Quantifying Nutrient and Sediment Load Reduction in a Constructed Wetland Complex Treating Riverine Storm Flows in Southwest Ohio

Abstract Submission: In response to recurring harmful algal blooms in Harsha Lake, Ohio, a riverine wetland complex was constructed in the floodplain of the East Fork Little Miami River, which drains to Harsha Lake and ultimately the Ohio River. The 5.5 acre constructed wetland consists of three contiguous treatment zones: a wintering pool; a detention basin developed with a retired drinking water reservoir; and a highly sinuous wetland. The wetland complex was instrumented at 5 locations to monitor water quality and hydrology to assess treatment effectiveness of the system’s distinct zones. Event-driven water quality data have been recorded for the first 1.5 years after wetland construction. Inflows have occurred during approximately 8 events/year, less frequent than the predicted 15-30 storm events/year due to inlet sedimentation and drought conditions. For the entire treatment system, nutrient concentration reductions are estimated at: 98% for TSS; 85% TP; 58% PO4; 16% NH3; 63% TN; 64% TKN; 64% NO2+NO3. The detention basin and sinuous wetland produced the most significant pollutant reductions. Calculating nutrient loads requires great care due to the complex wetland hydrology, e.g. backwater conditions at the wetland inlet and outlet when river stage is high. Preliminary nutrient loads were calculated using a stage-storage model developed for the wetland based on drone surveys. However, a more detailed hydraulic and hydrologic analysis in HEC-RAS will be undertaken to increase confidence in nutrient load estimations. This presentation will provide insight into challenges and successes of monitoring a nature-based constructed wetland treating large runoff volumes from an agricultural watershed.

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