FINAL 4/14/06 

FLORIDA DEPARTMENT OF ENVIRONMENTAL PROTECTION 

Division of Water Resource Management, Bureau of Watershed Management

CENTRAL DISTRICT • MIDDLE ST. JOHNS BASIN 

TMDL Report 

Nutrient and Unionized Ammonia 

TMDLs for Lake Jesup, 

WBIDs 2981 and 2981A 

Xueqing Gao 

April 14, 2006 
Florida Department of Environmental Protection 

ii

Executive Summary 

This report presents the Total Maximum Daily Load (TMDL) for nutrients and unionized 

ammonia for Lake Jesup (including Lake Jesup outlet), in the Middle St. Johns Basin. The lake 

was verified as impaired for nutrients and unionized ammonia due to elevated annual average 

Trophic State Index (TSI) values and exceedances of the unionized ammonia criterion (0.02 

milligrams per liter [mg/L]), and was included on the Verified List of impaired waters for the 

Middle St. Johns Basin that was adopted by Secretarial Order on May 27, 2004. According to 

the 1999 Florida Watershed Restoration Act (FWRA), Chapter 99-223, Laws of Florida, once a 

waterbody is included on the Verified List, a TMDL must be developed. The purpose of the 

TMDL is to establish the allowable loadings of pollutants to Lake Jesup that would restore the 

waterbody so that it meets its applicable water quality criteria for nutrients and unionized 

ammonia. 

The nutrient TMDL for Lake Jesup was developed through four major steps: 

1. Establish the TSI target 

2. Model the existing nutrient load 

3. Model the load that would achieve the TSI target (the TMDL) 

4. Estimate the percent load reduction needed to meet the TMDL. 

To establish the TSI target, the Department first estimated the background TSI for the lake by 

comparing results from several methods, including a historic sedimentation rate method, 

literature published values, a TSI defined by the Florida Impaired Waters Rule, an Ecoregion 

approach, a hydrogeomorphologic method, and a model simulated background condition. 

Results from these different methods all converged on a TSI of 60. To allow some assimilative 

capacity above background, the Department added 5 TSI units to the estimated background TSI 

and defined 65 as the target TSI for Lake Jesup. 

Nutrient loads from the watershed were simulated using a watershed pollutant loading model 

developed by PBS&J. This model is based on the Soil Conservation Service curve number 

approach, which takes into consideration the landuse, soil, and antecedent moisture condition of 

the soil in simulating the watershed loads. Nutrient loads from other sources, including 

groundwater input through baseflow and Artesian flow, loading from septic tanks, atmospheric 

deposition directly on to the lake surface, and nitrogen fixation were also considered. 

Estimates of nutrient loads from all these sources were entered into the Bathtub model to 

estimate in-lake TN, TP, and chlorophyll a concentrations, and model calibration was conducted 

through fitting model simulated concentrations with measured results. The watershed nutrient 

loads that resulted in existing TN, TP, and chlorophyll a concentrations were considered the 

existing nutrient loads. 

To estimate the nutrient TMDL, nutrient loads from different sources were adjusted using the 

calibrated PBS&J-Bathtub model suite until the target TSI was achieved. The nutrient loads that 

resulted in the target TSI were considered the TMDL. The percent load reduction required to 

achieve the TMDL was then calculated by dividing the difference between the existing load and 

the TMDL by the existing load. The following table provides the TN and TP TMDLs and 
Florida Department of Environmental Protection 

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required percent load reductions to achieve the TMDLs. As there are no major wastewater 

facilities discharging to surface waters in the Lake Jesup watershed, no wasteload allocation for 

conventional point source (WLANPDES wastewater ) was estimated. 

TMDL components for Lake Jesup 

WBID Parameter TMDL

(kg/year)* WLANPDES Stormwater LA MOS

2981 (including 2981A) TN 247,300 50% 50% Implicit 

2981 (including 2981A) TP 19,000 34% 34% Implicit 

* = Kilograms per year 

Un-ionized ammonia TMDL was not directly addressed in this TMDL report. Based on the 

observed relationship between pH and chlorophyll a concentrations, and the relationship 

between pH and un-ionized ammonia concentrations, the lake un-ionized ammonia 

concentration should meet the water quality criterion of 0.02 mg/L, once the target TSI of the 

lake is achieved. 
TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Acknowledgments 

This analysis could not have been accomplished without substantial support and assistance 

from the Seminole County Public Works Department and PBS&J. The county has very actively 

assisted the Florida Department of Environmental Protection (Department) in obtaining the 

needed information for total maximum daily load (TMDL) development and in providing valuable 

comments and suggestions. PBS&J, an environmental consulting company hired by Seminole 

County, played an indispensable role in coordinating information collection and assembly for the 

Department, providing the watershed model, and actively updating the model. The Department 

recognizes and appreciates the help received from Kim Ornberg, Mark Flomerfel, Gloria Eby, 

and Donald E. McKenna from Seminole County; and Joe Walter and his staff from PBS&J. 

The Department acknowledges the significant input of the St. Johns River Water Management 

District, especially the contributions of Troy Keller and Yanbing Jia. Both provided valuable 

suggestions on the modeling approach and constantly exchanged information with the 

Department on their efforts in developing the pollutant load reduction goal (PLRG) for Lake 

Jesup. The studies conducted by Troy Keller on potential water quality targets for the PLRG 

provided an important basis for the Department to develop its water quality targets for the 

TMDL. 

The Department would like to thank the Orange County Public Works Department and the cities 

of Orlando, Winter Park, Maitland, Altamonte Springs, Casselberry, Winter Springs, Oviedo, and 

Lake Mary. All these agencies provided valuable information to help the Department better 

understand the Lake Jesup watershed, and also provided a significant amount of data to 

support the modeling efforts. 

Our appreciation goes to the Friends of Lake Jesup (Robert King and his colleagues). The 

concerns raised by this environmental group helped the Department to consider various issues 

related to TMDL development and the management of the overall watershed and lake. The 

Department also thanks Doug Dycus (PBS&J) and Steve Lienhart (URS Corporation) for their 

valuable input and suggestions. 

Finally, this TMDL could not have been completed without close cooperation among 

Department staff. Mary Paulic did a great job of organizing local stakeholder meetings. 

Barbara Donner provided assistance in basin delineation and estimated the forest/wetlands ratio 

for background condition. Rick Hicks and Haizhi Chen provided data for ground water quality. 

Douglas Gilbert, Wayne Magley, Jan Mandrup-Poulsen, Fred Calder, and Paul Lee all provided 

valuable suggestions and comments regarding water quality modeling, ground water simulation, 

water management policy and regulation, and public involvement in TMDL development and 

implementation. 

Editorial assistance provided by 

Daryll Joyner, Jan Mandrup-Poulsen, and Linda Lord 

For additional information on the watershed management approach and impaired waters 

in the Middle St. Johns Basin, contact 

Xueqing Gao 

Florida Department of Environmental Protection 

Bureau of Watershed Management 

iv 
TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Watershed Planning and Coordination Section 

2600 Blair Stone Road, Mail Station 3565 

Tallahassee, FL 32399-2400 

Email: xueqing.gao@dep.state.fl.us

Phone: (850) 245-8464, SunCom: 205-8464 

Fax: (850) 245-8434 

Access to all data used in the development of this report can be obtained by contacting 

Xueqing Gao 

Florida Department of Environmental Protection 

Bureau of Watershed Management 

Watershed Planning and Coordination Section 

2600 Blair Stone Road, Mail Station 3565 

Tallahassee, FL 32399-2400 

Email: xueqing.gao@dep.state.fl.us

Phone: (850) 245-8464, SunCom: 205-8464 

Fax: (850) 245-8434 

Florida Department of Environmental Protection 

v
TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Contents 

Chapter 1: INTRODUCTION _____________________________________________ 1

1.1 Purpose of Report ________________________________________________ 1

1.2 Identification of Waterbody ________________________________________ 1

1.3 Background ______________________________________________________ 3

Chapter 2: DESCRIPTION OF WATER QUALITY PROBLEM __________________ 5

2.1 Statutory Requirements and Rulemaking History ______________________ 5

2.2 Information on Verified Impairment __________________________________ 5

Chapter 3. DESCRIPTION OF APPLICABLE WATER QUALITY STANDARDS 

AND TARGETS ______________________________________________ 7

3.1 Classification of the Waterbody and Criteria Applicable to the TMDL ______ 7

3.2 Applicable Water Quality Standards and Numeric Water Quality Target ___ 7

3.2.1 Unionized Ammonia Criterion _____________________________________ 7

3.2.2 Interpretation of Narrative Nutrient Criterion __________________________ 7

Chapter 4: ASSESSMENT OF SOURCES _________________________________ 15

4.1 Types of Sources ________________________________________________ 15

4.2 Potential Sources of Nutrients in the Lake Jesup Watershed ___________ 15

4.2.1 Point Sources ________________________________________________ 15

4.2.2 Nonpoint Sources _____________________________________________ 16

Chapter 5: DETERMINATION OF ASSIMILATIVE CAPACITY _________________ 51

5.1 Overall Approach ________________________________________________ 51

5.1.1 Entering Loading Estimates from All Sources into the Bathtub 

Eutrophication Model __________________________________________ 51

Chapter 6: DETERMINATION OF THE TMDL ______________________________ 84

6.1 Expression and Allocation of the TMDL _____________________________ 84

6.2 Load Allocation _________________________________________________ 85

6.3 Wasteload Allocation ____________________________________________ 85

6.3.1 National Pollutant Discharge Elimination System Wastewater 

Discharges __________________________________________________ 85

6.3.2 National Pollutant Discharge Elimination System Stormwater 

Discharges __________________________________________________ 85

6.4 Margin of Safety _________________________________________________ 86

6.5 Recommendations for Further Studies ______________________________ 86

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

6.5.1 How Sediment Dredging Will Influence Nutrient Loading into Lake 

Jesup ______________________________________________________ 86

6.5.2 Nitrogen Fixation ______________________________________________ 87

Chapter 7: NEXT STEPS: IMPLEMENTATION PLAN DEVELOPMENT AND 

BEYOND __________________________________________________ 89

7.1 Basin Management Action Plan ____________________________________ 89

References__________________________________________________________ 90

Appendices _________________________________________________________ 95

Appendix A: Background Information on Federal and State Stormwater 

Programs __________________________________________________________ 95

Appendix B: Vertical Distribution of DO Concentrations in Lake Jesup (mg/L), 

1995–1996. Data provided by the SJRWMD. _____________________________ 97

List of Tables 

Table 2.1. Verified impaired waterbody segments in the Lake Jesup 

watershed __________________________________________ 5

Table 2.2. Summary of annual TSI values for Lake Jesup in the verified 

period, 1996–2002 (WBIDs 2981 and 2981A) _______________ 6

Table 3.1. Summary of different SJRWMD approaches to determine 

Lake Jesup’s water quality target concentrations for TP and 

TN ________________________________________________ 8

Table 3.2. Summary of phosphorus/nitrogen concentrations 

(micrograms per liter [μg/L]) suggested as potential criteria 

for five different lake classes in Florida ___________________ 10

Table 3.3. Estimated percent of the time that chlorophyll concentrations 

will exceed the listed concentrations in lakes with a TN/TP 

ratio between 10 and 17 ______________________________ 12

Table 3.4. Summary of the SJRWMD’s Lake Jesup water quality 

targets kept for further analysis by the Department __________ 13

Table 4.1. Classification of land use categories in the Lake Jesup 

watershed _________________________________________ 18

Table 4.2. Classifications of AMCs _______________________________ 22

Table 4.3. Equivalent curve numbers under different AMC conditions ____ 22

Table 4.4. Curve numbers for different land use–hydrologic soil group 

combinations under AMC II ____________________________ 24

Table 4.5. Annual runoff coefficients for different land use–hydrologic 

soil group combinations, 1995–2002 _____________________ 25

Table 4.6. Annual precipitation at the city of Sanford weather station, 

1995–2002 _________________________________________ 27

Table 4.7. Areas of different land use–hydrologic soil group 

combinations in the Lake Jesup watershed ________________ 28

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 4.8. TN and TP EMCs for different land use categories (mg/L) ____ 30

Table 4.9. Areas covered by various BMPs in the Lake Jesup 

watershed _________________________________________ 31

Table 4.10. Pollutant and water removal efficiency for different BMPs _____ 31

Table 4.11-a. Annual surface runoff into Lake Jesup, 1995–2002 (acrefeet/year) __________________________________________ 37

Table 4.11-b. Annual TN loading into Lake Jesup, 1995–2002 (tons/year) ___ 37

Table 4.11-c. Annual TP loading into Lake Jesup, 1995–2002 (tons/year) ___ 37

Table 4.12-a. Annual surface runoff into Lake Jesup from different land 

use categories, 1995–2002 (acre-feet/year) _______________ 39

Table 4.12-b. Annual TP loading into Lake Jesup from different land use 

categories, 1995–2002 (tons/year) ______________________ 39

Table 4.12-c. Annual TN loading into Lake Jesup from different land use 

categories, 1995–2002 (tons/year) ______________________ 40

Table 4.13. Annual nutrient loadings into Lake Jesup through baseflow, 

1995–2002 _________________________________________ 40

Table 4.14. Population density for single-family residential and 

commercial areas in different planning areas of the Lake 

Jesup watershed (people/acre) _________________________ 41

Table 4.15. Nutrient load for household wastewater discharge into septic 

tanks (kilograms/capita/year) (cited from Reckhow, 1980) ____ 43

Table 4.16. Characteristic curve for Lake Jesup ______________________ 45

Table 4.17. Annual average lake stage and surface area, and annual 

flow from the Floridan aquifer to Lake Jesup, 1995–2002 _____ 47

Table 4.18. Annual TN and TP loadings from the Floridan aquifer into 

Lake Jesup, 1995–2002 (tons/year) _____________________ 47

Table 4.19. Annual TN and TP loadings through atmospheric deposition 

directly onto the surface of Lake Jesup, 1995–2002 

(tons/year) _________________________________________ 48

Table 4.20. Annual flow and TN and TP loadings from the St. Johns 

River into Lake Jesup, 1995–2002 ______________________ 48

Table 4.21-a. Annual flow from various sources into Lake Jesup, 1995–

2002 (acre-feet/year) _________________________________ 49

Table 4.21-b. Annual TN loading from various sources into Lake Jesup, 

1995–2002 (tons/year) ________________________________ 49

Table 4.21-c. Annual TP loading from various sources into Lake Jesup, 

1995–2002 (tons/year) ________________________________ 50

Table 5.1. Water quality stations from which TN, TP, and chl a 

concentrations were measured _________________________ 56

Table 5.2. Annual averages of TN, TP, chl a, and TSI values of Lake 

Jesup, 1995–2002. Data represent the mean ± 1 standard 

deviation (n = 4) _____________________________________ 57

Table 5.3. Seasonal variation of TN, TP, chl a, and TSI in Lake Jesup ___ 57

Table 5.4. Annual lake characteristics of Lake Jesup for the modeling 

period, 1995–2002 ___________________________________ 61

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 5.5. Annual meteorological data used for Bathtub modeling, 

1995–2002 _________________________________________ 62

Table 5.6. Annual areal atmospheric nutrient loadings to Lake Jesup, 

1995–2002 _________________________________________ 63

Table 5.7-a. Annual flow from major nonpoint sources into Lake Jesup, 

1995–2002 (cubic hectometers [HM3]/year) _______________ 63

Table 5.7-b. Annual TN concentration of flows from major nonpoint 

sources into Lake Jesup, 1995–2002 (parts per billion [ppb]) __ 64

Table 5.7c. Annual TP concentration of flows from major nonpoint 

sources into Lake Jesup, 1995–2002 (ppb) ________________ 64

Table 5.8. Long-term average TN concentrations for waterbodies 

discharging into Lake Jesup, calculated based on measured 

TN concentrations ___________________________________ 67

Table 5.9. Long-term average TP concentrations for waterbodies 

discharging into Lake Jesup, calculated based on measured 

TN concentrations ___________________________________ 68

Table 5.10. Annual DIN/DIP ratio of Lake Jesup, 1995–2002 (micro 

molar [μM]). DIN is the sum of NH4-N and NO3-N. DIP is 

primarily soluble reactive phosphorus (SRP). ______________ 72

Table 5.11. Annual nitrogen fixation rate used to match model-simulated 

TN concentrations to observed TN concentrations, 1995–

2002 (mg/m2

/day) ____________________________________ 72

Table 5.12. Nitrogen fixation rates and nutrient concentrations in Lake 

George, July and October 2000, and March and August 

2002 ______________________________________________ 73

Table 5.13-a. Annual TN loading from various sources into Lake Jesup, 

1995–2002 (tons/year) ________________________________ 74

Table 5.13 -b. Annual TP loading from various sources into Lake Jesup, 

1995–2002 (tons/year) ________________________________ 74

Table 5.14. Annual background TP, TN, and chl a concentrations and 

TSI for Lake Jesup, 1995–2002 _________________________ 78

Table 5.15-a. Annual TN background loading from various sources into 

Lake Jesup, 1995–2002 (tons/year) _____________________ 78

Table 5.15-b. Annual TP background loading from various sources into 

Lake Jesup, 1995–2002 (tons/year) _____________________ 79

Table 5.16. Annual water quality conditions that achieve the target TSI, 

1995–2002 _________________________________________ 80

Table 5.17-a. Annual TN loading from various sources into Lake Jesup, 

1995–2002 (tons/year) ________________________________ 80

Table 5.17-b. Annual TP loading from various sources into Lake Jesup, 

1995–2002 (tons/year) ________________________________ 81

Table 5.18-a. Annual TN load reductions required to achieve the water 

quality target for Lake Jesup, 1995–2002 _________________ 81

Table 5.18-b. Annual TP load reductions required to achieve the water 

quality target for Lake Jesup, 1995–2002 _________________ 81

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 6.1. TMDL components for Lake Jesup ______________________ 85

List of Figures 

Figure 1.1. Locations of Lake Jesup and major cities in the Lake Jesup 

watershed ________________________________________________ 2

Figure 1.2. Relative sizes of WBIDs 2981 and 2981A ________________________ 4

Figure 4.1. Principal land uses in the Lake Jesup watershed _________________ 17

Figure 4.2. Spatial distribution of hydrologic soil groups in the Lake 

Jesup watershed __________________________________________ 20

Figure 4.3. Spatial distribution of BMPs in the Lake Jesup watershed __________ 32

Figure 4.4. Locations of Department background ground water quality 

monitoring stations and a USGS gauging station _________________ 34

Figure 4.5. Results of the baseflow analysis, 1988–2003 ____________________ 35

Figure 4.6. Sub-basins of the Lake Jesup watershed _______________________ 36

Figure 4.7. Areas with septic tanks, areas within 200 meters of any 

receiving waters that discharge to Lake Jesup, and planning 

areas ___________________________________________________ 42

Figure 4.8. Characteristic curve between lake stage and lake surface 

area for Lake Jesup ________________________________________ 46

Figure 4.9. Characteristic curve between lake stage and lake cumulative 

volume for Lake Jesup ______________________________________ 46

Figure 5.1. Bathtub concept scheme ____________________________________ 52

Figure 5.2. Locations of water quality stations in Lake Jesup _________________ 55

Figures 5.3-a, 5.3-b, 5.3-c. Relationship between annual rainfall; 

concentrations of TN, TP, and chl a; and TSI in Lake Jesup, 

1995–2002 60

Figures 5.4-a, 5.4-b, 5.4-c. Results for flow and TN and TP calibrations, 

using the Bathtub model, 1995–2002 __________________________ 66

Figure 5.5. Annual average DO concentration in Lake Jesup, 1995–

2002 ____________________________________________________ 69

Figure 5.6. Dynamics of ammonia concentration in Lake Jesup, 1995–

2002 ____________________________________________________ 71

Figure 5.7. Correlation between nitrogen fixation rate and chl a 

concentration _____________________________________________ 75

Figure 5.8. Relationship between unionized ammonia and pH for Lake 

Jesup ___________________________________________________ 83

Figure 5.9. Relationship between pH and chl a concentration for Lake 

Jesup ___________________________________________________ 83

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Florida Department of Environmental Protection 

xi

Web sites 

Florida Department of Environmental Protection, Bureau of 

Watershed Management 

Total Maximum Daily Load (TMDL) Program 

http://www.dep.state.fl.us/water/tmdl/index.htm

Identification of Impaired Surface Waters Rule 

http://www.dep.state.fl.us/water/tmdl/docs/AmendedIWR.pdf

STORET Program 

http://www.dep.state.fl.us/water/storet/index.htm

2004 305(b) Report 

http://www.dep.state.fl.us/water/docs/2004_Integrated_Report.pdf

Criteria for Surface Water Quality Classifications 

http://www.dep.state.fl.us/water/wqssp/classes.htm

Basin Status Reports 

http://www.dep.state.fl.us/water/tmdl/stat_rep.htm

Water Quality Assessment Reports 

http://www.dep.state.fl.us/water/tmdl/stat_rep.htm

Allocation Technical Advisory Committee (ATAC) Report 

http://www.dep.state.fl.us/water/tmdl/docs/Allocation.pdf

U.S. Environmental Protection Agency 

Region 4: Total Maximum Daily Loads in Florida 

http://www.epa.gov/region4/water/tmdl/florida/

National STORET Program 

http://www.epa.gov/storet/

Chapter 1: INTRODUCTION 

1.1 Purpose of Report 

This report presents the Total Maximum Daily Load (TMDL) for nutrients and unionized 

ammonia for Lake Jesup, in the Middle St. Johns Basin. The lake was verified as impaired for 

nutrients and unionized ammonia due to elevated annual average Trophic State Index (TSI) 

values and exceedances of the unionized ammonia criterion (0.02 milligrams per liter [mg/L]), 

and was included on the Verified List of impaired waters for the Middle St. Johns Basin that was 

adopted by Secretarial Order on May 27, 2004. According to the 1999 Florida Watershed 

Restoration Act (FWRA), Chapter 99-223, Laws of Florida, once a waterbody is included on the 

Verified List, a TMDL must be developed. The purpose of the TMDL is to establish the 

allowable loadings of pollutants to Lake Jesup that would restore the waterbody so that it meets 

its applicable water quality criteria for nutrients and unionized ammonia. 

1.2 Identification of Waterbody 

Lake Jesup, located in central Florida (Figure 1.1), has a surface area of about 10,660 acres 

(16.7 square miles [mi2

]) and drains a watershed of about 87,331 acres (136.5 mi2) to the St. 

Johns River on the northeast side of the Middle St. Johns Basin. The majority of the watershed 

lies within Seminole County, but a small portion on the southwest end extends into Orange 

County. 

The lake is low-lying, with an average stage of about 1.86 feet National Geodetic Vertical Datum 

(NGVD). Lake elevations tend to follow the water surface elevations of the St. Johns River at its 

confluence with Lake Jesup. When local rainfall is lower than regional rainfall (particularly to the 

south), the river rises, and water flows from the St. Johns River into the lake (Keesecker, 1992). 

Surface runoff discharges into Lake Jesup primarily through three tributaries—Howell Creek, 

Gee Creek, and Soldier Creek—that are located to the south and southwest of the lake. The 

mean hydraulic residence time for lake waters has been estimated variously at approximately 

99 days (Brezonik and Fox, 1976), 82 days (U. S. Environmental Protection Agency [EPA], 

1977), and 87 days (Keesecker, 1992). 

The watershed occupies a highly urbanized area. Eleven municipalities are located within or 

associated with the watershed: Sanford and Lake Mary on the northwest end, and Oviedo, 

Winter Springs, Longwood, Casselberry, Altamonte Springs, Maitland, Winter Park, Eatonville, 

and Orlando in the southern part. According to 2003 data from the U. S. Census Bureau, the 

population densities in Seminole and Orange Counties were 1,184.9 and 987.8 persons/mi2

, 

respectively, which were significantly higher than the state average of 296.4 persons/mi2

. The 

area is also undergoing rapid population growth. From 1990 through 2000, the population of 

Seminole and Orange Counties increased by 27.0% and 32.3%, respectively, compared with a 

23.5% average increase for the state. 
TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Figure 1.1. Locations of Lake Jesup and major cities in the Lake Jesup 

watershed 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Both Seminole and Orange Counties, and all the other municipalities in the Lake Jesup 

watershed, are National Pollutant Discharge Elimination System (NPDES) Phase I municipal 

separate storm sewer system (MS4) permittees, and will be potentially affected by this TMDL. 

For assessment purposes, the Department has divided the Middle St. Johns Basin into water 

assessment polygons with a unique waterbody identification (WBID) number for each watershed 

or stream reach. Lake Jesup includes two WBIDs on the Middle St. Johns Verified List of 

impaired waters, WBIDs 2981 and 2981A, which are addressed by this TMDL. WBID 2981 

represents the majority of the lake. A very small portion of the lake directly connecting to the St. 

Johns River is designated as WBID 2981A. Figure 1.2 shows the relative sizes of both WBIDs. 

As WBID 2981A is very small compared with the size of the whole lake, the TMDL development 

effort mainly focuses on the lake, under the assumption that once the eutrophication problem for 

the lake is solved, the water quality problem of the outlet will also be addressed. 

1.3 Background 

This report was developed as part of the Department’s watershed management approach for 

restoring and protecting state waters and addressing TMDL Program requirements. The 

watershed approach, which is implemented using a cyclical management process that rotates 

through the state’s 52 river basins over a 5-year cycle, provides a framework for implementing 

the TMDL Program–related requirements of the 1972 federal Clean Water Act and the FWRA. 

A TMDL represents the maximum amount of a given pollutant that a waterbody can assimilate 

and still meet water quality standards, including its applicable water quality criteria and its 

designated uses. TMDLs are developed for waterbodies that are verified as not meeting their 

water quality standards, and provide important water quality restoration goals that will guide 

restoration activities. 

This TMDL report will be followed by the development and implementation of a Basin 

Management Action Plan, or BMAP, to reduce the amount of nutrients that caused the verified 

impairment of Lake Jesup. These activities will depend heavily on the active participation of the 

St. Johns River Water Management District (SJRWMD), local governments, businesses, and 

other stakeholders. The Department will work with these organizations and individuals to 

undertake or continue reductions in the discharge of pollutants and achieve the established 

TMDLs for impaired waterbodies. 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

 

Figure 1.2. Relative sizes of WBIDs 2981 and 2981A 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Chapter 2: DESCRIPTION OF WATER QUALITY 

PROBLEM 

2.1 Statutory Requirements and Rulemaking History 

Section 303(d) of the Clean Water Act requires states to submit to the EPA a list of surface 

waters that do not meet applicable water quality standards (impaired waters) and establish a 

TMDL for each pollutant source in each of these impaired waters on a schedule. The 

Department has developed these lists, commonly referred to as 303(d) lists, since 1992. The 

list of impaired waters in each basin is also required by the FWRA (Subsection 403.067[4)] 

Florida Statutes [F.S.]), and the list is amended annually to include updates for each basin 

statewide. 

Florida’s 1998 303(d) list included 22 waterbodies in the Middle St. Johns Basin. However, the 

FWRA (Section 403.067, F.S.) stated that all previous Florida 303(d) lists were for planning 

purposes only and directed the Department to develop, and adopt by rule, a new science-based 

methodology to identify impaired waters. After a long rulemaking process, the Environmental 

Regulation Commission adopted the new methodology as Chapter 62-303, Florida 

Administrative Code (F.A.C.) (Identification of Impaired Surface Waters Rule, or IWR), in April 

2001. The list of waters for which impairments have been verified using the methodology in the 

IWR is referred to as the Verified List. 

2.2 Information on Verified Impairment 

The Department used the IWR to assess water quality impairments in the Middle St. Johns 

Basin and verified impairments for Lake Jesup (Table 2.1). The lake was verified as impaired 

for nutrients based on the fact that, in the verified period (January 1, 1996, through June 30, 

2003), annual average TSI values exceeded 60 every year (Table 2.2). Based on the long-term 

median total phosphorus/total nitrogen (TN/TP) ratio of 17 (weight ratio), Lake Jesup 

phytoplankton communities were considered co-limited by phosphorus and nitrogen. The 

impairment for unionized ammonia was based on the fact that 27 out of 154 observations of 

unionized ammonia exceeded the water quality criterion (0.02 milligrams per liter [mg/L]). 

Table 2.1. Verified impaired waterbody segments in the Lake Jesup 

watershed 

WBID Waterbody Segment Parameters of Concern 

2981 Lake Jesup Nutrients 

2981 Lake Jesup Unionized Ammonia 

2981A Lake Jesup near St. Johns River Nutrients 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 2.2. Summary of annual TSI values for Lake Jesup in the 

verified period, 1996–2002 (WBIDs 2981 and 2981A) 

Year 

WBID 2981 WBID 2981A 

Mean Color 

(PCUs*) TSI Mean Color (PCUs*) TSI 

1996 113 76 127 69 

1997 61 74 97 68 

1998 58 77 71 68 

1999 123 81 132 74 

2000 65 83 69 - 

2001 70 84 - - 

2002 92 74 - - 

* = Platinum cobalt units 

- = Not enough data to calculate annual mean TSI 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Chapter 3. DESCRIPTION OF APPLICABLE WATER 

QUALITY STANDARDS AND TARGETS 

3.1 Classification of the Waterbody and Criteria Applicable to the TMDL 

Florida’s surface waters are protected for five designated use classifications, as follows: 

Class I Potable water supplies 

Class II Shellfish propagation or harvesting 

Class III Recreation, propagation, and maintenance of a healthy, wellbalanced population of fish and wildlife 

Class IV Agricultural water supplies 

Class V Navigation, utility, and industrial use (there are no state 

waters currently in this class) 

Lake Jesup is a Class III waterbody, 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 impairment addressed by this TMDL are unionized 

ammonia and nutrients. 

3.2 Applicable Water Quality Standards and Numeric Water Quality Target 

3.2.1 Unionized Ammonia Criterion 

The Class III water quality criterion for unionized ammonia is 0.02 mg/L. 

3.2.2 Interpretation of Narrative Nutrient Criterion 

Florida’s nutrient criterion is narrative only—i.e., 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. 

Accordingly, a nutrient-related target was needed to represent levels at which an imbalance in 

flora or fauna is expected to occur. While the IWR provides a threshold for nutrient impairment 

for lakes based on annual average TSI levels, the threshold is not a standard and need not be 

used as the nutrient-related water quality target for TMDLs. In fact, in recognition that the IWR 

threshold was developed using statewide average conditions, the IWR (Subsection 62-303.450, 

F.A.C.) specifically allows the use of alternative, site-specific thresholds that more accurately 

reflect conditions beyond which an imbalance in flora or fauna occurs in a waterbody. 

For this analysis, the Department established the TSI target by comparing the target TN and TP 

concentrations proposed by the SJRWMD (Table 3.1) with the target TN and TP concentrations 

estimated by the Department using watershed and water quality models. The SJRWMD used 

three major approaches to develop the target TN and TP concentrations. These include (1) 

examining Lake Jesup phytoplankton and zooplankton abundance and diversity against TN and 

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TP concentrations, analyzing historical sedimentation rates, and using water clarity targets 

defined by the lake area covered by submerged aquatic vegetation (SAV) to derive the TN and 

TP concentrations; (2) using literature methods to define the target concentration; and (3) using 

the ecoregion method or hydrogeomorphologic information to define the water quality targets. 

Table 3.1. Summary of different SJRWMD approaches to determine 

Lake Jesup’s water quality target concentrations for TP 

and TN 

Approach Period 

Nutrient Target Concentration 

(mg/L) Notes 

TP TN 

Lake-specific Approach 

Chlorophyll a (chl a) 2/95–7/02 - 2.0 Chl a uncorrected 

Zooplankton abundance 1/01–2/03 - 2.4 Rotifer abundance 

25% SAV coverage 2/01–7/03 0.044 0.61 Estimated from 

compensation depth 

Historical sedimentation rates Pre-1920 and 

1985–96 0.076 - Estimated using sediment cores and models 

Literature Reference Approach 

Literature 1993–97 0.040 1.1 Paul and Gerritsen, 2002 

Chl a bloom frequency 1989–2000 0.050 0.70 Bachmann et al., 2003 

Modified from IWR Pre-1980 0.070 1.2 Huber et al., 1982 

Florida Lake Comparisons 

Ecoregion 1980-1990 0.074 1.5 Hendrickson, unpublished 

Hydrogeomorphologic Pre-1980 0.041 0.80 Huber et al., 1982 

PLRG target 0.050 1.2 

- = Inconclusive in defining target concentrations 

Based on input from the SJRWMD (T. Keller, SJRWMD, personal communication), the major 

purpose of examining the abundance and biodiversity of phytoplankton and zooplankton 

communities against TN and TP concentrations in Lake Jesup was to find out whether there are 

threshold TN and TP concentrations above which dramatic changes in abundance or 

community diversity were observed. For the period of record examined, however, the TP 

concentration stayed constantly high, thus providing no hint of abundance and diversity under 

low TP concentrations. Therefore, no threshold could be identified for phosphorus using this 

method. 

Apparent changes of abundance for both phytoplankton (chl a concentration) and zooplankton 

(mainly rotifer abundance) were observed at TN concentrations of 2.0 and 2.4 mg/L, 

respectively (Table 3.1). Because of the possible existence of significant nitrogen fixation, 

however, the Department believes that it is difficult to determine whether nitrogen is the cause 

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of the change in the community structure, or the consequence of the change in the community 

structure. Therefore, the Department deemed inconclusive the target TN concentrations of 2.0 

or 2.4 mg/L for phytoplankton and zooplankton, respectively. 

Water quality targets were also studied by examining the relationship between TN and TP 

concentrations and water clarity (T. Keller, SJRWMD, personal communication). The major 

purpose of this study was to find out what TN and TP concentrations would provide sufficient 

water clarity to facilitate the growth of SAV over 25% of Lake Jesup’s benthos. The use of this 

method was stimulated by the Florida Fish and Wildlife Conservation Commission’s (FWCC) 

SAV goals for Lake Jesup. The 25% SAV coverage was thought to enhance fisheries and 

provide habitat for wildlife. SAV may also reduce the resuspension of flocculent organic 

sediments. 

Water clarity in this case is measured using compensation depth, which is considered to be the 

water depth at which the incoming photosynthetically active radiation (PAR) decreases to about 

5% of its original intensity (the definition of compensation depth adopted by the Department is 

1% of the incoming radiation). In this analysis, compensation depth could be determined using 

the characteristic curve of the lake and 25% area coverage. The target TN and TP 

concentrations could then be determined using regression curves between compensation depth 

and TN and TP concentrations. As shown in Table 3.1, TN and TP targets established using 

this method are 0.61 and 0.044 mg/L, respectively. 

Based on the SJRWMD, however, the TP target was established using a regression curve with 

a low regression coefficient (R2

 = 0.2). The Department believes that the TN target could once 

again be influenced by nitrogen coming from nitrogen fixation, which, by itself, is the 

consequence instead of the cause of the eutrophication. In addition, TN and TP targets 

established using this technique are tied to the usage of the lake, instead of the lake’s natural 

assimilative capacity. Therefore, the Department gave lesser weight to the TN and TP targets 

developed using the water clarity technique. 

The SJRWMD also used lake TN and TP concentrations prior to rapid development in the Lake 

Jesup watershed to infer the target concentrations. This technique assumed that the TN and 

TP sedimentation rates are proportional to the TN and TP concentrations in the water column. 

Therefore, if historical and current sedimentation rates and current lake TN and TP 

concentrations are known, assuming that the specific sedimentation rates for TN and TP are the 

same for current and historical conditions, TN and TP concentrations prior to the rapid 

development could be determined. Using this technique, the SJRWMD proposed a target TP 

concentration of about 0.076 mg/L (Table 3.1). No TN target was proposed using this 

technique, because possible denitrification in the sediment may lead to an underestimation of 

the actual TN sedimentation rate. 

TN and TP targets for Lake Jesup could also be derived from a study conducted by Tetra Tech, 

Inc., based on data collected from 200 Florida lakes between 1993 and 1997 (Paul and 

Gerritsen, 2002). A variety of exploratory analyses of these data suggested that the strongest 

organizing forces on the biota of the relatively undisturbed lakes were water color and pH 

(Gerritsen et al., 2000). On the basis of these results, the sampled lake regions were 

aggregated into 5 lake biological classes, such that the lakes within each class have similar 

biological assemblages. The lake classes were divided based on water color (greater than or 

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less than 20 PCUs, pH (greater than or less than 6.5), and ecoregion for acid clear lakes only 

(Omernik, 1987: Region 65 in northwest Florida and Region 75 in peninsular Florida). 

Several techniques were used in each lake class to establish TN and TP target concentrations. 

These included the reference lake technique, sediment diatom reconstructions, morphoedaphic 

indices, LOESS regression of lake trophic condition index (tLCI) versus nutrients, and multiple 

linear regression (Paul and Gerritsen, 2002). Among all the techniques used, the reference lake 

technique, LOESS regression of tLCI versus nutrients, and diatom reconstruction based on 

paleolimnological data provided meaningful results (Table 3.2).

Table 3.2. Summary of phosphorus/nitrogen concentrations 

(micrograms per liter [μg/L]) suggested as potential 

criteria for five different lake classes in Florida 

Lake Class 

Methodological Approach 

75th Percentile of 

Reference Distribution 

LOESS Regression 

(tLCI versus Nutrients) 

Paleolimnology 

(TROPH1 model) 

Acid clear lakes 

 EcoRegion 65 10/330 21/473 4*/NA 

 EcoRegion 75 10/470 23/776 67*/NA 

Acid colored lakes 42/910 43/1202 17*/NA 

Alkaline clear lakes 10/750 17/692 25/NA 

Alkaline colored lakes 73/1110 40/1148 32/NA 

* = N < 6 

NA = Not applicable 

Since the long-term average color of Lake Jesup is greater than 20 PCUs and the long-term 

average pH is greater than 6.5, the lake is considered an alkaline colored lake. Based on a 

study by the SJRWMD, the diatom reconstruction technique failed to predict accurately the 

current phosphorus concentration in Lake Jesup (T. Keller, SJRWMD, personal 

communication). Therefore, the Department did not use the result from this technique. While 

the reference lake technique and LOESS regression produced relatively similar results for TN, 

about 1,100 mg/L for alkaline colored lakes, the TP results from these 2 techniques appeared to 

be different (Table 3.2). Paul and Gerritsen (2002) believed that the difference could be caused 

by the different responses of different communities to the same nutrient concentration. These 

authors suggested that 40 μg/L of TP should be used as the target concentration, just to be 

more conservative and to protect downstream waters 

In contrast, the Department believes that as the LOESS regression relies heavily on the 

biological response to the nutrient concentration, and many factors other than nutrient 

concentration per se can influence the response, the results from LOESS are not as easily 

interpretable as the results from the reference lake technique. A large standard deviation is 

indeed associated with the regression curve between tLCI and TP concentration in Paul and 

Gerritsen (2002). The Department therefore believes that target TN and TP concentrations from 

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the reference lake technique, which are 1.100 and 0.073 mg/L, respectively, are more direct 

results and should be used for Lake Jesup. 

Potential TN and TP targets were also derived based on a relationship developed by Bachmann 

et al. (2003) between the frequency of algal blooms and TN and TP concentrations. These 

authors analyzed 1,473 lake-years of data on 438 Florida lakes to develop a series of tables. 

These can be used to predict the frequencies that phytoplankton chlorophylls will exceed 

concentrations of 10, 20, 30, 40, 50, and 60 μg/L in Florida lakes, based on the annual average 

concentrations of chlorophyll, TP, or TN. In their studies, the authors created different tables for 

lakes grouped by TN/TP ratios of > 17, < 17 but > 10, and < 10. Since the TN/TP ratio for Lake 

Jesup appears to fall between 10 and 17 most of the time, suggesting that the lake’s 

phytoplankton community is co-limited by nitrogen and phosphorus, the target TN and TP 

concentrations for Lake Jesup were developed based on the table for lakes with the 

corresponding TN/TP ratio. 

Table 3.3 shows the frequency of algal blooms at different TN and TP concentrations. A large 

standard deviation, however, is associated with the relationship between TN and TP 

concentrations and chl a concentration and bloom frequency when TN/TP ratios fall between 10 

and 17, especially when TN and TP concentrations are low (Bachmann et al., 2003). The 

Department therefore gave lesser weight to the TN and TP targets developed using this 

method. 

The SJRWMD proposed a critical chl a threshold to define a bloom as 40 μg/L and an 

exceedance frequency of 2 months per year, which is about 17% of the bloom frequency. The 

TP and TN associated with these target numbers, based on Table 3.3, are 50 μg/L (0.05 mg/L) 

and 700 μg/L (0.7 mg/L), respectively. 

Target TN and TP concentrations could also be derived, based on the threshold TSI of 60 for 

lakes with water color higher than 40 PCUs. This TSI target is defined in the IWR (Section 62-

303.352, F.A.C.). Huber et al. (1982) developed the equation on which the target TSI is based. 

For lakes with a TN/TP ratio falling between 10 and 30, the equation is as follows: 

TSI (TN) = 10*(5.6 + 1.98*LnTN) 

TSI (TP) = 10*(1.86*LnTP – 1.84) 

The TN and TP concentrations for a TSI of 60 are 1.22 and 0.068 mg/L, respectively. 

The SJRWMD used Hendrickson’s ecoregion approach (unpublished study) to select lakes that 

are considered to be in Lake Jesup’s ecoregion. In this analysis, Lake Jesup was classified as 

a 5th-order Atlantic Coast Forest Rivers lake. To improve the sample size, the analysis also 

included some 4th-order Atlantic Coast Forest Rivers lakes (T. Keller, SJRWMD, personal 

communication). Based on this information, the target TP is 0.074 mg/L, and the target TN is 

about 1.5 mg/L. 

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Table 3.3. Estimated percent of the time that chlorophyll 

concentrations will exceed the listed concentrations in 

lakes with a TN/TP ratio between 10 and 17 

Concentrations of chlorophyll (CHL), TN, and TP are in μg/L (Bachmann et al., 2003) 

TP CHL > 10 CHL > 20 CHL > 30 CHL > 40 CHL > 50 CHL > 60 

10 19 3 0 0 0 0 

25 23 4 0 0 0 0 

35 73 32 6 0 0 0 

40 79 47 15 6 2 1 

45 85 53 21 11 6 2 

50 87 61 27 17 10 5 

55 90 67 32 22 15 7 

60 92 70 38 30 19 10 

65 94 74 44 35 25 14 

70 95 78 48 40 28 17 

75 96 81 53 46 33 20 

80 96 85 60 52 38 24 

85 96 90 68 58 44 27 

90 96 92 72 64 49 32 

95 96 95 77 68 55 38 

103 96 95 85 78 65 44 

127 96 95 86 79 69 58 

139 96 95 86 80 70 65 

157 96 95 86 81 72 68 

197 96 95 86 82 75 73 

TN CHL > 10 CHL > 20 CHL > 30 CHL > 40 CHL > 50 CHL > 60 

124 10 2 0 0 0 0 

372 32 4 1 0 0 0 

500 74 38 12 4 1 1 

600 82 48 24 10 4 2 

700 85 55 35 16 10 3 

800 89 62 41 22 15 6 

900 93 69 46 26 22 11 

1,000 95 76 50 33 28 17 

1,100 97 81 58 44 40 26 

1,200 99 87 65 53 48 30 

1,221 99 89 70 55 50 32 

1,292 100 94 81 65 56 37 

1,400 100 100 96 84 67 46 

1,600 100 100 96 89 78 59 

1,800 100 100 96 89 78 67 

2,000 100 100 96 89 78 71 

2,491 100 100 96 89 78 73 

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In developing target TN and TP concentrations for Lake Jesup, the SJRWMD used a 

hydrogeomorphic approach that was also used to establish the Lake Apopka water quality 

target. Huber et al.’s (1982) data were used to define lakes that have hydrologic and 

morphologic characteristics similar to those of Lake Jesup. The criteria used to filter this data 

set included detention time (> 0.5* and < 4* 0.22 years; Lake Jesup detention time is about 0.25 

years), lake area (> 0.2* and < 5* 10,339 acres; Lake Jesup surface area is about 10,660 

acres), mean depth (> 0.3* and < 3* 1.96 feet NGVD; Lake Jesup mean surface elevation is 

about 1.86 feet NGVD), and drainage area/lake area (>0.2* and < 5* 8.97; Lake Jesup drainage 

area/lake area ratio is about 8.19) (here, * means multiplying). The TN and TP concentrations 

derived from this method are 0.80 and 0.041 mg/L, respectively. 

Table 3.4 lists some of the potential water quality targets from the SJRWMD’s target matrix. 

The Department believes these targets can be used for further analysis. 

 Table 3.4. Summary of the SJRWMD’s Lake Jesup water quality 

targets kept for further analysis by the Department 

Approach Period 

Nutrient Target Concentration 

(mg/L) Notes 

TP TN 

Lake-Specific Approach 

Historical sedimentation rates Pre-1920 and 

1985–96 0.076 - Estimated sediment 

cores and models 

Literature Reference Approach 

Literature 1993–97 0.073 1.100 Paul and Gerritsen, 

2002 

Modified from IWR Pre-1980 0.070 1.200 Huber et al., 1981 

Florida Lake Comparisons 

Ecoregion 1980–90 0.074 1.500 Hendrickson 

unpublished 

Hydrogeomorphologic Pre-1980 0.041 0.80 Huber et al., 1981 

- = No conclusive target was reached using the method. 

Based on Table 3.4, the potential target TP concentrations ranged from 0.041 to 0.076 mg/L, 

with the majority of the methods appearing to support a TP concentration above 0.070 mg/L. 

The target TN concentration ranged from 0.800 to 1.500 mg/L, with more methods supporting a 

concentration above 1.00 mg/L. The Department, therefore, averaged all the TP concentrations 

above 0.070 mg/L and all the TN concentrations above 1.00 mg/L, resulting in target TP and TN 

concentrations of 0.073 and 1.30 mg/L, respectively. Based on these TN and TP 

concentrations, the Department predicted a target chl a concentration of 22 μg/L, using the chl 

a–TP relationship developed by Huber et al. (1982) for lakes with a TN/TP ratio between 10 and 

30. Using the TSI calculation procedure defined in the 1996 305(b) report, the estimated target 

TSI is 61, based on concentrations of 1.30 mg/L TN, 0.073 mg/L TP, and 22 μg/L chl a. 

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Because this potential target TSI of 61 is primarily developed based on information not specific 

to Lake Jesup, it is reasonable to ask whether the target is even achievable for Lake Jesup. 

The Department examined this question by simulating background water quality conditions 

using watershed and water quality models. Later sections of this report describe the detailed 

model calibration and simulation. The simulated, long-term annual average background TN, 

TP, and chl a concentrations from these models are 1.20 mg/L, 0.070 mg/L, and 20.7 μg/L, 

respectively, which are similar to target values established using the methods mentioned above. 

The background TSI value, based on background TN, TP, and chl a concentrations, is 60. 

In modeling the background TSI value, the Department assumed that all human land use areas 

discharge TN and TP loadings in the same way as natural areas such as upland forests and 

wetlands. Considering the development that has already occurred in the Lake Jesup 

watershed, using this background condition as the water quality target for the lake may not be 

realistic. Furthermore, some increases over background condition should be allowable without 

causing an imbalance in the lake’s flora or fauna. 

The Department therefore set the target for Lake Jesup as a 5-unit increase in the TSI over the 

background condition, which allows for some increase in nutrient loading above the background 

condition but prevents a significant change in the trophic status of the lake. Using this 

approach, the water quality target for Lake Jesup is a TSI of 65, which corresponds to long-term 

annual average TN, TP, and chl a concentrations of 1.32 mg/L, 0.094 mg/L, and 30.5 μg/L, 

respectively. 

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Chapter 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 the pollutant of concern in the target 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, agriculture, 

silviculture, and 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 the EPA’s NPDES Program. These 

nonpoint sources included certain urban stormwater discharges, including those from local 

government master drainage systems, construction sites over five acres, and a wide variety of 

industries (see Appendix A for background information on the federal and state stormwater 

programs). 

To be consistent with Clean Water Act definitions, the term “point source” is 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 6.1 on Expression and Allocation of the TMDL). 

However, the methodologies used to estimate nonpoint source loads do not distinguish between 

NPDES and non-NPDES 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 Nutrients in the Lake Jesup Watershed 

4.2.1 Point Sources 

4.2.1.1 Wastewater Point Sources 

Lake Jesup was one of the most eutrophic bodies of water in Florida for many years, primarily 

due to the input of secondary wastewater effluent for over 20 years (EPA, 1977). By 1983, 

direct discharge from wastewater treatment plants to Lake Jesup had been either routed outside 

the watershed or discharged to land application systems or percolation ponds (Seminole 

County, 1991). At the time this analysis was conducted, no wastewater point sources were 

identified in the watershed that discharge directly to surface waters. 

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4.2.1.2 Municipal Separate Storm Sewer System Permittees 

Within the Lake Jesup watershed, the stormwater collection systems owned and operated by 

Seminole and Orange Counties; a number of municipalities, including Altamonte Springs, 

Casselberry, Eatonville, Lake Mary, Longwood, Maitland, Orlando, Oviedo, Sanford, Winter 

Garden, and Winter Springs; and the Florida Department of Transportation are covered by an 

NPDES MS4 Phase I permit. No Phase II permittees were identified in the watershed. 

4.2.2 Nonpoint Sources 

Additional nutrient loadings to Lake Jesup are primarily generated from nonpoint sources in the 

Lake Jesup watershed. As discussed earlier in this report, the watershed occupies a highly 

urbanized area. Eleven municipalities are located within or associated with the watershed: 

Sanford and Lake Mary on the northwest end of the watershed, and Oviedo, Winter Springs, 

Longwood, Casselberry, Altamonte Springs, Maitland, Winter Park, Eatonville, and Orlando in 

the southern part. According to 2003 data from the U. S Census Bureau, the population 

densities in Seminole and Orange Counties were 1,184.9 and 987.8 persons/mi2

, respectively, 

significantly higher than the state average of 296.4 persons/mi2

 (U. S. Census Bureau Web site, 

2003). The area is also growing rapidly. From 1990 through 2000, the population of Seminole 

and Orange Counties increased by 27.0% and 32.3%, which is greater than the state average of 

23.5%. 

Nonpoint sources addressed in this analysis primarily include loadings from surface runoff, 

baseflow from the surficial aquifer (including septic tank and sewer line contributions), input from 

the Floridan aquifer through either spring flow or lake bottom leakage, precipitation directly onto 

the lake’s surface, and inflow from the St. Johns River when regional rainfall is higher than local 

rainfall. Based on previous studies conducted in the Lake Jesup watershed, the majority of the 

nutrient load is generated from surface runoff or baseflow from the surficial aquifer (Keesecker, 

1992; Parsons Engineering Science, Inc., 2000). 

In this analysis, nutrient loadings from the watershed were estimated using a geographic 

information system (GIS) model provided by PBS&J. This model estimates the nutrient loadings 

from the watershed primarily based on the U. S. Soil Conservation Service (SCS, which is now 

the U. S. Department of Agriculture’s National Resources Conservation Service, or NRCS) 

curve number approach. This approach takes into account the influence on pollutant loadings 

from land use categories, soil types, antecedent soil moisture condition, and best management 

practices (BMPs). PBS&J (contracted by Seminole County) provided the Department with the 

SJRWMD’s 2000 land use GIS coverage (scale 1:40,000), after some recent updates from 

counties and municipalities were incorporated into the coverage. Soil hydrologic characteristics 

for the watershed were obtained from the SJRWMD’s SSURGO GIS coverage and updated with 

local information on the Type U soil hydrologic group. PBS&J collected and provided septic 

tank and BMP information to the Department . The following sections describe in detail the 

methods used to estimate nutrient loadings from various nonpoint sources. 

4.2.2.1 Land Uses 

The Lake Jesup watershed drains about 87,331 acres into Lake Jesup. Land use categories in 

the watershed were aggregated based on the classification system in Table 4.1. Figure 4.1

shows the distribution of the principal land uses in the watershed. 

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Figure 4.1. Principal land uses in the Lake Jesup watershed 

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Table 4.1. Classification of land use categories in the Lake Jesup 

watershed 

Land Use Acreage Percent 

Agriculture/Golf Course 6,264 7.2 

Forest and Rural Open 11,505 13.2 

Low-Density Residential 2,504 2.9 

Medium-Density Residential 22,697 26.0 

High-Density Residential 2,682 3.1 

Commercial 4,807 5.5 

Institutional 2,045 2.3 

Industrial/Utility 2,375 2.7 

Transportation Facilities 8,490 9.7 

Water 3,863 4.4 

Wetlands 1 (impacted) 11,745 13.4 

Wetlands 2 (unimpacted) 8,355 9.6 

Total 87,331 100 

Based on Table 4.1, human land use areas—including agriculture/golf course, residential, 

commercial, institutional, industrial/utility, and transportation facilities—occupy about 51,863 

acres of the Lake Jesup watershed, accounting for about 59% of the total watershed area. The 

remaining 41% of the watershed consists of natural areas, including forest and rural open, 

water, and wetlands. Although this analysis generally considered wetlands as natural areas, 

some of the wetland areas in the watershed that were previously used for agriculture are now 

recovering from human impacts. These areas were designated as Wetlands 1. The wetland 

areas that were not significantly affected by previous human activities were designated as 

Wetlands 2. PBS&J provided information regarding the locations (Figure 4.1) and scales 

(Table 4.1) of these 2 wetland subcategories. This analysis assumed different event mean 

concentrations (EMCs) of pollutants for impacted and unimpacted wetland areas; these will be 

discussed in detail in later sections of this report. 

About 53% of the human land use area is residential, with medium-density residential claiming 

22,697 acres, or about 44% of the total human land use area. Low-density and high-density 

residential areas both occupy about 5% of the total human land use area. Another major 

human land use category is transportation facilities, primarily roads, which occupy about 8,490 

acres of land, or about 16% of the total human land use area. As Figure 4.1 shows, urban and 

built-up areas almost cover the entire western and southern parts of the Lake Jesup watershed, 

with only some isolated lakes standing out among the human land use matrix. 

Agricultural land use accounts for about 7% of the total watershed, mostly the part that is 

immediately connected to Lake Jesup. Citrus groves, tree crops, row crops, field crops, horse 

farms, and improved or unimproved pasturelands are some of the major agricultural activities. 

Nutrient inputs to the lake through surface runoff due to the application of fertilizer in the area 

could be a significant source of pollutants leading to lake eutrophication. 

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The SJRWMD began working with local governments in 1984 to purchase land around the lake 

for preservation. This is important not only because the land provides public recreational 

opportunities, but also because the marshes that are part of those lands help to maintain animal 

habitat, improve water quality, and allow for the storage of large volumes of water during rainy 

periods, thus offering flood protection to surrounding communities. The SJRWMD subsequently 

brought about 3,850 acres around the lake into public ownership and has targeted additional 

areas for acquisition. Other government agencies have purchased an additional 4,700 acres of 

floodplain around the lake. 

4.2.2.2 Hydrologic Soil Groups 

The hydrologic characteristics of soil can significantly influence the capability of a given 

watershed to hold rainfall or produce surface runoff. Soils of the Lake Jesup watershed are 

classified as Types A, B, C, or D, according to the following criteria (Viessman et al., 1989): 

• Type A soil (low runoff potential): Soils having high infiltration rates even if thoroughly 

wetted and consisting chiefly of deep, well-drained to excessively drained sands or gravels. 

These soils have a high rate of water transmission. 

• Type B soil: Soils having moderate infiltration rates if thoroughly wetted and consisting 

chiefly of moderately deep to deep, moderately well-drained to well-drained soils with 

moderately fine to moderately coarse textures. These soils have a moderate rate of water 

transmission. 

• Type C soil: Soils having slow infiltration rates if thoroughly wetted and consisting chiefly of 

soils with a layer that impedes the downward movement of water, or soils with moderately 

fine to fine texture. These soils have a slow rate of water transmission. 

• Type D soil (high runoff potential): Soils having very slow infiltration rates if thoroughly 

wetted and consisting chiefly of clay soils with a high swelling potential, soils with a 

permanent high water table, soils with a clay pan or clay layer at or near the surface, and 

shallow soils over nearly impervious materials. These soils have a very slow rate of water 

transmission. 

Figure 4.2 shows the spatial distribution of the hydrologic soil groups in the Lake Jesup 

watershed. Type D soil dominates the northern part of the watershed, especially the areas that 

directly connect to the lake. Type A soil primarily dominates the southern part of the watershed. 

In this analysis, Type B soil is designated as Type B/D to indicate that when Type B soil is water 

saturated, its characteristics are similar to those of Type D soil. Soil hydrologic characteristics 

for the watershed were obtained from the SJRWMD’s SSURGO GIS coverage and were 

updated with local information about Type U soil.

4.2.2.3 Estimating Nonpoint Loadings from the Lake Jesup Watershed 

A. Estimating nonpoint TN and TP loadings using the PBS&J pollutant loading model. 

The PBS&J pollutant loading model is based on the SCS curve number approach, which 

calculates pollutant loadings from a given watershed based on the runoff coefficients for 

different land use–soil type combinations and EMCs for different land use categories and for 

different pollutants. The annual runoff coefficient for a given land use category for a given year 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

is estimated as the ratio between the annual runoff (QA) from the land use area and the annual 

rainfall (PA), which are the sum of daily runoff and daily rainfall, respectively. The daily runoff is 

calculated using Equation (1): 

 

Figure 4.2. Spatial distribution of hydrologic soil groups in the Lake 

Jesup watershed 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

 P > I (1) SIP

IP Q

+−

− = )()(

2

Where: 

Q is daily flow; 

P is daily rainfall; 

S is watershed storage; and 

I is defined as the initial abstraction of the rainfall, which, according to the SCS, mainly 

consists of interception, depression storage, and infiltration occurring prior to runoff 

(Suphunvorranop, 1985). 

To eliminate the necessity of estimating both S and I, the relation between I and S was 

developed by analyzing rainfall runoff data for many small and large watersheds from various 

parts of the United States. The empirical relationship is: 

 I = 0.2 S (2) 

Substituting Equation (2) into Equation (1) yields: 

 (3) SP

SP Q 8.0)2.0(

2

+

− =

S is related to the curve number (CN) by: 

Florida Department of Environmental Protection 

21

 (4) 

10 1000 −= CNS

CN is a function of hydrologic soil group, land use, and the antecedent moisture condition 

(AMC) of the soil. It is dimensionless and has values ranging from 0 to 100, with 0 representing 

no runoff being produced and 100 representing a situation in which all the rainfall is converted to 

runoff. 

CN associated with each land use–soil hydrologic group can vary with the AMC. The overall 

AMCs can be divided into three different classes, based on the growth season and the total fiveday antecedent rainfall (in centimeters [cm]). Table 4.2 lists the classification of AMCs, and 

Table 4.3 lists the equivalent curve numbers under different AMC conditions. 

PBS&J provided the Department with CNs for different land use–hydrologic soil group 

combinations. Table 4.4 lists these CNs at AMC II. Corresponding CNs under AMC I and AMC 

III can be derived based on Table 4.3. 
TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 4.2. Classifications of AMCs 

Total Five-day Antecedent Rainfall (cm) 

AMC Dormant Season Growing Season 

I Less than 1.3 Less than 3.6 

II 1.3 to 2.8 3.6 to 5.3 

III Over 2.8 Over 5.3 

Table 4.3. Equivalent curve numbers under different AMC conditions 

AMC I AMC II AMC III 

0.0 0.0 0.0 

2.0 5.0 17.0 

4.0 10.0 26.0 

7.0 15.0 33.0 

9.0 20.0 39.0 

12.0 25.0 45.0 

15.0 30.0 50.0 

19.0 35.0 55.0 

23.0 40.0 60.0 

23.8 41.0 61.0 

24.6 42.0 62.0 

25.4 43.0 63.0 

26.2 44.0 64.0 

27.0 45.0 65.0 

27.8 46.0 66.0 

28.6 47.0 67.0 

29.4 48.0 68.0 

30.2 49.0 69.0 

31.0 50.0 70.0 

31.8 51.0 71.0 

32.6 52.0 72.0 

33.4 53.0 73.0 

34.2 54.0 74.0 

35.0 55.0 75.0 

36.0 56.0 75.8 

37.0 57.0 76.6 

38.0 58.0 77.4 

39.0 59.0 78.2 

40.0 60.0 79.0 

41.0 61.0 79.8 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

AMC I AMC II AMC III 

42.0 62.0 80.6 

43.0 63.0 81.4 

44.0 64.0 82.2 

45.0 65.0 83.0 

46.2 66.0 83.8 

47.4 67.0 84.6 

48.6 68.0 85.4 

49.8 69.0 86.2 

51.0 70.0 87.0 

52.2 71.0 87.8 

53.4 72.0 88.6 

54.6 73.0 89.4 

55.8 74.0 90.2 

57.0 75.0 91.0 

58.2 76.0 91.6 

59.4 77.0 92.2 

60.6 78.0 92.8 

61.8 79.0 93.4 

63.0 80.0 94.0 

64.4 81.0 94.6 

65.8 82.0 95.2 

67.2 83.0 95.8 

68.6 84.0 96.4 

70.0 85.0 97.0 

71.6 86.0 97.2 

73.2 87.0 97.4 

74.8 88.0 97.6 

76.4 89.0 97.8 

78.0 90.0 98.0 

79.8 91.0 98.2 

81.6 92.0 98.4 

83.4 93.0 98.6 

85.2 94.0 98.8 

87.0 95.0 99.0 

89.6 96.0 99.2 

92.2 97.0 99.4 

94.8 98.0 99.6 

97.4 99.0 99.8 

100.0 100.0 100.0 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 4.4. Curve numbers for different land use–hydrologic soil 

group combinations under AMC II 

Land Use Hydrologic 

Soil Group AMC II 

Agriculture/Golf Course A 49 

Agriculture/Golf Course B/D 69 

Agriculture/Golf Course C 79 

Agriculture/Golf Course D 84 

Commercial A 89 

Commercial B/D 92 

Commercial C 94 

Commercial D 95 

High-Density Residential A 77 

High-Density Residential B/D 85 

High-Density Residential C 90 

High-Density Residential D 92 

Industrial/Utility A 81 

Industrial/Utility B/D 88 

Industrial/Utility C 91 

Industrial/Utility D 93 

Institutional A 77 

Institutional B/D 85 

Institutional C 90 

Institutional D 92 

Low-Density Residential A 51 

Low-Density Residential B/D 68 

Low-Density Residential C 79 

Low-Density Residential D 84 

Medium-Density Residential A 57 

Medium-Density Residential B/D 72 

Medium-Density Residential C 81 

Medium-Density Residential D 86 

Forest and Rural Open A 49 

Forest and Rural Open B/D 69 

Forest and Rural Open C 79 

Forest and Rural Open D 84 

Transportation Facilities A 83 

Transportation Facilities B/D 89 

Transportation Facilities C 92 

Transportation Facilities D 93 

Water A 95 

Water B/D 95 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Hydrologic Land Use AMC II Soil Group 

Water C 95 

Water D 95 

Wetlands A 95 

Wetlands B/D 95 

Wetlands C 95 

Wetlands D 95 

In this analysis, the annual average runoff coefficient for each land use–soil combination was 

calculated for each year for the period from 1995 through 2002. The runoff coefficient was 

calculated as the quotient between annual total runoff and annual total rainfall. The annual total 

runoff is the sum of runoff of all days within a given year. To calculate the daily runoff (Q) for 

each day using Equation (3), the total rainfall for a five-day period prior to the day under 

question was calculated and compared with the threshold values listed in Table 4.2 to 

determine the AMC. A CN was then assigned to the day in question, based on the AMC and 

the daily runoff calculated. The same process was applied to all the land use–hydrologic soil 

group combinations listed in Table 4.4, and runoff coefficients for all the land use–soil 

combinations for all the years from 1995 through 2002 were calculated (Table 4.5.)

Table 4.5. Annual runoff coefficients for different land use–

hydrologic soil group combinations, 1995–2002 

Land Use 

Hydrologic 

Soil 

Group 

1995 1996 1997 1998 1999 2000 2001 2002 

Agriculture/Golf Course A 0.015 0.026 0.013 0.019 0.009 0.044 0.026 0.017 

Agriculture/Golf Course B/D 0.069 0.112 0.071 0.103 0.061 0.097 0.096 0.100 

Agriculture/Golf Course C 0.131 0.206 0.145 0.192 0.125 0.142 0.176 0.207 

Agriculture/Golf Course D 0.184 0.275 0.207 0.257 0.176 0.178 0.240 0.288 

Commercial A 0.252 0.347 0.274 0.327 0.236 0.223 0.314 0.367 

Commercial B/D 0.313 0.406 0.330 0.387 0.291 0.267 0.377 0.429 

Commercial C 0.370 0.458 0.382 0.441 0.343 0.312 0.433 0.483 

Commercial D 0.405 0.490 0.414 0.474 0.376 0.342 0.467 0.515 

High-Density Residential A 0.116 0.184 0.127 0.172 0.110 0.132 0.157 0.182 

High-Density Residential B/D 0.198 0.292 0.223 0.273 0.190 0.188 0.258 0.308 

High-Density Residential C 0.269 0.364 0.290 0.344 0.251 0.235 0.332 0.385 

High-Density Residential D 0.313 0.406 0.330 0.387 0.291 0.267 0.377 0.429 

Industrial/Utility A 0.149 0.230 0.166 0.214 0.142 0.155 0.198 0.235 

Industrial/Utility B/D 0.236 0.332 0.259 0.312 0.223 0.212 0.298 0.351 

Industrial/Utility C 0.290 0.384 0.309 0.364 0.270 0.250 0.353 0.406 

Industrial/Utility D 0.340 0.430 0.354 0.412 0.315 0.288 0.403 0.454 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Hydrologic Land Use 1995 1996 1997 1998 1999 2000 2001 2002 Soil 

Group 

Institutional A 0.116 0.184 0.127 0.172 0.110 0.132 0.157 0.182 

Institutional B/D 0.198 0.292 0.223 0.273 0.190 0.188 0.258 0.308 

Institutional C 0.269 0.364 0.290 0.344 0.251 0.235 0.332 0.385 

Institutional D 0.313 0.406 0.330 0.387 0.291 0.267 0.377 0.429 

Low-Density Residential A 0.018 0.031 0.016 0.024 0.012 0.049 0.030 0.021 

Low-Density Residential B/D 0.065 0.104 0.066 0.096 0.057 0.094 0.091 0.093 

Low-Density Residential C 0.131 0.206 0.145 0.192 0.125 0.142 0.176 0.207 

Low-Density Residential D 0.184 0.275 0.207 0.257 0.176 0.178 0.240 0.288 

Medium-Density Residential A 0.030 0.050 0.029 0.043 0.023 0.064 0.046 0.039 

Medium-Density Residential B/D 0.084 0.136 0.089 0.126 0.077 0.109 0.116 0.127 

Medium-Density Residential C 0.149 0.230 0.166 0.214 0.142 0.155 0.198 0.235 

Medium-Density Residential D 0.209 0.304 0.234 0.285 0.200 0.195 0.270 0.322 

Forest and Rural Open A 0.015 0.026 0.013 0.019 0.009 0.044 0.026 0.017 

Forest and Rural Open B/D 0.069 0.112 0.071 0.103 0.061 0.097 0.096 0.100 

Forest and Rural Open C 0.131 0.206 0.145 0.192 0.125 0.142 0.176 0.207 

Forest and Rural Open D 0.184 0.275 0.207 0.257 0.176 0.178 0.240 0.288 

Transportation Facilities A 0.171 0.259 0.192 0.241 0.164 0.169 0.225 0.269 

Transportation Facilities B/D 0.252 0.347 0.274 0.327 0.236 0.223 0.314 0.367 

Transportation Facilities C 0.313 0.406 0.330 0.387 0.291 0.267 0.377 0.429 

Transportation Facilities D 0.340 0.430 0.354 0.412 0.315 0.288 0.403 0.454 

Water A 0.405 0.490 0.414 0.474 0.376 0.342 0.467 0.515 

Water B/D 0.405 0.490 0.414 0.474 0.376 0.342 0.467 0.515 

Water C 0.405 0.490 0.414 0.474 0.376 0.342 0.467 0.515 

Water D 0.405 0.490 0.414 0.474 0.376 0.342 0.467 0.515 

Wetlands 1 A 0.405 0.490 0.414 0.474 0.376 0.342 0.467 0.515 

Wetlands 1 B/D 0.405 0.490 0.414 0.474 0.376 0.342 0.467 0.515 

Wetlands 1 C 0.405 0.490 0.414 0.474 0.376 0.342 0.467 0.515 

Wetlands 1 D 0.405 0.490 0.414 0.474 0.376 0.342 0.467 0.515 

Wetlands 2 A 0.405 0.490 0.414 0.474 0.376 0.342 0.467 0.515 

Wetlands 2 B/D 0.405 0.490 0.414 0.474 0.376 0.342 0.467 0.515 

Wetlands 2 C 0.405 0.490 0.414 0.474 0.376 0.342 0.467 0.515 

Wetlands 2 D 0.405 0.490 0.414 0.474 0.376 0.342 0.467 0.515 

The PBS&J model calculates the gross runoff and gross TN and TP loadings before BMP 

removal is applied to the model, as well as the net runoff and net TN and TP loadings after BMP 

removal is applied. The equations used to calculate these values are as follows: 

Gross runoff = rainfall * runoff coefficient * area 

Gross loading = Gross runoff * EMC 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Net runoff = Gross runoff * (1 – water removal efficiency) 

Net loading = Gross loading (1 – pollutant removal efficiency) 

The model input required for simulating the watershed TN and TP loadings includes the 

following: 

• Annual precipitation, 

• Areas of all the land use–soil combinations, runoff coefficient for each land use–soil 

combination, and EMCs for each land use category, 

• Areas covered by different BMPs and pollutant removal efficiency for each BMP, and 

• Baseflow separation results to show the ratio between surface runoff and baseflow and 

ground water nutrient concentrations 

A. Daily rain precipitation data were obtained from a weather station located in the city of 

Sanford, Seminole County (UCAN: 4129 COOP: 087982). These data were retrieved from the 

Climate Interactive Rapid Retrieval User System (CIRRUS), hosted by the Southeast Regional 

Climate Center. Table 4.6 depicts annual average precipitation at this weather station from 

1995 through 2002.

Table 4.6. Annual precipitation at the city of Sanford weather station, 

1995–2002 

Year Total Rainfall 

(inches/year) 

1995 59.32

1996 62.82

1997 53.69

1998 48.83

1999 47.04

2000 32.83

2001 52.73

2002 66.24

B. Areas of different land use–hydrologic soil group combinations in Lake Jesup were 

obtained by overlaying the land use coverage with the coverage of hydrologic soil groups. 

Table 4.7 lists these areas. 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 4.7. Areas of different land use–hydrologic soil group 

combinations in the Lake Jesup watershed 

Land Use Hydrologic Soil 

Group Acreage 

Agriculture/Golf Course A 1,030 

Agriculture/Golf Course B/D 3,579 

Agriculture/Golf Course C 470 

Agriculture/Golf Course D 1,184 

Commercial A 3,393 

Commercial B/D 798 

Commercial C 390 

Commercial D 225 

High-Density Residential A 1,453 

High-Density Residential B/D 540 

High-Density Residential C 504 

High-Density Residential D 184 

Industrial/Utility A 802 

Industrial/Utility B/D 1,055 

Industrial/Utility C 142 

Industrial/Utility D 377 

Institutional A 1,257 

Institutional B/D 460 

Institutional C 181 

Institutional D 148 

Low-Density Residential A 329 

Low-Density Residential B/D 1,297 

Low-Density Residential C 133 

Low-Density Residential D 745 

Medium-Density Residential A 14,336 

Medium-Density Residential B/D 4,108 

Medium-Density Residential C 2,353 

Medium-Density Residential D 1,900 

Forest and Rural Open A 3,709 

Forest and Rural Open B/D 4,140 

Forest and Rural Open C 965 

Forest and Rural Open D 2,691 

Transportation Facilities A 4,429 

Transportation Facilities B/D 2,510 

Transportation Facilities C 827 

Transportation Facilities D 724 

Water A 148 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Hydrologic Soil 

Group Land Use Acreage 

Water B/D 61 

Water C 33 

Water D 3,621 

Wetlands 1 A 442 

Wetlands 1 B/D 2,218 

Wetlands 1 C 216 

Wetlands 1 D 8,870 

Wetlands 2 A 347 

Wetlands 2 B/D 2,450 

Wetlands 2 C 237 

Wetlands 2 D 5,321 

Runoff coefficients for each land use–hydrologic soil group combination were calculated using 

the SCS curve number procedures described above; Table 4.5 lists the values for different 

combinations.

TN and TP EMCs for different land use categories were mainly cited from Harper (1994). 

Agriculture/golf course land use in this analysis was considered the same as the general 

agriculture category in Harvey’s study. There is no forest/rural open category in Harvey’s 

system, and the forest/rural open category for this analysis was considered the same as 

recreational/open space in Harvey’s system. Medium- and high-density residential categories 

were considered the same as the single-family and multifamily categories in Harvey’s system. 

The high- and low-density commercial areas were combined into one single category called 

commercial, and its EMCs are considered to be the average values of the high- and low-density 

commercial category in Harvey’s system. The wetland TP EMC in Harvey’s analysis is 

commonly considered to be the result from wetlands that were influenced by human activities. 

The TP EMC for this type of wetland area (Wetlands 1) is about 0.09 mg/L. The TP EMC for 

relatively unimpacted wetlands (Wetlands 2), which is 0.06 mg/L, was cited from Fulton et al. 

(2003). Harvey’s work does not include institutional and transportation facilities, and PBS&J 

provided EMC values for these categories. In addition, PBS&J provided updated values for lowand medium-density residential. Table 4.8 lists the TN and TP EMCs for different land use 

categories. 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 4.8. TN and TP EMCs for different land use categories (mg/L) 

Land Use Category TN TP 

Agriculture/Golf Course 2.32 0.34 

Forest and Rural Open 1.25 0.05 

Low-Density Residential 1.97 0.44 

Medium-Density Residential 2.04 0.45 

High-Density Residential 2.42 0.49 

Commercial 2.01 0.29 

Institutional 2.29 0.15 

Industrial/Utility 1.79 0.31 

Transportation Facilities 1.87 0.28 

Water 1.25 0.11 

Wetlands 1 1.6 0.09 

Wetlands 2 1.6 0.06 

C. Areas covered by different BMPs: Table 4.9 lists the areas covered by various BMP 

structures, and Table 4.10 lists the water and pollutant removal efficiency of all the different 

BMPs. Figure 4.3 shows the spatial distribution of BMPs in the Lake Jesup watershed. PBS&J 

provided the Department with the information on pollutant removal efficiencies and BMP 

coverage. 

According to Table 4.9, some sort of BMP structure is used to treat stormwater from about 32% 

of the Lake Jesup watershed. The most common stormwater facilities appear to be dry and wet 

detention ponds, which address runoff from about 27% of the total Lake Jesup watershed area 

and account for about 83% of the area covered by BMPs. The drainage wells located in the 

southern end of the watershed are another major feature that retains water and nutrients. 

These drain about 3,081 acres of the watershed, accounting for about 3.5% of the total 

watershed area. Orange County has about 400 drainage wells, with 154 of them in the city of 

Orlando (Sheffield et al., March 1995). While these wells indeed contribute to pollutant removal 

and ground water recharge, their effect on ground water quality is a concern. 

It should be noted that, in Table 4.10, water removal efficiency for most of the stormwater 

facilities is 0, meaning that no water is removed by stormwater treatment facilities. This may not 

be a valid assumption. As most of the published removal efficiencies consider only pollutant 

removal, however, it becomes a commonly accepted practice that, when estimating the net 

pollutant loadings after BMP treatment, water discharge is considered unchanged. This 

assumption adds to the margin of safety because it tends to overestimate the net load when 

assuming a larger runoff from the watershed. 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 4.9. Areas covered by various BMPs in the Lake Jesup 

watershed 

BMP Type Area Covered 

(acres) 

Percent Area 

Covered 

No Stormwater Facility 59,418 68.04% 

Combination (Swale/Dry Pond) 311 0.36% 

Dry Detention 11,068 12.67% 

Swales 1,217 1.39% 

Wet Detention Pond 12,187 13.96% 

Orlando 100% On-Site Retention 33 0.04% 

Orlando Private BMPs 6 0.01% 

Lake Drainage Wells 3,081 3.53% 

Table 4.10. Pollutant and water removal efficiency for different 

BMPs 

BMP Type 

Pollutant Removal 

Efficiency Water Removal 

Efficiency TN TP 

No Stormwater Facility 0 0 0 

Combination (Swale/Dry Pond) 0.3 0.25 0 

Combination (Swale/Wet Pond) 0.86 0.63 0 

Dry Detention 0.2 0.2 0 

Swales 0.1 0.05 0 

Wet Detention Pond 0.8 0.6 0 

Orlando 100% On-Site Retention 1 1 0 

Orlando Private BMPs 0.2 0.2 0 

Lake Drainage Wells 0.64 0.64 0.64 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Figure 4.3. Spatial distribution of BMPs in the Lake Jesup watershed 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

D. Baseflow analysis and ground water TN and TP concentrations: Baseflow analysis is 

used to separate surface runoff from the ground water contribution. The separation is important 

in this analysis because surface runoff and baseflow come from different sources and may have 

different nutrient concentrations. Baseflow could come from both the surficial and Floridan 

aquifers. The Lake Jesup watershed is mostly confined by the Hawthorn Layer (Florida 

Geological Survey, 1991), and except for the areas immediately adjacent to the lake, the 

surface elevation of most of the watershed is higher than the potentiometric surface of the 

Floridan aquifer (Florida Geological Survey, 1991, U. S. Geological Survey [USGS] Quad Map 

1:24,000). The contribution from the Floridan aquifer to the total stream flow was therefore 

considered negligible, and base flow was mainly from the surficial aquifer. 

Baseflow analysis was conducted using the daily flow data from a USGS gauging station 

located on Howell Creek near Slavia, Florida (Station ID: 02234324, Figure 4.4). A graphical 

separation technique, based on the hydrograph, was used in the separation analysis. Because 

of the possible water detention effect from the many lakes located in the upper reach of Howell 

Creek, this analysis does not use day-to-day baseflow separation. Rather, it uses a modified 

local minimum method that targets the lowest flow measurement for each major flow period 

(Figure 4.5). Based on the daily flow measurements from 1988 through 2003, baseflow 

accounts for about 20% of the total stream flow on an annual average basis. 

Nutrient concentrations for the surficial aquifer were retrieved from the Department’s HydroPort 

database. Data were collected primarily from the 5 surficial aquifer background water quality 

monitoring wells located within or adjacent to the Lake Jesup watershed (Figure 4.4). Nutrient 

data from these wells were collected in 1990, 1992, 1993, 1994, 2002, and 2004. The longterm average TN concentration, which is 0.483, was calculated from all 5 wells due to the lack 

of nitrate/nitrite data for wells located upgrade of the lake. The long-term average TP 

concentration (0.155 mg/L) was calculated based on results from Wells 284052081212602, 

284317081182501, and 284320081090001, which are located upgrade of the lake. Since only 

limited amounts of water quality data were available when this analysis was conducted, longterm average TN and TP concentrations were used as constants for the entire modeling period 

(1995–2002). 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Figure 4.4. Locations of Department background ground water quality 

monitoring stations and a USGS gauging station 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

0

50

100

150

200

250

300

350

400

450

1988 1990 1993 1996 1998 2001

Year

Flow (cfs)

TOTAL

PBSJ Baseflow 1mo

Figure 4.5. Results of the baseflow analysis, 1988–2003 

E. Surface runoff and TN and TP loadings from the watershed into Lake Jesup: To 

simulate surface runoff and TN and TP loadings from the watershed into Lake Jesup, the entire 

watershed was delineated into five sub-basins: Gee Creek, Howell Creek, Lake Jesup (the area 

immediately connected to Lake Jesup), Little Lake Howell, and Soldiers Creek (Figure 4.6). 

Tables 4.11-a, 4-11b, and 4-11c list the annual surface runoff, TN loading, TP loading, and 

percent contribution from these sub-basins for 1995 through 2002. 

As shown in Table 4.11-a, a long-term average of about 70,052 acre-feet of surface runoff is 

discharged into Lake Jesup annually. The area directly connected to Lake Jesup has the 

highest discharge, accounting for about 45% of the total surface runoff. Because the sub-basin 

has one of the largest areas among the 5 sub-basins, this may contribute to the large quantity of 

surface runoff that it creates. In addition, it is predominately wetlands and has a relatively high 

potential to create surface runoff. 

Discharge from the Howell Creek sub-basin ranks second, creating 34% of the total surface 

runoff. Although the area of the Howell Creek sub-basin is the largest among all the sub-basins, 

a significant portion of the southern part of the sub-basin drains to drainage wells (Figure 3.4). 

The Soldiers Creek and Gee Creek sub-basins together contribute about 19% of the surface 

runoff, while Little Lake Howell contributes the smallest quantity of water, about 3% of the total 

runoff for the watershed. 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Figure 4.6. Sub-basins of the Lake Jesup watershed 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 4.11-a. Annual surface runoff into Lake Jesup, 1995–2002 (acrefeet/year) 

Sub-basin 1995 1996 1997 1998 1999 2000 2001 2002 Mean Percent 

Gee Creek 5,153 7,473 4,937 5,472 3,789 2,821 5,716 8,052 5,426 8% 

Howell Creek 22,602 32,478 21,530 23,812 16,570 12,376 24,945 34,912 23,653 34% 

Lake Jesup 30,898 42,140 29,089 31,293 22,753 15,514 32,897 46,061 31,330 45% 

Little Lake 

Howell 1,895 2,697 1,798 1,983 1,391 1,027 2,077 2,899 1,971 3% 

Soldiers Creek 7,390 10,467 7,044 7,709 5,447 3,879 8,063 11,370 7,671 11% 

Total 67,938 95,254 64,396 70,269 49,950 35,616 73,697 103,293 70,052 100% 

Table 4.11-b. Annual TN loading into Lake Jesup, 1995–2002 (tons/year) 

Sub-basin 1995 1996 1997 1998 1999 2000 2001 2002 Mean Percent 

Gee Creek 9.5 13.8 9.1 10.1 7.0 5.2 10.5 14.8 10.0 8% 

Howell Creek 40.2 58.3 38.4 42.6 29.5 22.3 44.6 62.5 42.3 33% 

Lake Jesup 60.4 82.6 56.8 61.3 44.4 30.4 64.4 90.1 61.3 47% 

Little Lake Howell 3.0 4.2 2.8 3.1 2.2 1.6 3.3 4.6 3.1 2% 

Soldiers Creek 12.7 18.0 12.1 13.2 9.3 6.6 13.8 19.5 13.2 10% 

Total 125.7 176.8 119.2 130.4 92.4 66.1 136.6 191.6 129.9 100% 

Table 4.11-c. Annual TP loading into Lake Jesup, 1995–2002 (tons/year) 

Sub-basin 1995 1996 1997 1998 1999 2000 2001 2002 Mean Percent 

Gee Creek 1.2 1.8 1.2 1.3 0.9 0.7 1.4 1.9 1.3 9% 

Howell Creek 5.3 8.0 5.1 5.8 3.9 3.1 6.1 8.5 5.7 41% 

Lake Jesup 4.9 7.0 4.7 5.2 3.6 2.6 5.4 7.6 5.1 37% 

Little Lake Howell 0.3 0.5 0.3 0.4 0.2 0.2 0.4 0.5 0.4 3% 

Soldiers Creek 1.4 2.1 1.4 1.5 1.0 0.8 1.6 2.2 1.5 10% 

Total 13.1 19.4 12.6 14.2 9.6 7.4 14.7 20.7 14.0 100% 

Contributions of TN and TP loadings from the different sub-basins show a trend similar to that of 

surface runoff. On an annual average basis, about 130 tons of TN are discharged into Lake 

Jesup through surface runoff. Of the total TN loading, the watershed immediately connected to 

Lake Jesup contributes about 47%; the Howell Creek sub-basin contributes 33%; the Soldiers 

Creek and Gee Creek sub-basins contribute about 10% and 8%, respectively; and the Little 

Lake Howell sub-basin contributes the smallest amount, about 2%. 

The long-term annual average TP discharge through surface runoff is about 14 tons. The 

watershed immediately connected to Lake Jesup produces about 37% of the total TP loading 

through surface runoff; the Howell Creek sub-basin contributes 41%; the Soldiers Creek and 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Gee Creek sub-basins contribute about 10% and 9%, respectively; and the Little Lake Howell 

sub-basin contributes the smallest amount, about 3%. 

Surface runoff and TN and TP loadings into Lake Jesup were also analyzed based on land use 

classification. Tables 4.12-a, 4.12-b, and 4.12-c list the annual surface runoff, TN loading, TP 

loading, and percent contribution from different land use categories for Lake Jesup from 1995 

through 2002. 

Total human land use areas contribute about 41.2% of the surface runoff, 73.1% of the annual 

TP loading, and 44.8% of the annual TN loading (Tables 4.12-a, 4.12-b, and 4.12-c). The 

human land use categories that contribute the largest quantity of surface runoff and nutrient 

loadings are commercial, medium-density residential, and transportation facilities, which 

contribute totally about 28.1% of the annual surface runoff, 47.5% of the annual TP loading, and 

28% of the annual TN loading, respectively. 

The single land use category with the largest contribution to runoff, TN, and TP loadings is 

wetlands, which contribute 44.9% of the total annual surface runoff, 20.2% of the watershed TP 

loading, and 44.6% of the watershed TN loading. The high loading from wetland areas mainly 

results from the high potential to produce surface runoff. It should be noted, however, that 

although the total nutrient loadings from the wetland area are high, the nutrient EMCs for the 

wetland area are lower than most of the human land use areas (Table 4.8). The water with low 

nutrient concentrations flowing from wetland areas to the lake ultimately dilutes the lake’s 

nutrient concentration, improving water quality. In addition, human activities currently affect the 

watershed’s wetlands and Lake Jesup significantly. If human influence in the watershed is 

efficiently controlled, wetland water quality should improve and nutrient concentrations will 

decrease even further. 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 4.12-a. Annual surface runoff into Lake Jesup from different land 

use categories, 1995–2002 (acre-feet/year) 

Land Use 

Category 1995 1996 1997 1998 1999 2000 2001 2002 Mean Percent 

Agriculture/Golf 

Course 2,115 3,514 2,051 2,515 1,531 1,449 2,558 3,545 2,410 3.4% 

Commercial 4,905 6,928 4,750 5,106 3,631 2,365 5,331 7,719 5,092 7.3% 

High-Density 

Residential 1,830 2,823 1,809 2,062 1,370 995 2,096 3,068 2,007 2.9% 

Industrial/Utility 2,113 3,124 2,075 2,288 1,577 1,069 2,357 3,454 2,257 3.2% 

Institutional 1,287 2,018 1,275 1,472 966 717 1,488 2,180 1,425 2.0% 

Low-Density 

Residential 968 1,580 946 1,137 708 634 1,160 1,632 1,096 1.6% 

MediumDensity 

Residential 

5,788 9,637 5,559 6,818 4,098 4,423 7,146 9,526 6,624 9.5% 

Open 3,796 6,257 3,708 4,458 2,752 2,574 4,588 6,392 4,316 6.2% 

Transportation 

Facilities 7,423 11,052 7,329 8,080 5,552 3,766 8,315 12,227 7,968 11.4% 

Water 5,573 7,140 5,156 5,369 4,103 2,604 5,712 7,913 5,446 7.8% 

Wetlands 1 18,760 24,036 17,357 18,073 13,811 8,767 19,229 26,638 18,334 26.2% 

Wetlands 2 13,381 17,145 12,380 12,891 9,851 6,254 13,715 19,000 13,077 18.7% 

Grand Total 67,938 95,254 64,396 70,269 49,950 35,616 73,697 103,293 70,052 100.0% 

Table 4.12-b. Annual TP loading into Lake Jesup from different land use 

categories, 1995–2002 (tons/year) 

Land Use 

Category 1995 1996 1997 1998 1999 2000 2001 2002 Mean Percent 

Agriculture/Golf 

Course 0.8 1.4 0.8 1.0 0.6 0.6 1.0 1.4 1.0 6.8% 

Commercial 1.5 2.2 1.5 1.6 1.1 0.7 1.7 2.4 1.6 11.4% 

High-Density 

Residential 1.0 1.5 1.0 1.1 0.7 0.5 1.1 1.6 1.1 7.6% 

Industrial/Utility 0.7 1.1 0.7 0.8 0.6 0.4 0.8 1.2 0.8 5.6% 

Institutional 0.2 0.3 0.2 0.2 0.2 0.1 0.2 0.3 0.2 1.5% 

Low-Density 

Residential 0.5 0.8 0.5 0.6 0.3 0.3 0.6 0.8 0.6 3.9% 

Medium-Density 

Residential 2.5 4.1 2.4 2.9 1.7 1.9 3.1 4.0 2.8 20.2% 

Open 0.2 0.3 0.2 0.2 0.2 0.1 0.3 0.4 0.2 1.7% 

Transportation 

Facilities 2.1 3.1 2.0 2.3 1.5 1.1 2.3 3.4 2.2 15.9% 

Water 0.7 0.9 0.7 0.7 0.5 0.3 0.7 1.0 0.7 4.9% 

Wetlands 1 1.9 2.5 1.8 1.8 1.4 0.9 2.0 2.7 1.9 13.4% 

Wetlands 2 1.0 1.2 0.9 0.9 0.7 0.5 1.0 1.4 1.0 6.8% 

Grand Total 13.1 19.4 12.6 14.2 9.6 7.4 14.7 20.7 14.0 100.0% 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 4.12-c. Annual TN loading into Lake Jesup from different land use 

categories, 1995–2002 (tons/year) 

Land Use 

Category 1995 1996 1997 1998 1999 2000 2001 2002 Mean Percent 

Agriculture/Golf 

Course 5.6 9.3 5.4 6.6 4.0 3.8 6.7 9.4 6.4 4.9% 

Commercial 10.4 14.7 10.1 10.8 7.7 5.0 11.3 16.4 10.8 8.3% 

High-Density 

Residential 4.6 7.1 4.6 5.2 3.5 2.5 5.3 7.7 5.1 3.9% 

Industrial/Utility 4.3 6.3 4.2 4.6 3.2 2.2 4.8 7.0 4.6 3.5% 

Institutional 3.0 4.8 3.0 3.5 2.3 1.7 3.5 5.1 3.4 2.6% 

Low-Density 

Residential 2.1 3.5 2.1 2.5 1.6 1.4 2.6 3.6 2.4 1.9% 

Medium-Density 

Residential 10.3 17.3 9.9 12.2 7.3 8.1 12.8 16.9 11.9 9.1% 

Open 5.2 8.5 5.0 6.1 3.7 3.5 6.2 8.7 5.9 4.5% 

Transportation 

Facilities 12.8 19.1 12.6 13.9 9.6 6.5 14.3 21.1 13.7 10.6% 

Water 8.1 10.3 7.5 7.8 5.9 3.8 8.3 11.4 7.9 6.1% 

Wetlands 1 33.5 42.9 31.0 32.2 24.6 15.6 34.3 47.5 32.7 25.2% 

Wetlands 2 25.8 33.1 23.9 24.9 19.0 12.1 26.5 36.7 25.3 19.4% 

Grand Total 125.7 176.8 119.2 130.4 92.4 66.1 136.6 191.6 129.9 100.0% 

F. Nutrient loadings from baseflow. As discussed in the previous section, nutrient loadings 

from baseflow were mainly through the surficial aquifer, because the majority of the watershed 

is confined and the surface elevation is higher than the potentiometric head of the Floridan 

aquifer in most parts of the watershed. Based on the baseflow separation analysis, about 20% 

of the total stream flow in the Lake Jesup watershed comes from baseflow. The total nutrient 

loading through baseflow was estimated by multiplying the baseflow quantity by the long-term 

average TN and TP concentrations, calculated based on the ground water quality data retrieved 

from the Department’s HydroPort database. These concentrations, which represent results 

from background wells, assume relatively low, if any, human impacts. Table 4.13 lists the 

annual flow, background TN and TP concentrations, and background TN and TP loadings 

through baseflow into Lake Jesup from 1995 through 2002. 

Table 4.13. Annual nutrient loadings into Lake Jesup through 

baseflow, 1995–2002 

1995 1996 1997 1998 1999 2000 2001 2002 Mean 

Flow 

(acre-feet/year) 16,985 23,814 16,099 17,567 12,487 8,904 18,424 25,823 17,513 

TP concentration 

(mg/L) 0.155 0.155 0.155 0.155 0.155 0.155 0.155 0.155 0.155 

TN concentration 

(mg/L) 0.483 0.483 0.483 0.483 0.483 0.483 0.483 0.483 0.483 

TP loading 

(tons/year) 3.2 4.5 3.1 3.3 2.4 1.7 3.5 4.9 3.3 

TN loading 

(tons/year) 10.1 14.2 9.6 10.5 7.4 5.3 11.0 15.4 10.4 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Background nitrogen and phosphorus are not the only nutrients carried through baseflow. 

Septic tanks may contribute significantly to the nutrients that can be carried through baseflow. 

In this analysis, septic tank nutrient loadings were estimated based on septic tank GIS coverage 

provided to the Department by PBS&J (Figure 4.7). The analysis considered only those septic 

tanks located within 200 meters of any receiving waterbodies that discharge to Lake Jesup. 

The nutrient loads contributed by septic tanks were calculated using the following equation: 

W = A * D * C * (1- R)

Where: 

A is the area (in acres) covered by septic tanks within 200 meters of any receiving 

waterbodies that discharge into Lake Jesup, 

D is the per-acre population density, 

C is the per-capita TN and TP generation rate, and 

R is the nutrient removal efficiency through septic tank systems and soil. 

About 789 acres of the watershed with septic tanks are located within 200 meters of receiving 

waters that discharge to Lake Jesup. Of the 789 acres, about 637 acres are primarily 

associated with single-family residential areas, and the remaining 153 acres are associated with 

commercial areas (occupied by institutions). Septic tank areas located in different planning 

areas have different population densities. Table 4.14 lists the population densities for singlefamily residential and commercial areas in each planning area in the Lake Jesup watershed. 

Table 4.14. Population density for single-family residential and 

commercial areas in different planning areas of the Lake 

Jesup watershed (people/acre) 

Planning Area Single-family 

Residential Commercial 

2 7.2 20.2 

3 10.9 31.3 

4 9.1 30.2 

6 8.6 32.9 

7 8.0 21.6 

8 8.2 21.5 

9 1.3 20.9 

10 1.7 67.2 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

 

Figure 4.7. Areas with septic tanks, areas within 200 meters of any 

receiving waters that discharge to Lake Jesup, and planning 

areas 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

The per-capita TN and TP generation rates (C) were cited from published literature values 

(Table 4.15). This analysis used the median values of these generation rates to estimate the 

final loads. The per-capita TN and TP generation rates for the commercial area are usually 

assumed to be one-third of those for the residential area (Tchobanoqlous et al., 2003). 

According to Reckhow (1980), it is a common practice to assume that within 200 meters of any 

receiving waterbody, septic tanks and soil can remove 70% of the phosphorus and 30% of the 

nitrogen generated. Beyond the 200-meter zone, septic tank influences are usually considered 

insignificant. 

The estimated septic tank annual TN and TP loadings are 19.7 and 2.7 tons, respectively. 

Because time-variable septic tank discharge rates were not available at the time of this analysis, 

these annual loading numbers were assumed to be constant throughout the modeling period 

(1995–2002). 

Table 4.15. Nutrient load for household wastewater discharge into 

septic tanks (kilograms/capita/year) (cited from 

Reckhow, 1980) 

TP TN Reference 

1.49 6.45 Ligman et al., 1974 

1.43 5.99 Laak, 1975 

N/A 2.65 Bennet and Linstedt, 1975 

0.74 4.61 Chan, 1978 

1.59 N/A Ellis and Childs, 1973 

1.49 2.15 Siegrist et al., 1976 

3 N/A Bernhard, 1975 

0.8 N/A Otis et al., 1975 

N/A 8.2 Walker et al., 1973 

1.28 3.2 EPA, 1974 

1.46 4.61 Median 

N/A = Not available 

G. Nutrient loadings from the Floridan aquifer: Lake Jesup lies in an artesian area for the 

Floridan aquifer. Water in the Floridan aquifer can rise to the ground surface through springs or 

leak through the confined layer in areas where the potentiometric head is higher than the 

surface elevation of ambient waters. Springs occur where there is a discrete breach in the 

relatively impermeable confining layer that overlies the aquifer, allowing water to flow to the 

surface. Water can also flow upward through the confining layer as diffuse leakage (Keesecker, 

1992). 

Two springs, Clifton Springs and Lake Jesup Spring, discharge Floridan aquifer water directly 

into Lake Jesup. Their flows are given by Rosenau et al. (1977, cited from Keesecker, 1992) as 

1.7 cubic feet per second (cfs) and 1.36 cfs, respectively. These values were from single 

measurements, since no long-term data were available. The sum of the individual flow rates 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

was assumed constant over the modeling period. The annual discharge from these springs is 

calculated using the following equation: 

Spring flow = (1.7 + 1.36 ) * convs * days/year 

Where: 

Spring flow is the annual total flow from the 2 springs, 

1.7 is the Clifton Springs discharge rate (cfs), 

1.36 is the Lake Jesup Spring discharge rate (cfs), and 

Convs is the conversion factor between cfs and acre-feet/year. 

Upward leakage through the confined layer is possible for Lake Jesup, because the 

potentiometric head of the Floridan aquifer directly below Lake Jesup is about 25 feet NGVD 

(Spechler et al., 1991, cited from Keesecker, 1992), while the surface elevation of Lake Jesup 

fluctuated between 0.23 and 7.04 feet NGVD during the modeling period (based on a USGS 

gauging station located at the outlet of Lake Jesup [Site ID 02234435]). The upward leakage 

into Lake Jesup through the confining layer was determined using an equation suggested by 

Keesecker (1992): 

LEAK = LEAKC * (PSURF – LJSTG) * days/year * A 

Where: 

LEAK is the annual upward leakage through the bottom of Lake Jesup (acre-feet/year), 

LEAKC is the leakage coefficient (feet per day per foot [ft/day/ft] head difference between 

PSURF and STG), 

PSURF is the average potentiometric surface of the Floridan aquifer (feet NGVD), 

STG is the Lake Jesup annual average stage (feet NGVD), and 

A is the annual average surface area for Lake Jesup (acres). 

LEAKC (0.000025 ft/day/ft) was set at the median of the range of values given by Tibbals (1990, 

cited from Keesecker, 1992) for the area occupied by Lake Jesup. PSURF (25 feet NGVD) is 

an estimate of the average potentiometric surface of the Floridan aquifer over the lake surface 

(Spechler et al., 1991, cited from Keesecker, 1992). This value was assumed to remain 

constant for the period of modeling. STG was estimated based on the daily lake stage data 

from USGS 02234435. A, the annual average surface area for Lake Jesup, was estimated 

based on the daily lake stage data and the characteristic curve for Lake Jesup. Table 4.16

shows the characteristic curve for Lake Jesup. 

Multinomial equations were developed, based on the data tabulated in Table 4.16, for 

calculating the daily lake surface area and daily cumulative lake volume. The annual average 

lake surface area and annual lake volume were calculated as the mean value of all the daily 

surface areas and lake volume in each calendar year. Figures 4.8 and 4.9 show the 

multinomial equations between lake stage and lake surface area, and between lake stage and 

cumulative lake volume, respectively. 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 4.16. Characteristic curve for Lake Jesup 

Lake Surface 

Elevation 

(feet NGVD) 

Lake Surface 

Area 

(acres) 

Cumulative Lake 

Volume 

(acre-feet) 

-8 0 0 

-7 50 25 

-6 140 120 

-5 380 380 

-4 1,331 1,236 

-3 3,194 3,498 

-2 6,007 8,099 

-1 7,578 14,891 

0 9,150 23,255 

1 10,011 32,836 

2 10,767 43,224 

3 11,523 54,369 

4 12,278 66,269 

5 13,034 78,926 

6 13,790 92,337 

7 14,546 106,505 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

y = -0.0751x6 - 0.1327x5 + 8.1083x4 - 0.11x3 - 254.29x2 + 1347.9x + 

9127.5

R2

 = 0.9988

0

2000

4000

6000

8000

10000

12000

14000

16000

-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8

Lake Stage (ft)

Surface area (acre)

Figure 4.8. Characteristic curve between lake stage and lake surface 

area for Lake Jesup 

y = 0.0047x6 + 0.5611x5 - 1.7927x4 - 54.314x3 + 698.47x2 + 8899x + 

23039

R2 = 1

0

20000

40000

60000

80000

100000

120000

-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8

Lake Stage (ft)

Lake Volume (ac-ft)

Figure 4.9. Characteristic curve between lake stage and lake cumulative 

volume for Lake Jesup 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 4.17 lists the annual calculated upward bottom leakage through the confining layer and 

spring discharge into Lake Jesup, as well as the average lake stage and lake surface area, from 

1995 through 2002. 

Nutrient loadings from the Floridan aquifer to Lake Jesup through both spring discharge and 

upward bottom leakage were calculated by multiplying the calculated flow by the TN and TP 

concentrations of the Floridan aquifer. These concentrations were retrieved from the 

Department’s HydroPort database. The long-term average TN and TP concentrations were 

0.611 and 0.11 mg/L, respectively (Parsons Engineering Science, Inc., 2000). These values 

represent summaries from several Floridan aquifer wells. Table 4.18 lists the annual nutrient 

loadings into Lake Jesup from 1995 through 2002. 

Table 4.17. Annual average lake stage and surface area, and annual 

flow from the Floridan aquifer to Lake Jesup, 1995–2002 

1995 1996 1997 1998 1999 2000 2001 2002 Mean 

Annual average lake 

stage (feet) 3.42 2.44 1.94 3.08 2.30 1.29 2.15 2.69 2.41 

Annual average lake 

surface area (acres) 11,688 11,163 10,890 11,493 11,089 10,467 11,007 11,291 11,136 

Calculated upward 

bottom leakage 

(acre-feet/year) 

2,370 2,341 2,305 2,365 2,332 2,234 2,322 2,353 2,328 

Calculated spring 

discharge 

(acre-feet/year) 

2,215 2,215 2,215 2,215 2,215 2,215 2,215 2,215 2,215 

Table 4.18. Annual TN and TP loadings from the Floridan 

aquifer into Lake Jesup, 1995–2002 (tons/year) 

Loading 1995 1996 1997 1998 1999 2000 2001 2002 Mean 

TN annual 

loading 3.5 3.4 3.4 3.5 3.4 3.4 3.4 3.4 3.4 

TP annual 

loading 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 

H. Atmospheric loading of TN and TP into Lake Jesup. TN and TP loading through 

atmospheric deposition directly on the surface of Lake Jesup were calculated using the 

following equation: 

ATMLoad = PRECP * A * CON 

Where: 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

ATMLoad is the annual atmospheric loading, 

PRECP is the annual precipitation (Table 4.6), 

A is the annual average surface area of the lake, which was calculated based on the lake 

stage data (Table 4.17), and 

CON is the atmospheric TN and TP concentrations. 

In this analysis, CON values were set at 0.630 mg/L for TN and 0.05 mg/L for TP. These values 

represent the concentrations in agricultural areas (similar to the land use types along the 

shoreline of Lake Jesup) and were cited from a study conducted by Ahn and James (1999). 

Table 4.19 lists the annual TN and TP loadings from atmospheric deposition directly onto the 

surface of Lake Jesup from 1995 through 2002. 

Table 4.19. Annual TN and TP loadings through atmospheric 

deposition directly onto the surface of Lake Jesup, 

1995–2002 (tons/year) 

Loading 1995 1996 1997 1998 1999 2000 2001 2002 Mean 

TN annual 

loading 46.2 46.3 38.1 37.4 34.3 22.0 38.0 49.6 39.0

TP annual 

loading 3.7 3.7 3.0 3.0 2.7 1.7 3.0 3.9 3.1

I. TN and TP input from the St. Johns River into Lake Jesup. The water level of the St. 

Johns River influences the level of Lake Jesup and water output from the lake to the St. Johns 

River. When regional rainfall is higher than local rainfall in the Lake Jesup area, inflow from the 

St. Johns River into Lake Jesup is expected. TN and TP loadings from the St. Johns River into 

Lake Jesup were calculated by multiplying the inflow from the St. Johns River by the TN and TP 

concentrations of the St. Johns River. The inflow measurements were obtained from a gauging 

station located at the outlet of Lake Jesup (USGS 02234435, now maintained by the SJRWMD). 

TN and TP concentrations in the St. Johns River above Lake Jesup were retrieved from the 

Department’s IWR database, Run 18.2. Table 4.20 lists the annual inflow from the St. Johns 

River into Lake Jesup, TN and TP concentrations in the St. Johns River above Lake Jesup, and 

TN and TP loadings from the St. Johns River into Lake Jesup, from 1995 through 2002. 

Table 4.20. Annual flow and TN and TP loadings from the St. Johns 

River into Lake Jesup, 1995–2002 

Florida Department of Environmental Protection 

48

1995 1996 1997 1998 1999 2000 2001 2002 Mean 

St. Johns River 

inflow 

(acre-feet/year) 

11,639 10,108 55,194 16,278 88,580 51,617 96,754 26,758 44,616 

TN (mg/L) 1.134 1.254 1.329 1.367 1.704 1.897 2.472 1.441 1.575 

TP (mg/L) 0.058 0.067 0.073 0.083 0.086 0.078 0.128 0.093 0.083 

TN loading 

(tons/year) 16.3 15.6 90.5 27.4 186.2 120.7 295.0 47.6 99.9 

TP loading 0.8 0.8 5.0 1.7 9.4 5.0 15.2 3.1 5.1 
TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

(tons/year) 

J. Summary of TN and TP loadings into Lake Jesup from various sources. As noted 

previously, there are no wastewater facilities that discharge directly to surface waters in the 

Lake Jesup watershed, and nutrient inputs to Lake Jesup are primarily from nonpoint sources. 

The nonpoint sources estimated in this analysis include the following: 

• Surface runoff from the watershed, 

• Background TN and TP loadings through baseflow from the surficial aquifer, 

• Septic tank TN and TP loadings, 

• Artesian input through springs and upward lake bottom leakage from the Floridan aquifer, 

• Atmospheric deposition directly onto the surface of Lake Jesup (including wet and dry 

deposition), and 

• St. Johns River inflow. 

Tables 4.21-a, 4.21-b, and 4.21-c summarize the annual flow and TN and TP loadings from all 

these sources into Lake Jesup for 1995 through 2002. 

Table 4.21-a. Annual flow from various sources into Lake Jesup, 1995–

2002 (acre-feet/year) 

Source 1995 1996 1997 1998 1999 2000 2001 2002 Mean Percent

Surface 

runoff 67,938 95,254 64,396 70,269 49,950 35,616 73,697 103,293 70,052 37.5% 

Baseflow 

(background) 16,985 23,814 16,099 17,567 12,487 8,904 18,424 25,823 17,513 9.4% 

Artesian 

input 4,585 4,556 4,520 4,580 4,547 4,449 4,537 4,568 4,543 2.4% 

Atmospheric 

deposition 59,513 59,535 49,003 48,114 44,136 28,259 48,921 63,801 50,160 26.8% 

St. Johns 

River inflow 11,639 10,108 55,194 16,278 88,580 51,617 96,754 26,758 44,616 23.9% 

Total 160,660 193,267 189,212 156,808 199,700 128,845 242,333 224,243 186,884 100.0% 

Table 4.21-b. Annual TN loading from various sources into Lake Jesup, 

1995–2002 (tons/year) 

Source 1995 1996 1997 1998 1999 2000 2001 2002 Mean Percent 

Surface runoff 125.7 176.8 119.2 130.4 92.4 66.1 136.6 191.6 129.9 43.0% 

Base flow 

(background) 10.1 14.2 9.6 10.5 7.4 5.3 11.0 15.4 10.4 3.4% 

Septic tanks 19.7 19.7 19.7 19.7 19.7 19.7 19.7 19.7 19.7 6.5% 

Artesian input 3.5 3.4 3.4 3.5 3.4 3.4 3.4 3.4 3.4 1.1% 

Atmospheric 

deposition 46.2 46.3 38.1 37.4 34.3 22.0 38.0 49.6 39.0 12.9% 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

St. Johns 

River inflow 16.3 15.6 90.5 27.4 186.2 120.7 295.0 47.6 99.9 33.0% 

Total 221.5 276 280.5 228.9 343.4 237.2 503.7 327.3 302.3 100.0% 

Table 4.21-c. Annual TP loading from various sources into Lake Jesup, 

1995–2002 (tons/year) 

Source 1995 1996 1997 1998 1999 2000 2001 2002 Mean Percent 

Surface runoff 13.1 19.4 12.6 14.2 9.6 7.4 14.7 20.7 14.0 48.6% 

Baseflow 

(background) 3.2 4.5 3.1 3.3 2.4 1.7 3.5 4.9 3.3 11.5% 

Septic tanks 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 9.4% 

Artesian input 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 2.1% 

Atmospheric 

deposition 3.7 3.7 3.0 3.0 2.7 1.7 3.0 3.9 3.1 10.8% 

St. Johns 

River inflow 0.8 0.8 5.0 1.7 9.4 5.0 15.2 3.1 5.1 17.7% 

Total 24.1 31.7 27 25.5 27.4 19.1 39.7 35.9 28.8 100.0% 

All totaled, about 186,884 acre-feet of water are discharged into Lake Jesup on a long-term 

annual average basis from various sources. About 46.9% of the discharge comes from the 

watershed through either surface runoff or baseflow. Contributions from rainfall directly onto the 

surface of the lake and inflow from the St. Johns River are comparable; these account for about 

26.8% and 23.9% of the annual discharge, respectively. The water contribution from the 

Floridan aquifer is relatively minor, at about 2.4% of the total annual discharge. 

The watershed contributes about 51.9% of the 302.3 tons of TN discharged into Lake Jesup 

annually through surface runoff, baseflow, and septic tanks. Another major source is the St. 

Johns River inflow, which accounts for about 33% of the TN input to the lake. Atmospheric 

deposition contributes about 12.9%, and again, the contribution from the Floridan aquifer is 

insignificant, about 1.1% of the total TN annual load. 

The majority of TP loading into Lake Jesup comes from the watershed, through surface runoff, 

baseflow, and septic tanks. Of the 28.8 tons of TP loading into the lake, about 69.5% comes 

from within the watershed. Of this 69.5%, surface runoff produces about 48.6%, and baseflow 

and septic tanks contribute 11.5% and 9.4%, respectively. Surface runoff is apparently the most 

important source of phosphorus for the lake. Another major source of phosphorus is the St. 

Johns River, which contributes about 17.7% of TP on a long-term annual average basis. 

Atmospheric deposition directly onto the surface of the lake and artesian input produce about 

10.8% and 2.1% of the total TP loading, respectively. 

Based on the above analyses, the watershed is the major contributor for TN and TP loadings 

into Lake Jesup. However, two other sources whose importance has not been evaluated so far 

are nutrient releases from lake sediments and nitrogen fixation; the next chapter discusses 

these sources in detail. 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Chapter 5: DETERMINATION OF ASSIMILATIVE 

CAPACITY 

5.1 Overall Approach 

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

loadings to the lake, so that the lake will meet water quality standards and maintain its functions 

and designated uses as a Class III water. Specifically, the water quality target in this analysis is 

a TSI of 65 (Chapter 3). The following three steps were taken to achieve this goal: 

1. TP and TN loadings from the Lake Jesup watershed were estimated using the 

PBS&J model (see Chapter 4). Loadings from other sources, including artesian 

input, atmospheric deposition directly onto the lake’s surface, and input from the 

St. Johns River were also considered in the loading estimation.

2. Loading estimates from all sources 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 by calibrating the Bathtub model against the 

measured in-lake TN, TP, and chl a concentrations. The calibrated Bathtub 

model was then used to predict in-lake TN, TP, and chl a concentrations and 

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

later in this report.

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

results reached the target TSI, and the TN and TP loadings that resulted in a TSI 

of 65 provided the TN and TP (nutrient) TMDLs for Lake Jesup. 

Ionized ammonia is part of the ammonia that is unionized, and its concentration usually 

increases along with an increase in pH (Wetzel, 2001). The pH of lake water can be 

significantly controlled by photosynthetic activity. Photosynthesis takes up the carbon dioxide 

(CO2) and bicarbonate (HCO3

-1) in the water and increases the pH. When nutrients, including 

TN and TP, decrease, the total algal biomass in the lake and photosynthetic activity are 

expected to decrease. This decreases the pH of the water and therefore the concentration of 

unionized ammonia. In this analysis, the target TN, TP, and chl a concentrations; the 

relationship between unionized ammonia and pH; and the relationship between pH and chl a

concentration were examined to ensure that the TN and TP targets will also decrease 

unionized ammonia. 

5.1.1 Entering Loading Estimates from All Sources into the Bathtub 

Eutrophication Model 

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51

Bathtub is a suite of models developed by the U. S. Army Corps of Engineers (USACOE) 

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 
TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

procedures for selecting the appropriate model(s) for a particular lake are described in the 

Users’ Manual (Walker, 1999). The model suite is composed of two types of models, as follows: 

• Nutrient balance models relate in-lake nutrient concentration to external nutrient loadings, 

morphometry, and hydrology, and 

• Eutrophication response models estimate chlorophyll concentration, transparency, and 

hypolimnetic oxygen depletion, based on the in-lake nutrient concentration established by 

the nutrient balance models. 

Figure 5.1 describes the conceptual scheme used by Bathtub to relate the 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. 

Florida Department of Environmental Protection 

52

Flow from 

various 

sources Nutrient balance model 

Figure 5.1. Bathtub concept scheme 

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 the losses of nutrients through whatever decay 

processes occur inside the lake, as follows: 

Net accumulation = Input - Decay - Output 

In this analysis, “input” included TN and TP loadings though stormwater surface runoff from 

various land use categories, baseflow contribution (including contributions from septic tanks), 

artesian input, atmospheric precipitation, and inflow from the St. Johns River. Nutrient output 

was considered primarily through the lake’s outflow to the St. Johns River. 

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

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

Storage Change = Input – Decay - Output

Eutrophication response model 

Chl a = f(TN, TP, light, flush, etc) 

Nutrients 

from 

various

sources 

Model simulated TN and TP concentration Model simulated chl a concentration

Measured TN and TP concentration

Calibration Calibration 

Measured chl a concentration
TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

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

actual sedimentation rate is the net difference between the gross sedimentation rate and 

sediment resuspension rate. 

The prediction of the eutrophication response model by Bathtub involves choosing one of 

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

or nitrogen, or co-limited by both nutrients. The suite of models also includes scenarios for algal 

communities limited by light intensity or controlled by the lake flushing rate. In addition, the 

response of chl 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 specific condition of an individual lake. 

The data requirements for the Bathtub model include the following: 

• The lake’s physical characteristics (surface area, mean depth, length, and mixed layer 

depth), 

• Meteorological data (precipitation and evaporation retrieved from CIRRUS), 

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

and TN and TP concentrations in precipitation), and 

• Loading data (flow combined with concurrent TN and TP concentrations from various 

sources). 

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

processes inherent to a lake. The calibration factor offered by Bathtub allows model users 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 the changes in lake conditions that are likely to result from specific management 

scenarios, under the assumption that the calibration factor remains constant for all prediction 

scenarios. 

5.1.1.1 Calculation of the Trophic State Index 

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

follows: 

TSI = (CHLATSI + NUTRTSI)/2 

Where: 

CHLATSI = 16.8 + 14.4 × LN (CHLA) 

TNTSI = 56 + [19.8 × LN(TN)] 

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

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

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 

5.1.1.2 TMDL Scenario Development for Lake Jesup 

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

A. TSI for current conditions. The current TSI for Lake Jesup was calculated based on the 

annual average TN, TP, and chl a concentrations obtained from the Department’s IWR 

database. 

B. Natural background TSI. This is the TSI calculated based on the TN, TP, and chl a

concentrations resulting from a watershed condition in which all human land uses—including 

agriculture/golf course; low-, medium-, and high-density residential; commercial; 

industrial/utilities; institutional; and transportation facilities—discharge pollutants with the same 

characteristics as those associated with natural land uses. In the actual modeling process, all 

the areas covered by human land uses were converted to forest/rural open and wetland areas. 

The target TSI is considered the background TSI plus 5 TSI units. 

The background TSI was estimated using the model settings calibrated against the measured 

data, which included the lake outflow measured at the lake outlet (Gauging Station ID: 

02234324) and measured TN, TP, and chl a concentrations for Lake Jesup. Because daily flow 

measurements at the outlet of Lake Jesup only started in 1995, the period of record used for 

model calibration was 1995 through 2002. 

5.1.1.3 Data Analysis 

Historical trends in the trophic status of Lake Jesup. TN, TP, and chl a concentrations for 

Lake Jesup from 1995 through 2002 were retrieved from the IWR database. Table 5.1 shows 

station IDs and the latitude and longitude of the water quality stations from which TN, TP, and 

chl a concentrations were measured. Figure 5.2 shows the locations of these water quality 

stations. 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Figure 5.2. Locations of water quality stations in Lake Jesup 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 5.1. Water quality stations from which TN, TP, and chl a 

concentrations were measured 

Station 

Number Station ID Latitude Longitude 

1 21FLSJWM44059 28.715 81.277 

2 21FLSJRWOW-6 28.715 81.274 

3 21FLSJWMWLJESS 28.716 81.273 

4 21FLSJRWOW-5 28.710 81.261 

5 21FLSJWM44057 28.705 81.254 

6 21FLSJRWOW-4 28.707 81.237 

7 21FLGFWFGFCCR0477 28.726 81.224 

8 21FLKWATSEM-JESUP-1 28.724 81.215 

9 21FLSJRWOW-7 28.739 81.216 

10 21FLSJWMELJESS 28.737 81.197 

11 21FLA 20010183 28.735 81.195 

12 21FLSJRWOW-3 28.735 81.183 

13 21FLSJWM44055 28.765 81.176 

14 21FLSJRWOW-2 28.766 81.175 

15 21FLSJWMUSJ027 28.777 81.180 

16 21FLVEMDSJ07 28.778 81.176 

Quarterly mean values for TN, TP, and chl a concentrations were calculated based on the TN, 

TP, and chl a concentrations measured for the sites listed in Table 5.1, 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 1995 through 2002. The long-term annual average values for the entire verified 

period were calculated based on the individual mean values of each year from 1996 through 

2002 (the verified period only includes a half-year for 2003, and the annual average values for 

2003 were not calculated). Seasonal trends for 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 from 1995 through 2002. The quarterly means for the verified period were calculated using 

the data from 1996 through 2002. Table 5.2 lists the individual annual mean TN, TP, chl a, and 

TSI values for Lake Jesup for 1995 through 2002, and Table 5.3 lists the quarterly mean for TN, 

TP, chl a, and TSI over the long term and for the verified period. 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 5.2. Annual averages of TN, TP, chl a, and TSI values of Lake 

Jesup, 1995–2002. Data represent the mean ± 1 

standard deviation (n = 4) 

Year TP (mg/L) TN (mg/L) Chl a (μg/L) TSI 

1995 0.19 ± 0.09 2.30 ± 1.09 64.0 ± 45.9 74.5 ± 8.2 

1996 0.17 ± 0.03 2.46 ± 0.35 64.8 ± 16.3 75.9 ± 2.9 

1997 0.14 ± 0.03 2.44 ± 0.40 51.3 ± 7.7 73.4 ± 2.4 

1998 0.15 ± 0.04 2.59 ± 0.72 87.8 ± 37.4 77.4 ± 5.7 

1999 0.16 ± 0.04 3.44 ± 0.86 110.2 ± 25.7 81.0 ± 4.5 

2000 0.14 ± 0.03 3.75 ± 0.72 159.9 ± 59.4 83.2 ± 4.2 

2001 0.14 ± 0.02 4.47 ± 1.21 143.5 ± 35.9 83.6 ± 3.8 

2002 0.16 ± 0.04 1.99 ± 0.50 64.8 ± 25.4 74.2 ± 4.8 

Mean 0.16 ± 0.02 2.93 ± 0.31 93.3 ± 16.5 77.9 ± 1.8 

Mean–

Verified Period 0.15 ± 0.01 3.02 ±0.30 97.5 ± 16.7 78.4 ± 1.1 

Table 5.3. Seasonal variation of TN, TP, chl a, and TSI in Lake 

Jesup 

Long-term Quarterly Mean 

TP (mg/L) TN (mg/L) Chl a (mg/L) TSI 

1st Quarter 0.15 ± 0.03 2.58 ± 1.10 77.4 ± 49.8 75.7 ± 6.0 

2nd Quarter 0.20 ± 0.05 3.66 ± 1.09 115.2 ± 38.3 82.3 ± 4.0 

3rd Quarter 0.16 ± 0.04 3.02 ± 1.07 102.5 ± 57.8 78.7 ± 5.9 

4th Quarter 0.12 ± 0.02 2.46 ± 0.72 77.9 ± 48.5 74.8 ± 4.7 

Verified Period Quarterly Mean 

TP (mg/L) TN (mg/L) Chl a (mg/L) TSI 

1st Quarter 0.15 ± 0.02 2.71 ± 1.12 83.37 ± 50.5 76.4 ± 6.1 

2nd Quarter 0.18 ± 0.02 3.62 ± 1.18 112.77 ± 40.7 81.7 ± 4.0 

3rd Quarter 0.16 ± 0.04 3.19 ± 1.05 110.17 ± 57.9 79.6 ± 5.8 

4th Quarter 0.12 ± 0.02 2.57 ± 0.71 83.50 ± 49.59 75.7 ± 4.3 

As shown in Table 5.2, the annual average TP concentration in Lake Jesup ranged from 0.14 to 

0.19 mg/L (1995–2002), and from 0.14 to 0.17 mg/L in the verified period (1996–2002). The 

variation in the annual average TP during the period of record appeared to be relatively small. 

The long-term and verified period annual average TP concentrations for the lake are both about 

0.16 mg/L. Larger variations were observed for annual average TN and chl a concentrations. 

Annual average TN and chl a concentrations from 1995 through 2002 ranged from 1.99 to 4.47 

mg/L, and from 51.3 to 159.9 μg/L, respectively. These variations were also observed in the 

verified period (1996–2002). The long-term annual average TN and chl a concentrations were 

2.93 and 93.3 μg/L, respectively, and for the verified period, these were 3.02 and 97.5 μg/L, 

Florida Department of Environmental Protection 

57
TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

respectively. The long-term annual average TSI for the lake is about 77.9, and the verified 

period TSI is about 78.4. 

Nutrient and chl a concentrations in the lake do not always correlate with the amount of rainfall. 

Figures 5.3-a, 5.3-b, 5.3-c, and 5.3-d show the relationships between TP, TN, and chl a

concentrations and TSI, respectively, with annual rainfall. No obvious relationship was 

observed between annual rainfall and annual average TP concentration (Figure 5.3-a). In 

contrast, correlations were observed between annual rainfall and annual average TN 

concentration, and between annual rainfall and annual average chl a concentration (Figures 

5.3-b and 5.3-c). It appears that whenever annual rainfall is high, TN and chl a concentrations 

are low, and when annual rainfall is low, TN and chl a concentrations are high. The highest TN 

and chl a concentrations observed in 1999, 2000, and 2001 appear to be associated with low 

annual rainfall in these three years. 

The concentration effect due to the decrease of lake volume could have caused the increase in 

concentration; however, the simple concentration effect could not fully explain nutrient dynamics 

under the low rainfall condition, because no significant increase of TP concentration was 

observed during these same dry years. Some in-lake chemical and biochemical processes 

must also be affecting the nutrients and algal biomass dynamics observed. Nitrogen fixation 

may be an important process contributing to nitrogen dynamics. The change in cellular chl a

content from some blue-green algal species could have contributed to the change in chl a

concentrations in the lake water. The following section explores these possibilities in more 

detail. 

Nutrient and chl a concentrations tend to be higher in the second and third quarters of the year. 

However, when taking into account the high standard deviations in these seasons, the quarterly 

mean values of TN, TP, and chl a concentrations in the second and third quarters are not 

significantly different from those of the first and fourth quarters (Table 5.3).

5.1.1.4 Establishing the Relationship between TN and TP Loadings 

and In-lake TN, TP, and Chl a Concentrations Using the Bathtub 

Model; Model Calibration 

The relationship between TN and TP loadings and in-lake TN and TP concentrations was 

established by fitting the Bathtub predictions with the measured TN and TP concentrations of 

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

• The lake’s physical characteristics, 

• Meteorological data (precipitation and evaporation), 

• Areal atmospheric deposition of nutrients directly onto the surface of the lake, 

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

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

The lake’s physical characteristics, including surface area, change of lake storage, mean depth, 

and mixed layer depth, were calculated based on the characteristic curve of Lake Jesup 

(Figures 4.8 and 4.9) and the lake stage measurements obtained from the gauging station 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

located at the outlet of Lake Jesup. Table 5.4 lists the annual lake stage, lake surface area, 

change of lake storage, mean depth, and mixed layer depth of Lake Jesup from 1995 through 

2002. 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

 c

0.0

20.0

40.0

60.0

80.0

100.0

120.0

140.0

160.0

180.0

1995 1996 1997 1998 1999 2000 2001 2002

Year

Chl a concentration 

(μg/L)

0

10

20

30

40

50

60

70

Annual rainfall (in/year)

Chla

Rainfall

a

0.00

0.05

0.10

0.15

0.20

0.25

1995 1996 1997 1998 1999 2000 2001 2002

Year

TP concentration (mg/L)

0

10

20

30

40

50

60

70

Annual rainfall (in/year)

TP

Rainfall

d

68.0

70.0

72.0

74.0

76.0

78.0

80.0

82.0

84.0

86.0

1995 1996 1997 1998 1999 2000 2001 2002

Year

TSI

0

10

20

30

40

50

60

70

Annual Rainfall (in/year)

TSI

Rainfall

b

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

1995 1996 1997 1998 1999 2000 2001 2002

Year

TN concentration (mg/L)

0

10

20

30

40

50

60

70

Annual rainfall (in/year)

TN

Rainfall

Figures 5.3-a, 5.3-b, 5.3-c. Relationship between annual rainfall; 

concentrations of TN, TP, and chl a; and TSI in Lake Jesup, 

1995–2002 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 5.4. Annual lake characteristics of Lake Jesup for the modeling period, 

1995–2002 

Year 

Annual 

Average 

Lake Stage 

(feet) 

Annual 

Average 

Lake 

Surface 

(square 

kilometers) 

Annual 

Average 

Lake 

Volume 

(square 

kilometers) 

Annual 

Average 

Mean Depth 

(meters) 

Annual 

Average 

Mixing Depth 

(meters) 

Annual Change of 

Lake Storage 

(meters) 

1995 3.42 47.30 0.071 1.51 1.51 -0.88 

1996 2.44 45.18 0.058 1.28 1.28 -0.61 

1997 1.94 44.07 0.051 1.16 1.16 1.37 

1998 3.08 46.51 0.067 1.43 1.43 -1.30 

1999 2.30 44.88 0.056 1.24 1.24 0.56 

2000 1.29 42.36 0.043 1.01 1.01 -0.64 

2001 2.15 44.54 0.054 1.21 1.21 0.33 

2002 2.69 45.69 0.061 1.34 1.34 0.46 

Mean 2.42 45.07 0.058 1.27 1.27 -0.09 

The annual change in lake storage shown in Table 5.4 was calculated as the difference 

between lake stage at the beginning (January 1) and the end (December 31) of each year 

(Walker, 1999). Because the mean depth of the lake is relatively low (a long-term average of 

1.27 meters), it was assumed that most of the time the lake was completely mixed vertically, 

and therefore the annual average mixing depth was assumed to be equal to the mean depth of 

the lake. 

Meteorological data were retrieved from CIRRUS, which has 2 weather stations in the area. 

Daily rainfall measurements were obtained from a weather station located in Sanford (UCAN: 

4129, COOP: 087982), and the daily evaporation data were obtained from Lisbon (UCAN: 4025, 

COOP: 085076). Table 5.5 lists annual rainfall and evaporation values for 1995 through 2002. 

The daily evaporation data were adjusted using a lake-pan evaporation coefficient of 0.86 

(Keesecker, 1992). 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 5.5. Annual meteorological data used for Bathtub modeling, 

1995–2002 

Year Annual rainfall 

(meters/year) 

Annual evaporation 

(meters/year) 

1995 1.51 1.07 

1996 1.60 1.18 

1997 1.36 1.11 

1998 1.24 0.92 

1999 1.19 1.00 

2000 0.83 1.17 

2001 1.34 1.01 

2002 1.68 1.10 

Mean 1.34 1.07

Areal atmospheric nutrient loadings: nutrient loads through atmospheric deposition directly 

onto the surface of the lake were calculated in Bathtub based on areal atmospheric loading 

rates, which in turn were calculated using the following equation: 

L = R * C 

Where: 

L is the areal atmospheric nutrient-loading rate, 

R is the annual rainfall, and 

C is the atmospheric TN and TP concentration (bulk concentration including both wet and 

dry deposition [Redfield, 2002]). 

Table 5.6 lists the areal atmospheric deposition rate for each year of the modeling period from 

1995 through 2002. Table 5.2 provides annual measured water quality data for Lake Jesup 

from 1995 through 2002. Tables 5.7-a, 5.7-b, and 5.7-c contain the annual flow and nutrient 

concentrations from each major nonpoint source into Lake Jesup from 1995 through 2002. 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 5.6. Annual areal atmospheric nutrient loadings to Lake Jesup, 

1995–2002 

Year 

Annual 

Rainfall 

(inches/year) 

Annual Mean 

Lake Surface 

(km2) 

Atmospheric TP 

concentration 

(mg/L) 

Atmospheric TN 

concentration 

(mg/L) 

Areal 

atmospheric 

load for TP 

(kg/km2/year) 

Areal 

atmospheric 

load for TN 

(kg/km2/year) 

1995 59.32 47.30 0.05 0.63 75 949 

1996 62.82 45.18 0.05 0.63 80 1005 

1997 53.69 44.07 0.05 0.63 68 859 

1998 48.83 46.51 0.05 0.63 62 781 

1999 47.04 44.88 0.05 0.63 60 753 

2000 32.83 42.36 0.05 0.63 42 525 

2001 52.73 44.54 0.05 0.63 67 844 

2002 66.24 45.69 0.05 0.63 84 1060 

Mean 52.94 45.07 0.05 0.63 67 847 

Table 5.7-a. Annual flow from major nonpoint sources into Lake 

Jesup, 1995–2002 (cubic hectometers [HM3]/year) 

Source 1995 1996 1997 1998 1999 2000 2001 2002 Mean 

Gee Creek 6.4 9.2 6.1 6.7 4.7 3.5 7.1 9.9 6.7 

Howell Creek 27.9 40.1 26.6 29.4 20.4 15.3 30.8 43.1 29.2 

Lake Jesup 38.1 52.0 35.9 38.6 28.1 19.1 40.6 56.8 38.6 

Little Lake Howell 2.3 3.3 2.2 2.4 1.7 1.3 2.6 3.6 2.4 

Soldiers Creek 9.1 12.9 8.7 9.5 6.7 4.8 9.9 14.0 9.5 

Baseflow 21.0 29.4 19.9 21.7 15.4 11.0 22.7 22.7 31.9 

Artesian leakage 

through the lake 

bottom 

3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 

Spring inflow 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 

St. John River’s 

inflow 14.0 12.0 68.0 20.0 109.0 64.0 119.0 33.0 55.0 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 5.7-b. Annual TN concentration of flows from major nonpoint 

sources into Lake Jesup, 1995–2002 (parts per billion 

[ppb]) 

Source 1995 1996 1997 1998 1999 2000 2001 2002 Mean 

Gee Creek 1,488 1,495 1,490 1,494 1,488 1,494 1,492 1,494 1,492 

Howell Creek 1,443 1,454 1,446 1,452 1,442 1,458 1,451 1,452 1,450 

Lake Jesup 1,584 1,588 1,584 1,588 1,583 1,591 1,586 1,586 1,586 

Little Lake Howell 1,275 1,275 1,276 1,276 1,277 1,266 1,274 1,277 1,275 

Soldiers Creek 1,390 1,390 1,391 1,390 1,390 1,387 1,390 1,391 1,390 

Baseflow* 1,423 1,154 1,475 1,392 1,762 2,276 1,350 1,102 1,492 

Artesian leakage 

through the lake 

bottom 

611 611 611 611 611 611 611 611 611 

Spring inflow 611 611 611 611 611 611 611 611 611 

St. John River 

inflow 1,134 1,254 1,329 1,367 1,704 1,897 2,472 1,441 1,575 

* Baseflow TN concentration includes the contribution from septic tanks. 

Table 5.7c. Annual TP concentration of flows from major nonpoint 

sources into Lake Jesup, 1995–2002 (ppb) 

Source 1995 1996 1997 1998 1999 2000 2001 2002 Mean 

Gee Creek 187 195 189 193 187 200 192 193 192 

Howell Creek 191 200 193 198 190 206 197 197 197 

Lake Jesup 128 135 130 134 128 137 132 133 132 

Little Lake Howell 140 148 142 146 140 151 145 146 145 

Soldiers Creek 153 161 156 160 153 163 158 160 158 

Baseflow* 282 246 289 278 328 398 272 238 291 

Artesian leakage 

through the lake 

bottom 

110 110 110 110 110 110 110 110 110 

Spring inflow 110 110 110 110 110 110 110 110 110 

St. John River’s 

inflow 58 67 73 83 86 78 128 93 83 

* Baseflow TP concentration includes the contribution from septic tanks. 

The flow and nutrient concentrations from the Lake Jesup watershed were divided into flows 

and nutrient concentrations from five sub-basins: Gee Creek, Howell Creek, the area directly 

connecting to Lake Jesup, Little Lake Howell, and Soldier Creek. Net flow and net TN and TP 

concentrations after the BMP treatments were used for model calibration because existing 

BMPs were considered as part of the current condition for the modeling. 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

5.1.1.5 Calibrating the Bathtub Eutrophication Model 

To calibrate the Bathtub model, each source of TN and TP previously identified was entered into 

the model as an independent tributary (Tables 5.7-a, 5.7-b, and 5.7-c). Figures 5.4-a, 5.4-b, 

and 5.4-c show the model results for lake volume and TP and TN calibration, respectively, for 

1995 through 2002. 

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

and TP concentrations (Walker, 1999). In this analysis, the Settling Velocity Model was chosen 

for both TN and TP. This model assumes that the sedimentation of TN and TP follows firstorder kinetics and should linearly correlate with in-lake TN and TP concentrations. The model 

also assumes that the depth of the lake influences sedimentation, i.e., the deeper the lake, the 

slower the sedimentation. This model fits the condition of Lake Jesup, because the lake is 

relatively shallow and large in surface area. Continued wind mixing prevents the lake from 

forming thermal stratification, which would otherwise prevent the particles from being 

resuspended once they settled to the bottom. Continued wind mixing through the entire water 

column also reduces the particle-settling rate by bringing the settled particles back into the 

water column. These processes produce a relatively low net settling rate in Lake Jesup. 

Other sedimentation models provided by Bathtub assume second-order kinetics, which fit 

reasonably well with lakes that form thermal stratification during the summer. However, these 

models would overestimate the net sedimentation of Lake Jesup, and in turn cause the in-lake 

TN and TP concentrations to be underestimated. 

Figure 5.4-a shows the calculated change of lake volume, based on model-simulated flow from 

the watershed, and the measured change of lake volume, based on the lake stage from the 

gauging station located at the outlet of Lake Jesup. The model-simulated lake volume change 

traces the measured volume change reasonably well for most years in the modeling period, 

except for 1995 and 2001. Increases in lake volume were apparently underestimated in 1995 

and overestimated in 2001. Based on the rainfall data analysis, 1995 was the end of three 

consecutive wet years, and the soil moisture condition designated by the SCS curve number 

technique may not fully reflect the actual soil moisture condition, resulting in an underestimate of 

runoff to the lake. In contrast, 2001 was the end of three consecutive dry years; the watershed 

model may not fully represent the moisture debt of the soil and may result in an overestimate of 

the amount of runoff discharged into the lake. 

The model-simulated TP concentrations were also reasonably consistent with the measured TP 

concentration (Figure 5.4-b). The long-term average error for model estimates was about 10%. 

In contrast, there was a large gap between model-simulated TN and measured TN 

concentrations for all the years in the modeled period (Figure 5.4-c). The following three 

possibilities were examined to address the difference between the measured and modelsimulated TN concentrations: 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

a

-150000

-100000

-50000

0

50000

100000

150000

1995 1996 1997 1998 1999 2000 2001 2002

Year

Lake Volume Change (ac-ft)

Calculated Difference Actual change of Lake Jesup volume

c

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

1995 1996 1997 1998 1999 2000 2001 2002

Year

TN concentration (mg/L)

Measured Model simulated

b

0.00

0.05

0.10

0.15

0.20

0.25

1995 1996 1997 1998 1999 2000 2001 2002

Year

TP concentration (mg/L)

Measured Model simulated

Florida Department of Environmental Protection 

66

-a, 5.4-b, 5.4-c. Results for flow and TN and TP calibrations, 

using the Bathtub model, 1995–2002 

Figures 5.4
TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Possibility A: The TN concentration of the flow (including surface runoff and baseflow) 

from the watershed could be underestimated. 

The TN concentration of the flow from the watershed was examined because (1) artesian inputs 

(spring flow and bottom leakage) only account for a very small portion of the total flow and 

nitrogen budget, and therefore an error in the artesian input should not influence the final lake 

concentration too dramatically; (2) since TN concentrations for the St. Johns River inflow are 

measured values, and the TN concentration is consistently and sufficiently higher than the 

minimum detection limit, long-term dramatic errors are not very likely; and (3) the rainfall TN 

concentration appears to be reasonable. Based on these analyses, if there is a major model 

error, the error should be associated with the watershed flow into the lake. 

If the SCS curve number approach underestimated the TN concentration of the flow from the 

watershed, observed tributary TN concentrations should be significantly higher than the modelsimulated TN concentration for the watershed flow. The long-term average TN concentrations 

for waterbodies discharging into Lake Jesup were analyzed, and Table 5.8 lists the results. 

Based on Table 5.8, the average TN concentration of the waterbodies discharging into Lake 

Jesup is about 1.181 ± 0.523 mg/L, which is not significantly different from the model-simulated 

average TN concentration of the flow from the watershed, or about 1.444 ± 0.115 mg/L (Table 

5.7-b). This result indicates that the SCS curve number–based PBS&J model did not 

underestimate the TN concentration from the watershed flow. Therefore, Possibility 1 is 

rejected. In fact, Table 5.8 also demonstrates that, even for measured TN values, the Lake 

Jesup TN concentration is about 2 times higher than the TN concentration for tributaries, 

suggesting that in-lake processes could be responsible for the higher TN concentration in Lake 

Jesup. 

Table 5.8. Long-term average TN concentrations for waterbodies 

discharging into Lake Jesup, calculated based on 

measured TN concentrations 

Waterbody WBID TN 

(mg/L) 

Soldier Creek 2986 0.970 

Gee Creek 2994A 0.907 

Howell Creek 2997 0.957 

Bear Creek 2999 0.960 

Lake Jesup Drain 2981E 1.041 

Sweetwater Creek 2996 1.689 

Lake Charm Shortcut 2995A 0.630 

Sweetwater Creek 2992 1.222 

Salt Creek 2990 1.710 

Chub Creek 2985 2.321 

Phelps Creek 2982 0.584 

Mean 1.181 

Lake Jesup 2981 2.445 

Florida Department of Environmental Protection 

67
TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Possibility B: Nutrient release from the sediment is responsible for the fact that TN 

concentrations in Lake Jesup are higher than those of the waterbodies discharging into 

the lake. 

Nutrient release from the sediment could take several different pathways: (1) the nutrient 

resuspension of sediment through wind mixing; (2) the direct diffusion of nutrients into the water 

column; and (3) nutrient translocation through biological activity. 

If the high TN concentration in Lake Jesup were mainly caused by the resuspension of 

sediment, a significant enhancement of this concentration should also be observed with TP, 

because TP is more electrically charged than TN and has a stronger tendency to bind with 

sediment particles and re-enter the water column together with suspended sediment. In 

contrast, TN is more soluble in the water and less easily associated with particulate materials. 

Table 5.9 shows the TP concentrations of Lake Jesup and the TP concentration of waterbodies 

discharging into Lake Jesup. The long-term average TP concentration for the waterbodies that 

discharge into Lake Jesup is about 0.199 ± 0.114 mg/L, which is not significantly different from 

the long-term average TP concentration for Lake Jesup of 0.142 mg/L. Apparently, the TP 

concentration of Lake Jesup is not significantly higher than the TP concentrations of the 

waterbodies that discharge into Lake Jesup, suggesting that the sediment resuspension may 

not be the major cause for TN enhancement. 

Table 5.9. Long-term average TP concentrations for waterbodies 

discharging into Lake Jesup, calculated based on 

measured TN concentrations 

Waterbody WBID TP 

(mg/L) 

Soldier Creek 2986 0.140 

Gee Creek 2994A 0.119 

Howell Creek 2997 0.120 

Bear Creek 2999 0.140 

Lake Jesup Drain 2981E 0.133 

Sweetwater Creek 2996 0.355 

Lake Charm Shortcut 2995A 0.300 

Sweetwater Creek 2992 0.302 

Salt Creek 2990 0.224 

Chub Creek 2985 0.351 

Phelps Creek 2982 0.005 

Mean 0.199 

Lake Jesup 2981 0.142 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Another observation working against the supposition that TN enhancement in Lake Jesup is due 

to sediment resuspension is the dissolved oxygen (DO) concentration of the lake. Figure 5.5

shows the annual average DO concentration in Lake Jesup from 1995 through 2002. If 

constant sediment resuspension were an important process in Lake Jesup, nutrients should not 

be the only materials being brought back to the water column. Organic materials in the 

sediment would also be resuspended in the water column, and this process should consume 

DO and cause the DO concentration to decrease. 

Of course, if the water can support a large quantity of phytoplankton, photosynthetic oxygen 

production may counterbalance DO consumption. When sediment resuspension becomes a 

problem, however, the enhanced turbidity could significantly shield light irradiance and limit algal 

growth, as well as DO production through photosynthesis. Therefore, the overall effect of the 

constant sediment resuspension should be a decrease of DO concentration in the water. 

However, this is not what is observed in Figure 5.5. The annual average DO concentrations of 

the lake were consistently higher than 8 mg/L from 1995 through 2002. DO concentrations did 

not change significantly when the TN concentration increased from around 2 mg/L to more than 

4 mg/L, indicating that the sediment resuspension may not be a very significant cause of the TN 

enhancement in Lake Jesup 

The assumption that nitrogen is being resuspended in Lake Jesup is also contradictory to the 

observation in a sediment core study conducted by the SJRWMD that the TN deposition rate for 

Lake Jesup is positive, i.e., the direction for TN in Lake Jesup is a net deposition to the 

sediment (Cable et al., 1997). 

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

1995 1996 1997 1998 1999 2000 2001 2002

Year

TN concentration (mg/L)

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

DO concentration (mg/L)

DO concentration Measured Model simulated

Figure 5.5. Annual average DO concentration in Lake Jesup, 1995–2002 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Diffusion could be another pathway through which nitrogen enters the water column from the 

sediment. Ammonia is the major form of nitrogen for sediment nutrient diffusion (Chiario and 

Burke, 1980; Hu et al., 2001). In sediments, the majority of ammonia is usually adsorbed on 

sediment particles (Wetzel, 2001). Under anaerobic conditions, however, the adsorptive 

capacity of the sediments is greatly reduced, and a marked release of ammonia from the 

sediments occurs (Kamiyama et al., 1977; Verdouw et al., 1985). 

During the time this analysis was conducted, no reliable data were available for sediment 

nutrient diffusion in Lake Jesup. Anaerobic conditions at the bottom of Lake Jesup are not likely 

to be a frequent phenomenon, however, because the lake is very shallow and its surface area is 

relatively large. The water column is well mixed vertically most of the time. The DO results 

from investigations conducted by the SJRWMD in 1995 and 1996 indicated no anaerobic 

conditions at the bottom of the lake. In most cases, the bottom DO concentration was not 

significantly different from the DO concentration measured at the surface of the lake (Appendix 

B). These observations are supported by an investigation conducted by the Department in 

January 2004, in which no DO hypoxia was found at the bottom of Lake Jesup in any part of the 

lake sampled. These observations rule out any long-lasting anaerobic condition, which would 

be required for a significant release of ammonia from the sediment through diffusion. 

In addition, the dynamics of ammonia concentration in Lake Jesup do not support the significant 

release of ammonia from the sediment. As shown in Figure 5.6, when the TN concentration 

increased from 1999 through 2001, no increase in ammonia concentration was observed. 

Instead, ammonia concentrations decreased during this period. While it is commonly accepted 

that ammonia is the form of nitrogen favored by phytoplankton in ambient water and can be 

taken up very rapidly when added into the water, if the increase of TN concentration were 

caused by the release of a large amount of ammonia into the water column, the ammonia 

concentration for 1999 through 2001 should at least have stayed the same as in the years 

before 1999. The decrease in ammonia concentration suggests that the TN concentration in 

Lake Jesup did not increase because ammonia was being added to the total amount of the TN 

pool. Instead, it is likely that the decrease in available ammonia caused the phytoplankton to 

find nitrogen from other sources, such as through atmospheric nitrogen fixation. 

At the time this analysis was conducted, no data were available to the Department on the scale 

of nutrient translocation that could be caused by zooplankton and/or fish that feed on the 

sediment and excrete in the water column. The activities of invertebrates in the sediment could 

increase severalfold the release of ammonia from the sediment (Henriksen et al., 1983; 

Fukuhara and Sakamoto, 1987; Svensson, 1997). All these processes, however, add ammonia 

back to the water column. If these processes are important in controlling the enhancement of 

the overall TN pool, the decrease of ammonia shown in Figure 5.6 should not have been 

observed. The decrease in ammonia concentration when the total TN pool increases suggests 

that nutrient translocation might not be responsible for the increase of TN concentration. 

Phytoplankton may obtain nitrogen from other sources, such as nitrogen fixation. 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

1995 1996 1997 1998 1999 2000 2001 2002

Year

TN concentration (mg/L)

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0.070

0.080

0.090

NH4 concentration (mg/L)

NH4 concentration Measured Model simulated

Figure 5.6. Dynamics of ammonia concentration in Lake Jesup, 1995–

2002 

Possibility C: Nitrogen fixation is the major factor responsible for the enhanced TN 

concentration observed in Lake Jesup. 

Based on the above analyses, the enhanced TN concentration in Lake Jesup is not very likely 

caused by sediment resuspension or direct diffusion. It is possible that nitrogen fixation caused 

the large gap between the measured and model-simulated TN concentration. Many studies 

have documented the importance of nitrogen fixation in eutrophic lakes (Keirn and Brezonik, 

1971; Horne and Goldman, 1972; Ashton, 1981). Up to 82% of the TN loading into eutrophic 

lakes could come from nitrogen fixation (Howarth et al., 1988). In freshwater lakes, blue-green 

algae, especially filamentous blue-green algae with a heterocystic structure, appear to be the 

most important organisms in nitrogen fixation (Stewart, 1969), although nitrogen fixation by 

other photosynthetic or heterotrophic bacteria has also been documented (Keirn and Brezonik, 

1971; Hill, 1992). 

The rates of nitrogen fixation are reasonably correlated with the biomass of nitrogen-fixing bluegreen algae (Wetzel, 1983; Goldman and Horne, 1983). The major blue-green algal taxa 

capable of fixing nitrogen include Anabaena, Anabaenopsis, Aphenezomenon, Nodularia, 

Cylindrospermopsis, and Lyngbya (D. Dobberfuhl, SJRWMD, personal communication). The 

critical condition that triggers nitrogen fixation by blue-green algae is when the molar ratio 

between dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) is lower 

than the Redfield ratio of 16:1 (Flett et al., 1980). 

No data were available regarding the importance and rate of nitrogen fixation in Lake Jesup 

when this analysis was conducted. However, the annual DIN/DIP ratio of Lake Jesup was 

always lower than 16 for the period of analysis, from 1995 through 2002 (Table 5.10). This low 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

ratio provides the necessary condition for a significant amount of nitrogen fixation in Lake 

Jesup. In addition, a phytoplankton community study conducted by the SJRWMD between 

1995 and 2002 in Lake Jesup identified several major nitrogen-fixing blue-green algae, including 

Anabeana, Anabeanopsis, Aphanizomenon, Cylindrospermopsis, and Lyngbya. The total cell 

counts for these nitrogen-fixing taxa typically account for about 20 to 30% of the total community 

cell counts during the growth season, and sometimes as high as 60% of the total cell counts for 

the phytoplankton community. The long-term average cell density for nitrogen-fixing blue-green 

algae from 1995 through 2002 is about 3.9 × 104

 cells/mL, and can reach as high as 1.8 × 105

cells/mL (data provided by D. Dobberfuhl, SJRWMD). This information suggests that nitrogen 

fixation could be a very important source of nitrogen in Lake Jesup. 

Table 5.10. Annual DIN/DIP ratio of Lake Jesup, 1995–2002 (micro 

molar [μM]). DIN is the sum of NH4-N and NO3-N. DIP is 

primarily soluble reactive phosphorus (SRP). 

Year 1995 1996 1997 1998 1999 2000 2001 2002 

DIN 2.89 4.60 5.14 6.53 4.23 2.27 4.50 4.41

DIP 1.93 1.65 1.65 1.80 1.88 1.69 1.58 1.77

Ratio 1.5 2.8 3.1 3.6 2.3 1.3 2.8 2.5

The annual nitrogen fixation rates listed in Table 5.11 for 1995 through 2002 were entered into 

the Bathtub model to make the model-simulated TN concentration match the measured TN 

concentration. The nitrogen fixation rates required to balance the model results ranged from 8.0 

to 37.4 milligrams per square meter per day (mg/m2

/day). For the majority of the year 

simulated, the nitrogen fixation rates ranged from 9.2 to 13.4 mg/m2

/day. High fixation rates 

were used in 1999 and 2001. 

Table 5.11. Annual nitrogen fixation rate used to match modelsimulated TN concentrations to observed TN 

concentrations, 1995–2002 (mg/m2/day) 

Year 1995 1996 1997 1998 1999 2000 2001 2002 

Fixation 

rate 9.5 13.4 12.4 12.0 23.5 13.2 39.0 9.2 

In 2000, 2001, and 2002, a research team led by Dr. Hans Paerl conducted nitrogen fixation 

studies in Lake George (WBID 2893A), which is located on the St. Johns River downstream of 

Lake Jesup. Dr. Paerl provided the Department with some results of nitrogen fixation rates from 

the study. Table 5.12 shows the nitrogen fixation rates and chl a concentration measured in 

Lake George in July and October 2002, and in March and August 2002. 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 5.12. Nitrogen fixation rates and nutrient concentrations in 

Lake George, July and October 2000, and March and 

August 2002 

July 2000 October 2000 March 2002 August 2002 

Nitrogen fixation rate 

(μg N/m3

/hr)* 18 78 63 23 

Nitrate (μg/L) 12 7 5 80 

NH4 (μg/L) below detection limit below detection limit 19 36 

PO4 (μg/L) 6 6 10 45 

Chl a (μg/L) 26 55 20 30 

** μg N/m3/hr - Micrograms of nitrogen per cubic meter per hour 

Taking into account the dynamics of the volume, surface area (Table 5.4), and chl a

concentration (Table 5.2) for Lake Jesup, the volumetric nitrogen fixation rate of Lake George 

can be translated into the nitrogen fixation rate for Lake Jesup, which ranged from 1.6 

mg/m2

/day to 10.7 mg/m2/day. This overlaps the range of nitrogen fixation rate required to 

balance model-simulated TN with measured TN in Lake Jesup (Table 5.11). 

Based on Paerl et al.’s study in Lake George, phosphate was found to be the factor regulating 

the nitrogen fixation rate. In most of their nutrient amendment studies, adding phosphate into 

the phytoplankton community stimulated the nitrogen fixation rate (H. Paerl, personal 

communication). For Lake George, the phosphate concentration ranged between 6 and 45 μg 

PO4/L (Table 5.12). For Lake Jesup, the phosphate concentration ranged between 157 and 

251 μg PO4/L, based on the data from IWR Run 19.1, which is almost an order of magnitude 

higher than the phosphate concentration for Lake George. The higher nitrogen fixation rate is 

therefore not unreasonable for Lake Jesup. 

Higher nitrogen fixation rates were used in the modeling in 1999 and 2001. These are the two 

dry years in which regional rainfall was higher than rainfall in the Lake Jesup area, leading to 

significant increases in the inflow from the St. Johns River into Lake Jesup (Table 5.7-a). The 

dissolved organic carbon concentration for the St. Johns River is usually high, as the river has a 

large riparian wetland area. It has been found that nitrogen fixation can correlate positively with 

concentrations of dissolved organic carbon, because the growth of heterotrophic bacteria can 

be stimulated by increased organic carbon availability. Some of these heterotrophic bacteria 

can colonize nitrogen-fixing blue-green algae and form bacteria–algae aggregates. Because of 

the oxygen consumption activity from bacteria, the bacteria–algae aggregates usually have a 

distinct low redox microenvironment, which helps to enhance the activity of oxygen-sensitive 

nitrogenase and increases the nitrogen fixation rate (Horne et al., 1972; Paerl and Prufert, 

1987). High nitrogen fixation rates in 1999 and 2001 could have resulted from the increase of 

dissolved organic carbon concentrations in the water. 

Tables 5.13-a and 5.13-b summarize the annual total nutrient loadings into Lake Jesup after the 

nitrogen fixation was taken into consideration, for 1995 through 2002. 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 5.13-a. Annual TN loading from various sources into Lake Jesup, 

1995–2002 (tons/year) 

Source 1995 1996 1997 1998 1999 2000 2001 2002 Mean Percent 

Surface runoff 125.7 176.8 119.2 130.4 92.4 66.1 136.6 191.6 129.9 23.5% 

Baseflow 

(background) 10.1 14.2 9.6 10.5 7.4 5.3 11.0 15.4 10.4 1.9% 

Septic tanks 19.7 19.7 19.7 19.7 19.7 19.7 19.7 19.7 19.7 3.6% 

Artesian input 3.5 3.4 3.4 3.5 3.4 3.4 3.4 3.4 3.4 0.6% 

Atmospheric 

deposition 46.2 46.3 38.1 37.4 34.3 22 38 49.6 39 7.0% 

St. Johns River 

inflow 16.3 15.6 90.5 27.4 186.2 120.7 295 47.6 99.9 18.0% 

Nitrogen fixation 164.1 221.1 199.6 203.9 385.2 204.2 634.4 153.5 270.8 48.9% 

Total 385.6 497.1 480.1 432.8 728.6 441.4 1,138.1 327.3 553.9 100.0% 

Table 5.13 -b. Annual TP loading from various sources into Lake Jesup, 

1995–2002 (tons/year) 

Source 1995 1996 1997 1998 1999 2000 2001 2002 Mean Percent 

Surface runoff 13.1 19.4 12.6 14.2 9.6 7.4 14.7 20.7 14.0 48.6% 

Baseflow 

(background) 3.2 4.5 3.1 3.3 2.4 1.7 3.5 4.9 3.3 11.5% 

Septic tanks 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 9.4% 

Artesian input 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 2.1% 

Atmospheric 

deposition 3.7 3.7 3.0 3.0 2.7 1.7 3.0 3.9 3.1 10.8% 

St. Johns River 

inflow 0.8 0.8 5.0 1.7 9.4 5.0 15.2 3.1 5.1 17.7% 

Total 24.1 31.7 27 25.5 27.4 19.1 39.7 35.9 28.8 100.0% 

Based on Table 5.13-a, TN loadings through nitrogen fixation in most years ranged from 153 to 

270 tons/year, which account for anywhere between 42 and 53% of the total nitrogen loading 

into Lake Jesup. This appears to be consistent with the observation in Lake George, where up 

to 50% of the TN loading came from nitrogen fixation (J. Hendrickson, personal 

communication). High nitrogen fixation was observed for 1999 and 2001. This may be related 

to the organic carbon input from the St. Johns River, as discussed previously. 

The nitrogen fixation rates that were required to make model-simulated annual TN 

concentrations match measured annual average TN concentrations were correlated with the 

chl a concentrations for 1995 through 2002. A tight correlation was found between the nitrogen 

fixation rate and chl a concentration (Figure 5.7). This observation is consistent with the 

hypothesis that the enhanced TN concentration in Lake Jesup is from biological activities. 

An interesting prediction from Figure 5.7 is that when the chl a concentration in Lake Jesup 

decreases to about 28 μg/L, the nitrogen fixation rate from phytoplankton communities becomes 

zero. The nitrogen fixation rate–chl a correlation equation was used in predicting the 

background TN concentration of Lake Jesup in the text below. 

Florida Department of Environmental Protection 

74
TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

This analysis tested the Bathtub eutrophication response model suite to calibrate the chl a

concentration. Bathtub provides two chl a–responding models based on the assumption of 

nitrogen and phosphorus co-limitation: Models 1 and 3. Model 1 assumes that algal 

communities are co-limited, not only by nitrogen and phosphorus, but also by light intensity. 

Model 3 assumes that the primary production of the lake is co-limited by nitrogen and 

phosphorus, but not by light intensity (Walker, 1999). In this analysis, neither model provided 

reasonable chl a predictions that matched with the measured values. Therefore, Bathtub’s 

model suite was not used to predict chl a concentrations. 

y = 0.307x - 8.7213

R2

 = 0.8653

0

5

10

15

20

25

30

35

40

45

0 20 40 60 80 100 120 140 160

Chlorophyll a concentration (μg/L)

Nitrogen fixation rates (mg/m2/day)

Figure 5.7. Correlation between nitrogen fixation rate and chl a

concentration 

A multivariate correlation analysis was conducted between the chl a concentration and the TN 

and TP concentration for Lake Jesup. Two factors, however, made the Department decide not 

to use the correlation equation to predict the chl a concentration under different nutrient 

scenarios. First, a significant portion of the TN concentration in Lake Jesup is not the cause of 

the chl a concentration. Instead, it comes from nitrogen fixation, which is the consequence 

of the chl a concentration. Second, TP concentrations remained stable and very high for the 

modeling period (Table 5.2). No TP concentrations below 0.100 mg/L were observed during 

this analysis. Using a multivariate equation developed based only on such a high and narrow 

range of TP concentrations may not give reliable chl a predictions when the TP concentration is 

decreased to a low level. Therefore, this analysis did not use the multivariate equation 

approach, either. 

The Department eventually decided to use the chl a–TP empirical equation developed by Huber 

et al. (1982) for lakes co-limited by phosphorus and nitrogen. These authors defined the 

phosphorus and nitrogen co-limitation using a TN/TP ratio between 10 and 30 (weight ratio), 

which fits the TN/TP ratio for Lake Jesup. Huber et al.’s equation is: 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Ln chl a = 1.29 * LnTP – 2.44 

Where: 

Ln is the natural log, 

chl a is the chlorophyll a concentration, and 

TP is the total phosphorus concentration of the lake. 

This equation, however, also fails to accurately predict the chl a concentrations that were 

recently measured in Lake Jesup. In most cases, the equation underestimates the chl a

concentration. A possible reason for the underestimation is that the phytoplankton community 

in Lake Jesup may have a different chl a-to-biomass ratio. Dobberfuhl (2003) observed a 

steady increase in the abundance of Cylindrospermopsis raciborskii in Lake Jesup since 1997, 

and the species makes up a significant portion of the blue-green algal cell counts. As discussed 

previously, it is also an important nitrogen fixation species. Phlips et al. (2004) found that under 

the same light and nutrient regimes, the chl a content of C. raciborskii was at least two times 

higher than that of Anabeana and Microsystis, suggesting that chl a concentration may not 

always be the best surrogate for algal biomass. 

Whether this observation applies to some other algal species in Lake Jesup is unknown but 

certainly a possibility. The underestimation from Huber’s equation is therefore not a surprise. 

The Department eventually decided to use this equation for chl a prediction because (1) the 

equation was derived based on 165 Florida lakes with TN/TP ratios similar to that of Lake Jesup 

and therefore should, to a certain extent, reflect the nutrient–chl a relationship of Lake Jesup; 

and (2) predicting the current chlorophyll concentration is not as important as predicting the 

chl a concentration when the overall water quality of the lake is improved. In fact, this analysis 

only used Huber’s equation to predict the chl a concentration under improved TN and TP 

loadings. Under these nutrient loadings, the amount of blue-green algae should be significantly 

decreased and the interference from the high chl a–biomass ratio described above is expected 

to be minimal. Huber’s equation should provide reasonably accurate chlorophyll predictions 

under the circumstances described. 

To summarize the calibration findings, the total flow into Lake Jesup and TP concentration 

predicted by PBS&J and the Bathtub model reasonably match the measured values. The 

Bathtub-simulated TN concentrations were made to match the measured TN concentrations by 

adding into Bathtub the component of nitrogen fixation. Chl a concentrations were not 

calibrated in this analysis because of potential interference from the higher chl a content of 

some algal species. The chl a concentration under improved water quality conditions was 

predicted using Huber’s equation. 

Evaluating the natural background TSI of Lake Jesup. Once the model was calibrated, the 

background TN and TP loadings without the loadings generated from the existing level of 

human activities were estimated using the following procedures: 

Florida Department of Environmental Protection 

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1. All the man-made land use categories (agriculture/golf course; commercial; low-, 

medium-, and high density residential; industrial/utility; and transportation 

facilities) were evaluated as forest/rural open or wetlands, with a ratio of 1:2 

between the forest/rural open to wetlands areas. This ratio was derived from late 

1930s’ aerial photographs of the area provided by Seminole and Orange 
TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Counties. The ratio between forest/rural open and wetlands areas for that period 

was assumed to represent, to some extent, the background ratio between 

forest/rural open and wetlands areas. In addition, the wetland TN and TP EMCs 

were changed from TN = 1.6 mg/L and TP = 0.09 mg/L (Harper, 1994) to TN = 

1.25 mg/L and TP = 0.06 mg/L (Fulton et al., 2003). The TN and TP EMCs were 

changed to relatively low values because Harper’s 1994 wetlands EMCs 

primarily represent the impacted wetlands area, while the unimpacted wetland 

EMCs should have low values (R. Fulton and J. Hendrickson, personal 

communication). 

2. The loading from septic tanks was removed, which decreases the TN and TP 

loading through baseflow. 

3. Background TN and TP loadings from the watershed were then re-estimated with 

the PBS&J model, using the rainfall measurements in the period from 1995 

through 2002. 

4. TN and TP concentrations of the inflow from the St. Johns River were assumed 

to meet the EMCs of relatively unimpacted wetlands (TN = 1.25 mg/L and TP = 

0.06 mg/L). 

5. TN and TP loadings from all the other sources were kept the same as for the 

current condition. 

6. Background TN and TP concentrations and chl a concentration were estimated 

using Bathtub and Huber’s equation described above. 

7. The in-lake TN concentration was adjusted by taking into account the residual 

nitrogen fixation using the regression curve developed between nitrogen fixation 

rate and chl a concentration (Figure 5.7).

8. The background TSI was calculated based on Bathtub-predicted background TP 

concentration, nitrogen fixation–adjusted background TN concentration, and 

background chl a concentrations predicted based on the Bathtub TP and Huber’s 

chl a–TP equation. Any further reduction of the TSI below the background TSI of 

the lake by additional reductions in the loadings was not considered. 

Table 5.14 lists annual concentrations of TN, TP, and chl a and TSI in Lake Jesup under 

background conditions for 1995 through 2002. 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 5.14. Annual background TP, TN, and chl a concentrations 

and TSI for Lake Jesup, 1995–2002 

Year TP (mg/L) TN (mg/L) Chl a (μg/L) TSI 

1995 0.073 1.111 22.0 61 

1996 0.074 1.12 22.4 61 

1997 0.071 1.163 21.3 60 

1998 0.072 1.105 21.7 60 

1999 0.069 1.178 20.5 60 

2000 0.076 1.287 23.2 61 

2001 0.069 1.158 20.5 60 

2002 0.072 1.11 21.7 60 

Mean 0.072 1.154 21.7 60 

As shown in Table 5.14, after all the human land use categories are “converted” to forest/rural 

open and wetlands area based on the 1:2 ratio, all the septic tank loadings are removed, and 

TN and TP concentrations of the St. Johns River inflow are decreased to the level of those 

found in unimpacted wetlands (TP = 0.06 mg/L and TN = 1.29 mg/L), the long-term annual 

average TN, TP, and chl a concentrations decreased from 3.02 mg/L, 0.15 mg/L, and 97.5 μg/L 

to 1.15 mg/L, 0.072 mg/L, and 21.7 μg/L, respectively. This represents a 62% decrease in TN, 

a 52% decrease in TP, and a 78% decrease in chl a concentrations over the existing condition 

(Table 5.2). The resultant calculated TSI value decreased from 78 to 60. Corresponding with 

these water quality indices, Tables 5.15-a and 5.15-b list the annual TN and TP loadings into 

Lake Jesup under background conditions for 1995 through 2002. 

 Table 5.15-a. Annual TN background loading from various sources into 

Lake Jesup, 1995–2002 (tons/year) 

Source 1995 1996 1997 1998 1999 2000 2001 2002 Mean Percent 

Surface runoff 121.9 160.8 113.3 120 89.5 60 127.3 176.1 121.1 49% 

Baseflow 

(background)* 14.4 18.9 13.4 14.1 10.6 7.0 15.0 20.7 14.3 6% 

Artesian input 3.5 3.4 3.4 3.5 3.4 3.4 3.4 3.4 3.4 1% 

Atmospheric 

deposition 46.2 46.3 38.1 37.4 34.3 22 38 49.6 39.0 16% 

St. Johns River 

inflow 17.9 15.6 85.1 25.1 136.6 79.6 149.2 41.3 68.8 28% 

Total 203.9 245 253.3 200.1 274.4 172 332.9 291.1 246.6 100% 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 5.15-b. Annual TP background loading from various sources into 

Lake Jesup, 1995–2002 (tons/year) 

Source 1995 1996 1997 1998 1999 2000 2001 2002 Mean Percent 

Surface runoff 6 7.9 5.6 5.9 4.4 2.9 6.2 8.6 5.6 33% 

Baseflow 

(background)* 4.6 6.0 4.3 4.5 3.4 2.3 4.8 6.6 4.6 25% 

Artesian input 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 4% 

Atmospheric 

deposition 3.7 3.7 3 3 2.7 1.7 3 3.9 3.0 18% 

St. Johns River 

inflow 0.9 0.7 4.1 1.2 6.6 3.8 7.2 2 3.5 21% 

Total 15.8 18.9 17.6 15.2 17.7 11.3 21.8 21.7 16.9 100% 

* The background TN and TP loadings from the baseflow in Tables 5.15-a and 5.15-b are higher than those in previous tables 

because of the increased baseflow volume when a significant portion of the watershed was converted to wetlands. 

The model-simulated background TN and TP concentrations, which are 1.150 and 0.072 mg/L, 

respectively (Table 5.14), appear to be very similar to the target TN and TP concentrations 

evaluated using approaches described in Chapter 3, which are 1.20 and 0.073 mg/L for TN and 

TP, respectively. When the errors in the modeling processes are taken into account, these 2 

sets of numbers should be considered not significantly different. 

Estimating assimilative capacity. To estimate the TN and TP TMDLs for Lake Jesup, nutrient 

concentrations from all the human land use categories and septic tanks were decreased in a 

stepwise manner, until in-lake TN and TP concentrations resulted in a TSI of 65.5. For this 

analysis, EMCs for waters and wetlands were set at 1.25 mg/L TN and 0.060 mg/L TP, which is 

considered a more natural condition for relatively unimpacted wetlands. As TN and TP 

concentrations for the St. Johns inflow have been very close to the unimpacted wetlands’ 

EMCs, for TMDL development purposes, TN and TP concentrations for the St. Johns River 

inflow were also set at 1.25 mg/L for TN and 0.060 mg/L for TP. It should be noted that the TN 

loading from nitrogen fixation will decrease along with the TP loading from the watershed 

because (1) the overall decrease of nutrient loading will decrease the overall biomass of the 

phytoplankton and also the biomass of nitrogen fixers, and (2) the decrease of TP loading into 

the system may make the system less nitrogen limited. 

Table 5.16 lists the annual in-lake TP, chl a, and TN concentrations (adjusted with residual 

nitrogen fixation using the nitrogen fixation rate–chl a equation) that achieve the target TSI, for 

1995 through 2002. 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 5.16. Annual water quality conditions that achieve the target 

TSI, 1995–2002 

Year TP 

(mg/L) 

TN Adjusted with 

Nitrogen Fixation 

(mg/L) 

Chl a

(μg/L) 

Remaining 

Nitrogen Fixation 

Rate (mg/m2/day) 

1995 0.096 1.26 31.4 0.91

1996 0.099 1.25 32.7 1.30

1997 0.093 1.26 30.1 0.53

1998 0.095 1.23 31.0 0.78

1999 0.094 1.29 30.5 0.65

2000 0.094 1.41 30.5 0.65

2001 0.098 1.26 32.2 1.17

2002 0.095 1.19 31.0 0.78

Mean 0.096 1.27 31.2 0.85

Tables 5.17-a and 5.17-b list the annual TN and TP loadings from all sources at the target TSI 

for 1995 through 2002. Tables 5.18-a and 5.18-b list the annual TN and TP load reductions 

required to achieve the water quality target, the TMDLs for TN and TP, and the long-term 

average annual load reductions required to achieve the TMDL. 

Table 5.17-a. Annual TN loading from various sources into Lake Jesup, 

1995–2002 (tons/year) 

Source 1995 1996 1997 1998 1999 2000 2001 2002 Mean Percent 

Surface runoff 96.8 135.6 91.7 100 71.1 50.7 104.9 147 99.7 40.3% 

Baseflow 

(background) 10.1 14.2 9.6 10.5 7.4 5.3 11 15.4 10.4 4.2% 

Septic tanks 13.8 13.8 13.8 13.8 7.5 5.3 13.8 13.8 12.0 4.8% 

Artesian input 3.5 3.4 3.4 3.5 3.4 3.4 3.4 3.4 3.4 1.4% 

Atmospheric 

deposition 46.2 46.3 38.1 37.4 34.3 22 38 49.6 39.0 15.8% 

St. Johns River 

inflow 17.9 15.6 85.1 25.1 136.6 79.6 149.2 41.3 68.8 27.8% 

Nitrogen fixation 15.7 21.5 8.5 13.3 10.7 10.1 19 13 14.0 5.7% 

Watershed Total 120.7 163.6 115.1 124.3 86 61.3 129.7 176.2 122.1 49.4% 

Total 204 250.4 250.2 203.6 271 176.4 339.3 283.5 247.3 100.0% 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 5.17-b. Annual TP loading from various sources into Lake Jesup, 

1995–2002 (tons/year) 

Source 1995 1996 1997 1998 1999 2000 2001 2002 Mean Percent 

Surface runoff 6.6 9.5 6.3 7 6.5 3.6 10 10.2 7.5 39.3% 

Baseflow 

(background) 3.2 4.5 3.1 3.3 2.4 1.7 3.5 4.9 3.3 17.5% 

Septic tanks 1.1 1.1 1.7 1.1 0.8 0.7 2.1 1.1 1.2 6.4% 

Artesian input 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 3.2% 

Atmospheric 

deposition 3.7 3.7 3 3 2.7 1.7 3 3.9 3.1 16.3% 

St. Johns River 

inflow 0.9 0.7 4.1 1.2 6.6 3.8 7.2 2 3.3 17.4% 

Watershed total 10.9 15.1 11.1 11.4 9.7 6 15.6 16.2 12.0 63.2% 

Total 16.1 20.1 18.8 16.2 19.6 12.1 26.4 22.7 19.0 100.0% 

Table 5.18-a. Annual TN load reductions required to achieve the 

water quality target for Lake Jesup, 1995–2002 

Year Current Loading Target Loading Required Load 

Reduction 

Percent Required 

Load Reduction 

1995 385.6 204 181.6 47% 

1996 497.1 250.4 246.7 50% 

1997 480.1 250.2 229.9 48% 

1998 432.8 203.6 229.2 53% 

1999 728.6 271 457.6 63% 

2000 441.4 176.4 265 60% 

2001 1,138.1 339.3 798.8 70% 

2002 327.3 283.5 43.8 13% 

Mean 553.9 247.3 306.6 50% 

Table 5.18-b. Annual TP load reductions required to achieve the water 

quality target for Lake Jesup, 1995–2002 

Year Current Loading Target Loading Required Load 

Reduction 

Percent Required 

Load Reduction 

1995 24.1 16.1 8.0 33% 

1996 31.7 20.1 11.6 37% 

1997 27 18.8 8.2 30% 

1998 25.5 16.2 9.3 36% 

1999 27.4 19.6 7.8 28% 

2000 19.1 12.1 7.0 37% 

2001 39.7 26.4 13.3 34% 

2002 35.9 22.7 13.2 37% 

Mean 28.8 19.0 9.8 34% 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Assimilative capacity for unionized ammonia. As noted previously, this TMDL analysis also 

represents the effort to determine the TMDL for unionized ammonia, because the reductions 

needed to meet the nutrient criteria are also expected to address exceedances of the unionized 

ammonia criterion. In ambient water, unionized ammonia is in a chemical equilibrium with 

ammonia, and the equilibrium coefficient is primarily controlled by pH, water temperature, and 

sometimes ionic strength, especially water pH (Wetzel, 2001). Usually, a higher fraction of the 

ammonia is in the form of unionized ammonia under a high pH condition. For many eutrophic 

freshwater lakes, phytoplankton biomass and photosynthetic activities can significantly influence 

pH. The photosynthetic consumption of carbon dioxide (CO2) and bicarbonate (HCO3

-

) creates 

hydroxide ions (OH-

) in the water and increases the pH of the water, which in turn raises the 

portion of unionized ammonia in the equilibrium. 

Figure 5.8 shows the relationship between the unionized ammonia concentration and pH in 

Lake Jesup. Based on the graph, when the pH of Lake Jesup water decreases below 8.5, in 

most cases the unionized ammonia concentration should be lower than 0.02 mg/L. Figure 5.9

shows the relationship between pH and the chl a concentration in Lake Jesup. As shown, when 

the chl a of the lake decreases to about 30 μg/L, the pH of the lake water can be expected to be 

lower than 9.0 standard units (SU), which, according to Figure 5.8, should create a pH 

environment that will result in unionized ammonia concentrations lower than 0.02 mg/L. 

Therefore, the established target TSI for this analysis not only addresses the nutrient 

impairment, but also addresses the impairment for unionized ammonia. Of course, the 

Department realized that water pH can be influence by factors other than algal photosynthetic 

activities. To make sure that unionized ammonia concentration will indeed decrease with the 

decrease of the overall phytoplankton biomass, the Department would encourage monitoring 

programs that track the change of pH and un-ionized ammonia concentration once the TMDL 

implementation starts. 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

y = 6E-11e2.1215x

R2 = 0.5165

0

0.02

0.04

0.06

0.08

0.1

0.12

5 6 7 8 91

pH

Un-ionized ammonia concentration (mg/L)

0

Figure 5.8. Relationship between unionized ammonia and pH for Lake 

Jesup 

y = 0.6011Ln(x) + 5.9023

R2

 = 0.2463

4

5

6

7

8

9

10

11

0.0 50.0 100.0 150.0 200.0 250.0 300.0

Chl a concentration (μg/L)

pH

Figure 5.9. Relationship between pH and chl a concentration for Lake 

Jesup 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Chapter 6: DETERMINATION OF THE TMDL 

6.1 Expression and Allocation of the 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 (wasteload allocations, or WLAs), nonpoint source loads (load allocations, or 

LAs), 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 discussed earlier, the WLA is broken out into separate subcategories for wastewater 

discharges and stormwater discharges regulated under the NPDES Program: 

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

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

to the value of the TMDL because (1) the WLA for NPDES stormwater is typically based on the 

percent reduction needed for nonpoint sources and is also accounted for within the LA, and (2) 

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

transport). The permitting of stormwater discharges also differs from 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 BMPs. 

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. TMDLs for Lake Jesup are expressed in terms of pounds per year 

(lbs/year) and percent reduction of TN and TP, and represent the maximum long-term annual 

average TN and TP loadings Lake Jesup can assimilate and maintain a balanced aquatic flora 

and fauna (Table 6.1). 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Table 6.1. TMDL components for Lake Jesup 

WBID Parameter TMDL

(kg/year)* WLANPDES Stormwater LA MOS

2981 (including 2981A) TN 247,300 50% 50% Implicit 

2981 (including 2981A) TP 19,000 34% 34% Implicit 

* = Kilograms per year 

6.2 Load Allocation 

Because there are no wastewater point sources that discharge directly into any surface water in 

the watershed, the TMDLs for TN and TP were assigned to the LA (and, as discussed below, to 

the MS4 as well). The long-term annual average LAs for TN and TP into Lake Jesup are 

247,300 and 19,000 kg/year, respectively. Nonpoint sources (including the loadings from 

MS4s) are responsible for all these loads. The current long-term annual average TN and TP 

loads are 553,900 kg/year for TN and 28,800 kg/year for TP; these figures include the loadings 

from surface runoff, baseflow from the surficial aquifer, septic tanks, artesian input, atmospheric 

deposition directly onto the surface of the lake, and the inflow from the St. Johns River. A 

significant portion of the TN load also comes from nitrogen fixation. 

To achieve the LA, current TN and TP loadings require a 50% and 34% reduction, respectively. 

The load reduction needs to apply to surface runoff, septic tanks, St. Johns River input for both 

TN and TP, and nitrogen fixation for TN. It should be noted that the load reduction for nitrogen 

fixation is associated with the watershed load reduction. As long as nutrient loadings from all 

human nonpoint sources are reduced, the TN loading from nitrogen fixation should decrease as 

well. 

6.3 Wasteload Allocation 

Because no wastewater facilities discharge to surface waters in the watershed, the only WLA 

considered in this analysis is the stormwater load from MS4 areas.

6.3.1 National Pollutant Discharge Elimination System Wastewater Discharges 

No NPDES-permitted wastewater discharges were identified in the Lake Jesup watershed. 

6.3.2 National Pollutant Discharge Elimination System Stormwater Discharges 

Florida Department of Environmental Protection 

85

Because no information was available to the Department at the time this analysis was 

conducted regarding the boundaries and locations of all the NPDES stormwater dischargers, 

the exact stormwater TN and TP loadings from MS4 areas were not explicitly estimated. Within 

the Lake Jesup watershed, the stormwater collection systems owned and operated by Seminole 

and Orange Counties; a number of municipalities, including Altamonte Springs, Casselberry, 

Eatonville, Lake Mary, Longwood, Maitland, Orlando, Oviedo, Sanford, Winter Garden, and 
TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Winter Springs; and the Florida Department of Transportation are covered by an MS4 Phase I 

permit. No Phase II permittees were identified in the watershed. The WLANPDESStormwater was set 

as the same percent reduction required to achieve the TMDL as for the other conventional 

nonpoint sources, which are 50% for TN and 34% for TP. 

6.4 Margin of Safety 

Consistent with the recommendations of the Allocation Technical Advisory Committee 

(Department, February 2001), an implicit MOS was used in the development of this TMDL. An 

implicit MOS was provided by the conservative decisions associated with a number of modeling 

assumptions, the development of site-specific alternative water quality targets, and the 

development of assimilative capacity. 

In this analysis, the MOS was primarily associated with the watershed pollutant loading 

estimation. The current TN and TP loadings were estimated as the loadings after existing BMP 

treatments. Since most of the published pollutant loading removal efficiencies target the 

decrease of pollutant concentration after BMP treatment, and very limited information was 

available regarding the water removal efficiency for stormwater BMPs, this analysis assumed 

that there was no water removal for most of the BMP facilities, except for drainage wells. This 

approach has the potential to overstate the current nutrient load estimation and therefore add to 

the MOS. In addition, this analysis considered only the pollutant load reduction from structural 

BMPs. The modeling process did not include loading reductions from other, nonstructural 

BMPs such as street sweeping. This could also lead to an overestimate of the current nutrient 

loadings into the lake and therefore adds to the MOS. 

In estimating nutrient loadings from septic tanks, it was assumed that all the septic tanks located 

within 200 meters of any receiving water that discharges to Lake Jesup contribute TN and TP to 

the lake to some extent, instead of just attributing the loading to failed septic tanks. This also 

has the potential to result in an overestimate of the current nutrient loadings into the lake and 

therefore adds to the MOS. 

6.5 Recommendations for Further Studies 

Because of the short period available for this analysis and also the lack of data regarding some 

functions of the lake system, the Department acknowledges the uncertainties associated with 

this TMDL. Further studies, described below, are recommended to address these issues to 

refine the TMDL goals developed in this analysis. 

6.5.1 How Sediment Dredging Will Influence Nutrient Loading 

into Lake Jesup 

Sediment dredging has been carried out as part of the Lake Jesup Restoration Program. The 

decision that dredging is important for the functions of the Lake Jesup ecosystem was based, in 

part, on the sediment study conducted by Cable et al. (1997). The study indicated that there are 

approximately 102 million cubic yards of soft sediment in Lake Jesup. Data from the study also 

showed that the soft sediment was characterized by high organic and nutrient content 

throughout the sediment volume. During the 2001–02 state legislative session, the Florida 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

legislature provided $2.9 million for the first 2 years of the 5-year Lake Jesup Enhancement 

Project. This project involves the hydraulic dredging of flocculent sediments from the lake 

bottom, removing organic sediments from 200 acres of former marsh, carrying out nuisance 

vegetation control, and revegetating with desirable native aquatic plant species. 

An important issue for the dredging program specifically related to TMDL development is that if 

the dredging spoil is disposed of in the Lake Jesup watershed, it will create an extra source of 

nutrients to the lake. Currently, funding for the dredging has expired, because the Department 

has not granted the permit for the program. It is not expected that the dredging will be finished 

before the final load allocation is completed as part of the BMAP for the TMDL. As such, the 

potential spoil disposal site may not have a load allocation by the time the dredging of the lake 

is finished. 

One possible solution is to give a nutrient reduction “credit” to the dredging activity, because 

sediment removal may decrease the nutrient loading from the sediment to the lake water 

column. The problem with this approach is that it will be difficult to quantify the credit that 

should be assigned to the dredging. As discussed previously, nutrient release from the 

sediment does not appear to be an important part of the nutrient budget for Lake Jesup. 

However, no direct information on the actual nutrient release rates was available to the 

Department when this analysis was conducted. Direct measurements of sediment nutrient 

release and the environmental factors that control sediment nutrient release could be very 

important for refining the TMDL, and will also help to quantify the possible credit assigned to 

sediment dredging. 

Sediment dredging, especially sediment dredging in the former marsh area, could also help to 

restore the littoral zone SAV, which will restore the habitats for fish and may also help prevent 

nutrients from entering the lake. Studies on the extent to which littoral zone aquatic plants can 

develop and their capability to hold nutrients from the watershed will also help to quantify the 

possible credit assigned to sediment dredging. 

6.5.2 Nitrogen Fixation 

Another issue worth further study is nitrogen fixation by blue-green algae. Based on this TMDL, 

nitrogen fixation could account for about 45% of the total TN budget for Lake Jesup. However, 

the nitrogen fixation rates used in this analysis were determined using the mass balance 

approach of the modeling process, instead of directly measured rates. The nitrogen fixation 

rates used in this analysis fall into the range of published values in the literature and overlap 

with the areal fixation rates calculated based on the volumetric fixation rates observed in Lake 

George, which is located relatively close to Lake Jesup. The DIN/DIP molar ratio of Lake Jesup 

is also consistently lower than the Redfield ratio of 16, which implies strong nitrogen limitation. 

The phosphate concentration, which was identified as the major driving factor for nitrogen 

fixation in Lake George, was almost an order of magnitude higher in Lake Jesup than in Lake 

George. In addition, a significant portion of the phytoplankton communities of Lake Jesup was 

composed of nitrogen fixers, and most of the nitrogen fixation taxa were identified in Lake 

Jesup. 

All these observations are consistent with the assumption that nitrogen fixation is important in 

the nitrogen budget of Lake Jesup. However, no directly measured nitrogen fixation rate data 

were available to the Department when this analysis was carried out. The Department believes 

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that, to address the nitrogen fixation in Lake Jesup more thoroughly, more studies need to be 

conducted, including analyses of the diurnal pattern, seasonal pattern, spatial pattern (including 

vertical and horizontal directions), and major factors that control the intensity of nitrogen fixation 

and the biomass of nitrogen fixers in Lake Jesup. The results of these studies will not only help 

to quantify nitrogen fixation more accurately, but will also help to evaluate the importance of 

residual nitrogen fixation when TN and TP loadings from the watershed are decreased. 

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Chapter 7: NEXT STEPS: IMPLEMENTATION PLAN 

DEVELOPMENT AND BEYOND 

7.1 Basin Management Action Plan 

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

implementation plan for the TMDL, referred to as the BMAP. This document will be developed 

over the next two years in cooperation with local stakeholders, who will attempt to reach 

consensus on detailed allocations and on how load reductions will be accomplished. The 

BMAP will include, among other things: 

• Appropriate load reduction allocations among the affected parties, 

• A description of the load reduction activities to be undertaken, including structural projects, 

nonstructural BMPs, and public education and outreach, 

• A description of further research, data collection, or source identification needed in order to 

achieve the TMDL, 

• Timetables for implementation, 

• Confirmed and potential funding mechanisms, 

• Any applicable signed agreement(s), 

• Local ordinances defining actions to be taken or prohibited, 

• Any applicable local water quality standards, permits, or load limitation agreements, 

• Milestones for implementation and water quality improvement, and 

• Implementation tracking, water quality monitoring, and follow-up measures. 

An assessment of progress toward the BMAP milestones will be conducted every five years, 

and revisions to the plan will be made as appropriate, in cooperation with basin stakeholders. 

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References 

Ahn, H., and T. James. 1999. Variability, uncertainty, and sensitivity of phosphorus deposition 

load estimates in south Florida. Ecosystem Restoration Department, South Florida Water 

Management District. 

Ashton, P. J. 1981. Nitrogen fixation and nitrogen budget of a eutrophic impoundment. Water 

Res. 15: 823-833. 

Bachmann, R. W., M. Hoyer, and D E. Canfield. 2003. Predicting the frequencies of high 

chlorophyll levels in Florida lakes from average chlorophyll or nutrient data. Lake and 

Reservoir Management 19(3): 229-241 

Bennett, E. R., and E. K. Linstedt. 1975. Individual home wastewater characterization and 

treatment. Colorado State University. Environmental Resources Center, Report #66. 

Bernhard, A. P. 1975. Return of effluent nutrients to the natural cycle through 

evapotranspiration and subsoil–infiltration of domestic wastewater. In: Proc. National 

Home Sewage Disposal Symposium, ASAE. 

Brezonik, P. L., and J. L. Fox. 1976. Analysis of eutrophication and water quality in the Middle 

St. Johns River Basin. Gainesville, Florida: Department of Environmental Engineering 

Sciences, University of Florida. 

 Brezonik, P. L., C. D. Hendry, E. S. Edgerton, Jr., R. L. Schulze, and T. L. Crisman. 1983. 

Acidity, nutrients, and minerals in atmospheric precipitation over Florida: Deposition 

patterns, mechanisms, and ecological effects. Report prepared for Environmental 

Research Laboratory, U. S. Environmental Protection Agency, Corvallis, Oregon. 

Cable, J. E. C L. Schelske, P. S. Hansen, W. F. Kenney, and T. J. Whitmore. 1997. Sediment 

and nutrient deposition in Lake Jesup, Florida (USA). Final report for St. Johns River Water 

Management District. Special Publication SJ98-SP18. 

Chan, H. T. 1978. Contamination of the Great Lakes by private wastes. PLUARG Technical 

Report Series. 

Chiaro, P. S. and D. A. Burke. 1980. Sediment oxygen demand and nutrient release. Journal 

of the Environmental Engineering Division 106(1): 177-195. 

Dobberfuhl, D. R. 2003. Cylindrospermopsis raciborskii in three central Florida lakes: 

population dynamics, controls, and management implications. Lake and Reservoir 

Management 19(4): 341-348. 

Ellis, B. G., and K. E. Childs. 1973. Nutrient movement from septic tanks and lawn fertilization. 

Michigan Department of Natural Resources. Technical Bulletin No. 73-5. Lansing, 

Michigan. 

Florida Department of Environmental Protection 

90
TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Flett, R. J., D. W. Schindler, R. D. Hamilton, R. D., and N. E. R. Campbell. 1980. Nitrogen 

fixation in Canadian Precambrian Shield lakes.” J. Fish. Aquat. Sci. 37: 494-505.

Florida Administrative Code. Chapter 62-302, Surface water quality standards. 

Florida Administrative Code. Chapter 62-303, Identification of impaired surface waters. 

Florida Watershed Restoration Act. Chapter 99-223, Laws of Florida. 

Florida Department of Environmental Protection. February 1, 2001. A report to the governor 

and the legislature on the allocation of total maximum daily loads in Florida. Tallahassee, 

Florida: Bureau of Watershed Management, Division of Water Resource Management. 

Available: http://www.dep.state.fl.us/water/tmdl/docs/Allocation.pdf. 

Fukuhara, H., and M. Sakamoto. 1987. Enhancement of inorganic nitrogen and phosphate 

release from lake sediment by tubificid worms and chironomid larvae. Oikos 48:312-320. 

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

and J. Richmond. 2003. Interim pollutant load reduction goals for seven major lakes in the 

Upper Ocklawaha River Basin. Palatka, Florida: St. Johns River Water Management 

District. 

Gerritsen, J. B., B. K. Jessup, E. Leppo, and J. White. 2000. Development of lake condition 

indexes (LCI) for Florida. Prepared for the Florida Department of Environmental Protection, 

Tallahassee, Florida. 

Goldman, C. R., and A. J. Horne. 1983. Limnology. New York: McGraw-Hill. 

Harper, H. H. 1994. Stormwater loading rate parameters for central and south Florida. 

Environmental Research & Design, Inc. Orlando, Florida. 

Hendrickson, J. Unpublished report Development of a surface water classification for 

evaluating water quality in northeast Florida. .

Henriksen, K., M. B. Rasmussen, and A. Jensen. 1983. Effect of bioturbation on microbial 

nitrogen transformations in the sediment and fluxes of ammonium and nitrate to the 

overlying water. Environ. Biogeochem. Ecol. Bull. (Stockholm) 35:193-205. 

Hill, S. 1992. Physiology of nitrogen fixation in free-living heterotrophs. In G. Stacey, R. H. 

Burris, and H. J. Evens, Eds. Biological nitrogen fixation. New York: Chapman & Hall. 

Horne, A. J., and C. R. Goldman. 1972. Nitrogen fixation in Clear Lake, California. 1. 

Seasonal variation and the role of heterocysts. Limnol. Oceanogr. 17: 678-692. 

Howarth, R. W., R. Marino, J. Lane, and J. J. Cole. 1988. Nitrogen fixation in freshwater, 

estuarine, and marine ecosystems. 1. Rates and importance. Limnol. Oceanogr. 33: 669-

687. 

Florida Department of Environmental Protection 

91
TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Hu, W. F., W. Lo, H. Chua, S. N. Sin, and P. H. Yu. 2001. Nutrient release and sediment 

oxygen demand in a eutrophic land-locked embayment in Hong Kong. Environ Int. 26 (5-6): 

369-375. 

Huber, W. C., P. L. Brezonik, and J. P. Heaney. 1982. A classification of Florida lakes. 

Prepared for the Florida Department of Environmental Regulation. Report ENV-05-82-1. 

Kamiyama, K., S. Okuda, and A. Kawai. 1977. Studies on the release of ammonium nitrogen 

from the bottom sediments in freshwater regions. II. Ammonium nitrogen in dissolved and 

absorbed form in the sediments. JPN. J. Limnol. 38: 100-106. 

Keesecker, D. H. 1992. Lake Jesup restoration diagnostic evaluation, water budget and 

nutrient budget. Prepared for the St. Johns River Water Management District. File: 91-

5057. 

Keirn, M. A., and P. L. Brezonik. 1971. Nitrogen fixation by bacteria in Lake Mize, Florida and 

in some lacustrine sediments. Limnol. Oceanogr. 16: 720-731. 

Laak, R. 1975. Relative pollution strengths of undiluted waste materials discharge in 

households and the dilution waters used for each. Manual of grey water treatment practice 

– Part II. Santa Monica, California: Monogram Industries, Inc. 

Lasi, M. A. 1999. An external nutrient budget for Orange Lake, effect of sinkhole seepage on 

in-lake nitrogen and phosphorus stores. Palatka, Florida: St. Johns River Water 

Management District. 

Ligman, K., N. Hutzler, and W. C. Boyle. 1974. Household wastewater characterization. J. 

Env. Eng. Div., ASCE 150 (EEI): 201-213. 

Omernik, J. M. 1987. Ecoregions of the conterminous United States. Annals of the Association 

of American Geographers 77: 118-125. 

Otis, R. J., W. C. Boyle, and D. K. Sauer. 1975. The performance of household wastewater 

treatment units under field conditions. Proc. National Home Sewage Disposal Symposium, 

ASAE. 

Parsons Engineering Science, Inc. 2000. Lake Jesup water and nutrient budget report. Final 

Submittal. Prepared for the St. Johns River Water Management District. 

Paerl, H. W., and L. E. Prufert. 1987. Oxygen-poor microzones as potential sites of microbial 

N2 fixation in nitrogen-depleted aerobic marine waters. Appl. Environ. Microbiol. 53:1078-

1087. 

Paul, M. J., and J. Gerritsen. 2002. Nutrient criteria for Florida lakes: A comparison of 

approaches. Prepared for the Florida Department of Environmental Protection, 

Tallahassee, Florida. 

Phlips, E. J., J. Frost, N. Yilmaz, M. Cichra. 2004. Factors controlling the abundance and 

composition of blue-green algae in Lake Griffin. Final Report to the St. Johns River Water 

Management District. Project # SF669AA. 

Florida Department of Environmental Protection 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Reckhow, K. H., M. N. Beaulac, and J. T. Simpson. 1980. Modeling phosphorus loading and 

lake response under uncertainty: A manual and compilation of export coefficients. U. S. 

Environmental Protection Agency. EPA 440/5-80-011. 

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. 

Scott, T. M., J. M. Lloyd, and G. Maddox. 1991. Florida’s Groundwater Quality Monitoring 

Program: Hydrogeological Framework. Florida Geological Survey. Special Publication No. 

32. 

Seminole County. 1991. Sanitary Sewer Element. Seminole County 1991 Comprehensive 

Plan. Seminole County Planning Office, Sanford, Florida. 

Sheffield, C. W., C. Rivero-DeAguilar, and K. McCann. March 1995. Drainage wells– dual 

Purpose. Florida Water Resources Journal. 

Siegrist, R. L., M. Witt, and W. C. Boyle. 1976. Characteristics of rural household wastewater. 

J. Env. Eng. Div., ASCE, 102(EE3): 533-548.

Spechler, R. M., L. C. Marray, L. A. Bradner, and G. F. Phelps. 1991. Potentiometric surface of 

the upper Floridan aquifer in the St. Johns River Water Management District and vicinity, 

September 1990. USGS Open-File Report 91-190. Denver, Colorado: U. S. Department 

of the Interior, U. S. Geological Survey. 

Stewart, W. D. P. 1969. Biological and ecological aspects of nitrogen fixation by free-living 

microorganisms. Proc. R. Soc. Lond. Ser. B 172: 367-388. 

Suphunvorranop, T. 1985. A guide to SCS runoff procedures. Department of Water 

Resources, St. Johns River Water Management District, Palatka, Florida. Project Number 

15/20 200 03. 

Svensson, J. M. 1997. Influence of Chironomus plumosus larvae on ammonium flux and 

denitrification (measured by the acetylene blockage- and the isotope pairing-technique) in 

eutrophic lake sediment. Hydrobiologia 346: 157-168. 

Tchobanoqlous, G., F. L. Burton, and H. D. Stensel. 2003. Wastewater engineering: Treatment 

and reuse. New York: McGraw-Hill. 

Tibbals, C. H. 1990. Hydrology of the Floridan aquifer system in east-central Florida. Regional 

aquifer-system analysis. U.S. Geological Survey Professional Paper 1403-E. Washington, 

D. C.: U. S. Government Printing Office. 

U. S. Census Bureau Web site. 2003. Available: http://www.census.gov/. 

U. S. Environmental Protection Agency. 1974. Nitrogen and phosphorus in wastewater 

effluents. Working Paper No. 22. National Eutrophication Survey. Corvallis, Oregon: 

Pacific Northwest Environmental Research Laboratory. GPO 697-032. 

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U.S. Environmental Protection Agency. 1977. Report on Lake Jesup, Seminole County, 

Florida. Working Paper No. 260. National Eutrophication Survey. Washington, D. C. 

Viessman, W., G. L. Lewis, and J. W. Knapp. 1989. Introduction to Hydrology. 3rd Ed. New 

York: HarperCollins Publishers. 

Verdouw, H., P. C. M. Boers, and E. M. J. Dekkers. 1985. The dynamics of ammonia in 

sediments and hypolimnion of Lake Vechten (The Netherlands). Arch. Hydrobiol. 105: 79-

92. 

Walker, W. W. 1999. Simplified Procedures for Eutrophication Assessment and Prediction: 

User Manual. Instruction Report W-96-2. Washington, D. C.: U. S. Army Corps of 

Engineers. 

Walker, W. C., J. Bouma, D. R. Keeney, and P. G. Olcott. 1973. Nitrogen transformations 

during subsurface disposal of septic tank effluent in sands: II. Ground water quality. J. 

Environ. Qual. 2: 521-525. 

Wetzel. R. G. 1983. Limnology. 2nd ed. Philadelphia, Pennsylvania: Saunders College 

Publishing. 

Wetzel. R. G. 2001. Limnology, lake and river ecosystems. 3rd ed. San Diego, California: 

Academic Press.

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Appendices 

Appendix A: Background Information on Federal and State Stormwater Programs 

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, F.S., was established as a technology-based program that relies on 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, F.A.C. In 1994, the Department’s 

stormwater treatment requirements were integrated with the stormwater flood control 

requirements of the state’s water management districts, along with wetland protection 

requirements, into the Environmental Resource Permit regulations. 

Chapter 62-40, F.A.C., also requires the water management districts 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, the Winter Haven Chain of Lakes, the Everglades, Lake Okeechobee, and Lake 

Apopka. No PLRG had been developed for Newnans Lake when this report was published. 

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 permitting 

program to designate certain stormwater discharges as “point sources” of pollution. The EPA 

promulgated regulations and began implementing the Phase I NPDES stormwater program in 

1990. These stormwater discharges include certain discharges that are associated with 

industrial activities designated by specific standard industrial classification (SIC) codes, 

construction sites disturbing 5 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, the EPA implemented Phase I of the MS4 permitting 

program on a countywide basis, which brought in all cities (incorporated areas), Chapter 298 

urban water control districts, and the Florida Department of Transportation throughout the 15 

counties meeting the population criteria. The Department received authorization to implement 

the NPDES stormwater program in 2000. 

An important difference between the federal NPDES and the state’s stormwater/environmental 

resource permitting programs is that the NPDES Program covers both new and existing 

discharges, while the state’s program focuses on new discharges only. Additionally, Phase II of 

the NPDES Program, implemented in 2003, expands the need for these permits to construction 

sites between 1 and 5 acres, and to local governments with as few as 1,000 people. 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 cannot be easily collected 

and treated by a central treatment facility, as are other point sources of pollution such as 

domestic and industrial wastewater discharges. It should be noted that all MS4 permits issued in 

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Florida include a re-opener clause that allows permit revisions to implement TMDLs when the 

implementation plan is formally adopted. 

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Appendix B: Vertical Distribution of DO Concentrations in Lake Jesup (mg/L), 1995–

1996. Data provided by the SJRWMD. 

Station Date Depth DO Temperature 

OW-01 02/15/95 0.5 9.40 18.47 

1 9.45 18.38 

1.5 9.30 18.40 

2 9.25 18.26 

2.5 9.23 18.25 

02/15/95 Mean 9.33 18.35 

03/15/95 0.5 9.26 20.11 

1 9.12 19.94 

1.5 8.98 19.98 

2 8.89 19.81 

03/15/95 Mean 9.06 19.96 

04/12/95 0.5 7.79 24.67 

1 7.73 24.56 

1.5 7.50 24.50 

2 7.44 24.47 

04/12/95 Mean 7.62 24.55 

05/17/95 0.5 8.91 30.73 

1 7.58 30.06 

1.5 7.40 29.91 

2 7.42 29.91 

05/17/95 Mean 7.83 30.15 

06/14/95 0.5 6.46 29.42 

1 6.16 29.41 

1.5 6.11 29.41 

06/14/95 Mean 6.24 29.41 

07/18/96 0.5 8.52 30.39 

1 7.05 29.96 

1.5 5.92 29.75 

2 5.73 29.72 

07/18/96 Mean 6.81 29.96 

08/15/96 0.5 5.45 29.51 

1 3.85 28.52 

08/15/96 Mean 4.65 29.02 

09/18/96 0.5 5.45 28.72 

1 5.44 28.71 

1.5 5.44 28.71 

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Station Date Depth DO Temperature 

2 5.44 28.72 

09/18/96 Mean 5.44 28.72 

10/16/96 0.5 6.47 23.94 

1 6.48 23.82 

1.5 6.42 23.79 

2 6.35 23.76 

2.5 6.29 23.74 

3 6.22 23.73 

3.5 6.18 23.74 

10/16/96 Mean 6.34 23.79 

11/13/96 0.5 9.18 17.10 

1 9.19 17.14 

1.5 9.20 16.89 

2 9.20 16.74 

2.5 9.08 16.69 

11/13/96 Mean 9.17 16.91 

12/11/96 0.5 14.04 16.43 

1 14.05 16.01 

1.5 13.88 15.91 

2 12.97 15.60 

12/11/96 Mean 13.74 15.99 

OW-01 Total 7.98 23.62 

OW-02 02/15/95 0.5 10.51 18.29 

1 10.49 18.26 

1.5 10.44 18.23 

02/15/95 Mean 10.48 18.26 

03/15/95 0.5 9.81 19.96 

1 9.43 19.74 

1.5 9.38 19.78 

03/15/95 Mean 9.54 19.83 

04/12/95 0.5 8.18 24.78 

1 7.94 24.76 

04/12/95 Mean 8.06 24.77 

05/17/95 0.5 11.98 31.06 

1 7.30 30.86 

05/17/95 Mean 9.64 30.96 

06/14/95 0.5 10.90 27.79 

1 10.39 27.59 

06/14/95 Mean 10.65 27.69 

07/18/96 0.5 10.00 30.69 

1 7.28 29.52 

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Station Date Depth DO Temperature 

1.5 2.32 29.49 

07/18/96 Mean 6.53 29.90 

08/15/96 0.5 10.93 29.47 

1 9.66 28.76 

08/15/96 Mean 10.30 29.12 

09/18/96 0.5 7.74 28.76 

1 7.72 28.76 

1.5 6.47 28.76 

09/18/96 Mean 7.31 28.76 

10/16/96 0.5 9.37 24.09 

1 9.37 24.07 

1.5 9.37 24.05 

10/16/96 Mean 9.37 24.07 

11/13/96 0.5 10.55 16.85 

1 10.57 16.84 

1.5 6.34 17.03 

11/13/96 Mean 9.15 16.91 

12/11/96 0.5 14.48 17.10 

1 12.52 16.16 

12/11/96 Mean 13.50 16.63 

OW-02 Total 9.34 23.98 

OW-03 03/15/95 0.5 10.40 19.48 

1 9.05 18.76 

1.5 8.20 18.69 

03/15/95 Mean 9.22 18.98 

04/12/95 0.5 9.03 24.55 

1 9.01 24.56 

04/12/95 Mean 9.02 24.56 

05/17/95 0.5 8.20 30.43 

1 2.05 30.31 

1.5 1.48 30.26 

05/17/95 Mean 3.91 30.33 

02/15/96 0.5 9.26 17.61 

1 9.26 17.55 

1.5 8.75 17.44 

02/15/96 Mean 9.09 17.53 

06/14/95 0.5 12.19 28.20 

1 11.43 27.60 

06/14/95 Mean 11.81 27.90 

07/18/96 0.5 7.83 29.65 

1 6.76 29.24 

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Station Date Depth DO Temperature 

1.5 1.55 29.21 

07/18/96 Mean 5.38 29.37 

08/15/96 0.5 8.69 28.32 

1 8.13 28.01 

08/15/96 Mean 8.41 28.17 

09/18/96 0.5 6.07 28.75 

1 6.06 28.75 

1.5 5.99 28.75 

09/18/96 Mean 6.04 28.75 

10/16/96 0.5 7.20 24.22 

1 7.20 24.22 

1.5 7.18 24.21 

10/16/96 Mean 7.19 24.22 

11/13/96 0.5 9.68 16.55 

1 9.67 16.55 

1.5 9.60 16.58 

11/13/96 Mean 9.65 16.56 

12/11/96 0.5 13.34 16.30 

1 10.70 15.09 

12/11/96 Mean 12.02 15.70 

OW-03 Total 8.07 23.79 

OW-04 03/15/95 0.5 8.65 19.10 

1 8.34 18.81 

1.5 8.01 18.75 

2 7.10 18.71 

03/15/95 Mean 8.03 18.84 

04/12/95 0.5 8.15 24.74 

1 8.12 24.75 

1.5 7.33 24.44 

2 7.07 24.37 

04/12/95 Mean 7.67 24.58 

05/17/95 0.5 7.07 29.92 

1 5.71 29.49 

1.5 3.94 29.39 

2 0.31 29.33 

05/17/95 Mean 4.26 29.53 

02/15/96 0.5 10.12 16.81 

1 9.71 16.71 

1.5 7.24 16.23 

02/15/96 Mean 9.02 16.58 

06/14/95 0.5 5.74 27.85 

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Station Date Depth DO Temperature 

1 3.96 27.58 

1.5 3.58 27.40 

2 2.15 27.42 

06/14/95 Mean 3.86 27.56 

07/18/96 0.5 8.26 30.68 

1 8.69 30.03 

1.5 6.78 29.56 

2 3.74 29.51 

07/18/96 Mean 6.87 29.95 

08/15/96 0.5 9.50 28.46 

1 8.19 28.08 

1.5 6.50 28.08 

08/15/96 Mean 8.06 28.21 

09/18/96 0.5 7.75 29.08 

1 6.83 29.09 

1.5 6.70 29.09 

2 6.65 29.07 

09/18/96 Mean 6.98 29.08 

10/16/96 0.5 8.32 24.04 

1 8.21 24.02 

1.5 8.05 24.00 

2 7.89 24.00 

2.5 4.12 24.05 

10/16/96 Mean 7.32 24.02 

11/13/96 0.5 9.85 16.87 

1 9.85 16.87 

1.5 9.85 16.85 

2 6.88 17.24 

11/13/96 Mean 9.11 16.96 

12/11/96 0.5 12.43 15.47 

1 12.42 15.17 

1.5 11.64 15.13 

2 9.09 15.28 

12/11/96 Mean 11.40 15.26 

OW-04 Total 7.45 23.76 

OW-05 02/15/95 0.5 10.09 17.05 

1 9.90 16.96 

1.5 9.55 16.80 

2 8.78 16.41 

2.5 8.36 16.26 

3 7.76 16.12 

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Station Date Depth DO Temperature 

3.5 6.09 15.65 

02/15/95 Mean 8.65 16.46 

03/15/95 0.5 8.42 19.28 

1 7.89 19.00 

1.5 7.86 18.92 

2 7.93 18.85 

2.5 7.94 18.82 

3 7.69 18.80 

3.5 7.51 18.80 

03/15/95 Mean 7.89 18.92 

04/12/95 0.5 7.30 24.69 

1 7.17 24.64 

1.5 7.13 24.59 

2 7.02 24.55 

2.5 5.85 24.33 

3 5.71 24.27 

04/12/95 Mean 6.70 24.51 

05/17/95 0.5 8.11 30.54 

1 8.04 30.33 

1.5 7.00 30.27 

2 7.70 30.21 

2.5 7.60 30.14 

3 7.37 30.07 

05/17/95 Mean 7.64 30.26 

06/14/95 0.5 7.75 27.78 

1 6.79 27.58 

1.5 5.49 27.32 

2 4.22 27.30 

2.5 1.72 27.30 

06/14/95 Mean 5.19 27.46 

07/18/96 0.5 8.25 30.47 

1 5.88 30.04 

1.5 4.55 29.97 

2 4.03 29.97 

2.5 5.10 29.92 

3 5.13 29.90 

3.5 4.86 29.90 

07/18/96 Mean 5.40 30.02 

08/15/96 0.5 7.87 29.25 

1 7.17 29.12 

1.5 7.27 29.00 

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Station Date Depth DO Temperature 

2 7.39 28.88 

2.5 4.68 28.80 

08/15/96 Mean 6.88 29.01 

09/18/96 0.5 7.57 29.04 

1 7.53 29.04 

1.5 7.04 29.01 

2 6.31 28.93 

2.5 5.76 28.90 

3 5.64 28.86 

3.5 5.51 28.85 

4 2.64 28.83 

09/18/96 Mean 6.00 28.93 

10/16/96 0.5 7.64 23.93 

1 7.67 23.93 

1.5 7.66 23.91 

2 7.58 23.90 

2.5 7.48 23.88 

3 7.31 23.87 

3.5 7.23 23.87 

4 5.62 23.88 

10/16/96 Mean 7.27 23.90 

11/13/96 0.5 9.12 17.35 

1 9.06 17.35 

1.5 9.04 17.36 

2 9.01 17.35 

2.5 8.98 17.34 

3 8.96 17.31 

3.5 7.68 17.37 

4 4.00 17.88 

11/13/96 Mean 8.23 17.41 

12/11/96 0.5 12.70 15.71 

1 12.67 15.33 

1.5 10.97 15.05 

2 8.19 14.83 

2.5 7.26 14.71 

3 6.99 14.65 

3.5 4.37 14.90 

12/11/96 Mean 9.02 15.03 

OW-05 Total 7.23 23.46 

OW-06 02/15/95 0.5 9.99 17.48 

1 9.57 17.43 

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Station Date Depth DO Temperature 

1.5 8.76 16.92 

02/15/95 Mean 9.44 17.28 

03/15/95 0.5 8.46 19.17 

1 8.11 18.96 

1.5 7.52 18.95 

03/15/95 Mean 8.03 19.03 

04/12/95 0.5 7.63 24.65 

1 7.62 24.66 

1.5 7.50 24.65 

04/12/95 Mean 7.58 24.65 

05/17/95 0.5 8.76 29.76 

1 8.71 29.76 

1.5 8.07 29.75 

05/17/95 Mean 8.51 29.76 

06/14/95 0.5 9.46 27.74 

1 9.33 27.74 

1.5 8.19 27.74 

06/14/95 Mean 8.99 27.74 

08/15/96 0.5 7.38 28.65 

1 6.85 28.55 

1.5 5.90 28.55 

08/15/96 Mean 6.71 28.58 

09/18/96 0.5 7.26 29.10 

1 7.24 29.10 

1.5 7.20 29.10 

2 4.32 29.10 

09/18/96 Mean 6.51 29.10 

10/16/96 0.5 7.89 24.40 

1 7.88 24.40 

1.5 7.86 24.40 

2 7.83 24.40 

10/16/96 Mean 7.87 24.40 

11/13/96 0.5 9.39 17.23 

1 9.39 17.22 

1.5 9.38 17.20 

11/13/96 Mean 9.39 17.22 

12/11/96 0.5 11.62 15.75 

1 11.66 15.62 

1.5 10.64 15.63 

2 5.83 16.23 

12/11/96 Mean 9.94 15.81 

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Station Date Depth DO Temperature 

OW-06 Total 8.28 23.33 

OW-07 02/15/95 0.5 8.33 17.50 

1 8.23 17.49 

1.5 7.91 17.42 

02/15/95 Mean 8.16 17.47 

03/15/95 0.5 8.96 19.32 

1 8.54 18.71 

1.5 7.73 18.69 

03/15/95 Mean 8.41 18.91 

04/12/95 0.5 7.94 24.34 

1 7.90 24.34 

04/12/95 Mean 7.92 24.34 

05/17/95 0.5 3.79 28.62 

1 3.56 28.60 

05/17/95 Mean 3.68 28.61 

06/14/95 0.5 10.05 27.20 

1 9.50 27.20 

06/14/95 Mean 9.78 27.20 

07/18/96 0.5 7.18 29.54 

1 5.79 29.17 

07/18/96 Mean 6.49 29.36 

08/15/96 0.5 10.05 28.90 

1 8.71 28.65 

08/15/96 Mean 9.38 28.78 

09/18/96 0.5 7.08 28.82 

1 6.98 28.82 

1.5 6.93 28.82 

09/18/96 Mean 7.00 28.82 

10/16/96 0.5 8.37 23.99 

1 8.34 23.99 

1.5 7.06 23.99 

10/16/96 Mean 7.92 23.99 

11/13/96 0.5 8.83 16.63 

1 8.83 16.63 

1.5 8.82 16.62 

11/13/96 Mean 8.83 16.63 

12/11/96 0.5 12.78 15.20 

1 12.88 14.81 

1.5 10.01 14.90 

12/11/96 Mean 11.89 14.97 

OW-07 Total 8.25 22.82 

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Station Date Depth DO Temperature 

OW-06 07/18/96 0.5 9.23 30.22 

1 8.92 30.13 

1.5 6.99 29.90 

07/18/96 Mean 8.38 30.08 

OW-06 Total 8.38 30.08 

OWSJR-1 07/18/96 0.5 5.49 29.72 

1 5.44 29.72 

1.5 5.50 29.72 

2 5.40 29.72 

07/18/96 Mean 5.46 29.72 

08/15/96 0.5 5.27 28.86 

1 5.23 28.76 

1.5 5.21 28.73 

08/15/96 Mean 5.24 28.78 

09/18/96 0.5 6.26 28.70 

1 6.19 28.70 

1.5 6.11 28.70 

2 6.09 28.70 

2.5 6.07 28.70 

09/18/96 Mean 6.14 28.70 

OWSJR-1 Total 5.69 29.06 

OWSJR-2 07/18/96 0.5 5.54 29.68 

1 5.39 29.68 

1.5 5.37 29.68 

2 5.11 29.66 

2.5 4.87 29.57 

3 4.78 29.56 

3.5 #DIV/0! 29.58 

4 4.76 29.58 

07/18/96 Mean 5.12 29.62 

08/15/96 0.5 5.38 29.13 

1 5.34 29.07 

1.5 5.28 29.05 

2 5.26 29.05 

2.5 5.27 29.04 

3 5.26 29.06 

3.5 5.22 29.07 

4 5.08 29.01 

4.5 5.08 28.98 

08/15/96 Mean 5.24 29.05 

09/18/96 0.5 6.71 28.15 

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Station Date Depth DO Temperature 

1 5.81 28.15 

1.5 5.60 28.14 

2 5.49 28.12 

2.5 5.40 28.12 

3 5.32 28.12 

3.5 5.22 28.12 

4 5.17 28.12 

09/18/96 Mean 5.59 28.13 

OWSJR-2 Total 5.32 28.94 

OW-SJR-1 10/16/96 0.5 6.34 23.56 

1 6.32 23.56 

1.5 6.31 23.56 

2 6.30 23.57 

2.5 6.29 23.57 

3 6.28 23.57 

10/16/96 Mean 6.31 23.57 

11/13/96 0.5 7.82 17.71 

1 7.82 17.71 

1.5 7.81 17.71 

2 7.81 17.71 

11/13/96 Mean 7.82 17.71 

12/11/96 0.5 9.08 15.68 

1 9.01 15.48 

1.5 8.97 15.48 

2 8.95 15.48 

12/11/96 Mean 9.00 15.53 

OW-SJR-1 Total 7.51 19.60 

OW-SJR-2 10/16/96 0.5 4.77 23.31 

1 4.77 23.33 

1.5 5.02 23.35 

2 5.25 23.32 

2.5 5.16 23.35 

3 5.32 23.39 

3.5 5.58 23.39 

4 5.76 23.39 

4.5 5.85 23.39 

5 5.90 23.39 

5.5 5.90 23.39 

10/16/96 Mean 5.39 23.36 

11/13/96 0.5 8.13 17.72 

1 8.11 17.72 

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Station Date Depth DO Temperature 

1.5 8.09 17.70 

2 8.07 17.72 

2.5 8.05 17.74 

3 8.01 17.77 

3.5 7.98 17.79 

4 7.96 17.80 

11/13/96 Mean 8.05 17.75 

12/11/96 0.5 9.42 15.24 

1 9.30 15.27 

1.5 9.17 15.33 

2 9.07 15.33 

2.5 9.00 15.32 

12/11/96 Mean 9.19 15.30 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

Appendix C: Comments from the public and responses from the Florida 

Department of Environmental Protection (DEP) on the nutrient and un-ionized 

ammonia TMDLs for Lake Jesup 

Comments from Seminole County via PBS&J:

The comments concern three areas: Water Budget including groundwater and St Johns River 

inflows; nutrient balance including wetland EMC and pristine wetlands assessment; and the 

sediment load to Lake Jesup supported by recent sampling events and Harvey Harper’s 

Sediment Loading Report.

WATER BUDGET

In revisiting the inflow to Lake Jesup, specifically the derivation of the groundwater contribution 

as compared to other studies, the 10% utilized (derived as a 17% of total surface runoff), is 

small compared to the typical 25% from USGS and others. Due to the lack of daily flow data in 

the tributaries to Lake Jesup, the baseflow percentage was derived a single tributary and 

extrapolated to the entire basin. The tributary however, drains an urbanized basin with high 

component of upstream storage and flow attenuation that can effectively masked the baseflow 

in with the storm flow. Additionally, the majority of the tributaries to Lake Jesup do not have 

storage characteristics and urban character. Further, other tributaries in the basin have 

significant groundwater influence, such as Salt Creek among others. In looking at the 

groundwater contribution as a calibration factor in the water balance it is revealed that a 20% 

groundwater contribution of total surface flow will provided a more accurate water balance than 

the 17% calculated from the Howell Creek Basin alone.

NUTRIENT LOAD

Another outstanding issue has been the wetlands EMC values in both the current period and the 

historic period. Historic conditions assume that wetlands are pristine and in as good of 

character as they can be, whereas it is assumed the current condition wetlands are impacted 

and discharge a higher level of nutrients. To investigate these issues further, a closer look was 

taken into Harvey Harper’s documentation of wetland EMCs and selected current wetlands 

were evaluated to determine if their character and functionality has changed since 

anthropogenic impacts. 

Closer look at the Harvey Harper report revealed that the wetland EMC of 0.19 mgl utilized as 

the anthropogenic wetland phosphorus contribution was derived based upon four values, (three 

from the Central Florida area and one from South Florida). The South Florida value was 

extremely high as compared to the Central Florida studies. When looking solely at the Central 

Florida values, the corresponding EMC for wetlands in an urban setting should be 0.14 mgl 

rather than 0.19 mgl. 

Then an assessment was made on selected wetlands adjacent to Lake Jesup to determine if 

their condition was pristine and comparable to historical wetland characteristics and 

functionality. In total seven wetlands were evaluated based upon hydroperiod, soils, invasive 

species and connectivity / habitat. Of the systems evaluated, three met all aspects of the 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

criteria while others were either determined to be some what impacted, separated, or 

improving. 

SEDIMENT LOADING

In previous discussions regarding the nutrient load to Lake Jesup from bottom sediments, it was 

noted that due to lack of applicable studies, aerobic lake conditions, lack of bottom samples, 

and similarity of Lake Jesup phosphorus concentrations to the adjacent WBIDs, the sediment 

load would be neglected. 

As stated in the Draft Lake Jesup TMDL table 5.9, an average of the concentrations of TP in 

Lake Jesup (0.142 mgl) is lower than in the WBIDs adjacent to Lake Jesup (0.199 mgl). 

However what this assessment failed to take into consideration is the flow weighted contribution 

of the adjacent WBIDs, when the magnitude of the contribution is considered, Lake Howell and 

Soliders Creek (0.11 and 0.14 mgl respectively), producing a weighted average of TP from 

these WBIDs the resulting TP concentration is at or below the concentration in the lake. Given 

the low concentrations present in direct rainfall on the lake and inputs from the SJR, it is 

plausible that the surface water contribution does not dictate the resulting concentration in the 

lake alone.

It was presumed that if sediments were resuspending and contributing to the nutrient load in the 

lake would lead to an oxygen demand and oxygen stratification in the lake. Observations and 

dissolved oxygen sampling has shown this not to be the case. However, given the shallow 

character of the lake relative to the large surface area has lead to significant mixing in the lake 

sufficient to elevate the DO in the entire water column. It was still inconclusive whether the 

nutrients were being released in the aerobic environment, so following the meeting on July 2nd

2005, Seminole County conducted samples of Lake Jesup at the sediment interface. 

Presumably, the concentrations would be in the range of expected values of Lake Jesup (0.14 

to 0.25 TP mgl). However, what the samples showed was concentrations in the central and 

northeast sampling locations of 1.5 and 3.1 mgl of TP. These values are more than an order of 

magnitude higher than was expected if the sediments did not contribute nutrients to the water 

column. Further, due to the significant mixing present in the lake, the release observed at the 

interface is assuredly entering the water column. To gain further insight on these revealing 

samples, Harvey Harper was consulted, in the conversation, Harvey confirmed the conclusion 

and indicated that he actually performed a study of sediment load in Lake Jesup during the 

1980’s. His study found that not only is the nutrient load from the sediments in Lake Jesup 

significant, it is in levels in excess of those found in Lake Apopka.

Responses from the DEP to comments from Seminole County via PBS&J 

Regarding the estimation of baseflow from tributaries into Lake Jesup, the original baseflow 

estimates was based on the local minimum method provided by the Hydrological Separation 

Program (HYSEP) developed by the USGS. Results of baseflow separation can indeed be 

influenced by the upstream storages. A separate modeling effort from the SJRWMD using 

HSPF revealed the similar findings. Their study indicated that relatively low baseflow was 

observed in the upper reach of the Howell Creek, but high baseflow could be observed in the 

area close to the lake. The DEP therefore decided to adopt the 20% baseflow rate suggested 

by Seminole County. This result is still based on the local minimum method but adjusted based 

on the stream flow results from the lower reach of Howell Creek. 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

As for the TP wetland EMC, the DEP used 0.19 mg/L In the first draft of TMDL report. This 

number was cited from Harvey Harper’s 1994 stormwater studies based on three central Florida 

wetlands and one south Florida wetland. Most of these wetlands are located in urbanized areas 

and was considered impacted by the surrounding urban area. Therefore, high TP 

concentrations from these wetlands were not beyond expectation. As the majority of wetland 

areas in the Lake Jesup watershed are located either downstream of urban areas or were 

converted from the previous agricultural lands, wetlands in Lake Jesup watershed were 

considered somewhat impacted or are recovering from previous impacts instead of being 

considered as under the pristine condition. This was the reason why 0.19 mg/L of TP was used 

as wetland EMC. The DEP later obtained an updated stormwater study by Harvey Harper 

(2003), which included 19 sets of wetland TP data. The mean TP EMC from these studies was 

0.09 mg/L. As this latter study contains a larger data set, the mean derived from the data set 

was deemed more reliable than that of the previous study. In addition, the PBS&J also 

evaluated the extent at which the wetlands in the Lake Jesup watershed are impacted by 

human activities and provided to DEP the area and location of the wetlands that were 

considered impact and those that are relatively unimpacted, the DEP decided to divide the 

whole wetland areas in the Lake Jesup watershed into two categories, wetland1 and wetland2, 

with wetland1 being somewhat impacted and wetland2 being relatively pristine. A TP EMC of 

0.06 mg/L has been used commonly for relatively unimpacted wetland area by the SJRWMD. 

This number was also adopted for unimpacted wetland in the Lake Jesup watershed. The 0.09 

mg/L was used as the TP EMC for impacted wetlands. Description of using different TP EMCs 

for wetland of different impact extents can be found in the TMDL report on page 29-30. 

As for the importance of sediment nutrient release, the DEP never denied that nutrient release 

from the sediment could be significant. However, the DEP explained in several public meeting 

the difference between gross sediment nutrient release and net sediment reslease. Bathtub, 

which is the model used to simulate TN, TP, and Chla in-lake mass balance processes, uses 

the net deposition rate (negative net release rate) for model simulation. The net nutrient 

deposition rate is the difference between the gross sediment nutrient release and gross nutrient 

deposition from the water column. While the gross sediment nutrient release could be 

significant, the release could be balanced out by nutrient deposition. The observations that the 

total nutrient output from Lake Jesup was not significantly different from the nutrient input into 

the lake, historic net accumulation of nutrient in the sediment, and nutrient concentrations in 

tributaries matching up with the in-lake nutrient concentration all supported the conclusion that 

the net sediment nutrient release is not a significant process in Lake Jesup. 

Although the DEP believes that net sediment nutrient release is not a significant process for 

Lake Jesup and therefore does not significantly influence the TMDL estimation, we also 

acknowledge that control of the gross sediment release may speed up the restoration of the 

favored nutrient condition. Because, at the time this TMDL was developed, we did not have 

enough information to quantify how activities like sediment dredging would help the lake 

restoration, the DEP suggested further studies on the gross sediment nutrient release in the 

lake. 

Comments from the St. Johns River Water Management District (SJRWMD):

Comments from the SJRWMD concerns four areas: the DEP should not add 5 TSI units on top of the 

background condition; Wetland TP EMC of 0.19 mg/L was too high; Several methods proposed by the 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

SJRWMD to establish the water quality target were rejected by the DEP; and the draft report allocate 

percent reduction of nutrient required to achieve water quality target to sediment nutrient release. 

The DEP should not add 5 TSI units on top of the background condition: The IWR defines 

natural background as “the condition of water in the absence of man-induced alterations based on 

the best scientific information available to the Department”. 

9 The paragraph on page 22 (note from DEP: this page number is changed because of the 

revision of the report based on comments from local stakeholder and report editing) is 

unclear and should be rewritten: “...some increase over the background condition should 

be allowable without causing an imbalance of flora or fauna of the lake”. 

There is no evidence that a target TSI of 65 will not cause an imbalance of flora and 

fauna in Lake Jesup. The Impaired Waters Rule states that a TSI of 60 is the threshold 

for lake impairment. 

9 The draft report states that a TSI of 65 “would prevent the lake from becoming impaired 

as defined by the IWR”. Is this statement supported by dta, as required by the IWR? 

9 In the draft report, the addition of TSI points to the target is justfified by saying that “This 

approach also reserves 5 TSI units to allow for future changes in the basin and as part of 

the implicit margin of safety in estimation of the assimilative capacity”. However, future 

landuse changes are dealt with in the pollutant allocation phase fo the BMAP process, not 

in the setting of concentration targets. Also, the margin of safety should be calculated in 

the TMDL, not outside of the TMDL. In this case, the draft report implies that the 

margin of safety is the 5 TSI units between 65 (TMDL) and 70, at which point the lake is 

impaired. 

Also, the Allocation Technical Advisory Committee report endorsed FDEP’s continued 

use of an “implicit margin of safety (MOS) based on conservative assumptions in the 

modeling” rather than the use of an explicit MOS. The lake Jesup target TSI of 60 

already contains an implicit MOS due to modeling assumptions. The fraft report, 

however, refers to an explicit MOS that is the difference between the TMDL and the 

lower limit of impairment. 

Wetland EMCs: 

9 Different wetland EMCs were used to predict current and historic loading. The wetland 

EMC (0.19 mg/L) used in the draft report to predict current TP loading is too high. It is 

based on Harper’s EMC calculated for “impacted” wetland, which are those that receive 

treated effluent. None of the wetlnads around Lake jesup fit this definition of 

“impacted”. 

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TMDL Report: Middle St. Johns Basin, Lake Jesup, WBIDs 2981/2981A, Nutrients/Unionized Ammonia

According to a study conducted by John Hendrickson at the SJRWMD, based on water 

quality data collected in recent years from more than 20 watersheds in the lower St. Johns 

River area, the EMC of TP for wetlands is 0.06 mg/L. 

Concentration target analyses provided by the SJRWMD: 

The nine analyses used by SJRWMD to develop recommended concentration targets for TP and 

TN are accurately represented in the TMDL report. The draft TMDL, however, did not consider 

almost half (four) of these paths of investigation for various reasons. 

The draft report states that: 

9 The relationship between nitrogen and plankton abundance is unclear because of the 

possibility of nitrogen fixation. The degree of nitrogen fixation in Lake Jesup has not 

been empirically measured, it is premature to disregard the possibility of a useful 

relationship between nitrogen and plankton abundance. 

9 The water clarity/SAV analysis is “tied to usage of the lake, instead of the natural 

assimilative capacity” because SAV coverage can be linked to sport fisheries. However, 

water clarity and SAV coverage are important metrics of a healthy lake, independent of 

links to recreational fishing use of the lake. 

9 There is a high standard deviation in TP/TN/chlorophyll-a/bloom frequency data. 

However, this should be considered part of the implicit margin of safety rather than 

eliminating the data from inclusion in the calculation of natural background condition. 

Allocation to Internal Recycling 

The draft TMDL report suggests allocating a nutrient reduction to FFWCC’s proposed lakebottom dredging projects “because the sediment removal may decrease the nutrient loading from 

the sediment to the lake water column”. The report acknowledges that nutrient relase from the 

sediments does not “appear to be an important part of the nutrient budget for Lake Jesup” but 

states that “there is no direct information on the actual nutrient release rates available”. 

The draft TMDL report should not allocate load reductions to internal recycling. Allocating a 

load reduction to a propose lake-bolttom dredging project would fall into this category. Lake 

bottom dredging designed to improve water quality should be handled as a project to help mee 

the TMDL, not as an allocation. In a statement agreed by both DEP and SJRWMD (dated May 

13, 2005), it is stated: 

 “A strategy that addresses internal nutrient recycling is appropriate to include in a 

BMAP, but it will not replace external load recution or come into play in pollutant trading.” 

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Responses from the DEP to comments from the SJRWMD: 

(1) Regarding comments that adding 5 TSI units on top of the natural background condition 

The DEP acknowledges that the IWR uses TSI 60 or 40 (depend on water color) as the 

assessment thresholds for lake nutrient impairment. However, the IWR did not define TSI 

60 or 40 as the only threshold for the assessment of lake nutrient impairment. In fact, the 

rule calls for developing, to the extent feasible, site-specific thresholds that more accurately 

reflect conditions beyond which an imbalance in flora or fauna occurs in the water segment. 

This was exactly why multiple methods were evaluated to establish nutrient targets instead 

of simply using TSI 60 or 40 as the target for the TMDL. 

All the methods described in the TMDL report characterize the relatively unimpacted lake 

condition. For this TMDL, the unimpacted condition established using these methods was 

very close to the background condition simulated by the calibrated model suit. For the 

nutrient assessment purpose, the IWR does not call for meeting the exact background 

condition. Instead, 10 TSI units above the historic minimum are allowed before a lake would 

be assessed as impaired for nutrient. In establishing the nutrient target for Lake Jesup, the 

DEP allows only 5 TSI units above the natural background condition, which is a more 

conservative target and more protective for the lake than allowing 10 TSI units, and 

therefore added margin of safety to the final TMDL. 

(2) Regarding the wetland TP EMCs 

The same as the response from the DEP to comments from Seminole County via PBS&J. 

The DEP decided to divide wetland areas in the Lake Jesup watershed into two categories 

including Wetland1 (somewhat impacted) and Wetland2 (relatively pristine), and assigned 

0.09 mg/L as TP EMC to Wetland1 and 0.06 mg/L as the TP EMC to Wetland2. 

(3) Regarding the rejection of several methods proposed by the SJRWMD in establishing 

the target water quality condition 

Several methods proposed by the SJRWMD in establishing the target water quality condition 

were not adopted by the DEP. The DEP described in the TMDL report in a very detailed 

fashion why these methods were not adopted. The reason why the DEP reject the method 

establishing the TN target based on a correlation between chlorophyll a and TN 

concentration was because of the possible complication from the nitrogen fixation. In this 

case, nitrogen is not the cause for the observed high chlorophyll a concentration. Instead, it 

is the consequence of the high chlorophyll a concentration. Although at the time the TMDL 

report was drafted there was no direct evidence showing that nitrogen fixation indeed 

existed in Lake Jesup, many indirect evidences, including the about two-fold higher TN 

concentration in the lake than in majority of the tributaries to the lake, the lack of evidence 

showing the significant release of nitrogen from the sediment, the existence and dominance 

of nitrogen fixing blue-green algae in the lake, and the low ratio between total inorganic 

nitrogen to total inorganic phosphorus that may trigger the nitrogen fixation, all support the 

possibility that nitrogen fixation not only exists in the lake, but could be very significant. All 

these details were discussed in the TMDL report. In fact, significant nitrogen fixation in Lake 

Jesup has been shown by Seminole County via PBS&J in a follow-up study. This was why 

the DEP reject the using the regression method to establish the TN target. 

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The major reason for the water clarity/SAV method to be rejected by the DEP was not 

because this method gears more toward usage instead of the natural assimilative capacity 

of nutrient of the lake although it is part of the reason. The major reason why this method 

was rejected was because it depends on the regression curve between the light extinction 

coefficient and TN and TP concentrations. The regression curve between light extinction 

coefficient and TP concentration established by the SJRWMD had a very low correlation 

coefficient and therefore was not deemed reliable. The correlation between the light 

extinction coefficient and TN concentration was again complicated by the possible existence 

of nitrogen fixation in the lake. In addition, it is very true that the final water quality target 

typically is the compromise between different designated uses, including the fishery. 

However, for nutrient target, the Florida Surface Water Quality Standard stresses the 

importance that the balance of natural aquatic flora and fauna should not be disturbed. It is 

also assumed that as long as the balance of natural aquatic flora and fauna is protected, 

healthy and well-balanced fish and wild life populations should also be maintained. 

Therefore, when considering the water quality target, the natural balance of aquatic flora 

and fauna was assigned the higher weight. 

The concept of using both frequency and intensity of algal bloom to establish the water 

quality target is a very important one because the frequency includes a very important 

parameter to characterize a bloom, the length of time of the bloom, not just the biomass 

intensity. If strong correlation could be defined between the bloom strength and frequency 

and nutrient concentrations, this technique would be very promising for nutrient target 

development for Lake Jesup. However, as it was shown in Bachmann et al. (2003), the 

variance between algal bloom strength and frequency and TN and TP concentration was 

very significant when the TN/TP ratio fall between 10 and 17 (in most case this is the range 

that TN/TP ratios of Lake Jesup fall in). The variance did not always point to the low nutrient 

concentration, which would be more protective for the lake function and add to the margin of 

safety. The variance fluctuates both ways. It is therefore difficult for the DEP to develop 

reliable water quality target using this method. 

(4) Sediment nutrient release and load allocation to lake-bottom sediment 

The SJRWMD claimed that the TMDL report allocated a certain portion of the pollutant 

loading to the lake sediment. In fact, the report did not allocate any pollutant load to the 

sediment. The report assumed sediment nutrients release was not significant in Lake Jesup 

based on the comparison of nutrient input into and output from the lake, the tributary nutrient 

concentration and in-lake nutrient concentration, nitrogen fixation, and etc. As the report 

assumes no sediment nutrient release, there is no way that pollutant load can be allocated 

to the sediment. The DEP recognized that the “sediment nutrient release is not significant” 

conclusion was reached based on indirect information instead of directly measured sediment 

nutrient release. Therefore, in the Recommendation for Future Studies section, the DEP 

recommended sediment nutrient release study. If the entity that will dredge the lake can 

quantify the influence of sediment nutrient release on the in-lake nutrient concentration and 

the in-lake nutrient effects from the sediment dredge, credit may be awarded to the entity. 

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Comments from the United States Environmental Protection Agency (EPA):

 

1. Given the relative sizes of the WBIDs and the fact that there do not seem to be any 

additional sources discharging to just WBID 2981A, it seems reasonable to assume that 

the nutrient TMDL for WBID 2981 will address the nutrient TMDL for WBID 2981A. 

2. The assumption that the nutrient TMDL for WBID 2981 will address the unionized 

ammonia impairment also seems reasonable. However, given the scatter in the 

relationships between pH and unionized ammonia, and between pH and chlorophyll, it is 

essential that implementation of the TMDL include continued monitoring for unionized 

ammonia to ensure that reductions are, in fact, being achieved. It might be a good idea 

to expressly state the intention for continued monitoring on page 82, when unionized 

ammonia is discussed. 

3. FDEP was very comprehensive in using different approaches to develop targets for TN 

and TP. This lends some confidence to the initial/natural background targets, because 

the different approaches mostly converge on the same numbers. It is reasonable to 

assume that some increase in TSI above natural background would not cause a nutrient 

problem. However, the rationale for a 5-unit increase in TSI to arrive at the TMDL 

targets should not be based solely on the IWR. Rather, the sufficiency of the final 

targets should be justified by some analysis showing that a TSI of 65 and nutrient 

concentrations of 1.32 mg/l TN and 0.094 mg/l TP will not cause an imbalance of flora 

and fauna in the lake. At the very least, Best Professional Judgment could be used to 

support the targets. Similarly, it is best to stay away from the “we wouldn’t consider it 

impaired based on the IWR” line of reasoning when discussing Margin of Safety and 

future assimilation capacity. 

4. The document could really use a brief summary that provides the big picture of what the 

final targets and TMDLs are and how they were determined. The length of the 

document, the copious amount of information it contains, and the fact that it describes 

some analyses that were ultimately not used to develop targets or determine the TMDL, 

make it easy to get confused. For example, the interpretation of the narrative nutrient 

standard is discussed in Chapter 3 (Water Quality Standards and Targets), but really the 

target you ended up using is described in Chapter 4 (Assessment of Sources), and 

some in Chapter 5 (Determination of Assimilative Capacity). Having a summary near 

the beginning of the document would provide some context for understanding the details 

of the analyses in chapters 3, 4, and 5. 

5. Please double-check the numbers for total phosphorus loading in all relevant tables. 

The estimates of total phosphorus loading coming from surface runoff in Table 4.11-c (p. 

42) and Table 4.12-c (p. 44) are different from the surface runoff estimates in Table 

4.21-c (p.53) and Table 5.13-b (p. 75). The estimates of TP loads for each year in each 

sub-watershed (Table 4.11-c), and from different land use categories (Table 4.12-c) do 

add to the total values provided. Also, even though the surface runoff values are 

different, the estimates for each source in Table 4.21-c and Table 5.13-b sum up to the 

totals provided. If the other two tables are correct, this implies that the totals in Table 

4.21-c and 5.13-b are incorrect (or vice versa). So which numbers are correct, and 

which were used for the TMDL? 

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6. Many of the tables are split between pages. It makes them hard to read when the 

column headings are on one page, and the numbers are on another. I know this is just a 

formatting issue, but in the final version, please make it so that the column headings 

repeat on different pages, or so that the tables are not split. 

Responses from the DEP to comments from the EPA: 

Regarding EPA’s Comment 2, the DEP added language on Page 84 to encourage monitoring 

programs that track the trend of water pH and un-ionized ammonia concentration when the 

TMDL implementation is started. 

Regarding EPA’s Comment 3, the DEP believes that direct application of natural background as 

the target TSI would not allow for any assimilative capacity. 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 DEP 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.

Regarding EPA’s Comment 4, the DEP created an Executive Summary for the report. 

Regarding EPA’s Comment 5, the DEP double checked the data in all the tables and text and 

removed some table fields that may cause confusion and made sure all the data and results are 

updated properly. 

Regarding EPA’s Comment 5, the DEP tried its best to ensure that majority of the tables are on 

just single one page except for the table in Appendix B, which is too long to fit in just one page. 

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Florida Department of Environmental Protection 

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www2.dep.state.fl.us/water/ 

2600 Blair Stone Road, Mail Station 3565 

Tallahassee, Florida 32399-2400 

(850) 245-8561 

Florida Department of Environmental Protection 

Division of Water Resource Management 

Bureau of Watershed Management