Detection of ionic contaminants in unsaturated soils using time domain reflectometry penetrometer

Previous investigations using time domain reflectometry (TDR) to detect ionic contaminants has mainly focused on saturated coarse-grained soils and conventional probes (e.g., coaxial, two- or three-rod probe). To explore the applicability of using the TDR penetrometer for the site investigation of ionically contaminated land, a theoretical framework was proposed to estimate the pore water electrical conductivity of both saturated and unsaturated silts based on measured soil dielectric permittivity and bulk electrical conductivity. The theoretical framework consists of three equations, the empirical equation relating soil dielectric permittivity and volumetric water content, the soil dielectric mixing model, and the modified Archie’s law considering the surface conductivity. To verify the theoretical framework, chamber tests using the TDR penetrometer were performed to detect CuSO₄ contaminants in silt columns. Results show that the theoretical framework provides an increasingly better calculation of the pore water electrical conductivity of saturated and unsaturated silt with increasing the concentration of CuSO₄ solution. This is because that the modified Archie’s law with a constant surface conductivity in the theoretical framework can well capture the relationship between soil bulk electrical conductivity and pore water electrical conductivity at high solution concentrations (i.e., 0.015 and 0.030 mol/L CuSO₄ solutions), but not at low solution concentrations (i.e., tap water). The theoretical framework and chamber tests are expected to provide scientific guidance for extending the capacity of TDR penetrometer to the site investigation of ionically contaminated land.