PNNL

The Science                                 

The movement of river water into riverbed sediments and the movement of groundwater into rivers have major impacts over local water quality and global cycling of greenhouse gases. These dynamics are controlled by the physical structure of subsurface sediments, but these structures are invisible when observing rivers from the surface. To overcome the challenge, Pacific Northwest National Laboratory (PNNL) researchers used PNNL-developed sensor technology and a new data analysis method to reveal these structures and dynamics underground.

The Impact

For this study, researchers developed a novel approach to studying the subsurface and it revealed the otherwise invisible: subsurface hydrologic flows through sediments that are spatially varied. The approach is particularly useful in highly dynamic river systems where the depth of the river changes significantly through time, which is common across both inland and coastal rivers that experience floods and tidal changes. And because the approach can be used in such dynamic environments, researchers anticipate using it across all types of rivers to help them understand governing processes that impact water quality and greenhouse gas fluxes. The resulting knowledge can be used to inform predictive models that aim to predict the health and function of rivers across future environmental disturbances associated with global change.

Summary

Groundwater surface water exchange plays a critical role in physical, biological, and geochemical function of coastal and riverine systems. Observing exchange flow behavior in heterogeneous systems is a challenge, particularly when flows are governed by dynamic river stage or tidal variations. In this paper, researchers demonstrate a novel approach that includes time-lapse electrical resistivity tomography and temperature monitoring where an array of thermistors installed beneath a riverbed double as resistivity electrodes. Researchers used the four-dimensional array to monitor subsurface water flows over a 6-day period in a large river impacted by hydrologic dam operations. They presented a method for addressing the otherwise confounding effects of the moving river-surface boundary on the raw resistivity data, thereby enabling successful tomographic imaging. Temperature time-series at each thermistor location and time-lapse 3D images of changes in bulk electrical conductivity together provide a detailed description of exchange dynamics over a 10-meter by 45-meter section of the riverbed, to a depth of approximately 5 m. Results reveal highly variable flux behavior throughout the monitored volume, including both horizontal and vertical exchange flows.

Funding

Pacific Northwest National Laboratory is operated by Battelle Memorial Institute for the DOE under Contract No. DE-AC05-76RL01830. This research was supported by the DOE Biological and Environmental Research program (BER), as part of BER’s Environmental System Science Program (ESS). This contribution originates from the River Corridor Scientific Focus Area at PNNL.