Vertical treatment efficiency of constructed wetlands: vegetation influence and caffeine removal

Constructed wetlands are widely implemented as a nature based solution for treating stormwater runoff, relying on the functions of vegetation, substrate and microbiome to remove pollutants. Horizontal subsurface flow constructed wetlands (HSCW) are often planted with macrophytes such as Phragmites australis and Arundo donax, which enhance stormwater treatment through root-zone interactions. However, treatment efficiency within a single wetland unit can vary vertically between the surface and bottom layers. Nevertheless, it remains unclear how vertical stratification in continuous subsurface flow systems influences the attenuation of conventional water quality parameters and chemicals of emerging concern (CECs), under outdoor environmental conditions. This study investigated how depth affected the performance of a three-stage vegetated and unvegetated HSCW system over 191 days. Distinct vertical stratification was observed, with surface layers favouring ammonia and phosphorus removal, while bottom layers enhanced the removal of suspended solids, turbidity, and caffeine. Caffeine removal exceeded 99% in the bottom layers, demonstrating the significant role of vertical gradients in treating conventional pollutants and CECs. These findings show that both vegetated and unvegetated HSCW systems perform effectively across the vertical profile, with surface layers favouring the removal of differing substances to the bottom layers in each case. Accounting for observed vertical treatment heterogeneity in wetland design and vegetation selection could improve treatment outcomes and support more efficient, targeted stormwater management strategies.