FLORIDA DEPARTMENT OF ENVIRONMENTAL PROTECTION 

Division of Environmental Assessment and Restoration, 

Bureau of Watershed Restoration 

SOUTHWEST DISTRICT • TAMPA BAY TRIBUTARIES BASIN 

TMDL Report 

Dissolved Oxygen and Nutrient 

TMDL for the Alafia River 

 Above Hillsborough Bay- Tidal Segment 

(WBID 1621G) 

Kevin Petrus 

Kristina Laskis

September 16, 2009
Florida Department of Environmental Protection 

ii
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

iii

Acknowledgments 

This TMDL analysis could not have been accomplished without significant contributions from 

staff in the Florida Department of Environmental Protection’s Southwest District Office, the 

Southwest Florida Water Management District, Tampa Bay Water, Hillsborough County, and the 

Watershed Evaluation and TMDL (WET) Section. Assistance provided by Charles Kovach and 

Jacki Champion, at the FDEP Southwest District Office, in obtaining point source facility 

information is especially appreciated. 

Editorial assistance provided by Jan Mandrup-Poulsen and Linda Lord. 

For additional information on the watershed management approach and impaired waters 

in the Tampa Bay Tributaries Basin, contact: 

Terry Hansen 

Florida Department of Environmental Protection 

Bureau of Watershed Management 

Watershed Planning and Coordination Section 

2600 Blair Stone Road, Mail Station 3565 

Tallahassee, FL 32399-2400 

Email: terry.hansen@dep.state.fl.us

Phone: (850) 245–8561 

Fax: (850) 245–8434 

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

Kevin Petrus 

Florida Department of Environmental Protection 

Bureau of Watershed Management 

Watershed Evaluation and TMDL Section 

2600 Blair Stone Road, Mail Station 3555 

Tallahassee, FL 32399-2400 

Email: kevin.petrus@dep.state.fl.us

Phone: (850) 245–8459 

Fax: (850) 245–8536 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

iv

Contents 

Chapter 1: INTRODUCTION___________________________________1

1.1 Purpose of Report ________________________________________________1

1.2 Identification of Waterbody _________________________________________1

1.2.1 Alafia River Tidal Segment (WBID 1621G) ___________________________1

1.3 Background______________________________________________________5

Chapter 2: DESCRIPTION OF WATER QUALITY PROBLEM ________6

2.1 Statutory Requirements and Rulemaking History ______________________6

2.2 Information on Verified Impairment __________________________________6

2.3 Factors That Influence Nutrient Concentrations_______________________11

2.4 Factors That Influence Dissolved Oxygen____________________________11

Chapter 3. DESCRIPTION OF APPLICABLE WATER QUALITY 

STANDARDS AND TARGETS ______________________13

3.1 Classification of the Waterbody and Criteria Applicable to the TMDL _____13

3.2 Applicable Water Quality Standards and Numeric Water Quality Targets __13

3.2.1 Interpretation of Dissolved Oxygen Criterion _________________________13

3.2.3 Interpretation of Narrative Nutrient Criterion _________________________13

Chapter 4: ASSESSMENT OF SOURCES_______________________16

4.1 Types of Sources________________________________________________16

4.2 Point Sources ___________________________________________________16

4.2.1 NPDES Permitted Wastewater Facilities ____________________________16

4.2.2 Municipal Separate Storm Sewer System Permittees __________________19

4.3 Land Uses and Nonpoint Sources__________________________________19

Chapter 5: DETERMINATION OF ASSIMILATIVE CAPACITY_______24

5.1 Determination of Loading Capacity_________________________________24

5.2 Analysis of Water Quality _________________________________________24

5.3 Relationship Between Nutrients and DO ____________________________29

5.4 Percent Reduction Approach______________________________________30

5.5 Critical Conditions_______________________________________________33

Chapter 6: DETERMINATION OF THE TMDL ____________________34

6.1 Expression and Allocation of the TMDL _____________________________34
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

v

6.2 Load Allocation _________________________________________________35

6.3 Wasteload Allocation ____________________________________________35

6.3.1 NPDES Wastewater Discharges__________________________________35

6.3.2 NPDES Stormwater Discharges __________________________________36

6.4 Margin of Safety_________________________________________________36

Chapter 7: TMDL IMPLEMENTATION __________________________37

TMDL Implementation _______________________________________________37

References _______________________________________________39

Appendices _______________________________________________41

Appendix A: Background Information on Federal and State Stormwater 

Programs__________________________________________________________41

Appendix B: Mosaic Fertilizer Riverview Chemical Complex - Monthy Flow and 

Total Nitrogen Data for Outfalls to the Alafia River _______________________42

Appendix C: Department Response Letter to Comments on TMDL Report ___45
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

vi

List of Tables 

Table 2.1 Verified Impairments for the Alafia River Tidal Segment (WBID 

1621G) _____________________________________________ 7

Table 2.2 Summary of Dissolved Oxygen Data Collected During 

Verification Period (January 2001 – June 2008) for the Alafia 

River Tidal Segment (WBID 1621G) ______________________ 7

Table 2.3 Summary of Chlorophyll-a Annual Averages During Verification 

Period (January 2001 – June 2008) for the Alafia River Tidal 

Segment (WBID 1621G) _______________________________ 8

Table 3.1 Lower Hillsborough Bay (WBID 1558D) Annual Average Total 

Nitrogen and Chlorophyll a Concentrations and Annual Rainfall. 14

Table 4.1 Classification of Land Use Categories in the Lower Alafia River 

Watershed Below Bell Shoals Road, 2004 ________________ 20

Table 5.1 Annual Average Total Nitrogen Values for the Alafia River Tidal 

Segment___________________________________________ 31

Table 5.2 Total Nitrogen Load Estimates for the Lower Alafia River _____ 32

Table 6.1.a TMDL Components Expressed as a Daily Load for the Alafia 

River Tidal Segment (WBID 1621G) _____________________ 35

Table 6.1.b TMDL Components Expressed as an Annual Load for the Alafia 

River Tidal Segment (WBID 1621G) _____________________ 35
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

vii

List of Figures 

Figure 1.1 Location of the Alafia River Tidal Segment (WBID 1621G) with 

Major Geopolitical Features in the Tampa Bay Tributaries 

Group 2 Basin _______________________________________ 3

Figure 1.2 Location of the Alafia River Tidal Segment (WBID 1621G) with 

Major Hydrologic Features______________________________ 4

Figure 2.1 Dissolved Oxygen Measurements in Cycle 2 Verification Period 

(January 2001 – June 2008) ____________________________ 9

Figure 2.2 Chlorophyll a Measurements in Cycle 2 Verification Period 

(January 2001 – June 2008) ____________________________ 9

Figure 2.3 Chlorophyll a Annual Averages in Cycle 2 Verification Period 

(2001 – 2007)_______________________________________ 10

Figure 3.1 Tampa Bay Watershed Alternative Chlorophyll a Thresholds __ 15

Figure 4.1. Location of the Lower Alafia River Point Source Facility _____ 18

Figure 4.2. Principal Land Uses in the Lower Alafia River Watershed 

Below Bell Shoals Road, 2004__________________________ 21

Figure 4.3. Distribution of Onsite Sewage Systems (Septic Tanks) in the 

Lower Alafia River Watershed Below Bell Shoals Road ______ 23

Figure 5.1 Monitoring Locations in the Lower Alafia River _____________ 26

Figure 5.2 Annual Average Chlorophyll a Concentrations in the Estuary 

and Freshwater Segments of the Lower Alafia River and Lower 

Hillsborough Bay WBIDs ______________________________ 27

Figure 5.3 Annual Average Chlorophyll a Concentrations in the Lower 

Alafia River at the Hillsborough County EPC Estuary and 

Freshwater Stations __________________________________ 27

Figure 5.4 Annual Average Total N Concentrations in the Estuary and 

Freshwater Segments of the Lower Alafia River and Lower 

Hillsborough Bay WBIDs ______________________________ 28

Figure 5.5 Annual Average Total N Concentrations in the Lower Alafia 

River at the Hillsborough County EPC Estuary and Freshwater 

Stations ___________________________________________ 28

Figure 5.6 Relationship between Chlorophyll a and Total N at the 

Hillsborough County EPC Estuary Stations ________________ 29

Figure 5.7 Relationship between Biochemical Oxygen Demand and 

Chlorophyll a at the Hillsborough County EPC Estuary Stations 30
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

viii

Web sites 

Florida Department of Environmental Protection, Bureau of 

Watershed Restoration 

TMDL Program 

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

Identification of Impaired Surface Waters Rule 

http://www.dep.state.fl.us/legal/Rules/shared/62-303/62-303.pdf

STORET Program 

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

2008 305(b) Report 

http://www.dep.state.fl.us/water/docs/2008_Integrated_Report.pdf

Criteria for Surface Water Quality Classifications 

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

Basin Status Report for the Tampa Bay Tributaries Basin 

http://www.dep.state.fl.us/water/basin411/tbtribs/status.htm

Water Quality Assessment Report for the Tampa Bay Tributaries Basin 

http://www.dep.state.fl.us/water/basin411/tbtribs/assessment.htm

U.S. Environmental Protection Agency 

Region 4: Total Maximum Daily Loads in Florida 

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

National STORET Program

http://www.epa.gov/storet/
Chapter 1: INTRODUCTION 

1.1 Purpose of Report 

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

for the Alafia River Above Hillsborough Bay (WBID 1621G) water segment located in the Tampa 

Bay Tributaries Group 2 Basin – Alafia River Planning Unit (Figure 1.1). This water segment is 

the marine tidal segment of the river, tributary to Hillsborough Bay, and will be referred to in this 

TMDL report as the Alafia River tidal segment. This reach of the river was verified impaired for 

dissolved oxygen and nutrients (based on elevated chlorophyll-a), and was included on the 

Verified List of impaired waters for the Tampa Bay Tributaries Group 2 Basin that was adopted 

by Secretarial Order in May 2004. The TMDL establishes the allowable loadings to the Alafia 

River Tidal Segment (WBID 1621G) that would restore the waterbody so that it meets its 

applicable water quality criterion for dissolved oxygen and nutrients. 

1.2 Identification of Waterbody 

To provide a smaller-scale geographic basis for assessing, reporting, and documenting water 

quality improvement projects, FDEP divides basin groups into smaller areas called planning 

units. Planning units help organize information and management strategies around prominent 

sub-basin characteristics and drainage features. To the extent possible, planning units were 

chosen to reflect sub-basins that had previously been defined by the Southwest Florida Water 

Management District (SWFWMD). The tidal segment of the Alafia River is located within the 

Alafia River Planning Unit. For assessment purposes, the Department has divided the Alafia 

River Planning Unit into water assessment polygons with a unique waterbody identification 

(WBID) number for each water segment. The estuary of the Alafia River is identified as WBID 

1621G (Figure 1.1). 

1.2.1 Alafia River Tidal Segment (WBID 1621G) 

The Alafia River Tidal Segment is approximately 7.5 miles long and extends from the river’s 

confluence with Buckhorn Creek downstream to the mouth of the river, Figure 1.2. This river 

reach is located in the south central area of Hillsborough County. The watershed area flowing 

directly to this segment, comprises about 5,504 acres (8.6 square miles). The predominant 

landuses are approximately 3,228 acres of urban and built-up, 825 acres of water, and 478 

acres of agriculture. Rice Creek flows into the estuarine reach of the river approximately 5.6 

miles upstream from the river mouth and Buckhorn Creek enters the river at the upper end of 

the tidal segment. Baseflow in Buckhorn Creek is supplemented by ground water discharge 

from Buckhorn Springs, which provides significant flow to the lower river in the dry season 

(SWFWMD, 2008). Upstream of the tidal segment, the river receives flow from Bell Creek, 

Fishhawk Creek, Little Fishhawk Creek, and Lithia Springs all of which are located below the 

USGS flow gaging station at State Highway 640, 16 miles upstream from the river mouth. 

 

The area near the river mouth has been extensively modified by dredge and fill activities that 

were completed by the 1930s (Fehring,1985). A deep-water channel was dredged from the 

main ship channel in Tampa Bay through uplands north of the river mouth to intersect the river 

channel some distance upstream (Stoker et al., 1996). This channel was dredged to provide 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

2

access to the still active fertilizer processing plant near the river mouth downstream of US 

Highway 41. The facility includes a major docking site for the shipping of fertilizer. The 

former river mouth, south of the existing channel was partially filled with the excavated material 

as part of the construction of the barge channel and its’ turning basin. Over the years, sediment 

from a spoil area has accumulated in the historic river mouth, reducing the former river mouth to 

a small tidal creek with little or no connection to the river (SWFWMD, 2008). 

The Alafia River is a tributary to Hillsborough Bay and a large portion of the watershed is 

located in Hillsborough County, with the headwaters extending into Polk County. The Alafia is 

the second largest river watershed that contributes flow to Tampa Bay, encompassing about 19 

percent of the total watershed area of the bay. The river flows in a westerly direction and has 

two major tributaries, the North Prong and South Prong, that join near Alderman’s Ford to form 

the main stem of the river. The river’s total watershed area drains 422 square miles. 

The climate of the Alafia River watershed, is sub-tropical with annual rainfall averaging 

approximately 44.9 inches, although rainfall amounts can vary greatly from year to year 

(CLIMOD, 2009). Based on data from a 30-year period (1971 – 2000), the average summer 

temperature is 89.9o

F, and the average winter temperature is 72.8oF (CLIMOD, 2009). The 

topography of the lower Alafia River watershed reflects its location within the Southwestern 

Florida Flatwoods or Southwestern Coastal Plains ecoregion. Elevations range in the western 

part of the watershed from around 5 – 10 feet above sea level, and in the eastern part of the 

watershed around 35 – 40 feet above sea level (FDEP, 2008). This watershed had three 

predominant soil types. Downstream the soil type is shelly sand and clay. In the middle of the 

watershed the soil type is sandy clay and clay. In the upstream portion of the watershed, the 

soil type is medium fine sand and silt (FDEP, 2008). Population centers in the watershed 

include the areas of Gibsonton and Riverview. 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

3

Figure 1.1 Location of the Alafia River Tidal Segment (WBID 1621G) with 

Major Geopolitical Features in the Tampa Bay Tributaries Group 

2 Basin 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

4

Figure 1.2 Location of the Alafia River Tidal Segment (WBID 1621G) with 

Major Hydrologic Features 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

5

1.3 Background 

This report was developed as part of the Florida Department of Environmental Protection’s 

(Department) watershed management approach for restoring and protecting state waters and 

addressing TMDL Program requirements. The watershed approach, which is implemented 

using a cyclical management process that rotates through the state’s fifty-two river basins over 

a five-year cycle, provides a framework for implementing the TMDL Program–related 

requirements of the 1972 federal Clean Water Act and the 1999 Florida Watershed Restoration 

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

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

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

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

water quality standards. TMDLs provide important water quality restoration goals that will guide 

restoration activities. 

This TMDL report will be followed by the development and implementation of a restoration plan, 

that would restore the waterbody so that it meets its applicable water quality criteria for 

dissolved oxygen and nutrients. These activities will depend heavily on the active participation 

of the Southwest Florida 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 

impaired waterbodies. 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

6

Chapter 2: DESCRIPTION OF WATER QUALITY 

PROBLEM 

2.1 Statutory Requirements and Rulemaking History 

Section 303(d) of the federal Clean Water Act requires states to submit to the U.S. 

Environmental Protection Agency (EPA) lists of surface waters that do not meet applicable 

water quality standards (impaired waters) and establish a TMDL for each pollutant causing 

impairment of 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 FWRA (Subsection 403.067[4], Florida 

Statutes [F.S.]); the state’s 303(d) list is amended annually to include basin updates. 

Florida’s 1998 303(d) list included several waterbodies in the Tampa Bay Tributaries Basin. 

The Alafia River Tidal Segment (WBID 1621G) is 1998 303(d) listed. However, the FWRA 

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

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

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

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

Code (F.A.C.) (Identification of Impaired Surface Waters Rule, or IWR), in April 2001; the rule 

was modified in 2006 and 2007. 

2.2 Information on Verified Impairment 

The Department used the IWR methodology to assess water quality impairments and has 

verified the impairments for dissolved oxygen (DO) and nutrients in the Alafia River Tidal 

Segment (Table 2.1). The IWR assessment was based on samples collected during the Cycle 

2 verified period (January 2001 to June 2008), and are sample results made available in IWR 

Database Run 36. Table 2.2 provides the assessment results for dissolved oxygen following 

the IWR methodology. The WBID was verified as impaired for DO because more than 10 

percent of the values were below the Class III marine criterion minimum value of 4.0 

milligrams/liter (mg/L) over the course of the verified period. The individual DO measurements 

used in the assessment are displayed in Figure 2.1. For the verified nutrient impairment, 

annual average chlorophyll-a values serve as the primary means for assessing nutrient 

impairment in estuaries in the IWR. The individual chlorophyll a results used to calculate the 

annual averages are displayed in Figure 2.2. During the verified period, the annual average 

chlorophyll-a values for the tidal segment (WBID 1621G) were above the site specific 

chlorophyll a threshold of 15 micrograms per liter (μg/L) in five of the seven calendar years, 

averaging between 16.5 μg/L in 2003 and 28.8 μg/L in 2006 (Table 2.3 and Figure 2.3). 

According to the IWR, if the annual mean chlorophyll-a for any one year in the verified period is 

greater than the chlorophyll-a threshold, the water is verified as impaired. For the estuary 

segments of waterbodies tributary to Hillsborough Bay, the Hillsborough Bay chlorophyll a target 

of 15 ug/L, developed by the Tampa Bay Estuary Program to protect and restore seagrass, is 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

7

used as the alternative chlorophyll a threshold. Further discussion of water quality target 

development for this TMDL is provided in Chapter 3. 

The main sources of data for the IWR assessment came from stations sampled by the 

Hillsborough County Environmental Protection Commission (21FLHILL...), Florida Game and 

Freshwater Fish Commission (21FLGFWF...), USGS (112WRD...), Florida DEP Southwest 

District (21FLA...), Florida Lake Watch (21FLKWAT...), PBSJ (PBSJ...), and Tampa Bay Water 

(21FLTBW...). The Florida Lake Watch data, however, can not be used for verification 

purposes, but the nutrient data were included in the analysis used for TMDL development in 

Chapter 5. The water quality measurements used in this analysis are available in IWR 

Database Run 36, and are available upon request. 

Table 2.1 Verified Impairments for the Alafia River Tidal Segment (WBID 

1621G) 

WBID 

Waterbody 

Segment Name 

Parameters 

Included on the 

1998 303(d) List 

Parameter 

Causing 

Impairment 

Projected 

Year for 

TMDL 

Development* 

1621G 

Alafia River 

Above 

Hillsborough Bay Nutrients (Chla) Nutrients (Chla) 2008 

1621G 

Alafia River 

Above 

Hillsborough Bay 

Dissolved 

Oxygen Nutrients (TN) 2008 

*The projected year for TMDL development was 2008, but the Settlement Agreement between 

EPA and Earthjustice, which drives the TMDL development schedule for waters on the 1998 

303(d) list, allows an additional nine months to complete the TMDLs. As such, these TMDLs must 

be adopted and submitted to EPA by September 30, 2009. 

Table 2.2 Summary of Dissolved Oxygen Data Collected 

During Verification Period (January 2001 – June 

2008) for the Alafia River Tidal Segment (WBID 

1621G) 

Waterbody 

Segment 

Name 

Total 

Number 

of 

Samples 

IWR-required 

number of 

exceedances for 

the Verified List 

Number of 

observed 

exceedances 

Number of 

observed 

nonexceedances 

Number of 

seasons 

data was 

collected 

Alafia River 

Above 

Hillsborough 

Bay 2101 252 304 1797 4 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

8

Table 2.3 Summary of Chlorophyll-a Annual Averages During Verification 

Period (January 2001 – June 2008) for the Alafia River Tidal 

Segment (WBID 1621G) 

Year Chlorophyll-a Annual 

Average (ug/L) 

2001 11.5 

2002 27.9 

2003 16.5 

2004 10.8 

2005 17.7 

2006 28.8 

2007 18.3 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

9

Figure 2.1 Dissolved Oxygen Measurements in Cycle 2 Verification 

Period (January 2001 – June 2008) 

Figure 2.2 Chlorophyll a Measurements in Cycle 2 Verification 

Period (January 2001 – June 2008) 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

10

Figure 2.3 Chlorophyll a Annual Averages in the Cycle 2 

Verification Period (2001 – 2007)

As part of the verified listing process, the Department attempts to identify the limiting nutrient or 

nutrients for the impaired waterbody. The limiting nutrient, generally nitrogen or phosphorus, is 

defined as the nutrient that limits plant growth when it is not available in sufficient quantities. A 

limiting nutrient is a chemical that is necessary for plant growth, but available in quantities 

smaller than those needed for algae, represented by chlorophyll-a, and macrophytes to grow. 

Once the limiting nutrient in a waterbody is exhausted, algae stop growing. If more of the 

limiting nutrient is added, larger algal populations will result until nutrients or other 

environmental factors again limit their growth. 

In Florida waterbodies, nitrogen and phosphorus are most often the limiting nutrients, and 

nitrogen is typically the limiting nutrient in most Florida estuaries. There is a general 

understanding in the marine scientific community that nitrogen is the principal cause of nutrient 

over enrichment in coastal systems (National Research Council, 1993) and an analysis of the 

data from the Alafia River tidal segment supports this conclusion. 

Determining the limiting nutrient in a waterbody can be accomplished by calculating the ratio of 

nitrogen to phosphorus in the waterbody, with water column ratios of total nitrogen (TN) to total 

phosphorus (TP) of less than 10 indicating that nitrogen is limiting. The median TN to TP ratio 

is 1.9 (computed from n=560 values), indicating that nitrogen is the limiting nutrient in the tidal 

segment of the Alafia River. The median value for total nitrogen (TN) in the verified period is 

1.41 mg/L; (n = 561). 

Since nitrogen is the limiting nutrient, reductions in TN loadings would be expected to result in 

decreases in algal growth, and are measured as decreases in chlorophyll a levels. Reductions 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

11

in TN loading are also expected to result in additional benefits, including improvement in DO 

conditions and decreases in biochemical oxygen demand (BOD). BOD is defined as the 

amount of oxygen required by bacteria while stabilizing decomposable organic matter under 

aerobic conditions (Sawyer & McCarty, 1967). Reductions in nutrients will result in lower algal 

biomass levels in the water column, and lower algal biomass levels will result in smaller diurnal 

fluctuations in DO, fewer algal-based total suspended solids, and reduced BOD. 

2.3 Factors That Influence Nutrient Concentrations 

Recent data compiled by Harper & Baker (2007) demonstrated that increasing intensity of 

landuse generally results in increasing nutrient runoff concentration (Harper & Baker, 2007, 

Table 4-17) and an increase in runoff volume (Harper & Baker, 2007, Figure 4-4). Both 

urbanization and increasing intensity of agricultural landuse (e.g. conversion from rangeland to 

a managed pasture) can result in increased delivery of nutrients to local receiving waters. 

Specific agricultural activities that can contribute to the declining health of the system include 

water flow changes due to the creation of secondary and tertiary canal systems for use in 

irrigation and flood control and the introduction of nutrients via fertilization. Urbanization can 

result in reduction of pervious areas for infiltration of runoff, which contributes significantly to 

increased runoff and nutrient load (Harper & Baker, 2007). Other activities associated with 

urbanization also increase nutrient inputs such as the installation of septic tanks, sewage 

overflows, fertilizer usage, and the use of irrigation quality water in sprinkler systems in golf 

courses and new housing developments. The impact of agricultural and urban activities on 

eutrophication of receiving waters can be decreased through the implementation of best 

management practices (BMPs). 

2.4 Factors That Influence Dissolved Oxygen 

The availability of DO in a marine or freshwater system is highly variable due to several factors. 

Oxygen is produced in the water column by photosynthesis and is consumed by respiration of 

plants, animals and aerobic bacteria, and by chemical reactions that occur in brackish waters 

due to the interaction of sunlight, humic and fulvic materials, as well as oxidation and reduction 

reactions. The ability of a system to absorb oxygen from the atmosphere is dependent on flow 

factors such as water depth and turbulence. Elevated nitrogen and phosphorus compounds 

contribute to excess algae growth. Under high nutrient levels, algae grow rapidly and raise DO 

concentrations during daylight hours. Respiration by the dense algal populations and other 

consumers reduce DO concentrations during the night. When phytoplankton cells die, they sink 

towards the bottom, and are decomposed by bacteria, a process that further reduces DO in the 

water column. 

Factors that may cause significant oxygen depletion include BOD and sediment oxygen demand 

(SOD). BOD, including carbonaceous BOD (CBOD) and nitrogenous BOD (NBOD) may be the 

product of both naturally occurring oxygen use from the decomposition of organic materials, and 

the stabilization of waste products associated with nonpoint source runoff. The significance of 

any of these factors depends on the specific stream conditions. BOD related to microorganisms 

is called CBOD. The source material for CBOD is organic matter. CBOD results when oxygen 

is consumed by microorganisms in converting organic material into CO2, H20, nutrients, energy, 

and new cells. Algal cells contain organic chemicals that consume oxygen during 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

12

decomposition. BOD related to chemical oxidation is called NBOD. The source materials for 

NBOD include organic matter that decays to ammonia, and ammonia entering the system 

through stormwater systems or runoff. Nitrification, the process of oxidizing ammonia to nitrates 

by microorganisms, requires almost 5 mg/L of DO (NBOD) for every milligram per liter of 

ammonia that is oxidized. 

SOD is the overall demand for DO from the water column that is exerted by the combination of 

biological, biochemical, and chemical processes at the sediment-water interface. The primary 

sources of SOD are anaerobic (low-oxygen) chemical compounds in the riverbed sediments and 

particulate BOD (including algae and other sources of organic matter) that settle out of the water 

column. SOD is generally composed of biological respiration from benthic organisms and the 

biochemical (i.e., bacterial) decay processes in the top layer of deposited sediments. In addition 

to DO depletion, degradation of organic matter in the sediment results in the release of oxygendemanding (i.e., reduced) nutrients, metals, ammonium, iron, manganese, sulfide, and 

ammonia (Price et al, 1994). These soluble chemicals are released into the water and exert a 

relatively rapid (i.e., it occurs on a timescale of hours) oxygen demand as the reduced 

chemicals are oxidized. Some oxidation processes, such as nitrification of ammonia to nitrate, 

require bacteria and may be slower (i.e., days). In stratified waters, the sediment and the 

bottom layer of water are somewhat "trapped" and the oxygen is depleted as a result of decay 

of organic matter and lack of exchange of oxygenated water from upper layers (EPA, 2007). 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

13

Chapter 3. DESCRIPTION OF APPLICABLE WATER 

QUALITY STANDARDS AND TARGETS 

3.1 Classification of the Waterbody and Criteria Applicable to 

the TMDL 

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

Class I Potable water supplies 

Class II Shellfish propagation or harvesting 

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

Class IV Agricultural water supplies 

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

waters currently in this class) 

3.2 Applicable Water Quality Standards and Numeric Water Quality Targets 

The Alafia River Tidal Segment (WBID 1621G) is a Class III waterbody, with a designated use 

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

wildlife. The Class III water quality criteria applicable to the impairment addressed by this TMDL 

are for DO and the narrative nutrient criteria. 

3.2.1 Interpretation of Dissolved Oxygen Criterion 

The Class III marine criterion for DO, as established by Subsection 62-302.530(30), F.A.C., 

states that DO shall not average less than 5.0 mg/L in a 24-hour period, and shall not be less 

than 4 mg/L, and that normal daily and seasonal fluctuations above these levels shall be 

maintained. 

3.2.3 Interpretation of Narrative Nutrient Criterion 

Florida’s nutrient criterion is narrative only, i.e. nutrient concentrations of a body of water shall 

not be altered so as to cause an imbalance in natural populations of aquatic flora or fauna. 

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

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

for estuaries based on annual average chlorophyll a 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 (Section 62-303.450, F.A.C.) specifically allows the use of alternative, site-specific 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

14

thresholds that more accurately reflect conditions beyond which an imbalance in flora or fauna 

occurs in the waterbody. 

In translating the narrative nutrient criterion to develop a nutrient target for this TMDL, the first 

step in this process was to identify a chlorophyll a target. The Department selected a 

chlorophyll a target of 15 ug/L, that was developed for the adjacent Hillsborough Bay segment 

by the Tampa Bay Estuary Program for making resource-based management decisions (Janicki 

and Wade, 1996; Janicki et al., 2000). The chlorophyll a target was developed as part of the 

nitrogen management strategy to protect and restore seagrass meadows in the bay. The 

Department uses this water quality target as a site specific alternative chlorophyll a threshold for 

the Hillsborough Bay segments (and the estuary segments tributary to the bay) to perform 

nutrient assessments following the IWR methodology. Applying the bay threshold to the 

adjacent estuary segments provides for consistency in assessing nutrients in this area of Tampa 

Bay. Figure 3.1 displays the water segments in the Tampa Bay watershed where the 

resource-based chlorophyll a targets are used as alternative chlorophyll a thresholds. 

As the lower Hillsborough Bay segment (WBID 1558D), that the Alafia River flows into, currently 

has annual average chlorophyll a values less than the alternative threshold, the recently 

measured total nitrogen (TN) concentrations in the bay were used to derive the water quality 

target used for TMDL development. Table 3.1 provides the lower Hillsborough Bay annual 

average chlorophyll a and TN values along with rainfall from 1995 to 2006. Over this time 

frame, as annual average TN concentrations have exhibited a general decrease in the bay, 

there has been a similar decreasing trend in annual average chlorophyll a concentrations. This 

decreasing trend in Hillsborough Bay TN and chlorophyll a values has occurred in years with 

above and below average rainfall. 

Table 3.1 Lower Hillsborough Bay (WBID 1558D) Annual 

Average Total Nitrogen and Chlorophyll a 

Concentrations and Annual Rainfall. 

Year 

 Annual 

Average 

Total N 

(mg/L) 

Annual 

Average 

Chlorophyll a 

(ug/L) 

Hillsborough 

Bay 

Watershed 

Annual 

Rainfall (in.)1

1995 0.87 21.2 56 

1996 0.76 16.3 51 

1997 0.74 14.3 65 

1998 0.86 21.3 58 

1999 0.64 11.8 44 

2000 0.92 8.6 37 

2001 0.87 17.3 55 

2002 0.79 14.2 64 

2003 0.58 14.1 59 

2004 0.56 14.7 65 

2005 0.52 10.7 55 

2006 0.46 11.6 43 

1

 From Tampa Bay Estuary Program 2004 - 2007 Pollutant Loading Update Report 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

15

Figure 3.1 Tampa Bay Watershed Alternative Chlorophyll a 

Thresholds 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

16

Chapter 4: ASSESSMENT OF SOURCES 

4.1 Types of Sources 

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

source subcategories, or individual sources of pollutants in the impaired waterbody and the 

amount of pollutant loadings 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 System (NPDES) Program. These nonpoint sources included certain urban 

stormwater discharges, including those from local government master drainage systems, 

construction sites over five acres, and a wide variety of industries (see Appendix A for 

background information on the federal and state stormwater programs). 

To be consistent with Clean Water Act definitions, the term “point source” 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. 

4.2 Point Sources 

4.2.1 NPDES Permitted Wastewater Facilities

There is one industrial facility, the Mosaic Fertilizer, L.L.C. Riverview Chemical Complex, that 

has an NPDES surface water discharge permit with four effluent outfall locations to the tidal 

segment of the Alafia River, NPDES No. FL0000761. The facility manufactures sulfuric acid, 

phosphoric acid, fluoride products, ammoniated phosphates and micronutrient added fertilizers, 

and animal feed phosphates. Other operations at the site include electric cogeneration, 

phosphogypsum storage, raw material storage and handling, wastewater storage and handling, 

process material handling, and product shipping facilities, that includes a major vessel docking 

site for fertilizer shipping. 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

17

The four outfalls are located in close proximity to each other on the north side of the river, 

approximately 0.75 miles upstream of the river mouth. Outfalls designated as D-001, D-005A, 

and D-005B discharge non-process wastewater, treated process wastewater, and stormwater. 

Outfall D-021 discharges treated stormwater runoff from the southern basin of the facility 

complex into the Alafia River. The location of the facility and point source discharge area is 

shown in Figure 4.1. 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

18

Figure 4.1. Location of the Lower Alafia River Point Source Facility 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

19

4.2.2 Municipal Separate Storm Sewer System Permittees 

Municipal separate storm sewer systems (MS4s) may also discharge pollutants to waterbodies 

in response to storm events. To address stormwater discharges, the EPA developed the 

NPDES stormwater permitting program in two phases. Phase 1, promulgated in 1990, 

addresses large and medium-size MS4s located in incorporated areas and counties with 

populations of 100,000 or more. Phase 2 permitting began in 2003. Regulated Phase 2 MS4s 

are defined in Section 62-624.800, F.A.C., and typically cover urbanized areas serving 

jurisdictions with a population of at least 10,000, or discharging into Class I or Class II waters, or 

into Outstanding Florida Waters. 

The stormwater collection systems owned and operated by Hillsborough County and CoPermittees (FDOT District 7 & Florida’s Turnpike Enterprise, and City of Plant City) are covered 

by a Phase I NPDES municipal separate storm sewer system (MS4) permit (FLS000006). 

There are no Phase II MS4 permits identified for the tidal segment of the Alafia River. 

4.3 Land Uses and Nonpoint Sources 

Nonpoint source pollution, unlike pollution from industrial and sewage treatment plants, comes 

from many diffuse sources. Nonpoint pollution is caused by rainfall moving over and through 

the ground. As the runoff moves, it picks up and carries away natural and human-made 

pollutants, finally depositing them into lakes, rivers, wetlands, coastal waters, and even our 

underground sources of drinking water (EPA, 1994). 

Nutrient loading from urban areas is most often attributable to multiple sources, including 

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

waste, runoff from improper disposal of waste materials, leaking septic systems, and domestic 

animals. In agricultural areas, agricultural fertilizing or nutrients from wildlife and agricultural 

livestock wastes are sources contributing to the pollutant load. 

Land Uses 

The spatial distribution and acreage of different land use categories were identified using the 

SWFWMD 2004 land use coverage (scale 1:40,000) contained in the Department’s geographic 

information system (GIS) library, as this time frame represents land cover near the midpoint of 

the verified assessment period. The land use information analyzed for this TMDL is the 

watershed area draining to the Lower Alafia River downstream of Bell Shoals Road. Bell Shoals 

Road crosses the river 11.3 miles upstream of the mouth. This drainage area was used as the 

loads generated are in close proximity to the tidal segment and is consistent with the watershed 

area applied by the SWFWMD for minimum flow and level development for the lower Alafia 

River (SWFWMD, 2008). Land use categories were aggregated using the simplified Level 1 

codes and are tabulated in Tables 4.1. Figure 4.2 shows the principal land uses in the 

watershed area of the lower river. 

The lower river watershed drains 30,140 acres (47.1 miles2

), Table 4.1. The dominant land use 

is urban, which covers over 47 percent of the watershed. The majority of the urban area is 

residential land use, accounting for about 39 percent of the watershed area. Medium density 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

20

residential covers over 20 percent of the urban area. Agriculture comprises 18 percent of the 

area, and cropland and pastureland make up 72 percent of the agriculture. Rangeland and 

forest cover 16.5 percent of the watershed and water and wetlands make up over 16 percent of 

the area. 

Table 4.1 Classification of Land Use Categories in the Lower Alafia River 

Watershed Below Bell Shoals Road, 2004 

Level 1 Code Landuse Acreage Percent of 

Total 

1000 Urban Open 2,604 8.6 

1100 Low Density Residential 2,898 9.6 

1200 Medium Density Residential 6,173 20.5 

1300 High Density Residential 2,587 8.6 

2000 Agriculture 5,437 18.0 

3000+4000 Rangeland + Forest/Rural Open 4,972 16.5 

5000 Water 1,448 4.8 

6000 Wetlands 3,483 11.6 

7000 Barren Land (Transportation) 198 0.7 

8000 Communication and Transportation 340 1.1 

Total 30,140 100.0 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

21

Figure 4.2. Principal Land Uses in the Lower Alafia River Watershed Below 

Bell Shoals Road, 2004 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

22

Septic Tanks 

Septic tanks are another potentially important source of pollution. Information on the location of 

septic systems was obtained from Hillsborough County, which is based on an ongoing study to 

identify septic tanks. The septic tank database was developed from records obtained from the 

Florida Department of Health and sewer billing records (David Glicksberg, personal 

communication) 

The septic tanks located in the lower Alafia River watershed are displayed in Figure 4.3. 

Currently, the number of septic tanks in this watershed area is estimated to be 2,331. The map 

shows that a large number of systems are in close proximity to the river channel, between US 

Highway 41 and Bell Shoals Road. 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

23

Figure 4.3. Distribution of Onsite Sewage Systems (Septic Tanks) in the 

Lower Alafia River Watershed Below Bell Shoals Road 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

24

Chapter 5: DETERMINATION OF ASSIMILATIVE 

CAPACITY 

5.1 Determination of Loading Capacity 

The goal of the TMDL development process is to identify a total nitrogen (TN) target for the 

estuarine area of the Alafia River, so that the tidal segment will meet the applicable DO and 

nutrient water quality criteria and maintain its function and designated use as a Class III water. 

In this case, information used to develop the TMDL was based on water quality changes in the 

lower Hillsborough Bay segment and relationships between water quality variables in the Alafia 

River tidal segment. The existing water quality in lower Hillsborough Bay, which is not impaired 

for DO and nutrients, is used as a reference condition for establishing the TMDL. 

5.2 Analysis of Water Quality 

Water quality data collection in the Alafia River tidal segment (WBID 1621G), available to the 

Department, have been performed by four different agencies and the sampling locations are 

shown in Figure 5.1. The majority of data collection in the lower river is performed by the 

Hillsborough County Environmental Protection Commission (HCEPC) and by Tampa Bay 

Water. The HCEPC collects samples at two fixed locations in the estuary, Station 74 at US 

Highway 41 and Station 153 at US Highway 301, that are sampled approximately monthly. The 

county also samples monthly in the freshwater reach of the lower river at Bell Shoals Road 

(Station 114), Figure 5.1. Tampa Bay Water conducts water quality sampling in the lower river 

as part of the Hydrobiological Monitoring Program (HBMP). The HBMP consists of using a 

probabilistic design in which sampling is randomized within seven strata that are oriented along 

the longitudinal axis of the lower river between the river mouth and Bell Shoals Road. Data 

collection within each stratum occurs on a monthly basis. Data collected by the U.S. Geological 

Survey and Florida Lake Watch are also included in this assessment. The Florida Lake Watch 

data, however, can not be used for verification purposes, but have been included in the analysis 

of nutrient data used for TMDL development. 

Trends in chlorophyll a for two segments of the lower Alafia River (WBIDs 1621G and 1621A) 

and for the lower Hillsborough Bay segment (WBID 1558D), are presented in Figure 5.2. 

Chlorophyll a annual averages in the lower Hillsborough Bay WBID have exhibited a decreasing 

trend, with the values in the last five years being less than the alternative chlorophyll a threshold 

of 15 ug/L, Figure 5.2. The annual average chlorophyll a values in the Alafia River tidal 

segment (WBID 1621G), suggest a decreasing trend up to the year 1999, and since the year 

2000, the values have exhibited greater variability fluctuating above and below the chlorophyll a 

threshold, Figure 5.2. It is problematic to compare the more recent tidal segment annual 

averages to annual averages prior to 2000, since additional monitoring has been conducted in 

more recent years, with the addition of the HBMP monitoring and sampling by the HCEPC at US 

Highway 301. 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

25

A time series of chlorophyll a results measured at the HCEPC stations in the lower Alafia River 

are presented in Figure 5.3. In Figure 5.3, the annual averages suggest a slight decreasing 

trend over time at the US Highway 41 sampling location (Station 74). Further upstream in the 

tidal segment, annual average values have fluctuated considerably from year to year since 

monitoring began in 1999 at US Highway 301 (Station 153), and the values are typically well 

above the chlorophyll a threshold. Chlorophyll a annual averages at the Bell Shoals Road 

sampling location (Station 114) are much lower than the values in the tidal segment, with 

averages being well below 10 ug/L since 1994. 

Trends in annual average TN values by WBID and by HCEPC station are displayed in Figure 

5.4 and Figure 5.5, respectively. The annual average TN concentrations by WBID shows a 

decreasing trend over time in the lower Hilllsborough Bay segment, whereas, a slight increasing 

trend is evident in the tidal segment and freshwater segment of the Lower Alafia River, Figure 

5.4. In Figure 5.5, the annual averages have been lower in the last three years at the US 

Highway 41 sampling location (Station 74), and during the same period decreases in annual 

averages have also occurred in the lower Hillsborough Bay WBID, Figure 5.4. Total N 

concentrations are on average lower at the downstream sampling location (1.5 mg/L and less), 

compared to the HCEPC stations further upstream, Figure 5.5. 

A statistical evaluation of the data was performed to investigate the relationship between total 

nitrogen and chlorophyll a concentrations, but no strong relationships were found between the 

variables. The comparison, however, does indicate that generally lower chlorophyll a 

concentrations are associated with lower total N concentrations at the HCEPC sampling 

locations in the tidal segment. The linear regression analysis using data collected at US 

Highway 41 and US Highway 301 during the verified period, indicates that 19 percent of the 

variation in chlorophyll a can be explained by total N, Figure 5.6. 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

26

Figure 5.1 Monitoring Locations in the Lower Alafia River 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

27

Annual Average Chlorophyll a Values 

Calculated Using IWR Methodology

0

10

20

30

40

50

60

70

80

90

Chlorophyll a (ug/L)

Alafia River Estuary (WBID 1621G) Alafia River Freshw ater (WBID 1621A)

Low er Hillsborough Bay (WBID 1558D) Estuary Chlorophyll a Threshold

Figure 5.2 Annual Average Chlorophyll a Concentrations in the Estuary 

and Freshwater Segments of the Lower Alafia River and 

Lower Hillsborough Bay WBIDs 

Lower Alafia River Annual Average Chlorophyll a 

at Hillsborough County EPC Stations

0

10

20

30

40

50

60

70

80

90

Year

Uncorrected Chlorophyll a (ug/L)

HCEPC 74 (Estuary) HCEPC 153 (Estuary)

HCEPC 114 (Stream) Estuary Chlorophyll a Threshold

Figure 5.3 Annual Average Chlorophyll a Concentrations in the Lower 

Alafia River at the Hillsborough County EPC Estuary and 

Freshwater Stations
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

28

Annual Average Total Nitrogen

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Total Nitrogen (mg/L)

Alafia River Estuary (WBID 1621G) Alafia River Freshw ater (WBID 1621A)

Low er Hillsborough Bay (WBID 1558D)

Figure 5.4 Annual Average Total N Concentrations in the Estuary and 

Freshwater Segments of the Lower Alafia River and Lower 

Hillsborough Bay WBIDs

Lower Alafia River Annual Average TN 

at Hillsborough County EPC Stations

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Total Nitrogen (mg/L)

HCEPC 74 (Estuary) HCEPC 153 (Estuary) HCEPC 114 (Stream)

Figure 5.5 Annual Average Total N Concentrations in the Lower Alafia 

River at the Hillsborough County EPC Estuary and 

Freshwater Stations 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

29

Alafia River Estuary HCEPC Stations 74 & 153 

Chlorophyll a vs. Total Nitrogen (2001 - 2007)

y = 30.661x - 15.334

R2 = 0.1909

0

50

100

150

200

250

300

350

012345

Total Nitrogen (mg/L)

Uncorrected Chlorophyll 

a

(ug/L)

Figure 5.6 Relationship between Chlorophyll a and Total N at the 

Hillsborough County EPC Estuary Stations

5.3 Relationship Between Nutrients and DO 

Reductions in TN concentrations are also expected to result in additional benefits for other 

parameters of concern, including DO and BOD. As described in Chapter 2, reduced algal 

biomass, as measured by chlorophyll a, should result in lower BOD levels. During daylight 

hours, algal photosynthesis consumes nutrients and organic matter. The organic matter acts as 

an energy reserve, and oxygen is released. The reverse process, respiration, may occur 

simultaneously and dominate during dark periods of the day. During respiration, algae consume 

oxygen and their energy reserve to produce carbon dioxide and water. Because photosynthesis 

creates oxygen and respiration depletes oxygen, the algae affect the estuary’s oxygen sources. 

Swings in oxygen can be induced by diurnal light patterns where oxygen levels rise during 

daylight and become depleted at night. 

Figure 5.7 shows that 51 percent of the variation in BOD at the US Highway 41 and US 

Highway 301 locations can be explained by the chlorophyll a concentration, suggesting that 

reductions in chlorophyll a levels would result in improvements to dissolved oxygen. Lower 

algal biomass should lower the BOD levels in the creek, and sediment oxygen demand (SOD) in 

the area should also decrease over time as algal biomass reduction reduces the accumulation 

of organic matter in the sediments. Sediment processes play an important role in regulating 

water quality and are particularly important in a shallow estuary. A portion of the organic matter 

produced in the water column settles to the sediment surface. Sediment processes influence 

DO in the water column by serving as a long-term repository of oxygen demand. A reduction of 

both algal BOD and SOD will have a positive impact on DO concentrations in the water column. 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

30

Alafia River Estuary HCEPC Stations 74 & 153 

Biochemical Oxygen Demand (5 Day) vs. Chlorophyll a 

(2001 - 2007)

y = 0.0298x + 1.4447

R2 = 0.5134

0

2

4

6

8

10

12

14

0 50 100 150 200 250 300 350

Uncorrected Chlorophyll a (ug/L)

BOD5 ( mg/L)

Figure 5.7 Relationship between Biochemical Oxygen Demand and 

Chlorophyll a at the Hillsborough County EPC Estuary 

Stations 

5.4 Percent Reduction Approach 

The TMDL calculation was developed using the “percent reduction” approach. For this method, 

the selected water quality target for TMDL development is the average of the annual average 

TN concentrations from the eight years when chlorophyll a in lower Hillsborough Bay was below 

the threshold value of 15 ug/L. The annual average TN concentrations are from the years 1997, 

1999, 2000, and 2002 to 2006, Table 3.1. The average of these annual average values is 0.65 

mg/L. The percent reduction was calculated using the following equation. 

 [measured exceedance – target] X 100 

measured exceedance 

The measured exceedances used in the percent reduction calculation are the Alafia River tidal 

segment annual average TN concentrations from 2000 to 2006, Table 5.1. The percent 

reduction in the average of these TN annual averages, 54 percent, is the TN reduction used for 

establishing the TMDL. In other words, to achieve the TN target of 0.65 mg/L in the Alafia River 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

31

tidal segment, the percent reduction needed in the current TN concentration of 1.41 mg/L is 54 

percent. Annual average values were used in establishing the TMDL in this case as the IWR 

nutrient assessment is performed using annual average chlorophyll a values. 

Table 5.1 Annual Average Total Nitrogen Values for the 

Alafia River Tidal Segment 

Year 

Alafia River 

Tidal Segment 

Annual Average 

TN 

Concentration 

(mg/L) 

2000 1.16 

2001 1.14 

2002 1.39 

2003 1.44 

2004 1.82 

2005 1.34 

2006 1.55 

Average 1.41 

Total nitrogen loads entering the lower river from the Mosaic Fertilizer facility discharges and 

from upstream watershed loads were calculated to determine the magnitude of the point source 

load. Mosaic Fertilizer loads were calculated by the Department’s Southwest District Office 

using results on the facility discharge monitoring reports. Total nitrogen loads from the 

watershed upstream of the lower river were calculated using estimated flows at Bell Shoals 

Road obtained from Tampa Bay Water and nitrogen concentration data collected at this location 

by the HCEPC. Loadings calculated for the 2003-2007 period indicate that, on average, the 

point source load is less than one percent (0.7 percent) of the upstream river load, Table 5.2. 

The monthly average facility load varied between 21 lbs/month and 3,436 lbs/month over this 

time frame. The point source load percentage is actually less than this amount, since nonpoint 

source loadings downstream of Bell Shoals Road were not included in this analysis. As the 

point source load is a small fraction of the total load, the point source TN load discharged to the 

river for the 2001 to 2007 period was used to establish the facility’s wasteload allocation in the 

TMDL. Appendix B provides the data used to calculate the Mosaic Fertilizer facility’s TN load 

to the Alafia River. 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

32

Table 5.2 Total Nitrogen Load Estimates for the Lower 

Alafia River 

Year Month 

Alafia River at 

Bell Shoals 

Road - Total 

Nitrogen Load 

(lbs/month) 

Mosaic 

Fertilizer 

Riverview 

Complex - 

Total N Load to 

Alafia River 

(lbs/month) 

Point Source 

Load as 

Percentage of 

River Load 

2003 3 175290.7 600.9 0.34 

2003 4 84012.3 73.3 0.09 

2003 5 84568.8 83.4 0.10 

2003 6 301440.9 218.2 0.07 

2003 7 316836.8 452.8 0.14 

2003 8 185273.2 603.4 0.33 

2003 9 221924.6 194.2 0.09 

2003 10 137997.6 197.6 0.14 

2003 11 55696.5 85.8 0.15 

2003 12 48786.4 73.4 0.15 

2004 1 88990.8 112.0 0.13 

2004 2 90604.5 180.9 0.20 

2004 3 73971.1 71.5 0.10 

2004 4 41863.3 128.9 0.31 

2004 5 31010.1 98.5 0.32 

2004 6 81436.6 171.8 0.21 

2004 7 110865.1 1636.2 1.48 

2004 8 384469.9 1132.6 0.29 

2004 9 496475.8 2192.0 0.44 

2004 10 660963.7 142.6 0.02 

2004 11 149145.0 21.0 0.01 

2004 12 194143.8 38.3 0.02 

2005 1 137840.7 82.5 0.06 

2005 2 61875.2 224.3 0.36 

2005 3 198965.3 230.4 0.12 

2005 4 131718.7 85.5 0.06 

2005 5 90468.6 163.0 0.18 

2005 6 213021.1 3436.4 1.61 

2005 7 318624.3 306.4 0.10 

2005 8 164576.8 615.1 0.37 

2005 9 79613.1 328.0 0.41 

2005 10 147515.7 1253.6 0.85 

2005 11 95525.5 1119.6 1.17 

2005 12 49316.9 336.1 0.68 

2006 1 35867.3 50.7 0.14 

2006 2 132067.6 330.8 0.25 

2006 3 40876.5 28.6 0.07 

2006 4 21496.9 37.1 0.17 

2006 5 24021.0 787.2 3.28 

2006 6 52781.2 678.2 1.28 

2006 7 64624.2 1357.7 2.10 

2006 8 175431.3 382.0 0.22 

2006 9 242619.8 343.7 0.14 

2006 10 82504.5 114.6 0.14 

2006 11 48477.9 953.0 1.97 

2006 12 38808.7 123.5 0.32 

2007 1 44552.9 1496.2 3.36 

2007 2 62048.3 581.6 0.94 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

33

2007 3 33548.8 328.3 0.98 

2007 4 24492.8 367.7 1.50 

2007 5 13063.6 306.1 2.34 

2007 6 24242.7 368.5 1.52 

2007 7 43667.4 301.3 0.69 

2007 8 50608.3 1425.0 2.82 

2007 9 45423.8 687.3 1.51 

2007 10 121997.5 615.5 0.50 

2007 11 40149.0 631.2 1.57 

2007 12 33446.8 321.9 0.96 

 

Minimum 13063.6 21.0 0.01 

Median 83258.4 314.1 0.31 

Mean 124166.4 505.3 0.69 

Maximum 660963.7 3436.4 3.36 

5.5 Critical Conditions 

The TMDL was based on annual average conditions (i.e., values from all four seasons in a 

calendar year) rather than critical/seasonal conditions because of the following: 

a) The methodology used to determine assimilative capacity does not lend itself very well to 

short-term assessments, 

b) The net change in overall primary productivity, which is better addressed on an annual 

basis, is generally a better indicator of an imbalance in flora or fauna, and 

c) The methodology used to determine impairment is based on an annual average and 

requires data from all four quarters of a calendar year. 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

34

Chapter 6: DETERMINATION OF THE TMDL 

6.1 Expression and Allocation of the TMDL 

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

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

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

point source loads (Wasteload Allocations, or WLAs), nonpoint source loads (Load Allocations, 

or LAs), and an appropriate margin of safety (MOS), which takes into account any uncertainty 

concerning the relationship between effluent limitations and water quality: 

TMDL = ∑ WLAs +∑ LAs + MOS

As discussed earlier, the WLA is broken out into separate subcategories for wastewater 

discharges and stormwater discharges regulated under the NPDES Program: 

TMDL ≅ ∑ WLAs

wastewater + ∑ WLAsNPDES Stormwater + ∑ LAs + MOS

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

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

percent reduction needed for nonpoint sources and is also accounted for within the LA, and (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 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 also differs from the permitting of most 

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

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

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

treatment to the “maximum extent practical” through the implementation of best management 

practices (BMPs). 

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

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

appropriate measure. The TMDL for the Alafia River Tidal Segment (WBID 1621G) is 

expressed in terms of pounds/day for point sources and percent reduction for nonpoint sources, 

Table 6.1.a, and pounds/year for point sources and percent reduction for nonpoint sources, 

Table 6.1.b, and represent the maximum TN load the surface water can assimilate to meet both 

the nutrient and DO criteria. The TMDL to be implemented is the one expressed on a mass per 

year basis, and the expression of the TMDL on a mass per day basis is for information purposes 

only. 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

35

Table 6.1.a TMDL Components Expressed as a Daily Load for the Alafia 

River Tidal Segment (WBID 1621G) 

 WLA 

TMDL 

(mg/L) Wastewater 

NPDES 

Stormwater 

LA 

(% Reduction)2

WBID Parameter (lbs/day)1 (% Reduction)2 

MOS 

1621G Total N 0.65 14.3 54% 54% Implicit 

1

 Represents Mosaic Fertilizer’s average daily load (2001-2007) to Alafia River calculated using monthly average data. 2

 As the TMDL represents a percent reduction, it also complies with EPA requirements to express the TMDL on a daily basis. 

Table 6.1.b TMDL Components Expressed as an Annual Load for the Alafia 

River Tidal Segment (WBID 1621G) 

 WLA 

TMDL 

(mg/L) Wastewater 

NPDES 

Stormwater 

LA 

(% Reduction) 

WBID Parameter (lbs/year)1 (% Reduction) 

MOS 

1621G Total N 0.65 5,140 54% 54% Implicit 

1

 Represents Mosaic Fertilizer’s average annual load (2001-2007) to Alafia River calculated using monthly average data.

6.2 Load Allocation 

A total nitrogen reduction of 54 percent is required from nonpoint sources. It should be noted 

that the load allocation includes loading from stormwater discharges that are not part of the 

NPDES Stormwater Program. 

6.3 Wasteload Allocation 

6.3.1 NPDES Wastewater Discharges 

The one point source with outfalls discharging to the Alafia River, the Mosaic Fertilizer, L.L.C. 

Riverview Chemical Complex, NPDES No. FL0000761, was considered not to be a significant 

load to the lower Alafia River relative to the total nonpoint source load, as the current TN load 

being discharged is less than one percent of the total load entering the lower Alafia River. The 

facility’s current load to the river, for the 2001-2007 period, was applied as the WLA. Since the 

facility load is small relative to the total load entering the estuary, the reductions will be focused 

on MS4 facilities and nonpoint sources. 

Any future discharge permits issued in the watershed will also be required to meet the state’s 

Class III criteria for DO and nutrients and contain appropriate discharge limitations on nitrogen 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

36

that will comply with the TMDL as well as existing state requirements related to discharges to 

Outstanding Florida Waters. 

6.3.2 NPDES Stormwater Discharges 

Hillsborough County and Co- Permittees (FDOT District 7 & Florida’s Turnpike Enterprise, and 

City of Plant City) are covered by a Phase I NPDES municipal separate storm sewer system 

(MS4) permit (FLS000006) and areas within their jurisdiction contributing loads to the lower 

Alafia River watershed may be responsible for a 54 percent reduction in current anthropogenic 

TN loading. It should be noted that any MS4 permittee is only responsible for reducing the 

anthropogenic loads associated with stormwater outfalls that it owns or otherwise has 

responsible control over, and it is not responsible for reducing other nonpoint source loads in its 

jurisdiction. There are no Phase II MS4 permits identified in the Lower Alafia River watershed. 

6.4 Margin of Safety 

Consistent with the recommendations of the Allocation Technical Advisory Committee 

(Department, February 2001), an implicit MOS was used in the development of this TMDL. A 

MOS was included in the TMDL by selecting, as the nutrient target, the average of the annual 

average TN concentrations that occurred when chlorophyll a in lower Hillsborough Bay was 

below the threshold value of 15 ug/L. 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

37

Chapter 7: TMDL IMPLEMENTATION 

TMDL Implementation 

Following the adoption of this TMDL by rule, the Department will determine the best course of 

action regarding its implementation. Depending upon the pollutant(s) causing the waterbody 

impairment and the significance of the waterbody, the Department will select the best course of 

action leading to the development of a plan to restore the waterbody. Often this will be 

accomplished cooperatively with stakeholders by creating a Basin Management Action Plan, 

referred to as the BMAP. Basin Management Action Plans are the primary mechanism through 

which TMDLs are implemented in Florida [see Subsection 403.067(7) F.S.]. A single BMAP 

may provide the conceptual plan for the restoration of one or many impaired waterbodies. 

If the Department determines a BMAP is needed to support the implementation of this TMDL, a 

BMAP will be developed through a transparent stakeholder-driven process intended to result in 

a plan that is cost-effective, technically feasible, and meets the restoration needs of the 

applicable waterbodies. Once adopted by order of the Department Secretary, BMAPs are 

enforceable through wastewater and municipal stormwater permits for point sources and 

through BMP implementation for nonpoint sources. Among other components, BMAPs typically 

include: 

• Water quality goals (based directly on the TMDL); 

• Refined source identification; 

• Load reduction requirements for stakeholders (quantitative detailed allocations, if technically 

feasible); 

• A description of the load reduction activities to be undertaken, including structural projects, 

nonstructural BMPs, and public education and outreach; 

• A description of further research, data collection, or source identification needed in order to 

achieve the TMDL; 

• Timetables for implementation; 

• Implementation funding mechanisms; 

• An evaluation of future increases in pollutant loading due to population growth; 

• Implementation milestones, project tracking, water quality monitoring, and adaptive 

management procedures; and 

• Stakeholder statements of commitment (typically a local government resolution). 

BMAPs are updated through annual meetings and may be officially revised every five years. 

Completed BMAPs in the state have improved communication and cooperation among local 

stakeholders and state agencies, improved internal communication within local governments, 

applied high-quality science and local information in managing water resources, clarified 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

38

obligations of wastewater point source, MS4 and non-MS4 stakeholders in TMDL 

implementation, enhanced transparency in DEP decision-making, and built strong relationships 

between DEP and local stakeholders that have benefited other program areas. 

However, in some basins, and for some parameters, particularly those with fecal coliform 

impairments, the development of a BMAP using the process described above will not be the 

most efficient way to restore a waterbody, such that it meets its’ designated uses. Why? 

Because fecal coliform impairments result from the cumulative effects of a multitude of potential 

sources, both natural and anthropogenic. Addressing these problems requires good old 

fashioned detective work that is best done by those in the area. There are a multitude of 

assessment tools that are available to assist local governments and interested stakeholders in 

this detective work. The tools range from the simple – such as Walk the WBIDs and GIS 

mapping - to the complex such as Bacteria Source Tracking. Department staff will provide 

technical assistance, guidance, and oversight of local efforts to identify and minimize fecal 

coliform sources of pollution. Based on work in the Lower St Johns River tributaries and the 

Hillsborough River basin, the Department and local stakeholders have developed a logical 

process and tools to serve as a foundation for this detective work. In the near future, the 

Department will be releasing these tools to assist local stakeholders with the development of 

local implementation plans to address fecal coliform impairments. In such cases, the 

Department will rely on these local initiatives as a more cost-effective and simplified approach to 

identify the actions needed to put in place a roadmap for restoration activities, while still meeting 

the requirements of Chapter 403.067(7), F.S. 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

39

References 

Climate Information for Management and Operational Decisions. 2009. Chapel Hill, NC: 

Southeast Regional Climate Center. Available: http://www.sercc.net/. 

Fehring, W. K. 1985. History of the Port of Tampa, In Treat, S.F., J.L. Simon, and R. R. Lewis 

(Eds.), Proceedings of the Tampa Bay Area Scientific Information Symposium, May 1982. 

Bellweather Press 

Florida Department of Environmental Protection. February 1, 2001. A Report to the Governor 

and the Legislature on the Allocation of Total Maximum Daily Loads in Florida. 

Tallahassee, Florida: Bureau of Watershed Management. 

Florida Department of Environmental Protection (FDEP), Water Resources Restoration and 

 Preservation Act of 1999, Florida Statute Chapter 403, Part 1, Section 403.0615. 

 Available at: 

http://www.leg.state.fl.us/statutes/index.cfm?mode=View%20Statutes&SubMenu=1&App

 _mode=Display_Statute&Search_String=RESTORATION&URL=CH0403/CH0403.HTM

Florida Department of Environmental Protection, Surface Water Quality Standards, Florida 

 Administrative Code (F.A.C.), Chapter 62-302.530, (effective 12-7-06). Available at: 

http://www.dep.state.fl.us/legal/Rules/shared/62-302/62-302.pdf

Florida Department of Environmental Protection, Identification of Impaired Surface Waters Rule 

(IWR), F.A.C. Chapter 62-303.100, (New 6-10-02, Repromulgated 1-2-07). Available at: 

http://www.dep.state.fl.us/legal/Rules/shared/62-303/62-303.pdf

Florida Department of Environmental Protection. 2008. Tallahassee, FL: Division of 

Environmental Assessment and Restoration, Bureau of Information Systems, Geographic 

Information Systems Section. Available: http://www.dep.state.fl.us/gis/contact.htm.

Harper, H. and D. Baker. 2007. Evaluation of Current Stormwater Design Criteria within the 

State of Florida. Prepared by Environmental Research and Design for the Florida Department 

of Environmental Protection. Available at: 

http://www.dep.state.fl.us/water/nonpoint/docs/nonpoint/SW_TreatmentReportFinal_71907.pdf 

Janicki, A. and D. Wade. 1996. Estimating Critical Nitrogen Loads for the Tampa Bay Estuary: 

An Empirically Based Approach to Setting Management Targets. Tampa Bay Estuary Program 

Technical Publication No. 06-96. 

Janicki, A., D. Wade, and R.J. Pribble. 2000. Developing and Establishing a Process to Track 

the Status of Chlorophyll a Concentrations and Light Attenuation to Support Seagrass 

Restoration Goals in Tampa Bay. Tampa Bay Estuary Program Technical Report No. 04-00. 

National Research Council, 1993. Managing Wastewater in Coastal Urban Areas. National 

Academy Press, Washington, D.C. 
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

40

Price, C. B., C. Cerco, and D. Gunnison. 1994. Sediment Oxygen Demand and Its Effects on 

Dissolved Oxygen Concentrations and Nutrient Release; Initial Laboratory Studies. U. S. Army 

Corps of Engineers. Water Quality Research Program. Technical Report W-94-1. 

Sawyer and McCarty. 1967. Chemistry For Sanitary Engineers. McGraw-Hill Book Company. 

Southwest Florida Water Management District, 2004 Land Use, September 2006 

 Available at: http://www.swfwmd.state.fl.us/data/gis/layer_library/category/physical_dense

Southwest Florida Water Management District. 2008. The Determination of Minimum Flows for 

the Lower Alafia River Estuary. Technical Report of the Southwest Florida Water Management 

District. Brooksville. Florida. 

Stoker, Y. E., V.A. Levesque, and W. A. Woodham.1996. The effect of discharge and water 

quality of the Alafia River, Hillsborough River, and the Tampa Bypass Canal on nutrient loading 

to Hillsborough Bay, Florida. U.S. Geological Survey Water-Resources Investigation Report 95-

4107. Tallahassee, FL. 

U.S. Environmental Protection Agency. 1991. Guidance for water quality–based decisions: The 

TMDL process. EPA-440/4-91-001. Washington, D.C.: Office of Water. 

_____________. 2007. Sediment Oxygen Demand Studies. Office of Environmental Measurement and 

Evaluation. Last updated on Monday, May 14th, 2007. Available at: http://www.epa.gov/
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and 

Nutrients

Florida Department of Environmental Protection 

41

Appendices 

Appendix A: Background Information on Federal and State Stormwater Programs 

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

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

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

in Chapter 403, F.S., was established as a technology-based program that relies on the 

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

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

The rule requires the state’s 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, the Winter Haven Chain of Lakes, the Everglades, Lake Okeechobee, and Lake 

Apopka. 

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 stormwater 

permitting program 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, the EPA has implemented Phase 1 of the MS4 permitting program 

on a countywide basis, which brings in all cities (incorporated areas), Chapter 298 urban water 

control districts, and the Florida Department of Transportation throughout the fifteen counties 

meeting the population criteria. 

An important difference between the federal and 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 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 

cannot 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 Department 

recently accepted delegation from the EPA for the stormwater part of the NPDES Program. It 

should be noted that most MS4 permits issued in Florida include a re-opener clause that allows 

permit revisions to implement TMDLs once they are formally adopted by rule. 

Appendix B: Mosaic Fertilizer Riverview Chemical Complex - Monthy Flow and Total Nitrogen D

the Alafia River 

MonthYear 

D001 

Flow 

mo 

av 

mgd 

D001 

TN 

mo 

av 

mg/l 

D001 

TN 

mo av 

lbs/mo 

D005 

Flow 

mo 

av 

mgd 

D005

TN 

mo 

av 

mg/l 

D005 

TN 

mo av

lbs/mo 

D005A

Flow 

mo av

mgd 

D005A

TN 

mo av

mg/l 

D005A

TN 

mo av

lbs/mo 

D005B 

Flow 

mo av 

mgd 

D005B

TN 

mo av

mg/l 

D005B

TN 

mo av

lbs/mo 

D021

Flow

mo 

av 

mgd 

D021

TN 

mo 

av 

mg/l 

D

m

lb

Jan-01 0 0 0.002 3.825 1.9791 0 

Feb-01 0 0 0 0 0 Mar-01 0 0 0.016 5.5 22.766 1.47 5.5 2

Apr-01 0 0 0 0 0.002 5.5 

May-01 0 0 0 0 0 

Jun-01 0 3.2 0 0 0 0.143 3.2 1

Jul-01 0 0 0.098 4.3 109.02 0.022 4.3 2

Aug-01 0 0 0.066 4 68.299 0.129 4 1

Sep-01 0 0 0.099 4.2 104.1 0.815 4.2 8

Oct-01 0 0 0.038 2.2 21.628 0 

Nov-01 0 0 0.028 2.7 18.927 0 

Dec-01 0 0 0.024 4.5 27.94 0 

Jan-02 0 0 0.008 3.6 7.4508 0 

Feb-02 0 0 0 0 0 Mar-02 0 0 0 0 0 

Apr-02 0 0 0.007 1.3 2.2783 0 

May-02 0 0 0 3.3 0 0 

Jun-02 0 0 0.041 1.7 17.45 0 

Jul-02 0 0 0.045 5.2 60.538 0 

Aug-02 0 0 0.048 2.4 29.803 0 

Sep-02 0 0 0.081 4.1 83.145 0 

Oct-02 0 0 0.16 2.4 99.344 0 

Nov-02 0 0 0.063 3 47.318 0 

Dec-02 0.075 0.6 11.642 0.057 4.1 60.46 0 

Jan-03 1.723 5.4 2407.1 0.029 2.8 21.007 0.118 2.8 8

Feb-03 0 0 0.047 3.8 41.734 0.112 3.8 9Mar-03 0 0 0.082 7.9 167.59 0.212 7.9 4

Apr-03 0 0 0.08 2.9 58.084 0.021 2.9 1

May-03 0 0 0.107 2.6 71.972 0.017 2.6 1

Jun-03 0 0 0.411 2.1 216.09 0.004 2.1 

Jul-03 0 0 0.537 3.2 444.56 0.01 3.2 8

Aug-03 0 0 0.587 3.9 592.26 0.011 3.9 1
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and Nutr

Florida Department of Environmental Protection 

2

Sep-03 0 0 0.267 2.8 187.17 0.01 2.8 7

Oct-03 0 0 0.193 3.8 189.74 0.008 3.8 7

Nov-03 0 0 0.149 2.3 85.799 0 

Dec-03 0 0 0.086 3.3 73.421 0 

Jan-04 0 0 0.117 3.7 111.99 0 3.7 

Feb-04 0 0 0.202 3.7 180.88 0 3.7 Mar-04 0 0 0.079 3.5 71.533 0 3.5 

Apr-04 0 0 0.103 3.6 92.834 0.04 3.6 3

May-04 0 0 0.112 3.4 98.516 0 3.4 

Jun-04 0 0 0.143 4.8 171.85 0 4.8 

Jul-04 0 0 0.967 6.5 1626.1 0.006 6.5 

Aug-04 0 0 0.732 5.5 1041.6 0.064 5.5 9

Sep-04 0 0 1.466 5.7 2092.1 0.07 5.7 9

Oct-04 0 0 0.163 3.3 139.16 0.004 3.3 3

Nov-04 0 0 0.024 3 18.026 0.004 3 3

Dec-04 0 0 0.053 2.6 35.65 0.004 2.6 2

Jan-05 0 0 0.106 2.9 79.527 0.004 2.9 

Feb-05 0 0 0.06 15 210.3 0.004 15 Mar-05 0 0 0.172 4.2 186.89 0.04 4.2 4

Apr-05 0 0 0.105 2.8 73.606 0.017 2.8 1

May-05 0 0 0.089 4.6 105.91 0.048 4.6 5

Jun-05 0 0 0.618 4.2 649.84 2.65 4.2 2

Jul-05 0 0 0.47 2.1 255.34 0.094 2.1 5

Aug-05 0 0 0.133 3.2 110.11 0.61 3.2 

Sep-05 0 0 0.047 3.3 38.831 0.35 3.3 2

Oct-05 0 0 0.146 3.6 135.98 1.2 3.6 1

Nov-05 0 0 0.1 4.3 107.66 0.94 4.3 

Dec-05 0 0 0.074 5.8 111.04 0.15 5.8 2

Jan-06 0 0 0 0 0.056 3.5 50.707 

Feb-06 0 0 0 0 0.183 3.9 166.77 3.9 1Mar-06 0 0 0 0 0.048 2.3 28.561 

Apr-06 0 0 0 0 0.037 4 37.054 

May-06 0 0 0 0 0.091 4.9 115.36 4.9 6

Jun-06 0 0 0 0 0.22 4.3 236.84 4.3 4

Jul-06 0 0 0 0 0.579 5.1 763.94 5.1 5

Aug-06 0 0 0 0 0.172 2.3 102.34 2.3 2

Sep-06 0 0 0 0 0.528 2.6 343.7 2.6 

Oct-06 0 0 0 0 0.164 2.7 114.56 2.7 

Nov-06 0 0 0 0 0.146 3.9 142.56 3.9 8

Dec-06 0 0 0 0 0.059 3.7 56.476 3.7 6

Jan-07 0 0 0 0 0.121 5.4 169.04 5.4 1

Feb-07 0 0 0 0 0.125 3.8 110.99 3.8 4Mar-07 0 0 0 0 0.083 3.7 79.449 3.7 2
TMDL Report: Alafia River Above Hillsborough Bay (WBID 1621G), DO and Nutr

Florida Department of Environmental Protection 

3

Apr-07 0 0 0 0 0.092 5.4 124.38 5.4 2

May-07 0 0 0 0 0.028 3.4 24.629 3.4 2

Jun-07 0 0 0 0 0.08 2.3 46.067 2.3 3

Jul-07 0 0 0 0 0.144 3.2 119.21 3.2 1

Aug-07 0 0 0 0 0.401 4.3 446.09 4.3 9

Sep-07 0 0 0 0 0.192 3.7 177.86 3.7 5

Oct-07 0 0 0 0 0.165 2.6 110.99 2.6 5

Nov-07 0 0 0 0 0.064 3.3 52.876 3.3 5

Dec-07 0 0 0 0 0.089 2.9 66.772 2.9 2

Minimum 0.00 0.60 0.00 0.00 1.30 0.00 0.00 0.00 0.00 0.03 2.30 24.63 0.00 2.10 Median 0.00 3.20 0.00 0.08 3.60 76.57 0.00 #NUM! 0.00 0.12 3.70 112.77 0.00 3.70 Mean 0.02 3.07 28.79 0.16 3.87 177.95 0.00 #DIV/0! 0.00 0.16 3.63 153.63 0.16 4.02 2Maximum 1.72 5.40 2407.07 1.47 15.00 2092.08 0.00 0.00 0.00 0.58 5.40 763.94 2.65 15.00 27

Appendix C: Department Response Letter to Comments on TMDL Report 

August 6, 2009 

Winston K. Borkowski 

Hopping Green & Sams 

119 S. Monroe Street, Suite 300 

Tallahassee, FL 32301 

Subject: Response to Comments on the Dissolved Oxygen and Nutrient TMDLs for Alafia River 

above Hillsborough Bay (WBID 1621G) 

Dear Mr. Borkowski: 

The Department has reviewed the comments provided on the proposed Dissolved Oxygen and Nutrient 

TMDL for the Alafia River Above Hillsborough Bay - Tidal Segment, (WBID 1621G) submitted in the July 

20, 2009 letter on behalf of the Mosaic Company. After evaluating these comments on the June 2009 

TMDL report, we believe that the TMDL reduction for total nitrogen as proposed by the Department does 

not have to be revised; however, we will make revisions to the report as appropriate based on comments 

provided. We appreciate your involvement in reviewing and commenting on the TMDL report and the 

following are our responses to your comments. 

The Department does recognize that the Hillsborough Bay chlorophyll target of 15 ug/L, used as a target 

for establishing the TMDL, was originally developed for Hillsborough Bay as a water quality target for 

making resource-based management decisions for seagrass restoration. As the Alafia River tidal 

segment has direct exchange of water with Hillsborough Bay, we believe that the bay chlorophyll a target 

is the best available information for establishing a target for this segment of the river. We would also like 

to point out that the Department is using the Hillsborough Bay chlorophyll a target as a threshold for 

performing nutrient assessments, following the IWR methodology, for all estuary segments tributary to the 

bay as this provides a consistent approach for determining nutrient impairment in this area of Tampa Bay. 

The Department also recognizes that the bay’s chlorophyll a target was developed based on relationships 

with total nitrogen loading; however, there is also a relationship, although not as strong, between 

chlorophyll a and total nitrogen concentrations both in Hillsborough Bay and the adjacent Alafia River 

estuary, as explained in the TMDL report. Additionally, the Tampa Bay Estuary Program presented 

results at the June 2008 Estuary and Coastal Waters Nutrient Criteria Development meeting showing that 

there is a positive relationship between Hillsborough Bay total nitrogen and chlorophyll a concentrations 

and both variables are exhibiting a decreasing trend over time. The Estuary Program presentation is 

available at the following web site: 

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

The available information indicates the total nitrogen concentrations are elevated in the river’s tidal 

segment compared to the bay, and it is expected that reductions in nitrogen concentrations will lead to 

reductions in chlorophyll a levels, as has been observed in the bay. The intent of this TMDL is not to 

address water quality in the bay, but instead to use the existing information from the bay to serve as a 

reference condition for TMDL development in the adjacent river segment. 
Florida Department of Environmental Protection 

2

Winston K. Borkowski 

Hopping Green & Sams 

August 6, 2009 

Page Two 

The Department recognizes that many of the relationships between water quality variables in the Alafia 

River estuary are not that strong, and that chlorophyll a and DO are typically influenced by a number of 

factors in surface waters. The reference condition approach using observed water quality in Lower 

Hillsborough Bay, which is not impaired for nutrients and DO, was used to establish the TMDL. The 

reference condition approach is an acceptable method for establishing TMDLs and we believe that there 

is a sufficient weight of evidence that this approach is the best way to move forward in addressing water 

quality problems in the Alafia River tidal segment. As Hillsborough Bay chlorophyll a and total nitrogen 

concentrations have decreased over time, it is reasonable to expect that activities aimed at reducing 

nitrogen concentrations in the adjacent Alafia River estuary would result in lower chlorophyll a 

concentrations there. 

The comment letter suggests that the nitrogen concentration target should be derived from the highest 

value where the chlorophyll a threshold is met. The average of the total nitrogen annual average values, 

during years the chlorophyll a averages were less than the target of 15 ug/L, that are used for 

establishing the TMDL, is appropriate to apply in order to provide for a sufficient margin of safety in the 

TMDL. 

The Department agrees that the TMDL should be implemented based on annual average conditions and 

as such the load allocated to the Mosaic Fertilizer, L.L.C. Riverview Chemical Complex should also be 

expressed as an annual average load. The report will be revised to indicate that the TMDL will be 

implemented on an annual average basis and will include a table expressing the TMDL on an annual 

average basis. The facility’s monthly average total nitrogen load to the river of 428.34 pounds, calculated 

based on monthly average discharge monitoring report data for the 2001 to 2007 period, will be multiplied 

by 12 to derive an annual average load allocation for the facility of 5,140 pounds/year. 

 

If you have any questions concerning our responses, please do not hesitate to contact me or Kevin 

Petrus at (850) 245-8449. 

Sincerely, 

Jan Mandrup-Poulsen, Environmental Administrator 

Watershed Evaluation and TMDL Section 

cc: Jim Giattina, Director, Water Protection Division, USEPA Region 4 

 Holly Greening, Director, Tampa Bay Estuary Program 

 Drew Bartlett 

Jeff Greenwell 

 Charles Kovach 

 Terry Hansen 
Florida Department of Environmental Protection 

3
Florida Department of Environmental Protection 

4

Florida Department of Environmental 

Protection 

Division of Environmental Assessment 

 and Restoration 

Bureau of Watershed Restoration 

2600 Blair Stone Road, Mail Station 3565 

Tallahassee, Florida 32399-2400 

(850) 245-8561 

www2.dep.state.fl.us/water/