Exploring limitations in the induced polarization versus surface conductivity relationship in the case of wetland soils

Abstract

Recent induced polarization studies suggest that the real part of surface conductivity ([Formula: see text]) scales linearly with the imaginary conductivity ([Formula: see text] = [Formula: see text]) or normalized chargeability (Mn) for a range of soil types. The coefficients of this relationship l and l_Mn ( l = [Formula: see text]/[Formula: see text] or l_Mn = Mn/[Formula: see text]) allow the separation of the surface and electrolytic conductivities from the bulk conductivity. However, the dependence of these constants on varying soil physicochemical properties, including under unsaturated conditions, is yet to be assessed. Using estimates of [Formula: see text] from 18 undisturbed soil samples from a restored wetland and [Formula: see text] measured over a frequency range of 0.01 Hz to 10 kHz, the [Formula: see text] and [Formula: see text] were compared with the laboratory measurements of soil properties. Also, l and l_Mn were calculated for each soil sample and regressed them against the soil properties. We find an apparent dependence of l on soil texture, bulk density, organic matter, and moisture contents, with coefficients of determination ([Formula: see text]) ranging from 0.5 to 0.65 at low frequencies (e.g., 1 Hz) but not at high frequencies (e.g., 936 Hz). This dependence of l on soil texture results from the insensitivity of [Formula: see text] at low frequency to [Formula: see text] and, by implication, to the soil properties controlling [Formula: see text]. In contrast, l_Mn indicates no correlation with the soil properties because Mn is linearly correlated with [Formula: see text] and correlated with the soil properties controlling [Formula: see text]. Our results call for caution on the application of [Formula: see text] at a single frequency as a proxy of [Formula: see text] because [Formula: see text] is not necessarily correlated with [Formula: see text] across all soil types. Although using l_Mn derived from multifrequency measurements overcomes this limitation, field acquisition of spectral information (e.g., up to 1000 Hz) remains a challenge.