Imaging peatland hydrogeological structure using electrical impedance tomography

Peatland management, conservation, and restoration rely on a thorough understanding of peatland hydrogeology and carbon dynamics. To date, most of the information about peatland subsurface properties is derived from laboratory analyses of borehole samples, which allow direct measurement of hydrogeological and geochemical parameters but provide only point-wise information. Here, we present the use of the low-frequency electrical impedance tomography (EIT) to investigate hydrogeological properties and carbon dynamics in alpine peatlands in an imaging framework. In particular, we aim to map biogeochemical hotspots and to delineate flow paths controlling surface-groundwater interactions and nutrient cycles. The EIT method is an extension of the widely-used electrical resistivity tomography (ERT), which deploy electrodes (placed on the ground) to inject current to resolve the conductive (conductivity) and capacitive (polarization) properties of the subsurface along 2D planes or 3D volumes. While the electrical conductivity (due to migration of charges in the pore space) has been commonly used to investigate variations in saturation, porosity and fluid electrical conductivity (EC); the polarization (i.e., capacitive effect, resulting from accumulation and polarization of charges in the fluid-grain interface), provides a unique opportunity to gain information about changes in pore-space geometry, cations exchange capacity and soil organic carbon (SOC). To resolve the frequency-dependence of the electrical properties, imaging measurements were collected in the range between 0.1 and 75 Hz, while vertical soundings were conducted between 0.1 and 1000 Hz. This information is needed to discriminate between SOC and clay content and quantify changes in hydraulic properties. The geophysical investigations are supported by the analysis of material extracted from boreholes.  

The EIT surveys were conducted across six alpine peatlands in Austria to characterize the variability in electrical properties at both local (site-specific) and regional (inter-site) scales. The investigated peatlands mainly originated through terrestrialisation of lakes that had formed in glacially eroded depressions at the end of the Last Glacial Maximum. Accordingly, substratum is dominated by lacustrine fines in direct proximity to glacial sediments and various types of alpine bedrock.

Our results reveal that the electrical conductivity and polarization are consistent for data collected across the different peatlands, supporting the relevance of the EIT method for peatlands investigations. We demonstrate that incorporating polarization as an additional parameter alongside electrical conductivity improves the resolution of peat thickness compared to interpretations based solely on electrical conductivity. Moreover, the polarization response reveals clear spatial variations related to geochemical variations in the organic soil. While the electrical properties are consistent, important changes in the polarization response can be observed in degraded peatlands, demonstrating the potential of the EIT method for the design of remediation and conservation strategies. Ongoing tracer experiments aim to validate petrophysical models connecting electrical and hydraulic properties in an imaging framework.