January 4, 2006 

Final 

Nutrient Total Maximum Daily Load 

For Trout Lake, 

Lake County, Florida 

Xueqing Gao and Douglas Gilbert 

Watershed Assessment Section 

Florida Department of Environmental Protection 

2600 Blair Stone Road, MS 3555 

Tallahassee, FL 32399-2400 

January 4, 2006 
January 4, 2006 

 

Table of Contents 

Table of Contents....................................................................................... i 

List of Figures............................................................................................. ii 

List of Tables.............................................................................................. ii 

Acknowledgements.................................................................................... iii 

1. Introduction....................................................................................... 1 

1.1 Purpose of report............................................................................ 1 

1.2 Identification of water body............................................................. 1 

2. Statement of Problem........................................................................... 3 

3. Description of Standards....................................................................... 4 

4. Assessment of Sources......................................................................... 7 

4.1 Types of Sources............................................................................ 7 

4.2 Potential sources of TN and TP in Trout Lake watershed.............. 7 

Point sources............................................................................ 7 

Nonpoint sources......................................................................... 7 

Estimating Watershed TN and TP................................................... 8 

5. Loading Estimates.................................................................................. 9 

Overall approach................................................................................... 9 

Bathtub Model....................................................................................... 10 

Bathtub Data Requirements ................................................................. 11 

Calculation of Trophic State Index........................................................ 11 

TMDL Scenario Development.............................................................. 12 

6. Results................................................................................................... 12 

 6.1 Historical trend of trophic status of Trout Lake............................... 12 

 Critical Conditions............................................................................ 15 

6.2 Estimating TN and TP sub-basin loadings Using WMM................. 16 

TN and TP loadings from Trout Lake watershed............................. 28 

Atmospheric Loading to the Lake.................................................... 31 

Summary of the TN and TP loadings from different sources.......... 32 

6.3 Establishing Relationship between TN and TP loading to 

the Lake and In-Lake Concentrations.............................................. 33 

Calibrating Bathtub Model............................................................... 35 

Natural Background......................................................................... 36 

7. Trout Lake TMDL.................................................................................... 37 

7.1 Load Allocation................................................................................ 39 

7.2 Wasteload Allocation....................................................................... 40 

7.3 Margin of Safety.............................................................................. 40 

8. Next Steps:............................................................................................. 40 

References................................................................................................. 42 

Appendix A................................................................................................. 43 

i
January 4, 2006 

 

List of Figures 

Figure 1-A Comparison of Hicks Ditch and Trout Lake Watersheds......... 3 

Figure 1-B Trout Lake Watershed Land Uses............................................ 6 

Figure 2 Bathtub Conceptual Scheme...................................................... 10 

Figure 3 Location of Data Stations............................................................ 13 

Figure 4-A Lake TN Concentration vs. Precipitation................................. 17 

Figure 4-B Lake TP Concentration vs. Precipitation................................. 17 

Figure 4-C Lake Chl a Concentration vs. Precipitation............................. 18 

Figure 4-D Lake TSI vs. Precipitation....................................................... 18 

Figure 5-A Quarterly TN vs. Precipitation................................................. 19 

Figure 5-B Quarterly TP vs. Precipitation.................................................. 19 

Figure 5-C Quarterly Chl a vs. Precipitation.............................................. 20 

Figure 5-D Quarterly TSI vs. Precipitation................................................ 20 

Figure 6 Trout Lake Sub-basins...................................................... 22 

Figure 7 TSI-TN Loading Curve...................................................... 38 

Figure 8 TSI-TP Loading Curve...................................................... 38 

List of Tables 

Table 1 Lake Basin Landuse and acreage................................................. 8 

Table 2 Annual Average Lake TN, TP, Chl a, and TSI............................... 14 

Table 3 Seasonal Variation of Lake TN, TP, Chl a, and TSI...................... 15 

Table 4 Annual Precipitation...................................................................... 21 

Table 5 Quarterly Precipitation................................................................... 21 

Table 6 Watershed percent distribution of Landuse.................................. 23 

Table 7 Percent Directly Connected Impervious Area by Landuse .......... 24 

Table 8 TN and TP Landuse based Event Mean Concentration............... 24 

Table 9 Percent suspended TN and TP by Landuse ............................... 25 

Table 10-1 Annual Runoff from Trout Lake sub-basin............................... 26 

Table 10-2 Annual Runoff from Pine Meadow sub-basin........................... 27 

Table 10-3 Annual Runoff from Springhill sub-basin.................................. 27 

Table 11-1 Annual TN Loading from Trout Lake sub-basin........................ 28 

Table 11-2 Annual TN Loading from Pine Meadow sub-basin................... 29 

Table 11-3 Annual TN Loading from Springhill sub-basin.......................... 29 

Table 12-1 Annual TP Loading from Trout Lake sub-basin........................ 30 

Table 12-2 Annual TP Loading from Pine Meadow sub-basin................... 30 

Table 12-3 Annual TP Loading from Springhill sub-basin.......................... 31 

Table 13 Atmospheric Loading to the Lake............................................... 32 

Table 14 Annual Volume and TN TP Loading to the Lake........................ 32 

Table 15 Precipitation and Evaporation.................................................... 33 

Table 16 Measured TN, TP, Chl a Concentrations in Lake........................ 34 

Table 17 Predicted Flow, TN, and TP Concentrations by Source............ 34 

Table 18 Bathtub Calibration Results........................................................ 36 

Table 19 Flow, TN, TP from Forest/Rural and Water/Wetland.................. 36 

Table 20 TN, TP, Chl a, and TSI Concentrations Natural Background...... 37 

Table 21 TMDL Components.................................................................... 39 

ii
January 4, 2006 

 

Acknowledgments 

This study could not have been accomplished without the significant 

contributions from staff within the Watershed Assessment Section. Particular 

appreciation goes to Barbara Donner for her contribution, as all of the basin 

delineation’s, landuse aggregations, and much of the data gathering are a result 

of her efforts. Additional appreciation is extended to Dr. TS Wu for his 

assistance and insights on modeling and to Dr. Paul Lee for his assistance in 

understanding the ground water system. The Department also recognizes the 

substantial support and assistance provided by the St. Johns River Water 

Management District staff, particularly their contributions towards understanding 

the issues, history, and processes at work in the Trout Lake watershed. 

iii
January 4, 2006 

 

1. INTRODUCTION 

1.1 Purpose of Report 

This report represents the efforts to develop a nutrient TMDL for Trout Lake (Lake). The Lake, 

located in central Lake County near Eustis (Figure 1), was verified as impaired by nutrients 

based on elevated levels of the Trophic State Index for lakes, and was included on the verified 

list of impaired waters for the Ocklawaha Basin that was adopted by Secretarial Order on 

August 28, 2002. 

According to Section 303(d) of the federal Clean Water Act (CWA) and the Florida Watershed 

Restoration Act, Chapter 403, Florida Statutes, the Florida Department of Environmental 

Protection (DEP) is required to submit on a recurring basis lists of surface waters that do not 

meet applicable water quality standards (impaired waters). The methodologies used by the 

state for the determination of impairment are established in Chapter 62-303, Identification of 

Impaired waters (IWR), Florida Administrative Code (FAC). Once a waterbody or waterbody 

segment has been verified as impaired and referenced in the Secretarial Order Adopting the 

Verified List of Impaired Waters, work on establishment of the Total Maximum Daily Load 

(TMDL) begins. The TMDL process establishes the allowable loadings of pollutants or other 

quantifiable parameters for a waterbody based on the relationship between pollution sources 

and in-stream water quality conditions, so that states can establish water quality based controls 

to reduce pollution from both point and nonpoint sources and restore and maintain the quality of 

their water resources (USEPA, 1991) 

1.2 Identification of Water Body 

Trout Lake is located in the Hicks Ditch watershed (central Lake County, Florida; Latitude 

280

52’0”, Longitude 81040’56”, Figures 1-A and 1-B). The original 2003 TMDL study by the 

Department utilized the entire Hicks Ditch Watershed. Subsequent to the adoption of the 

nutrient TMDL for Trout Lake, the SJRWMD provided significant comments regarding the area 

of the Hicks Ditch watershed that contributed runoff to Trout Lake. The Department, 

incorporating the comments of the SJRWMD, revised the Trout Lake watershed to reflect the 

changes recommended by the WMD. Figure 1-A shows the original watershed (Hicks Ditch) 

and the current revised watershed (Trout Lake) side by side. The Lake has a surface area of 

about 102 acres (Florida Lake Watch) and a mean depth of about 9 feet (SJRWMD, 1996). A 

stream (Hicks Ditch) northeast of the lake collects surface runoff from the majority of the Hicks 

Ditch watershed and discharges into the lake. Trout Lake has a single major surface-water 

outlet, which discharges into Lake Eustis. 

For assessment purposes, the State of Florida has been divided into water body assessment 

polygons that are identified using Waterbody Ids and are referred to as WBIDs. Trout Lake is 

part of WBID 2819A. Additional information about derivation and use of these WBIDs is 

provided in the “Documentation For The 2002 Update To The State Of Florida’s 303(d) List” 

dated October 1, 2002, and GIS shapefiles of the WBIDs can be obtained from the following 

website: 

http://www.floridadep.org/water/watersheds/basin411/downloads.htm

The climate of the watershed is generally humid and subtropical. Average annual rainfall is 

about 51 inches, based on records from 1969 to 1999. The rainfall may enter aquifers through 

1
January 4, 2006 

 

infiltration, fall onto the surface of lakes and streams, enter surface waterbodies as runoff from 

adjacent land, or return to the atmosphere through evaporation and transpiration. The potential 

annual evapotranspiration of the area ranges from 41 to 45 inches, with the remaining 6 to 10 

inches of rainfall either recharging ground water or entering surface waters through runoff. 

Based on the information from Florida’s ground water quality monitoring program, the Hicks 

Ditch watershed is isolated from the Floridan Aquifer by the Hawthorn formation. Therefore, 

influence from Floridan Aquifer was not considered in this study. The soil of the Hicks Ditch 

watershed is dominated primarily by seven soil series including Astatula sands, Eureka sands, 

Wauchula sands, Brighton soils, Fellowship soils, Myakka sands, and Albany sands. All these 

soil series, except for Astatula sands, are poorly drained. The majority of the watershed area is 

drained relatively poorly; therefore it is reasonable to consider the surface runoff as the major 

source of water for Trout Lake. 

The watershed contains several land use types, with the predominate land uses consisting of 

agriculture, wetlands, pine flatwoods, lakes, and residential/urban development (SJRWMD, 

1996). Agricultural land use consists of pasture, citrus, a small muck farm, and a few ferneries. 

The predominate wetland types are marshes, cypress, bayheads, forested depressions, and 

shrub swamps. There are two types of lakes in the watershed: those that are part of the primary 

drainage systems and those that are internally drained (landlocked) and only discharge during 

major storm events. The communities of Eustis and Umatilla are the two significant urban areas 

in the study area. Residential development is located in the vicinity of these two cities and 

around several of the lakes in the study area. 

2
January 4, 2006 

 

Lake Eustis

Trout Lake

Hicks Ditch

N

1 0 1 2 Miles

Trout

Lake 

Trout Lake Watershed 

Hicks Ditch Watershed

Figure 1-A 

Original Hicks Ditch Watershed and 

Revised Trout Lake Watershed 

(Notice location of Trout Lake in each watershed) 

2. STATEMENT OF PROBLEM 

Prior to regulatory surface water controls, development in the Trout Lake watershed created 

flooding and water quality problems. These changes in land use increased the amount of runoff 

and lowered water quality. The increase of nutrient loading into Trout Lake was one of the 

consequences of these changes in landuse. In fact, Hicks Ditch is part of the upper Ocklawaha 

River basin (UORB), which has been identified by the St. Johns River Water Management 

District (SJRWMD) as a high priority basin for restoration. As an initial effort to reduce nutrient 

runoff into the Lake, a 670-acre muck farm in the watershed was purchased by the SJRWMD in 

1992 through an active muck farm acquisition program (Environmental Science & Engineering, 

Inc. 1996). At this point in time, data indicate that further reduction of nutrient loading from the 

watershed into the lake is necessary because the lake is still impaired by the nutrients. 

Based on data contained in the DEP database (data), the long-term (1993 – 2000) average 

concentrations of total phosphorus (TP), total nitrogen (TN), and chlorophyll a (Chl a) were 

0.187 mg/L, 1.67 mg/L, and 68.5 μg/L, respectively. The long term TSI calculated from these 

3
January 4, 2006 

 

data according to the procedures adopted in the IWR is 74 (Table 2). For the verified period 

(June, 1995 – December, 2000), the TP, TN, and Chl a concentrations averaged 0.192 mg/L, 

1.62 mg/L, and 59.4 ug/L, respectively, with an average TSI of 73 for the verified period. The 

mean color of the lake was calculated as 252 platinum-cobalt units. Exceeding the threshold 

TSI of 60 in any year of the verified period would be sufficient to verify the lake as impaired. Not 

only is the average TSI of the verified period (73) greater than 60, but each annual mean TSI 

from the verified period is greater than 60 (Table 2). Based on this information, Trout Lake was 

verified as impaired by nutrients. 

3. Description of Applicable Water Quality Standards and Criteria 

Trout Lake is classified as a Class III freshwater body, with a designated use of recreation, 

propagation and maintenance of a healthy, well-balanced population of fish and wildlife. The 

Class III water quality criteria applicable to the observed impairment is the narrative nutrient 

criterion (nutrient concentrations of a body of water shall not be altered so as to cause an 

imbalance in natural populations of aquatic flora or fauna). Because the nutrient criterion is 

narrative only, a nutrient related target was needed to represent levels at which imbalance in 

flora or fauna are expected to occur. For lakes, the IWR threshold for impairment is based on a 

trophic state index (TSI) and the average color (platinum cobalt units) of the lake water. Since 

the Lake has a mean color greater than 40 platinum cobalt units, the IWR threshold for 

impairment is an annual mean TSI of 60, unless paleolimnological or other information such as 

modeling indicates the natural TSI of the lake was different than 60.

The TSI originally developed by R. E. Carlson (1977) was calculated based on Secchi depth, 

chlorophyll a concentration, and total phosphorus concentration and was used to describe a 

lake’s trophic state. Carlson’s TSI was developed based on the assumption that the lakes were 

all phosphorus limited. In much of Florida, because the local geology produced a phosphorus 

rich soil, nitrogen can be the sole or co-limiting factor for phytoplankton population in some 

lakes. In addition, because of the existence of dark-water lakes in the state, using Secchi depth 

as an index to represent lake trophic state can produce misleading results. Therefore, the TSI 

was revised to be based on chlorophyll a, total nitrogen, and total phosphorus concentrations. 

The Florida-specific TSI was determined based on the analysis of data from 313 Florida lakes. 

The index was adjusted so that a chlorophyll a concentration of 20 ug/L was equal to a TSI 

value of 60. A TSI of 60 was then set as the threshold for nutrient impairment for most lakes 

(for those with a color higher than 40 platinum cobalt units) because, generally, the 

phytoplankton may switch to communities dominated by blue-green algae at chlorophyll a levels 

above 20 ug/L. These blue-green algae are often an unfavorable food source to zooplankton 

and many other aquatic animals. Some blue-green algae may even produce toxins, which 

could be harmful to fish and other animals. In addition, excessive growth of phytoplankton and 

the subsequent death of these algae may consume large quantity of dissolved oxygen and 

result in anaerobic condition in lakes, which makes conditions in the impacted lake unfavorable 

for fish and other wildlife. All of these processes may negatively impact the health and balance 

of native faunal and floral communities. 

Because of the amazing diversity and productivity of Florida lakes, some lakes have a natural 

background TSI that is different from 60. In recognition of this natural variation, the IWR allows 

for the use of a lower TSI (40) in very clear lakes, a higher TSI if paleolimnological data indicate 

the lake was naturally above 60, and the development of site-specific thresholds that better 

represent the levels at which nutrient impairment occurs. As Trout Lake has a mean color 

4
January 4, 2006 

 

greater than 40 platinum cobalt units, the default IWR threshold for impairment is 60. For this 

study, the Florida Department of Environmental Protection (DEP) used modeling to estimate the 

natural background TSI by setting land uses to represent natural conditions, and then compared 

the resulting natural background TSI to the IWR thresholds. The IWR uses as one measure of 

impairment in lakes, a 10 unit change in TSI from “historical” levels. This 10 unit increase is 

assumed to represent the transition of a lake from one trophic state (say mesotrophic) to 

another nutrient enriched condition (eutrophic). The Department has assumed that allowing a 5 

unit increase in TSI over the natural background condition would prevent a lake from becoming 

impaired (changing trophic states) and reserve 5 TSI units to allow for future changes in the 

basin and as part of the implicit margin of safety in establishing the assimilative capacity. 

5
January 4, 2006 

 

Trout Lake

Figure 1-B. The general location and landuse types of Trout Lake watershed. 

6
January 4, 2006 

 

4. ASSESSMENT OF SOURCES 

4.1 Types of Sources 

An important part of the TMDL analysis is the identification of pollutant source categories, 

source subcategories, or individual sources of nutrients in the Trout Lake watershed and the 

amount of pollutant loading contributed by each of these sources. Sources are broadly 

classified as either “point sources” or “nonpoint sources.” Historically, the term point sources 

has meant discharges to surface waters that typically have a continuous flow via a discernable, 

confined, and discrete conveyance, such as a pipe. Domestic and industrial wastewater 

treatment facilities (WWTFs) are examples of traditional point sources. In contrast, the term 

“nonpoint sources” was used to describe intermittent, rainfall driven, diffuse sources of pollution 

associated with everyday human activities, including runoff from urban land uses, runoff from 

agriculture, runoff from silviculture, runoff from mining, discharges from failing septic systems, 

and atmospheric deposition. 

However, the 1987 amendments to the Clean Water Act redefined certain nonpoint sources of 

pollution as point sources subject to regulation under EPA’s National Pollutant Discharge 

Elimination Program (NPDES). These nonpoint sources included certain urban stormwater 

discharges, including those from local government master drainage systems, construction sites 

over five acres, and from a wide variety of industries (see Appendix A for background 

information about the State and Federal Stormwater Programs). 

To be consistent with Clean Water Act definitions, the term “point source” will be used to 

describe traditional point sources (such as domestic and industrial wastewater discharges) AND 

stormwater systems requiring an NPDES stormwater permit when allocating pollutant load 

reductions required by a TMDL (see Section 6). However, the methodologies used to estimate 

nonpoint source loads do not distinguish between NPDES stormwater discharges and nonNPDES stormwater discharges, and as such, this source assessment section does not make 

any distinction between the two types of stormwater. 

4.2 Potential Sources of TN and TP in Trout Lake Watershed 

Point Sources 

There are no wastewater facilities authorized to discharge to the lake. Based on the information 

provided by Department NPDES Stormwater Program staff, the City of Eustis is on the Phase II 

MS4 list. However, none of the watershed currently lies in an area covered under an MS4 area. 

As such, there are currently no point sources authorized to discharge to the lake under the 

NPDES Program. 

Nonpoint Sources 

The TN and TP loadings to Trout Lake are generated from nonpoint sources. Nonpoint sources 

addressed in this study primarily include TN and TP loadings from surface runoff, precipitation 

directly on the surface of the lake, and the contribution from leaking septic tanks. TN and TP 

loadings through surface runoff were estimated using the Watershed Management Model 

(WMM) based on the imperviousness and event mean concentration (EMC) of TN and TP from 

different landuse types of the watershed. The spatial distribution and acreage of different 

landuse categories were identified using the St. Johns River Water Management District 

(SJRWMD) 1995 landuse coverage (scale 1:40,000) contained in the DEP GIS library. Methods 

7
January 4, 2006 

 

used to estimate the TN and TP loadings precipitation directly on the surface of the lake, and 

the contribution from leaking septic tanks are described in detail in Section 5.2. 

Landuse of Trout Lake Watershed

Based on information provided by the SJRWMD, the area of Hicks Ditch watershed that drains 

directly to Trout Lake was subdivided into a Trout Lake watershed. The Trout Lake watershed 

drains an area of about 9,686 acres into Trout Lake. Landuse categories of the watershed were 

aggregated using the simplified level 1 codes tabulated in Table 1. The spatial distribution of 

different landuse types of Trout Lake watershed is given in Figure 1-B. 

Table 1. Classification of landuse categories of Trout Lake watershed 

Code Landuse Acreage 

1000 Urban Open 460 

Low density residential 458 

Medium density residential 1137 

High density residential 81 

2000 Agriculture 1846 

3000 Rangeland 704 

8000 Transportation, communication, and utilities 66 

4000 Forest/rural open 1267 

5000/6000 Water/Wetland 3667 

Estimating Nonpoint TN and TP loadings using WMM 

WMM development was originally funded by DEP under contract to Camp Dresser and McKee 

(CDM). Subsequently, CDM has refined the model. WMM is a watershed model designed to 

estimate annual or seasonal pollutant loadings from a given watershed and evaluate the effect 

of watershed management strategies on water quality (WMM User’s Manual: 1998). While the 

strength of the model is its capability to characterize pollutant loadings from nonpoint sources, 

such as those through stormwater runoff, stream baseflow, and leakage of septic tanks, the 

model handles point sources such as discharge from wastewater treatment facilities. Estimation 

of pollution load reduction due to partial or full-scale implementation of onsite or regional best 

management practices (BMP) is also part of this model. The fundamental assumption of the 

model is that the amount of stormwater runoff from any given landuse is in direct proportion to 

annual rainfall. The quantity of runoff is controlled by that fraction of the landuse category that 

is characterized as impervious and the runoff coefficients of both pervious and impervious area. 

8
January 4, 2006 

 

The governing equation is: 

(1) RL = [Cp + (CI – Cp) IMPL] * I 

Where: 

RL = total average annual surface runoff from land use L (in/yr); 

IMPL = fractional imperviousness of land use L; 

I = long-term average annual precipitation (in/yr); 

CP = pervious area runoff coefficient; and 

CI = impervious area runoff coefficient. 

The model estimate of pollutant loadings is based on nonpoint pollution loading factors 

(expressed as lbs/ac/yr) that vary by land use and the percent imperviousness associated with 

each land use. The pollution loading factor ML is computed for each land use L by the following 

equation: 

(2) ML = EMCL * RL * K 

Where: 

ML = loading factor for land use L (lbs/ac/yr); 

EMCL = event mean concentration of runoff from land use L (mg/L); EMC varies 

by land use and pollutant; 

RL = total average annual surface runoff from land use L computed from 

Equation (1) (in/yr); and 

K = 0.2266, a unit conversion constant. 

Data required for WMM application include: 

• Area of all the landuse categories and the area served by septic tanks 

• Percent impervious area of each landuse category 

• EMC for each pollutant type and landuse category 

• Percent EMC of each pollutant type that is in suspended form 

• Annual precipitation 

• Annual baseflow and baseflow concentrations of pollutants 

• Point source flows and pollutant concentrations.

Because no flow data were available for Hicks Ditch or the outlet from the Lake for the verified 

period, WMM model calibration was not conducted in this study. Additionally, due to the lack of 

data, TN and TP contribution from ground water was not considered. 

5.0 Loading Estimates 

Overall Approach

The goal of the TMDL development for Trout Lake is to identify the maximum allowable TP and 

TN loadings to the lake so that the lake will meet the water quality standard and maintain its 

function and designated use as a Class III water. It was initially anticipated that the following 

three steps would be taken to achieve this goal. The target in the third step was modified, as 

described in Section 6. 

9
January 4, 2006 

 

1. TN and TP loadings from various sources in the Trout Lake watershed were estimated 

using the Watershed Management Model (WMM). 

2. Loading estimates from the WMM were entered into the Bathtub eutrophication model to 

establish the relationship between TN and TP loadings and in-lake TN, TP, and Chl a

concentrations. The model results for in-lake TN, TP, and Chl a were used to calculate 

TSI-predicted (TSI-P) for several different loading scenarios discussed later. 

3. The loadings to the lake were adjusted until the TSI-P calculated from the model results 

was less or equal to the target TSI for the Lake, and the TN and TP loadings that 

resulted in the target TSI become the nutrient TMDL for Trout Lake. 

Lake Modeling Using the Bathtub Model

The Bathtub eutrophication model is a suite of empirically derived steady state models 

developed by the U. S. Army Corps of Engineering (ACOE) Waterways Experimental Station. 

The primary function of these models is to estimate nutrient concentrations and algal biomass 

resulting from different patterns of nutrient loadings. The procedures for selection of the 

appropriate model for a particular lake are described in the Users Manual. The empirical 

prediction of lake eutrophication using this approach typically can be described as a two stage 

procedure using the following two categories of models (Walker 1999): 

• Nutrient balance model. This type of model relates in-lake nutrient concentration to 

external nutrient loadings, morphometry, and hydrology. 

• Eutrophication response model. This type of model describes relationships among 

eutrophication indicators within the lake, including nutrient levels, Chl a, transparency, 

and hypolimnetic oxygen depletion.

Figure 2 describes the concept scheme used by Bathtub to relate external loading of nutrients to 

the in-lake nutrient concentrations and the physical, chemical, and biological response of the 

lake to the level of nutrients. 

Figure 2. Bathtub concept scheme

 Loading of nutrients 

(Flow and Concentration) 

 

Physical characters of the lake In-lake nutrient Chl a, Secchi 

(Surface area and mean depth) Concentrations (TN&TP) DO 

Hydraulic characteristics of the lake 

 (Water residence time) 

The nutrient balance model adopted by Bathtub assumes that the net accumulation of nutrients 

in a lake is the difference between nutrient loadings into the lake from various sources and the 

nutrients carried out through outflow and losses of nutrient through whatever decay process 

occur inside lake: 

(3) Net accumulation = Inflow – Outflow – decay 

10
January 4, 2006 

 

Equation (3) is solved by assuming that the pollutant dynamics in the lake are at a steady state, 

i.e., the net accumulation of the pollutant in the lake equals zero. 

In this study, “inflow” included TN and TP loadings though stormwater surface runoff from 

various landuse categories, leakage of septic tanks, and atmospheric precipitation. Nutrient 

outflow was considered primarily through the outflow to Lake Eustis. However, because there 

was no flow gauging station located in the outflow stream of Trout Lake, the outflow was not 

specifically defined in the Bathtub modeling process. In this case, the outflow was automatically 

calculated by Bathtub assuming the inflow and outflow were at a steady state. 

To address nutrient decay within the lake, Bathtub provided several alternatives depending on 

the inorganic/organic nutrient partitioning coefficient and reaction kinetics. The major pathway 

of decay for TN and TP in the model is through sedimentation to the bottom of the lake. 

Prediction of the eutrophication response by Bathtub involves choosing one of several 

alternative models, depending on whether the algal communities are limited by phosphorus or 

nitrogen, or co-limited by both nutrients. Scenarios that include algal communities limited by 

light intensity or controlled by the lake flushing rate are also included in the suite of models. In 

addition, the response of chlorophyll a concentration to the in-lake nutrient level is characterized 

by two different kinetic processes: linear or exponential. The variety of models available in 

Bathtub allows the user to choose specific models based on the particular condition of the 

project lake. 

One feature offered by Bathtub is the “calibration factor.” The empirical models implemented in 

Bathtub are mathematical generalizations about lake behavior. When applied to data from a 

particular reservoir, measured data may differ from predictions by a factor of two or more. Such 

differences reflect data limitations (measurement or estimation errors in the average inflow and 

outflow concentrations), unique features of the particular lake (Walker 1999), and unexpected 

processes inherent to the lake. The calibration factor offered by Bathtub provides model users 

with an ability to calibrate the magnitude of lake response predicted by the empirical models. 

The model calibrated to current conditions against measured data from the lake can then be 

applied to predict changes in lake conditions likely to result from specific management scenarios 

under the condition that the calibration factor remains constant for all prediction scenarios. 

Data Requirements for Running Bathtub

Data requirements for the Bathtub model include: 

• Physical characteristics of the lake (surface area, mean depth, length, and mixed layer 

depth) 

• Meteorological data (precipitation and evaporation retrieved from Climate Interactive 

Rapid Retrieval Users System of National Climate Data Center) 

• Measured water quality data (TN, TP, and Chl a concentrations of the lake water, TN 

and TP concentrations in precipitation, etc.) 

• Loading data (flow and TN and TP concentrations of the flow from various sources) 

• Coefficient of variance (CV) of all the measured data 

Calculation of Trophic State Index (TSI)

TSI values were calculated using the procedures outlined in Florida’s 1996 305(b) report: 

TSI = (CHLATSI + NUTRTSI)/2 

11
January 4, 2006 

 

Where: 

CHLATSI = 16.8 + 14.4 × LN (CHLA)] 

TNTSI = 56 + [19.8 × LN(TN)] 

TN2TSI = 10 × [5.96 + 2.15 × LN(TN + 0.0001)] 

TPTSI = [18.6 × LN(TP × 1000)] –18.4 

TP2TS = 10 × [2.36 × LN(TP × 1000) – 2.38] 

The procedure addresses limiting nutrient considerations by calculating NUTRTSI: 

If TN/TP > 30 then NUTRTSI = TP2TSI

If TN/TP < 10 then NUTRTSI = TN2TSI 

If 10 < TN/TP < 30 then NUTRTSI = (TPTSI + TNTSI)/2 

TMDL Scenario Development for Trout Lake

The TMDL of the lake was developed through evaluating TSIs for the following scenarios: 

A. The TSI for current conditions. 

B. Natural background. TSI after the loadings from all human activities (urban open, low, 

medium, and high density residential, agriculture and rangeland, and transportation, 

communication, utilities, and septic tanks) were removed. 

Scenario B was considered the natural background condition of the lake. If the TSI of Scenario 

B is greater than 60, then natural background will become the new target TSI threshold for the 

lake. 

6. Results 

6.1 Historical trend of trophic status of Trout Lake 

Monthly TN, TP, and Chl a concentrations for Trout Lake from 1993 through 2000 were 

retrieved from the DEP database. The general locations of the individual stations from which 

water quality data were collected are shown in Figure 3. Analysis of the data indicated that the 

spatial variation between stations across Trout Lake is not significant. Therefore, data from all 

the stations within Trout Lake were pooled together and treated as data collected from one 

station. Quarterly mean values for TN, TP, and Chl a concentrations were calculated based on 

the monthly data, and quarterly TSIs were calculated based on the quarterly mean values of TN, 

TP, and Chl a concentrations. Quarterly TN, TP, Chl a, and TSI values were then used to 

calculate annual mean values. The long term annual average values of these data were 

calculated based on annual mean values of each year from 1993 through 2000. The long-term 

annual average values for the entire verified period were calculated based on the individual 

mean values of each year from 1995 through 2000. The seasonal trend of TN, TP, Chl a, and 

TSI were examined by calculating the long-term quarterly mean values based on the quarterly 

mean values of each year (1993 – 2000). The quarterly means for the verified period were 

calculated using the data from 1995 through 2000. The individual annual mean TN, TP, Chl a, 

and TSI values are listed in Table 2, and the long-term quarterly TN, TP, Chl a, and TSI results 

are listed in Table 3. 

12
January 4, 2006 

 

 

N

#

#

Trout Lake

Lake Eustis

Hicks Ditch watershed

0 1 Miles

Figure 3. General locations of water quality stations in Trout Lake

13
January 4, 2006 

 

As shown in Table 2, the long-term annual average of TN, TP, and Chl a concentrations are 

1.67 mg/L, 0.187 mg/L, and 68.5 μg/L, respectively. The long-term TSI is 72. Long-term 

average TN/TP ratio is about 9.4, which is not significantly different from 10, indicating that the 

algal communities in this lake may be either borderline co-limited by both nitrogen and 

phosphorus, or nitrogen limited. For the verified period, the TN, TP, and Chl a concentrations 

are 1.62 mg/L, 0.192 mg/L, and 59.4 μg/L, respectively. The TSI of the verified period is 71. 

Based on these data, the lake is eutrophic and exceeded the IWR TSI threshold of 60 for lakes 

in each year of the verified period. 

Table 2. Annual averages of TN, TP, Chl a, and TSI values of Trout Lake from 1993 

through 2000. Data represent mean ± 1SE (n=4) 

TN 

(mg/L) 

TP 

(mg/L) 

Chl a

(μg/L) TSI 

1993 1.87 ± 0.19 0.205 ± 0.044 110.4 ± 19.3 78 ± 2 

1994 1.73 ± 0.14 0.137 ± 0.016 81.5 ± 27.7 71 ± 6 

1995 1.41 ± 0.07 0.138 ± 0.014 50.8 ± 10.8 70 ± 2 

1996 1.31 ± 0.04 0.153 ± 0.026 45.6 ± 6.5 68 ± 1 

1997 1.54 ± 0.12 0.128 ± 0.024 76.0 ± 16.1 73 ± 2 

1998 1.62 ± 0.04 0.231 ± 0.024 23.5 ± 4.3 66 ± 1 

1999 1.61 ± 0.04 0.242 ± 0.030 48.6 ± 12.1 70 ± 2 

2000 2.25 ± 0.26 0.261 ± 0.052 111.9 ± 35.5 79 ± 4 

Mean-L 1.67 ± 0.10 0.187 ± 0.019 68.5 ± 11.3 72 ± 2 

Mean-V 1.62 ± 0.12 0.192 ± 0.021 59.4 ± 10.8 71 ± 2 

Mean-P 1.80 ± 0.07 0.171 ± 0.034 95.9 ± 14.4 74 ± 3 

Note: 

•

•

•

•

Mean-L represents results of record mean (1993 through 2000)

Bolded data were annual means for the verified period.

Mean-V: mean values for the modified verified period (January of 1995 through December of 2000)

Mean-P: mean values for the pre-verified period (1993 through 1994) 

14
January 4, 2006 

 

Table 3. Seasonal variation of TN, TP, Chl a, and TSI in Trout Lake 

TN 

(mg/L) 

TP 

(mg/L) 

Chl a

(μg/L) TSI 

General long-term quarterly mean 

1st quarter 1.78 ± 0.13 0.244 ± 0.026 81.1 ± 16.2 74 ± 2 

2nd quarter 1.80 ± 0.15 0.211 ± 0.031 89.9 ± 15.3 75 ± 2 

3rd quarter 1.63 ± 0.15 0.140 ± 0.016 54.5 ± 20.3 68 ± 3 

4th quarter 1.46 ± 0.06 0.153 ± 0.015 48.6 ± 8.0 70 ± 1 

Quarterly mean for the verified period 

1st quarter 1.63 ± 0.10 0.256 ± 0.030 61.7 ± 11.5 72 ± 2 

2nd quarter 1.78 ± 0.20 0.215 ± 0.035 83.3 ± 19.3 75 ± 3 

3rd quarter 1.66 ± 0.20 0.140 ± 0.019 55.8 ± 25.1 69 ± 3 

4th quarter 1.42 ± 0.08 0.158 ± 0.020 36.8 ± 2.9 68 ± 1 

Data represent mean ±1SE and sample size (n) equals 8 years for the general long-term 

quarterly mean values and 6 for quarterly mean values for the verified period.

Examination of the values for TN, TP, Chl a, and TSI in Trout Lake from 1993 through 2000 

indicates that all of these water quality indices are relatively higher in 1993 and 2000, and 

relatively lower in the period from 1994 through 1999. The amount of rainfall appears to explain 

at least some portion of the variance (Figures 4-A, B, C, and D). Lower rainfall was observed in 

1993 and 2000 than the rest of the years during the study period. This is consistent with the 

trend of the water quality indices. 

Except for the TN concentration, the general trend for the seasonal variation is that TP and Chl 

a concentrations and TSI tend to be at the lowest level in the third and fourth quarter of the year 

(Table 3). The lowest TN concentration appears in the fourth quarter of the year. While low 

temperature might contribute to the low Chl a in the fourth quarter of the year, and high rainfall 

during the third quarter might dilute out the TP concentration in lake, the precipitation alone can 

not account for the observed seasonal pattern of TN, TP, and Chl a concentrations (Figures 5-

A, B, C, and D). 

Critical Conditions

Nutrient reductions in the UORB lakes proposed by the SJRWMD were based upon a 10-year 

average loads. Fulton et al. (2003) note that load reduction goals should be treated as longterm average annual loads and that there is substantial year to year variation in the loadings to 

the UORB lakes. 

Based on Department regulations for nutrient impairment of lakes (62-303, FAC), the overall 

critical condition (worst case) is based on the annual mean TSI unless other conditions such as 

unusual relationships between season and TSI or the presence of NPDES point source facilities 

indicate a different critical condition would be required to ensure that water quality standards will 

be attained in the water body. As there are no point sources discharging into the Lake, and no 

unusual seasonal differences in TSI, the critical condition (worst case) for this TMDL is annual 

mean TSI. 

15
January 4, 2006 

 

6.2 Estimating TN and TP Sub-basin loadings using WMM 

Data required for estimating TN and TP loadings Trout Lake watershed using WMM

To estimate TN and TP loadings from the Trout Lake watershed using WMM, the following data 

were collected: 

A. Precipitation data from the weather station located in Lisbon [Lake County (UCAN: 

4025,COOP: 085076)]. These data were retrieved from the Climate Interactive Rapid 

Retrieval User System (CIRRUS) hosted by the Southeast Regional Climate Center. Annual 

average precipitation and seasonal variation are depicted in Table 4 and Table 5, 

respectively. 

16
January 4, 2006 

 

0.000

0.500

1.000

1.500

2.000

2.500

3.000

1993 1994 1995 1996 1997 1998 1999 2000

Year

TN concentration (mg/L)

0

10

20

30

40

50

60

70

80

Precipitation (in/year)

TN Concentration

Rainfall

Figure 4-A TN concentration of Trout Lake vs. precipitation 

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

1993 1994 1995 1996 1997 1998 1999 2000

Year

TP concentration (mg/L)

0

10

20

30

40

50

60

70

80

Precipitation (in/year)

TP concentration

Precipitation

Figure 4-B TP concentration of Trout Lake vs. precipitation 

17
January 4, 2006 

 

0.0

20.0

40.0

60.0

80.0

100.0

120.0

140.0

160.0

1993 1994 1995 1996 1997 1998 1999 2000

Year

Chla concentration (μg/L)

0

10

20

30

40

50

60

70

80

Precipitation (in/year)

Chla Concentration

Precipitation

Figure 4-C. Chl a concentration of Trout Lake vs. precipitation 

0

10

20

30

40

50

60

70

80

90

1993 1994 1995 1996 1997 1998 1999 2000

Year

TSI

0

10

20

30

40

50

60

70

80

Precipitation (in/year)

TSI

Precipitation

Figure 4-D. TSI of Trout Lake vs. precipitation

18
January 4, 2006 

 

0.00

0.50

1.00

1.50

2.00

2.50

1st quarter 2nd quarter 3rd quarter 4th quarter

Year

Quarterly mean TN concentration

(mg/L)

0.0

5.0

10.0

15.0

20.0

25.0

Quarterly rainfall (in/quarter)

TN concentration

Precipitation

 

Figure 5-A. Quarterly mean TN concentration of Trout Lake vs. quarterly mean rainfall

0.00

0.05

0.10

0.15

0.20

0.25

0.30

1st quarter 2nd quarter 3rd quarter 4th quarter

Year

Quarterly mean TP concentration 

(mg/L)

0.0

5.0

10.0

15.0

20.0

25.0

Quarterly rainfall (in/quarter)

TP concentration

Precipitation

 

Figure 5-B. Quarterly mean TP concentration of Trout Lake vs. quarterly mean rainfall 

 

19
January 4, 2006 

 

0.00

20.00

40.00

60.00

80.00

100.00

120.00

1st quarter 2nd quarter 3rd quarter 4th quarter

Year

Quarterly mean Chla concentration

(μg/L)

0.0

5.0

10.0

15.0

20.0

25.0

Quarterly rainfall (in/quarter)

Chla concentration

Precipitation

 

Figure 5-C. Quarterly mean Chl a concentration of Trout Lake vs. quarterly mean rainfall 

55.00

60.00

65.00

70.00

75.00

80.00

1st quarter 2nd quarter 3rd quarter 4th quarter

Year

Quarterly mean TSI

0.0

5.0

10.0

15.0

20.0

25.0

Quarterly rainfall (in/quarter)

TSI

Precipitation

 

Figure 5-D. Quarterly mean TSI of Trout Lake vs. quarterly mean rainfall

20
January 4, 2006 

 

Table 4. Annual precipitation 

Year 

Annual Precipitation 

(in/year) 

1995 52.12 

1996 57.90 

1997 56.06 

1998 37.18 

1999 54.13 

2000 29.26 

Table 5. Long-term average quarterly precipitation. Data represent mean ±SE (n=6) 

Quarter 

Quarterly Precipitation 

(in/quarter) 

1st quarter 9.92 ± 2.11 

2nd quarter 10.64 ± 1.96 

3rd quarter 19.55 ± 1.48 

4th quarter 9.62 ± 2.11 

For modeling purposes, the Trout Lake watershed was further divided into three sub-basins 

including the Trout Lake sub-basin (TL) with 8,834 acres, the Pine Meadow sub-basin (PM) 

with 623 acres, and the Springhill sub-basin (SH) with 229 acres (Figure 6). The subdivision 

of the watershed into three subbasins was required as the SJRWMD had site-specific 

EMC’s for landuses within PM and SH that were different from those generally used. Pine 

Meadow was a muck farm purchased by SJRWMD and converted to wetlands. Areas of 

different landuse categories in the three sub-basins were obtained by aggregating GIS 

landuse coverage based on the simplified level 1 code and are listed in Table 6. In all three 

sub-basins, Water/Wetland occupies the highest percentage acreage (32% in TL, 100% in 

PM, and 78% in SH). The second largest landuse category in TL is claimed by agriculture, 

which accounts for about 20% of the TL sub-basin. Forest coverage in TL is about 14%, 

and the rest of the TL sub-basin was occupied by urban, residential area, and transportation 

facilities. In SH, Water/Wetland occupies the majority of the landuse and about 18% of the 

land is occupied by agriculture. Forest coverage of the sub-basin is only about 2%.

Based on a study conducted by SJRWMD (Fulton et. al., 2003) the TN and TP 

concentrations of the discharge from the Water/Wetland areas of the Pine Meadow 

restoration area and Springhill muck farm were substantially higher than the event mean 

concentrations (EMC) of the corresponding landuse category in other parts of the Trout 

Lake watershed (Table 8). 

21
January 4, 2006 

 

 

SH 

Sub-basin 

TL 

Sub-basin

PM 

Sub-basin 

Figure 6. Three sub-basins of the Trout Lake watershed. 

22
January 4, 2006 

 

Table 6. Distribution of landuse categories of three Trout Lake sub-watersheds 

Area (acre) Code Landuse 

TL PM SH 

1000 Urban Open 460 0 0 

Low density residential 457 0 1 

Medium density residential 1137 0 0 

High density residential 81 0 0 

2000 Agriculture 1804 0 42 

3000 Rangeland 702 0 2 

8000 

Transportation, 

communication, and 

utilities 

66 0 0 

4000 Forest/Rural open 1261 0 6 

5000/6000 Water/Wetland 2866 623 178 

Percent impervious area of each landuse category is a very important parameter in 

estimating surface runoff using WMM. Nonpoint pollution monitoring studies throughout 

the U.S. over the past 15 years have shown that annual “per acre” discharges of urban 

stormwater pollution are positively related to the amount of imperviousness in the 

landuse (WMM User’s Manual 1998). Ideally, impervious area is considered as the area 

that does not retain water and therefore, 100% of the precipitation falling on the 

impervious area should become surface runoff. In practice, the runoff coefficient for 

impervious area typically ranges from 95 to 100%. Impervious runoff coefficients lower 

than this range were observed in the literature, but usually this number should not be 

lower than 80%. For pervious area, the runoff coefficient usually ranges between 10 to 

20%. However, values lower than this range were also observed (WMM User’s Manual: 

1998). In this study, runoff coefficients of 90% and 1.24% were used for impervious and 

pervious area, respectively, according to a study conducted previously by SJRWMD 

(Fulton, et al. 2003). 

B. 

It should be noted that the impervious area percentages do not necessarily represent 

directly connected impervious area (DCIA). Using a single-family residence as an 

example, rain falls on rooftops, sidewalks, and driveways. The sum of these areas may 

represent 30% of the total lot. However, much of the rain that falls on the roof drains to 

the grass and infiltrates to the ground or runs off the property and thus does not run 

directly to the street. For WMM modeling purpose, whenever the area of the watershed 

that contributes to the surface runoff was considered, DCIA was used in place of 

impervious area. Because local values were not available, DCIAs used in this study 

were from literature published values or results from other studies (Table 7). 

23
January 4, 2006 

 

Table 7 Percent direct connected impervious area for different landuse categories 

Landuse Categories DCIA Reference 

Forest/Rural Open 0.5% WMM User’s Manual: 1998 

Urban Open 0.5% WMM User’s Manual: 1998 

Agriculture 3.7% Brown 1995 

Low Density Residential 12.40% Brown 1995 

Medium Density Residential 18.70% Brown 1995 

High Density Residential 29.60% Brown 1995 

Communication and Transportation 36.20% Brown 1995 

Rangeland 3.7% CDM 

Water/Wetlands 30% Harper and Livingston 1999 

C. Event mean concentrations (EMC) of TN and TP for different landuse categories were 

provided by SJRWMD (Fulton, personal communication) and are listed in Table 8. 

Table 8. Event mean concentration of TN and TP for different landuse categories 

Unit: mg/L 

Landuse Categories TN TP 

Forest/Rural Open 1.25 0.057 

Urban Open 2.03 0.313 

Agriculture 2.98 0.421 

Low Density Residential 1.77 0.177 

Medium Density Residential 2.29 0.300 

High Density Residential 2.42 0.490 

Communication and Transportation 2.83 0.430 

Rangeland 1.25 0.057 

Water/Wetlands in TL sub-basin 0.82 0.03 

Water/Wetlands PM and SH subbasins 2.54 0.47 

EMCs of TN and TP for Urban Open, Agriculture, and Communication and Transportation 

landuse categories were not specified in the data provided by the SJRWMD. The EMCs for 

Urban Open used in this study were calculated as the average between the Low Density 

Commercial area and the High Density Commercial area characterized by SJRWMD. The 

EMCs for Agriculture landuse were the mean values for Pasture, Tree Crops, Cropland, and 

24
January 4, 2006 

 

Other Agriculture provided by SJRWMD. The EMCs for Communication and Transportation 

took the value for the High Density Commercial area provided by SJRWMD. 

Based on a study conducted by SJRWMD, TN and TP concentrations of the Water/Wetland 

area in the Pine Meadow Restoration area and Springhill muck farm were significantly 

higher than the EMCs of the corresponding landuse category in the TL sub-basin. Based on 

their suggestion, the TN and TP EMCs for the Water/Wetland area in PM and SH were set 

at 2.54 mg/L and 0.47 mg/L, respectively (Table 8). 

D. Not all of the TN and TP are transported by the stormwater in the dissolved form. The 

percentage of the total EMC represented by TN and TP attached to suspended particles can 

be defined in WMM. The percent suspended TN and TP values used in this study were 

provided by SJRWMD and assigned to land uses following the same procedures as used for 

determining EMCs (Table 9). 

 

Table 9. Percent TP and TN in suspended form for different landuse categories. 

Landuse Categories TP TN 

Forest/Rural Open 50% 25% 

Urban Open 41% 29% 

Agriculture 34% 9% 

Low Density Residential 50% 25% 

Medium Density Residential 50% 25% 

High Density Residential 50% 25% 

Communication and Transportation 41% 29% 

Rangeland 50% 25% 

Water/Wetlands 69% 41% 

E. 

F. 

The Sediment delivery ratio determines how much TN and TP attaching to suspended 

particles will be delivered to the destination waterbody. In this study, the sediment delivery 

ratio was estimated using the correlation between delivery ratio and watershed area 

developed by Roehl (1962), which in this case is 0.18. 

To estimate the TN and TP loadings from leakage of septic tanks, WMM incorporates the 

concept of “septic tank failure loading rate.” The annual failure rate reported for the country 

is 3 – 5 percent. Pollutant loading rates reported in the WMM Users Manual assume 50 

gallons per capita per day usage. The mid-range of loading rates for failing septic tanks is 

2.0 mg/L for TP (about a 160% to 250% increase) and for TN is 15.0 mg/L (about a 140% to 

200% increase). To provide a Margin of Safety, this study adopted the high end of the 

range in the User Manual, which were 30.0 mg/L for TN and 4.0 mg/L for TP (WMM User 

Manual: 1998). 

Another value required by WMM to estimate the influence from leaking septic tanks on TN 

and TP loading is the “septic tank failure rate,” which defines the frequency at which septic 

tanks may fail. Studies conducted on the water quality of the Ocklawaha River Basin found 

25
January 4, 2006 

 

that annual frequency of septic tank repairs was about 0.97% (Ocklawaha Basin Status 

Report 2001). For average annual conditions, it is conservative to assume that septic tank 

systems failures would be unnoticed or ignored for five years before repair or replacement 

occurred (WMM User Manual: 1998). Therefore, the septic tank failure rate used in this 

study was calculated by multiplying repairing frequency (0.97%) by 5 (years) and was about 

5%. 

The quantity of surface runoff into Trout Lake estimated using WMM 

The year by year surface runoff into Trout Lake from TL, PM, and SH predicted using WMM is 

listed in Tables 10-1, 10-2, and 10-3, respectively. 

Table 10-1. Annual surface runoff from TL. 

Unit: acre-feet/year 

Landuse Categories 1995 1996 1997 1998 1999 2000

Forest/Rural Open 92 103 99 66 96 52

Urban Open 34 37 36 24 35 19

Agriculture 354 394 381 253 368 199

Low Density Residential 243 270 262 173 253 137

Medium Density Residential 881 978 947 628 915 494

High Density Residential 97 108 104 69 101 54

Communication and Transportation 96 106 103 68 100 54

Rangeland 138 153 148 98 143 77

Water/Wetlands 3469 3854 3731 2473 3603 1947

Total 5404 6003 5812 3852 5612 3034

26
January 4, 2006 

 

Table 10-2. Annual surface runoff from PM. 

Unit: acre-feet/year 

Landuse Categories 1995 1996 1997 1998 1999 2000

Forest/Rural Open 0 0 0 0 0 0

Urban Open 0 0 0 0 0 0

Agriculture 0 0 0 0 0 0

Low Density Residential 0 0 0 0 0 0

Medium Density Residential 0 0 0 0 0 0

High Density Residential 0 0 0 0 0 0

Communication and Transportation 0 0 0 0 0 0

Rangeland 0 0 0 0 0 0

Water/Wetlands 1474 1638 1586 1051 1531 828 

Total 1474 1638 1586 1051 1531 828 

Table 10-3. Annual surface runoff from SH. 

Unit: acre-feet/year 

Landuse Categories 1995 1996 1997 1998 1999 2000

Forest/Rural Open 0 1 0 0 0 0 

Urban Open 0 0 0 0 0 0 

Agriculture 8 9 9 6 9 5 

Low Density Residential 1 1 1 1 1 0 

Medium Density Residential 0 0 0 0 0 0 

High Density Residential 0 0 0 0 0 0 

Communication and Transportation 0 0 0 0 0 0 

Rangeland 0 0 0 0 0 0 

Water/Wetlands 422 469 454 301 438 237 

Total 431 480 464 308 448 242 

According to Table 10, among the three sub-basins, the largest surface runoff was from TL, 

which ranged from 3,034 to 6,003 acre-foot/year from 1995 through 2000. The surface runoff 

produced in PM and SH in the same period was 828 to 1,638 acre-foot/year and 242 to 480 

acre-foot/year, respectively. In all the three sub-basins, surface runoff from the Water/Wetland 

dominated, and accounted for about 64%, 100%, and 98% of the total stormwater produced in 

TL, PM, and SH, respectively. The Medium Density Residential area in TL contributed about 

16% of the flow. The individual flow contributions from all the other landuse categories of all the 

three sub-basins were less than 10%. 

27
January 4, 2006 

 

Considering that 1996 has the highest annual rainfall and 2000 has the lowest annual rainfall, 

and that no difference was found between the percent surface runoff between these two years, 

it appears that the percent distribution of surface runoff from different landuse categories is not 

significantly influenced by the annual rainfall. 

TN and TP loadings from Trout Lake watershed

The annual TN and TP loadings from the three sub-basins are listed in Table 11-1, 11-2, and 

11-3 and 12-1, 12-2, and 12-3. 

According to Table 11, TL at 8,834 acres contributed the largest total amount of TN among the 

three sub-basins, with loads ranging from 8,634 to 17,086 lbs/year (0.977 to 1.934 lbs/acre TN) 

from 1995 through 2000. In comparison, PM at 623 acres contributed between 4,971 to 9,837 

lbs of TN/year (7.979 to 15.790 lbs/acre TN), and SH at 229 acres contributed 1,130 to 2,233 

lbs TN/year (4.934 to 9.751 lbs/acre TN), in the same time period. For all the three sub-basins, 

Water/Wetland area was the largest contributor of TN among all the landuse categories, 

contributing about 34%, 100% and 97% of the TN load in TL, PM, and SH, respectively. In TL, 

the Medium Density Residential area was the second largest TN contributor. The TN 

contribution from this landuse area was about 28%. Agriculture contributed about 17% of the 

TN in TL sub-basin, and the individual contributions from all the other landuse categories in all 

three sub-basins were less than 10%. 

Table 11-1. Annual TN loadings from TL. 

Unit: lbs/year 

Landuse Categories 1995 1996 1997 1998 1999 2000

Forest/Rural Open 250 278 269 178 260 140

Urban Open 142 157 152 101 147 80

Agriculture 2656 2950 2856 1893 2758 1491

Low Density Residential 933 1037 1004 665 969 524

Medium Density Residential 4374 4859 4704 3117 4542 2455

High Density Residential 508 565 547 362 528 285

Communication and Transportation 563 626 606 401 585 316

Rangeland 374 415 402 266 388 210

Water/Wetlands 5158 5730 5548 3676 5357 2896

Septic tanks 422 469 454 301 439 237

Total 15380 17086 16542 10960 15973 8634

28
January 4, 2006 

 

Table 11-2. Annual TN loadings from PM. 

Unit: lbs/year 

Landuse Categories 1995 1996 1997 1998 1999 2000

Forest/Rural Open 0 0 0 0 0 0

Urban Open 0 0 0 0 0 0

Agriculture 0 0 0 0 0 0

Low Density Residential 0 0 0 0 0 0

Medium Density Residential 0 0 0 0 0 0

High Density Residential 0 0 0 0 0 0

Communication and Transportation 0 0 0 0 0 0

Rangeland 0 0 0 0 0 0

Water/Wetlands 8855 9837 9524 6312 9197 4971

Septic tanks 0 0 0 0 0 0

Total 8855 9837 9524 6312 9197 4971

Table 11-3. Annual TN loadings from SH. 

Unit: lbs/year 

Landuse Categories 1995 1996 1997 1998 1999 2000

Forest/Rural Open 1 1 1 1 1 1

Urban Open 0 0 0 0 0 0

Agriculture 62 69 67 44 64 35

Low Density Residential 3 3 3 2 3 2

Medium Density Residential 0 0 0 0 0 0

High Density Residential 0 0 0 0 0 0

Communication and Transportation 0 0 0 0 0 0

Rangeland 1 1 1 1 1 1

Water/Wetlands 1944 2159 2091 1385 2019 1091

Septic tanks 0 0 0 0 0 0

Total 2011 2233 2163 1433 2088 1130

From 1995 through 2000 the TL sub-basin was the largest source of TN, with loads ranging 

from 8,634 to 17,086 lbs TN/year. During the same period, the largest TP contribution was from 

the PM sub-basin with loads ranging from 793 to 1,550 lbs TP/year. TL contributed from 641 to 

1,270 lbs TP/year, and SH contributed, 136 to 269 lbs TP/year, in the same time period (Figure 

12). For both PM and SH, the largest TP contributor was the Water/Wetland area. In TL, 

29
January 4, 2006 

 

however, the Medium Density Residential area was the largest TP source, accounting for about 

37% of the total TP created in the TL sub-basin. The second largest TP contributor was 

agriculture, contributing 26%. The TP contribution from the Water/Wetland area only accounted 

for about 11% of the total TP loading in TL, and individual contributions from all the other 

landuses categories were less than 10%. 

Table 12-1. Annual TP loadings from TL. 

Unit: lbs/year 

Landuse Categories 1995 1996 1997 1998 1999 2000

Forest/Rural Open 8 9 9 6 9 5

Urban Open 19 21 20 14 20 11

Agriculture 293 325 315 209 304 164

Low Density Residential 69 77 74 49 72 39

Medium Density Residential 425 472 457 303 441 238

High Density Residential 76 85 82 54 79 43

Communication and Transportation 74 83 80 53 77 42

Rangeland 13 14 14 9 13 7

Water/Wetlands 130 145 140 93 135 73

Septic tanks 35 39 37 25 36 19

Total 1142 1270 1228 815 1186 641

Table 12-2. Annual TP loadings from PM. 

Unit: lbs/year 

Landuse Categories 1995 1996 1997 1998 1999 2000

Forest/Rural Open 0 0 0 0 0 0

Urban Open 0 0 0 0 0 0

Agriculture 0 0 0 0 0 0

Low Density Residential 0 0 0 0 0 0

Medium Density Residential 0 0 0 0 0 0

High Density Residential 0 0 0 0 0 0

Communication and Transportation 0 0 0 0 0 0

Rangeland 0 0 0 0 0 0

Water/Wetlands 1395 1550 1500 994 1449 783

Septic tanks 0 0 0 0 0 0

Total 1395 1550 1500 994 1449 793

30
January 4, 2006 

 

Table 12-3. Annual TP loadings from SH. 

Unit: lbs/year 

Landuse Categories 1995 1996 1997 1998 1999 2000

Forest/Rural Open 0 0 0 0 0 0

Urban Open 0 0 0 0 0 0

Agriculture 7 8 7 5 7 4

Low Density Residential 0 0 0 0 0 0

Medium Density Residential 0 0 0 0 0 0

High Density Residential 0 0 0 0 0 0

Communication and Transportation 0 0 0 0 0 0

Rangeland 0 0 0 0 0 0

Water/Wetlands 235 261 253 168 244 132

Septic tanks 0 0 0 0 0 0

Total 242 269 260 173 251 136

Like the percent distribution of TN loading, the distribution of TP loadings from different landuse 

categories is not significantly influenced by annual precipitation. 

Atmospheric loading of TN and TP into Trout Lake

One source of TN and TP loading to Trout Lake that is not considered by WMM is the TN and 

TP falling directly into the lake through precipitation. In this study, atmospheric loading of TN 

and TP was calculated by multiplying the amount of precipitation directly falling on to the lake 

surface (calculated by multiplying annual precipitation by surface area of the lake) by the TN 

and TP concentration of the rainfall. Because no TN and TP concentrations of the rainfall were 

found for the project area, published values were adopted in this study, which were 0.1 mg/L 

and 0.05 mg/L for TN and TP, respectively (Stites, et al 2001). Calculated annual TN and TP 

loadings are tabulated in Table 13. 

31
January 4, 2006 

 

Table 13. Atmospheric Loading of TN and TP into Trout Lake 

Unit: lbs/year 

Year TN TP 

1995 121 60 

1996 134 67 

1997 130 65 

1998 86 43 

1999 125 63 

2000 68 34 

Mean 110 55 

SE 11 6 

CV 10% 10% 

Summary of the TN and TP loadings into Trout Lake from the various sources

The annual average TN and TP loadings (over the period from 1995 through 2000) from runoff 

in the Trout Lake watershed, septic tank leakage, and atmospheric deposition is summarized in 

Table 14. 

Table 14. Average annual quantity of surface runoff and TN and TP loadings into Trout 

Lake. Data represent mean ± 1 SE (n=6) 

Item Quantity 

Surface runoff from Trout Lake watershed (acre-foot/year) 6700 ± 671 

Precipitation (acre-foot/year) 406 ± 41 

TN loading from Trout Lake watershed (lbs/year) 23668 ± 2369

TN loading from atmospheric precipitation (lbs/year) 110 ± 11 

TN loading from septic tank (lbs/year) 387 ± 39 

TP loading from Trout Lake watershed (lbs/year) 2516 ± 252 

TP loading from atmospheric precipitation (lbs/year) 55 ± 6 

TP loading from septic tank (lbs/year) 32 ± 3 

Total amount of water into Trout Lake (acre-foot/year) 7106 ± 711 

Total TN loading into Trout Lake (lbs/year) 24165 ± 2419

Total TP loading into Trout Lake (lbs/year) 2603 ± 261 

32
January 4, 2006 

 

6.3 Establishing the relationship between TN and TP loading and in-lake TN, TP, and 

Chl a concentrations using the Bathtub model 

Data required for calibrating Bathtub eutrophication model

The relationship between TN and TP loading and the in-lake TN and TP concentrations was 

established through fitting the Bathtub predictions with the measured TN and TP concentrations 

of the lake. To calibrate the model, the following data were required: 

1. Physical characteristics of the lake (surface area, mean depth, and mixed layer depth) 

2. Meteorological data (precipitation and evaporation) 

3. Measured water quality data (TN, TP, and Chl a concentrations of the lake water) 

4. Loading data (flow and TN and TP concentrations of the flow from various sources). 

Because Bathtub allows both error and variability analysis, whenever there were historical data, 

long-term average values and the corresponding coefficient of variance (CV) of the average 

values were calculated and entered into the model as inputs. All the data that were required for 

model calibration are shown in Tables 15 through 17. 

Because no lake stage data were available at the time this study was conducted, a fixed lake 

volume was adopted for the Bathtub modeling. The long-term average surface area was 

obtained from Florida LAKEWATCH (http://lakewatch.ifas.ufl.edu/), as 102 acres (about 0.413 

km2

). The mean depth is about 9 feet (about 2.74 m, SJRWMD 1996). Because the lake is 

relatively shallow, the mixed layer depth is assumed equal to the mean depth of the lake. 

Table 15. Precipitation and evaporation 

Unit: ft/year 

Year Precipitation Evaporation 

1995 4.33 4.07 

1996 4.82 4.49 

1997 4.66 4.23 

1998 3.08 3.51 

1999 4.49 3.81 

2000 2.43 4.46 

Mean 3.97 4.10 

SE 0.12 0.05 

CV 10.0% 3.8% 

33
January 4, 2006 

 

Table 16. Measured TN, TP, and Chl a concentrations of Trout Lake 

Year TN (mg/L) TP (mg/L) Chl a (ug/L) 

1995 1.40 0.138 50.8 

1996 1.31 0.153 45.6 

1997 1.54 0.128 76.0 

1998 1.62 0.231 23.5 

1999 1.62 0.242 48.6 

2000 2.25 0.261 111.9 

Mean 1.62 0.192 59.4 

SE 117 21 10.8 

CV 7% 11% 18% 

Table 17. Predicted Flow and TN and TP concentrations of different sources 

(hm3

/year = cubic hectometers/year) 

Land Use 

Flow 

(hm3/year)

TN 

(mg/L)

TP 

(mg/L)

Forest/Rural Open 0.10 0.997 0.034 

Urban Open 0.04 1.547 0.208 

Agricultural 0.41 2.755 0.304 

Low density residential 0.28 1.412 0.105 

Medium density residential 1.00 1.826 0.177 

High density residential 0.11 1.930 0.290 

Transportation, Communications, and Utilities 0.11 2.162 0.286 

Rangeland 0.16 0.997 0.034 

Water/Wetlands 6.07 1.094 0.121 

Note: 

a) Bathtub does not allow direct input of loading, therefore the data presented in Table 17 are 

the simulated flow and TN and TP concentrations. 

b) The flow and TN and TP concentrations presented here are the mean values simulated for 

the verified period (1995 through 2000). 

c) The TN and TP concentrations are different from the EMC’s listed in Table 8. The EMC 

concentrations in Table 8 represent the concentrations at the beginning of the overland 

transport to the Lake. Because some fraction of the TN and TP from each landuse category 

is in the particulate form and thus subject to attenuation during transport to the water body, 

34
January 4, 2006 

 

the final concentrations entering the Lake (Table 17) will be different from the starting 

concentration (EMC). 

Calibrating Bathtub eutrophication model

To calibrate the model, each source of TN and TP previously identified (except septic tanks) 

was input to the model as an independent tributary (Table 17). The TN and TP contributions 

from septic tanks are characterized with areal flux rates, which are 1.17 mg/m2

-day for TN and 

0.1 mg/m2

-day for TP. 

Bathtub provides alternative models for estimating the influence of sedimentation on the in-lake 

TN and TP concentrations. In this study, the settling velocity model was chosen for both TN 

and TP. This model assumes that the sedimentation of TN and TP is in first-order kinetics and 

should linearly correlate with the in-lake TN and TP concentration. The model also assumes 

that the sedimentation is influenced by the depth of the lake, i.e., the deeper the lake, the slower 

the sedimentation. This model fit the condition of Trout Lake because the lake is relatively 

shallow. Continued wind mixing prevents the lake from forming thermal stratification, a process 

that prevents the particles from being re-suspended once settled down to the bottom. 

Continued wind mixing through the entire water column also reduces the particle settling rate by 

continuously bringing the settled particle back in to water column. These processes could 

produce a relatively low settling rate in the lake. Other sedimentation models provided by 

Bathtub assume second-order kinetics, which fit reasonably well with lakes that thermally stratify 

during the summer, but could overestimate the sedimentation of Trout Lake, and in turn cause 

underestimation of the in-lake TN and TP concentration. 

Bathtub provides two chlorophyll a models based on the assumption of nitrogen and 

phosphorus co-limitation: Model 1 and 3. Model 1 assumes that algal communities are colimited, not only by nitrogen and phosphorus, but also by light intensity. This model seemed to 

fit Trout Lake because the lake has a high chlorophyll concentration and the self-shading effect 

was possible. However, application of this model yielded a Chl a concentration much lower 

than the measured data. Therefore, in simulating Chl a response, Model 3 was adopted. This 

model assumes that the primary production of the lake was co-limited by nitrogen and 

phosphorus, but not by light intensity (Walker 1999). This could be the case in Trout Lake 

because the lake was large and shallow. Wind mixing could constantly stir the entire water 

column and bring the algal cells in the deep water up to the surface so that no cells would be 

permanently shaded. Using this model, a reasonable fit between predicted and measured Chl a

was achieved. 

 

Calibration factors were applied to fit TN and TP predictions to the measured data. Two 

calibration methods are provided by Bathtub for phosphorus and nitrogen: Method 0 calibrates 

decay rates and Method 1 calibrates concentration. In the first case, the calibration factors are 

applied to estimated sedimentation rates in computing nutrient balances. In the second case, 

the factors are applied to estimated concentrations. In Method 0, it is assumed that the error is 

attributed primarily to the sedimentation model. In Method 1, the error source is unspecified 

(some combination of input error and sedimentation model error). The latter may be used when 

predicted nutrient profiles are insensitive to errors in predicted sedimentation rate because the 

mass balance is dominated by inflow and outflow terms (low hydraulic residence times) (Walker 

1999). In this study, Method 1 was adopted to calibrate the concentration. Typical calibration 

factors for TN and TP recommended by the Bathtub user’s manual are 0.5 – 2.0 for TP and 0.33 

– 3 for TN. In this study, 1.4 and 1.3 were adopted for calibrating TP and TN, respectively. 

Results of model calibration are listed in Table 18. 

35
January 4, 2006 

 

Table 18. Bathtub calibration results 

Parameter Measured Estimated Error 

TP (mg/l) 0.192 0.191 1% 

TN (mg/l) 1.622 1.649 2% 

Chl a (μg/l) 59.4 60.2 1% 

TSI 73 74 1% 

TSI was calculated using the procedures outlined in Florida’s 1996 305(b) report for both 

measured and Bathtub predicted data. Using the measured TN, TP, and Chl a concentration, 

the calculated TSI is 73. When Bathtub predicted TN, TP, and Chl a concentrations were used, 

the resulted TSI is 74. The error between these two TSIs is only 1%, indicating the TSI 

calculated based on Bathtub predicted TN, TP, and Chl a concentrations properly predicted the 

TSI resulting from the measured data. 

Evaluating Natural Background TSI of Trout Lake

Once the model was calibrated, the background TN and TP loading without the loadings 

generated from the existing level of human activities were estimated using the following 

procedures: 

1. All the man-made landuse categories (Urban open, Agricultural, Low-Density 

Residential, Medium Density Residential, High Density Residential, Transportation and 

Communication, and Rangeland) were evaluated as Forest/Rural open. The loading 

from septic tanks was also removed. 

2. The TN and TP concentrations of the Water/Wetland area in PM and SH were reduced 

to the level of the natural Water/Wetland. 

3. TN and TP loadings through surface runoff were then estimated using the calibrated 

WMM, and a long-term average precipitation of 47.77 inches/year (over the period from 

1995 through 2000). 

4. TN and TP concentrations from Forest/Rural Open and Water/Wetland were calculated 

by dividing the total loadings from the Trout Lake watershed by the total flow (Table 19). 

Table 19. Flow and TN and TP concentrations of surface runoff from Forest/rural open 

and Water/Wetland area 

Land Use 

Flow 

(Hm3/year) 

TN concentration 

(mg/L) 

TP concentration 

(mg/L) 

Forest/Rural Open 0.5 0.997 0.034 

Water/Wetland 6.07 0.592 0.016 

5. The flow and TN and TP concentrations of surface runoff from Forest/rural open and 

Water/wetland area were then entered into Bathtub to estimate the in-lake TN, TP, and 

Chl a concentrations. 

36
January 4, 2006 

 

6. The TSI was calculated based on the predicted TN, TP, and Chl a concentrations. The 

resulting TSI of 49 was considered the natural background TSI of Trout Lake and any 

further reduction of the TSI of the lake by additional reductions in the loadings was not 

considered. The resulting TN, TP, and Chl a concentration and TSI are listed in Table 

20. 

Table 20. TN, TP, and Chl a concentrations and TSI after all the human landuse 

categories were treated as Forest/Rural Open and Water/Wetland 

TP (mg/l) 0.028 

TN (mg/l) 0.78 

Chl a (μg/l) 9.9 

TSI 49 

According to Table 20, after all the human landuse categories were treated as Forest/Rural 

Open area, TN, TP, and Chl a concentrations decreased about 53%, 85%, and 84%, 

respectively, over the existing condition (Table 18). The TSI value decreased from 74 to 49. 

The results in Table 20 compare favorably to those obtained by the SJRWMD for the Upper 

Oklawaha River Basin Pollutant Load Reduction Goal. The SJRWMD used the ecoregional 

approach and found a TP range of 0.021 – 0.025 mg/L (Table 20 value of 0.028 mg/L) and a TN 

range of 0.72 – 0.82 mg/L (Table 20 value of 0.78 mg/L). 

According to the IWR, a lake is only considered impaired by nutrients when (1) The TSI of the 

lake is higher than 60 (if the water color of the lake is higher than 40 pcu.); (2) The current TSI is 

10 units higher than the background TSI; (3) There is a positive increase in TSI over the study 

period. As previously stated, when natural background can be established, the TMDL target will 

be the natural background TSI of 49 plus 5 TSI units, which in this case is a TSI of 54. 

TSI-nutrient loading curves were developed for both TN and TP in this study (Figure 7 and 

Figure 8). According to these curves, the TN and TP loadings that result in a TSI of 54 are 

9,733 and 521 lbs/year respectively. These loadings represent a 60% and 80% loading 

reduction from the current loadings of 24,165 lbs/year TN and 2,603 lbs/year TP. 

7. Determination of TMDL 

The objective of a TMDL is to provide a basis for allocating acceptable loads among all of the 

known pollutant sources in a watershed so that appropriate control measures can be 

implemented and water quality standards achieved. A TMDL is expressed as the sum of all 

point source loads (Waste Load Allocations), nonpoint source loads (Load Allocations), and an 

appropriate margin of safety (MOS), which takes into account any uncertainty concerning the 

relationship between effluent limitations and water quality: 

TMDL = ∑ WLAs + ∑ LAs + MOS

As mentioned in Section 4.1, the WLA is broken out into separate subcategories for wastewater 

discharges and stormwater discharges regulated under the NPDES Program: 

37
January 4, 2006 

 

 

TMDL ≅ ∑ WLAswastewater + ∑ WLAsNPDES Stormwater + ∑ LAs + MOS 

TSI vs. TN loading

y = -1E-07x2 + 0.0052x + 12.863

R2

 = 0.9777

0

10

20

30

40

50

60

70

80

0 5000 10000 15000 20000 25000 30000

TN loading (lbs/year)

TSI

Figure 7. TSI-TN loading curve 

TSI vs. TP loading

y = -3E-06x2 + 0.0204x + 44.195

R2

 = 0.9946

0

10

20

30

40

50

60

70

80

0 500 1000 1500 2000 2500 3000

TP loading (lbs/year)

TSI

Figure 8. TSI-TP loading curve 

38
January 4, 2006 

 

It should be noted that the various components of the TMDL equation may not sum up to the 

value of the TMDL because a) the WLA for NPDES stormwater is typically based on the percent 

reduction needed for nonpoint sources and is accounted for within the LA, and b) TMDL 

components can be expressed in different terms [for example, the WLA for stormwater is 

typically expressed as a percent reduction and the WLA for wastewater is typically expressed as 

a mass per day]. 

WLAs for stormwater discharges are typically expressed as “percent reduction” because it is 

very difficult to quantify the loads from MS4s (given the numerous discharge points) and to 

distinguish loads from MS4s from other nonpoint sources (given the nature of stormwater 

transport). The permitting of stormwater discharges is also different than the permitting of most 

wastewater point sources. Because stormwater discharges cannot be centrally collected, 

monitored and treated, they are not subject to the same types of effluent limitations as 

wastewater facilities, and instead are required to meet a performance standard of providing 

treatment to the “maximum extent practical” through the implementation of Best Management 

Practices. 

This approach is consistent with federal regulations [40 CFR § 130.2(I)], which state that TMDLs 

can be expressed in terms of mass per time (e.g. pounds per day), toxicity, or other 

appropriate measure. The nutrient TMDL for Trout Lake (Table 21) is expressed in terms of 

pounds per year and percent reduction. 

The total annual average loadings to Trout Lake for the current condition are 24,165 lbs/year for 

TN and 2,603 lbs/year for TP. These loadings result in a current annual average TSI of 74. The 

target TSI for this study is 54, which is 5 units above the background TSI of 49. The annual TN 

and TP loadings that result in the target TSI are 9,733 lbs/year for TN and 521 lbs/year for TP, 

respectively, which represent a 60% reduction of TN and a 80% reduction of TP loadings from 

the current condition. The TN and TP loadings that result in the TSI of 54 are considered as the 

TMDL for Trout Lake. 

Table 21 TMDL Components 

WLA 

WBID Parameter Wastewater 

(lbs/year) 

NPDES 

Stormwater 

(1) 

LA 

(lbs/year) MOS TMDL (lbs/year) 

Percent 

Reduction 

2819A TN None 60% 

reduction 9,733 Implicit 9,733 60 

2819A TP None 80% 

reduction 521 Implicit 521 80 

(1) Required if during development of the City of Eustis MS4 permit it is determined that the 

Eustis MS4 contributes TN or TP to Trout Lake. 

7.1 Load Allocation 

The allowable LA is 521 lbs/year for TP and 9,733 lbs/year for TN. This corresponds to 

reductions from the existing loadings of 60 percent for TN and 80 percent for TP. It should be 

noted that the LA includes loading from stormwater discharges regulated by the Department 

39
January 4, 2006 

 

and the Water Management Districts that are not part of the NPDES Stormwater Program (see 

Appendix A). 

7.2 WasteLoad Allocation

NPDES Stormwater Discharges

As noted in Sections 4 and 7.1, loadings from stormwater discharges permitted under the 

NPDES Stormwater Program are placed in the WLA, rather than the LA. This includes loads 

from municipal separate storm sewer systems (MS4). Based on comments from the US 

Environmental Protection Agency (EPA), Trout Lake is within the city limits of Eustis, which is 

scheduled to receive a Phase II permit under EPA’s NPDES stormwater permitting program in 

2004. EPA has stated that if the City of Eustis MS4 area contributes any TN or TP to Trout 

Lake the TMDL must include a WLANPDES Stormwater. Therefore, if during preparation of the MS4 

permit it is determined that Eustis contributes TN or TP to Trout Lake, the WLA for stormwater 

discharges will be a 60 percent reduction in TN and a 80 percent reduction in TP in the current 

loading from the MS4. It should be noted that any MS4 permittees will only be responsible for 

reducing the loads associated with stormwater outfalls for which it owns or otherwise has 

responsible control, and is not responsible for reducing other nonpoint source loads within its 

jurisdiction. 

NPDES Wastewater Discharges

There are no known NPDES point source discharges within the watershed. 

7.3 Margin of Safety

The implicit margin of safety exists due to conservative assumptions used in the modeling 

process. For example, the estimates of septic tank failures were set to the maximum values 

instead of the mean values. 

8. NEXT STEPS: IMPLEMENTATION PLAN DEVELOPMENT AND BEYOND 

Following adoption of this TMDL by rule, the next step in the TMDL process is to develop an 

implementation plan for the TMDL, which will be a component of the Basin Management Action 

Plan for the Trout Lake Basin. This document will be developed in cooperation with local 

stakeholders and will attempt to reach consensus on more detailed allocations and on how load 

reductions will be accomplished. 

The Basin Management Action Plan (B-MAP) will include: 

• Appropriate allocations among the affected parties. 

• A description of the load reduction activities to be undertaken. 

• Timetables for project implementation and completion. 

• Funding mechanisms that may be utilized. 

• Any applicable signed agreements. 

• Local ordinances defining actions to be taken or prohibited. 

• Local water quality standards, permits, or load limitation agreements. 

• Monitoring and follow-up measures. 

40
January 4, 2006 

 

It should be noted that TMDL development and implementation is an iterative process, and this 

TMDL will be re-evaluated during the BMAP development process and subsequent Watershed 

Management cycles. The Department acknowledges the uncertainty associated with TMDL 

development and allocation, particularly in estimates of nonpoint source loads and allocations 

for NPDES stormwater discharges, and fully expects that it may be further refined or revised 

over time. If any changes in the estimate of the assimilative capacity AND/OR allocation 

between point and nonpoint sources are required, the rule adopting this TMDL will be revised, 

thereby providing a point of entry for interested parties. 

41
January 4, 2006 

 

REFERENCES 

Basin Status Report. 2001. Ocklawaha. Division of Water Resource Management, Florida 

Department of Environmental Protection 

Brown, M. T. 1995. South Dade Watershed Project. University of Miami/SWFWMD 

CDM. 1990. Point and non-point source loading assessment, phase I. Report to Sarasota Bay 

National Estuary Program. 

Fulton, R. S., C. Schluter, T. A. Keller, S. Nagid, W. Godwin, D. Smith, D. Clapp, A. Karama, 

and J. Richmond. 2003, Draft Interim pollutant load reduction goals for seven major 

Lakes in the Upper Ocklawaha River Basin. St. Johns River Water Management district, 

Palatka, Florida. 

Harper, H. H. 1992. Estimation of stormwater loading rate parameters for Central and South 

Florida. Environmental Research & Design. Orlando, Florida. 

Harper, H. H. and E. H. Livingston. 1999. Everything you always wanted to know about 

stormwater management practices but were afraid to ask. Biennial Stormwater Research 

Conference, Tampa, Florida. 

Roehl, J. W. 1962. Sediment source areas, delivery ratios, and influencing morphological 

factors. International association of Scientific Hydrology. 59: 202-213. Symposium of 

Bari, October 1-8, 1962 

St. Johns River Water Management District and Lake County. 1996. Water quality analysis of 

the Hicks Ditch watershed of Lake County, Florida. 

Stites, D. L., M. Coveney, L. Battoe, E. Lowe, and V. Hoge. 2001. An external phosphorus 

budget for Lake Apopka, 1989-1994. Division of Environmental Sciences, Department of 

Water Resources, St. Johns River Water Management district, Palatka, Florida. 

User’s Manual: Watershed management model, Version 4.1 1998. Rouge River national wet 

weather demonstration project. Wayne County, Michigan. PRO-NPS-TM27.02. 

Walker, W. W. 1999. Simplified procedures for eutrophication assessment and prediction: User 

manual. Instruction Report W-96-2. U.S. Army Corps of Engineers. Washington, DC. 

42
January 4, 2006 

 

Appendix A 

In 1982, Florida became the first state in the country to implement statewide regulations to 

address the issue of nonpoint source pollution by requiring new development and 

redevelopment to treat stormwater before it is discharged. The Stormwater Rule, as authorized 

in Chapter 403, Florida Statutes (F.S.), was established as a technology-based program that 

relies upon the implementation of BMPs that are designed to achieve a specific level of 

treatment (i.e., performance standards) as set forth in Chapter 62-40, Florida Administrative 

Code (F.A.C.). 

The rule requires Water Management Districts (WMDs) to establish stormwater pollutant load 

reduction goals (PLRGs) and adopt them as part of a SWIM plan, other watershed plan, or rule. 

Stormwater PLRGs are a major component of the load allocation part of a TMDL. To date, 

stormwater PLRGs have been established for Tampa Bay, Lake Thonotosassa, Winter Haven 

Chain of Lakes, the Everglades, Lake Okeechobee, and Lake Apopka. No PLRG has been 

developed for Trout Lake at the time this study was conducted.

In 1987, the U.S. Congress established section 402(p) as part of the Federal Clean Water Act 

Reauthorization. This section of the law amended the scope of the federal NPDES to designate 

certain stormwater discharges as “point sources” of pollution. These stormwater discharges 

include certain discharges that are associated with industrial activities designated by specific 

Standard Industrial Classification (SIC) codes, construction sites disturbing five or more acres of 

land, and master drainage systems of local governments with a population above 100,000 

[which are better known as “municipal separate storm sewer systems” (MS4s)]. However, 

because the master drainage systems of most local governments in Florida are interconnected, 

EPA has implemented Phase 1 of the MS4 permitting program on a county-wide basis, which 

brings in all cities (incorporated areas), Chapter 298 urban water control districts, and the DOT 

(Department of Transportation) throughout the 15 counties meeting the population criteria. 

An important difference between the federal and the state stormwater permitting programs is 

that the federal program covers both new and existing discharges while the state program 

focuses on new discharges. Additionally, Phase 2 of the NPDES stormwater permitting 

program will expand the need for these permits to construction sites between one and five 

acres, and to local governments with as few as 10,000 people. These revised rules require that 

these additional activities obtain permits by 2003. While these urban stormwater discharges are 

now technically referred to as “point sources” for the purpose of regulation, they are still diffuse 

sources of pollution that can not be easily collected and treated by a central treatment facility 

similar to other point sources of pollution, such as domestic and industrial wastewater 

discharges. The DEP recently accepted delegation from EPA for the stormwater part of the 

NPDES program. It should be noted that most MS4 permits issued in Florida include a reopener clause that allows permit revisions to implement TMDLs once they are formally adopted 

by rule. 

43