New insights into particle transport in karst conduits using comparative tracer tests with natural sediments and solutes during low-flow and high-flow conditions

Colloidal particles are an important vector for the transport of contaminants in karst aquifers, characterized by a high degree of hydrologic variability. Understanding this heterogeneity and hydrogeological functioning is of particular importance for water management strategies. Until now, transport parameters for particles were mostly determined by injection of solutes or surrogates for natural sediments. But suspended particles are easier to analyse, less expensive and better represent natural conditions. Therefore, tracer tests with sediments represent natural transport and a conservative solute dye were conducted simultaneously in an active and partly accessible cave system in Vietnam during constant high-flow and low-flow conditions. Breakthrough curves (BTCs) for 10 different particle sizes classes ranging from 1 to 15 μm and the solute were recorded in situ at the two main resurgences of the cave stream at high temporal resolution. This study gives new insights into the transport processes of suspended particles and the highly dynamic exchange between mobile and immobile regions. Major findings include: (1) at low-flow conditions, inflow from the surrounding aquifer matrix and the velocity distribution inside the karst conduits themselves lead to hydrodynamic focusing, mainly affecting particles; (2) this highly advective, preferential transport of particles in the centre of the karst conduits results in narrow BTC. ... mehrThe more dispersive transport of solutes results in a longer tailing; (3) at high-flow conditions a more homogeneous distribution of the particles indicates a reversal of the conduit-matrix interaction; (4) by comparing the results at different hydraulic conditions, the activation of additional flow paths with increasing discharge could be identified. This study also presents the first tracer test in the field resulting in pronounced double peaks for the particulate tracer but only a single, right-skewed BTC for the simultaneously injected solute, including a conceptual model for particle transport.