Visualizing Hyporheic Flow Paths in Three Dimensions Using Time-Lapse Electrical Resistivity Tomography

Abstract:

The hyporheic zone, the region underneath/surrounding a stream where surface and subsurface waters – and subsequently solutes – are exchanged and interact, is important for many biogeochemical, hydrological, and ecological processes. However, it has remained difficult for researchers to sufficiently describe solute transport within the hyporheic zone, due, in part, to the great degree of heterogeneity of the subsurface. Thus, more direct and invasive sampling techniques are limited in their usefulness. We used an indirect approach for measuring the hyporheic zone, employing 3D time-lapse electrical resistivity tomography (ERT), with a pole-dipole configuration, downstream of a constant-rate addition of an electrically conductive salt tracer (Cl^-) as a solution via a high-precision peristaltic pump. This method allowed us to measure the extent of subsurface dynamics of streams in Wyoming’s Laramie and Snowy Range mountains, as it yields a three-dimensional view of solute transport and exchange within the hyporheic zone. We found that the physical size of the hyporheic zone and the rate of exchange between the hyporheic and surface waters, as estimated from 3D ERT, are largely related to sediment properties (i.e. grain size distribution) and the extent of tailing of the solute’s breakthrough curve (length of time for the solute to flush from the subsurface post cessation of the pump upstream). Coarser sediments with a relatively large porosity, such as gravels and sands, allowed for more subsurface exchange, and larger flow paths, than finer sediments with tighter packing structures, such as clays. Stream reaches that showed a higher degree of tailing in the breakthrough curve, traditionally implying a large transient storage zone, had larger and more active hyporheic zones as measured by 3D ERT. We therefore believe further investigations with 3D ERT will better our understanding of hyporheic exchange and stream solute transport.