Hydrologic balance for a subtropical treatment wetland constructed for nutrient removal

Abstract

This paper reports on an analysis of a water budget for the Everglades Nutrient Removal (ENR) Project in South Florida, USA, for the first 2 years of operation. Estimates of nominal hydraulic retention time (HRT) based on average monthly values are compared with HRT obtained from steady-state two-dimensional hydrodynamic simulations, and show good agreement. Statistical analysis is performed to develop stage- and depth-duration curves for the ENR Project. The ENR Project was constructed south of Lake Okeechobee by the South Florida Water Management District to begin the process of removing nutrients (especially phosphorus) from agricultural drainage and stormwater run-off before entering the Everglades. The State of Florida's Everglades Forever Act of 1994 mandates, among other things, completion of stormwater treatment areas (STAs), and research to optimize phosphorus retention capacity, and to define threshold phosphorus concentrations that lead to an imbalance of biota. The ENR Project, a 1544-ha wetland, was designed and constructed as a pilot project to gain experience on design, construction, and operation of the STAs. It began operation in August 1994. For the 732 days analyzed (19 August 1994–19 August 1996), the average water inputs into the project were as follows: 86.2% from the inflow pumps, 11.2% from rainfall, and 2.6% as emerging measured and estimated seepage from an adjacent area with higher stages (Water Conservation Area 1). The average water outputs from the project consisted of 85.1% from the outflow pumps, 8.9% as evapotranspiration, and 6.0% as a net seepage and groundwater component. This net component accounts for elements of the surface/subsurface water interaction, either entering or leaving the project, which are unknown at this time. Considering monthly average values, there were only 3 months within the study period with positive values of this net component. These months were June 1995, and March and July 1996. The two most important elements included in the net seepage and groundwater component are expected to be the surficial aquifer recharge (outflow), and the unmeasured seepage (inflow) from Water Conservation Area 1 (subsurface seepage). The unmeasured subsurface seepage has recently been determined from computer model simulations.