Geophysically-based analysis of BTCs and ion exchange processes in soil
Abstract. Breakthrough curves (BTCs) are a valuable tool for qualitative and quantitative examination of transport patterns in porous media. Although breakthrough (BT) experiments are simple, they often require extensive sampling and multi-component chemical analysis. In this work, we examine spectral induced polarization (SIP) signals measured along a soil column during a BT experiment in a homogeneous and heterogeneous soil profiles. Soil profiles were equilibrated with an NaCl background solution and then a constant flow of CaCl2 solution was applied. SIP signature was recorded, and complementary ion analysis was performed on the collected outflow samples. Our results confirm that changes to the pore-water composition, ion exchange processes and profile heterogeneity are detectable by SIP: the real part of the conductivity-based BTCs clearly indicated the BT of the non-reactive ions as well as the retarded BT of Ca2+. The imaginary part of the conductivity-based curves reacted to the changes in ion mobility around the electrical double layer (EDL) and indicated the initiation and the termination of the Na+–Ca2+ exchange reaction. Finally, both the real and imaginary components of the complex conductivity reacted to the presence of a coarser textured layer in the heterogeneous profile.