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The Everglades Nutrient Removal (ENR) Project was constructed to reduce nutrient concentrations in stormwater runoff water from the Everglades Agricultural Area. Although nutrient concentrations of influent water ranged from 66 to 201 p.g TP L ~ and 140 to 541 ~g TN L-' and varied substantially over time, the outlet concentrations remained low, 9 to 39 p,g TP L ~ and 99 to 286 ~.g TN L ', during the first three years of operation (from August 1994 through August 1997), Nutrient removal efficiency was calculated in terms of decrease in both loads and nutrient concentrations. Nutrient loading rates averaged 1.17 g TP m -~ year ' (-+0.12 SE) and 31.56 g TN m-: year ' (-+2.92 SE) at the inflow and 0.23 g TP m -~ year (+0.02 SE) and 20.71 g TN m : year t (-+1.63 SEt at the outflow. TP load removal ranged from 66% to 91% and averaged 82% for the period of record, while TN load removal ranged from 11% to 76% and averaged 55%. Rainfall contribution to the ENR Project nutrient budgets was small, averaging only 4% and 3% for TP and TN, respectively. The ENR Project performance results during the first three years suggest that additional Stormwater Treatment Areas (STAs) will be effective in removing TP from stormwater runoff.

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Abstract:The Florida (USA) Everglades ecosystem has been impacted due to increased loading of nutrients, in particular phosphorus (P), primarily from adjacent agricultural areas. Consequently, restoration measures involve the establishment of stormwater treatment areas (STAs) comprising a series of constructed wetlands. A series of mesocosms were established at the inflow of the Everglades Nutrient Removal Project wetland, the first such STA constructed. These mesocosms were designed to mimic STAs, as they operated as flow-through systems and were packed with native soil. The objective of the study was to determine the effects of vegetation and hydrologic fluctuations on P retention/release by the wetland soil and on effluent water quality. Four treatment combinations consisted of continuously flooded with emergents (Typha), intermittently flooded with emergents, continuously flooded with no emergents, and intermittently flooded with no emergents. Intermittently flooded treatments underwent two 1 month drawdown events during the year. Soils were collected to determine the various pools of P and surface water samples were collected twice weekly to determine mass P flux in and out of the mesocosms. Results showed that the majority of the P was stored in the calcium-and magnesium-bound fraction, as well as the refractory pool in the soil. Approximately 91% of the inflow soluble reactive P (SRP) mass was retained within the mesocosms for the continuously flooded treatment, and 80% was retained in the treatments subjected to periodic drawdown events, regardless of vegetation type. There was a net annual flux of dissolved organic P (DOP) out of the mesocosms for the drawdown treatments, whereas the net reduction in the DOP concentrations for the continuously flooded treatments was just 17%. These results demonstrate that, although these wetland systems perform well in reducing surface water SRP, additional research may need to focus on improving the reduction of DOP in order to reduce further the P loads to the nutrient-sensitive Everglades system.

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