IS THE HYPORHEIC ZONE A USEFUL CONCEPT FOR KARST AQUIFERS WITH CONDUITS?

Hydraulic and geochemical coupling between the conduits and matrix of the fully phreatic (beneath the water table) portion of karst aquifers has been poorly quantified. A similar coupling exists between surface streams and their underlying sediments and bedrock. In streams a very important aspect of this coupling is the hyporheic zone, where stream water enters the sediment and bedrock, mixes with groundwater, geochemically reacts, and is then returns to the stream. In fluvial research it has only been in the last couple of decades that this coupling has been recognized as a fundamental process affecting the chemistry and biology of streams, and the transport and transformation of stream nutrients and contaminants. Since karst conduits are analogous to surface streams, we suggest that a karst conduit hyporheic zone exists and that it is a critical hydraulic and geochemical coupling between the conduits and matrix of a karst aquifer. Drawing from previous work in surface streams, laboratory experiments mimicking karst conduit systems, and modeling efforts, we describe the mechanisms driving karst conduit hyporheic exchange. The effects of the exchange are discussed with an emphasis on their hypothesized impact on geochemistry, speleogenesis, biology, contaminant sequestration and hydraulics in a karst aquifer.