Wednesday, May 20, 2009: 2:15 PM
Traditional characterization of hyporheic zone processes uses conservative tracers and in-stream breakthrough curves to characterize physical extent and solute transport through modeling; however, capturing spatial and temporal dynamics of surface water-groundwater exchange and sub-reach scale heterogeneity with these data is difficult. Widely used conservative solute tracer studies rely on the stream as an integrator of exchange processes and provide reach-scale estimates of hyporheic exchange rate and extent. Installation of extensive, invasive monitoring well networks provides point-verification of exchange, but ultimately lacks local resolution and extensive spatial coverage of the solute transport processes. Here, we highlight how the use of near-surface electrical resistivity tomography methods, coupled with a conservative salt tracer test, provides in-situ imaging of hyporheic zone spatial and temporal dynamics. Tracer-labeled water is more electrically conductive than groundwater and in-stream background levels, reducing resistance of the subsurface actively communicating with the stream. Comparison of background measurements with those recording active exchange of the tracer provides temporal resolution of hyporheic zone exchange rates. Inverted resistivity tomograms map the extent of hyporheic exchange throughout the reach of interest, and provide insight to subsurface heterogeneity and localized processes, which would otherwise be averaged over the reach using traditional characterization methods.
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