PNNL

Abstract

Recent induced polarization (IP) studies suggest that the real part of surface conductivity (s'surf) scales linearly with the imaginary conductivity (s" = s"surf) or normalized chargeability (Mn) for a range of soil types. The coefficients of this relationship l and l_Mn (l = s"/s'surf or l_Mn = Mn/s'surf), allow for separating the surface and electrolytic conductivities from the total measured conductivity. However, the dependence of these constants on varying soil physicochemical properties, including under unsaturated conditions, is yet to be assessed. In this study, we measured s'surf of eighteen undisturbed soil samples from a restored wetland using multi-salinity conductivity measurements. We also acquired s" measurements over a frequency range of 0.01 Hz to 10 kHz. The s'surf and s" were compared with laboratory measurements of soil properties, while l and l_Mn calculated individually for each soil sample were regressed against the soil properties. We found an apparent dependence of l on soil texture, bulk density, organic matter, and moisture contents, with coefficients of determination (R2) ranging from 0.5 to 0.65 at low frequencies (e.g., 1 Hz). This correlation did not exist at the high frequencies (e.g., 936 Hz). This apparent dependence of l on soil texture results from the insensitivity of s" at low frequency to s'surf and, by implication, to the soil properties controlling s'surf. l_Mn, on the other hand, shows no correlation with the soil properties because Mn is linearly correlated with s'surf and correlated to the soil properties controlling s'surf. Our results call for caution on the application of s" at a single frequency as a proxy of s'surf, as s" will not necessarily be correlated with s'surf across all soil types. Although using l_Mn derived from multi-frequency measurements overcame this limitation, field acquisition of spectral information (e.g., up to 1000 Hz) remains a major challenge.