Abstract
Interaction between surface water and groundwater plays a fundamental role in influencing aquatic chemistry, where hyporheic exchange processes, distribution of flow paths and residence times within the hyporheic zone will influence the transport of mass and energy in the surface-water/groundwater system. Geomorphological conditions greatly influence hyporheic exchange, and heterogeneities such as rocks and clay lenses will be a key factor for delineating the hyporheic zone. Electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) were used to investigate the streambed along a 6.3-m-long reach in order to characterise geological layering and distinct features which may influence parameters such as hydraulic conductivity. Time-lapse ERT measurements taken during a tracer injection demonstrated that geological features at the meter-scale played a determining role for the hyporheic flow field. The penetration depth of the tracer into the streambed sediment displayed a variable spatial pattern in areas where the presence of highly resistive anomalies was detected. In areas with more homogeneous sediments, the penetration depth was much more uniformly distributed than observed in more heterogeneous sections, demonstrating that ERT can play a vital role in identifying critical hydraulic features that may influence hyporheic exchange processes. Reciprocal ERT measurements linked variability and thus uncertainty in the modelled resistivity to the spatial locations, which also demonstrated larger variability in the tracer penetration depth, likely due to local heterogeneity in the hydraulic conductivity field.
Highlights
The interaction between surface water and groundwater is an important process in water resources management, influencing aquatic ecosystems (Findlay 1995; Brunke and Gonser 1997; Briggs et al 2015) as well as water quality (Boulton et al 1998; Boano et al 2013; Lewandowski et al 2019)
Streambed geomorphology exerts a strong control on the hyporheic exchange processes (Findlay 1995; Azizian et al 2017) and spatial heterogeneities and local anomalies in sediment composition are thought to constrain the distributions in flow paths and residence times within the hyporheic zone (Mermillod-Blondin et al 2015; Malenda et al 2019)
The apparent resistivity of the subsurface during the tracer injection plateau period (ERT 1 to Electrical resistivity tomography (ERT) 4, Fig. 4) was roughly stable with less than 10% variation seen between measurements (Fig. 6), but the results demonstrated that the tracer following termination of the injection was removed rapidly (ERT 5, Figs. 4, 5 and 6)
Summary
The interaction between surface water and groundwater is an important process in water resources management, influencing aquatic ecosystems (Findlay 1995; Brunke and Gonser 1997; Briggs et al 2015) as well as water quality (Boulton et al 1998; Boano et al 2013; Lewandowski et al 2019). The physical extent of the hyporheic zone and the degree to which it varies temporally and spatially is often both highly variable and dependent on the in-situ conditions such as bedform geometry (Cardenas et al 2004; Stonedahl et al 2010; Boano et al 2013; Mermillod-Blondin et al 2015; Malenda et al 2019), stream channel discharge (Maier and Howard 2011; Ward et al 2010; Stonedahl et al 2018; Wu et al 2018) and regional groundwater flow patterns (Marzadri et al 2016; Schmadel et al 2016) Heterogeneities in both streambed topography and sediment composition influence the hyporheic exchange (Malenda et al 2019). In order to successfully describe an environmental system where the interactions between surface water and groundwater play an important role in transport and ecosystem processes, characterization of these interactions requires an understanding of the spatial nature of the hydraulic behaviour of the hyporheic zone (Ward et al 2010)
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