Water and solute movement in air pollution control residues measured by time domain reflectometry.

The mobility of pollutants from air pollution control (APC) residues is strongly influenced by its contact with water. This study aims to verify the applicability of time-domain reflectometry (TDR) to infer water and solute movement in situ in an APC residue–compost mixture in a laboratory column. The calibrated dielectric permittivity–volumetric water content relationship (K−thetav) for the material shows a distinct plateau at midrange volumetric water contents, which are underestimated by Topp's K−thetav model throughout the studied range. The coating of probes enables a clear TDR signal collection and analysis up to electrical conductivity values of 280 mS cm−1. Heterogeneities in packing and channel flow in the column are revealed by TDR measurements. Water contents inferred by TDR signal analysis require careful interpretation when used in materials such as APC residues, due to the high amount of water bound in the hydration shells of ions released into the pore water.