污染物可能会回流到主流路径中，并在很长一段时间内以低（但显著）浓度到达喀斯特泉。这是对在喀斯特系统中经常观察到的示踪剂突破曲线陡峭上升支和长尾下降支的传统解释。在这里，对奥地利高山喀斯特系统的野外测量进行了检查，在该系统中，观察到了一系列独特的、长尾的示踪剂突破曲线（BTC），距离长达 7400 米。认识到传统的对流-弥散方程（ADE）通常无法量化这种行为，考虑了另外两种建模方法，即两个区域非平衡（2RNE）模型，该模型明确包括移动和不可移动区域，以及连续时间随机游走（CTRW）模型，该模型基于物理基础的概率方法，描述了非均相系统（如喀斯特）特有的异常（或非菲克）传输行为。在大多数情况下，ADE 和 2RNE 模型无法量化较长传输时间的低浓度。相比之下，CTRW 解释了在这个喀斯特系统中发现的长尾突破行为。
