Hyporheic zone turnover time from 3D geophysics

A stream's balance of exchange with the hyporheic zone and the reaction rate within hyporheic flowpaths controls its nutrient removal capacity. However, our ability to quantify rates of exchange between surface and hyporheic waters is often limited to techniques describing in-stream breakthrough curves of a conservative tracer. Combining constant-rate conductive tracer (salt) additions with geophysical imaging of the subsurface allows for the direct measurement of hyporheic exchange and turnover. We used time-lapse 3D electrical resistivity tomography (ERT) to estimate the turnover time of the hyporheic zone at two headwater streams following the completion of a constant-rate addition of a conservative tracer, sodium chloride (NaCl). Though the streams were similar in geomorphology, they produced contrasting in-stream breakthrough curves of large and small storage zones. We measured hyporheic exchange using 3D ERT at the stream with elongated breakthrough curves but not at the more dynamic stream. Results from 3D ERT showed a delay in loading of the tracer in to the hyporheic zone and a >60 h return to ambient conditions. The estimated turnover time of the tracer in the hyporheic zone that was measured was over an order of magnitude longer from 3D ERT (9.7 h) than by transient storage modeling (0.73 h). Exchange with the hyporheic zone was mostly constrained to a small portion of the ERT grid which is consistent with the heterogeneous exchange seen in other ERT tracer studies. Our results highlight how using geophysical imaging techniques, such as 3D ERT, can describe the turnover of solutes in hyporheic flowpaths below the sensitivity of in-stream breakthrough curve analysis.