Middle St. Johns - Florida Department of Environmental Protection

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Water Quality Assessment Report: Middle St. Johns
Total Maximum Daily Load Development
During Phase 3 of the watershed management cycle, TMDLs will be
developed for both point and nonpoint sources of pollutants in impaired
waterbodies and will be adopted by rule at the end of this phase.
TMDL development involves determining the maximum amount of
a given pollutant that a waterbody can assimilate and still meet the applicable
numeric or narrative water quality criterion for the pollutant. In
most cases, this “assimilative” capacity will be determined using computer
modeling (both hydrodynamic and water quality models) that predicts
the fate and transport of pollutants in the receiving waters. Modeling for
the typical TMDL will include model setup, calibration, and verification,
followed by a variety of model runs that determine the assimilative capacity
of the water under worst-case conditions.
State law and federal regulations require that TMDLs include a margin
of safety (MOS) that takes into account “any lack of knowledge concerning
the relationship between effluent limitations and water quality.” The EPA
has allowed states to establish either a specific MOS (typically some percentage
of the assimilative capacity) or an implicit MOS based on conservative
assumptions in the modeling. To date, the Department has elected to
establish an implicit MOS based on predictive model runs that incorporate
a variety of conservative assumptions (they examine worst-case ambient
flow conditions and worst-case temperature, and assume that all permitted
point sources discharge at their maximum permitted amount).
It is important to note that TMDLs will be developed only for the
actual pollutants causing the impairment in the listed waterbody. These are
called the “pollutants of concern.” In Florida, the most commonly listed
pollutants of concern are nutrients, sediments, and coliforms. TMDLs
will not be developed for impairments not due to pollutant discharges—for
example, natural conditions, physical alterations such as dams and channelization,
or changes in the flow of the water. In other cases, a waterbody
may be deemed potentially impaired based on bioassessment data or toxicity
data. In these cases, the Department must determine the actual pollutant
causing the impairment before a TMDL can be developed.
Total Maximum Daily Load Allocation and
Implementation
Initial Allocation of Pollutant Loadings
The Florida Watershed Restoration Act (FWRA) requires that a
TMDL include the “establishment of reasonable and equitable allocations
. . . among point and nonpoint sources . . . .” The Department refers to
this as the “initial allocation,” which is adopted by rule. For the purposes
of allocating the required pollutant loadings, the term “point sources”
primarily includes traditional sources such as domestic and industrial
wastewater discharges.