Hyporheic exchange in a karst conduit and sediment system – A laboratory analog study

Karst conduits are often partly filled by clastic sediments. Flow through such sediments can have a strong impact on the fate of sediment-entrapped contaminants. In contrast to stream bed sediments, hyporheic flow in karst sediments has received little attention so far. For karst sediments, conduit bends could induce hyporheic flow, in addition to bedforms or obstacles. The main aim of this study was to investigate flow processes in a conduit-sediment system using a laboratory model resembling a siphon and numerical modeling. In the laboratory system, zones with forward and reserve flow occurred in the sediment due to the conduit bends. As demonstrated with sediment-source tracer test, entrapped solutes were generally flushed out more rapidly at a higher flow rate and steeper conduit angle. Numerical modeling assuming pressure continuity across the conduit-sediment interface reproduced the flow patterns and breakthrough curves (BTCs) well. Based on the model, the magnitude of hyporheic exchange is expected to increase linearly with the flow rate and was higher for a steeper conduit angle. However, the increase in flushing intensity was not evenly distributed throughout the sediments but occurred mainly adjacent to the conduit bends consistent with observations from tracer tests. This study confirms that conduit bends could have a strong influence on hyporheic flow in karst sediments.