NUTRIENT REMOVAL POTENTIAL OF CONSTRUCTED WETLANDS IN GRAND LAKE ST. MARYS WATERSHED

Constructed wetlands are becoming an increasingly important management tool to reduce nutrient rich agricultural runoff in the Great Lakes region. The objective of this study was to assess the removal efficiency of two constructed wetlands operating on tributaries of Grand Lake St. Marys (Prairie Creek and Coldwater Creek) located in northwest Ohio. Water samples were collected weekly during summer and fall 2017 (n = 24) from inflow and outflow points. They were analyzed for nutrient (nitrate-N, total phosphorus, dissolved reactive phosphorus) concentrations following standard EPA colorimetric methods. Overall, while both wetlands experienced high mean nutrient inputs (concentrations in mg/L) across both fall and summer seasons ranging from 0.4 to 4.3 NO3", 1.1 to 1.7 TP, and 0.2 to 0.6 DRP, respectively, high removal efficiencies (often in excess of 75%) produced significantly reduced outflow concentrations (paired t tests; p < 0.05) largely consistent with EPA recommended TMDL target values for watersheds of these size (~20 mi2: 1.0 mg/L NO3, 0.10 mg/L TP). Extending these concentration reductions to effect size and loading impact, mean monthly stream discharge rates compared with wetland discharge data (continuously monitored using pressure transducers) revealed that PC and CC Treatment Train Wetlands were found to have processed an average of 10% to 29% of flows during summer and 35% to 40% of flows during the fall season, respectively. This study demonstrates the importance of constructed wetlands towards freshwater conservation strategies.