FINAL

 

FLORIDA DEPARTMENT OF ENVIRONMENTAL PROTECTION 

Division of Water Resource Management

South West DISTRICT • Tampa Bay Tributaries Basin 

TMDL Report 

Nutrient TMDL for 

Lake Hunter 

Dr. Andrzej Baniukiewicz 

Douglas Gilbert 

September 2004 

Final TMDL Report Lake Hunter, September, 2004 

Acknowledgments 

This study could not have been accomplished without the significant contributions from 

other staff within the Watershed Assessment Section. The authors have particular 

appreciation for Barbara Donner’s contributions. Most of the basin delineation’s, 

landuse aggregations, and much of the data gathering are a result of her efforts. The 

Department also recognizes the substantial support and assistance provided by the City 

of Lakeland staff, particularly Bal Sukhraj and Karen Valler for their contributions 

towards understanding the basin boundaries, issues, history, and processes at work in 

the Lake Hunter watershed. 

Editorial assistance provided by Jan Mandrup-Poulsen and Daryll Joyner. 

Map production assistance provided by Barbara Donner. 

For additional information on the watershed management approach and impaired 

waters in the Lake Hunter Basin, contact: 

Tom Singleton, Environmental Consultant 

Florida Department of Environmental Protection 

Bureau of Watershed Management 

Watershed Planning and Coordination Section 

2600 Blair Stone Road, Mail Station 3565 

Tallahassee, FL 32399-2400 

Jan.Mandrup-Poulsen@dep.state.fl.us 

Phone: (850) (245-8448); Suncom: (205-8448) 

Fax: (850) 245-8434 

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

Douglas Gilbert 

Florida Department of Environmental Protection 

Bureau of Watershed Management 

Watershed Assessment Section 

2600 Blair Stone Road, Mail Station 3555 

Tallahassee, FL 32399-2400 

douglas.gilbert@dep.state.fl.us 

Phone: (850) 245-8450; Suncom: 205-8450 

Fax: (850) 245-8536 

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Final TMDL Report Lake Hunter, September, 2004 

Contents 

CHAPTER 1: INTRODUCTION................................................................................................. 1 

1.1 Purpose of Report ............................................................................................................1 

1.2 Identification of Waterbody .............................................................................................1 

1.3 Background Information..................................................................................................1 

CHAPTER 2: STATEMENT OF WATER QUALITY PROBLEM ........................................... 3 

2.1 Legislative and RuleMaking History ...............................................................................3 

2.2 Information on Verified Impairment................................................................................3 

CHAPTER 3. DESCRIPTION OF APPLICABLE WATER QUALITY STANDARDS AND 

TARGETS....................................................................................................................................... 7 

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

3.2 Interpretation of the Narrative Nutrient Criterion ..........................................................7 

CHAPTER 4: DETERMINATION OF CURRENT LOADING ................................................ 9 

4.1 Overview............................................................................................................................9 

4.2 Types of Sources..............................................................................................................9 

4.3 Potential Sources of Nutrients in the Lake Hunter Watershed ....................................9 

4.4 Modeling of Nonpoint Sources......................................................................................11 

Discharge from Lake Hunter.................................................................................................15 

Runoff from Lake Hunter Basin............................................................................................18 

Volume of Water in Lake Hunter ..........................................................................................18 

Discharges from Lakes Beulah and Wire............................................................................19 

Other Sources of water: Groundwater................................................................................20 

Mass from Lakes Wire and Beulah ......................................................................................24 

Mass from Lake Hunter watershed ......................................................................................25 

Mass from Ground water ......................................................................................................26 

Annual Water and Mass Balance Results ...........................................................................27 

CHAPTER 5: DETERMINATION OF ASSIMILATIVE CAPACITY ................................... 30 

5.1 Critical Conditions..........................................................................................................30 

5.2 Determination of Natural Background..........................................................................31 

5.3 Determination of Assimilative Capacity .......................................................................37 

CHAPTER 6: DETERMINATION OF THE TMDL................................................................ 42 

6.1 Expression and Allocation of the TMDL.......................................................................42 

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Final TMDL Report Lake Hunter, September, 2004 

6.2 Load Allocation (LA).......................................................................................................43 

6.3 Wasteload Allocation (WLA)..........................................................................................43 

6.4 Margin of Safety (MOS) ..................................................................................................43

CHAPTER 7: NEXT STEPS: IMPLEMENTATION PLAN DEVELOPMENT AND 

BEYOND ...................................................................................................................................... 44 

APPENDICES ............................................................................................................................. 46 

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Final TMDL Report Lake Hunter, September, 2004 

Tables 

Table 3.1 Land use in Lake Hunter Watershed............................................................................... 12 

Table 3.2 Rainfall and other data applied to water budget 1988 – 2002........................................ 28 

Table 3.3 Summary of Groundwater Component.(1988 – 2002).................................................. 29 

Table 4.1 Annual Average TN, TP, Chl a, TSI, missing TN and TP for Lake Hunter after 80 

 Percent reduction............................................................................................................ 49 

Table 4.2 Summary Loads, Concentrations, and TSI for Current, Natural Background, 

 Critical worst and best years, and TMDL (80 percent reduction)................................... 52 

Table 4.3 Summary of Loads From All Sources (Lakes Wire/Beulah, Groundwater, Lake Hunter 

Watershed, Septic Tanks, Direct Rainfall On Lake, and Internal Recycling) for 

 Current, Natural Background and TMDL (80% Reduction)............................................. 53 

Table 4.4 Mass of TN and TP discharged from Lakes Wire and Beulah into Lake Hunter 

 (1996 – 2002).................................................................................................................... 55 

Table 4.5 Lake Hunter Watershed Loadings by Source Category (1996 – 2002)............................. 56 

Table 5.1 TMDL Allocation.............................................................................................................., 59 

Figures 

Figure 1.1 City of Lakeland with Lakes Hunter, Wire, and Beulah................................................. 4 

Figure 2.1 Level of TP in Lake Hunter last 25 years...................................................................... 5 

Figure 2.2 Level of TN in Lake Hunter last 15 years.................................................................... 6 

Figure 2.3 Level of Chl a in Lake Hunter last 19 years.................................................................. 6 

Figure 2.4 Annual TSI for Lake Hunter 1991-2002...................................................................... 8 

Figure 3.1 Map of Lake Hunter Watershed with Land use........................................................... 13 

Figure 3.2 Turbidity in Lake Hunter 1978 – 2002........................................................................ 18 

Figure 3.3 Correspondence in Calculated outflows from Lake Hunter....................................... 20 

Figure 3.4 Annual rainfall versus outflow from Lake Hunter...................................................... 22 

Figure 3.5 Bathymetric Map of Lake Hunter............................................................................... 23 

Figure 3.6 Annual discharges from Lakes Wire and Beulah......................................................... 25 

Figure 3.7 Cumulative discharges from Lakes Wire and Beulah and accretion to groundwater.. 27 

Figure 3.8 Groundwater interactions with Lake Hunter................................................................ 30 

Figure 3.9 Comparison of modeled TP load to measured load for Lake Hunter (1989 – 2002)... 33 

Figure 3.10 Comparison of modeled TN load to measured load for Lake Hunter (1989 – 2002).. 33 

Figure 4.1 Annual Average Color Lake Hunter (1984 – 2002)...................................................... 37 

Figure 4.2 Annual Average TSI (1991 – 2002).............................................................................. 38 

Figure 4.3 Approximation of Groundwater Component by Eq 6 (1989 – 2002)........................... 39 

Figure 4.4 Idealization of natural outflow from Lake Hunter........................................................ 40 

Figure 4.5 Comparision of modeled Secchi to measured (1989 – 2002)........................................ 44 

Figure 4.6 Comparision of modeled Chl a to measured (1989 – 2002)........................................... 45 

Figure 4.7 Variation f TSI with Rainfall Natural Background Conditions..................................... 46 

Figure 4.8 Comparision of Lake Hunter TSI to Cumulative discharge 

 from Lakes Wire and Beulah......................................................................................... 51 

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Final TMDL Report Lake Hunter, September, 2004 

 v

Web sites 

FLORIDA DEPARTMENT OF ENVIRONMENTAL PROTECTION, BUREAU OF 

WATERSHED MANAGEMENT 

TMDL Program 

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

Identification of Impaired Surface Waters Rule 

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

STORET Program 

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

2000 305(b) Report 

http://www.dep.state.fl.us/water/305b/index.htm

Criteria for Surface Water Quality Classifications 

http://www/dep.state.fl.us/legal/legaldocuments/rules/ruleslistnum.htm

Basin Status Report for the Lake Hunter Basin 

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

Assessment Report for the Lake Hunter Basin 

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

Allocation Technical Advisory Committee (ATAC) Report 

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

U.S. ENVIRONMENTAL PROTECTION AGENCY, NATIONAL STORET PROGRAM 

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

Final TMDL Report Lake Hunter, September, 2004 

Chapter 1: INTRODUCTION 

1.1 Purpose of Report 

This report presents the TMDL for nutrients for Lake Hunter (Lake) in the Hillsborough River 

Basin (Figure 2.1). The Lake was verified as impaired by nutrients using the methodology in the 

Identification of Impaired Surface Waters Rule (IWR, Rule 62-303, Florida Administrative Code), 

and was included on the Verified List of impaired waters for the Hillsborough River Basin that 

was adopted by Secretarial Order on May 27, 2004. The TMDL establishes the allowable 

loadings to the Lake that would restore the waterbody so that it meets its applicable water 

quality criteria for nutrients. 

1.2 Identification of Waterbody 

Lake Hunter is located inside the City of Lakeland, Polk County, Florida, at Latitude 28o 02” 06’ 

and Longitude 81o 58” 00’ (Figure 1.1). The estimated annual average surface area of the Lake 

is less than 100 acres, with an average depth of six feet. The entire surface water drainage 

basin of the Lake is approximately 0.8 square miles. The topographic elevation of the water 

surface is near 160 feet NGVD. There are no tributaries flowing into the Lake, although there 

are a number of stormwater outfalls that discharge runoff from the basin into the Lake. 

Additionally, stormwater discharges from two other lakes (Lakes Beulah and Wire) are piped 

into Lake Hunter (Figure 1.1). 

For assessment purposes, the Department has divided the Hillsborough River Basin into water 

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

or stream reach. The Lake has been given the WBID number of 1543. 

1.3 Background Information 

The TMDL Report for the Lake is part of the implementation of the Florida Department of 

Environmental Protection’s (Department) watershed management approach for restoring and 

protecting water resources and addressing Total Maximum Daily Load (TMDL) Program 

requirements. The watershed approach, which is implemented using a cyclical management 

process that rotates through the state’s fifty-two river basins over a five-year cycle, provides a 

framework for implementing the requirements of the 1972 federal Clean Water Act and the 1999 

Florida Watershed Restoration Act (Chapter 99-223, Laws of Florida). 

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

and still meet the waterbody’s designated uses. A waterbody that does not meet its designated 

uses is defined as impaired. TMDLs must be developed and implemented for each of the 

state’s impaired waters, unless the impairment is documented to be a naturally occurring 

condition that cannot be abated by a TMDL or unless a management plan already in place is 

expected to correct the problem. 

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Final TMDL Report Lake Hunter, September, 2004 

Lake Hunter

Lake Beulah

Lake Wire

Figure 1.1: Lakeland area with drainage basins of the lakes including Lakes Hunter WBID 1543, 

Wire, and Beulah. 

The development and implementation of a Basin Management Action Plan, or B-MAP, to reduce 

the amount of pollutants that caused the impairment will follow this TMDL Report. These 

activities will depend heavily on the active participation of the water management district, local 

governments, businesses, and other stakeholders. The Department will work with these 

organizations and individuals to undertake or continue reductions in the discharge of pollutants 

and achieve the established TMDLs for the impaired Lake. 

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Final TMDL Report Lake Hunter, September, 2004 

Chapter 2: STATEMENT OF WATER QUALITY 

PROBLEM 

2.1 Legislative and RuleMaking History 

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

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

establish a TMDL for each pollutant causing the impairment of the listed waters on a schedule. 

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

The list of impaired waters in each basin, referred to as the Verified List, is also required by the 

Florid Watershed Restoration Act (Subsection 403.067[4)] Florida Statutes [F.S.]), and the 

state’s 303(d) list is amended annually to include basin updates. 

Florida’s 1998 303(d) list included 21 waterbodies in the Hillsborough River Basin. However, 

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

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

methodology to identify impaired waters. After a long rule-making process, the Environmental 

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

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

2001.

2.2 Information on Verified Impairment 

The Department used the IWR to assess water quality impairments in the Lake Hunter 

watershed and verified that the Lake was impaired for nutrients. Lake Hunter was verified as 

impaired based on elevated annual average Trophic State Index (TSI) values over the 

verification period (the Verified Period for the Group 2 basins is from January 1, 1996 to June 

30, 2003). The TSI is calculated based on concentrations of TP, TN, and Chl a as follows: 

 

CHLATSI = 16.8 + 14.4 * LN(Chl a) Chl a in μg/L 

TNTSI = 56 + 19.8 * LN(N) N in mg/L 

TN2TSI = 10 * [5.96 + 2.15 * LN(N + 0.0001)] 

TPTSI = 18.6 * LN(P * 1000) – 18.4 P in mg/L 

TP2TSI = 10 * [2.36 * LN(P * 1000) – 2.38] 

If N/P > 30, then NUTRTSI = TP2TSI

If N/P < 10, then NUTRTSI = TN2TSI

if 10< N/P < 30, then NUTRTSI = (TPTSI + TNTSI)/2 

TSI = (CHLATSI + NUTRTSI)/2 (TSI has no units) 

The following analysis of the eutrophication-related data for Lake Hunter used all of the 

available data from 1988 – 2002 for which records of TP, TN, and Chl a were sufficient to 

calculate annual average conditions. To calculate the TSI for a given year, there must be at 

least one sample in each season of the year. This caused the elimination of the years 1988, 

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Final TMDL Report Lake Hunter, September, 2004 

1989, 1990, and 1995 from the analysis of TSI for Lake Hunter. Additionally, as the verified 

period ends in the middle of 2003, annual averages were not determined for 2003. 

Annual average TSI values exceeded the IWR threshold level of 60 in all of the years with 

sufficient data (Figure 2.5), and averaged 79.7 over the period. Exceeding 60 in any one year 

of the verified period would have been sufficient to determine the Lake was impaired for 

nutrients. 

TSI during period 1991 - 2002

68

70

72

74

76

78

80

82

84

86

88

1991 1992 1993 1994 1996 1997 1998 1999 2000 2001 2002

Years

TSI

Figure 2.1. TSI for Lake Hunter calculated from annual average concentrations of TP, TN, and Chl a.

Temporal trends in individual constituents of the TSI were examined using plots of the annual 

average concentrations and 2 point moving average of total phosphorous (TP), total nitrogen 

(TN), and chlorophyll a (Chl a) (Figures 2.2, 2.3, and 2.4). From the late 1980’s through 1994, 

TN and TP concentrations appear fairly stable. However, both TN and TP concentrations 

began to increase starting in 1994. For Chl a, the 1990’s appear to be a period of gradually 

increasing concentrations. 

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Final TMDL Report Lake Hunter, September, 2004 

Annual concentration of phosphorus in 1978 - 2002

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

1978198019821984198619881990199219941996199820002002

Year

Concentration (mg/L)

Figure 2.2. Level of phosphorus in Lake Hunter during past 25 years. 

Concentration of nitrogen in 1988 - 2002

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

Years

Concentration (mg/L)

Figure 2.3. Level of nitrogen in Lake Hunter during past 15 years. 

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Final TMDL Report Lake Hunter, September, 2004 

 6

Annual chlorophyll concentration during 1984 - 2002

0

20

40

60

80

100

120

140

160

180

1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

Year

Concentration (ug/L)

Figure 2.4. Chlorophyll a measured in Lake Hunter during past 19 years. 
Final TMDL Report Lake Hunter, September, 2004 

Chapter 3. DESCRIPTION OF APPLICABLE WATER 

QUALITY STANDARDS AND TARGETS 

3.1 Classification of the Waterbody and Criteria Applicable to the TMDL 

Florida’s surface water is protected for five designated use classifications, as follows: 

Class I Potable water supplies 

Class II Shellfish propagation or harvesting 

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

Class IV Agricultural water supplies 

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

currently in this class) 

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

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

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

criterion [Rule 62-302.530(48)(b), FAC]. 

3.2 Interpretation of the Narrative Nutrient Criterion 

Florida’s nutrient criterion is narrative only — nutrient concentrations of a body of water shall not 

be altered so as to cause an imbalance in natural populations of aquatic flora or fauna. 

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

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

for lakes based on annual average TSI levels, these thresholds are not standards and need not 

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

IWR thresholds were developed using statewide average conditions, the IWR (Subsection 62-

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

accurately reflect conditions beyond which an imbalance in flora or fauna occurs in the 

waterbody. 

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

chlorophyll concentration, and total phosphorus concentration and was used to describe a lake’s 

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

phosphorus limited. In Florida, because the local geology produced a phosphorus rich soil, 

nitrogen can be the sole or co-limiting factor for phytoplankton population in some lakes. In 

addition, because of the existence of dark-water lakes in the state, using Secchi depth as an 

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

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

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

The index was adjusted so that a Chl a concentration of 20 ug/L was equal to a TSI value of 60. 

A TSI of 60 was then set as the threshold for nutrient impairment for most lakes (for those with a 

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Final TMDL Report Lake Hunter, September, 2004 

color higher than 40 platinum cobalt units) because, generally, the phytoplankton may switch to 

communities dominated by blue-green algae at Chl a levels above 20 ug/L. These blue-green 

algae are often an unfavorable food source to zooplankton and many other aquatic animals. 

Some blue-green algae may even produce toxins, which could be harmful to fish and other 

animals. In addition, excessive growth of phytoplankton and the subsequent death of these 

algae may consume large quantities of dissolved oxygen and result in anaerobic condition in 

lakes, which makes conditions in the impacted lake unfavorable for fish and other wildlife. All of 

these processes may negatively impact the health and balance of native fauna and flora. 

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

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

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

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

represent the levels at which nutrient impairment occurs. For this study, the Florida Department 

of Environmental Protection (DEP) used modeling to estimate the natural background TSI by 

setting land uses to natural or forested land, and then compared the resulting TSI to the IWR 

thresholds. If the natural background TSI can be determined, then an increase of 5 TSI units 

above natural background will be used as the water quality target for the TMDL. Otherwise, the 

IWR threshold TSI of 60 will be established as the target for TMDL development. 

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Final TMDL Report Lake Hunter, September, 2004 

Chapter 4: DETERMINATION OF CURRENT LOADING 

4.1 Overview 

The external load assessment was intended to determine the loading characteristics of the 

various sources of pollutants to the Lake. Assessing the external load entailed assessing land 

use patterns and rainfall to determine the volume, concentration, timing, location, and 

underlying nature of the nonpoint and atmospheric sources of nutrients to the Lake. 

4.2 Types of Sources 

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

source subcategories, or individual sources of the pollutant of concern in the watershed and the 

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

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

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

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

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

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

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

silviculture, and mining; discharges from failing septic systems; and atmospheric deposition. 

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

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

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

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

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

the federal and state stormwater programs). 

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

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

stormwater systems requiring an NPDES stormwater permit when allocating pollutant load 

reductions required by a TMDL (see Section 6.1). However, the methodologies used to 

estimate nonpoint source loads do not distinguish between NPDES stormwater discharges and 

non-NPDES stormwater discharges, and as such, this source assessment section does not 

make any distinction between the two types of stormwater. 

4.3 Potential Sources of Nutrients in the Lake Hunter Watershed 

Point Sources 

There are no permitted wastewater treatment facilities that discharge nutrient loads into the 

Lake. 

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Final TMDL Report Lake Hunter, September, 2004 

Municipal Separate Storm Sewer System Permittees 

Within the Lake Hunter Basin, the stormwater systems owned and operated by local 

governments and the Florida Department of Transportation within the City of Lakeland are 

covered by an NPDES MS4 permit. Several other local governments in the basin have also 

applied for coverage under the Phase 2 NPDES MS4 permit. 

Land Uses and Nonpoint Sources 

Unlike traditional point source effluent loads, nonpoint source loads enter at so many locations 

and exhibit such large temporal variation that a direct monitoring approach is often infeasible. 

For this project, all nonpoint sources were evaluated by use of a watershed and lake modeling 

approach. Table 4.1 shows the acreage of the various land use categories examined. Figure 

4.1 shows the drainage basin of the Lake and the spatial distribution of the land uses shown in 

Table 4.1. The predominate land coverage is Medium density Residential (59%) with general 

Urban second (27.3%) and Transportation, Communication, and Utilities following at 9.1 

percent. The 93.4 acres of the Lake itself were not included as part of the watershed in Table 

4.1. 

Table 4.1: Land Cover Distribution 

CODE LANDUSE ACRES 

1000 Urban and Built-Up 167.07

Low Density Residential 0.00

Medium Density Residential 360.96

High Density Residential 11.72

2000 Agriculture 0.00

3000 Rangeland 0.00

4000 Forest 3.42

5000 Water 0.00

6000 Wetlands 12.84

7000 Barren Land 0.00

8000 Transportation, Communications, and Utilities 55.45

TOTAL SUB-WATERSHED ACRES: 611.44

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Final TMDL Report Lake Hunter, September, 2004 

Figure 4.1. Map of drainage basin of Lake Hunter and land uses 

4.4 Modeling of Nonpoint Sources 

A spreadsheet model based on the governing equations contained in the Watershed 

Assessment Model (WMM) was used to estimate loading from nonpoint sources (except septic 

tanks). For the evaluation of septic tanks, the methods of Douglas Haith, Ross Mandel, and 

Ray Shyan Wu [1] were used. It was assumed that the effluent from septic systems infiltrates 

into the soil and enters the shallow saturated zone. Effluent nitrogen is converted to nitrate, and 

except for removal by plant uptake, the nitrogen is transported to the receiving water by 

groundwater discharge. Conversely, phosphate in the effluent is adsorbed and retained by the 

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Final TMDL Report Lake Hunter, September, 2004 

soil and hence does not reach the lake. The nitrogen load to groundwater in a month (m) as 

(kg) is: 

 Nm = 0.001 am dm(e – um) (1) 

Where: 

am = population served by septic tanks, 

dm = number of days in month m, 

e = per capita daily nutrient load in septic tank effluent (assumed 12 g/day), and 

um = per capita daily nutrient uptake by plants in month m (assumed 1.6 g/d during 

growing season and zero during non-growing season). 

It was assumed that there are seven months of growing season in Florida. The City of Lakeland 

provided the estimates of the number of septic tanks. It was further assumed that the average 

household has four persons. 

As noted previously, the governing equations from the Watershed Management Model (WMM) 

were incorporated into a spreadsheet model for use in the Lake Hunter Basin to estimate TN 

and TP loading. Development of the WMM model was originally funded by DEP under contract 

to Camp Dresser and McKee (CDM). The equations in the WMM are designed to estimate 

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

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

fundamental assumption of the equations is that the stormwater runoff from any given land use 

is in direct proportion to annual rainfall and is dictated by the portion of the land use category 

that is impervious and the runoff coefficients of both pervious and impervious area. 

The governing equation is: 

(1) RL = [Cp + (CI – Cp) IMPL] * I 

Where: 

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

IMPL = fractional imperviousness of land use L; 

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

CP = pervious area runoff coefficient; and 

CI = impervious area runoff coefficient. 

The equation estimates pollutant loadings based on nonpoint pollution loading factors 

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

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

following equation: 

(2) ML = EMCL * RL * K 

Where: 

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

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

by land use and pollutant; 

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

Equation (1) (in/yr); and 

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Final TMDL Report Lake Hunter, September, 2004 

K = 0.2266, a unit conversion constant. 

Data required include: 

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

• Percent impervious area of each land use category 

• EMC for each pollutant type and land use category 

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

• Annual precipitation, and 

• Point source flows and pollutant concentrations.

Calibration is normally conducted on both runoff quantity and quality. This was a two-step 

procedure since the water quality calibration is a function of the predicted runoff volumes. 

Calibration of water quantity is usually achieved through adjusting the pervious and impervious 

area runoff coefficients. Typical ranges of runoff coefficients are 0.05 – 0.30 for pervious area 

(WMM User’s Manual: 1998) and 0.85 – 1.0 for impervious area (Linsley and Franziani, 1979). 

After the water quantity calibration, water quality should be calibrated by adjusting the pollutant 

delivery ratio – the percent quantity of pollutant in the surface runoff that is eventually delivered 

to the destination waterbody. 

Atmospheric Deposition 

Rainfall records for the watershed were taken from the NOAA database, CIRRUS. The rainfall 

station used was located in Lakeland, FL. (UCAN: 4017, COOP: 084797). In 2000, the rainfall 

station was relocated to the Lakeland Linder Regional Airport. This resulted in a few months of 

missing data. These rainfall data gaps were filled by taking records from the weather station in 

Plant City, located about eight miles from the Lake. Annual rainfall averages 54.48 inches, with 

values ranging from 38.26 inches to 67.13 inches over the period under study (Table 4.2). 

Evaporation rate data were primarily obtained from a University of Florida Experimental Station 

about 15 miles northeast of the Lake. Some gaps were filled by data obtained from the 

Archibald Biological Station, located southeast of Lake Hunter. The available data were Class A 

pan evaporation data. Class A Pan’s have the following characteristics: 4 foot diameter, 10 

inches deep, well-ventilated (bottom raised 10 inches off the ground), and filled an eight inch 

depth. The “Pan” is undoubtedly the most widely used evaporation instrument today. However, 

hydrologists and scientists are generally interested in the evaporation rate over land or from an 

open water body like a lake. The pan evaporation rate should be adjusted to estimate the 

actual evaporation from land or open water due to pan boundary effects (greater radiant energy 

intercepted by the side wall and shallow bottom of the pan) and a lower water vapor pressure in 

the air above the land than in the air above a lake’s surface. For these reasons, evaporation 

from an open water surface (E) is often estimated from the pan evaporation (Ep) as : E = KEp, 

where K is the pan coefficient. This adjustment of the measured pan rate was incorporated into 

this study. The average Pan coefficient for the United States is 0.7 [2]. For Lake Hunter, the 

value of 0.8 followed the estimate of 0.81 for Lake Okeechobee, Florida. Annual evaporation 

rates varied from 54.72 inches to 63.52 inches (Table 4.2). 

Wet deposition data were obtained from the Polk County Natural Resource Division, Bartow, 

Florida. The data were collected at a station located at Lake Cannon in the Winter Haven Chain 

of Lakes, around 10 miles east of Lake Hunter. The nearly 40 TN and 40 TP rainfall samples 

collected during 1988 – 2002 averaged 0.680 and 0.026 mg/L, respectively. 

 13
Final TMDL Report Lake Hunter, September, 2004 

Sediment Flux 

For calculation of the sedimentation rate, the Vollenweider model described in [5] and two W. 

Walker models included in the Bathtub model [6] were considered. After reviewing a report by 

Knight on improvement of water quality in South Florida wetlands [7] (primarily due to 

sedimentation) the second order Walker models for phosphorus and nitrogen sedimentation 

were selected. In the case of phosphorus; 

 Cp = [(1 + 4 a cf Cpo τ)

0.5 – 1]/ (2 a cf τ) (3) 

Where: 

Cpo is the phosphorus concentration for incoming water, 

τ is residence time, 

cf is a calibration factor, and 

a is the intercept of the phosphorus sedimentation term, in this case 

 0.17 Qs / (Qs + 13) (4) 

Symbol Qs stands for the surface overflow rate, defined as the discharge from the lake per one 

unit of surface area. 

The sedimentation of sediment-attached nitrogen was estimated by an equation similar to Eq. 

(3). However, this time the intercept of the nitrogen sedimentation term was 

 0.0045 Qs / (Qs + 7.2) (5) 

In this report, the term internal recycling refers to internally derived phosphorus. The most 

important source comes from bottom sediments that release phosphorus when the surrounding 

area goes anoxic. Additionally, in relatively shallow prairie reservoirs, phosphorus is recycled 

from bottom sediments from circular eddies generated by wind. Yousef reported that a 75-hp 

boat motor could re-suspend fine clay sediments to a depth of over 10 feet [8]. However, resuspension may be negligible in Lake Hunter because the amount of accumulated sediments in 

the lake were reduced by a lake drawdown in 1983. The total drawdown of the Lake, conducted 

by the Division of Freshwater Fisheries of the Florida Fish and Wildlife Conservation 

Commission, exposed the bottom sediments through the spring of 1984. Bottom sediments 

were removed, followed by chemical treatment and mechanic removal of cattails [8]. 

The annual-average phosphorus concentration and turbidity of lake water usually drops during 

the years following this type of treatment. For example, when nutrient-rich sediments were 

removed from Lake Trummen in Sweden, the total phosphorus concentration dropped sharply 

and remained fairly stable for at least 18 years [9]. However, a similar lowering of the 

phosphorus concentration and turbidity did not take place in Lake Hunter, as shown in Figures 

2.2 and 4.2. 

 14
Final TMDL Report Lake Hunter, September, 2004 

Annual turbidity during 1978 - 2002

0

5

10

15

20

25

30

35

40

45

50

1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

Years

Turbidity

Figure 4.2 Turbidity in Lake Hunter during 1978 - 2002 

Water Budget: Flows and Volumes 

An essential part of the analysis of a water body is a water budget and mass balance for each 

year. The water budget for Lake Hunter was formulated as 

Point and nonpoint source inflow + groundwater flow + precipitation = outflow + 

evaporation 

 + change of volume 

While there are no NPDES facilities that discharge wastewater within the Lake Hunter 

watershed, the City of Lakeland, Division of Lakes & Stormwater staff provided information that 

two other lakes, Beulah and Wire, discharge stormwater through pipes into Lake Hunter. In this 

analysis, those lakes were evaluated as point sources in developing the water budget. 

Discharge from Lake Hunter 

Lake Hunter discharges through a double gate into an unnamed creek. Although the discharge 

flows were never recorded, the Public Works Department for the City has limited monitoring 

data for the opening of the gates (adjustments) during the period 1997 – 2003. Using these 

data, a statistical approach was used to establish a relationship between water stages in the 

Lake and the flow through the outfall structure. The records of water stage in the Lake were 

matched with the available records of the gates’ adjustments. The discharge from the gate was 

simulated by equation (Eq.) 2 [3]: 

 Q = C a L (2 g h)0.5 (2) 

 15
Final TMDL Report Lake Hunter, September, 2004 

Where:I 

a is the height of an opening, 

L is the width of the opening, 

g is the gravitational constant, 

h is the hydraulic head on the center of the opening, and 

C is the coefficient of discharge. 

During the calculations, it was allowed that C varied with the depth of the opening and the 

hydraulic head. From Eq. (2) the discharge, Q, can be estimated knowing the water level in the 

lake, height of the opening of the gate, and the dimensions of the gate. For the double gate in 

Lake Hunter, the calculated flow through a single gate was doubled, disregarding any mutual 

influence of both gates. 

The Public Works Department provided nearly 450 records of the water stage in Lake Hunter 

during the years 1988 – 2002, but with only 55 records of the gates’ adjustments. Moreover, 

prior to October 1997, a double weir was in place instead of the double gate present now. 

Given the situation, the estimate of historical outflows from the Lake may not be very accurate. 

The principal assumption of this portion of the analysis was that, whether the water was 

discharged through weirs or gates, the City’s objective to maintain a specific, safe water stage in 

the Lake remained the same. Therefore, over a long time span such as a year, the total volume 

of water expected to be discharged depended more on the inflows to the Lake rather than on 

the method of discharge from the Lake. 

To estimate the complete outflows from the record of 450 water stages (with only 55 records of 

adjustments of the gates or weirs), an approximate relationship between stages, some other 

easily measured parameters, and the known (or assumed) outflows needed to be developed. 

After some experimentation, it was found that the outflows correlate reasonably well with water 

stages, the amount of rain in a given day, and the sum of rainfall during the 13 prior days. The 

total rainfall that occurred during the previous two weeks was included into the estimation of the 

daily outflow. The gate adjustments, provided by the Public Works Department, were not 

consistently recorded during the early years. However, with time, the recordings showed a 

more systematic two-week schedule. Therefore, for the majority of outflows, the rainfall 

aggregated from the previous 13 days and the current day encompassed practically all rainfall 

recorded at the Lake. In other words, the sum of the rain (1+ 13 days) was related to the 

predominant time span of the records of adjustments (two week intervals). The least-square 

technique available with the software As-Easy-As provided the coefficients of polynomials 

relating estimated outflows with three independent variables. The coefficient of determination, 

R2

, was 0.86 (see Figure 4.3). 

 16
Final TMDL Report Lake Hunter, September, 2004 

D is c ha rg e s c a lc u la te d ve rs us s im ula te d by s to c h a s tic e qu a tion

0

5

1 0

1 5

2 0

2 5

3 0

3 5

4 0

4 5

0 5 10 15 20 25 30 35 40 45

C alcu late d b y h yd r au lic e q u atio n (cfs )

Stochastic simulation (cfs)

Figure 4.3 Correspondence between outflows calculated from known adjustment of the gates and 

estimated by regression equation with water stage, rainfall at this day, and total rain in 

preceding 13 days as independent variables. Ideally, the points should plot along a 

diagonal. 

The regression developed was applied to the complete record of 450 water stages (1988 – 

2002) and the record of daily rainfalls to estimate the outflows from Lake Hunter. The 

calculated outflows were then multiplied by the number of days between consecutive dates 

(mostly in 14-day intervals) and summed up into annual discharges. 

The lack of measured data on the actual outflows during 1988 – 2002 prevented estimating the 

accuracy of the calculated outflows. The implied accuracy was judged intuitively, plotting 

outflows and rainfalls and looking for the expected correspondence between both parameters 

(Figure 4.4). The estimated annual outflows are provided in Table 4.2. 

 17
Final TMDL Report Lake Hunter, September, 2004 

Correspondence between rainflow and estimated outflow from Lake 

Hunter

0

10

20

30

40

50

60

70

80

1986 1988 1990 1992 1994 1996 1998 2000 2002 2004

Years

Rainfall (in) or outflow 

(100,000 cu m)

rainfall outflow

Figure 4.4 Plotted annual rainfalls and outflows from Lake Hunter 

Runoff from Lake Hunter Basin 

The other component of the water balance equation is surface runoff volume. The spreadsheet 

model based on the equations from the WMM was applied to calculate discharges from the 

mostly residential basin of 611 acres. The runoff coefficients were taken from Harvey Harper 

[4], complimented by estimates provided by other authors (Haith, [1], and Brown, [15]) wherever 

it was necessary. The land uses and acreage in the Lake Hunter drainage basin is shown in 

Figure 4.1 and Table 4.1. Estimates of surface runoff are provided in Table 4.2. 

Volume of Water in Lake Hunter 

One of the components of the water budget, and a quantity playing a dominant role in the mass 

balance equation as well, is the volume of water detained in the Lake. The records of water 

stages during 1988 – 2002 were provided by the Public Works Department, City of Lakeland. 

However, for the estimate of detained water volume, two pieces of information were still needed: 

lake morphometry and at least one pair of water stage and water volume measurements as a 

reference point. This information was taken from the bathymetric map for Lake Hunter (Figure 

4.5). The area and volume of Lake Hunter on the day the aerial photo was taken are shown in 

the lower right corner of the map are. In the records of water stages, it was found that the water 

level in that month was stabilized at 161.60 feet above see water level. 

 18
Final TMDL Report Lake Hunter, September, 2004 

The scale of the bathymetric map (Figure 4.5) was found by overlapping that map on a 

cartographic map of the Lake held in the Department’s GIS files. Knowing the scale of Figure 

4.5, it was possible to calculate the areas inside the contour lines. Analysis of the contour lines 

assumed a simple dependency of the surface area and volume of the detained water on the 

depth of the Lake. Two types of volume have been calculated: one is the average volume 

during a year, and the other is the volume at the end of the year. The latter volume was used to 

estimate the change of water volume in a given year, which is one component of the water 

budget equation. 

Figure 4.5. Bathymetric map for Lake Hunter with magnified legend to the right 

Discharges from Lakes Beulah and Wire 

As mentioned previously, Lakes Beulah and Wire occasionally discharge to Lake Hunter via 

hydraulic structures and underground pipes. The Lakes & Stormwater Division (Department of 

Public Works, Lakeland, Florida) kept records of water stages and the adjustment of the 

openings in the hydraulic structures in these two lakes. Lake Wire discharges through a single 

weir that holds up to three six inch flushboards, and Lake Beulah discharges through a gate. 

Both structures were redesigned at least once between 1988 and 2002 and were not functional 

during some periods. 

The stormwater discharges from those two lakes constitute major inflows to Lake Hunter. Due 

to the uncertainty of the data, the estimates of the stormwater contribution of both lakes to Lake 

Hunter can not currently be verified. However, both of the other drainage basins are of a similar 

type, mostly medium density residential, are located close to one and another, and are receiving 

almost the same rainfalls. Additionally, the same objective applies to both lakes: to maintain a 

certain, safe, water level. The resemblance of one hydrograph to another is the only measure 

 19
Final TMDL Report Lake Hunter, September, 2004 

of accuracy that can be applied to the estimates (see Figure 4.6). The estimated cumulative 

outflows are included in Table 4.2. 

Estimated outflows from Lake Beulah and Wire during 1988 - 2002

0

1,000,000

2,000,000

3,000,000

4,000,000

5,000,000

6,000,000

7,000,000

8,000,000

1986 1988 1990 1992 1994 1996 1998 2000 2002 2004

Years

Outflow (cu m)

Beulah Wire

Figure 4.6. Annual discharges from Lake Wire and Beulah estimated from recorded water stages 

and adjustments. 

Other Sources of water: Groundwater 

Within the Central Florida area, the surficial aquifer system ranges from 50 to 300 feet in 

thickness and is composed of unconsolidated sand and clay. The surficial aquifer system is 

hydrologically separated from the underlying Upper Floridan aquifer by the intermediate 

confining unit. However, sinkholes and subsidence features often modify and breach the 

confining unit, thereby diminishing the hydrologic confinement [11]. 

Groundwater can be a very important component of water sources to lakes in Central Florida. 

In this mantled karst terrain, unconsolidated sands and clays overlie an irregular limestone 

surface. The surface is highly permeable, so rainfall quickly percolates through the sands to the 

water table, favoring groundwater flow over surface water drainage. As a result, 70 percent of 

Florida’s lakes are seepage lakes, having no natural stream flow into or out of them. 

Groundwater typically enters a lake in shallow areas near the shore [12]. 

Data on the groundwater interaction with Lake Hunter were not available. Traditionally, these 

data are one of the most difficult to collect. The recharge of Lake Hunter by the shallow 

groundwater aquifer was estimated from the water balance made for the Lake for each year. 

 20
Final TMDL Report Lake Hunter, September, 2004 

The water budget for the Lake was on a negative side (on an annual average basis, more water 

was lost to the groundwater from the lake than water gained by the lake from the aquifer) mostly 

due to the substantial water supply from Lakes Wire and Beulah. It appears that accretion to 

the surficial aquifer is related to the amount of discharge from Lakes Wire and Beulah. When 

discharges from these other two lakes stop playing a dominant role in the total inflow to Lake 

Hunter, the groundwater system supplies the Lake with water. When these lakes are 

discharging to Lake Hunter, the aquifer becomes a ‘sink’ from the over-supplied Lake Hunter. 

The cumulative discharges from Lakes Wire and Beulah and accretion to the surficial aquifer 

are compared in Figure 4.7. 

Discharge from both lakes and accretion to ground water 

aquifer

-4

-2

0

2

4

6

8

10

12

14

16

1986 1988 1990 1992 1994 1996 1998 2000 2002 2004

Years

Discharge or accretion, both in 

million cu m

Discharge accretion

Figure 4.7 Cumulative discharge from Lakes Wire and Beulah with loss to groundwater aquifer. 

The red line falling below level zero corresponds to groundwater seeping into Lake 

Hunter. 

 21
Final TMDL Report Lake Hunter, September, 2004 

Surface Inflow from Average Volume at 

Year Rainfall Evap Outflow Runoff Lake Wire & Beulah Volume year's end 

in/y in/y cu m Ac- ft cu m cu m. cu m. 

597,311 

1988 60.54 60.96 3,265,172 1,001.3 13,388,753 671,643 666,044 

1989 52.36 61.84 1,627,951 866.0 3,317,435 637,902 615,549 

1990 39.43 62.40 794,500 652.1 339,607 545,978 579,138 

1991 58.21 56.80 2,566,142 962.7 732,438 666,958 675,278 

1992 50.51 56.24 628,170 835.4 2,798,808 564,129 561,032 

1993 52.38 55.12 1,887,566 866.3 1,486,330 609,919 581,406 

1994 67.13 54.72 4,496,325 1,110.3 3,410,846 629,888 647,625 

1995 60.06 57.12 4,093,349 993.3 5,817,687 659,252 453,772 

1996 52.85 57.12 767,620 874.1 4,339,355 590,449 617,834 

1997 58.14 61.84 1,437,865 961.6 5,930,325 595,036 638,440 

1998 54.41 63.52 3,671,542 899.9 7,021,634 648,457 626,982 

1999 48.66 57.84 3,155,802 804.8 3,271,311 669,416 724,027 

2000 38.26 58.59 1,910,111 632.8 6,494 638,707 657,977 

2001 57.67 57.66 3,381,921 953.8 266,217 666,132 691,477 

2002 66.58 56.17 3,104,547 1,101.2 832,367 656,464 626,982 

Table 4.2. Rainfall and other data applied to water budget during years 1988 – 2002 

It is unlikely that periods during which the shallow groundwater aquifer discharges to Lake 

Hunter are accidentally coincidental to seasons of reduced rainfall. Comparison of Figure 4.4 

with Figure 4.7 shows that periods of net groundwater inflow to the lake or relatively low losses 

from the Lake to the groundwater corresponds to, or closely follows, annual precipitation that 

was below average. 

The contribution of groundwater to Lake Hunter (Figure 4.8) or Lake Hunter to the shallow 

aquifer (red line in Figure 4.7) showed large variations over time. Also, the percentage of the 

groundwater component in the total water supply to Lake Hunter or in the discharge of water 

from Lake Hunter varied in such a wide range that the average value would not have had 

practical meaning. The variability of the groundwater component is shown in Figure 4.8. 

 22
Final TMDL Report Lake Hunter, September, 2004 

Year Ground water

(cu m) 

Ground water 

as percent 

1988 -11,286,080 -- 

1989 -2,651,724 -- 

1990 -179,311 -- 

1991 721,902 23

1992 -3,263,913 -- 

1993 -601,408 -- 

1994 -329,854 -- 

1995 -3,136,600 -- 

1996 -4,417,866 -- 

1997 -5,643,038 -- 

1998 -4,290,527 -- 

1999 -679,050 -- 

2000 1,238,178 50

2001 1,972,624 50

2002 755,157 21

Table 4.3 Summary of groundwater component. Years in which aquifer discharged to the Lake 

are underscored. Percent of groundwater is related to total water supplied to Lake. 

Surficial ground water interaction with Lake Hunter

-12

-10

-8

-6

-4

-2

0

2

4

1986 1988 1990 1992 1994 1996 1998 2000 2002 2004

Years

In flow, million cubic meters

 Figure 4.8 Supply of groundwater aquifer by Lake Hunter. The positive volumes refer to seepage 

of ground water into the Lake. 

 23
Final TMDL Report Lake Hunter, September, 2004 

Water Budget: Mass Calculations 

To calculate nutrient loads into and out of the Lake, annual average concentrations for nutrients 

and chlorophyll a were calculated from the seasonal concentrations of TN, TP, and Chl a. 

Figures 2.2, 2.3, and 2.4 summarize those data as the annual average concentrations in Lake 

Hunter. The major sources of nutrients to the Lake are surface runoff from the drainage basin 

that picks up and transports soil, nutrients, organic matter, toxins, and other pollutants; 

stormwater discharges from lakes Wire and Beulah; direct rainfall falling onto the Lake’s 

surface; and groundwater inflow to the Lake. 

Mass from Lakes Wire and Beulah 

A spreadsheet model based on the WMM was used to balance the mass of TP and TN 

measured in the Lake. The mass balance equation is 

 Sedimentation – internal load = point and nonpoint source load + groundwater 

 load + rainfall load – export load – increase of suspended mass 

The term Export load in the mass balance equation is that mass of nutrients that leaves Lake 

Hunter with the outflow. It is always calculated as a product of outflow volume and the 

concentration in the Lake. 

The last term in the mass balance equation is an increase of suspended mass. This term was 

calculated as the difference between the mass of either nutrient contained in the Lake at the 

beginning and end of each year. 

Water quality data for Lakes Wire and Beulah consisted of a monthly sampling of TP and TN 

from 1989 – 1991. Beginning in 1992, only one sample per season of both nutrients was 

collected. Products of the concentrations and outflows during matching months or seasons 

were aggregated into annual totals. 

The combined loads from Lake Wire and Lake Beulah were added into the annual mass 

balance equation as a point source to Lake Hunter. Missing water quality data for Lakes Wire 

and Beulah for 1988 eliminated that year from further analysis. See Table 4.4 for results of 

these mass load calculations. 

 24
Final TMDL Report Lake Hunter, September, 2004 

Mass of TN and TP in kgs pounds(lbs) discharged from Lakes Beulah 

and Wire into Lake Hunter during current conditions: 

 Lake Beulah Lake Wire 

Year TN TP TN TP 

1996 6,077 (13,397) 332 (732) 737 (1,625) 76 (168)

1997 3,570 (7,870) 519 (1,144) 3,066 

(6,759)

857 (1,889)

1998 2,983 (6,576) 597 (1,316) 2,553 

(5,628)

733 (1,616)

1999 2,974 (6,556) 411 (906) 345 (761) 71 (156)

2000 3 (6.6) 0 (0) 4 (8.8) 1 (2.2)

2001 0 (0) 0 (0) 171 (377) 45 (99)

2002 222 (489) 35 (77) 680 (1,499) 156 (344)

Average 2,261 (4,985) 271 (597) 1,079 

(2,379)

277 (611)

Table 4.4 Mass of TN and TP in kgs & pounds (lbs) discharged from Lakes Beulah and Wire into 

Lake Hunter during current conditions: 

Mass from Lake Hunter watershed 

The mass of nutrients washed off by surface runoff was estimated using loading functions 

provided by Harvey Harper for different land uses [4]. It should be noted that the loading 

function for forest was assumed as 1.09 mg/L for nitrogen and 0.046 mg/L for phosphorus. 

Those loading function became particularly important during estimation of natural background 

conditions when all man-made alterations to natural conditions were arbitrarily replaced by 

forest land use. See Tables 4.5 and 4.6 for results of mass load calculations. 

 25
Final TMDL Report Lake Hunter, September, 2004 

 

Current Condition 

Natural Background 

Condition 

80% reduction 

TMDL Condition 

Sources TN 

Kgs (lbs) 

TP 

 Kgs (lbs)

TN 

 kgs (lbs)

TP 

 kgs (lbs)

TN 

 kgs (lbs)

TP 

kgs (lbs) 

Lakes Wire and 

Beulah combined 

3,341 

(7,366) 

548

(1,208)

0

(0)

0

(0)

668 

(1,473) 

109 

(240) 

Ground water 503 

(1,109) 

35

(77)

252

(556)

8

(18)

503 

(1,109) 

35 

(77) 

Nonpoint Sources 

Lake Watershed 

2,385 

(5,259) 

349

(769)

1,135

(2,502)

49

(108)

495 

(1,091) 

71 

(157) 

Septic Tanks 340 

(749) 

0

(0)

0

(0)

0

(0)

0 

(0) 

0 

(0) 

Direct Rainfall on 

Lake 

331 

(730) 

13

(29)

319

(703)

12

(26)

331 

(730) 

13 

(29) 

Net internal 

recycling 

1,872 

(4,127) 

0

(0)

89

(196)

0

(0)

137 

(302) 

0 

(0) 

Table 4.5 Load in kgs and lbs from various sources (Current is average for the period 1996 - 

2002): 

Note: Reduction of load for internal recycling for natural background and the 80% reduction scenarios 

further increases the implicit margin of safety. 

Lake Hunter watershed average nutrients contribution during 1996 - 2002 (in kgs): 

 Current conditions Background condition 80% reduction 

TN TP TN TP TN TP 

Septic Tank 339.7 0.0 0.0 0.0 0.0 0.0 

Forest/Rural 

Open 

6.4 0.3 1,118.5 47.3 6.4 0.3 

Urban Open 88.8 9.8 0.0 0.0 17.8 2.0 

Medium Density 1,633.4 251.1 0.0 0.0 326.7 50.2 

High Density 105.8 21.4 0.0 0.0 21.2 4.3 

Highway 535.4 64.8 0.0 0.0 107.1 13.0 

Wetlands 16.1 1.4 16.1 1.4 16.1 1.4 

Sum 2,725.6 348.8 1,134.6 48.7 495.3 71.2 

Please note that septic tanks were completely eliminated in 80% reduction alternative, 

And forest and wetlands were not reduced at all 

Table 4.6 Lake Hunter Watershed Loadings by Source Category during 1996 - 2002 (in kgs). 

Mass from Ground water 

No data on ground water quality was available at the time of this analysis. However, out of the 

14 years analyzed, ground water was found to supply Lake Hunter in only four years. For those 

four years, the nutrient concentrations in ground water were set at values from studies on Lake 

 26
Final TMDL Report Lake Hunter, September, 2004 

Morton by Environmental Research & Design [13]. Lake Morton is a 40.8-acre urban lake 

located in the central downtown area of the City of Lakeland, less than a mile northeast of Lake 

Hunter. The project included a detailed study of ground water interaction using seepage 

meters. This provided an annual estimate of the mass loading from ground water seepage as 

57.8 kg of nitrogen and 4.07 kg of phosphorus. Together with the annual ground water seepage 

estimated as 52.74 ac-ft, the average concentration of nutrients in the groundwater was 0.8885 

mg/L of TN and 0.06256 mg/L of TP. These concentrations were used in the analysis of Lake 

Hunter. See Tables 4.5 and 4.6 for results of mass load calculations. 

Annual Water and Mass Balance Results 

An annual water balance and mass budget were conducted for the years 1989 through 2002 

using the spreadsheet model. The calculated mass entering the Lake was added to the mass 

residing in the Lake at the beginning of each year and then divided by the total volume of water 

that entered the Lake. The estimated average concentration of either TP or TN was then 

reduced by sedimentation, according to Eq.s (3), (4), and (5). No data on sedimentation in Lake 

Hunter was available during this analysis. Therefore, the calibration coefficient cf was manually 

adjusted to match the total calculated mass of nutrients coming into the Lake to the measured 

data. Figures 4.9 and 4.10 provide insight into the reliability of the modeling effort. Tables 5.2, 

4.4, 4.5,and 4.6 contain the mass loading, concentration, and TSI results of the various 

scenarios. 

Annual incoming phosphorus from collected data and simulated

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

1988 1990 1992 1994 1996 1998 2000 2002 2004

Years

Mass of phosphorus, kgs

modeled measured

Figure 4.9 Comparison of modeled phosphorus load into Lake Hunter during 1989 – 2002 to the 

measured load. Average difference is 0.7 kg/year. 

In general, the accuracy of the TP load estimates seems to be reasonable. While the model 

estimates did not match the spike of load in 1995, the high load measured in 1995 is an artifact 

of the collection method and a storm induced washout from a construction site. According to 

 27
Final TMDL Report Lake Hunter, September, 2004 

City of Lakeland staff, there was a severe rain event (more than 2.0" rain) in August 1995 during 

the construction of a school near Lake Wire. This rain caused a construction washout, and silt 

deposits (whitish water discharge) covered almost five acres of the Lake. Because lake 

samples were only taken quarterly and the sampling corresponded to the month with the spill, 

this one spill accident impacted the average phosphorus concentration during the entire season 

preceding the spill and during the following season as well. 

The mass of TN coming into Lake Hunter provided a greater challenge. While estimates for 

1987 through 1995 were relatively accurate, concentrations measured in the Lake consistently 

exceeded the model estimates for 1996 through 2002, in spite of applying a separate negative 

calibration coefficient (cf) for those years. Unless the estimate of loads originating from Lakes 

Wire and Beulah is erroneous, there is an additional source of nitrogen to Lake Hunter that has 

yet to be identified. The negative calibration coefficient may reflect net nutrient releases from 

bottom sediments or fixation of atmospheric nitrogen by blue green algae. A search for algae 

species composition data from 1989 through 2002 proved to be unsuccessful. The City of 

Lakeland, Florida Fish & Wildlife Conservation Commission, Southwest Florida Water 

Management District, and the Polk County Division of Natural Resource were unable to provide 

any such data. 

Annual incoming nitrogen from collected data and simulated

0

5,000

10,000

15,000

20,000

25,000

1988 1990 1992 1994 1996 1998 2000 2002 2004

Years

Mass of nitrogen, kgs

modeled measured

Figure 4.10 Comparison of nitrogen load into Lake Hunter during 1989 – 2002 to the measured 

load. Average difference is 1,844 kg/year. 

Another potential source of nutrients to the Lake that was not included into this analysis could 

be waterfowl. Lake Hunter is situated inside the City with a recreational park and trail around it, 

and this park is home to a variety of waterfowl species such as swans, herons, ducks, and 

geese. The majority of the waterfowl are actively maintained and fed by the City of Lakeland 

Parks and Recreation Department as well as local residents and park visitors [13]. For nearby 

Lake Morton (with a surface area less than 50% of Lake Hunter), Environmental Research & 

Design estimated that waterfowl contributed 776 kg of TN and 156 kg of TP per year load to the 

lake. 

 28
Final TMDL Report Lake Hunter, September, 2004 

Summary of mass balance in Lake Hunter 

The spreadsheet model applied to the Lake Hunter data facilitated calculation of the water 

balance (volume and mass) for all water incoming and leaving the Lake. Parameter cf in Eq. (3) 

was optimized in the simulation of sedimentation/internal recycling to match the predicted 

annual average mass of both nutrients with the measured data. The ground water interaction 

with Lake Hunter was a major missing component of the water budget. The outflow from Lakes 

Wire, Beulah, and Hunter were estimated from water stages read in two-week or even longer 

time intervals. Therefore, it was not possible to assure a truly balanced water budget. The 

mass balance equation revealed a missing nitrogen mass loading to the Lake during the period 

1996 –2002 (see Figure 4.10). 

From the quantity of missed nutrients, especially of nitrogen, it appears that some source of 

nitrogen is still unidentified. In addition to the sources already mentioned (birds, bluegreen 

algae, or internal recycling) there may be unaccounted sources discharging to Lake Hunter. 

There are a total of 28 pipes that discharge to Lake Hunter. It is the Departments 

understanding that these are storm drainage systems serving the drainage basin of the Lake. It 

is possible that some of the pipes may serve municipal areas from outside the drainage basin. 

 29
Final TMDL Report Lake Hunter, September, 2004 

Chapter 5: DETERMINATION OF ASSIMILATIVE 

CAPACITY 

Nutrient enrichment and the resulting problems related to eutrophication tend to be widespread 

and are frequently manifested far (in both time and space) from their source. Addressing 

eutrophication involves relating water quality and biological effects (such as photosynthesis, 

decomposition, and nutrient recycling), as acted upon by hydrodynamic factors (including flow, 

wind, tide, and salinity) to the timing and magnitude of constituent loads supplied from various 

categories of pollution sources. 

5.1 Critical Conditions 

To evaluate nutrient impairment in a lake, a reference condition is needed. For lakes in Florida, 

the reference condition is represented by the TSI. Lake Hunter was included on the list of 

impaired waters because at least one annual mean TSI exceeded the IWR threshold during the 

verified period. For Lake Hunter, the verified threshold was different in different years due to 

variations in the Lake’s color (Figure 5.1). In years with a mean color less than 40, the TSI 

threshold would be 40. In years with a mean color greater than 40, the TSI threshold would be 

60. Since the annual TSIs of Lake Hunter were always greater than 60 (Figure 5.2), the Lake 

would be listed as impaired regardless of the color. Due to the uncertainty of the appropriate 

target TSI for development of the TMDL, the Department used modeling to evaluate various 

background scenarios for the Lake. 

Annual color during 1984 - 2002

0

5

10

15

20

25

30

35

40

45

1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

Years

Color

Figure 5.1 Annual-average color in Lake Hunter during years 1984 – 2002 

The assimilative capacity should be related to some specific hydro-meteorological condition 

such as an ‘average’ during a selected time span or to cover some range of expected variation 

in these conditions. In this analysis, rather than use average conditions, the Department 

 30
Final TMDL Report Lake Hunter, September, 2004 

examined the existing data to determine a set of “critical years” for meteorological conditions. 

The TSIs from Figure 5.2 were arranged in ascending order: 74.9, 75.3, 75.7, 77.6, 78.6, 81.2, 

81.4, 81.8, 82.43, 82.6, 85.6. The lower tenth percentile for this set of numbers is 75.3 (1991), 

the 50th percentile is 81.2, and 90th percentile is 82.6 (2001). The meteorological conditions of 

the year 2001 were selected as the “worst case” critical condition (90th percentile worst case 

TSI for the period 1989 – 2002). The meteorological conditions of 1991 were selected as the 

“best case” critical condition (10th percentile TSI for the period 1989 – 2002). 

TSI during 1991 - 2002

68

70

72

74

76

78

80

82

84

86

88

1991 1992 1993 1994 1996 1997 1998 1999 2000 2001 2002

Years

TSI

Figure 5.2 Annual-average TSI during years 1991 – 2002. During those years, samples of TN, TP, 

and Chl a were taken in every season. 

5.2 Determination of Natural Background 

To evaluate the natural background condition for the Lake, the current land uses listed in Table 

4.1 under urban, and transportation categories were evaluated as forest, loading from septic 

tanks was set at zero, and the connections between Lake Hunter and Lakes Wire and Beulah 

were eliminated. 

Under these background conditions, it is likely that the ground water inflow/outflow to the Lake 

was different from that of the current condition, which is highly impacted by the roughly 740 

thousand cubic meters of water flowing annually from Lakes Wire and Beulah. The removal of 

this volume of water from Lake Hunter would lower the hydraulic head in the Lake and increase 

ground water inflow to the Lake. 

To estimate the ground water inputs to the lake under background conditions, several 

relationships were tested, and the total annual inflow to the Lake correlated best with the 

groundwater component listed in Table 4.3. The approximation function was 

 groundwater inflow to Lake Hunter = 2.404 – 0.86 * total inflow (6) 

where both inflows are in million cubic meters. Figure 5.3 shows this correlation [coefficient of 

determination (R2

, was 0.73)]. 

 31
Final TMDL Report Lake Hunter, September, 2004 

Groundwater inflow to Lake Hunter estimated from water balance and 

regressed

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

1988 1990 1992 1994 1996 1998 2000 2002 2004

Years

Volume in 10^6 cu m / y

w ater balance regression

Figure 5.3. Approximation of groundwater component by regression Eq. (6). During most of 

period 1989 – 2002 Lake Hunter discharged to groundwater aquifer (negative 

numbers). 

Using the same climatological conditions as occurred in 1991 (10th percentile year), the natural 

background total inflow to Lake Hunter was estimated as 1.636 million cubic meters, and for this 

inflow Eq. (6) provided the expected groundwater inflow as 891,600 cubic meters. Comparing 

estimates of the groundwater component for current conditions in 1991, to the natural 

background estimates, the total groundwater contribution increased by 162,000 cubic meters. 

The estimate from Eq. (6) suggests that, as expected, the surficial aquifer bled down more 

abundantly to Lake Hunter before the basin was populated. 

Having estimates of inflows from all identified sources, a water budget for the pre-populated 

Lake was still not complete because the volume of water detained in the Lake and discharges 

from the Lake were not known. Discharge from the Lake under background conditions is not 

known because the morphometry of the Lake outlet is not known and discharge is a straight 

function of depth of water and morphometry of the outlet. 

Given the lack of information about the Lake depth and outlet, discharges, water stages, and the 

volume of detained water were calculated for the natural condition in daily steps until equilibrium 

was reached in the Lake. The daily inflows were total volumes divided by the number of days in 

a year. The secondary advantage of such a simulation was the elimination of the influence of 

not knowing the initial condition of the Lake for volume, depth, and discharge at steady state. 

 32
Final TMDL Report Lake Hunter, September, 2004 

Figure 5.4 Idealization of water discharging from Lake Hunter 

The outfall of Lake Hunter in the pre-populated basin was then approximated by a rectangular, 

top – open orifice, Figure 5.4. The theoretical formula for a stream of water issuing from such 

an orifice is [3] 

 Q = 2/3 L h (2 g h)0.5 (7) 

where 

L = width of the opening, m 

h = water level above the crest of the orifice, m, and 

g = gravitational constant, 9.80665 m2

/s. 

In the derivation of Eq. (7) from the Bernoulli energy conservation equation, the energy loss 

term has been dropped. The convenient way to account for the energy loss and the effect of 

contraction of the nappe is by introducing a coefficient of discharge, C, that is usually close to 

0.6, but not necessarily 2/3. For wide orifices, C stabilizes near 0.6, and this value was used in 

the calculation. Steady-state was achieved with the Lake’s volume at 195,648 cu m, a depth of 

0.91 m, and a discharge of 5,566 cu m/d or two million cu m per year. With reference to the 

current average condition, the natural background depth of Lake Hunter was about a meter less 

than current. Possibly, the Lake was shallow most of the time and the groundwater aquifer 

supplied the Lake in most years. 

With the assumption of steady – state, the calculated natural background discharge from the 

Lake becomes less influenced by the shape and width of the outlet (that were only roughly 

estimated from available data). However, the volume of detained water depends on the 

elevation of the outlet. In turn, the residence time varies with the volume of the Lake, and the 

resident mass of Chl a (and other constituents) through the process of sedimentation may be 

impacted as well. 

Background Mass Balance 

Determination of the background condition of the Lake required the mass balance equation to 

be modified. Outflow from the Lake was assessed by Eq. (7). However, water level above the 

crest of the orifice, h, was not known a priori. The water level, h, needed to be approximated for 

 33
Final TMDL Report Lake Hunter, September, 2004 

consecutive days until the discharge from the Lake stabilized at a steady-state level. This made 

the calculation of the discharge for background conditions different from the current condition 

described in Section 4, and in which the discharge was regressed upon known water stages 

and antecedent precipitation. 

These changes were incorporated into the spreadsheet model and the following mass continuity 

equation was solved: 

 Initial mass in lake + mass in rainfall + mass in nonpoint source + mass in

ground water inflow – sedimentation = (lake’s volume + Vout ) X 

In this equation, Vout denotes the volume of annual outflow, and X is the unknown 

concentration of TP or TN in Lake Hunter. Since sedimentation is closely related to the 

concentration in the lake, the unknown was calculated in two steps. First, the estimate of 

sedimentation was assumed to be zero, and the mass continuity equation solved for 

concentration X. This value was then applied to refine the estimate of sedimentation using W. 

Walker’s [6] algorithm and then to correct the concentration of X. 

Before the Lake Hunter watershed was developed, groundwater from the surficial aquifer 

seeped into the Lake more frequently. The quality of this groundwater is not known. As the 

best approximation, the concentrations established by the U.S. Eutrophication Survey, and 

quoted in [6], was substituted for the missing data. These data are mean concentrations 

computed from 12 monthly stream flow samples in watersheds free of point sources. Since 

such limited sampling is unlikely to capture nutrient fluxes from storm runoff, the stream flow 

concentrations were used to represent groundwater discharges to streams. For an area in the 

above survey that was at least 75% forested, the concentration of TP was 0.007 mg/L and the 

concentration of TN was 0.23 mg/L. 

The effect of eliminating Lakes Wire and Beulah, septic tanks, and converting all human 

landuses to forest, was that the concentration of TP in Lake Hunter dropped from 0.130 mg/L 

(current) to 0.033 mg/L (natural background). Similarly, the current concentration of TN of 1.750 

mg/L dropped to 0.843 mg/L (natural background). 

The regression equation of Dr. Reckhow from the EUTROMOD model and five models 

developed by Dr. Walker were tested against the annual average Chl a concentrations in Lake 

Hunter. The best fit was a function of TP, TN, turbidity, and flushing rate [6] 

Xpn = {p-2 + [(n - 150)/12]-2}

-.05 

Bx = Xpn1.33 / 4.31 (8) 

G = Zmix (0.14 + 0.0039 Fs) 

Chl a = Cc Bx / [(1 + 0.025 Bx G) (1 + G a)] 

where 

Chl a is chlorophyll a concentration in μg/L 

 34
Final TMDL Report Lake Hunter, September, 2004 

p is phosphorus concentration in μg/L 

n is nitrogen concentration in μg/L, 

Zmix is mean depth of mixed layer, m 

a is nonalgal turbidity, m-1, and 

Cc is calibration coefficient. 

The estimate of nonalgal turbidity was also provided by Walker as 

 a = S-1 - 0.025 (Chl a) (9) 

In this equation, S is Secchi depth, in m, that was approximated as [6] 

 S = Cs 16.2 Xpn-0.79 (10) 

With a calibration coefficient Cs equal to one, approximation (10) fit the data from Lake Hunter 

as seen on Figure 5.5. 

Measured and modeled Secchi depth

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

1988 1990 1992 1994 1996 1998 2000 2002 2004

Years

Depth, m

Figure 5.5 Equation (10) fits the data on Secchi depth from Lake Hunter. Pink line connects 

measured data. 

The set of equations (8) – (10) were validated with Chl a data from Lake Hunter, as shown on 

Figure 5.6. The predicted Chl a concentrations matched data collected during 1991 – 2002 

(with 1995 dropped), with the calibration coefficient, Cc, equal to 1.6. 

Walker’s procedure has been used to approximate Chl a for the natural background condition. 

Since the natural background Secchi depth is unknown, the concentration was calculated in 

iterations until an acceptable fit was obtained. 

 35
Final TMDL Report Lake Hunter, September, 2004 

Chlorophyll a measured and m odeled by W alker's m ethod

0

50

100

150

200

250

1991 1992 1993 1994 1996 1997 1998 1999 2000 2001 2002

Year

Concentration, ug/L

Measured

Walker's model

Figure 5.6. Chlorophyll concentration in Lake Hunter approximated by Walker’s model 

During 1991 – 2002, the range of the TN to TP ratio (N/P) was 9.8 – 16.9. In Florida, a lake is 

considered nitrogen-limited if the ratio of N/P is less than 10, and phosphorus-limited if the ratio 

of N/P exceeds 30. According to such a classification, Lake Hunter is neither clearly 

phosphorus nor nitrogen limited. The correlation of Chl a with either of the nutrients was found 

to be weak. 

Based on the Walker model, the natural background chlorophyll a, TN, TP, and secchi depth 

were estimated as 28.9 μg/L, 0.843 mg/L, 0.033 mg/L, and 1.14 m, respectively. The resultant 

estimated natural background TSI for the meteorological conditions of 1991 (lowest 10th

percentile TSI in period of record) was equal to 57.4. For the comparison, and to establish the 

upper range of TSIs occurring prior to settlement in the basin, a similar analysis was conducted 

for the year 2001, which coincides with the 90th percentile TSI. For that condition, the 

estimated concentration of TP in Lake Hunter was 0.033 mg/L, the estimated TN was 0.873 

mg/L, the estimated Chl a was 29.2 μg/L, and the estimated TSI was 57.5. The TSI for 1991 

under current conditions is 75.2 and for 2001 is 82.6. The predicted narrow variation between 

worst and best case meteorological conditions for background would be expected due to 

evaluating the land as forest instead of the current residential/urban nature of the watershed. 

This is primarily related to (1) evaluating all land area as a single landuse, (2) using a lower 

loading function for forest over residential/urban results in an overall reduction in loadings, and 

(3) the use of a lower runoff coefficient for forest results in less water and less variation in runoff 

 36
Final TMDL Report Lake Hunter, September, 2004 

than use of multiple coefficients for multiple landuses. Both parameters (loading rate and runoff 

coefficient) are used in multiplication [Eq.’s (1) and (2)]. Therefore, any amount of rain that fell 

on residential land will cause a greater incremental increase in load than from forested land. In 

other words, the range in variation of loads (and concentrations in the Lake) coming from a 

watershed predominated by forest is expected to have a smaller range in variation of those 

loads than the same rain falling on the watershed if dominated by residential/urban landuse. 

The ease of use of the spreadsheet model allowed for the calculation of natural background 

TN, TP, Chl a, and TSIs for the range of actual meteorological conditions over the period of 

1989 – 2002. Those calculations showed moderate variability of all three parameters with the 

TSIs ranging between 53 and 59 during the 14 years modeled (Figure 5.7). It would appear 

that in it’s natural state, the TSI of the Lake was always well above 40. Mass loading results 

for natural background are contained in Tables 5.2, 4.4, and 4.6. 

Dependency of Natural Background TSI on Rainfall

53

54

55

56

57

58

59

60

35 40 45 50 55 60 65 70

Rainfall, in

TSI

Figure 5.7 Variation of TSI with total annual rainfall in pre-developed basin of Lake Hunter. The 

TSIs were approximated by polynomial of second order (pink line). 

5.3 Determination of Assimilative Capacity 

It should be recognized that the direct application of natural background as the target TSI would 

not allow for any assimilative capacity. The IWR uses as one measure of impairment in lakes, a 

10 unit change in TSI from “historical” levels. This 10 unit increase is assumed to represent the 

transition of a lake from one trophic state (say mesotrophic) to another nutrient enriched 

condition (eutrophic). The Department has assumed that allowing a 5 unit increase in TSI over 

the natural background condition would prevent a lake from becoming impaired (changing 

 37
Final TMDL Report Lake Hunter, September, 2004 

trophic states) and reserve 5 TSI units to allow for future changes in the basin and as part of the 

implicit margin of safety in establishing the assimilative capacity. This raises the target TSI for 

Lake Hunter to 63 (57.6 + 5 rounded). 

Determination of Loading Capacity: Scenarios Evaluated 

Several scenarios were evaluated in an attempt to determine the load that will meet the target 

TSI of 63 (Tables 5.2 and 4.4). For the first scenario, the meteorological conditions of 2001 

were used as worst case conditions, and the spreadsheet model was run without the discharges 

from Lakes Wire and Beulah. This condition induced more groundwater into the Lake. This 

resulted in a moderate drop of the TSI for critical conditions from 82.6 to 76.3. However, in that 

calculation, the nutrient mass from “unknown” sources was not included. If the loads from 

“unknown” sources is included, the TSI would rise to 78.7. 

The second scenario evaluated consisted of successive simulations in which the concentrations 

of TN and TP in the various sources of nutrients were reduced. The loading functions of the 

most influential land uses, i.e. urban open, medium density residential, high density residential, 

and highways were reduced by 90 percent, the septic tanks were removed, and the inflow from 

Lakes Wire and Beulah shut off. This lowered the TSI for the critical year (2001) to 61.6 (target 

TSI equals 63). In another attempt, an 85 percent reduction of the loading functions increased 

the TSI to 62.9, the target value. 

However, there is some uncertainty related to that calculation. Although inflow from both 

contributing lakes was eliminated in the scenario, the volume and outflow from Lake Hunter was 

kept the same as estimated for the actual year 2001. There is not enough data to reliably 

estimate discharge, groundwater inflow, and the detention volume of the Lake at the same time. 

Therefore, an additional set of simulations was conducted in which the water inflow from both 

contributing lakes was preserved, but the nutrient content reduced in the same proportion as the 

watershed loading functions. In these model runs, the water volumes (and whole water budget) 

was the same as that resulting from the analysis of actual years, only the concentration of 

nutrients carried by that water was reduced. Trying several reductions, it was determined that 

an 80 percent reduction of nutrients in discharges from land uses - urban open, medium density 

residential, high density residential, and highways, and an 80 percent reduction of the loads 

from the two connected lakes (Wire and Beulah) should lower the nutrient concentrations in 

Lake Hunter to the required level. This analysis presumed that all the septic tanks in the basin 

were eliminated (Table 5.1). 

 38
Final TMDL Report Lake Hunter, September, 2004 

Year TP in lake TN in lake Chla in lake TSI Unknown 

Sources 

 P 

Unknown 

Sources 

 N 

mg/L mg/L mg/L Kg Kg 

1989 0.026 0.301 8.3 42.2 169 1,704 

1990 0.048 0.567 26.0 56.5 -73 -792 

1991 0.051 0.634 29.0 58.1 -90 -836 

1992 0.032 0.492 18.0 51.3 204 -1,028 

1993 0.037 0.494 19.8 52.5 -34 -1,328 

1994 0.034 0.414 15.2 49.5 109 2,434 

1996 0.038 0.425 16.4 50.6 653 5,069 

1997 0.049 0.328 10.8 43.4 426 6,758 

1998 0.043 0.254 5.7 36.1 -105 5,626 

1999 0.042 0.357 12.6 45.4 416 4,004 

2000 0.061 0.856 39.5 62.6 134 169 

2001 0.061 0.871 39.2 62.6 118 2,326 

2002 0.059 0.717 33.7 60.4 306 1,587 

Table 5.1 Average nutrient concentrations in Lake Hunter and TSI after 80% reduction of strength 

of surface runoff and discharge from Lake Wire and Beulah. 

The resulting estimates of concentrations in Lake Hunter are shown in Table 5.1. All years were 

calculated in sequence to preserve initial conditions of the Lake at the beginning of each year. 

A reduction of nutrient concentration by 80 percent should protect the mesotrophic condition of 

the Lake during all years used in this analysis. The maximum TSI would occur in 2000 and 

2001 at the level of 62.6. However, these estimates assume that the unidentified mass of 

nutrients, tabulated in the last two columns of Table 5.1 are located and eliminated. For 

example, based on the water budget, during the year 2001, 118 kg of phosphorus and 2,326 kg 

of nitrogen entered the Lake from “unknown” source(s). This study was unable to assign that 

mass to any known source of pollution. 

Importance of Inflows from Lakes Wire and Beulah 

For most of the years during the period 1989 – 2002, loads into Lake Hunter stabilized at a 

similar level, yet the TSI rose in 1990, 1991, and during the last three years (2000-2002). The 

simple explanation for that increase was found by comparing the TSI of the Lake to the total 

inflow from the two northern lakes [Figure 5.8. Note that the scale for the volume discharged 

from Lakes Wire and Beulah (pink line) is inversed]. For years when the total inflow from Lakes 

Wire and Beulah was much reduced, the TSI in Lake Hunter jumped above normal. It may not 

be coincidental that just for the years 1991 and 2000 – 2002, the analysis predicted a net inflow 

of groundwater into the Lake. This indicates the importance of the inflow of water from both 

lakes (Wire and Beulah) to provide dilution, elevated stage, and decreased residence time in 

Lake Hunter. By this evaluation, it is important to keep the volume of the discharge from Beulah 

 39
Final TMDL Report Lake Hunter, September, 2004 

and Wire at current levels as a part of any management plan for Lake Hunter. Keeping the 

water stage in Lake Hunter high should prevent ground water, which is high in nutrients, from 

entering the Lake. This conclusion is valid as long as the estimated nutrient concentrations in 

groundwater have not been exaggerated. 

Similarity between TSI and discharge from Lakes Wire and Beulah

0

10

20

30

40

50

60

70

1988 1990 1992 1994 1996 1998 2000 2002 2004

years

TSI

0

2

4

6

8

10

12

TSI measure of volume

Figure 5.8 Comparison between Lake Hunter TSI and water volume discharging from Lakes Wire 

and Beulah into Lake Hunter. Note that the scale for the discharges from Lakes Wire and Beulah is 

inverted

 40
Final TMDL Report Lake Hunter, September, 2004 

Average During Verified 

Period 

Year 2001 (worst case) Year 1991 (best case) 

Concentration Mass (kgs) 

(lbs) 

Concentration Mass (kgs) 

(lbs) 

Concentration Mass (kgs) 

(lbs) 

Current Condition 

TN (mg/L) 2.113 8,780 

(19,356)

2.324 7,388 

(16,288)

TP (mg/L) 0.173 944 

(2,081)

0.142 556 

(1,226)

Chl a (ug/L) 77.514 346

 (763)

79.02 270 (595)

TSI 76.8 76.5

Natural 

Background 

TN (mg/L) 0.853 1,799 

(3,966)

0.843 1,824 

(4,021)

TP (mg/L) 0.032 69

 (152)

0.033 73

 (161)

Chl a (ug/L) 28 59

 (130)

29 62

 (137)

TSI 57.1 57.5 (58) 

All Sources Reduced 80 % 

TN (mg/L) 0.544 2,135 

(4,707)

0.871 2,984 

(6,579)

TP (mg/L) 0.05 243

(536)

0.061 222 (489)

Chl a (ug/L) 22.529 81

 (179)

39 134 (295)

TSI 55.4 62.6 

(63)

Table 5.2 Summary table providing loads and concentrations in current, natural background, 

and with 80% reduction in all sources: 1991 is Critical Condition “good” year, 2001 is 

Critical Condition “worst” year. 

Note: The meteorological conditions of 1991 were used for calculating the natural background condition 

(best case). The year 1991 was one of least impaired years for Lake Hunter (10th percentile year). 

Note: The meteorological conditions of 2001 were used for calculating the worst case condition for 

determining the TMDL for the Lake. These conditions were among those that produced the highest 

level of impairment (90th percentile year). 

Note: All sources reduced equally by 80%, except septic tank loads that were eliminated. 

Note: The combination of these conditions (best case background and worst case current) was used as 

the critical conditions and this added to the implicit margin of safety. 

 41
Final TMDL Report Lake Hunter, September, 2004 

Chapter 6: DETERMINATION OF THE TMDL 

6.1 Expression and Allocation of the TMDL 

A TMDL can be expressed as the sum of all point source loads (wasteload allocations or 

WLAs), nonpoint source loads (load allocations or LAs), and an appropriate margin of safety 

(MOS) that takes into account any uncertainty about the relationship between effluent limitations 

and water quality: 

As mentioned previously, the WLA is broken out into separate subcategories for wastewater 

discharges and stormwater discharges regulated under the NPDES Program: 

 

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

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

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

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

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

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

a mass per day]. 

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

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

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

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

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

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

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

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

Practices. 

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

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

appropriate measure. TMDLs for Lake Hunter are expressed in terms of pounds/year and 

percent reductions, and represent the maximum annual average load of TN and TP the 

waterbody can assimilate and maintain the Class III nutrient criterion (Table 6.1).

WLA 

WBID Parameter Wastewater 

(lbs/year) 

Stormwater 

(% reduction) 

LA 

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

Percent 

Reduction 

1543 TN NA 80% 6,579 Implicit 6,579 80 

1543 TP NA 80% 489 Implicit 489 80 

Table 6.1 TMDL Allocation 

 42
Final TMDL Report Lake Hunter, September, 2004 

6.2 Load Allocation (LA) 

The allowable LA is 489 lbs/year for TP and 6,579 lbs/year for TN. This corresponds to 

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

noted that the LA may include loading from stormwater discharges regulated by the Department 

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

Appendix A). 

6.3 Wasteload Allocation (WLA) 

NPDES Wastewater Discharges 

The WLAwastewater is not applicable because there are no NPDES wastewater facilities present in 

the Lake Hunter watershed. 

NPDES Stormwater Discharges 

The wasteload allocation for stormwater discharges is an 80% reduction in loading, which is the 

required percent reduction in nonpoint sources. It should be noted that the LA may include 

loading from stormwater discharges regulated by the Department and the Water Management 

District that are not part of the NPDES Stormwater Program (see Appendix A). 

6.4 Margin of Safety (MOS) 

Consistent with the recommendations of the Allocation Technical Advisory Committee (Florida 

Department of Environmental Protection, February 2001), an implicit margin of safety (MOS) 

was used in the development of this TMDL. An implicit MOS was provided by the conservative 

decisions associated with a number of modeling assumptions. These include use of event 

mean concentrations for estimating runoff water quality, estimating the maximum load from 

septic tanks rather than average loading, use of total rainfall instead of effective rainfall for 

runoff calculations, use of a probabilistic approach (described in Section 5.1) to determine the 

worst case meteorological conditions instead of using average rainfall conditions, and the 

development of a site-specific alternative water quality target that estimates the assimilative 

capacity. 

 43
Final TMDL Report Lake Hunter, September, 2004 

Chapter 7: NEXT STEPS: IMPLEMENTATION PLAN 

DEVELOPMENT AND BEYOND 

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

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

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

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

reductions will be accomplished. 

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

• Appropriate allocations among the affected parties. 

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

• Timetables for project implementation and completion. 

• Funding mechanisms that may be utilized. 

• Any applicable signed agreements. 

• Local ordinances defining actions to be taken or prohibited. 

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

• Monitoring and follow-up measures. 

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

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

Management cycles. The Department acknowledges the uncertainty associated with TMDL 

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

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

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

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

thereby providing a point of entry for interested parties. 

 44
Final TMDL Report Lake Hunter, September, 2004 

References 

1. Haith D., Mandel R., and Wu R., ‘ Generalized Watershed Loading Functions’, Version 2.0, 

User’s Manual, Department of Agricultural & Biological Engineering, Cornell University, 

Ithaca NY, December 1992 

2. Linsley R., Kohler M., and Paulhus J., ‘Hydrology for Engineers’, McGraw-Hill Book Co., 

1975 

3. Brater E., and King H., ‘Handbook of Hydraulics’, McGraw-Hill Book Co., 1976 

4. Harper, H., ‘Evaluation of Alternative Stormwater Regulations for Southwest Florida’, Draft 

Final Report, Environmental research & Design, Inc., Orlando, Fl, 2002 

5. Chapra S., ‘Surface Water-Quality Modeling’, The McGraw-Hill Companies, Inc., 1997 

6. Walker W., ‘Simplified Procedures for Eutrophication Assessment and Prediction: User 

Manual’, US Army Corps of Engineers Waterways Experiment Station, Instruction Report W96-2, 1996 

7. Knight, B., 'Ten Mile Creek Water Preserve Area - Updated Water Quality Assessment', 

Draft Report prepared for South Florida Water Management District, May 2002 

8. Dames & Moore, ‘City of Lakeland Comprehensive Stormwater Management and Lake 

Pollution Study’, December 1990 

9. North American Lake Management Society and Terrene Institute, ‘Managing Lakes and 

Reservoirs’, 2001 

10. Reckhow, K., ‘EUTROMOD – Version 3.0 Watershed and Lake Modeling Software’, North 

American Lake Management Society, 1990 

11. U.S. Geological Survey, ‘Evaluation of Nitrate Sources Using Nitrogen-Isotope Techniques 

in Shallow Ground Water Within Selected Lake Basin in the Central Lakes District, Polk and 

Highlands Counties, Florida’, Water-Resources Investigation Report 97-4207 

12. U.S. Geological Survey, ‘Estimating Ground-Water Exchange with Lakes Using WaterBudget and Chemical Mass-Balance Approaches for Ten Lakes in Ridge Areas of Polk and 

Highlands Counties, Florida’, Water-Resources Investigations Report 98-4133 

13. Environmental Research & Design, Inc.,‘Lake Morton Diagnostic Evaluation’, prepared for 

the City of Lakeland, 2002 

14. Camp Dresser & McKee, ‘Watershed Management Model Version 4.0. User’s Manual’, 1998 

15. Brown, M.,T., ‘The South Dade Watershed Project’, Center for Urban and Community 

Design, University of Miami, School of Architecture/South Florida Water Management 

District 

16. Florida Department of Environmental Protection. February 2001. A 

Report to the Governor and the Legislature on the Allocation of Total 

Maximum Daily Loads in Florida. Tallahassee, Florida: Bureau of 

Watershed Management. 

 45
Final TMDL Report Lake Hunter, September, 2004 

 46

Appendices 

Appendix A 

Stormwater Regulations 

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

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

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

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

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

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

Code (F.A.C.). 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

by rule.