Isotopically tracing the impact of water contaminant “cocktails” on nitrogen pathways in constructed treatment wetlands

Constructed wetlands mitigate nitrogen (N) loads in surface runoff, yet implications of common contaminant “cocktail” mixtures to wetland N removal are relatively unexplored. A ¹⁵N isotopic tracer was used to assess the impact contaminants from urban (imidacloprid, caffeine, and PFOS) and rural (atrazine, glyphosate, and sulfate) environments have on nitrogen (N) pathways in free-water surface wetlands (FWS) and floating treatment wetlands (FTW). Nitrate-N removal rates ranged from 0.20 to 0.68 d^-1 and were dependent on the contaminant mixture and wetland design. Of the enriched ¹⁵N, biomass uptake accounted for 36.5 ± 30.7 %, 6.3 ± 4.2 % was denitrified and lost through air-water exchange, and 55.9 ± 24.4 % was assumed to have been denitrified and transmitted through plant tissues or lost due to ebullition (bubbling). The FTWs had increased temporary storage of N while the FWSs had increased permanent removal. In the presence of contaminant mixtures, both designs observed a shift in the NO_3[sbnd]N removal pathway toward increased biomass uptake. Caffeine (59.7 ± 3.6 %), PFOS (65.9 ± 8.8 %), atrazine (92.0 ± 0.6 %), and glyphosate (>89.1 %) were effectively removed from the water with limited removal for SO₄^2- (2.3 ± 12.3 %). Findings elucidate implications to N fate and maintenance required (e.g., biomass harvesting) in two constructed pulse-flow wetlands positioned in urban and rural landscapes while providing a clear understanding of how system design and water quality characteristics impact biogeochemical cycling.