Internal hydraulics and wind effect in a surface flow constructed wetland receiving agricultural drainage water

Variations of internal water velocities in surface flow constructed wetlands (SF-CWs) affect the hydraulic efficiency and are responsible for heterogeneous distribution of the biogeochemical processes. Internal obstructions, short circuiting due to the presence of immobile zones, and incomplete mixing are among the main culprits. In this study, a high resolution spatial analysis of solute transport is presented aiming at identifying internal flow preferentiality of the system and the influence of wind effects. Dye (Uranine) and salt (Bromide) tracer tests were performed in a compartment-designed SF-CW constituted of alternated deep and shallow zones. Water samples were collected from the outflow and a number of internal stations located according to a 3-transect system at two different depths. Bromide outflow and internal breakthrough curves (BTCs) were fitted using the tank-in-series (TIS) model. The results showed that the hydraulic efficiencies at internal stations (0.08-1.63) generally decreased with increasing distances from the tracer injection point. Both spline interpolation and dye tracer results showed preferential flow alongside the southern side of the SF-CW. Analysis further showed that the shallow zones and the wind blowing in opposite direction to the flow facilitate water mixing and recirculation. The method is a valuable exploiting tool for already operating SF-CWs.