U.S. Department of Energy

Pacific Northwest National Laboratory

Quantifying Shallow Water Hydrologic Exchanges

The study’s river reach. Temperature data were measured by iButton ® sensors at sites A-E marked as red triangles.

The Science 

Hydrologic exchange between rivers and subsurface zones is a crucial component of the water cycle. From centimeter to kilometer scales, the magnitude of exchange affects biogeochemical and ecological processes that occur in the hyporheic zone at a river’s edge and in the aquifer below. 

Hydrologic exchange fluxes (HEFs) – the way river water and subsurface water interact – can be quantified, but there are limitations of scale. In relatively large rivers (a scale of greater than a kilometer in length), quantification is difficult because of site accessibility, the logistics of representative sampling, and the complexities of geomorphologic features and subsurface properties.

In a recent special issueof the journal Water, co-authorsTian Zhou, Maoyi Huang, and 11 other researchers at Pacific Northwest National Laboratory reported on a new approach to estimating shallow-water HEF rates. They developed and validated the approach, a synthesis of HEF measuring methods.

The Impact 

This approach can be easily employed in other regulated river reaches to facilitate large scale river corridor studies or to inform biogeochemical and ecological studies in highly dynamic river reaches.

Summary 

River water interacts with subsurface water through hydrologic exchange fluxes (HEFs). Hydrologic exchange encompasses surface water-groundwater interactions at a number of spatiotemporal scales. All are vital, and include hyporheic exchange, bank storage, and regional groundwater discharge and recharge. These fluxes facilitate nutrient and carbon cycling, organic biodegradation, fish spawning, metal transport, and other key biogeochemical and hydroecological processes.

The direction, path, magnitude, and residence times of HEFs define when and where nutrient cycling and biogeochemical reactions occur in the beds and banks of rivers. In the past 50 years, more than 50,000 large dams have been built across the globe, which have modulated the direction and rates of HEFs as much as hundreds of kilometers downstream, and thus impacted biogeochemical processes and terrestrial water resources. Therefore, measurements of HEFs are critical to support accurate simulation of hydrobiogeochemical activity in the world’s increasing number of dam-regulated rivers.

A number of HEF studies have been performed in smaller rivers based on measurements or modeling at the point or transect scale. However, researchers have not fully explored HEFs at the scale of large rivers, primarily because of monitoring challenges.

A recent paper by 11 researchers at PNNL devised a synthesis of methods that use heat as a naturally occurring tracer to estimate HEFs and overcome the challenges of applying such methods to large and highly regulated river systems.

Using a 7-km stretch of the Columbia River in southeastern Washington State as a field site, they devised an approach that provides continuous measurement of HEF rates that requires less (and more easily accessible) data than previous methods. The approach was successfully applied to infer the sub-daily dynamics of vertical HEF rates at five shallow water sites along the shoreline.

The experiment had two objectives: 1) to demonstrate a new approach that combines field measurements, physical principles, and statistical analyses to infer long-term riverbed HEF rates, and 2) to understand and quantify spatial and temporal distributions of HEFs in the shallow water area along the river reach. 

The authors say the new approach could provide point measurements of HEF rates for large scale groundwater monitoring evaluations, and could provide guidance for field studies intended to identify strong HEF hotspots and active biogeochemical reaction areas at the river bed. Using this method, they revealed that the HEF rates along a large river reach are highly variable in space and time due to influences of local hydrodynamic and hydromorphic settings. The magnitude of HEFs in the primary channel could be six to nine times higher than that in the secondary channel.

Funding 

This research was supported by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research (BER), as part of BER’s Subsurface Biogeochemistry Research Program (SBR). This contribution originates from the SBR Scientific Focus Area of the Pacific Northwest National Laboratory.

Publication

T Zhou, et al., “A New Approach to Quantify Shallow Water Hydrologic Exchanges in a Large Regulated River Reach.”Water2017, 9(9), 703; doi:10.3390/w9090703

 

Date: 
September 2017
| Pacific Northwest National Laboratory