Mixing between hyporheic flow cells and upwelling groundwater: laboratory simulations and implications for mixing-dependent reactions

Abstract:

The hyporheic zone is the region where surface water and groundwater interact beneath and adjacent to stream and river systems. The hyporheic zone has been shown to affect water quality constituents such as heat, pollutants, and nutrients in this interfacial region. Recent modeling studies have shown that some compounds upwelling from groundwater toward surface water only react when water originating in surface water and groundwater mix at the edge of the hyporheic zone. These “mixing-dependent” reactions may be important for addressing upwelling pollution, yet depend on the extent of the mixing zone. Recent modeling studies have also shown this mixing zone to be thin, but this has not been confirmed in lab or field studies. Our current study took the first step toward such confirmation by simulating mixing between hyporheic flow cells and upwelling groundwater in a 1.7 m x 1m flow-through laboratory sediment mesocosm. We used tracer dyes and digital photographs taken through the glass wall of the mesocosm to quantify mixing zone thickness. We varied key hydraulic controls such as the surface water head drop that drives the hyporheic flow cells. Steady-state results confirm the thin mixing zones predicted by the earlier modeling. Mixing zone length increases with increasing surface water head drop, but mixing zone thickness appears to remain relatively constant. Furthermore, if the head drop in surface water changes rapidly, the mixing zone moves through riverbed sediments but does not appear to change appreciably in thickness. As a result, we conclude that mixing zones are probably thin under a range of field conditions. This has implications for one common definition of the hyporheic zone as an area of substantial mixing of surface water and groundwater. Thin mixing zones also may limit mixing-dependent reactions of upwelling contaminants, which bears further study.