TMDLs for Sanibel Slough discussed
A public workshop was held at the Sanibel Public Library to discuss the proposed total maximum daily loads for the Sanibel Slough, the central river system that flows through the island by J.N. “Ding” Darling National Refuge.
The Sanibel Slough has three weir structures that control the outflow into San Carlos Bay. There is a weir control structure in Sanibel Slough West at Sanibel Captiva Road that discharges into Tarpon Bay, and then eventually into San Carlos Bay. The Sanibel Slough East has a weir control system at Beach Road that drains into canals and then eventually San Carlos Bay. According to DEP, there were less than 45 days of recorded discharges in seven years.
The TMDL stemmed from the Clean Water Act to set water quality standards.
According to the Florida Department of Environmental Protection a TMDL is “a scientific determination of the maximum amount of a given pollutant that surface water can absorb and still meet the water quality standards that protect human health and aquatic life.” When water bodies do not meet the quality standards, a specific TMDL is developed.
Ansel Bubel, with the Florida Department of Environmental Protection, said they are always checking to make sure they are on the right path to return bodies of water to a healthy state again. Computer models are used for TMDLs by calculating the pattern of where the water is going and being released.
Pamela Flores, also with the Florida Department of Environmental Protection, said the Sanibel Slough is a slow body of water, rather than a fast river located in the Charlotte Harbor Basin. She said the slough is on a five year acceptance and has been reclassified as a stream system, and now an estuary due to the brackish water, after originally being classified as a lake.
When classified as an estuarine system, the Sanibel Slough was divided into the Sanibel Slough West and Sanibel Slough East. The East Basin has 40 percent wetlands and has 34 percent low, medium and high density residential development. The West Basin is covered by 55 percent wetlands and 29 percent low, medium and high density residential development.
For a healthy estuary, chlorophyll has a target of 11 micrograms per liter or below. The Sanibel Slough between Jan. 1, 2007 to June 30, 2014 exceeded the threshold during every year, resulting in the body of water remaining on a verified list.
With nitrogen and phosphorus often the limiting nutrients, limiting plant growth, in Florida waterbodies, they were also a part of the study.
For the purpose of the TMDL of the Sanibel Slough, the BATHTUB model was used to simulate estuary nitrogen, phosphorus and chlorophyll. The nutrient loading placed into the model were from atmospheric deposition loads, groundwater loads and stormwater runoff loads.
According to the model, the Sanibel West Slough needs to have a reduction of 18 percent in the existing nitrogen loads and 27 percent reduction in the phosphorus loads to meet the target. The Sanibel East Slough needs a 31 percent reduction for nitrogen and 74 percent reduction for phosphorus.
These percentages are used for a three-year rolling average of nitrogen and phosphorus to achieve the 11 micrograms per liter or below of chlorphyll.