Low-frequency electrical properties of peat

Electrical resistivity/induced polarization (0.1-1000 Hz) and vertical hydraulic conductivity (K-nu) measurements of peat samples extracted from different depths (0-11 m) in a peatland in Maine were obtained as a function of pore fluid conductivity (sigma(w)) between 0.001 and 2 S/m. Hydraulic conductivity increased with sigma(w) (K-nu proportional to sigma(w)(0.3) between 0.001 and 2 S/m), indicating that pore dilation occurs due to the reaction of NaCl with organic functional groups as postulated by previous workers. Electrical measurements were modeled by assuming that "bulk'' electrolytic conduction through the interconnected pore space and surface conduction in the electrical double layer (EDL) at the organic sediment-fluid interface act in parallel. This analysis suggests that pore space dilation causes a nonlinear relationship between the "bulk'' electrolytic conductivity (sigma(el)) and sigma(w) (sigma(el) proportional to sigma(w)(1.3)). The Archie equation predicts a linear dependence of sigma(el) on sigma(w) and thus appears inappropriate for organic sediments. Induced polarization (IP) measurements of the imaginary part (sigma"(surf)) of the surface conductivity (sigma*(surf)) show that sigma"(surf) is greater and more strongly sigma(w)-dependent (sigma"(surf) proportional to sigma(w)(0.5) between 0.001 and 2 S/m) than observed for inorganic sediments. By assuming a linear relationship between the real (sigma'(surf)) and the imaginary part (sigma"(surf)) of the surface conductivity, we develop an empirical model relating the resistivity and induced polarization measurements to sigma(w) in peat. We demonstrate the use of this model to predict (a) sigma(w) and (b) the change in K-nu due to an incremental change in sigma(w) from resistivity and induced polarization measurements on organic sediments. Our study has implications for noninvasive geophysical characterization of sigma(w) and K-nu with potential to benefit studies of carbon cycling and greenhouse gas fluxes as well as nutrient supply dynamics in peatlands.