Runoff pH Influences Nutrient Removal Efficacy of Floating Treatment Wetland Systems

Abstract

Floating treatment wetlands (FTWs), a modified constructed wetland technology, can be deployed in ponds for the treatment of nursery and greenhouse irrigation runoff. The pH of nursery and greenhouse operation irrigation water varies from 3.3 to 10.4 across the United States. Water flow rate, plant species selection, and variable nutrient inputs influence the remediation efficacy of FTWs and may interact with the pH of inflow water to change nutrient remediation dynamics. Therefore, an experiment was designed to quantify the effect of pH on the growth and nutrient uptake capacity of three macrophyte species using a mesocosm FTW system. ‘Rising Sun’ japanese iris (Iris ensata), bushy bluestem (Andropogon glomeratus), and maidencane (Panicum hemitomon) were grown for two 6-week periods and exposed to five pH treatment levels representing the range of nursery and greenhouse irrigation runoff, 4.5, 5.5, 6.5, 7.2, and 8.5, for a total of 15 plant and pH combinations. Water was treated with either hydrochloric acid to decrease the pH or sodium hydroxide to increase the pH. The pH-adjusted solutions were mixed with 12 mg·L−1 nitrogen (N) and 6 mg·L−1 phosphorus (P) fertilizer (64.8 g·m−3 N and 32.4 g·m−3 P). Differences in pH impacted both N and P removal from the FTW systems for two of the three species studied, maidencane and bushy bluestem. Higher pH treatments reduced nutrient removal efficacy, but plants were still capable of consistently removing nutrients across all pH treatments. Conversely, ‘Rising Sun’ japanese iris maintained similar remediation efficacies and removal rates across all pH treatments for both N and P, possibly due to the ability to acidify its rhizosphere and modify the pH of the system. Average N and P loads were reduced by 47.3 g·m−3 N (70%) and 16.6 g·m−3 P (56%). ‘Rising Sun’ japanese iris is a promising plant for use in highly variable conditions when the pH of irrigation runoff is outside the typical range (5.5–7.5). Results from model simulations poorly predict the nutrient availability of P and ammonium in effluent, most likely due to the inability to determine plant and biological contributions to the system, such as N-fixing bacteria.