Dr. Don Rosenberry, United States Geological Survey

Thursday, April 7, 2022 in BE 241/ Zoom, 4-5PM

Groundwater-surface-water Exchange Unrelated to Measured Hydraulic Gradients: Unraveling the Complexities at the Sediment-water Interface

Abstract: Hydrogeologists are taught that exchange between groundwater and surface water is driven by hydraulic gradients and governed by hydraulic conductivity.  Although true in most settings, quantifying the exchange with shallow piezometers is often difficult and gradient-based data can conflict with results from seepage meters deployed at the same location.  Automated seepage meters with temporal resolution of seconds to minutes can resolve conflicting data.  Sometimes the hydraulic gradients are simply too small to measure accurately.  Other times, the piezometers are not located properly and the implied vertical hydraulic gradients are not relevant to the flowpaths controlling the seepage.  The timing of gradient-based measurements also may not relate well with the time-integrated values from manual seepage meters.  Seiches and waves are an example of short-term seepage variability commonly detected by automated meters and commonly missed when hydraulic-head sensors are programmed to output data at 30-minute intervals or longer.  A seepage bias unrelated to gradients can also exist when upward flow buoys fine-grained sediment in the interstices of a coarser matrix, enhancing exchange rates, but downward flow pushes fines into intergranular traps, reducing flow.  Seepage also responds to biological processes.  Numerous species that live on or beneath the sediment-water interface alter the sediment structure, creating conduits for preferential flow.  Some actively filter water and create substantial exchanges across the sediment-water interface.  Biogenic gas can either accumulate and occlude pore spaces, reducing transmission of water, or be released episodically to the surface, abruptly changing seepage.  Devices that can measure fluxes at these time scales have thus far been rare.  They reveal processes that have been little measured and are poorly understood, processes that could be very important when quantifying transmission of water, nutrients, and anthropogenic compounds between groundwater and surface water.

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