Application of heat pulse injections for investigating shallow hyporheic flow in a lowland river

Hyporheic zone processes can have significant impact on groundwater and surface water resources. Detailed knowledge of exchange flow patterns is crucial for understanding the ecohydrological and biogeochemical functioning of river corridors. In particular, small-scale hyporheic exchange flow is still poorly understood, partially because of the lack of adequate in situ monitoring technology. This paper investigates the spatial heterogeneity of hyporheic exchange flow in a lowland river at multiple scales. It demonstrates the conjunctive use of active heat pulse tracing at shallow depths (15 cm) and vertical hydraulic gradients (VHG) at 120-150 cm streambed depth for improving the understanding of hyporheic exchange flow processes. Generally positive VHG indicated a regional dominance of groundwater up-welling. High and temporally variable VHG were used to identify confined conditions caused by low conductivity layers in the subsurface (low connectivity), while locations with lower and temporally less variable VHG indicated free groundwater up-welling (high connectivity) in highly conductive sediments. A heat pulse sensor (HPS) was applied for identifying shallow hyporheic flow at three locations representative for high versus low streambed connectivity. Shallow hyporheic flow patterns were found to be spatially heterogeneous. Subsurface flow could only partially be explained by streambed topography. Surface water infiltration and horizontal flow coincided with inhibited groundwater upwelling, whereas locations with high streambed connectivity were characterized by increased up-welling. The combined information of spatiotemporal VHG variability and flow vector frequency distribution by HPS has the potential to improve the understanding of impacts of streambed topography and subsurface stratification on hyporheic flow patterns.