Alkalinity of Irrigation Return Water Influences Nutrient Removal Efficacy of Floating Treatment Wetland Systems1

Water quality concerns often prevent reuse of captured irrigation return water for irrigation of specialty crops. Prior research indicated alkalinity of specialty crop operation irrigation varies from 0 to >500 mg.L−1 ( >0.06 oz.gal−1) CaCO3 across the United States. Floating treatment wetlands (FTWs) are an option for remediation of nutrients in irrigation return water, but effects of variable alkalinity on nutrient removal efficiency of FTWs are unknown. An experimental FTW system was developed to quantify the effect of alkalinity on the growth and nutrient uptake capacity of three plant species. ‘Rising Sun' Japanese iris (Iris ensata ‘Rising Sun’ Thunb.), upright sedge (Carex stricta Lam.);, and switchgrass (Panicum virgatum L.). were grown for 6 weeks at one of five alkalinity treatment levels, representing the alkalinity range of nursery and greenhouse irrigation runoff: 0, 100, 200, 300, and 400 mg.L−1 CaCO3 (0, 0.01, 0.02, 0.04, 0.05 oz.gal−1 CaCO3). Overall, Japanese iris demonstrated consistent remediation across each alkalinity treatment for both nutrient load reduction and plant accumulation. Species of iris warrant greater consideration and use in bioremediation systems. Both upright sedge and switchgrass could be used in systems with appropriate alkalinity levels. Future work should consider assessing novel plants at different points within their growth cycle, extended exposure durations, and decreased hydraulic retention time. Index words: Aquatic plant, nitrogen, phosphorus, sodium bicarbonate, nitrogen speciation. Species Used in this study: ‘Rising Sun' Japanese iris (Iris ensata ‘Rising Sun' Thunb.); upright sedge (tussock sedge) (Carex stricta Lam.); switchgrass (Panicum virgatum L.).