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Environmental Significace of the Groundwater-Surface Water Interaction Zone

Field Site

  • the Hanford Reach

    Our field work is being conducted in a section of the Columbia River in eastern Washington State called the Hanford Reach. The Hanford Reach was protected by Presidential proclamation in 2000. This area of the river is free flowing.

  • local-scale field site

    Our first local-scale field site is located in Hanford's 300 Area along the Columbia River north of Richland, Washington. We are in the process of planning and developing two additional highly-instrumented sites at other locations within the Hanford Reach.

  • 300-Area study site

    The 300-Area study site is located over a paleo erosional channel (turquoise-blue-magenta) in the underlying, fine-grained Ringold aquitard that facilitates groundwater - river exchange. The Ringold aquitard surface (red) is 10-15 m below the ground surface.

  • Freeze coring utilizing liquid nitrogen

    Freeze coring utilizing liquid nitrogen is one method used to extract structurally intact materials from the hyporheic zone and underlying sediments in the groundwater-surface water interaction zone. Core is being collected in this instance from below the Columbia River high water mark.

  • sampling water in the hyporheic zone

    Pacific Northwest National Laboratory staff install an aquifer tube at the Hanford 300 Area field site. ~70 such tubes provide a 3D array for sampling water in the hyporheic zone. Water samples are analyzed to monitor the mixing of river water and groundwater, to define the geochemical characteristics of hyporheic zone water over time, and to identify biogeochemical processes in the hyporheic zone.

Three geological strata are located within the groundwater-surface water interaction zone depending on spatial location and river elevation. Enlarge Image

The Hanford Reach is the last free-flowing stretch of the Columbia River. Water flows are strongly controlled by upstream dams. The Columbia River above Priest Rapids Dam drains mountainous regions in Canada, Idaho, Montana, and Washington, which has a major impact on the distribution of precipitation in the drainage basin and the timing and magnitude of river flows. The Columbia River flow through the Hanford Reach displays multiple time scales, a key feature for addressing research objectives for this Science Focus Area.

The Hanford Reach is an 80-km stretch of the Columbia River that defines the north and east boundaries of the DOE Hanford Site. It is a classic example of a hyporheic corridor, and it affords important advantages to address our grand challenges. Groundwater and surface water in this system each possess key nutrient limitations for subsurface microorganisms that are relieved upon mixing of waters from these two sources to stimulate biogeochemical activity. Upstream dams provide hydrologic control to support planned experiments, while the Hanford Reach is probably the most highly characterized long stretch of any major river worldwide, with unique multiscale datasets available for integration into modeling activities.

Dynamic hydrologic conditions in the Columbia River cause seasonal changes in the groundwater-surface water interaction zone. Enlarge Image

The biogeochemical functioning of the subsurface interaction zone and solute transport both within and through it is controlled by the annual riverine water cycle, with large increases in river stage occurring in spring from snowmelt. The overall system is highly sensitive to climate change which influences the distribution between rainfall and snow in the Columbia River basin, the timing and amount of recharge and runoff, and the height and dynamics of the water table in the near shore aquifer. On shorter (hourly to weekly) time scales, dynamic flows associated with variable dam releases (hydropeaking) lead to changes of river stage as large as 1-2 meters, which drives frequent reversals in river-groundwater exchange direction. The Hanford Reach is an ideal location to investigate and understand these interactions.Our field research has focused on a 400-m stretch of riverbank located in the Hanford 300 Area. This site has a rich history of scientific study including the impacts of river-groundwater interactions on the fate of a uranium contaminant plume, and an extensive dataset describing groundwater levels and composition is available. We have recently installed a dense array of ~70 aquifer tubes in the hyporheic zone which provide multilevel sampling ports within the alluvial layer of the riverbed as well as temperature observations and electrodes for 3D electrical resistivity tomography. We are in the process of planning two similarly-instrumented sites that will be located in diverse hydromorphic settings at different locations in the Hanford Reach.

Electrical Resistivity Tomography (ERT) image showing the structure of the riverbed along a section of the Columbia River. Enlarge Image

We have also installed and are operating two eddy covariance flux towers, one in the 300 Area adjacent to the aquifer tube array and a second in the Hanford 100H Area upriver. These towers provide data that are leading to new insights into the role of hydrologic exchange flows on land surface fluxes and riparian zone processes.

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