Contaminant mixtures and their impact on nitrate removal in wetlands: A mesocosm study

Constructed wetlands are used extensively to mitigate surface runoff. While wetland treatment for nitrogen (N) has been comprehensively studied, a knowledge gap remains regarding the implications of other contaminants (e.g., pesticides, pharmaceuticals) on nitrate-N (NO₃-N) removal. This study sought to fill that gap by determining the impact imidacloprid, caffeine, perfluorooctane sulfonic acid (PFOS), atrazine, glyphosate, and sulfate (SO₄^2-) have on NO₃-N removal rates. The contaminants were determined based on their occurrence across Kentucky surface waters in urban (imidacloprid, caffeine, and PFOS) and rural (atrazine, glyphosate, and SO₄^2-) environments. Two constructed wetland designs, floating treatment wetlands (FTWs), and free-water surface wetlands (FWSs), were evaluated along with planted and un-planted controls, equating to 24 mesocosms. Individual contaminants in both designs inhibited the rate of NO₃-N removal, while the presence of the contaminants in their mixtures decreased N removal rates in FWSs compared to FTWs. However, by the end of each trial, 72-99 % of the NO₃-N was removed despite the wetland design or the presence of contaminant(s). The FWSs outperformed the FTWs earlier in the growing season (May-June) when the water temperatures were colder, while the FTWs outperformed the FWSs when the plants reached maturity (July-September). Both FTWs and FWSs observed significant removal of contaminants with 28-89 %, 63-70 %, >90 %, and >92 % removal for PFOS, caffeine, glyphosate, and atrazine, respectively. Limited removal of SO₄^2- was observed (≤57.9 %). These findings improve our understanding of water treatment for contaminant mixtures and their impact on NO₃-N removal, guiding treatment wetland design and placement.