Abstract
Understanding the hydrology of a created or restored wetland is a critical component in understanding the overall functioning of its design, and in determining if the original goals of the wetland are being met. Freedom Park, a 20-ha park constructed in Naples, Florida in 2009, includes a 4.6-ha subtropical stormwater wetland treatment system for improving the water quality of the downstream impaired Gordon River and estuaries that flow into the Gulf of Mexico. Subtropical wetlands often present significant issues of sustaining viable wetland vegetation communities because of long wet and then long dry patterns in the wetlands’ hydroperiods. The watershed includes 1,766 ha of an urbanized Naples metro area. The goals of the stormwater wetlands are to improve the water quality, reduce peak flows discharging into Gordon River and further to Naples Bay, and provide an aesthetic water park for the public. A monthly hydrologic budget was developed for the period August 1, 2016 to January 31, 2018, which included both wet and dry seasons. For the 18-month study period, average surface water pumped into the system was 50.1 cm week-1; average surface outflow was 38.8 cm week-1. During this time, average precipitation inflow was 3.76 cm week-1. By comparison, average potential evapotranspiration (PET) was estimated to be 5.34 cm week-1. Residual water fluxes identified as not included in the budget (i.e., not accurately quantified) included actual evapotranspiration, seepage, and additional runoff. Residual fluxes averaged 18.9 cm week-1 leaving the system, much higher than the PET, suggesting groundwater seepage plays a large role in the wetlands’ water budget. Average water level at the wetland outflow basin decreased gradually throughout the dry season from November 1, 2016 to May 31, 2017, then rapidly spiked 1.4 m during the week of June 6, 2017 when the wet season began. The wetland received an additional pulse of 34 cm of precipitation during Hurricane Irma on September 10, 2017. A stormwater pulsing study during Hurricane Irma showed that after an initial inflow flush due primarily to rainfall, total phosphorus and total nitrogen concentrations were higher at the outflow than at the inflow for several days before returning to normal conditions. Similarly, the mass of nutrients leaving the system was 130% and 37% higher for TN and TP, respectively, than the mass entering the system during this 6-day period during and following the hurricane. The design of the inflow with only pumps, while not allowing natural ditch overflow as well, meant that the hurricane-caused inflow was much less than it could have been during such a dramatic hydrologic pulse. Samples taken at the inflow and outflow of the system for four other storm events throughout the 2017 wet season (July through October) were also analyzed for their effects on water quality.
