Characterizing Physical Properties of Streambed Interface Sediments Using In Situ Complex Electrical Conductivity Measurements

Abstract

Streambed sediment physical properties such as surface area, are difficult to quantify in situ but exert a high‐level control on a wide range of biogeochemical processes and sorption of contaminants. We introduce the use of complex electrical conductivity (CC) methods (also known as spectral‐induced polarization (SIP)) that measure both real and imaginary conductivity to noninvasively and efficiently characterize shallow streambed sediments. We explore the method through synthetic modeling, laboratory, and field measurements to demonstrate the sensitivity of imaginary conductivity to sediment surface area, controlled in part by fine‐grained iron oxides produced by anoxic groundwater discharge. Laboratory measurements verify expected relationships between CC parameters and sediment properties. Synthetic modeling using a 1D analytical model illustrates the influence of water layer depth and conductivity on the field CC measurements made at the streambed‐stream water interface. Specifically, the inverted sediment imaginary conductivity is less impacted by uncertainty in the water layer depth and conductivity relative to the real conductivity and phase shift. Field CC measurements along a landfill‐impacted river reveal discrete streambed zones with enhanced bulk surface area generally corresponding to anoxic groundwater discharge zones with high concentrations of fine‐grained iron oxide precipitates.

Evaluation of riverbed magnetic susceptibility for mapping biogeochemical hot spots in groundwater‐impacted rivers

“…Analysed sediments were sampled from Mashpee River and sieved to exclude grains larger than 2 mm. Part of the Fe(III) data were reported in Wang et al (2021)…”

“…However, these techniques are generally based on the natural temperature contrast between groundwater and surface water and do not directly indicate biogeochemical hot spots. Wang et al (2021) explored the application of the complex electrical conductivity method to characterize shallow riverbed sediment surface area, from which the distribution of high surface area Fe oxides may be inferred. As Fe oxides are magnetic materials, a non‐invasive magnetic susceptibility (MS) method might allow for a more rapid and direct assessment of river sediment Fe oxide distributions, providing information on biogeochemically reactive groundwater/surface water exchange, although MS has not previously been utilized for river corridor investigation.…”