Integrating transient groundwater control of the hyporheic zone
Malzone, Jonathan M.
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The hyporheic zone is a physical volume just below the sediment water interface where mixing between downwelling surface water and upwelling groundwater occurs. Dissolved constituents moving through the hyporheic zone are metabolized by a variety of organisms, which can result in the biogeochemically mediated attenuation of non-point source nutrient pollution and immobilization of contaminants such as metals. The potential for biogeochemical transformation is related to the hyporheic volume and supply of nutrients, which underscores the importance of quantifying hyporheic volume. Previous work has quantified the increase in the volume of the hyporheic zone as a result of increases in stream depth, stream velocity, and streambed topography, but groundwater controls are rarely included. Increases in groundwater discharge generally decrease the volume of the hyporheic zone, but how groundwater temporally controls the volume and function of the hyporheic zone is un-quantified. As a result, the objective of this research is to quantify important transient physical processes in the hyporheic zone by determining how temporal fluctuations in groundwater flow impacts both the source of hyporheic water and the volume of the hyporheic zone. Geochemical analyses demonstrated that top down controls on the hyporheic zone were caused by changes in surface water concentrations that then propagated down into the hyporheic zone. These changes to the source of hyporheic water were observed through characterization of groundwater and surface water endmembers and monitoring of the hyporheic zone. Key end members that controlled the system included temporally stable focused groundwater discharge and temporally fluctuating diffuse groundwater discharge. While temporally fluctuating diffuse groundwater was depleted during dry periods, the stable focused groundwater discharge became a more dominant endmember source to the surface water. This change in source to surface water in turn affected the hyporheic zone by increasing the concentration of nitrate in hyporheic flowpaths. A conceptual model of groundwater feedback was created to explain this temporal process: as focused groundwater discharge increases in dominance and alters the composition of surface water, downwelling of surface water results in a top down “feedback” into the hyporheic zone. The volume of the hyporheic zone is dominated by bottom up groundwater controls from annual and storm induced gradient changes between the stream and the riparian water table. Groundwater controlled changes to the volume of the hyporheic zone on the seasonal and storm event time scales in Elton Creek was quantified with specific conductance and hydraulic gradients. A comparison of planar and riffle beds indicated that the hyporheic zone expanded seasonally due to riparian water table declines for a longer duration and larger depth compared to storm events on the planar bed. Conversely, the riffle bed exhibited opposite behavior (more expansion due to storm events) because the local gradient induced by the riffle increased the dominance of surface water processes. The conceptual model of groundwater-dominated differential expansion of the hyporheic zone was created to explain the role of seasonal versus storm related groundwater processes that change the volume of the hyporheic zone. This represents a bottom up dynamic groundwater control of the hyporheic zone. Annual and storm event groundwater fluctuations controlled dynamic expansion and contraction of the hyporheic zone volume. A numerical model coupling the Navier-Stokes equation with the Brinkman-Darcy equation was used to simulate a range of storm and annual hydrologic scenarios that represented typical surface water and groundwater fluctuations. Simulations revealed that the magnitude of annual water table fluctuation controlled the volume hyporheic zone. These results indicated that the importance of transient processes between stream reaches fall into two categories: (1) Stream reaches that have an annually stable hyporheic zone where fluctuation only occurs during storm events, and (2) Stream reaches that have an annually fluctuating hyporheic zone where changes occur on the annual and storm event time scale. The seasonal position of the water table also determined the impact of storm event fluctuations on the hyporheic zone. When the pre-event water table elevation was at the lowest annual level, storm event induced expansion of the hyporheic zone increased in duration compared to the annual maximum water table. This relationship suggests the supply of a biogeochemically important constituent to the hyporheic zone, such as dissolved oxygen, is larger during the annual minimum water table elevation.