Utilizing water level draw-down to remove excess organic matter in a constructed treatment wetland

Constructed treatment wetlands are commonly used to enhance surface water nutrient removal following traditional wastewater treatment. However, the constant inflow may necessitate continuous wetland inundation, leading to persistent anaerobic conditions and the accumulation of organic matter (OM) as suspended detrital flocculent (floc) and soil OM. This study investigated if temporary water level draw-down (WLDD) could promote OM consolidation and oxidation without impacting nutrient removal efficiency. A large-scale, 2-y, before-after-control-impact field experiment at the Orlando Easterly Wetland (Christmas, FL, USA) was complemented by an intact soil core laboratory experiment with varied WLDD regimes. Changes in floc thickness, soil elevation, and surface water and soil nutrients were quantified. Field experiment results demonstrated an average floc thickness reduction of 60 % and soil elevation decline of 2.7 cm persisted after return to normal flow operation. This reduction was achieved with one ∼3-week dry event for two consecutive years and removed an estimated 7.5 years' worth of accumulated floc. Intact soil core results showed a direct relationship (R² = 0.93) between days of WLDD and cumulative CO₂-C loss, despite oxidation only accounting for 4-5 % of OM loss (and consolidation accounting for the remaining 95-96 %). While soil nitrogen (N) and phosphorus (P) concentrations did tend to increase during WLDD, outflow surface water N was not affected by the WLDD. Soluble reactive P increased for ∼36 days following reflooding, then returned to baseline. Incorporating WLDD into wetland management every few years could significantly reduce the frequency of costly cell renovation projects aimed at removing accumulated OM.