Solute transport and nitrate removal in full-scale subsurface flow constructed wetlands of various designs treating agricultural drainage water

Abstract

Subsurface flow constructed wetlands (SSF-CWs) consisting of woodchip based filter matrixes are promising measures targeting agricultural N loss via subsurface tile drains. Optimization of these systems may include the selection of appropriate hydraulic designs (i.e. horizontal and vertical flow), which may affect the solute residence time. In this study a bromide tracer experiment was performed in three full scale SSF-CWs, consisting of a woodchip-based filter matrix. Three different hydraulic designs (horizontal (H), vertical upward (Vup) and vertical down flow (Vdown)) and two flow rates were investigated (0.49 and 1.83Ls−1). Additionally, batch experiments investigating the intra-granular diffusion into woodchips using two tracer solutes (tritium and bromide) were carried out. Non-equilibrium solute transport, including a mass exchange between a mobile and an immobile domain, was found in all SSF-CWs. The Vup demonstrated the most pronounced non-equilibrium and the lowest N removal rate. In contrast the largest N removal rate was observed in the Vdown. The higher NO3 removal rates were attributed to a longer solute residence time. Tailing of the tracer BTC indicated the influence of diffusive exchange in solute residence time, and this was further supported by the intra-granular diffusion of tracer solutes. Generally, the results suggested the vertical downwards SSF-CW as the best performing SSF-CW in terms of solute transport behaviour and N removal efficiency.