Advances in understanding long-tailed breakthrough curves from tracer tests in karst aquifers

Karst aquifers are important drinking water resources. However, they are highly vulnerable to contamination. Contaminants can be transported rapidly through the network of conduits or fractures with only limited sorption or degradation. This leads to a fast and strong response at the karst springs and, along with it, a rapid decrease in the water quality. In addition to this, contaminants can also enter immobile zones, such as pools and riffles, or move into the adjacent fractured rock matrix. As the concentrations in the main flowpath decrease, contaminants may migrate back into the main flowpath and reach the karst springs at lower concentrations, but for a longer time-span. This is the conventional interpretation for the steep rising limb and the long-tailed falling limb of tracer breakthrough curves which are often-observed phenomena in karst systems. Such behavior cannot be quantified by the conventional advection-dispersion equation (ADE). The two-region, non-equilibrium model (2RNE), which includes mobile and immobile zones, delivers a relatively good approximation of the breakthrough curves. However, in most cases, even the 2RNE fails to simulate the lowermost concentrations at the longest travel times. In this context, the continuous time random walk (CTRW) approach can be applied to understand such long-tailed breakthrough curves. CTRW accounts for the anomalous (or non-Fickian) transport behavior which characterizes heterogeneous systems, such as karst. We examined examples from an alpine karst system in Austria where we observed distinctive longtailed breakthrough curves of the conservative tracers. We present several modeling approaches – (i) ADE, (ii) 2RNE and (iii) CTRW – and show that CTRW describes the observed tracer behavior. The CTRW approach is a physically-based framework that accounts for the various transport mechanisms and simulates even the late and low concentrations.