Organic Matter Reduction via Periodic Water-Level Drawdown in a Constructed Treatment Wetland Cell of the Orlando Easterly Wetlands

Constructed treatment wetland (CTW) efficiency can decline over time due to the accumulation of organic matter (OM). The Orlando Easterly Wetlands (OEW), a CTW in a high productivity ecoregion (central Florida, USA) requires treatment cell renovations every 15-30 years to optimize hydrologic retention times and nutrient removal efficiency. Renovating cells by manual OM removal is expensive and ecologically disruptive. This study evaluated applying periodic water level drawdown (WLD) to facilitate soil consolidation and oxidation, thereby reducing renovation frequency of OEW cells. The effect of WLD on two CTW cells, Cell 11 (subjected to WLD) and Cell 12 (control), at OEW (Christmas, FL) were compared during a field before-after-controlled-impact (BACI) study. The effectiveness of WLD in reducing soil and unconsolidated flocculent elevation was quantified at 15 points across both wetland Cells before, during, and following WLD. Relative soil elevation was persistently lower in drawdown Cell 11, as compared to Cell 12. After reflooding, the flocculent depth averaged 59% less in Cell 11 than Cell 12. Nutrient concentrations within the soil and flocculent spiked during WLD but returned to initial concentrations during reflooding. The reflooding of Cell 11 resulted in a spike of ortho-phosphate concentrations in the surface water (SW) that subsided within 40 days. Furthermore, an intact core study revealed how WLD interval and native soil mixing affects OM and nutrient dynamics. Replicate cores from an unrenovated Cell of the OEW were treated with the following WLD intervals for 57 days: long, short, multi, long with added native mineral soil, or control (continuously flooded). Core CO2 flux rate, SW nutrient concentrations, soil elevation, and flocculent depth were quantified. CO2 flux in the intact cores peaked in the long WLD cores (472% higher than the control cores) and was elevated in all cores during WLD. Multiple WLD intervals did not accelerate CO2 loss, and added native mineral soils reduced CO2 flux from the cores. Flocculent resuspension did not occur after reflooding of any treatment and mean flocculent depth of all treatments was at least 33% lower than the control cores. These findings suggest regular periodic WLD that reaches a subsurface water table reduces soil and flocculent elevation and persists following reflooding. Effluent from CTW cells receiving WLD should be restricted for approximately 40 days following reflooding to allow for reassimilation of mineralized nutrients released during oxidation. Future studies on the use of WLD as an OM management technique at OEW should include tracer studies to quantify the effect on hydrologic retention times