Seasonal shifts in surface water-groundwater connections in a ferricrete-impacted stream estimated from electrical resistivity

Abstract

The efficacy of the hyporheic zone (HZ) — where surface water and groundwater mix — for processing nutrients or the uptake of metals is dependent on the streambed hydraulic conductivity and stream discharge, among other characteristics. Here, we have explored electrical resistivity tomography (ERT) of hyporheic exchange in Cement Creek near Silverton, Colorado, which is affected by ferricrete precipitation. To quantify flows through the HZ, we have conducted 4 h salt injection tracer tests and collected time-lapse ERT of the streambed and banks of Cement Creek at high and low flow. We have installed piezometers to conduct slug tests, which suggest a low-permeability zone at 44 cm depth likely composed of ferricrete that cemented the cobbles together. Based on the ERT, the tracer released into the stream is constrained within the shallow streambed with little subsurface flow through the banks. The tracer is detected in the HZ for a longer time at high flow compared to low flow, suggesting that more flow paths were available to connect the stream to the HZ. The tracer is confined above the ferricrete layer during the high- and low-flow tests. Mass transfer and storage area parameters are calculated from combined analysis of apparent bulk conductivity derived from ERT and numerical modeling of the tracer breakthrough curves. The hyporheic storage area estimated at low discharge ([Formula: see text]) is smaller than that at high discharge ([Formula: see text]) and residence times are 2.7 h at low discharge and 4.1 h at high discharge. During high discharge, in-stream breakthrough curves display slower breakthrough and longer tails, which is consistent with the time-lapse electrical inversions and 1D transport with inflow and storage modeling. Our findings indicate that ferricrete reduces the hydraulic conductivity of the streambed and limits the areal extent of the HZ, which may lower the potential for pollutant attenuation from the metal-rich waters of Cement Creek.