Temporal responses of groundwater‐surface water exchange to successive storm events

Abstract

Groundwater-surface water exchange within the hyporheic zone is widely recognized as a key mechanism controlling the fate of nutrients within catchments. In gaining river systems, groundwatersurface water interactions are constrained by upwelling groundwater but there is increasing evidence that a rapid rise in river stage during storm events can result in a temporary reversal of vertical hydraulic gradients, leading to surface water infiltration into the subsurface and supply of surface-borne reactive solutes to this biogeochemically active interface. At a UK study site, using logged hydraulic heads in the surface water, riverbed, and riverbanks and logged electrical conductivity at multiple depths in the riverbed we show that storm events can lead to a temporary reversal of vertical hydraulic gradient with mixing evident up to 30 cm beneath the riverbed. Cross-channel variability is evident, with the center of the channel consistently having shorter reversals of hydraulic gradient, compared to the channel margins. The direction of shallow subsurface riverbank flow at the site is also reactive to storm events, temporarily aligning with the surface flow direction and then reverting back to preevent conditions. Such a transition of flow paths during events is also likely to lead to expansion of lateral hyporheic exchange. This study provides evidence that storm events can be a key driver of enhanced hyporheic exchange in gaining river systems, which may support nutrient reactions beyond the duration of event-driven change. Our observations demonstrate the dynamic nature of the hyporheic zone, which should be considered when evaluating its biogeochemical function.

Stream solute tracer timescales changing with discharge and reach length confound process interpretation

“…Likewise, we expect CV to increase with discharge. Subsurface short‐term storage volume can increase with discharge in stream systems with relatively coarse (e.g., sand to gravel) streambeds [ Dudley‐Southern and Binley , ; Schmid et al ., ; Zimmer and Lautz , ], which would likely cause an increase in prolonged tracer tailing or γ . Conversely, streams with low subsurface short‐term storage potential (i.e., low‐streambed hydraulic conductivity and valley slope) are less likely to undergo substantial storage flow path changes with discharge [ Wondzell , ].…”

Hyporheic exchange controlled by dynamic hydrologic boundary conditions

“…Shifts in timing and magnitudes of exchange are also controlled by interactions at smaller temporal and spatial scales. For example, while groundwater inflow can reduce the extent of bed form‐induced exchange [ Boano et al ., ; Cardenas and Wilson , ], a rise in stream discharge due to a storm event can overwhelm groundwater inflow and increase the extent of exchange [ Dudley‐Southern and Binley , ; Sawyer et al ., ]. Increased stream discharge can also mute the influence of bed forms [ Boano et al ., ].…”

How does reach‐scale stream‐hyporheic transport vary with discharge? Insights from rSAS analysis of sequential tracer injections in a headwater mountain stream

“…However, the internal variability of hyporheic zones has received limited study, because subsurface observations are difficult to make in the field [ Bencala et al ., ]. Still, several recent studies demonstrate apparent internal variability in hyporheic zones, observing apparent flow path‐scale responses to hydrologic dynamics [ Voltz et al ., ; Dudley‐Southern and Binley , ].…”