Hydrodynamic evaluation of table salt used as a tracer for hydraulic characterization in wetlands

Wetlands are commonly used for wastewater treatment, with table salt frequently used as a tracer. This study aimed to evaluate the behaviour of table salt as a tracer for hydrodynamic characterization of wetlands using low-cost Arduino sensors to collect data and computational imaging to monitor the flow in a laboratory-scale wetland. A 100 L circular-shaped tank filled with gravel was used, and sampling points were placed at 56 different points. Five tests were conducted: three with only water in the tank, and two with water and a 0.27 m coarse aggregate layer (void ratio 45%, typical for wetlands). Water-only tests used a flow rate of 7.2 L·h^-1 for an ascendant configuration, and 3.6 and 7.2 L·h^-1 for a descendant configuration. Hydraulic diagnosis data were collected from electrical signals by electrodes in the circulation tank using the signal conductivity-concentration correlation. To analyse the tracer plume evolution over time, a control volume was created using processing software. For different tank levels, concentration curves, and hydraulic efficiency parameters, including short circuit, hydraulic efficiency (λ), theoretical residence time (t_n), and average residence time (t_m), were calculated. Colour map graphs exhibited densimetric deposition of the tracer at the bottom of the tank for all tested flow configurations. The imagery showed the behaviour of table salt as a tracer, highlighting deposition and diffusion for the tested configurations. The imaging method obtained in this study may be applied to other studies and adapted to different scenarios.