TOTAL MAXIMUM DAILY LOAD (TMDL) 

For Nutrients, Dissolved Oxygen, 

and Coliforms 

In 

Sarasota Bay/Charlotte Harbor Basin Groups 

(WBIDs 1982A, 1984, 1984A, 2009A, 1975A, 1975, 

2049, 2039, 2042, and 2030) 

Prepared by: 

US EPA Region 4 

61 Forsyth Street SW 

Atlanta, Georgia 30303 

March 2006

 
ii

In compliance with the provisions of the Federal Clean Water Act, 33 U.S.C §1251 et. seq., as 

amended by the Water Quality Act of 1987, P.L. 400-4, the U.S Environmental Protection 

Agency is hereby establishing the Total Maximum Daily Load (TMDL) for nutrients, dissolved 

oxygen, and coliforms in the Sarasota Bay/Charlotte Harbor Basin (WBIDs1982A, 1984, 1984A, 

2009A, 1975A, 1975, 2049, 2039, 2042, and 2030). Subsequent actions must be consistent 

with this TMDL. 

__________________________________________ ___________ 

 James D. Giattina, Director Date 

 Water Management Division 
iii

TABLE OF CONTENTS 

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1. INTRODUCTION ...................................................................................................................1

2. PROBLEM DEFINITION........................................................................................................2

3. WATERSHED DESCRIPTION ..............................................................................................4

3.1 SARASOTA BAY PLANNING UNIT..................................................................................................................4

3.2 LEMON BAY PLANNING UNIT........................................................................................................................5

4. WATER QUALITY STANDARD AND TARGET IDENTIFICATION .......................................9

5. WATER QUALITY ASSESSMENT AND DEVIATION FROM TARGET..............................10

5.1 DISSOLVED OXYGEN DATA........................................................................................................................12

5.2 CHLOROPHYLL DATA .................................................................................................................................14

5.3 ALGAL GROWTH POTENTIAL DATA............................................................................................................19

5.4 TOTAL COLIFORM DATA.............................................................................................................................20

5.5 FECAL COLLIFORM DATA...........................................................................................................................22

6. SOURCE ASSESSMENT....................................................................................................24

6.1 POINT SOURCES ........................................................................................................................................24

6.1.1 NPDES Facilities...................................................................................................................................24

6.1.2 MS4 areas ..............................................................................................................................................24

6.2 NONPOINT SOURCES .................................................................................................................................25

6.2.1 Wildlife...................................................................................................................................................25

6.2.2 Agricultural animals ..............................................................................................................................25

6.2.3 Onsite Sewerage Treatment and Disposal Systems (Septic Tanks)........................................................26

6.2.4 Urban Development...............................................................................................................................27

7. ANALYTICAL APPROACH..................................................................................................27

7.1. NUTRIENT AND BIOLOCHEMICAL OXYGEN DEMAND (BOD)......................................................................27

7.1.1 Determination of Pollutant Loading Capacity.......................................................................................28

7.1.1.1. Hydrologic Data and Runoff Generation ..........................................................................................28

7.1.1.2. Pollutant Loading Rates for Land Use Categories............................................................................29

7.1.1.3. Load Attenuation Factors..................................................................................................................30

7.1.1.4. Computations of Runoff Volumes and Pollutant Loadings ................................................................31

7.1.2 Determination of Target Loads..............................................................................................................33

7.2. COLIFORMS ................................................................................................................................................36

7.1.2.1. Percent Reduction Approach.............................................................................................................37

8. DEVELOPMENT OF TOTAL MAXIMUM DAILY LOAD.......................................................38

8.1. WASTE LOAD ALLOCATIONS......................................................................................................................39

8.1.1 NPDES Wastewater Discharges ............................................................................................................39

8.1.2 NPDES Stormwater Discharges ............................................................................................................39

8.2. LOAD ALLOCATIONS...................................................................................................................................40

8.3. MARGIN OF SAFETY (MOS).......................................................................................................................40

8.4. CRITICAL CONDITIONS AND SEASONAL VARIATIONS .................................................................................42

9. CONCLUSION AND RECOMMENDATIONS ......................................................................42

REFERENCES ...........................................................................................................................44

APPENDIX A PHOTOS OF IMPAIRED WBIDS .....................................................................1

APPENDIX B TMDL DEVELOPMENT FOR SARASOTA BAY/LEMON BAY WBIDS USING 
iv

THE REFERENCE WATERSHED APPROACH............................................................................I
v

LIST OF TABLES 

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Table 1. TMDLs Developed By EPA in Sarasota Bay/Charlotte Harbor Basins ...........................2

Table 2. Current Land Use/Cover Distribution in Impaired WBIDs1 .............................................6

Table 3. Aggregate Land Use/Cover Categories Derived from the Florida Land Use, Cover, and 

Forms Classification System (FLUCCS). ..............................................................................7

Table 4. Water Quality Monitoring Stations in the impaired WBIDs...........................................11

Table 5. Summary of Dissolved Oxygen Data (mg/l) for Impaired WBIDs.................................13

Table 6. Summary of Chlorophyll Data (μg/l) for Impaired WBIDs. ...........................................15

Table 7. Results of Algal Growth Potential Tests.......................................................................19

Table 8. Total Coliform Data for Impaired WBIDs......................................................................21

Table 9. Fecal Coliform Data for Impaired WBIDs.....................................................................22

Table 10. Livestock Inventory for Sarasota County, Florida. .....................................................26

Table 11. Estimates of Septic Tanks and Repair Permits for Sarasota County.........................27

Table 12. Percent Impervious and DCIA by Land Use for Impaired WBIDs..............................29

Table 13. Nutrient and BOD Concentrations in Stormwater by Land Use .................................29

Table 14. Treatment Potential Factors.......................................................................................30

Table 15. Stormwater System Attenuation Factors ...................................................................31

Table 16. Conveyance System Attenuation Factors..................................................................31

Table 17. Annual Total N Loads by Land Use Under Existing Conditions.................................32

Table 18. Annual BOD Loads by Land Use Under Existing Conditions.....................................33

Table 19. Land Cover Distribution in the Impaired WBIDs Under Natural Conditions ...............34

Table 20. Annual Total N Loads by Land Cover Under Natural Conditions...............................35

Table 21. Annual BOD Loads by Land Cover Under Natural Conditions ..................................36

Table 22. Fecal Coliform Measurements for Elligraw Bayou .....................................................37

Table 23. TMDL Components for Nutrient-Impaired Waters. ....................................................40

Table 24. TMDL Components for Waterbodies Impaired by DO ...............................................41

Table 25. TMDL Components for Waterbodies Impaired by Coliform. ......................................41

LIST OF FIGURES 

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Figure 1. Location of 303(d) listed WBIDs in the Sarasota Bay and Lemon Bay Basins.............3

Figure 2. Dissolved Oxygen Measurements in Gottfried Creek, WBID 2049.............................14

Figure 3. Dissolved Oxygen Measurements in Elligraw Bayou, WBID 1975. ............................14

Figure 4. Chlorophyll measurements in WBID 1975 (Elligraw Bayou)........................................15

Figure 5. Chlorophyll measurements in WBID 1975A (Clowers Creek)......................................16

Figure 6. Chlorophyll measurements in WBID 1982A (South Creek). .......................................17

Figure 7. Chlorophyll measurements in WBID 1984A (North Creek).........................................17

Figure 8. Chlorophyll measurements in WBID 1984 (Catfish Creek).........................................18

Figure 9. Chlorophyll measurements in WBID 2030 (Alligator Creek)........................................18

Figure 10. Total Coliform Measurements in WBID 1975 (Elligraw Bayou). ...............................21

Figure 11. Total Coliform Measurements in WBID 1975A (Clowers Creek). .............................22

Figure 12. Fecal Coliform Measurements in WBID 1975 (Elligraw Bayou)................................23

Figure 13. Fecal Coliform Measurements in WBID 1975A (Clowers Creek). ............................23
vi

LIST OF ABBREVIATIONS 

________________________________________________________________________________________ 

AWT Advanced Waste Treatment 

BMP Best Management Practices 

BPJ Best Professional Judgment 

CFS Cubic Feet per Second 

CFU Colony Forming Units 

DEM Digital Elevation Model 

DMR Discharge Monitoring Report 

EPA Environmental Protection Agency 

FAC Florida Administrative Code 

GIS Geographic Information System 

HUC Hydrologic Unit Code 

LA Load Allocation 

MGD Million Gallons per Day 

MOS Margin of Safety 

MPN Most Probable Number 

MS4 Municipal Separate Storm Sewer Systems 

NASS National Agriculture Statistics Service 

NLCD National Land Cover Data 

NPDES National Pollutant Discharge Elimination System 

NRCS Natural Resources Conservation Service 

OSTD Onsite Sewer Treatment and Disposal Systems 

PLRG Pollutant Load Reduction Goal 

Rf3 Reach File 3 

RM River Mile 

STORET STORage RETrieval database 

TMDL Total Maximum Daily Load 

USDA United States Department of Agriculture 

USGS United States Geological Survey 

WBID Water Body Identification 

WCS Watershed Characterization System 

WLA Waste Load Allocation 

WMP Water Management Plan 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basins 

September 2005 

Page v

SUMMARY SHEET 

Total Maximum Daily Load (TMDL) 

1. 303(d) Listed Waterbody Information 

State: Florida 

County: Sarasota 

Major River Basin: Sarasota Bay Basin (HUC 03100201) 

WBID Segment Name Type Constituents 

1975 Elligraw Bayou Freshwater Nutrients, Dissolved Oxygen (DO), Coliforms 

1975A Clowers Creek Freshwater Coliforms 

1982A South Creek Estuarine Nutrients 

1984 Catfish Creek Estuarine Nutrients 

1984A North Creek Estuarine Nutrients 

2009A Curry Creek Estuarine Nutrients 

2030 Alligator Creek Estuarine Nutrients 

2039 Forked Creek Estuarine Nutrients 

2042 Direct Runoff to 

Bay Estuarine Nutrients 

2049 Gottfried Creek Estuarine Nutrients, Dissolved Oxygen 

2. TMDL Development Approach and Modeling 

- Nutrient and DO Impairment 

Because of insufficient and/or lack of recent monitoring data of nutrients and DO for 

impaired WBIDs, pollutant loading capacity and target loads were determined using the 

Lemon Bay Watershed Model. The model used pollutant loading rates typical for land 

uses in these WBIDs and runoff volumes to compute long-term average annual loads of 

TN and BOD. The target loads were assumed to be those that existed under “natural 

conditions”, that is prior to anthropogenic influences in the impaired WBIDs. 

 - Coliforms 

Due to the lack of long-term record of bacteria samples and flow data to establish the 

relationship between bacterial concentrations and flow using load duration curves, 

coliform TMDLs were expressed as percent reductions. 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basins 

September 2005 

Page vi

3. TMDL Endpoints and Load Allocations (LAs) 

 - Total N for Waterbodies Impaired by Nutrients 

WLA2

Waterbody Parameter1

Existing 

Load 

(kg/year) 

Target 

Load-LA 

(kg/year) 

Continuous 

(kg/year) 

MS4 

(Percent 

reduction)

MOS4 

(kg/year) 

Percent 

Reduction5

Elligraw 

Bayou TN 2525.86 1799.13 N/A3

 28.8 Implicit 28.8 

Curry Creek TN 1300.99 482.60 N/A3 62.9 Implicit 62.9 

North Creek TN 1116.10 589.95 N/A3 47.1 Implicit 47.1 

Catfish Creek TN 2279.7 1110.0 N/A3 51.3 Implicit 51.3 

South Creek TN 1816.58 941.89 N/A3 48.2 Implicit 48.2 

Gottfried 

Creek TN 3025.13 2965.58 N/A3

 2.0 Implicit 2.0 

Forked Creek TN 4234.90 3386.74 N/A3 20.0 Implicit 20.0 

Direct Runoff 

to Bay TN 1414.23 640.51 N/A3

 54.7 Implicit 54.7 

Alligator 

Creek TN 5370.41 3857.43 N/A3

 28.2 Implicit 28.2 

Notes: 1TN = total nitrogen

2

WLA component is separated into load from continuous NPDES facilities (e.g., WWTPs) and load 

from MS4s. 

3

N/A = not applicable 

4

Margin of Safety is implicit due to conservative assumptions made in determining TMDL targets. 

5

Percent reduction from current conditions to achieve the target concentration for that nutrient. 

 - TN and BOD Loads for Waterbodies Impaired by DO 

Target LoadLA (kg/year) WLA2

Waterbody Parameter1

Existing 

Load 

(kg/year) Anthropogenic Natural Continuous 

(kg/year) 

MS4 

(Percent 

reduction) 

MOS4 

(kg/year)

Percent 

Reduction5

TN 2525.86 0 1799.13 N/A3 28.8 Implicit 28.8 

Elligraw Bayou 

BOD 12931.64 0 3800.00 N/A3

 70.6 Implicit 70.6 

TN 5370.41 0 3857.43 N/A3

28.2 Implicit 28.2 

Alligator Creek 

BOD 15727.53 0 6632.32 N/A3

57.8 Implicit 57.8 

1

TN = total nitrogen, BOD = Biochemical Biological Demand 

2

WLA component is separated into load from continuous NPDES facilities (e.g., WWTPs) and load 

from MS4s. 

3

N/A = not applicable 

4

Margin of Safety is implicit due to conservative assumptions made in determining TMDL targets. 

5

Percent reduction from current conditions to achieve the target concentration for that 

nutrient.
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basins 

September 2005 

Page vii

 - Coliforms: 

Endpoints: 

Fecal Coliform: 400 MPN/100ml 

 Total Coliform: 2400 MPN/100ml 

 For Waterbodies Impaired by Coliform 

WLA2

WBID Parameter1

TMDL 

(MPN/day) 

LA 

(MPN/day) Continuous 

MS4 

(percent 

reduction 

Percent 

Reduction3

Elligraw TCOLI 3910 3910 N/A 39 39 

Bayou FCOLI 1323 1323 N/A 70 70 

Clowers TCOLI 2615 2615 N/A 85 85 

Creek FCOLI 1678 1678 N/A 76 76 

Notes: 

1. WLA component is separated into load from continuous NPDES facilities and load from MS4s. 

2. N/A = not applicable 

3. Percent reduction from current conditions to achieve the target concentration for that nutrient. 

4. Endangered Species (yes or blank): Yes 

5. EPA Lead on TMDL (EPA or blank): EPA 

6. TMDL Considers Point Source, Nonpoint Source, or both: Both 

7. Major NPDES Discharges to surface waters addressed in EPA TMDL: None 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 1 

TOTAL MAXIMUM DAILY LOAD (TMDL) FOR NUTRIENTS, D.O., FECAL AND TOTAL 

COLIFORMS in WBIDs 1975, 1975A, 1982A, 1984, 1984A, 2009A, 2030, 2039, 2042, 2049 

(SARASOTA BAY/CHARLOTTE HARBOR BASINS)

1. INTRODUCTION 

Section 303(d) of the Clean Water Act requires each state to list those waters within its boundaries 

for which technology-based effluent limitations are not stringent enough to protect any water quality 

standard applicable to such waters. Listed waters are prioritized with respect to designated use 

classifications and the severity of pollution. In accordance with this prioritization, states are 

required to develop Total Maximum Daily Loads (TMDLs) for those water bodies that are not 

meeting water quality standards. The TMDL process establishes the allowable loadings of 

pollutants or other quantifiable parameters for a waterbody based on the relationship between 

pollution sources and in-stream water quality conditions, so that states can establish water quality 

based controls to reduce pollution from both point and non-point sources and restore and maintain 

the quality of their water resources (USEPA, 1991). 

The State of Florida Department of Environmental Protection (FDEP) has developed 303(d) lists 

since 1992. The process by which Florida implements section 303(d) requirements is set forth in 

the Florida Watershed Restoration Act (FWRA) of 1999 (s. 403.067, Florida Statutes). The FDEP 

list of impaired waters in each basin, referred to as the “Verified List”, is also adopted pursuant to 

the FWRA (Subsection 403.067[4], Florida Statutes). However, the FWRA also states 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-rule-making process, the Florida Environmental Regulation Commission adopted the new 

methodology as Chapter 62-303, Florida Administrative Code (Identification of Impaired Surface 

Waters Rule, or IWR), in April 2001. These TMDLs are being established pursuant to EPA 

commitments in the 1998 Consent Decree in the Florida TMDL lawsuit (Florida Wildlife Federation, 

et al. v. Carol Browner, et al., Civil Action No. 4: 98CV356-WS, 1998). 

 

FDEP developed a statewide, watershed-based approach to water resource management. 

Following this approach, water resources are managed on the basis of natural boundaries, such as 

river basins, rather than political boundaries. The watershed management approach is the 

framework DEP uses for implementing TMDLs. The state’s 52 basins are divided into 5 groups. 

Water quality is assessed in each group on a rotating five-year cycle. The Group 3 basin includes 

waters in the basins of the Caloosahatchee River, Choctawhatchee Bay and River, Saint Andrews 

Bay, Peace River, Sarasota Bay, Myakka River, Lake Worth Lagoon/Palm Beach Coast, and Saint 

Johns River. Group 3 waters were first assessed in 2002 with plans to revisit water management 

issues in 2007. 

FDEP established five water management districts (WMD) responsible for managing ground and 

surface water supplies in the counties encompassing the districts. For the purpose of planning and 

management, the WMDs divided the district into planning units defined as either an individual 

primary tributary basin or a group of adjacent primary tributary basins with similar characteristics. 

These planning units contain smaller, hydrological based units called drainage basins, which are 

further divided into “water segments”. A water segment usually contains only one unique waterbody 

type (stream, lake, cannel, etc.) and is about 5 square miles. Unique numbers or waterbody 

identification (WBIDs) numbers are assigned to each water segment. 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 2 

2. PROBLEM DEFINITION 

Florida’s final 1998 Section 303(d) list identified numerous WBIDs in the Sarasota Bay/Charlotte 

Harbor basin groups as potentially not supporting water quality standards (WQS). After assessing 

all readily available water quality data, EPA is responsible for determining whether TMDLs are 

needed for coliforms, nutrients and/or dissolved oxygen (D.O.) in various WBIDs of the basin. The 

TMDLs described in this report are listed in Table 1, and the geographic location of each listed 

WBID is provided in Figure 1. A request to de-list WBID 1975A (Clowers Creek) for nutrients and 

turbidity will be approved. 

Table 1. TMDLs Developed By EPA in Sarasota Bay/Charlotte Harbor Basins 

WBID Segment Name Type Planning Unit Parameter(s) of Concern 

1975 Elligraw Bayou Freshwater Sarasota Bay Nutrients, Dissolved Oxygen, 

Coliforms 

1975A Clowers Creek Freshwater Sarasota Bay Coliforms 

1982A South Creek Estuarine Sarasota Bay Nutrients 

1984 Catfish Creek Estuarine Sarasota Bay Nutrients 

1984A North Creek Estuarine Sarasota Bay Nutrients 

2009A Curry Creek Estuarine Sarasota Bay Nutrients 

2030 Alligator Creek Estuarine Lemon Bay Nutrients 

2039 Forked Creek Estuarine Lemon Bay Nutrients 

2042 Direct Runoff to Bay Estuarine Lemon Bay Nutrients 

2049 Gottfried Creek Estuarine Lemon Bay Nutrients, Dissolved Oxygen 

The waterbodies listed in Table 1 are Class III waters with the designated use of recreation, and 

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

waters are further categorized as either freshwater or marine. 

To determine the status of surface water quality in the state, three categories of data – chemistry 

data, biological data, and fish consumption advisories – were evaluated to determine potential 

impairments. The level of impairment is defined in the Identification of Impaired Surface Waters 

Rule (IWR), Section 62-303 of the Florida Administrative Code (F.A.C.). The IWR defines the 

threshold for determining if waters should be included on the state’s planning list and verified list. 

Potential impairments are determined by assessing whether a waterbody meets the criteria for 

inclusion on the planning list. Once a waterbody is on the planning list, additional data and 

information will be collected and examined to determine if the water should be included on the 

verified list. 

The format of the remainder of this report is as follows: Chapter 3 is a general description of the 

impaired watersheds; Chapter 4 describes the water quality standard and target criteria for the 

TMDLs; and Chapter 5 describes the development of the TMDLs.
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 3 

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S #S

#S

#S

#S

#S

#S#S

#S

Sarasota

North Port

Port

Charlotte

Venice

Charlotte

 Harbor

Little Sarasota Bay

Lemon Bay

Gulf

of

Mexico

Englewood

2049

2039

2042

2030

2009A

1982A

1984A

1975A

1975

1984

7 0 7 14 Miles

Catalog Units

Cities and Towns

Counties

Sarasota B./Charlotte H. WBIDs

Rivers and Streams

#S NPDES Facilities

N

W E

S

area of detail

LEGEND

Figure 1. Location of 303(d) listed WBIDs in the Sarasota Bay and Lemon Bay Basins 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 4 

3. WATERSHED DESCRIPTION 

The Sarasota Bay basin is defined by USGS Hydrologic Unit Code (HUC) 03100201. The following 

description of the impaired watersheds in the Sarasota Bay Planning Unit is from the Basin Status 

Report for Sarasota Bay and Peace and Myakka Rivers (FDEP, 2003). The descriptions of the four 

WBIDs that drain to Lemon Bay are from the Charlotte Harbor Basin Status Report (FDEP, 2002). 

These documents should be consulted for additional details. Pictures of the impaired WBIDs in 

Sarasota and Lemon Bay Planning Units may be found in Appendix A. 

3.1 Sarasota Bay Planning Unit 

The Sarasota Bay watershed is located in southwest Florida near the cities of Sarasota and Venice. 

 The basin includes several important estuaries as well as many wetlands and small coastal 

streams that are predominantly tidal in nature. A few freshwater streams are also located in the 

basin. Sarasota Bay itself is the largest estuary in the basin, with 52 square miles of open water 

(FDEP, 2003). The total watershed draining to Sarasota Bay, from its northern extent at Anna 

Maria Sound to its southern extent at Venice Inlet, has been estimated at 150 square miles 

(SWFWMD, 2002). 

Six of the estuaries in the Sarasota Bay watershed have been designated Estuaries of National 

Significance by the U.S. Environmental Protection Agency (EPA) and Outstanding Florida Waters 

(OFWs) by the Florida Department of Environmental Protection (FDEP). These include Anna Maria 

Sound, Palma Sola Bay, Sarasota Bay, Roberts Bay, Little Sarasota Bay, and Blackburn Bay. In 

1995, the South Florida Water Management District (SWFWMD) named Sarasota Bay as a priority 

water body for its Surface Water Improvement and Management (SWIM) Program. 

Population in the Sarasota Bay watershed has grown rapidly in recent decades, especially during 

the 1980's. Between 1980 and 1990, Manatee County's population grew 43%, Sarasota County's 

grew 37%, and Charlotte County's went up 90% (FDEP, 2003). Continued population growth in 

these counties is anticipated. 

Near the coast and along the barrier islands, the land use is largely urban and residential. There 

are several major urban areas in the watershed including the cities of Sarasota, Venice and 

Bradenton. The eastern portion of the watershed contains more agricultural uses such as range 

land, pasture, and row crops. Freshwater and saltwater wetlands and open waters are also 

important landuses, although the acreage of tidal and freshwater wetlands has declined significantly 

since World War II (SWFWMD, 2002). 

North Creek, WBID 1984A, is located in Sarasota County near the city of Osprey. It is a Class III 

Marine waterbody that was listed on the 1996 303(d) list for nutrients (FDEP, 2003). North Creek is 

also considered potentially impaired for D.O. and coliforms. The creek is divided into freshwater 

(WBID 1984) and marine (WBID 1984A) segments. 

Catfish Creek, WBID 1984, is located near the City of Vamo. It also drains into Little Sarasota Bay. 

 It is also Class III Marine waterbody that was listed on the 1996 303 (d) list for nutrients. 

South Creek, WBID 1982A, is located in Sarasota County near the city of Laurel. South Creek is 

one of the series of creeks and bayous on the mainland that drains stormwater runoff to the bay. 

South Creek is a Class III Marine water that was included on the 1996 303(d) list for nutrients 

(FDEP, 2003). The freshwater portion of South Creek is in WBID 1982, while WBID 1982B, South 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 5 

Creek in Park, is a small, elongate piece of WBID 1982A that was cut out along the creek as it flows 

through Oscar Scherer State Park. South Creek then empties into Dryman Bay. There are a 

number of nonforested freshwater wetlands in the upper South Creek watershed. 

Curry Creek, WBID 2009A, is located in Sarasota County near the cities of Venice and Nokomis. 

Curry Creek was included on the 1996 303(d) list for nutrients (FDEP, 2003). It is also considered 

potentially impaired for D.O. Curry Creek terminates at Roberts Bay (the Roberts Bay located more 

to the south, near Dona Bay). Curry Creek provides hydraulic connection between the Myakka 

River and Roberts Bay during periods of high flow. 

Elligraw Bayou, WBID 1975, and Clowers Creek, WBID 1975A, are both Class III freshwater 

streams located in Sarasota County near the city of Sarasota. Elligraw Bayou is 303(d)-listed for 

coliforms, nutrients, and dissolved oxygen, and Clowers Creek is listed for coliforms. The estuarine 

portion of Clowers Creek is covered by WBID 1975AA. 

3.2 Lemon Bay Planning Unit 

The 97 square miles of the Lemon Bay planning unit are encompassed by Sarasota and Charlotte 

Counties. The bay itself extends from South Venice to the southern end of Gasperilla Island. The 

watershed includes a system of interconnected embayments and their watersheds. Four of these 

estuarine creeks have been identified as impaired by nutrients and/or dissolved oxygen: Alligator 

Creek, Direct Runoff, Gottfried Creek, and Forked Creek. 

The Lemon Bay Estuarine System is designated an Outstanding Florida Water (OFW) by the FDEP, 

and there is a 7,667 acre state aquatic preserve in the bay. Lemon Bay is one of five individual 

preserves that make up the Charlotte Harbor Aquatic Preserves. Each preserve has a 

management plan to assess its condition and resource management needs. The management plan 

for Lemon Bay was adopted in April 1992. Although Lemon Bay is included in the Sarasota Bay 

hydrologic unit, FDEP has decided to consider it part of the Charlotte Harbor Basin because of 

Lemon Bay’s direct connection through Placida Harbor (FDEP, 2002). Table 2 contains the land 

use/cover distribution of the Sarasota and Lemon Bay WBIDs. 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 6 

Table 2. Current Land Use/Cover Distribution in Impaired WBIDs1 

Land Use/Cover Area (Acres) 

Land Use Clowers 

Creek 

Elligraw 

Bayou 

Curry 

Creek 

North 

Creek 

Catfish 

Creek 

South 

Creek 

Gottfried 

Creek 

Forked 

Creek 

Alligator 

Creek 

Direct 

Runoff to 

Bay 

Agriculture 0 186 102 13 96 2 584 1,272 0 0 

Commercial 118 307 63 40 164 152 197 94 423 15 

Extractive 0 0 0 0 0 0 0 15 0 0 

Industrial 0 332 0 0 1 0 0 19 0 3 

Low-Density 

Residential 

10 

153 25 79 37 102 344 459 72 166 

MediumDensity 

Residential 

8 

1,758 66 417 121 316 559 947 1,566 807 

High-Density 

Residential 

198 

451 509 202 871 364 202 480 1,610 110 

Transportation 20 64 18 17 21 37 6 63 153 0 

Institutional 13 163 25 0 3 47 70 7 129 0 

Open 111 418 78 258 250 1,168 3,589 2,446 692 298 

Recreational 0 209 38 347 304 7 231 114 187 5 

Wetland 5 97 111 106 291 214 1,024 1,093 478 186 

Water (Ponds 

and Lakes) 

9 

158 26 113 256 70 300 303 399 27 

Water 

(Conveyance) 

38 

92 70 60 11 44 120 65 51 65 

Total 530 4,390 1,131 1,651 2,426 2,524 7,227 7,375 5,761 1,683 

1

Data source is 1995-Land cover from the SWFWMD. The land use classes in the Table were obtained by 

aggregating detailed land use categories of Land Use, Cover, and Forms Classification System (FLUCCS) as 

shown in Table 3. 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 7 

Table 3. Aggregate Land Use/Cover Categories Derived from the Florida Land Use, Cover, and 

Forms Classification System (FLUCCS). 

Land Use/Cover FLUCCS Code 

Description 

FLUCCS 

Code 

Low-Density Residential Residential Low Density < 2 Dwelling Units 110 

Medium-Density Residential Residential Med Density 2->5 Dwelling Unit 120 

High-Density Residential Residential High Density 130 

Commercial Commercial And Services 140 

Industrial Industrial 150 

Extractive 160 

Beaches Other Than Swimming Beaches 710 

Sand Other Than Beaches 720 

Exposed Rock 730 

Extractive 

Disturbed Land 740 

Institutional Institutional 170 

Recreational Recreational 180 

Cropland and Pastureland 210 

Row Crops 214 

Tree Crops 220 

Feeding Operations 230 

Nurseries And Vineyards 240 

Vineyards 244 

Specialty Farms 250 

Tropical Fish Farms 255 

Agriculture 

Other Open Lands <rural> 260 

Herbaceous 310 

Shrub and Brushland 320 

Mixed Rangeland 330 

Upland Coniferous Forest 410 

Pine Flatwoods 411 

Longleaf Pine – Xeric Oak 412 

Upland Hardwood Forests - Part 1 420 

Xeric Oak 421 

4300 Upland Hardwood Forests - Part 2 430 

Tree Plantations 440 

Open Land 

Open land 190 

WATER (Ponds and Lakes) Lakes 520 

Reservoirs 530 

 

Water (Conveyance) Bays And Estuaries 540 

Streams And Waterways 510 

 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 8 

Land Use/Cover FLUCCS Code 

Description 

FLUCCS Code 

Wetland Hardwood Forests 610 

Bay Swamps 610 

Mangrove Swamps 612 

Titi Swamps 614 

Stream And Lake Swamps (bottomland) 615 

Wetland Coniferous Forests 620 

Cypress 621 

Wetland Forested Mixed 630 

Vegetated Non-forested Wetlands 640 

Freshwater Marsh 641 

Saltwater Marshes 642 

Wet Prairies 643 

Emergent Aquatic Vegetation 644 

Non-Vegetated 650 

Tidal Flats/submerged Shallow Platform 651 

Shorelines 652 

Wetlands 

Intermittent Ponds 653 

Transportation 810 

Communications 820 

Transportation 

Utilities 830 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 9 

4. WATER QUALITY STANDARD AND TARGET IDENTIFICATION 

The waterbodies addressed in this document are Class III waters, with a designated use of 

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

 The water quality criteria for protection of Class III waters are established by the State of Florida in 

the Florida Administrative Code (F.A.C.), Section 62-302.530. The individual criteria should be 

considered in conjunction with other provisions in water quality standards, including Section 62-

302.500 F.A.C. [Surface Waters: Minimum Criteria, General Criteria] that apply to all waters unless 

alternative criteria are specified in F.A.C. Section 62-302.530. In addition, unless otherwise stated, 

all criteria express the maximum not to be exceeded at any time. While the State of Florida does 

not have numeric criteria for nutrients, a narrative criterion exists as described below. The specific 

criteria for the impaired WBIDs addressed in this TMDL are as follows: 

Nutrients (Class III Waters- Fresh and Marine)

The discharge of nutrients shall continue to be limited as needed to prevent violations of other 

standards contained in this chapter [Section 62.302 F.A.C.]. In no case shall nutrient 

concentrations of a body of water be altered so as to cause an imbalance in natural populations of 

aquatic flora and fauna [Section 62.302.530 F.A.C.] 

Dissolved Oxygen (Class III Waters)

For Fresh Water: Dissolved Oxygen (D.O.) shall not be less than 5.0 (milligrams/liter) in a 24-hour 

period. Normal daily and seasonal fluctuations above these levels shall be maintained. 

For Marine Water: Dissolved Oxygen (D.O.) shall not average less than 5.0 (milligrams/liter) in a 

24-hour period and shall never be less than 4.0 (mg/l). Normal daily and seasonal fluctuations 

above these levels shall be maintained. All the waterbodies are estuarine creek systems and 

therefore fall in this category. 

Biochemical Oxygen Demand (Class III Waters- Fresh and Marine)

Biochemical Oxygen Demand (B.O.D.) shall not be increased to exceed values which would cause 

dissolved oxygen to be depressed below the limit established for each class and, in no case, shall it 

be great enough to produce nuisance conditions. 

Fecal Coliform Bacteria (Class III Waters- Fresh and Marine)

The most probable number (MPN) or membrane filter (MF) counts per 100 ml of fecal coliform 

bacteria shall not exceed a monthly average of 200, nor exceed 400 in 10 percent of the samples, 

nor exceed 800 on any one day. Monthly averages shall be expressed as geometric means based 

on a minimum of 10 samples taken over a 30-day period. 

When flow data are available in the waterbody, fecal coliform TMDLs are typically expressed as 

daily loads in units of counts per day. The target for the TMDLs is 400 MPN/100ml, which is the 

level not to be exceeded in more than 10 percent of the data, as this is the more stringent of the 

acute criteria. Fecal coliform TMDLs may also expressed in terms of the percent reduction 

required to achieve water quality standards when flow data are not available in the WBID or due to 

hydrologic and/or geologic conditions it is not possible to estimate flow (i.e., tidal influence or karst 

geologic formation). The percent reduction is calculated based on those observations exceeding 

the 400 MPN criteria. 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 10 

It is appropriate to use the 10 percentile and maximum-based criteria for fecal coliform TMDL 

development as the data indicates violations of the standard are typically related to storm events, 

which are short-term in nature. Violations of the geometric mean criteria are typically associated 

with point sources or non-point source continuous discharges (e.g., leaking septic systems) and 

typically occur during all weather conditions. Targeting the 10 percentile and maximum-based 

criteria should be protective of the chronic criteria. 

Total Coliform Bacteria (Class III Waters- Fresh and Marine)

The MPN per 100 ml of total coliform bacteria shall be less than or equal to 1,000 as a monthly 

average nor exceed 1,000 in more than 20 percent of the samples examined during any month, and 

less than or equal to 2,400 at any time. Monthly averages shall be expressed as geometric means 

based on a minimum of 10 samples taken over a 30-day period. 

The target for the total coliform TMDLs is the one-day maximum concentration of 2400 

counts/100mL, as less than 10 samples were collected in a 30-day period to determine violations of 

the geometric mean. Total coliform bacteria generally indicate the presence of soil-associated 

bacteria and result from natural influences on a water body such as rainfall runoff as well as 

sewage inflows (i.e., acute conditions). By protecting the acute criteria (i.e., one-day maximum) 

bacteria concentrations in the stream should meet the chronic criteria. The not to exceed criteria is 

not selected as the target because there are no known health risks associated with soil-born 

bacteria and using this criterion for the target would likely result in overly conservative reductions. 

5. WATER QUALITY ASSESSMENT AND DEVIATION FROM TARGET 

FDEP maintains ambient monitoring stations throughout the Sarasota Bay and Lemon Bay drainage 

basins. Table 4 provides a list of these monitoring stations. Each station is identified, and the time 

period of record for the individual stations is provided. Data collected during the Group 3 listing 

cycle (i.e. January 1997 through December 2003) and any more recent data, if available, are 

considered in the data assessment. However, for those instances where no (or insufficient) post1997 data are available for a particular parameter, older data are considered in the assessment. 

Additional tables of water quality data measured in these WBIDs are provided in Appendix A. 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 11 

Table 4. Water Quality Monitoring Stations in the impaired WBIDs. 

WBID WATERBODY STATION STATION NAME 

PERIOD OF 

RECORD 

1975 Elligraw Bayou 

21FLMML 

271433082303100/ 

21FLSARA24010644 

Elligraw Bayou@ US41 Bridge 

NEPWQ-82 2003 

1975A Clowers Creek 21FLTPA 

27134838230100 

Clowers Creek at Brooks 

House 2003 

21FLTPA 

27135458229507 TP190- Clowers Creek 2003-2004 

1984A North Creek 

21FLMML 

271234082292500 

North Creek @ US41 Bridge 

NEPWQ-92 1992-1994 

1984 Catfish Creek 21FLMML 

271301082292600 

CATFISH CREEK @ VAMO 

WAY BRIDGE NEPWQ-88 2003 

21FLSARA24010587 North Ck US 41 Bridge 1992 

21FLMML 

271004082283800 

South Creek @ US41 Bridge 

NEPWQ-95 

1982A South Creek 1992-1994 

21FLSARA24010588 South Ck US 41 Bridge 1992 

SC1 South Creek @ Canoe Launch 

in Oscar Sherer Park 2005 

21FLGW FLO0099 / 

21FLSWFDFLO0099 Curry Creek @ Albee Farm 1995-1998 

21FLSARA24010617 Curry Creek @ Albee Farm 

Rd. Bridge 1992 

2009A Curry Creek

21FLTPA 

27070318226004 TP197- Curry Creek 2003 

CC1 Curry Creek at US 41 2005 

2030 Alligator Creek 21FLSARA24010620 Alligator Creek at US 41 1991-1992 

21FLSARA24010676 Alligator Crk at Jacaranda 

Blvd. 1991-1992 

21FLCHARLBV001 Alligator Crat Yacht Club Dr. 

Venice 1998-2000 

21FLA 56112SEAS Canal West of ICWW 31 1991-1998 

21FLSARA24010567 Lemon B Up Forked CR 1991-1992 

21FLSARA24010623 Forked Crk at RT #775 Bridge 1991-1992 

2039 Forked Creek 

21FLCHARLBFOR1 Forked Creek at 775 Bridge 2000 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 12 

WBID WATERBODY STATION STATION NAME 

PERIOD OF 

RECORD 

FC1 Forked Creek @ SR 776 

(Englewood Rd.) 2005 

21FLSARA24010621 Alligator Crk at Shamrock 

Blvd. 2042 1991-1992 Direct Runoff to 

Bay 

WC1 

Woodmere Creek @ 

Ponderosa Rd. Gauging 

Station 

2005 

AC1 Outlet of small creek 

(Woodmere Cr.?) to Bay 2005 

21FLA 24010591 Gottfried Crk at 775-776-45A 

Brd 1991-1998 

21FLSARA24010593 Gottfried Crk at End 

Wentworth S 1991-1992 

21FLSARA24010624 Deer Creek at Norton Av 

Englewood 1991-1992 

21FLCHARLBGOT2 Gottfried Creek at SR77 1998-2000 

2049 Gottfried Creek 

CHNEPLLB515 CHNEP515POR 2002-2003 

 GC1 Gottfried Creek @ Dearborn 

Avenue 2005 

5.1 Dissolved Oxygen Data 

Although the State of Florida does not have numeric criteria for nutrients, a narrative criterion exists 

as described in the Water Quality Standards section of this report. Chlorophyll and dissolved 

oxygen are generally used to indicate whether nutrients are present in excessive amounts. 

However CHLA and DO monitoring data in the waterbodies are old and/or insufficient to perform a 

defendable analysis/modeling of current water quality condition. 

Natural dissolved oxygen (D.O.) levels in a waterbody are a function of water temperature, water 

depth and velocity, and relative contribution of groundwater. Oxygen can be introduced to streams 

by wind, diffusion, tributaries, and photosynthesis. Warm water holds less oxygen than cool water, 

and slower-flowing, less turbulent water has less diffusion of atmospheric oxygen into it. Because it 

is not in contact with air, groundwater naturally has lower concentrations of oxygen dissolved in it. 

D.O. levels naturally fluctuate over the course of a day. During the daylight, aquatic plants produce 

oxygen as a by-product of photosynthesis. At night, respiration may consume dissolved oxygen 

from the water. Decomposition of organic matter, such as dead plants and animals, also uses up 

dissolved oxygen. 

For Class III marine waters, the water quality criteria state that dissolved oxygen should not be less 

than 5.0 mg/l in a 24-hour period, and should never be less than 4.0 mg/l. For Class III fresh 

waters, the dissolved oxygen should not be less than 5.0 mg/l. Table 5 summarizes the D.O. data 

for the Sarasota Bay and Lemon Bay (Charlotte Harbor) WBIDs. 

The available D.O. data indicate that dissolved oxygen concentrations in many of the Sarasota Bay 

and Charlotte Harbor WBIDs are frequently below the Class III Water Quality Criterion that applies 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 13 

to them (Table 5). In Clowers Creek, WBID 1975A, D.O. is potentially impaired, ranging from 1.1 to 

7.8 mg/l and averaging 4.2 mg/l. Thirty eight percent of the data from Clowers Creek is below the 

Class III Fresh Water Criterion of 5 mg/l D.O. Although there are no D.O. data available for South 

Creek, WBID 1982A, D.O. was measured between 1992 and 1994 in WBID 1982B of South Creek. 

 WBID 1982B is located entirely within the boundaries of WBID 1982A. Those D.O. data range 

from 1.5 to 7.6 mg/l and average 5.4 mg/l, with 26% of the values below the 4 mg/l standard. In 

North Creek, WBID 1984A, the only D.O. concentrations available are from 1992-1994. Those data 

range from 2.6 mg/l to 13.5 mg/l, and average 5.0 mg/l, with about 40% of the available data below 

the standard of 4 mg/l. More recent data are available for Curry Creek, WBID 2009A. The D.O. 

measurements range from 1.34 to 7.57 mg/l and average 4.54 mg/l. This represents an excursion 

rate of about 54%. 

Table 5. Summary of Dissolved Oxygen Data (mg/l) for Impaired WBIDs. 

WBID Waterbody Time 

Period Obs Min Max Mean StDev Median WQC % 

1975 Elligraw Bayou 2003 20 2.1 10.8 5.8 3.1 5.6 5 55%

1975A Clowers Creek 2003 13 1.1 7.8 4.2 2.4 3.7 5 38%

1982A1 South Creek 1992-1994 19 1.5 7.6 5.4 1.9 5.3 4 26%

1984 Catfish Creek 2003 25 3.1 15.4 7.4 6.6 3.4 4 80%

1984A North Creek 1992-1994 15 2.6 13.5 5.0 2.8 4.2 4 40%

2009A Curry Creek 1997-2003 13 1.3 7.6 4.5 2.0 4.5 4 54%

2030 Alligator Creek 1998-2000 28 1.2 6.0 3.4 1.0 3.2 4 75%

2039 Forked Creek 2000 9 3.2 6.4 4.5 1.2 4.4 4 33%

2049 Gottfried Creek 1998-2000 27 1 8.7 3.7 1.8 3.3 4 68%

Notes: 

 Obs= number of observations; Min= minimum value in the dataset; Max= maximum value in the dataset; 

Mean= average of all values in the dataset; StDev= standard deviation; Median= median of all values in the 

dataset; WQC= applicable water quality criterion (marine or fresh); % = percentage of samples below the 

applicable water quality criterion for D.O. 

1

Data summary for South Creek is from WBID 1982B, South Creek in Park. 

In Alligator Creek, WBID 2030, the dissolved oxygen concentrations range from 1.2 to 6.0 mg/l and 

average only 3.4 mg/l. Fully 75% of the measurements are below the Class III Marine Water 

Quality Criterion of 4 mg/l. In Forked Creek, WBID 2039, one-third of the data are below the 

criterion. The D.O. concentrations in Forked Creek range from 3.2 to 6.4 mg/l and average 4.5 

mg/l. Dissolved oxygen could not be assessed for WBID 2042, Direct Runoff to Bay, because no 

D.O. data are available. 

Gottfried Creek and Elligraw Bayou have been formally 303(d)-listed for dissolved oxygen. In 

Gottfried Creek, WBID 2049, the D.O. data range from 1 mg/l to 8.7 mg/l, and average at 3.7 mg/l. 

Over two-thirds of the measured D.O. values in WBID 2049 are below the applicable standard of 4 

mg/l. The D.O. data for Gottfried Creek are plotted in Figure 2. In Elligraw Bayou, WBID 1975, 

D.O. ranges from 2.1 to 10.8 mg/l, and averages 5.8 mg/l. Approximately 55% of the data is below 

the water quality criterion (Figure 3). 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 14 

WBID 2049 Water Quality Data

0

2

4

6

8

10

1/5/1998

7/24/1998

2/9/1999

8/28/1999

3/15/2000

10/1/2000

4/19/2001

11/5/2001

5/24/2002

12/10/2002

6/28/2003

Dissolved Oxygen (mg/l) WaterQualityCriteria

Figure 2. Dissolved Oxygen Measurements in Gottfried Creek, WBID 2049. 

WBID 1975 Water Quality Data

0

2

4

6

8

10

12

01/29/03

03/20/03

05/09/03

06/28/03

08/17/03

10/06/03

11/25/03

01/14/04

Dissolved Oxygen Water Quality Criteria

Figure 3. Dissolved Oxygen Measurements in Elligraw Bayou, WBID 1975. 

5.2 Chlorophyll Data 

Chlorophyll is the green pigment in plants that allows them to create energy from light. In a water 

sample, chlorophyll is indicative of the presence of algae, and chlorophyll-a is simply a measure of 

the active portion of total chlorophyll. There are limited chlorophyll data for many of the Sarasota 

Bay and Charlotte Harbor WBIDs that are 303(d)-listed for nutrients. Table 6 summarizes the 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 15 

available chlorophyll data. 

Table 6. Summary of Chlorophyll Data (μg/l) for Impaired WBIDs. 

WBID Waterbody Time Period Obs Min Max Mean StDev Median 

1975 Elligraw Bayou 1992-1993 7 10.4 47.3 19.9 12.8 15.3 

1975A Clowers Creek 2003 10 1 58 11.9 21.8 1.55 

1982A South Creek 1992-93, 2005 9 3.1 46.1 10.1 13.7 6.1 

1984A North Creek 1992-1993 8 3.8 30.5 14.4 11.5 11.1 

2009A Curry Creek 2003-2005 3 1 5.4 2.5 2.5 1 

2030 Alligator Creek 1998-2000 25 1 48.7 9.5 10.1 6.0 

2039 Forked Creek 2005 1 - - 9.3 - 9.3 

2042 Direct Runoff to Bay 2005 2 2.0 2.5 2.25 0.35 2.25 

2049 Gottfried Creek 2002-2005 3 1 13.1 6.1 6.3 4.2 

Notes: 

Obs= number of observations; Min= minimum value in the dataset; Max= maximum value in the 

dataset; Mean= average of all values in the dataset; StDev= standard deviation; Median= median of 

all values in the dataset. 

1 μg/l is the lower quantification limit. 

Chlorophyll measurements available for Elligraw Bayou (WBID 1975) were collected in 2003. 

Those concentrations range from 1.0 to 56.0 μg/l, and average 19.9 μg/l (Figure 4). Based on this 

data, it appears that Elligraw Bayou may not be subject to algal blooms. 

WBID 1975 Water Quality Data

0

10

20

30

40

50

60

06/28/03

07/18/03

08/07/03

08/27/03

09/16/03

10/06/03

10/26/03

11/15/03

12/05/03

12/25/03

Chlorophyll A Water Quality Criteria

Figure 4. Chlorophyll measurements in WBID 1975 (Elligraw Bayou).

The chlorophyll concentrations measured in Clowers Creek (WBID 1975A) range from 1 to 58 μg/l, 

and average 11.9 μg/l. The high range and standard deviation of the data indicates that chlorophyll 

concentrations are frequently low in Clowers Creek, but the stream is subject to occasional algal 

blooms that increase chlorophyll concentrations considerably (Figure 5). 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 16 

WBID 1975A Water Quality Data

0

10

20

30

40

50

60

70

1/29/2003

3/20/2003

5/9/2003

6/28/2003

8/17/2003

10/6/2003

11/25/2003

1/14/2004

Chlorophyll Corrected (ug/l)

Figure 5. Chlorophyll measurements in WBID 1975A (Clowers Creek).

Most of the chlorophyll measurements available for South Creek (WBID 1982A) were collected in 

1992-1993 (Figure 6). In March, EPA collected another sample in South Creek and measured its 

chlorophyll concentration. Only one of the 9 chlorophyll measurements appears elevated. In Curry 

Creek (WBID 2009A), there are only three measurements for corrected chlorophyll, two of which 

were at the lower quantification limit of 1 μg/l. The third value was also low (5.4 μg/l). The 

chlorophyll concentrations for both of these streams do not indicate the presence of algal blooms. 

However, nutrient TMDLs are provided for both streams, because their chlorophyll data are limited, 

median nutrient levels in both streams are higher than FDEP’s program threshold, both have low 

dissolved oxygen, and both are potentially influenced by urban stormwater runoff.
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 17 

WBID 1982A Water Quality Data

0

10

20

30

40

50

5/7/1990

1/31/1993

10/28/1995

7/24/1998

4/19/2001

1/14/2004

10/10/2006

Chlorophyll (ug/l) Water Quality Comparison

Figure 6. Chlorophyll measurements in WBID 1982A (South Creek).

The only chlorophyll concentrations available for North Creek (WBID 1984A) were eight 

measurements made in 1992-1993. Those concentrations appear elevated, ranging from 3.8 to 

30.5 `μg/l and averaging 14.4 μg/l (Figure 7). 

WBID 1984A Water Quality Data

0

5

10

15

20

25

30

35

12/28/1991

4/6/1992

7/15/1992

10/23/1992

1/31/1993

5/11/1993

8/19/1993

11/27/1993

3/7/1994

Chlorophyll (ug/l) Water Quality Comparison

Figure 7. Chlorophyll measurements in WBID 1984A (North Creek). 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 18 

There were eight measurements for chlorophyll concentrations made in 2003 for Catfish Creek 

(WBID 1984). Five of those measurements were higher than 11 μg/l. Concentrations ranged from 

0.9 to 40.0 μg/l and averaged 12.6 μg/l (Figure 8). 

WBID 1984 Water Quality Data

0

5

10

15

20

25

30

35

40

45

5/9/2003

6/28/2003

8/17/2003

10/6/2003

11/25/2003

1/14/2004

Chlorophyll (ug/l) Water Quality Comparison

Figure 8. Chlorophyll measurements in WBID 1984 (Catfish Creek). 

In Alligator Creek (WBID 2030), chlorophyll concentrations ranged from 1 to 48.7 μg/l. The average 

concentration was 9.5 μg/l. Many of the observations appear to be elevated, indicating the likely 

presence of algal blooms (Figure 9). 

WBID 2030 Water Quality Data

0

10

20

30

40

50

60

4/15/1998

7/24/1998

11/1/1998

2/9/1999

5/20/1999

8/28/1999

12/6/1999

3/15/2000

6/23/2000

10/1/2000

1/9/2001

4/19/2001

Chlorophyll (ug/l) Water Quality Comparison

Figure 9. Chlorophyll measurements in WBID 2030 (Alligator Creek). 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 19 

There is only one recent measurement of chlorophyll in Forked Creek (WBID 2039). This sample 

was collected in March 2005, and had a value of 9.3 μg/l. Two measurements of chlorophyll were 

made in WBID 2042 (Direct Runoff to Bay) in March 2005. The sample taken from Woodmere 

Creek (which is a freshwater stream) had a chlorophyll concentration of 2.0 μg/l, and the sample 

taken from a small, unnamed watercourse entering the bay had a concentration of 2.5 μg/l. Three 

recent measurements of chlorophyll are available for Gottfried Creek (WBID 2049). These 

concentrations range from 1 to 13.1 μg/l, and average 6.1μg/l. 

5.3 Algal Growth Potential Data 

Algal Growth Potential (AGP) tests measure the levels of biologically available nutrients by 

determining the maximum amount of algal growth that nutrients in the water can support. This 

indicates the potential for algal blooms or nuisance aquatic weed growth, which can lead to 

eutrophication. Studies have shown that algal dry weight concentrations exceeding 6.1 mg/l are 

associated with highly productive waters (Miller et. al., 1974) and algal dry weight concentrations 

exceeding 10 mg/l are associated with eutrophic waters which are subject to nuisance algal blooms 

(EPA, 1975; and Raschke, et. al., 1987). 

Table 7 summarizes the results of the algal growth potential tests for the impaired WBIDs. Results 

of AGP tests in WBID 2009A (Curry Creek), WBID 1982A (South Creek), and WBID 2049 (Gottfried 

Creek) yielded algal dry weight concentrations of 5.4 mg/l, 5.0 mg/l, and 5.1 mg/l, respectively. 

These values are slightly lower than the threshold considered to indicate highly productive waters. 

The AGP test result for WBID 1975 (Elligraw Bayou) was just above this threshold. Algal dry weight 

concentrations for WBID 2030 (Forked Creek) and both samples for WBID 2042 (Direct Runoff to 

Bay) are above 10 mg/l, indicating that those waters are very productive and may be eutrophic. 

Table 7. Results of Algal Growth Potential Tests 

WBID Waterbody Date AGPT (mg/l) 

1975 Elligraw Bayou (McGraw Creek) 3/22/2005 6.1 (7.1 for duplicate) 

1982A South Creek 3/22/2005 5.0 

2009A Curry Creek 3/22/2005 5.4 

2039 Forked Creek 3/22/2005 12.4 A 

2042 Direct Runoff to Bay (outlet 

creek) 3/22/2005 11 

2042 Direct Runoff to Bay (Woodmere 

Creek) 3/22/2005 16.8 A 

2049 Gottfried Creek 3/22/2005 5.1 

 NOTES: A = Analyte analyzed in replicate. Reported value is “average” of replicates. 

 AGPT= maximum standing crop (dry weight) 

Based on the available data for chlorophyll, nutrients, and algal growth potential, etc, TMDLs are 

provided for nutrients in the following 303(d)-listed waters: Elligraw Bayou (WBID 1975), South 

Creek (WBID 1982A), North Creek (WBID 1984A), Curry Creek (WBID 2009A), Alligator Creek 

(WBID 2030), Forked Creek (WBID 2039), Direct Runoff to Bay (WBID 2042), and Gottfried Creek 

(WBID 2049). The D.O. listings for Elligraw Bayou (WBID 1975) and Gottfried Creek (WBID 2049) 

will also be addressed. EPA will accept the delisting request for nutrients in WBID 1975A (Clowers 

Creek). 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 20 

5.4 Total Coliform Data 

Total coliform bacteria generally indicate the presence of soil-associated bacteria. One sub-group 

of total coliforms is fecal coliform bacteria, which are used as indicators of fecal contamination since 

they are abundant in the intestines and feces of animals and people. As such, total coliform 

bacteria concentrations may result from natural influences on a water body, as well as unnatural 

influences such as sewage inflows. 

In Elligraw Bayou (WBID 1975), 4 of the 20 samples (25%) were above the 2400 MPN/100ml 

standard that is not to be exceeded at any time. The data are illustrated in Figure 9.

In Clowers Creek (WBID 1975A), 5 of the 20 samples (20%) were above the 2400 MPN/100ml 

standard that is not to be exceeded at any time (Figure 10). 

Summary statistics for total coliforms in both WBIDs are provided below in Table 8. 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 21 

Table 8. Total Coliform Data for Impaired WBIDs. 

WBID Waterbody Time 

Period Obs Min Max Mean StDev Median Percent Excursions

1975 Elligraw 

Bayou 2003 20 1 5800 1325 1685 690 20 

1975A Clowers 

Creek 2003 20 1 3700 1107 1139 780 25 

Notes: 

Obs= number of observations; Min= minimum value in the dataset; Max= maximum value in the dataset; 

Mean= average of all values in the dataset; StDev= standard deviation; Median= median of all values in the 

dataset. 

WBID 1975 Water Quality Data

0

1000

2000

3000

4000

5000

6000

7000

01/29/03

03/20/03

05/09/03

06/28/03

08/17/03

10/06/03

11/25/03

01/14/04

Total Colifirm Water Quality Criteria

Figure 10. Total Coliform Measurements in WBID 1975 (Elligraw Bayou). 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 22 

WBID 1975A Water Quality Data

0

500

1000

1500

2000

2500

3000

3500

4000

1/29/2003

3/20/2003

5/9/2003

6/28/2003

8/17/2003

10/6/2003

11/25/2003

1/14/2004

Total Coliform (/100ml) WaterQualityCriteria

Figure 11. Total Coliform Measurements in WBID 1975A (Clowers Creek). 

5.5 Fecal Colliform Data 

The fecal coliform data for Elligraw Bayou (WBID 1975) and Clowers Creek (WBID 1975A) are 

summarized in Table 9. 

In Elligraw Bayou (WBID 1975), 8 of the 20 samples (40%) were above the 800 MPN/100ml 

standard that is not to be exceeded at any time, and 70% of the data were above 400 MPN/100ml. 

The data are illustrated in Figure 12. In Clowers Creek (WBID 1975A), 7 of the 10 samples (35%) 

were above the 800 MPN/100ml standard that is not to be exceeded at any time and 70% of all the 

data are above 400 MPN/100 ml (Figure 13). 

Table 9. Fecal Coliform Data for Impaired WBIDs. 

WBID Waterbody Time 

Period Obs Min Max Mean StDev Median Percent Excursions1

1975 Elligraw 

Bayou 2003 20 65 2500 758 575 602 70 

1975A Clowers 

Creek 2003 20 75 4400 975 986 628 70 

NOTES: 

Obs= number of observations; Min= minimum value in the dataset; Max= maximum value in the 

dataset; Mean= average of all values in the dataset; StDev= standard deviation; Median= median of all 

values in the dataset. 

1. Percent excursions above the TMDL target of 400 MPN/100ml. 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 23 

WBID 1975 Water Quality Data

0

500

1000

1500

2000

2500

3000

01/29/03

03/20/03

05/09/03

06/28/03

08/17/03

10/06/03

11/25/03

01/14/04

Fecal Coliform Water Quality Criteria

Figure 12. Fecal Coliform Measurements in WBID 1975 (Elligraw Bayou). 

WBID 1975A Water Quality Data

0

1000

2000

3000

4000

5000

1/29/2003

3/20/2003

5/9/2003

6/28/20038/17/200310/6/2003

11/25/2003

1/14/2004

Fecal Coliform (/100mll) WaterQualityCriteria

Figure 13. Fecal Coliform Measurements in WBID 1975A (Clowers Creek). 

Based on the available data for coliforms, TMDLs are needed for total and fecal coliforms in 

Elligraw Bayou (WBID 1975) and Clowers Creek (WBID 1975A). 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 24 

6. SOURCE ASSESSMENT 

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

subcategories, or individual sources of pollutants in the watershed and the amount of loading 

contributed by each of these sources. Sources are broadly classified as either point or non-point 

sources. Nutrients enter surface waters from both point and nonpoint sources. 

A point source is defined as a discernable, confined, and discrete conveyance from which pollutants 

are or may be discharged to surface waters. Point source discharges of industrial and treated 

sanitary wastewater must be authorized by National Pollutant Discharge Elimination System 

(NPDES) permits. NPDES permitted facilities, including certain urban stormwater discharges such 

as municipal separate stormwater systems (MS4s), certain industrial facilities, and construction 

sites over one acre, are storm-water driven sources that are considered “point sources” in this 

report. 

Non-point sources of pollution are diffuse sources that cannot be identified as entering a waterbody 

through a discrete conveyance at a single location. These include runoff of agricultural fields, golf 

courses, and lawns, septic tanks, and residential developments outside of MS4 areas. These 

sources generally, but not always, involve accumulation of nutrients and/or bacteria on land 

surfaces and wash off as a result of storm events. 

6.1 Point Sources 

A wasteload allocation (WLA) is given only to NPDES facilities discharging to surface waters and to 

permitted Municipal Separate Storm Sewer Systems (MS4s). It should be noted that permits for 

wastewater facilities authorize a discharge only if the applicant provides reasonable assurance that 

the discharge will not cause or contribute to violations of the water quality criteria. 

6.1.1 NPDES Facilities 

Although many of the 303(d)-listed WBIDs in the Sarasota Bay and Lemon Bay planning units 

contain domestic and industrial wastewater facilities, most of them do not discharge directly to 

surface water and therefore, do not have NPDES permits (although they may have state permits). 

Only one facility, Snowbirdland Vistas (FL0041785), currently discharge a pollutant of concern in 

the listed WBIDs. Snowbirdland Vistas is a mobile home site that discharges to Curry Creek (WBID 

2009A), which is 303(d)-listed for nutrients. It is a minor facility that has an annual average 

discharge limit of only 20,000 gallons per day (gpd). Its permit (#FL0041785) also contains limits 

for dissolved oxygen (D.O.), pH, radium 226 + radium 228, and alpha- gross particle activity. In 

addition, Snowbirdland Vistas is required to conduct toxicity testing. There are no specific limits for 

any nutrient. The limit for D.O. is a daily minimum of 4 mg/l. This is an end-of-pipe limit equivalent 

to the applicable water quality standard for Curry Creek, the receiving waterbody. 

6.1.2 MS4 areas 

According to 40 CFR 122.26(b)(8), a municipal separate storm sewer (MS4) is “a conveyance or 

system of conveyances (including roads with drainage systems, municipal streets, catch basins, 

curbs, gutters, ditches, man-made channels, or storm drains): 

(i) Owned or operated by a State, city, town, borough, county, parish, district, association, or 

other public body (created by or pursuant to State law)...including special districts under 

State law such as a sewer district, flood control district or drainage district, or similar entity, 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 25 

or an Indian tribe or an authorized Indian tribal organization, or a designated and approved 

management agency under section 208 of the Clean Water Act that discharges into waters 

of the United States. 

(ii) Designed or used for collecting or conveying storm water; 

(iii) Which is not a combined sewer; and 

(iv) Which is not part of a Publicly Owned Treatment Works.” 

Municipal Separate Storm Sewer Systems (MS4s) may discharge nutrients, coliform bacteria, and 

other pollutants to waterbodies in response to storm events. Large and medium MS4s serving 

populations greater than 100,000 people are required to obtain a NPDES storm water permit under 

the Phase I storm water regulations. After March 2003, small MS4s serving urbanized areas were 

required to obtain a permit under the Phase II storm water regulations. An urbanized area is 

defined as an entity with a residential population of at least 50,000 people and an overall population 

density of 1,000 people per square mile. 

The Phase I MS4 permit #:FL000004, which covers Sarasota County, FDOT District 1 and the cities 

of North Port, Sarasota, and Venice, and Longboat Key, may affect listed WBIDs in the Sarasota 

Bay watershed. A requirement of the MS4 permit is the development of a Master Stormwater 

Management Plan. Implementation of this plan addresses water quality and flooding problems 

according to a priority list. 

6.2 Nonpoint Sources 

Nonpoint sources that ultimately contribute to depletion of in-stream dissolved oxygen include 

sources of nutrients such as animal waste, waste-lagoon sludge, fertilizer application to agricultural 

fields, lawns, and golf courses, and malfunctioning onsite sewage treatment and disposal systems 

or septic tank systems. Many of these sources are also potential contributors of coliform bacteria. 

The major landuses in the Sarasota Bay Planning Unit WBIDs are residential and 

commercial/industrial, along with lesser amounts of open water and wetlands (Table 2). Residential 

and commercial/industrial developments are also important in the Lemon Bay Planning Unitespecially in WBIDs 2042 (Alligator Creek) and 2030 (Direct Runoff to Bay). Agriculture and 

rangeland are significant portions of WBIDs 2049 (Gottfried Creek) and 2039 (Forked Creek). 

6.2.1 Wildlife 

Wildlife deposits their feces onto land surfaces where it can be transported during storm events to 

nearby streams. Water fowl (e.g., egrets, ducks, wood storks, herons) often frequent stormwater 

ponds. Generally, the nutrient and bacterial load from wildlife is assumed to be background, as the 

contribution from this source is small relative to the load from urban and agricultural areas in most 

watersheds.

6.2.2 Agricultural animals 

Agricultural activities, including fertilizer application and runoff from pastureland and cattle in 

streams, can impact water quality. Between 1997 and 2002, Sarasota County experienced a 6% 

decline in farmland. Many of the waterbodies listed for nutrients and/or coliforms are located in 

urban areas and livestock are not a relevant source of impairment. However, agriculture and 

rangeland are significant landuses in some of the impaired watersheds, including WBIDs 2049 

(Gottfried Creek) and 2039 (Forked Creek). 

The Florida Department of Agriculture and Consumer Services (FDACS), Office of Agricultural 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 26 

Water Policy developed a manual outlining best management practice for cow/calf operations 

(FDACS, 1999). In this report the authors state “implementation of the practices described in this 

manual provides a good argument that you have made reasonable efforts to reduce pollutants from 

your ranch by the maximum practicable amount”. The manual acknowledges “after implementation 

of these BMPs it may be necessary to add more stringent guidelines for site specific areas that 

continue to exceed water quality standards”. 

 

The USDA National Agricultural Statistics Service (NASS) compiles Census of Agriculture data by 

county for virtually every facet of U.S. agriculture (USDA, 2002). The “Census of Agriculture Act of 

1997” (Title 7, United States Code, Section 2204g) directs the Secretary of Agriculture to conduct a 

census of agriculture on a 5-year cycle collecting data for the years ending in 2 and 7. Livestock 

inventory from the 1997 and 2002 Census of Agriculture reports for Sarasota County are listed in 

Table 10. As shown in this table, cattle and beef cows are the predominate livestock. Confined 

Animal Feeding Operations (CAFOs) are not known to operate in the county. 

Table 10. Livestock Inventory for Sarasota County, Florida. 

Livestock Number of Farms1 Inventory 

1997 2002 1997 2002 

Cattle and calves 276 176 26,248 22,741 

Beef Cows 240 161 16,368 15,147 

Dairy Cows 21 4 377 9 

Swine 47 15 526 181 

Poultry (broilers sold) - - - - 

Sheep 10 7 46 68 

Goats - 5 - 23 

Horses and Ponies - 126 - 882 

 Notes: (D) – data withheld to avoid disclosing data for individual farms. 

1. A farm is defined as any place from which $1,000 or more of agricultural products were 

produced and sold, or normally would have been sold, during the census year. 

In 2002, NASS reported 371 farms in Sarasota County covering an area of 121,310 acres, as 

compared to 463 farms in 1997 covering an area of 129,311 acres. The average size farm in 

Sarasota County in 2002 was 327 acres. In both 1997 and 2002, farms in Sarasota County applied 

more commercial fertilizer to cropland, pastureland, and rangeland, as compared to manure. 

Despite the decline in the number of farms and their total acreage, use of chemical fertilizers in 

Sarasota County increased between 1997 and 2002. In 1997, 3,416 acres of pastureland and 

rangeland were treated with chemical fertilizer, compared to 4,237 acres in 2002.

6.2.3 Onsite Sewerage Treatment and Disposal Systems (Septic Tanks) 

Onsite sewage treatment and disposal systems (OSTDs) including septic tanks are commonly used 

where providing collection and central treatment are not cost effective or practical. When properly 

sited, designed, constructed, maintained, and operated, OSTDs are a safe means of disposing of 

domestic waste. The effluent from a well-functioning OSTD is comparable to secondarily treated 

wastewater from a sewage treatment plant. When not functioning properly, OSTDs can be a source 

of nutrients (nitrogen and phosphorus), pathogens, and other pollutants to both ground water and 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 27 

surface water. 

The State of Florida Department of Health (www.doh.state.fl.us/environment/ostds/statistics/) 

publishes septic tanks data on a county basis. Table 11 summarizes the number of septic systems 

installed in Holmes County since the 1970 census and the total number of repair permits issued 

between 1997-98 and 2003-04. The data does not reflect septic tanks removed from service. 

Table 11. Estimates of Septic Tanks and Repair Permits for Sarasota County. 

County Number of Septic 

Tanks (2002) 

Number of Repair Permits 

Issued (1997 – 2004)

Sarasota 74,774 3,491 

6.2.4 Urban Development 

Nutrient loading from urban areas is attributable to multiple sources including storm water runoff, 

leaks and overflows from sanitary sewer systems, illicit discharges of sanitary waste, runoff from 

improper disposal of waste materials, leaking septic systems, and domestic animals. 

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 outlined in Chapter 403 Florida 

Statutes (F.S.), was established as a technology-based program that relies upon the 

implementation of BMPs that are designed to achieve a specific level of treatment (i.e., performance 

standards) as set forth in Chapter 62-40, F.A.C. 

Florida’s stormwater program is unique in having a performance standard for older stormwater 

systems that were built before the implementation of the Stormwater Rule in 1982. This rule states: 

“the pollutant loading from older stormwater management systems shall be reduced as needed to 

restore or maintain the beneficial uses of water” (Section 62-4-.432 (5)(c), F.A.C.). 

Nonstructural and structural BMPs are an integral part of the State’s stormwater programs. 

Nonstructural BMPs, often referred to as “source controls”, are those that can be used to prevent 

the generation of NPS pollutants or to limit their transport off-site. Typical nonstructural BMPs 

include public education, land use management, preservation of wetlands and floodplains, and 

minimizing impervious surfaces. Technology-based structural BMPs are used to mitigate the 

increased stormwater peak discharge rate, volume, and pollutant loadings that accompany 

urbanization. 

7. ANALYTICAL APPROACH 

7.1. Nutrient and Biolochemical Oxygen Demand (BOD) 

In order to develop nutrient TMDLs, the narrative nutrient criterion must first be translated to 

numeric targets using appropriate analytical tools. Because of the lack of sufficient and/or recent 

nutrient and flow data to allow appropriate calibration/validation of a distributed model, a watershed 

loading model, the Lemon Bay Watershed Model (LBMW), was used to determine existing and 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 28 

target loads of total nitrogen and BOD for the impaired WBIDs. Nitrogen is the only nutrient 

addressed in these TMDLs because it is the limiting factor for phytoplankton productivity in 

Sarasota, Charlotte Harbor, and Lemon Bay ecosystems (Dixon and Kirkpatrick, 1991, SWFWMD, 

1999). The next section describes watershed loading models in general and the LBWM modeling 

application to generate loads for the impaired WBIDs. 

7.1.1 Determination of Pollutant Loading Capacity 

During the last fifteen years, several WQ modeling efforts were undertaken to estimate pollutant 

loadings originating from upland watersheds to several Florida bays including Hillsborough Bay 

(Johansson, 1991), Charlotte Harbor (Squires et al., 1998), Sarasota Bay (Heyl, 1992), Tampa Bay 

(Harper, 1991), and Lemon Bay (ERD, 2004). These modeling activities were carried out by 

Watershed Management Districts often in collaboration with national estuaries programs in these 

areas, Florida Department of Environmental Protection, and local counties. These models are 

simple loading functions in which annual average runoff and loadings to bays are derived using 

simple algorithms and land use loading rates to generate long term annual averages of runoff 

volumes and pollutant loadings to the Bay. 

The Lemon Bay Watershed Model is an excel worksheet that estimates total annual runoff 

stormwater runoff volumes and pollutant loadings from seven watersheds draining to Lemon Bay 

(ERD, 2004). First, Runoff volumes are estimated using site specific land use and hydrologic soil 

groups, percent impervious, and percent directed connected impervious areas (DCIA) calculated as 

a percent of the total impervious area. Then pollutant concentrations specific to each land use are 

used with runoff volumes to generate pollutant loads. Finally, attenuation factors are applied to total 

generated runoff volumes and pollutant loads to account for losses in stormwater and conveyance 

systems. The model is an improvement of previous similar efforts in Sarasota, Charlotte and 

Lemon Bays (Heyl, 1992, Squires et al., 1998, Tomasko et al., 2001) because land use loading 

rates and attenuation factors used in this model are considered to be more accurate. When 

compared to measured data from contributing watersheds, modeled results differed only by 11%, 

24%, 1%, and 7%, respectively for total N, total P, BOD and TSS. More information on the model is 

provided in Environmental Research and Design, Inc. (2004). 

7.1.1.1. Hydrologic Data and Runoff Generation 

This section presents hydrologic data used for runoff generation in the impaired WBIDs using the 

LBWM model. Land use and soil information were obtained from the SWFWMD GIS data 

(http://www.swfwmd.state.fl.us/data/gis/). FLUCCS were consolidated into 14 land use categories 

as shown in Table 12 and are the same as those presented in Table 3. Percent impervious for 

each land use in a WBID was determined based on measurements from 1999 1-meter resolution 

orthophoto quads. Percent DCIA is computed as a percentage of impervious areas using values 

typical for the given land use (Table 12). Wet and dry season rainfall distributions used in the 

model were derived from the National Climatic Data Center (NCDC) hourly rainfall for St. 

Petersburg and Ft. Myers, and daily rainfall from Englewood and Cape Haze. Weighted seasonal 

curve numbers (CNs) under average antecedent moisture conditions (AMC II), dry (AMC I) and wet 

conditions (AMCIII) are computed using the fraction of events falling into each AMC, hydrologic soil 

group (HSG) and land use/cover distributions in WBIDs. 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 29 

Table 12. Percent Impervious and DCIA by Land Use for Impaired WBIDs. 

LAND USE PERCENT 

IMPERVIOUS 

% IMPERVIOUS THAT IS 

DCIA 

Agriculture 0 0 

Commercial 65 75 

Extensive 50 25 

Industrial 50 25 

Low-Density Residential 15 20 

Medium Density Residential 40 40 

High-Density Residential 60 65 

Transportation 50 90 

Institutional 45 60 

Open 0 0 

Recreational 6 0 

Wetlands 0 0 

7.1.1.2. Pollutant Loading Rates for Land Use Categories 

LBWM uses loading concentrations in runoff emanating from each land use. Nutrient and BOD 

concentrations in stormwater were primarily based on long-term monitoring data that were obtained 

as part of the model development and on data collected by counties, universities, and private 

agencies in Lemon Bay, Sarasota and Charlotte Bay contributing watersheds (ERD, 2004). 

Loading rates for total N and BOD used in this analysis are presented in Table 13. 

Table 13. Nutrient and BOD Concentrations in Stormwater by Land Use 

Typical Loading Rates (mg/l) Land Use 

Total N BOD 

Agriculture 1.24 2.40 

Commercial 0.88 2.30 

Extractive 1.18 9.60 

Industrial 1.64 9.60 

Low-Density Residential 0.94 1.50 

Medium-Density Residential 1.17 2.00 

High-Density Residential 1.17 3.10 

Transportation 2.23 6.70 

Institutional 1.17 2.00 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 30 

Open 0.70 1.00 

Recreational 1.17 2.00 

Wetland 1.44 2.63 

Water (Ponds and Lakes) 0.31 2.00 

Water (Conveyance) 0.31 2.00 

7.1.1.3. Load Attenuation Factors 

A series of load attenuation factors is used in the model for each land use to represent the effect of 

stormwater system treatment. The first attenuation factor is the treatment potential value of each 

land use. It is the ratio of water (ponds, lakes, and reservoirs) falling in each land use to that land 

use area in the watershed. The attenuation values for this factor are presented by land use for the 

8 impaired WBIDs in Table 14. The second factor is the stormwater attenuation factor (Table 15). 

It is applied to pollutant load to represent load losses over the watershed landscape and load 

reductions associated with in-stream processes. The third factor is the conveyance attenuation 

factor which is applied to the entire watershed and is function of the ratio of conveyance system 

(streams, estuaries) area to total watershed area (Table 16). 

Table 14. Treatment Potential Factors 

Land Use Ratio Land Use Area to Treatment Area 

Elligraw 

Bayou 

Curry 

Creek 

North 

Creek 

Catfish 

Creek 

South 

Creek 

Gottfried 

Creek 

Forked 

Creek 

Alligato

r Creek 

Direct 

Runoff 

to Bay 

Agriculture 0.05 0.02 0.00 --- --- 0.20 0.13 --- --- 

Commercial 0.03 0.10 0.00 0.40 --- 0.03 0.05 0.05 --- 

Extractive --- --- --- --- --- --- --- --- --- 

Industrial 0.04 --- --- --- --- --- --- 0.00 --- 

Low-Density 

Residential 0.00 --- 0.03 0.05 0.05 0.06 0.02 0.48 --- 

MediumDensity 

Residential 0.06 0.23 0.04 0.02 0.05 0.04 0.02 0.02 

High-Density 

Residential 0.04 0.00 0.20 0.40 0.08 0.02 0.03 0.11 0.14 

Transportatio

n --- --- --- --- --- --- --- --- --- 

Institutional 0.01 --- --- --- 0.15 0.00 0.75 0.03 --- 

Open 0.00 0.03 --- 0.05 0.02 0.03 0.02 0.13 --- 

Recreational 0.03 0.00 0.16 0.20 0.00 0.11 0.09 0.08 --- 

Wetland --- --- --- 0.10 --- --- --- --- --- 

Water (Ponds 

and Lakes) 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 

Water 

(Conveyance

) 0.02 0.06 0.04 0.00 0.02 0.02 0.01 0.01 0.04 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

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Table 15. Stormwater System Attenuation Factors 

Land Use Runoff 

Volume Total N BOD 

Agriculture 0.00 0.00 0.00 

Commercial 0.10 0.25 0.20 

Extractive 0.00 0.25 0.20 

Industrial 0.10 0.25 0.20 

Low-Density Residential 0.15 0.25 0.00 

Medium-Density Residential 0.15 0.25 0.20 

High-Density Residential 0.15 0.25 0.20 

Transportation 0.10 0.25 0.20 

Institutional 0.15 0.25 0.20 

Open 0.00 0.00 0.00 

Recreational 0.15 0.25 0.20 

Wetland 0.00 0.00 0.00 

Water (Ponds and Lakes) 1.00 1.00 1.00 

Table 16. Conveyance System Attenuation Factors 

Land Use Runoff 

Volume Total N BOD 

Water (Conveyance) 0.00 0.25 0.10 

7.1.1.4. Computations of Runoff Volumes and Pollutant Loadings 

The runoff volume for each WBID was generated for wet and dry seasons using watershed area, 

percent impervious, DCIA, and CN for each combination of land use and hydrologic soil group. 

Total N and BOD loads for dry and wet seasons were computed by multiplying the land use 

pollutant concentrations in Table 14 with appropriate generated runoff volumes. Attenuated 

pollutant loads for each season were obtained by multiplying generated load pollutants by 

stormwater and conveyance attenuation factors respectively in Tables 15 and 16. Annual 

attenuated total N and BOD loads in the impaired WBIDs were obtained by summing loads 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 32 

generated in wet and dry seasons and are presented in Tables 17 and 18, respectively. 

Table 17. Annual Total N Loads by Land Use Under Existing Conditions 

Land Use Annual Total N (kg) 

Land use 

Elligraw 

Bayou 

Curry 

Creek 

North 

Creek 

Catfish 

Creek 

South 

Creek 

Gottfried 

Creek 

Forked 

Creek 

Alligator 

Creek 

Direct 

Runoff 

to Bay 

Agriculture 42.9 16.4 1.7 41.7 0.2 120.5 248.9 0.0 0.0 

Commercial 407.5 69.1 65.5 232.9 260.3 328.4 150.7 532.6 29.8 

Extractive 0.0 0.0 0.0 0.0 0.0 0.0 25.1 0.0 0.0 

Industrial 452.7 0.0 0.0 1.0 0.0 0.0 41.7 0.0 9.0 

Low-Density 

Residential 60.2 10.5 30.7 13.6 35.0 142.4 213.3 15.9 82.0 

Medium-Density 

Residential 1417.5 50.9 405.7 155.8 309.4 554.3 1078.1 1313.6 952.8 

High-Density 

Residential 672.3 846.9 289.4 1405.5 579.6 398.2 947.5 2003.0 177.0 

Transportation 213.5 61.5 64.5 92.5 144.6 26.0 257.6 511.4 0.0 

Institutional 188.5 27.6 0.0 4.4 51.1 109.2 7.3 143.3 0.0 

Open 51.1 6.5 47.6 41.2 173.9 466.1 317.3 73.5 47.9 

Recreational 69.5 4.6 77.4 87.1 2.1 49.3 32.9 43.8 4.0 

Wetland 49.2 115.8 87.6 187.7 192.2 738.6 786.4 252.8 129.3 

Water (Ponds and 

Lakes) 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Water (Conveyance) 73.8 105.4 90.8 16.3 72.0 132.0 127.0 75.3 89.8 

Total 2525.9 1301.0 1116.1 2279.7 1816.6 3025.1 4234.9 5370.4 1414.2 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 33 

Table 18. Annual BOD Loads by Land Use Under Existing Conditions 

Land Use Annual BOD (kg) 

Land Use 

Elligraw 

Bayou 

Curry 

Creek 

North 

Creek 

Catfish 

Creek 

South 

Creek 

Gottfried 

Creek 

Forked 

Creek 

Alligator 

Creek 

Direct 

Runoff 

to Bay 

Agriculture 104.9 36.9 3.4 82.0 0.4 236.6 499.0 0.0 0.0 

Commercial 1370.7 223.9 175.1 660.6 725.8 884.1 418.5 1844.0 70.3 

Extractive 0.0 0.0 0.0 0.0 0.0 0.0 211.5 0.0 0.0 

Industrial 3417.4 0.0 0.0 6.2 0.0 0.0 253.1 0.0 47.4 

Low-Density 

Residential 121.4 19.4 53.4 25.2 67.9 274.4 367.4 43.6 118.2 

Medium-Density 

Residential 3162.0 107.9 722.9 271.0 569.7 991.4 1954.6 2924.4 1483.5 

High-Density 

Residential 2301.8 2607.3 835.8 4041.6 1724.5 1079.2 2643.4 7289.4 452.2 

Transportation 810.2 214.9 198.1 282.6 463.3 79.3 801.6 1978.6 0.0 

Institutional 408.4 54.8 0.0 7.6 99.5 189.6 13.8 320.8 0.0 

Open 91.7 10.7 69.2 59.6 263.9 672.4 467.5 134.6 61.6 

Recreational 152.6 9.1 144.3 161.6 3.9 91.1 61.7 101.2 6.2 

Wetland 113.5 245.7 163.6 348.8 374.5 1368.2 1487.7 594.4 213.5 

Water (Ponds and 

Lakes) 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Water (Conveyance) 877.0 648.3 555.4 106.4 423.1 1146.3 626.1 496.4 602.5 

Total 12931.6 4178.8 2921.1 6053.4 4716.3 7012.5 9806.0 15727.5 3055.4 

7.1.2 Determination of Target Loads 

Target loads for total N and BOD were determined using the same model (LBWM), loading rates , 

and attenuation factors except land cover distribution in the impaired WBIDs. Land cover 

categories used for determining target loads are those considered to have existed in the impaired 

WBIDs prior to anthropogenic influences. They are Open Land, Wetlands and Water (ponds, lakes, 

and conveyances), in addition to streams and rivers. This assumption is very conservative because 

impairement in these watersheds more likely occured sometime after the start of the human 

influence. This scenario was compared to the Reference Watershed Approach, a method 

considered as more approapriate for TMDL computations in the abscence of WQ criteria (See 

Appendix B). 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 34 

Soil properties are a good indicator of land cover prior to human influnces because soils are 

mapped in their ‘natural’ state. Therefore, pre-development land cover categories in each WBID 

was determined using primarily hydrologic soil groups (HSG) as follows: 

First, agriculture and developed land (residential, commercial industrial, extractive, etc.) that 

presently occurs on HSG D have existed as Wetlands before human settlements. This 

assumption makes sense because all soil series with HSG D in the Sarasota County Soil 

survey Manual are hydric soils, one of the main indicators of wetlands. 

Second, agriculture and developed land presently on soils with HSGs A, B and C was 

assigned to Open Land cover and are assumed to have occurred on same HSGs. 

Third, agriculture and developed land presently on soils with HSGs B/D and C/D was 

assigned to Open Land cover or Wetlands with the assumption that 20% of these soils were 

initially wetlands with HSG D. The same assumption was used in the Lemon Bay Watershed 

Model (ERD, 2004). The Open land use currently on B/D and C/D was assumed to have 

existed on HSG D. This assumption is valid because soils with HSGs B/D and C/D, which 

existed as poorly drained soils (HSG D) in their original state are currently drained and as a 

result exhibit characteristics of soils with HSGs B and C. These soils are currently mapped in 

Florida as hydric soils. 

Finally, agriculture and developed land occurring on HSG designation W existed as 

waterbodies such as lakes and ponds and therefore were assigned to Water (ponds and 

lakes). 

The land cover categories estimated in each WBID for natural conditions are presented in Table 19. 

 Results obtained using the LBWM model using these categories are presented in Tables 20 and 21 

for total nitrogen and BOD respectively. 

Table 19. Land Cover Distribution in the Impaired WBIDs Under Natural Conditions

Land Use Area (acres) 

Land Use Elligraw 

Bayou 

South 

Creek 

Catfish 

Creek 

North 

Creek 

Curry 

Creek 

Gottfried 

Creek 

Forked 

Creek 

Alligato

r Creek 

Direct 

Runoff 

to Bay 

Agriculture 0 0 0 0 0 0 0 0 0 

Commercial 0 0 0 0 0 0 0 0 0 

Extractive 0 0 0 0 0 0 0 0 0 

Industrial 0 0 0 0 0 0 0 0 0 

Low-Density 

Residential 0 0 0 0 0 0 0 0 0 

Medium-Density 

Residential 0 0 0 0 0 0 0 0 0 

High-Density 

Residential 0 0 0 0 0 0 0 0 0 

Transportation 0 0 0 0 0 0 0 0 0 

Institutional 0 0 0 0 0 0 0 0 0 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 35 

Open 2,984 1,950 1,304 1,198 740 5,324 4,947 3,736 1,141 

Recreational 0 0 0 0 0 0 0 0 0 

Wetland 1,126 451 843 281 287 1,461 2,022 1,574 449 

Water (Ponds and 

Lakes) 188 79 267 112 35 322 341 399 28 

Water (Conveyance) 92 44 11 60 70 120 65 51 65 

Total 4,390 2,524 2,426 1,651 1,132 7,227 7,375 5,760 1,683 

Table 20. Annual Total N Loads by Land Cover Under Natural Conditions 

Land Use Annual Total N (kg) 

Land Use 

Elligraw 

Bayou 

South 

Creek 

Catfish 

Creek 

North 

Creek 

Curry 

Creek 

Gottfried 

Creek 

Forked 

Creek 

Alligator 

Creek 

Direct 

Runoff 

to Bay 

Agriculture 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Commercial 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Extractive 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Industrial 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Low-Density 

Residential 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Medium-Density 

Residential 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

High-Density 

Residential 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Transportation 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Institutional 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Open 910.4 531.2 456.4 323.8 147.4 1683.8 1696.9 2430.1 278.1 

Recreational 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Wetland 758.7 349.7 637.3 186.4 242.7 1110.3 1594.8 1351.9 276.4 

Water (Ponds and 

Lakes) 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Water 

(Conveyance) 130.0 61.1 16.3 79.7 92.5 171.5 95.1 75.4 86.0 

Total 1799.1 941.9 1110.0 590.0 482.6 2965.6 3386.7 3857.4 640.5 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 36 

Table 21. Annual BOD Loads by Land Cover Under Natural Conditions 

Land Use Annual BOD (kg) 

Land Use 

South 

Creek 

Catfish 

Creek 

North 

Creek 

Curry 

Creek 

Gottfried 

Creek 

Forked 

Creek 

Alligator 

Creek 

Direct 

Runoff 

to Bay 

Agriculture 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Commercial 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Extractive 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Industrial 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Low-Density 

Residential 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Medium-Density 

Residential 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

High-Density 

Residential 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Transportation 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Institutional 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Open 850.4 660.5 541.1 250.3 2561.4 2497.9 3579.3 470.9 

Recreational 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Wetland 718.8 1184.3 399.8 529.4 2168.5 3014.2 2556.6 600.8 

Water (Ponds and 

Lakes) 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Water 

(Conveyance) 417.6 106.4 555.5 649.0 1146.3 626.1 496.4 602.5 

Total 1986.8 1951.3 1496.4 1428.7 5876.2 6138.3 6632.3 1674.2 

7.2. Coliforms 

The approach for calculating coliform TMDLs depends on the number of bacteria samples and the 

availability of flow data. When long-term records of water quality and flow data are available, load 

duration curves, that establish a relationship between flow and concentration, are used to develop 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 37 

TMDLs. Load duration curves utilize a mass balance approach to estimate loadings transported in 

the stream. Otherwise, when water quality and flow data are scarce or non-existent as it is the case 

here, TMDLs are expressed as percent reductions. 

7.1.2.1. Percent Reduction Approach 

Coliform TMDLs for Elligraw Bayou and Clowers Creek WBIDs were developed using the Percent 

Reduction Approach. As shown in Tables 8 and 9, there is a limited number of coliform samples in 

these WBIDs. In addition, flow data for the two waterbodies is not available. Under this approach, 

the percent reduction needed to meet the applicable criterion is calculated based on the 90th

percentile of all measured concentrations. The 90th percentile concentration represents existing 

conditions and implies that 90 percent of the measured values are lower than this concentration or 

10 percent are higher. The percent reduction required to meet the coliform criteria is based on the 

following equation: 

Percent Reduction (%) = (existing concentration – criteria) / existing concentration * 100 

As an example, the calculated 90th percentile concentration and percent reduction required to meet the 

acute criterion for Elligraw Bayou WBID is shown in Table 22. The 90th percentile concentration is 

MPN/100ml and a % reduction is necessary to meet the instream water quality target of 400 counts/100 

ml. 

Table 22. Fecal Coliform Measurements for Elligraw Bayou 

Station Date Concentration 

(MPN/100ml) 

21FLTPA 27151668230388 6/9/2003 1200 

 3/4/2003 1300 

 7/22/2003 790 

 10/13/2003 65 

 4/8/2003 2500 

 5/20/2003 210 

 9/23/2003 30 

 8/25/2003 440 

 12/15/2003 940 

 12/1/2003 400 

21FLTPA 27152258230197 5/20/2003 470 

 12/15/2003 1210 

 8/25/2003 1530 

 3/4/2003 235 

 12/1/2003 360 

 9/23/2003 680 

 7/22/2003 395 

 6/9/2003 480 

 10/13/2003 670 

 4/8/2003 1260 

90th Percentile Concentration 1323 

Percent Reduction 70% 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 38 

8. DEVELOPMENT OF TOTAL MAXIMUM DAILY LOAD 

The TMDL process quantifies the amount of a pollutant that can be assimilated in a waterbody, 

identifies the sources of the pollutant, and recommends regulatory or other actions to be taken to 

achieve compliance with applicable water quality standards based on the relationship between 

pollution sources and in-stream water quality conditions. A TMDL can be expressed as the sum of 

all point source loads (Waste Load Allocations), non-point source loads (Load Allocations), and an 

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

relationship between effluent limitations and water quality: 

TMDL = Σ WLAs + Σ LAs + MOS 

The objective of a TMDL is to allocate loads among the known pollutant sources throughout a 

watershed so that appropriate control measures can be implemented and water quality standards 

achieved. 40 CFR §130.2 (i) states that TMDLs can be expressed in terms of mass per time (e.g. 

pounds per day), toxicity, or other appropriate measure. The TMDLs are expressed as the longterm annual average loads in units of kilograms (kg) per year. TMDL components for nutrientimpaired water are provided in Table 23. For waterbodies impaired by DO, the process to allocate 

total N and BOD loads is described below. 

TMDL established also a load for nutrients and BOD for Elligraw Bayou (WBID 1975) and Alligator 

Creek (WBID 230) to address DO-listed impairments. EPA has evaluated the waterbody to 

determine 1) existing loading to the waterbody and 2) loading to the waterbody with a natural 

condition land use (forested). There are no point sources in the waterbody. Modeling results under 

the natural condition loading showed that the applicable dissolved oxygen criteria could not be 

achieved. 

The water quality standards for Florida at 62-302.200(15) FAC state that, “’Natural Background’ 

shall mean the condition of waters in the absence of man-induced alterations based on the best 

scientific information available to the Department. The establishment of natural background for an 

altered waterbody may be based upon a similar unaltered waterbody or on historical pre-alteration 

data.” Florida standards also state at 62-302.300(15) FAC, “Pollution which causes or contributes 

to new violations of water quality standards or to continuation of existing violations is harmful to the 

waters of this State and shall not be allowed. Waters having water quality below the criteria 

established for them shall be protected and enhanced. However, the Department shall not strive to 

abate natural conditions.” 

The purpose of a TMDL is to allocate pollutant loading such that water quality standards are 

achieved. The Clean Water Act at Section 303(d)(1)(C) states, “Such load shall established at a 

level necessary to implement the applicable water quality standards ...” In expressing this TMDL, 

EPA must apply the applicable dissolved oxygen water quality standard while also considering the 

definition of natural background and the directive not to abate natural conditions. 

The natural background definition established a narrative criteria in cases where there is an 

absence of man-induced alterations, but then states that a natural background level can be 

established (through 62-302.800 FAC) based on a similar unaltered waterbody, or on historical pre-
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 39 

alteration data. Therefore the Agency can apply a narrative natural background condition 

(unaltered) as the water quality criteria. In the TMDL context, without a site specific criteria, 

calculating allocations would be targeted to achieve the natural background. Using a model, this 

can be achieved by removing all anthropogenic loading. Any allowance of increased loading 

beyond natural background would likely cause other than natural, and likely lower, dissolved 

oxygen levels which would not be a proper application of the Florida definition. Therefore, the 

allocations must be established with no allowance for anthropogenic loading in order to properly 

“implement the applicable water quality standards.” However, because of the standard that 

prevents abatement of natural conditions (62-302.300(15) FAC), the TMDL must provide an 

allocation, where necessary, to allow for natural condition loading. 

The TMDL was calculated to allow natural condition loading, but did not provide an allocation for 

anthropogenic loading. The Agency strongly encourages the development of site specific criteria 

such that anthropogenic loadings may be expressed as part of the TMDL. In the absence of site 

specific criteria, implementation of loading reductions should strive to achieve a natural background 

loading condition for the waterbody. Nutrient and BOD TMDL components to address DO 

impairment in Elligraw Bayou and Alligator Creek are presented in Table 24. Table 25 presents 

TMDL components for waterbodies impaired by coliforms. 

8.1. Waste Load Allocations 

Only facilities discharging directly into streams and MS4 areas are assigned a WLA. The WLAs, if 

applicable, are expressed separately for continuous discharge facilities (e.g., WWTP) and MS4 

areas as the former discharges during all weather conditions whereas the later discharges in 

response to storm events. 

8.1.1 NPDES Wastewater Discharges 

Snowbirdland Vistas is a mobile home site that discharges to Curry Creek, which is 303(d)-listed for 

nutrients. However the facility does not have limits for nutrients. 

8.1.2 NPDES Stormwater Discharges 

The WLA for the MS4s are expressed in terms of percent reduction. Given the available data, it is 

not possible to estimate loadings in units of MPN/day coming exclusively from the MS4 areas. 

Although the aggregate wasteload allocations for storm water discharges are expressed in numeric 

form, i.e. percent reduction, based on the information available today, it is infeasible to calculate 

numeric WLAs for individual storm water outfalls because discharges from these sources can be 

highly intermittent, are usually characterized by very high flows occurring over relatively short time 

intervals, and carry a variety of pollutants whose nature and extent varies according to geography 

and local land use. For example, municipal sources such as those covered by these TMDLs often 

include numerous individual outfalls spread over large areas. Water quality impacts, in turn, also 

depend on a wide range of factors, including the magnitude and duration of rainfall events, the time 

period between events, soil conditions, fraction of land that is impervious to rainfall, other land use 

activities, and the ratio of storm water discharge to receiving water flow. 

These TMDLs assume for the reasons stated above that it is infeasible to calculate numeric water 

quality-based effluent limitations for storm water discharges. Therefore, in the absence of 

information presented to the permitting authority showing otherwise, these TMDLs assume that 

water quality-based effluent limitations for storm water sources of coliforms and/or nutrients derived 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 40 

from this TMDL can be expressed in narrative form (e.g., as best management practices), provided 

that (1) the permitting authority explains in the permit fact sheet the reasons it expects the chosen 

BMPs to achieve the aggregate wasteload allocation for these storm water discharges; and (2) the 

state will perform ambient water quality monitoring for coliform and/or nutrients for the purpose of 

determining whether the BMPs in fact are achieving such aggregate wasteload allocation. 

The percent reduction calculated for nonpoint sources is assigned to the MS4 as violations from 

both sources typically occur in response to storm events. Permitted MS4s will be responsible for 

reducing only the loads associated with stormwater outfalls which it owns, manages, or otherwise 

has responsible control. MS4s are not responsible for reducing other nonpoint source loads within 

its jurisdiction. All future MS4s permitted in the area are automatically prescribed a WLA equivalent 

to the percent reduction assigned to the LA. 

8.2. Load Allocations 

The load allocations are expressed as the annual average loads nutrient and BOD less the MOS. 

In this analysis, load allocation equals the target load because MOS is implicit. 

8.3. Margin of Safety (MOS) 

There are two methods for incorporating a MOS in the analysis: a) implicitly incorporate the MOS 

using conservative model assumptions to develop allocations; or b) explicitly specify a portion of the 

TMDL as the MOS and use the remainder for allocations. An implicit MOS is incorporated in the 

conservative decision associated with the TMDL development approach used. This approach 

established TMDLs at loads that are expected to have existed under natural land cover conditions 

before human influences. 

Table 23. TMDL Components for Nutrient-Impaired Waters. 

WLA2

Waterbody Parameter1

Existing 

Loads 

(kg/year) 

Target 

Load-LA 

(kg/year) 

Continuous 

(kg/year) 

MS4 

(Percent 

reduction)

MOS4 

(kg/year) 

Percent 

Reduction6

Elligraw 

Bayou TN 2525.86 1799.13 N/A3

 28.8 Implicit 28.8 

Curry Creek TN 1300.99 482.60 N/A3 62.9 Implicit 62.9 

North Creek TN 1116.10 589.95 N/A3 47.1 Implicit 47.1 

Catfish TN 2279.7 1110.0 N/A3 51.3 Implicit 51.3 

South Creek TN 1816.58 941.89 N/A3

48.2 Implicit 48.2 

Gottfried 

Creek TN 3025.13 2965.58 N/A3

2.0 Implicit 2.0 

Forked 

Creek TN 4234.90 3386.74 N/A3

20.0 Implicit 20.0 

Direct Runoff 

to Bay TN 1414.23 640.51 N/A3

54.7 Implicit 54.7 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 41 

WLA2

Waterbody Parameter1

Existing 

Loads 

(kg/year) 

Target 

Load-LA 

(kg/year) 

Continuous 

(kg/year) 

MS4 

(Percent 

reduction)

MOS4 

(kg/year) 

Percent 

Reduction6

Alligator 

Creek TN 5370.41 3857.43 N/A3

28.2 Implicit 28.2 

Notes: 

1

TN = total nitrogen 

2

WLA component is separated into load from continuous NPDES facilities (e.g., WWTPs) and load 

from MS4s. 

3

N/A = not applicable 

4

Margin of Safety is implicit due to conservative assumptions made in determining TMDL targets. 

5

Percent reduction from current conditions to achieve the target concentration for that nutrient. 

Table 24. TMDL Components for Waterbodies Impaired by DO 

Target LoadLA (kg/year) WLA2

Waterbody Parameter1

Existing 

Load 

(kg/year) Anthropogenic Natural Continuous 

(kg/year) 

MS4 

(Percent 

reduction) 

MOS4 

(kg/year)

Percent 

Reduction5

TN 2525.86 0 1799.13 N/A3 28.8 Implicit 28.8 

Elligraw Bayou 

BOD 12931.64 0 3800.00 N/A3

 70.6 Implicit 70.6 

TN 5370.41 0 3857.43 

N/A3

28.2 Implicit 28.2 

Alligator Creek 

BOD 15727.53 0 6632.32 N/A3

57.8 Implicit 57.8 

1

TN = total nitrogen, BOD = Biochemical Biological Demand 

2

WLA component is separated into load from continuous NPDES facilities (e.g., WWTPs) and load 

from MS4s. 

3

N/A = not applicable 

4

Margin of Safety is implicit due to conservative assumptions made in determining TMDL targets. 

5

Percent reduction from current conditions to achieve the target concentration for that nutrient. 

Table 25. TMDL Components for Waterbodies Impaired by Coliform. 

Waterbody Parameter1 TMDL LA WLA2
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 42 

(MPN/day) (MPN/day) 

Continuous 

MS4 

(percent 

reduction) 

Percent 

Reduction3

Elligraw TCOLI 3910 3910 N/A 39 39 

Bayou FCOLI 1323 1323 N/A 70 70 

Clowers TCOLI 2615 2615 N/A 85 85 

Creek FCOLI 1678 1678 N/A 76 76 

Notes: 

1. TCOLI= total coliforms; FCOLI= fecal coliforms 

2. WLA component is separated into load from continuous NPDES facilities (e.g., WWTPs) and load 

from MS4s. N/A = not applicable. MS4 load represented as percent reduction. If future residential 

development densities result in a change to the MS4 boundary, the MS4 will be required to meet 

the established TMDL in the form of BMPs. 

3. Percent reduction from current conditions to achieve the target concentration. 

8.4. Critical conditions and Seasonal Variations 

The critical conditions can be defined as the environmental conditions requiring the largest 

reduction to meet standards. By achieving the reduction for critical conditions, water quality 

standards should be achieved during all other times. Seasonal variation must also be considered to 

ensure that water quality standards will be met during all seasons of the year. 

The critical condition for non-point source loadings is typically an extended dry period followed by a 

rainfall runoff event. During the dry weather period, pollutants build up on the land surface, and are 

washed off by rainfall. The critical condition for point source loading occurs during periods of low 

stream flow when dilution is minimized. The expression of a nutrient impairment is more likely to 

occur during warmer months. However, because nutrients (especially phosphorus) can 

accumulate, nutrient loadings are usually considered over longer periods (e.g. annual instead of 

daily loads). Critical conditions and seasonal variation were incorporated into the TMDL 

development by using dry and wet seasonal variations of rainfall. 

9. CONCLUSION AND RECOMMENDATIONS 

Determining the source of nutrients in waterbodies is the initial step to implementing a nutrient 

TMDL. FDEP employs the Basin Management Action Plan (B-MAP) as the mechanism for 

developing strategies to accomplish the necessary load reductions. Components of a B-MAP are: 

• Allocations among stakeholders 

• Listing of specific activities to achieve reductions 

• Project initiation and completion timeliness 

• Identification of funding opportunities 

• Agreements 

• Local ordinances 

• Local water quality standards and permits 

• Follow-up monitoring 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 43 

TMDLs for impaired WBIDs presented in this document were based on limited nutrient and BOD 

monitoring data. As a result, existing and target loads were computed using a conservative 

modeling approach in which long-term estimated loading rates rather than actual rates were 

used. A more robust modeling analysis, coupled with appropriate model calibration and 

verification, could have been used if sufficient monitoring data was available. 

Sarasota County, in cooperation with Sarasota Bay National Estuary Program (SBNEP), is 

currently planning an aggressive water quality monitoring of these waterbodies as part of the 

development of its Basin Management Action Plan (B-MAP). The data collected as part of this 

monitoring program can be used to better assess site-specific allowable loads necessary to 

protect aquatic communities in these waterbodies. At the same time, new pollutant percent 

reductions may be computed and used to address these TMDLs during the B-MAP 

implementation phase. 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 44 

REFERENCES 

Dixon, L.K. and G.J. Kirkpatrick. 1999. Biological effects of atmospheric deposition on algal 

assemblages. Sarasota Bay National Estuary Program. Sarasota, FL. 

Environmental Research and Design, Inc (ERD). 2004. Nonpoint source model development 

and basin management strategies for Lemon Bay. Final Report to Southwest Florida Water 

Management District. May 2004. 

Florida Administrative Code (F.A.C.). Chapter 62-302, Surface Water Quality Standards. 

Florida Department Agriculture and Consumer Services (FDACS), 1999. Water Quality Best 

Management Practices for Cow/Calf Operations in Florida, Office of Agricultural Water Policy, 

June 1999. 

Florida Department of Environmental Protection (FDEP), 2002. Basin Status Report, Charlotte 

Harbor, FDEP Division of Water Resource Management, South District, Group 2 Basin, June 

2002. 

Florida Department of Environmental Protection (FDEP), 2003. Basin Status Report, Sarasota Bay 

and Peace and Myakka Rivers, FDEP Division of Water Resource Management, Southwest 

District, Group 3 Basin, September 2003. 

Harper, H. H. 1991. Estimation of loading rate parameters for the Tampa Bay Watershed. 

SWFWMD, Brooksville, FL. 

Heyl, M.G. Point and nonpoint source pollutant loading assessment. P. 12.1-12.1. In P. Roat, C. 

Cicolella, H. Smith, and D. Tomasko (eds). Sarasota Bay Framework for Action. Sarasota Bay 

National Estuary Program, Sarasota, FL. 

Johansson, J.O.R. 1991. Long-term trends in N loadings and biological indicators in 

Hillsborough Bay, Fl. P. 157-176. In S.F. Treat and P.A. Clark (eds). Proc. Tampa Bay Area 

Scient. Information Symposium. St Petersburg, FL. 

Sarasota Bay Estuary Program, 2004. Sarasota Bay Watershed Management Plan for the 

Sarasota Bay Planning Unit, Florida, December 2004. 

Southwest Florida Water Management District (SWFWMD), 2002. Sarasota Bay Surface Water 

Improvement and Management (SWIM) Plan, Resource Management Department, Tampa, 

Florida. December 2002. 

Squires, A.P. et al. 1998. Loadings of TN, TP, and TSS to Charlotte Harbor. P. 187-200. In 

S.F. Treat (ed.). Proc. of the Charlotte Harbor Public Conference and Technical Symposium 

March 13-16, 1997. Punta Gorda, FL. NEP and SFWMD. 

Tomasko, D.A., D.L. Bristol, and J.A. Ott. 2001. Assessment of present and future nitrogen 

loads, water quality, and seagrass depth distribution in Lemon Bay. Estuaries 24(6A): 925-938. 
Draft Nutrient/D.O. TMDLs

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page 45 

United States Department of Agriculture (USDA). National Agricultural Statistics Service (NASS), 

2002 Census of Agriculture, U.S. Department of Agriculture. 

USEPA, 1991. Guidance for Water Quality –based Decisions: The TMDL Process. U.S. 

Environmental Protection Agency, Office of Water, Washington, DC. EPA-440/4-91-001, April 

1991.
Draft Nutrient/D.O. TMDLs 

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page B-1

APPENDIX A PHOTOS OF IMPAIRED WBIDS 
Draft Nutrient/D.O. TMDLs 

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page B-2

Small Creek in WBID 1975 (Elligraw Bayou). 

WBID 1982A, South Creek at US41 east, looking upstream. 
Draft Nutrient/D.O. TMDLs 

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page B-3

WBID 1984, Catfish Creek (Upstream) 

WBID 1984A, North Creek at US41. 
Draft Nutrient/D.O. TMDLs 

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page B-4

WBID 2009A, Curry Creek at Colonia and Albee Farm 

WBID 2030, Alligator Creek at US41, facing east 
Draft Nutrient/D.O. TMDLs 

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page B-5

WBID 2039, Forked Creek at mobile home park. 

WBID 2042 (Direct Runoff to Bay), ditch behind residences, Florida and Rutgers Streets. 
Draft Nutrient/D.O. TMDLs 

Sarasota Bay/Charlotte Harbor Basin 

September 2005 

Page B-6

WBID 2049, Gottfried Creek at Dearborn St., facing downstream. 
APPENDIX B TMDL Development for Sarasota Bay/Lemon Bay WBIDs Using the 

Reference Watershed Approach 
Watershed Loading Modeling and the Reference Watershed Approach 

For 

Nutrient/BOD/DO TMDLs

In 

Sarasota Bay/Charlotte Harbor Basins 

A Comparison of Two Approaches: 

Reference Watershed versus “Natural Conditions” Scenario 

Prepared by: 

US EPA Region 4 

61 Forsyth Street SW 

Atlanta, Georgia 30303 

September 2005

 
Table of Contents 

1. Introduction 1 

2. Reference Watershed Approach (RWA) 1 

2.1. Concept of a Reference Watershed Approach 1 

2.2. Selection of a Reference Watershed 2 

3. Water Quality Modeling for Reference Watersheds 2 

4. TMDL Computations and Comparison of Approaches 6 

References 7 

List of Tables 

Table 1. Land Use/Cover distributions in the Reference Watersheds 3 

Table 2. Comparison Between Impaired and Reference Watersheds/WBIDs 4 

Table 3. Annual Total Nitrogen Loads by Land Cover for the 

 Reference Watersheds 5 

Table 4. Annual Biochemical Oxygen Demand (BOD) Loads 

 by Land Cover for the Reference Watersheds 5 

Table 5. TMDL Components Using the Reference Watershed Approach 6 

Table 6. Percent Reductions Needed To Achieve Water Quality 

 Objectives- Comparison of Approaches 7 
Loading Function Models and Reference Watershed Approach for TMDL Development Plans 

1

1. Introduction 

The Lemon Bay Watershed Model (LBWM) was used to generate long-term average nutrient and 

BOD loads from nine watersheds in the Sarasota and Lemon Bay Basins. These watersheds were 

selected to represent “reference“ conditions for TMDL development and their predicted loads are 

used as targets for impaired WBIDs in these basins. Existing loads for the impaired watersheds 

were derived using the same model and are presented in the main body of this document. 

The present appendix describes the Reference Watershed Approach (RWA) to TMDL development, 

computes nutrient and BOD TMDLs based on modeling results for impaired and reference 

watersheds in Sarasota/Charlotte Harbor Bay Basins, and finally compares the “natural conditions” 

scenario to RWA approach in terms of percent reductions needed to attain water quality objectives 

in impaired watersheds. In this analysis, the RWA approach is considered to be the most accurate 

since the impairment occurred sometimes after human influences started in the WBIDs. 

2. Reference Watershed Approach (RWA) 

2.1. Concept of a Reference Watershed Approach 

Several states, including Florida, do not have water quality criteria for nutrients, sediments, and 

BOD. For this reason, a number of them (e.g., Pennsylvania, Maryland, New York) have adopted a 

reference watershed approach to identify TMDL endpoints or water quality objectives for these 

parameters in impaired watersheds. The general approach to TMDL development involves 

comparing pollutant loads of the impaired watershed to those (target loads) of a reference 

watershed. The procedure consists at reducing pollutant loadings in the impaired watershed to 

levels consistent with the reference watershed. Such load reductions will ensure that the aquatic 

life use in the impaired watershed is achieved and maintained as evidenced in the reference 

watershed. Further description and example of a Reference Watershed Approach and its use in 

TMDL development plans can be found in Nizeyimana-Louis et al. (2002). 

The reference watershed approach can be used in TMDL determinations in three different ways 

depending on the availability of monitoring data: 

1) If the reference watershed has sufficient monitoring data for the pollutant of interest, the 

target load (or TMDL) is determined by computing average annual loads (target) and 

comparing them to modeling results of impaired watersheds. 

2) In most cases however, both reference and impaired watersheds do not have monitoring 

data. In this instance, impaired and reference watersheds are modeled the same way and 

the resulting predicted pollutant loads are compared to determine load reductions needed to 

remove the impairment. 

3) Sometimes the time when the impairment took place and its cause are known based on 

expert and/or stakeholder knowledge. In this case, rather than using a reference watershed, 

the reference “target” is pollutant loads obtained by modeling the impaired watershed but 

using land use conditions before the impairment occurred (e.g., prior to urbanization) some 

time in the past (reference conditions). Load reductions are performed by comparing 

modeled loads obtained using “present” and “past” land use distributions. 
Loading Function Models and Reference Watershed Approach for TMDL Development Plans 

2

2.2. Selection of a Reference Watershed

In general, three factors should be considered when selecting a suitable reference watershed. 

1) A reference watershed should be a watershed that has been assessed by the State and 

has been determined to attain water quality standards or objectives. If not assessed yet, the 

reference watershed should exhibit lower levels of the pollutant for which TMDL (e.g., 

nutrient) is being determined and satisfy other indicators of impairment for that parameter. 

For example, if a nutrient TMDL is being determined, Chlorophyll A should be at acceptable 

levels and the watershed should not be impaired by dissolved oxygen (DO). 

2) The reference watershed should closely resemble the impaired watershed in terms of 

proximity and physical properties such as land cover/land use, physiographic province, soils, 

and geology. However, the difference in land use distribution is not always taken into 

account because the land use change (e.g. from forest to agriculture or urban land) is often 

the cause of impairment. That is removing the impairment via the use of BMPs and 

engineering structures becomes equivalent to returning the impaired watershed to land use 

conditions similar to those of the reference watershed. 

3) Finally, the size of the reference watershed should be within 0 to 30% of the impaired 

watershed area to minimize the impact of watershed size on loading differences. 

The search for a reference watershed that would satisfy the above characteristics is 

commonly done by means of a desktop screening using several GIS coverages including 

land use distribution maps, State’s 305(b) assessed stream databases, soils, ecoregions, 

and geologic rock types. Since finding a suitable reference watershed for use each time a 

TMDL need to be developed may be time consuming, it is suggested that potential reference 

watersheds be identified ahead of time in different physiographic provinces, ecological 

regions, and geologic formations using statewide or regional spatial datasets (See an 

example in Evans et al., 2002). However, once potential reference watersheds are 

identified, a field investigation should be done to determine the most suitable reference 

watershed to use. 

3. Water Quality Modeling for Reference Watersheds 

The LBWM was previously run for the impaired WBIDs for current and natural land use/cover 

conditions (see main body of the document). The modeling was also performed for the reference 

watersheds as part of this analysis using the same loading rates and attenuation factors as for the 

impaired watersheds. The watersheds used as references for the impaired WBIDs are shown in 

Table 2. A map showing the location of impaired reference watersheds in relation to impaired 

WBIDs is indicated in Figure 1. Detailed land use/cover distributions in the reference watersheds 

are presented in Table 1. All impaired and reference watersheds are located in Sarasota and 

Lemon Bay Basins and within the Southwestern Florida Flatwoods ecoregion, Lowlands, Gaps, and 

Valleys physiographic province, and Shelly Clays, and Sands geologic formation. The impaired and 

corresponding reference watersheds compare well except in land use distributions as evidenced in 

Table 2. 

LBWM modeling results are presented in Tables 3 and 4 respectively for total N and BOD. 
Loading Function Models and Reference Watershed Approach for TMDL Development Plans 

3

Table 1. Land Use/Cover distributions in the Reference Watersheds 

Land Cover Area (acres) 

Land Use 

Ainger 

Creek 

Coral Creek Unnamed 

Ditch 

Blackburn 

Canal 

Catfish 

Bayou 

Agriculture 202 42 754 493 46 

Commercial 85 36 32 56 0 

Extractive 0 0 219 0 0 

Industrial 25 2 0 0 0 

Low-Density Residential 904 73 0 261 0 

Medium-Density Residential 359 144 16 140 0 

High-Density Residential 29 11 0 6 0 

Transportation 11 0 0 38 24 

Institutional 23 0 0 0 0 

Open 3619 781 21 226 2,060 

Recreational 7 89 0 26 0 

Wetland 1133 246 223 37 2,471 

Water (Ponds and Lakes) 135 64 74 72 354 

Water (Conveyance) 104 251 0 0 101 

Total 6637 1,740 1,339 1,355 5,055 
Loading Function Models and Reference Watershed Approach for TMDL Development Plans 

4

Table 2. Comparison Between Impaired and Reference Watersheds/WBIDs 

ATTRIBUTE IMPAIRED WBID REFERENCE 

WATERSHED/WBID 

Elligraw Bayou Catfish Bayou 

Area (acres) 4390 5055 

Dominant Land Use/Covers Residential (54%), Open (10%) Wetland (49%); Open (41%) 

Dominant Hydrologic Soil Groups B/D (69%); D (15%) B/D (44); D (48%) 

South Creek Coral Creek 

Area (acres) 2525 1740 

Dominant Land Use/Covers Open (46%); Residential (31%) Open (45%); Wetlands (14%) 

Dominant Hydrologic Soil Groups B/D (60); C(18%) B/D (50); C (18%) 

North Creek Blackburn Canal 

Area (acres) 1651 1355 

Dominant Land Use/Covers Residential (41); Recreational (21) Agriculture (36%); Residential 

(30%) 

Dominant Hydrologic Soil Groups B/D (65%); D (15%) B/D (75%); D (19%) 

Curry Creek Unnamed Ditch 

Area (acres) 1131 1339 

Dominant Land Use/Covers Residential (53%); Agriculture 

(10%) 

Agriculture (56%); Wetlands 

(17) 

Dominant Hydrologic Soil Groups B/D (57%); A (18%) B/D (59%); D (35%) 

Gottfried Creek Ainger Creek 

Area (acres) 7207 6637 

Dominant Land Use/Covers Open (49%), Wetland (17%) Open (54%), Wetlands (17%) 

Dominant Hydrologic Soil Groups B/D (59%); D (23%) B/D (70%) and D (21%) 

Forked Creek Ainger Creek 

Area (acres) 7360 6637 

Dominant Land Use/Covers Open (32%); Residential (26%) Open (54%), Wetlands (17%)

Dominant Hydrologic Soil Groups B/D (65%), D (20%) B/D (70%) and D (21%) 

Alligator Creek Ainger Creek 

Area (acres) 5761 6637 

Dominant Land Use/Covers Residential (56); Open (12%) Open (54%), Wetlands (17%) 

Dominant Hydrologic Soil Groups B/D (73%), D (14%) B/D (70%) and D (21%) 

Direct Runoff to Bay Coral Creek 

Area (acres) 1683 1740 

Dominant Land Use/Covers Residential (65%); Open (18%) Open (45%); Wetlands (14%) 

Dominant Hydrologic Soil Groups B/D (57%); C (18%) B/D (50); C (18%) 
Loading Function Models and Reference Watershed Approach for TMDL Development Plans 

5

Table 3. Annual Total Nitrogen Loads by Land Cover for the Reference Watersheds 

Total Nitrogen (Kg) 

Land Use Ainger 

Creek 

Coral 

Creek 

Unnamed 

Ditch 

Blackburn 

Canal 

Catfish 

Bayou 

Agriculture 140.8 4.7 147.1 99.4 9.4 

Commercial 133.7 51.2 46.9 60.4 0.0 

Extractive 0.0 0.0 285.3 0.0 0.0 

Industrial 32.6 3.1 0.0 0.0 0.0 

Low-Density Residential 547.4 34.8 0.0 94.4 0.0 

Medium-Density Residential 292.6 148.6 22.4 106.2 0.0 

High-Density Residential 43.9 19.6 0.0 9.6 0.0 

Transportation 40.8 0.0 0.0 125.0 88.5 

Institutional 23.9 0.0 0.0 0.0 0.0 

Open 1228.9 116.5 3.7 26.6 272.2 

Recreational 1.5 46.6 0.0 5.7 0.0 

Wetland 825.7 144.3 123.1 17.7 1589.7 

Water (Ponds and Lakes) 0.0 0.0 0.0 0.0 0.0 

Water (Conveyance) 148.8 331.0 0.0 0.0 142.3 

Total 3460.7 900.3 628.4 545.0 2102.1 

Table 4. Annual Biochemical Oxygen Demand (BOD) Loads by Land Cover for the 

Reference Watersheds 

Annual BOD Loads (Kg) 

Land Use Ainger 

Creek 

Coral 

Creek 

Unnamed 

Ditch 

Blackburn 

Canal 

Catfish 

Bayou 

Agriculture 11.0 11.0 341.6 230.8 19.7 

Commercial 160.4 160.4 147.0 202.2 0.0 

Extractive 0.0 0.0 2898.8 0.0 0.0 

Industrial 21.8 21.8 0.0 0.0 0.0 

Low-Density Residential 66.6 66.6 0.0 201.7 0.0 

Medium-Density Residential 304.9 304.9 45.9 224.3 0.0 

High-Density Residential 62.2 62.2 0.0 30.5 0.0 

Transportation 0.0 0.0 0.0 450.7 288.6 

Institutional 0.0 0.0 0.0 0.0 0.0 

Open 198.9 198.9 6.3 45.4 420.1 

Recreational 95.6 95.6 0.0 12.5 0.0 

Wetland 316.2 316.2 269.9 38.8 3150.7 

Water (Ponds and Lakes) 0.0 0.0 0.0 0.0 0.0 

Water (Conveyance) 2326.3 2326.3 0.0 0.0 957.8 
Loading Function Models and Reference Watershed Approach for TMDL Development Plans 

6

Total 3563.8 3563.8 3709.3 1436.8 4837.0 

4. TMDL Computations and Comparison of Approaches 

Total N and BOD loads computed for the reference watersheds were used as target loads (Table 5, 

Column 4). A 10% margin of safety (MOS) was applied to compute load allocations (LAs). The 

existing loads from impaired watersheds (see main document) are presented in Table 5 Column 2, 

below. Total phosphorus loads were not computed since nitrogen is the limiting factor in these 

estuarine creek systems. 

Table 6 compares predevelopment land use cover conditions and the Reference Watershed 

Approach in terms of percent reductions needed to meet water quality objectives for nutrient and 

water quality standard for DO. As it appears, the two approaches compare well. At the average, 

there is close agreement between total N percent reductions obtained using natural conditions 

(36.5%) and the Reference watershed Approach (32.8%). The difference between the two methods 

for BOD, 16 percent, can be considered relatively low given the high uncertainty associated 

assessment assumptions used in the two approaches. 

Table 5. TMDL Total N and BOD Components Using the Reference Watershed Approach 

WLA2

Waterbody 

(WBID) Parameter1

Existing 

Load 

(kg/year) 

Target 

Load-TMDL 

(kg/year) LA 

(kg/year) 

Continuous 

(kg/year) 

MS4 

(Percent 

reduction) 

MOS4 

(kg/year)

 

Percent 

Reduction5

TN 2525.9 2102.1 1891.8 N/A3 25.1 210.2 25.1 Elligraw Bayou

6

BOD 12931.6 4837.0 4353.3 N/A3

 70.6 483.7 66.3 

Curry Creek TN 1301.0 628.4 565.6 N/A3 56.5 62.8 56.5 

North Creek TN 1116.1 545.0 490.5 N/A3 56.1 54.5 56.1 

South Creek TN 1816.6 1740.0 1566.0 N/A3 13.8 174 13.8 

Gottfried Creek TN 3025.1 3460.7 3114.7 N/A3 0.0 346.0 0.0 

Forked Creek TN 4234.9 3460.7 3114.7 N/A3 26.5 346.0 26.5 

Direct Runoff 

to Bay TN 1414.2 900.30 810.3 N/A3

 41.7 90.3 41.7 

TN 5370.4 3460.7 3114.7 N/A3

A 42.0 346.0 42.0 lligator 

Creek 6

BOD 15727.5 7200.4 6480.4 N/A3

 58.8 720.0 58.8 

Notes: 1TN = total nitrogen; BOD = Biological Oxygen Demand 

2

WLA component is separated into load from continuous NPDES facilities (e.g., WWTPs) and load 

from MS4s. 

3

N/A = not applicable 

4

Margin of Safety is implicit due to conservative assumptions made in determining TMDL targets. 

5

Percent reduction from current conditions to achieve the target concentration for that nutrient. 

6

Elligraw Bayou and Alligator Creek are impaired by DO. Reducing nutrient and BOD as indicated in 

the table will assure that these waterbodies meet DO water quality standards. 
Loading Function Models and Reference Watershed Approach for TMDL Development Plans 

7

Table 6. Percent Reductions Needed To Achieve Water Quality Objectives- Comparison 

of Approaches 

Percent Reduction In Total Nitrogen (N) Loads 

Approach 

IMPAIRED 

WATERBODY 

Natural Conditions Reference Watershed 

Elligraw Bayou 28.8 25.1 

South Creek 48.2 13.8 

North Creek 47.1 56.1 

Curry Creek 62.9 56.5 

Runoff to Bay 54.7 42.7 

Gottfried 2.0 0.0 

Forked 20.0 26.5 

Alligator 28.2 42.0 

 - Mean 36.5 32.8 

Percent Reduction in Biochemical Oxygen Demand (BOD) Loads 

Approach 

IMPAIRED 

WATERBODY 

Natural Conditions Reference Watershed 

Elligraw Bayou 70.6 66.3 

South Creek 57.9 32.0 

North Creek 48.8 52.7 

Curry Creek 65.8 20.1 

Runoff to Bay 45.2 0.0 

Gottfried 16.2 7.6 

Forked 37.4 33.9 

Alligator 57.8 58.8 

 - Mean 50.0 34.0 

References

Evans, B.M.. et al. 2003. A Comprehensive GIS-based modeling approach for predicting nutrient 

loads in watersheds. J. Spatial Hydrology. 2 (2). 234-264. 

Nizeyimana-Louis, E. and B.M. Evans. 2002. Total maximum daily load (TMDL) development 

plans for urbanized watersheds in Pennsylvania. Case Study: Neshaminy watershed, Southeaster 

PA. 282 pp + appendices.