The Symposium for the Application of Geophysics to Environmental and Engineering Problems (SAGEEP) will be held in Tucson, AZ this March. I will be chairing the session on Geophysics in Rivers and Streams, and presenting the following paper:
Comparison of Time-lapse Resistivity and Well Data in the Hyporheic Zone beneath Streams
Jonathan Nyquist, Temple University
Laura Toran, Temple University
David O’Donnell, Temple University
Robert Ryan, Temple University
Characterization of the hyporheic zone beneath streams, where surface water and groundwater mix, is critical for understanding stream ecology and contaminant fate and transport. Tracer tests can identify storage zones, but cannot distinguish between tracer lingering in pools and subsurface storage. Wells monitor the subsurface, but prohibitively many may be required to avoid spatially under-sampling the plume, and with mixed porosity sediment wells preferentially sample the more hydraulically conductive sediments. Additional information can be obtained by monitoring saline tracer using time-lapse resistivity – continuous spatial coverage and sensitivity to the tracer distribution irrespective of the hydraulic conductivity – at the expense of a more ambiguous interpretation.
Tracer tests performed in two streams and in two reaches of the same stream led to surprisingly different results. At Crabby Creek, in an urbanizing area outside Philadelphia, time-lapse resistivity data showed zones in a reach with thin sediment where tracer lingered long after well concentrations had returned to background. This is indicative of dual-porosity sediment where the tracer in the large pores flushes quickly, but the tracer slowly diffuses into smaller pores and is even more gradually released by back-diffusion. At a second Crabby Creek reach with thick sediment both the wells and the resistivity showed lingering tracer, but the wells returned to background sooner.
In a nearby Valley Creek experiment no tracer in the hyporheic zone was apparent in time-lapse images or well water samples. However, geophysics detected a gradual decrease in the resistivity of the top layer of stream sediments. Modeling of diurnal temperature fluctuations measured in the stream sediments suggests that this reach a groundwater discharge zone. The gradual decrease in the shallow resistivity could be a change in either the mixing zone thickness or pore fluid conductivity.
In each case time-lapse resistivity monitoring provided useful information about the hyporheic zone that complements well data but the results can vary significantly from stream to stream depending on sediments, stream structures, and groundwater gradients.