Characterizing hyporheic transport processes — Interpretation of electrical geophysical data in coupled stream–hyporheic zone systems during solute tracer studies

a r t i c l e i n f o Quantifying hyporheic solute dynamics has been limited by our ability to assess the magnitude and extent of stream interactions with multiple domains: mobile subsurface storage (MSS, e.g., freely flowing pore water) and immobile subsurface storage (ISS, e.g., poorly connected pore water). Stream-tracer experiments coupled with solute transport modeling are frequently used to characterize lumped MSS and ISS dynamics, but are limited by the ability to sample only " mobile " water and by window of detection issues. Here, we couple simulations of near-surface electrical resistivity (ER) methods with conservative solute transport to directly compare solute transport with ER interpretations, and to determine the ability of ER to predict spatial and temporal trends of solute distribution and transport in stream–hyporheic systems. Results show that temporal moments from both ER and solute transport data are well correlated for locations where advection is not the dominant solute transport process. Mean arrival time and variance are especially well-predicted by ER interpretation, providing the potential to estimate rate-limited mass transport (i.e. diffusive) parameters from these data in a distributed domain, substantially increasing our knowledge of the fate and transport of subsurface solutes. The movement of stream water into relatively slow-moving domains increases reach residence time of water and solutes [1,2]. By slowing the movement of water within a reach, the potential for processing of nutrients and other solutes is increased through longer exposure of stream solutes to microbial communities and biogeochemi-cally active sediment [3–5]. Understanding the transport and fate of stream solutes is critical to understanding the temporal and spatial distribution of biogeochemical cycling associated with hyporheic exchange in streams. Stream solute transport studies frequently yield concentration time series from in-stream and subsurface monitoring locations with long tails that are not explained by the in-stream processes of advection and dispersion alone. The movement of solute from the highly advective mobile domains into less-mobile domains with heterogeneous residence times is a common explanation of the observed tailing behavior in hydrological sciences. This type of tailing behavior has been observed in both flow through porous media [e.g., 6,7] and the exchange of stream water with the less-mobile hyporheic zone [e.g., 8,9]. Movement of tracer into the slower moving in-stream dead zones and into the hyporheic zone is typically considered responsible for this tailing behavior in stream–hyporheic studies. However, most stream solute studies overlook the range of …