Laboratory and numerical study of hyporheic flow-mediated DNAPL dissolution in karst conduits

Abstract

Dense non-aqueous phase liquids (DNAPLs) can easily penetrate into karst aquifers, may accumulate in the sediment-filled depressions of conduits, and constitute a long-term source of groundwater contamination. In contrast to porous media, DNAPL dissolution in karst settings has not received much attention. This study investigated the dissolution of sediment-entrapped DNAPLs using a laboratory analogue model corresponding to a syphon structure partly filled by sediments and using numerical simulations. The mass transfer rate increased with an increasing conduit Reynolds number (Re) and increasing conduit angle, as both of these factors led to a more intense hyporheic flow in the sediments. In addition, the mass transfer rate increased linearly with the average flow velocity in the sediment and was higher than expected for an infinite porous medium considered as a reference case. The enhanced mass transfer rate can be explained by the curved streamlines associated with hyporheic flow, which transport solutes away from the DNAPL interface. This study demonstrates that hyporheic flow through karst sediment is a possible mechanism for the mobilization of DNAPL components and might explain the persistent occurrence of such contaminants in karst springs.