A field method to quantify exchange with less-mobile porosity in streambeds using electrical hysteresis

Abstract:

Heterogeneous streambed materials may be expected to develop two general porosity domains: a more-mobile porosity dominated by advective exchange, and a less-mobile porosity dominated by diffusive exchange. Less-mobile porosity containing unique redox conditions or contaminant mass may be invisible to traditional porewater sampling methods, even using “low-flow” techniques, because these methods sample water preferentially from the mobile porosity domain. Further, most tracer breakthrough curve analyses have only provided indirect information (tailing) regarding the prevalence and connectivity of less-mobile porosity, typically over experimental flowpath scales between 1-10 meters. To address the limitations of conventional methods, we use electrical geophysical methods to aid in the inference of less-mobile porosity parameters. Unlike traditional fluid sampling, electrical methods can directly sense less-mobile solute and can target specific points along subsurface flowpaths. We demonstrate how the geophysical methodology developed for dual-domain groundwater transport can be scaled to the streambed through synthetic, laboratory column, and field experiments; further we show how previously-used numerical modeling techniques can be replaced by a more-simple analytical approach. The new analytical method is based on electrical theory, and involves characteristics of electrical hysteresis patterns (e.g. hinge point values) that are used to quantify (1) the size of paired mobile and less-mobile porosities, and (2) the exchange rate coefficient through simple curve fitting. Results from the analytical approach compare favorably with results from calibration of numerical models and also independent measurements of mobile and less-mobile porosity. Lastly, we demonstrate a method of focused solute streambed injection to quantify less-mobile porosity and explain redox zonation in contrasting stream environments.