Are We There Yet? A Critical Experimental Assessment of the Application of Induced Polarization for Monitoring Geochemical Processes

Spectral induced polarization (SIP) can provide valuable information about (bio)geochemical processes taking place in the poorly accessible subsurface. The method is sensitive to reactions that alter the solid-water interface. Here, we critically evaluate the effectiveness of SIP to monitor geochemical processes by focusing on a model-supported analysis of cation exchange dynamics in sediments containing organic matter. Organic matter is a crucial substrate for contaminant immobilization that exhibits a strong SIP response. We compare the SIP response of columns during the injection of cations (Na+, Ca2+ and Zn2+) with different sorption strengths. We assess whether a change in surface ion mobility due to cation exchange is reflected by an increasing (Na+, high surface mobility) or decreasing (Zn2+, low surface mobility) imaginary conductivity. Our work demonstrates how we can qualitatively monitor reactive solute fronts using (S)IP, thus, helping to target sampling events. Furthermore, we explore the quantitative value of SIP data sets in constraining reactive transport models. We use the imaginary conductivity as a proxy for sorbed concentrations by separating the contributions of ion exchange and bulk electrical conductivity to changes in imaginary conductivity. By integrating a Bayesian parameter-estimation scheme, we test whether the use of SIP can replace geochemical sampling and improve reaction-parameter estimates. While inverting SIP-data alone does not yield better results than breakthrough samples, their integration reduces the uncertainty of some parameters, highlighting their potential value. Finally, we discuss opportunities and limitations for reaction monitoring using SIP and provide an outlook for its successful application by non-geophysicists. Plain Language Summary Understanding and monitoring geochemical processes in the subsurface is crucial for managing environmental challenges. However, the subsurface is difficult to access, and traditional sampling methods can be time-consuming and invasive. Spectral induced polarization (SIP) offers a promising alternative by providing indirect but detailed insights into these processes. It can detect changes in the electrical properties of sediment particles on which many critical reactions occur. This study investigates the potential of SIP to monitor the adsorption dynamics of cations as they flow through sediments containing organic matter (OM). OM is key for the immobilization of contaminants. By exploring how different cations (sodium, calcium and zinc) interact with OM and alter SIP signals, this research aims to assess SIP's ability to track reactive zones and enhance our understanding of geochemical reactions. We also explore whether SIP could replace traditional sampling methods and improve the accuracy of models used to predict contaminant transport in groundwater. Combining SIP data with traditional sampling helps to improve the accuracy of parameter predictions, showing SIP's potential as a complementary tool for environmental monitoring. Finally, we discuss how SIP could be more widely applied, even by non-specialists, while acknowledging its current limitations and areas for further development.