A Robust, Low‐Cost Conductance Sensor for High Resolution Real‐Time Monitoring of Streambed Pore Water Dynamics

Abstract

The groundwater—surface‐water interface is an important regulator of many biogeochemical processes along river corridors. One of the main drivers of these processes is the pore water travel time along hyporheic flow paths. However, our understanding and ability to predict the spatial‐temporal dynamics of subsurface travel times are limited because current techniques are restricted to a few locations or constant subsurface flows. To overcome these limitations, we designed and field‐tested a small, easy‐to‐build, low‐cost pore water conductance sensor to monitor subsurface travel times in real‐time. Leveraging the simple design and low cost, we built and installed 93 sensors as profiles (here measuring up to 20 cm depth) in a first‐order stream and conducted repeated tracer injections during baseflow conditions and a storm event. We quantified the dynamics of pore water travel times at unprecedented high vertical (cm‐scale), horizontal (dm‐scale) and temporal (minutes) resolution during a period of almost 2 months, moving beyond the usual snapshot view of these processes. We observed small‐scale deviations from the general flow field along a riffle and inconsistent patterns of pore water travel times during the storm event. Measured travel times, combined with targeted pore water sampling of reactive or conservative solutes, allows calculation of solute transformation rates at a high spatial resolution and extent in future sensor applications. These insights could help to understand the relative contribution of the different drivers (e.g., residence time, redox conditions, substrate availability) controlling biogeochemical processes at the groundwater—surface‐water interface and the impact of these drivers on the metabolism and solute fluxes along river corridors.