Dam Operations and Subsurface Hydrogeology Control Dynamics of Hydrologic Exchange Flows in a Regulated River Reach

Abstract

Hydrologic exchange flows (HEFs) across the river‐aquifer interface have important implications for biogeochemical processes and contaminant plume migration in the river corridor, yet little is known about the hydrogeomorphic factors that control HEFs dynamics under dynamic flow conditions. Here, we developed a 3‐D numerical model for a large regulated river corridor along the Columbia River to study how HEFs are controlled by the interplays between dam‐regulated flow conditions and hydrogeomorphic features of such river corridor system. Our results revealed highly variable intra‐annual spatiotemporal patterns in HEFs along the 75‐km river reach, as well as strong interannual variability with larger exchange volumes in wet years than dry years. In general, the river was losing during late spring to early summer when the river stage was high, and river was gaining in fall and winter when river stage was low. The magnitude and timing of river stage fluctuations controlled the timing of high exchange rates. Both river channel geomorphology and the thickness of a highly permeable river bank geologic layer controlled the locations of exchange hot spots, while the latter played a dominant role. Dam‐induced, subdaily to daily river stage fluctuations drove high‐frequency variations in HEFs across the river‐aquifer interfaces, resulting in greater overall exchange volumes as compared to the case without high‐frequency flows. Our results demonstrated that upstream dam operations enhanced the exchange between river water and groundwater with strong potential influence on the associated biogeochemical processes and on the fate and transport of groundwater contaminant plumes in such river corridors.

Controls of River Dynamics on Residence Time and Biogeochemical Reactions of Hydrological Exchange Flows in A Regulated River Reach

“…The influences of the high-frequency stage fluctuation on HEFs has been evaluated in several studies. High-frequency flow variations induce more flow reversals, higher mass exchange, and deeper penetration depth of river thermal signals into the aquifer Shuai et al, 2019;Song et al, 2018). The increased mass exchange might accelerate the subsurface reactions by providing more reactants (Song et al, 2018;Trauth & Fleckenstein, 2017) or decrease specific reactions by inducing strong inhibition (Knights et al, 2017).…”

“…Even short-term perturbations (e.g., flooding) can have long-lasting influences (Gomez-Velez et al, 2017). The shapes of RTDs in real dynamic river corridors can be very complex since RTDs are influenced by both subsurface physical features and hydrologic forcings (Boulton et al, 1998;Gomez & Wilson, 2013), including sediment permeability and porosity (Cardenas et al, 2004;Liu & Chui, 2018;Salehin et al, 2004), river morphology (e.g., riffles, bars, and dunes) (Buffington & Tonina, 2009;Cardenas, 2008;Cardenas & Wilson, 2007;Stonedahl et al, 2013), naturally occurring hydrologic processes and events (e.g., flooding, evapotranspiration, recharge, snowmelt and tidal cycles) (Gomez-Velez et al, 2017;Gomez-Velez & Wilson, 2013;Larsen et al, 2014), and anthropogenic activities (e.g., dam-induced stage fluctuations) (Shuai et al, 2019;Song et al, 2018). The dynamic hydrologic fluctuations can produce equivalently complex pathways and RTDs as complex geomorphic features (Schmadel et al, 2016).…”

“…Dynamic flow variations are common in most large river systems, which exhibit multi-frequency patterns that are influenced by natural processes and anthropogenic activities (Graf, 1999;Nilsson et al, 2005). These stage variations cause significant pressure changes along the river shoreline and significant lateral exchange flow and bank storage (Shuai et al, 2019;Zachara et al, 2016), which enhance biogeochemical processes within the river corridors (Briody et al, 2016;Knapp et al, 2017;Shuai et al, 2017;Trauth & Fleckenstein, 2017). However, there have been few studies that link HEFs, RTDs and their biogeochemical consequences within large river corridors due to challenges in data collection and lack of modeling capability (Boano et al, 2014).…”

“…The simulated RTDs were then used to evaluate the impacts of river dynamics on generalized river corridor biogeochemical reactions. We chose the Hanford Reach of the Columbia River as an example in this study, which serves as an ideal testbed because of its highly dynamic HEFs (Shuai et al, 2019;Song et al, 2018). In addition, extensive hydrologic and geologic data collected from more than thirty monitoring wells within this site provides detailed site characteristics.…”

Kilometer‐Scale Hydrologic Exchange Flows in a Gravel Bed River Corridor and Their Implications to Solute Migration

“…Similar hydrologic exchange behavior appears to occur at other locations in the Columbia River corridor that exhibit comparable geomorphic and geologic characteristics. Variations in the physical template along the shoreline and in associated subsurface sediments lead to an apparent patchwork distribution of macroscopic exchange zones that vary in the detail of their hydrologic exchange behavior (Shuai et al, 2019). Two other well-studied Cr groundwater plumes (100 D and 100 H) exist in the river corridor some 35 km upstream of our study site that discharge to sensitive salmon spawning areas (Pacific Northwest National Laboratory, 1999).…”