SARASOTA BAY 

NATIONAL ESTUARY PROGRAM 

POINTINON-POINT SOURCE -- I 

POLLUTION LOADING ASSESSMENT 

PHASE I 

JANUARY 10. 1992 

Camp Dresser & McKee 
TABLE OF CONTENTS 

Section 

1.0 INTRODUCTION 

1.1 Background of the Sarasota Bay 

National Estuary Program 

1.2 Purpose and Scope of Point and 

Nonpoint Pollution Loading Analysis 

1.3 Organization of Report 

2.0 STUDY AREA CHARACTERISTICS 2-1 

2.1 Introduction 

2.2 Watershed Delineation 

2.3 Existing Land Use 

2.3.1 Sarasota County 

2.3.2 Manatee County 

2.3.3 City of Sarasota 

2.3.4 Barrier Islands 

2.4 Future Land Use 

2.4.1 5-Year Future. 

2.4.2 Buildout Future 

2.5 Soils Characteristics 

2.6 Water Quality Characteristics 

2.7 Sources of Pollution 

2.7.1 Surface Runoff 

2.7.2 Baseflow 

2.7.3 Point Source Discharges 

2.7.4 Septic Tanks 

2.7.5 Rainfall 

3.0 METHODOLOGY FOR POLLUTION LOADING PROJECTIONS 3-1 

3.1 Introduction 

3.2 RainfalllRunoff Relationships 

3.2.1 Rainfall and Streamflow Data 

3.2.2 Pervious Area Runoff 

3.2.3 Impervious Area Runoff 

3.2.4 Baseflow 

3.3 Nonpoint Pollution Loading Factors 

3.3.1 Surface Runoff 
TABLE OF CONTENTS 

(Continued) 

Section 

3.3.2 Baseflow 

3.4 Point Source Loadings 

3.5 Septic Tank Loadings 

3.5.1 Failing Septic Tanks 

3.5.2 Working Septic Tanks 

3.6 Rainfall Loadings 

4.0 LOADING TRANSPORT TO BAY 

4.1 Model Description 

4.2 Model Parameters 

4.2.1 Tidal Influence 

4.2.2 Tributary flows and Loadings 

4.2.3 Tributary Geometries 

4.2.4 Pollutant Settling Parameters 

4.3 Modeling Procedure 

4.4 Model Results 

5.0 POLLUTION LOADING PROJECTIONS FOR EXISTING 

LAND USE CONDITIONS 5-1 

5.1 Average Annual Loading Result 

5.1.1 Loadings by Watershed 

5.1.2 Loadings by Source 

5.1.3 Loadings by Jurisdiction 

5.2 Wet Season and Dry Season Results 

5.3 Wet Year and Dry Year Results 

5.4 Summary 

6.0 POLLUTION LOADING PROJECTIONS FOR FUTURE LAND USE 

CONDITIONS 6-1 

6.1 Five-Year Future Land Use 

6.2 Buildout Future Land Use 

7.0 SUMMARY 7-1 

8.0 REFERENCES 8-1 
CONTENTS 

(Continued) 

Section & 

APPENDICES 

APPENDIX A - STORET Water Quality Data for Tributaries to 

Sarasota Bay 

APPENDIX B - Technical Memorandum on Septic Tank Literature Review 

and Analysis 

APPENDIX C - Statistical Analysis of Banier Island and Mainland Rainfall 

Data 
LIST OF TABLES 

Watersheds in Sarasota Bay NEP Study Area 

Land Use Categories and Associated Percent Imperviousness 

Existing Land Use by Jurisdiction in Sarasota Bay NEP 

Study Area 

Existing Land Use by Watershed in Sarasota ~ay NEP Study 

Area - Sarasota County 

Existing Land Use by Watershed in Sarasota Bay NEP Study 

Area - Manatee County 

Existing Land Use by Watershed in Sarasota Bay NEP Study 

Area - City of Sarasota 

Existing Land Use by Watershed in Sarasota Bay NEP Study 

Area - Barrier Islands 

Five-Year Future Land Use by Watershed in the Sarasota Bay 

NEP Study Area - Sarasota County 

Buildout Future Land Use by Watershed in the Sarasota Bay 

NEP Study Area - Sarasota County 

Buildout Future Land Use by Watershed in the Sarasota Bay 

NEP Study Area - Manatee County 

Septic Tank Coverage for Sarasota Bay NEP Study Area 

Precipitation Data for Gages in Sarasota Bay NEP Study Area 

Streamflow Data for Gages in Sarasota Bay NEP Study Area 

Event Mean Concentration Values for Sarasota Bay NEP Study 

Point Source Discharges in Sarasota Bay NEP Study Area 

Point Source Flows and Concentrations for Existing Conditions 

Point Source Flows and Concentrations for Five-Year 

Conditions 

Point Source Flows and Concentrations for Buildout Future 

Conditions 

Septic Tanks Flow Rates for Various Land Uses 

Parameters for Analysis of Working Septic Tanks 

Delivery Ratios for Major Tidal Tributaries 

iii 
LIST OF TABLES 

(Continued) 

Average Annual Loadings by Watershed for Existing Land Use 

Conditions 

Average Annual Loadings by Source for Existing Land Use 

Conditions 

Average Annual Loadings by Jurisdiction for Existing Land 

Use Conditions 

Wet Season Loadings by Watershed for Existing Land Use 

Conditions 

Dry Season Loadings by Watershed for Existing Land Use 

Conditions 

Wet Season and Dry Season Loadings by Source for Existing 

Land Use Conditions 

Wet Season and Dry Season Loadings by Jurisdiction for 

Existing Land Use Conditions 

Wet Year Loadings by Watershed for Existing Land Use 

Conditions 

Dry Year Loadings by Watershed for Existing Land Use 

Conditions 

Wet Year and Dry Year Loadings by Source for Existing Land 

Use Conditions 

Wet Year and Dry Year Loadings by Jurisdiction for Existing 

Land Use Conditions 

Average Annual Loadings by Watershed for Five-Year Future 

Land Use Conditions 

Average Annual Loadings by Source for Five-Year Future 

Land Use Conditions 

Average Annual Loadings by Jurisdiction for Five-Year 

Future Land Use Conditions 

Wet Season Loadings by Watershed for Five-Year Future 

Land Use Conditions 

Dry Season Loadings by Watershed for Five-Year Future 

Land Use Conditions 

Wet Season and Dry Season Loadings by Source for Five-Year 

Future Use Conditions 

Wet Season and Dry Season Loadings by Jurisdiction for 

Five-Year Land Use Conditions 

Wet Year Loadings by Watershed for Five-Year Future Land 

Use Conditions 
LIST OF TABLES 

(Continued) 

Dry Year Loadings by Watershed for Five-Year Future Land 

Use Conditions 

Wet Year and Dry Year Loadings by Source for Five-Year 

Future Land Use Conditions 

Wet Year and Dry Year Loadings by Jurisdiction for 

Five-Year Future Land Use Conditions 

Average Annual Loadings by Watershed for Buildout Future 

Land Use Conditions 

Average Annual Loadings by Source for Buildout Future 

Land Use Conditions 

Average Annual Loadings by Jurisdiction for Buildout Future 

Land Use Conditions 

Wet Season Loadings by Watershed for Buildout Future Land 

Use Conditions 

Dry Season Loadings by Watershed for Buildout Future Land 

Use Conditions 

Wet Season and Dry Season Loadings by Source for Buildout 

Future Land Use Conditions 

Wet Season and Dry Season Loadings by Jurisdiction for 

Buildout Future Land Use Conditions 

Wet Year Loadings by Watershed for Buildout Future Land 

Use Conditions 

Dry Year Loadings by Watershed for Buildout Future Land 

Use Conditions 

Wet Year and Dry Year Loadings by Source for Buildout 

Future Land Use Conditions 

Wet Year and Dry Year Loadings by Jurisdiction for Buildout 

Future Land Use Conditions 
LIST OF FIGURES 

Figure 

2- 1 Sarasota Bay NEP Study Area 

Septic Tank Total N Concentration at Various Distances 

from Receiving Water 

WASP4 Model Segmentation for Philippi Creek 

WASP4 Model Segmentation for Whitaker Bayou 

WASP4 Model Segmentation for Hudson Bayou 

WASP4 Model Segmentation for Bowless Creek 

WASP4 Model Segmentation for Cedar Hammock 

Runoff Distribution by Watershed in Sarasota Bay NEP 

Study Area 

Total P Distribution by Watershed in Sarasota Bay NEP 

Study Area 

Total N Distribution by Watershed in Sarasota Bay NEP 

Study Area 

Lead Distribution by Watershed in Sarasota Bay NEP 

Study Area 

Zinc Distribution by Watershed in Sarasota Bay NEP 

Study Area 

Runoff Distribution by Source in Sarasota Bay NEP 

Study Area 

Total P Distribution by Source in Sarasota Bay NEP 

Study Area 

Total N Distribution by Source in Sarasota Bay NEP 

Study Area 

Lead Distribution by Source in Sarasota Bay NEP Study Area 

Zinc Distribution by Source in Sarasota Bay NEP Study Area 
1.0 INTRODUCTION 

1.1 BACKGROUND OF THE SARASOTA BAY NATIONAL ESTUARY PROGRAM 

Sarasota Bay was identified in Section 317 of the Water Quality Act of 1987 as an 

estuary to be given priority consideration for inclusion in the National Estuary Program 

(NEP). As a result, Governor Bob Martinez nominated Sarasota Bay to the NEP, in a 

May 19, 1987 letter to EPA Administrator Lee Thomas. According to the Act, 

documentation was required to support the nomination. This documentation would 

evaluate the need for a management conference, likelihood of program success, and need 

to protect water quality beyond existing controls. 

In 1987, the EPA established a cooperative agreement with Mote Marine Laboratory to 

gather data regarding environmental status, trends and problems in Sarasota Bay. These 

data were to be used as the documentation to support the Bay's nomination to the NEP. 

The report developed by Mote was presented at a workshop of federal, state and local 

government representatives on March 17, 1988. Through the workshop, additional 

information on Sarasota Bay was obtained. 

The Florida Department of Environmental Regulation (FDER) used the Mote report as the 

basis for the completion of the Governor's nomination document. The supporting 

documentation was submitted on May 31, 1988 to the EPA Office of Marine and Estuary 

Protection. After EPA's evaluation, the Bay was designated as a member of the NEP on 

July 18, 1988. 

In May of 1989, the Sarasota Bay NEP released a Request for Proposal (RFP) for work 

related to the preservation, restoration and enhancement of the Bay. The RFP included 

the following areas: 
0 Baywide Segmentation and Mapping 

0 Wetland Habitat Assessment 

0 Estuarine Bottom Habitat Assessment 

0 Regional Beachhlet Management 

0 Impacts of Sea Level Rise 

0 Fishery Resource Assessment 

0 Shellfish Contamination Assessment 

0 Point and Nonpoint Pollution Loading Assessment, Calibration, Verification, and 

Projections 

0 Resource Access and Use Assessment 

0 Data Management 

The information gathered in these areas will lead to the development of a comprehensive 

conservation and management plan. 

1.2 PURPOSE AND SCOPE OF POINT AND NONPOINT POLLUTION LOADING 

ANALYSIS 

CDM was selected to conduct the analysis of point and nonpoint source loadings to 

Sarasota Bay. Basically, the objective of the analysis is to quantify the loadings of 

nutrients and metals to the Bay, to identify the areas that are contributing the largest share 

of the total load, and to analyze alternative measures for reducing the loadings. 

For purposes of this study, assessment of pollutant loadings was restricted to total 

nitrogen, total phosphorus, lead and zinc. Sources of loadings evaluated include baseflow, 

storm event runoff, atmospheric, and point source loadings. While it is fully recognized 

that there are other contaminating substances entering Sarasota Bay through a variety of 

other sources, these parameters were chosen as surrogates for & non-point and point 

source loadings. These parameters represent contaminants which are well characterized in \ 

the literature, and for which documented relationships with land-use exist. As such, this \ 

\ 
study does not attempt to characterize all pollutant loadings, nor all sources of pollution, 

but rather focuses on the major loadings delivered to Sarasota Bay. 

The analysis is divided into three distinct phases. In Phase 1, existing data are used to 

estimate current loading levels. Phase 2 is designed to collect field measurements or other 

data required to improve the estimates developed in Phase 1. Analysis of future loadings, 

based on projected changes in population, land use, and other factors, is performed in 

Phase 3. Evaluation of alternative management programs is also part of Phase 3. 

This report documents the results generated in Phase 1. This phase includes the 

estimation of existing point and nonpoint source loadings. Even though future loading 

estimates are part of the Phase 3 analysis, preliminary estimates of future loadings also are 

included here in Phase 1. 

1.3 ORGANIZATION OF REPORT 

Chapter 2 summarizes the study area characteristics that are pertinent to the analysis of 

point and nonpoint sources loadings. The methodology used to develop loading estimates 

is presented in Chapter 3. Chapter 4 documents the modeling of water quality 

constituents in the tidal tributaries of the major watersheds. The estimated loadings for 

existing and future land uses are presented in Chapter 5 and 6, respectively. Chapter 7 

evaluates and summarizes the results presented in Chapters 5 and 6. The references used 

in the study are listed in Chapter 8. 
2.0 STUDY AREA CHARACTERISTlCS 

2.1 INTRODUCTION 

This chapter describes the NEP study area characteristics that are essential to the pollutant 

loading analyses. These characteristics include watershed boundaries, existing and future 

land use, soil classifications , water quality, and sources of pollution. 

2.2 WATERSHED DELINEATION 

The Sarasota Bay NEP study area is shown in Figure 2-1. The area extends to the north 

as far as Anna Maria Island and Perico Island, and to the south as far as Casey Key. The 

watersheds in the study drain not only to Sarasota Bay itself, but to several smaller bays 

to the south including Roberts Bay, Little Sarasota Bay, Dryman Bay, and Blackbum Bay. 

In this report, the term "Sarasota Bay" will include all of the bays listed above. 

Delineation of the total study area draining to Sarasota Bay, and delineation of major 

watersheds within the study area, was done on 1" = 2000' USGS quadrangle maps. These 

maps were combined in a mosaic to create one large map which was used as a project 

work map for the study. In some cases, additional information was required to delineate 

watershed boundaries due to the lack of topography. Previous studies (CDM, 1987; 

Briley and Wild, 1984) were used to check the delineation and to provide guidance where 

map topography was lacking. 

Table 2-1 lists the watersheds that were defined in the delineation process. In addition, 

the table shows the drainage area and jurisdiction or jurisdictions in which each watershed 

is located. In all, the study area contains approximately 150 square miles of land area, 

plus 52 square miles of water surface. 


TABLE 2-1 

WATERSHEDS IN SARASOTA BAY NEP STUDY AREA 

Jurisdiction(s) 

City of Sarasota 

Sarasota County 

Manatee County 

City of Sarasota 

Ciiy of Sarasota 

Sarasota County 

Manatee County 

City of Sarasota 

Sarasota County 

Manatee County 

City of Sarasota 

Sarasota County 

City of Sarasota 

Sarasota County 

Sarasota County 

Sarasota County 

Sarasota County 

Sarasota County 

Manatee County 

Manatee County 

Manatee County 

Manatee County 

Manatee County 

Barrier Islands 

Barrier Islands 

Barrier Islands 

Barrier Islands 

Barrier Islands 

Watershed 

Phillippi Creek 

Hudson Bayou 

Bowlees Creek 

West Bowlees 

Whitaker Bayou 

Direct to Bay 

Matheny Creek 

Catfish Creek 

North Creek 

South Creek 

Palma Sola Creek 

Palma Sola 2 

West Bradenton 

South Bradenton 

Cedar Hammock 

Siesta Key 

Anna Maria Island 

Perico Island 

Longboat Key 

Other Islands 

Drainage area 

(Acres) 

36.41 7 

1,595 

6,489 

1,559 

5,015 

4,241 

3,800 

3,360 

1,920 

12,995 

900 

1,120 

4,395 

4,635 

1,930 

1,385 

919 

860 

1,697 

900 
. 2.3 EXISTING LAND USE 

Existing land use was established in the study area based on Real Estate Data Inc. 

(REDI) maps. These maps include aerial photographs and corresponding zoning maps. 

From the aerials, developed and undeveloped areas were identified. The type of 

development in urban areas was determined based on the zoning maps. 

The study area was classified according to the following land uses: 

Cropland 

Forested Uplands 

RangelandlWoodlands 

Openhtecreation 

Wetland 

Citrus 

Low Density Single Family Residential 

Medium Density Single Family Residential 

High Density Single Family Residential 

Multi-Family Building 

Mobile Home 

CommerciallServices 

Institutional 

Industrial 

Transportation 

Waterbody 

Sewage Treatment and Power Plants 

Table 2-2 summarizes the assumed directly connected impervious area (DCIA) established 

for each urban land use category. The DCIA value is important because it determines the 

amount of surface runoff that is generated by precipitation. The values in the table were 

selected based on literature values and previous CDM studies. For urban land use, 

industrial and commercial areas tend to have the highest percentage of impervious area. 

In residential areas, the DCIA value increases with the density of the development (i.e., 

number of dwellings per acre). 
TABLE 2-2 

IMPERVIOUS COVER FOR URBAN LAND USE CATEGORIES 

Land Use 

Low Density Single Family Residential 

Medium Density Single Family Residential 

High Density Single Family Residential 

Multi-family Building 

Mobile Home 

CommerciaVServices 

Extractive 

Institutional 

Industrial 

Transportation 

STP and Power Plants 

Directly Connected 

Impervious Area 

("4 

20 

30 

40 

50 

60 

85 

70 

40 

70 

90 

40 
Table 2-3 presents the existing land use by jurisdiction for the study area. As shown in 

the table, Sarasota County has the largest contributory area to the Bay, accounting for 65 

percent of the total land area. The City of Sarasota, Manatee County, and the barrier 

islands make up 8, 21 and 6 percent of the total land area, respectively. 

In the study area, slightly more than half of the land area consists of urban development 

(residential, commercial, industrial and institutional land uses), and the rest is rural. Of 

the urban development, about 81 percent is residential, primarily medium density and high 

density. For the rural areas, about 18 percent is either cropland or citrus, and the rest is 

primarily rangeland/woodlands, openlrecreation and forested uplands. 

The following sections discuss land use distributions for each jurisdiction. 

2.3.1 SARASOTA COUNTY 

The existing land use distribution for Sarasota County, broken down by watershed, is 

presented in Table 2-4. The values in the table indicate that Phillippi Creek accounts for 

over half of the total drainage area in the County. South Creek accounts for about 21 

perc_e&gf$e L~~-~c~ 7%*---~c,L-- 

of the total area. 

In the County, approximately 42 percent of the land area consists of urban development 

and the remaining 58 percent is rural. The urban development is most prevalent in the 

land areas closest to the Bay (i.e., the western sections of each watershed), and rural areas 

are typically located in the eastern sections of each watershed. Of the urban development, 

about 87 percent is residential, primarily split between low, medium and high density 

single family residential. For the rural areas, about 17 percent is either cropland or citrus, 

and the rest is primarily rangelandlwoodlands or openlrecreation. 
TABLE 2-3 

EXISTING LAND USE BY JURISDICTION IN 

SARASOTA BAY NEP STUDY AREA 

Land Use 

Cropland 

Forested Uplands 

Rangeland~Woodlands 

OpenIRecreation 

Wetland 

Citrus 

Low Density Single Family Residential 

Medium Density Single Family Residential 

High Density Single Family Residential 

Multi-family Building 

Mobile Home 

CommerciaVSe~ices 

Institutional 

Industrial 

Transportation 

Waterbody 

Sewage Treatment and Power Plants 

Total 

Ci of 

Sarasota 

0 

168 

126 

650 

0 

4 

368 

1,320 

2,164 

946 

0 

926 

529 

521 

17 

75 

30 

7,844 

Sarasota 

County 

3,756 

673 

15,544 

11,688 

1,449 

2,278 

7,375 

6,827 

5,163 

1,983 

1,065 

1,485 

647 

1,106 

203 

1,028 

30 

62,301 

AREA (Acres) 

Manatee 

County 

1,912 

1.052 

1,253 

2.222 

41 5 

209 

292 

2,012 

3,422 

2,677 

862 

654 

365 

1,989 

633 

228 

30 

20,226 

Barrier 

Islands 

130 

44 

19 

1,373 

403 

0 

388 

1.641 

741 

754 

37 

169 

0 

40 

15 

2 

5 

5,761 

Total 

5,798 

1,937 

16,942 

15,933 

2,267 

2,491 

8,423 

11,800 

11,490 

6,360 

1,964 

3,234 

1,541 

3,656 

868 

1,333 

95 

96,132 
TABLE 2-4 

EXISTING LAND USE BY WATERSHED IN SARASOTA BAY NEP STUDY AREA - 

SARASOTA COUNTY 

Land Use 

Cropland 

Forested Uplands 

RangelandMloodlands 

OpenIRecreation 

Wetland 

Citrus 

Low Density Single Family Residential 

Medium Density Single Family Residential 

High Density Single Family Residential 

Multi-family Building 

Mobile Home 

CommerciaVSewices 

Institutional 

Industrial 

Transportation 

Waterbody 

Sewage Treatment and Power Plants 

Total 

Total 

3,756 

673 

15,544 

11,688 

1,449 

2,278 

7,375 

6,827 

5,163 

1,983 

1,065 

1,486 

647 

1,106 

203 

1,028 

30 

62,301 

Phillippi 

Creek 

3,612 

570 

5,304 

5,353 

436 

1,938 

4.770 

4.997 

3,685 

896 

423 

588 

496 

542 

65 

542 

30 

34,247 

Matheny 

Creek 

0 

15 

675 

444 

14 

104 

1 45 

114 

1,176 

307 

30 

435 

0 

221 

0 

120 

0 

3,800 

Catfish 

Creek 

0 

20 

2,202 

171 

199 

50 

227 

257 

0 

73 

42 

30 

20 

0 

0 

69 

0 

3,360 

AREA 

Norlh 

Creek 

48 

20 

728 

655 

57 

10 

191 

70 

0 

5 

35 

35 

0 

25 

0 

41 

0 

1,920 

Bowlees 

Creek 

0 

0 

7 

7 

0 

0 

0 

0 

0 

0 

0 

0 

7 

0 

135 

4 

0 

160 

(Acres) 

South 

Creek 

65 

0 

6.41 0 

4,502 

565 

120 

636 

133 

140 

109 

80 

30 

75 

10 

0 

120 

0 

12,995 

West 

Bowlees 

0 

0 

0 

0 

0 

0 

0 

38 

0 

0 

0 

13 

0 

0 

0 

0 

0 

51 

Whitaker 

Bayou 

31 

29 

29 

499 

3 

4 

578 

291 

80 

317 

31 4 

105 

21 

308 

2 

98 

0 

2,710 

Direct 

toBay 

0 

19 

189 

57 

175 

52 

828 

927 

82 

276 

141 

250 

28 

0 

0 

34 

0 

3,058 
2.3.2 MANATEE COUNTY 

The existing land use distribution for the watersheds in Manatee County is presented in 

Table 2-5. The largest watersheds in the County include Bowlees Creek, South 

Bradenton, and West Bradenton. These three watersheds account for 75 percent of the 

total drainage area in the County. 

In Manatee County, about 64 percent of the land area consists of urban development and 

the other 36 percent is rural. Generally, the areas immediately south and west of the City 

of Bradenton are undeveloped, whereas the remaining areas are developed. Roughly 72 

percent of the urban area is residential, primarily divided between medium and high 

density single family and multi-family building. In the rural areas, about 29 percent is 

either cropland or citrus, and the remaining 71 percent is typically either 

rangeland/woodlands or openlrecreation. 

2.3.3 CITY OF SARASOTA 

Table 2-6 presents the existing land use distribution for the watersheds in the City of 

Sarasota. The largest watersheds in the City include Whitaker Bayou, Phillippi Creek and 

Hudson Bayou. These watersheds account for 75 percent of the City drainage area. In 

addition, another 15 percent drains directly to Sarasota Bay. 

In the City, 87 percent of the land area consists of urban development and the other 13 

percent is rural. Most of the rural area is located in the far eastern part of the City, and 

the majority of the rural area actually consists of golf course and park land that will not 

be urbanized in the future. Residential development accounts for 70 percent of the total 

urban land use, and it is primarily split between medium density and high density single 

family residential and multi-family building. 
EXISTING LAND USE BY WATERSHED IN SARASOTA BAY NEP STUDY AREA - 

MANATEE COUNTY 

Land Use 

Cropland 

Forested Uplands 

RangelandMloodlands 

OpenIRecreation 

Wetland 

Cirus 

Low Density Single Family Residential 

Medium Density Single Family Residential 

High Density Single Family Residential 

Multi-family Building 

Mobile Home 

CommerciaVSe~ices 

Institutional 

Industrial 

Transportation 

Waterbody 

Sewage Treatment and Power Plants 

Total 

Total 

1,912 

1,052 

1,253 

2,222 

41 5 

209 

292 

2,012 

3.422 

2,677 

862 

654 

365 

1,989 

633 

228 

30 

20,226 

Phillippi 

Creek 

0 

0 

87 

0 

0 

0 

63 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

150 

Palma 

Sola 2 

0 

65 

45 

385 

0 

0 

0 

0 

295 

180 

70 

0 

0 

80 

0 

0 

0 

1,120 

West 

Bowlees 

0 

0 

41 

77 

0 

0 

0 

319 

246 

41 

101 

25 

0 

31 

0 

0 

0 

881 

Bowlees 

Creek 

16 

32 

345 

365 

0 

204 

24 

958 

621 

481 

461 

294 

95 

1,663 

633 

24 

0 

6,215 

Palma 

Sola 

Creek 

250 

135 

20 

55 

0 

0 

0 

20 

35 

310 

0 

0 

75 

0 

0 

0 

0 

900 

South 

Bradenton 

1,486 

625 

525 

61 0 

110 

0 

0 

315 

285 

250 

130 

85 

0 

40 

0 

144 

30 

4,635 

AREA (Acres) 

West 

Bradenton 

160 

180 

160 

720 

305 

5 

125 

280 

1.540 

550 

20 

1 45 

80 

80 

0 

45 

0 

4,395 

Cedar 

Hammock 

0 

15 

30 

10 

0 

0 

80 

120 

400 

865 

80 

105 

115 

95 

0 

15 

0 

1.930 
TABLE 2-6 

EXISTING LAND USE BY WATERSHED IN SARASOTA BAY NEP STUDY AREA - 

CITY OF SARASOTA 

Land Use 

Cropland 

Forested Uplands 

RangelandMloodlands 

OpenlRecreation 

Wetland 

Citrus 

Low Density Single Family Residential 

Medium Density Single Family Residential 

High Density Single Family Residential 

Multi-family Building 

Mobile Home 

CommerciallSe~ices 

Institutional 

Industrial 

Transportation 

Waterbody 

Sewage Treatment and Power Plants 

Total 

Direct 

to Bay 

0 

10 

0 

52 

0 

0 

39 

484 

229 

0 

0 

279 

39 

30 

15 

6 

0 

1.183 

Total 

0 

168 

126 

650 

0 

4 

368 

1,320 

2,164 

946 

0 

926 

529 

521 

17 

75 

30 

7,844 

AREA (Acres) 

Bowlees 

Creek 

0 

9 

35 

0 

0 

0 

0 

3 

1 

25 

0 

19 

19 

0 

0 

3 

0 

114 

Phillippi 

Creek 

0 

38 

27 

370 

0 

4 

55 

356 

574 

306 

0 

99 

113 

38 

0 

39 

0 

2,020 

West 

Bowlees 

0 

0 

6 

59 

0 

0 

77 

21 0 

170 

5 

0 

46 

54 

0 

0 

0 

0 

627 

Hudson 

Bayou 

0 

0 

0 

65 

0 

0 

0 

150 

635 

240 

0 

305 

180 

20 

0 

0 

0 

1,595 

Whitaker 

Bayou 

0 

111 

57 

104 

0 

0 

197 

117 

555 

370 

0 

178 

124 

433 

2 

27 

30 

2,305 
2.3.4 BARRIER ISLANDS 

The existing land use distribution for the barrier islands draining to Sarasota Bay is 

presented in Table 2-7. Siesta Key, Anna Maria Island, Perico Island and Longboat Key 

are the major land areas contributing to the Bay. A group of smaller islands, including 

Lido Key, St. Armands Key, Coon Key, Bird Key, Otter Key and Casey Key, was 

combined under the category of "Other Islands". 

For the barrier islands, 66 percent of the land area consists of urban development and the 

other 34 percent is rural. Most of the rural area is located on Longboat Key and Perico 

Island, whereas Siesta Key and Anna Maria Island are predominantly urbanized. Of the 

urban area, 94 percent is residential. Medium density single family makes up almost haIf 

of the residential area, and the rest is split primarily between low and high density single 

family residential and multi-family building. Cropland and citrus account for only 7 

percent of the total rural area on the islands. Over 70 percent of the rural area is 

openlrecreation. 

2.4 FUTURE LAND USE 

Even though the focus of Phase 1 is determination of existing loads to Sarasota Bay, 

preliminary projections of future loadings were developed to get a preliminary estimate of 

the incremental increases in loadings that are expected due to future development. 

Consequently, future land use data were developed as part of the analysis. Two future 

land use scenarios were developed: 5-year future and a buildout future. 

The 5-year future land use was developed based on the Developments of Regional Impact 

(DM) data provided by Sarasota and Manatee Counties. The DRI information was 

screened to determine which projects were located in the Sarasota Bay study area. The 

appropriate projects were located on the existing land use maps to determine the 

watershed in which the development would occur, and the type of land that would be 
TABLE 2-7 

EXISTING LAND USE BY WATERSHED IN SARASOTA BAY NEP STUDY AREA - 

BARRIER ISLANDS 

Land Use 

Cropland 

Forested Uplands 

RangelandMloodlands 

OpenIRecreation 

Wetland 

Citrus 

Low Density Single Family Residential 

Medium Density Single Family Residential 

High Density Single Family Residential 

Multi-family Building 

Mobile Home 

Commercial/Sewices 

Institutional 

Industrial 

Transportation 

Waterbody 

Sewage Treatment and Power Plants 

Total 

- 

Other 

Islands 

'0 

6 

19 

137 

57 

0 

155 

394 

0 

65 

0 

30 

0 

20 

15 

2 

0 

900 

Longboat 

Key 

0 

38 

0 

689 

8 

0 

34 

459 

95 

31 1 

25 

38 

0 

0 

0 

0 

0 

1,697 

Perico 

Island 

130 

0 

0 

330 

290 

0 

0 

0 

15 

80 

0 

0 

0 

15 

0 

0 

0 

860 

AREA(Acres) 

Anna 

Maria 

Island 

0 

0 

0 

172 

0 

0 

0 

0 

621 

55 

12 

59 

0 

0 

0 

0 

0 

919 

Total 

130 

44 

19 

1.373 

403 

0 

388 

1,641 

741 

754 

37 

169 

0 

40 

15 

2 

5 

5,761 

Siesta 

Key 

0 

0 

0 

45 

48 

0 

199 

788 

10 

243 

0 

42 

0 

5 

0 

0 

5 

1,385 
converted to urbanized area. Because some of the DRI projects are already underway and 

some are not due to be completed 5 years from now, the increase in urban area during the 

5-year planning horizon was estimated to reflect the amount of development expected to 

occur between 1991 and 1996. For example, a DRI with a scheduled construction start 

date of 1991 and buildout date of 2001 would be assumed 50 percent complete in the 5- 

year planning horizon. 

The Comprehensive Plans for Manatee and Sarasota Counties were used to develop the 

buildout future land use scenario. Development of rural areas in the two counties was 

evaluated by locating rural areas on the existing land use maps, and consulting the 

comprehensive plans to determine the future land use. 

For both scenarios, the City of Sarasota and the barrier islands were not included in the 

analysis. The City of Sarasota is approaching build-out, and future development will be 

limited. The barrier islands were excluded because they represent only 6 percent of the 

land area in the study, and because comprehensive plans for the island communities were 

not available when the analysis was conducted. 

2.4.1 5-YEAR FUTURE 

After screening the DRI data for Sarasota and Manatee Counties, the following 

developments were included in the 5-year future land use analysis: 

. Palmer Ranch, Increments I-V, in Catfish and Matheny Creek watersheds, 

of Sarasota County 

. Gateway Development, in Phillippi Creek Watershed, of Sarasota County 

. Sawgrass Hollow Development, in Phillippi Creek Watershed, of Sarasota 

County 
Based on information in the DRI reports and the existing land use maps, the development 

of 1,686 acres of rural land was projected. Note that all of these developments are in 

Sarasota County. 

The 5-year future land use for Sarasota County is presented in Table 2-8. Overall, 248 

acres of open/residential, 1,350 acres of rangeland/woodlands and 88 acres of citrus are 

developed to create 1,319 acres of medium density single family residential, 120 acres of 

industrial, 20 acres of institutional and 227 acres of commercial area. In addition, 87 

acres are shifted from low density to medium density residential to account for increased 

densities in developments which .were only partly developed under existing land use 

conditions. 

2.4.2 BUILDOUT FUTURE 

Tables 2-9 and 2-10 present the buildout future land use distributions for Sarasota and 

Manatee Counties, respectively. Between the two counties, almost 31,000 acres of 

undeveloped land are converted to urban uses in going from the existing to the buildout 

future scenario. Of the 31,000 acres, about 85 percent is developed into residential land 

* 

use, and the remaining 15 percent is primarily converted to commercial or industrial use. 

The Comprehensive Plans for Manatee and Sarasota Counties do not specify the planning 

horizon that is represented by the buildout scenario. However, by comparing the 23,000 

acres of development for Sarasota County in the buildout scenario to the 1,686 acres of 

d&elopment in the 5-year scenario, the buildout scenario is estimated to represent a 

planning horizon of roughly 68 years (23,000 acres divided by 1,686 acres, times 5 years). 

This estimate involves two assumptions. One is that the current rate of development is 

representative of the long-term rate, and the other is that the 5-year estimate of land 

development is accurate. While the 5-year estimate is believed to be accurate, it may be 

less than the overall long-term rate of development due to the weakness of the economy at 

this time. 
TABLE 2-8 

FIVE - YEAR FUTURE LAND USE BY WATERSHED IN THE SARASOTA BAY NEP STUDY AREA - 

SARASOTA COUNTY 

Land Use 

Cropland 

Forested Uplands 

RangelandMloodlands 

OpenlRecreation 

Wetland 

Citrus 

Low Density Single Family Residential 

Medium Density Single Family Residential 

High Density Single Family Residential 

Multi-family Building 

Mobile Home 

CommerciaVSe~ices 

Institutional 

Industrial 

Transportation 

Waterbody 

Sewage Treatment and Power Plants 

Total 

Total 

3,756 

673 

14,194 

11,439 

1,449 

2,190 

7,288 

8,233 

5,163 

1,983 

1,065 

1,712 

667 

1,226 

203 

1,028 

30 

62,301 

Phillippi 

Creek 

3,612 

570 

5,114 

5,118 

436 

1,850 

4,770 

5,408 

3,685 

896 

423 

689 

496 

542 

65 

542 

30 

34,247 

Matheny 

Creek 

0 

15 

403 

430 

14 

104 

121 

304 

1,176 

307 

30 

435 

0 

341 

0 

120 

0 

3,800 

AREA 

North 

Creek 

48 

20 

728 

655 

57 

10 

191 

70 

0 

5 

35 

35 

0 

25 

0 

41 

0 

1,920 

Catfish 

Creek 

0 

20 

1,314 

171 

199 

50 

164 

1,062 

0 

73 

42 

156 

40 

0 

0 

69 

0 

3,360 

(Acres) 

South 

Creek 

65 

0 

6,410 

4,502 

565 

1 20 

636 

133 

140 

109 

80 

30 

75 

10 

0 

120 

0 

12,995 

' 

Bowlees 

Creek 

0 

0 

7 

7 

0 

0 

0 

0 

0 

0 

0 

0 

7 

0 

135 

4 

0 

160 

West 

Bowlees 

Creek 

0 

0 

0 

0 

0 

0 

0 

38 

0 

0 

0 

13 

0 

0 

0 

0 

0 

51 

Whitaker 

Bayou 

31 

29 

29 

499 

3 

4 

578 

291 

80 

317 

314 

1 05 

21 

308 

2 

98 

0 

2,710 

Direct 

toBay 

0 

19 

189 

57 

175 

52 

828 

927 

82 

276 

141 

250 

28 

0 

0 

34 

0 

3,058 
TABLE 2-9 

BUILDOUT FUTURE LAND USE BY WATERSHED IN THE SARASOTA BAY NEP STUDY AREA 

SARASOTA COUNTY 

Land Use 

Cropland 

Forested Uplands 

RangelandNVoodlands 

OpenlRecreation 

Wetland 

Citrus 

Low Density Single Family Residential 

Medium Density Single Family Residential 

High Density Single Family Residential 

Multi-family Building 

Mobile Home 

CommerciaVSewices 

Institutional 

Industrial 

Transportation 

Waterbody 

Sewage Treatment and Power Plants 

Total 

Total 

1,695 

33 

3,582 

5,221 

1,449 

369 

12,738 

22,114 

5,163 

1,983 

1,065 

1.775 

667 

3,186 

203 

1,028 

30 

62,301 

Phillippi 

Creek 

1,655 

6 

1,167 

677 

436 

353 

7,763 

13,448 

3,685 

896 

423 

765 

496 

1,840 

65 

542 

30 

34,247 

Matheny 

Creek 

0 

5 

90 

21 

14 

0 

1 45 

729 

1,176 

307 

30 

455 

0 

708 

0 

120 

0 

3,800 

Catfish 

Creek 

0 

20 

64 

65 

199 

0 

227 

2,285 

0 

73 

42 

51 

40 

225 

0 

69 

0 

3,360 

AREA 

North 

Creek 

9 

0 

0 

0 

57 

0 

339 

1,364 

0 

5 

35 

45 

0 

25 

0 

41 

0 

1.920 

(Acres) 

South 

Creek 

0 

0 

2,249 

4,382 

565 

12 

2,868 

2,355 

140 

109 

80 

30 

75 

10 

0 

120 

0 

12,995 

Bowlees 

Creek 

0 

0 

0 

0 

0 

0 

0 

14 

0 

0 

0 

0 

7 

0 

135 

4 

0 

1 60 

West 

Bowlees 

Creek 

0 

0 

0 

0 

0 

0 

0 

38 

0 

0 

0 

13 

0 

0 

0 

0 

0 

51 

Whitaker 

Bayou 

31 

2 

2 

49 

3 

4 

578 

725 

80 

31 7 

314 

105 

21 

378 

2 

98 

0 

2,710 

Direct 

toBay 

0 

0 

10 

27 

175 

0 

818 

1,156 

82 

276 

141 

31 I 

28 

0 

0 

34 

0 

3,058 
TABLE 2-10 

BUILDOUT FUTURE LAND USE BY WATERSHED IN THE SARASOTA BAY NEP STUDY AREA - 

MANATEE COUNTY 

Land Use 

Cropland 

Forested Uplands 

RangelandMloodlands 

OpenlRecreation 

Wetland 

Citrus 

Low Density Single Family Residential 

Medium Density Single Family Residential 

High Density Single Family Residential 

Multi-family Building 

Mobile Home 

CommerciallSewices 

Institutional 

Industrial 

Transportation 

Waterbody 

Sewage Treatment and Power Plants 

Total 

Total 

0 

39 

88 

325 

415 

93 

685 

4,132 

5,650 

3.240 

862 

824 

365 

2,452 

633 

228 

195 

20,226 

Phillippi 

Creek 

0 

0 

0 

0 

0 

0 

63 

0 

0 

0 

0 

87 

0 

0 

0 

0 

0 

150 

Palma 

Sola 2 

0 

0 

20 

0 

0 

0 

0 

108 

495 

315 

70 

32 

0 

80 

0 

0 

0 

1,120 

Bowlees 

Creek 

0 

0 

0 

0 

0 

88 

24 

1,151 

713 

61 6 

461 

294 

95 

2,116 

633 

24 

0 

6,215 

AREA (Acres) 

West 

Bradenton 

0 

21 

18 

315 

305 

5 

518 

658 

1,579 

555 

20 

196 

80 

80 

0 

45 

0 

4,395 

Cedar 

Hammock 

0 

15 

30 

10 

0 

0 

80 

120 

400 

865 

80 

105 

115 

95 

0 

15 

0 

1,930 

Palma 

Sola 

Creek 

0 

3 

20 

0 

0 

0 

0 

20 

41 7 

365 

0 

0 

75 

0 

0 

0 

0 

900 

West 

Bowlees 

Creek 

0 

0 

0 

0 

0 

0 

0 

437 

246 

41 

101 

25 

0 

31 

0 

0 

0 

881 

South 

Bradenton 

0 

0 

0 

0 

110 

0 

0 

1,638 

1,800 

483 

130 

85 

0 

50 

0 

1 44 

195 

4,635 
With the exception of the West Bradenton watershed in Manatee County and the Phiippi 

and South Creek watersheds in Sarasota County, all of the watersheds are almost 

completely developed in the buildout scenario. The West Bradenton watershed includes 

area near Perico Island that is planned for conservation purposes. Both the Philippi and 

South Creek watersheds include area east of 1-75 that is designated as rural land use in the 

Sarasota County Comprehensive Plan. 

2.5 SOILS CHARACTERISTICS 

The soils characteristics for the study area were determined using the Soils Conservation 

Service Soil Surveys for Manatee and Sarasota Counties. The SCS hydrologic soils 

groups (A, B, C, and D) indicate the relative inf~ltration characteristics of a soil after 

prolonged wetting. In the SCS system, an 'A' soil is the most well-drained and a 'D' soil 

is the most poorly-drained. 

Review of the soils data indicated that most of the area is characterized by BID soils. In 

the Manatee County part of the study area, three soils associations make up most of the 

total area. These are the Eau Gallie-Floridana, Wabasso-Bradenton-Eau Gallie, and 

Delray-Floridana associations. The soil types in these associations are classified as B/D 

by SCS. For Sarasota County, the major soil types shown on the soil survey maps were 

Eau Gallie and Myakka fine sands, and Pineda fine sand. Each of these soils is classified 

as B/D by SCS. Based on these data, the entire study area was considered to be B/D soils 

for this analysis. 

2.6 WATER OUALlTY CHARACTERISTICS 

Based on reports from the Florida Department of Environmental Regulation (FDER, 1990; 

FDER, 1987), Sarasota Bay is generally characterized as having "fair" water quality, and 

its tributaries are characterized as having "poor" to "fair" water quality. The 

classifications were established by analyzing water quality data in EPA's Storage and 
Retrieval System (STORET), as well as USGS data. The monitoring data were 

transformed by FDER into Water Quality Indices (WQI) and Trophic State Indexes (TSI) 

for tributaries and various areas of the Bay. 

Tributaries with "fair" water quality ratings include Phiilippi Creek, Matheny Creek and 

Catfish Creek. Whitaker Bayou has a "poor" water quality rating. In most cases, elevated 

nutrient levels are the reason for the poor to fair ratings. The elevated nutrient levels are 

usually attributed to urban runoff and wastewater treatment plant effluent. 

Several studies (Hey1 and Dixon, 1987; CDM, 1990) have explored the trends in Sarasota 

Bay water, quality since the mid-1960s. These studies have identified a downward trend 

in salinity and nutrient concentrations over time. Rainfall patterns, changes in agricultural 

land use, changes in point source loadings and other factors were considered as reasons 

for the identified trends. 

The changes in land use appear to be one plausible reason for declining salinity and 

nutrients in the Bay. The transformation of certain types of agricultural land to urban 

residential land use results in increased surface runoff, which in turn results in more 

freshwater dilution in the Bay. In addition, a previous local study (CDM, 1987) for the 

Braden River projected a decrease in nutrients as agricultural land was converted to low 

density residential use. It should be noted that the study also projected an increase in 

heavy metals loading as a result of urbanization. Unfortunately, water quality trends for 

metals in Sarasota Bay could not be assessed due to a lack of monitoring data. 

Recent improvements in wastewater treatment and disposal should continue to improve 

water quality in the Bay. The City of Sarasota Wastewater Treatment Plant (WWTP), 

which discharges to Whitaker Bayou, has recently upgraded treatment from secondary to 

advanced waste treatment (AWT). In addition, the quantity of effluent discharged to 

Whitaker Bayou has been reduced through a recently implemented reuse irrigation system. 

Manatee County has developed a deep-well injection system to accept excess effluent 
from the WWTP with the largest flow in the Sarasota Bay study area, and has improved 

its reuse irrigation system. 

2.7 SOURCES OF POLLUTION 

In this study, five different sources of pollution to Sarasota Bay have been considered. 

They include the following: 

. Surface runoff 

. Baseflow 

. Point source discharges 

. Septic tanks 

. Rainfall 

A brief summary of each pollution source is presented below. 

2.7.1 SURFACE RUNOFF 

During a rainfall event, the volume of rainfall that cannot infitrate into the soil becomes 

surface runoff which enters numerous tributaries and ultimately is transported to Sarasota 

Bay. En route to the tributaries, the surface runoff picks up pollutants that have 

accumulated on the land surface. Examples of such pollutants include nutrients such as 

nitrogen and phosphorus, which are applied to lawn areas for fertilization, and metals such 

as lead and zinc, which are deposited on streets by automobiles. 

Because 60 percent of the study area is currently characterized by improved land uses 

(e.g., agricultural, residential, industrial, commercial), it is likely that surface runoff is a 

significant contribution to the total pollution loading to the Bay. Cropland, citrus, 

commercial, industrial, and the more dense residential land uses can be expected to 

contribute high concentrations of nitrogen and phosphorus to tributaries. With the 

exception of cropland and citrus, these same land uses will also contribute relatively high 

metals concentrations. 
The baseflow loading accounts for pollution conveyed by groundwater. The fraction of 

total watershed loading that is due to baseflow becomes smaller as the watershed 

develops, because more of the rainfall is converted to surface runoff and less infiltrates 

into the soil. The concentration of pollutants in the groundwater is based on the natural 

composition of the soil. 

2.7.3 POINT SOURCE DISCHARGES 

Point source discharges in the study area include wastewater treatment plants (WWTPs) 

that discharge treated wastewater from municipal and industrial sources. Municipal 

sources account for almost all of the total point source discharge. 

Anna Maria Island, Longboat Key and the portion of Manatee County within the study 

area are all served by the Southwest Regional WWTP. The plant currently has an average 

daily flow rate of 12.8 million gallons per day (mgd). The effluent is used for irrigation 

purposes, primarily at the Manatee Fruit Company site and several golf courses. In 

addition, effluent can also be discharged into a deep well injection system during wet 

weather conditions. In effect, no effluent is directly discharged to surface waters. 

The City of Saraso-ta and some areas in Sarasota County are served by the City of 

Sarasota WWTP. In 1990, the average discharge from the plant was 6.9 mgd. The plant 

has recently been upgraded from secondary treatment to AWT. Disposal of the effluent 

consists of irrigation on pasture land and golf course property with intermittent surface 

water discharge. Loadings to Sarasota Bay from discharge to Whitaker Bayou have 

decreased considerably since these improvements were implemented. Additional reuse 

sites, which will further reduce loadings from this point source, are planned. 
Siesta Key and parts of Sarasota County are served by a number of small package plants 

and privately-owned wastewater treatment utilities. The total flow for these plants is 

approximately 7.4 mgd. Some of these facilities achieve AWT standards. Most of the 

facilities discharge via irrigation, drain fields, and percolation ponds, though several of the 

larger plants discharge directly to surface water. 

2.7.4 SEPTIC TANKS 

Septic tanks (also referred to as onsite disposal systems) are used in some cases to treat 

waste from individual homes, multi-family buildings, and commercial and industrial areas. 

Basically, a septic tank achieves primary treatment (i.e., settling) in an anaerobic 

environment, and discharges the effluent to a drainfield. Presumably, further pollutant 

transformation and removal occurs as the effluent percolates downward through the 

drainfield to the water table. Further dilution and removal is expected to occur as the 

effluent mixes with and moves along with the groundwater flow. 

Table 2-1 1 shows septic tank coverage information for the Sarasota Bay NEP study area. 

Septic tanks are used throughout the Sarasota County mainland and in the barrier islands. 

For Casey Key, all of the residential development (157 acres) is served by septic tanks. 

In Sarasota County, the percentage of land use served by septic tanks depends on the type 

of land use. For residential land use, about 58 percent of the low and medium single 

family residential land use is served by septic tanks, whereas the percent served for high 

density single family residential, multi-family building and mobile homes are estimated to 

be 38, 13, and 3 respectively. The percent served for commercial, industrial and 

institutional land uses are estimated to be 21, 23, and 9, respectively. 

The pollutants of concern from septic tanks are total nitrogen and, to a lesser extent, total 

phosphorus. Both total nitrogen and total phosphorus are discharged at high 

concentrations from a septic tank. Typical effluent concentrations are 40 to 80 mg/L for 
TABLE 2-1 1 

SEPTIC TANK COVERAGE FOR SARASOTA BAY NEP STUDY AREA 

NOTES: 

1. Septic tank areas in Sarasota County estimated basedon mapping provided by County staff. 

2. Small number of septic tanks are still adve In Manatee County and the City of Sarasota, but were excluded from the analysis. 

Land Use 

Low Density Slngle Family Resldentlal 

Medium Density Single Family ResidenUal 

Hlgh Density Single Family Residential 

Multl-family Building 

Moblle Home 

CommerciallSe~ices 

InstiMional 

industrial 

Manatee County 

Area 

Sewed by 

Septic 

Tanks 

(acres) 

0 

0 

0 

0 

0 

0 

0 

0 

Percent 

of 

Total 

Area 

0% 

0% 

0% 

0% 

0% 

0% 

0% 

0% 

Ciy of Sarasota 

Area 

Sewed by 

Septic 

Tanks 

(acres) 

0 

0 

0 

0 

0 

0 

0 

0 

Percent 

of 

Total 

Area 

0% 

0% 

0% 

0% 

0% 

0% 

0% 

0% 

Barrier Islands 

Area 

Sewed by 

Septic 

Tanks 

(acres) 

155 

0 

0 

0 

0 

0 

0 

0 

Percent 

of 

Total 

Area 

40% 

0% 

0% 

0% 

0% 

0% 

0% 

0% 

Sarasota County 

Area 

Sewed by 

Septic 

Tanks 

(acres) 

4272 

3897 

1963 

252 

34 

31 1 

59 

252 

Total 

Percent 

of 

Total 

Area 

58% 

57% 

38% 

13% 

3% 

21% 

9% 

23Y9 

Area 

Sewed by 

Septic 

Tanks 

(acres) 

4427 

3897 

1963 

252 

34 

31 1 

59 

252 

Percent 

of 

Total 

Area 

53% 

33% 

17% 

4% 

2% 

10% 

4% 

7% 
total N and about 15 mgL for total P, as compared to 3 mg/L total N and 1 mgL total P 

for AWT. In most instances, soil is effective in removing total P, such that 90 percent or 

more is retained in the soil through adsorption. For total N, however, much of the mass 

in the effluent reaches the water table, with ammonia nitrogen being converted to nitrate 

nitrogen under aerobic conditions in the soil. Nitrate nitrogen is known to be very mobile 

in groundwater and can have serious health effects if drinking water concentrations are too 

high. 

Loadings to Sarasota Bay also are contributed by rainfall on the Bay surface. Considering 

that the water surface is about 52 square miles (34 percent of the total drainage area to the 

Bay), rainfall could have a significant impact on pollution loading. Precipitation totals for 

the study area are discussed in Section 3.2, and rainfall quality is discussed in Section 3.6. 
3.0 METHODOLOGY FOR POLLUTION LOADING PROJECTIONS 

3.1 INTRODUCTION 

This section presents the methodology and assumptions used to estimate pollution loadings 

to Sarasota Bay. The discussion includes the relationship between rainfall and 

streamflow, and the selection of nonpoint pollution loading factors. Data regarding 

quantity and quality of point sources, septic tanks and rainfall are also presented. 

3.2 RAINFALL/RUNOFF RELATIONSHIPS 

The annual loadings for surface runoff and baseflow are based upon the streamflow from 

the watershed, mean pollutant concentrations of the surface runoff and baseflow, and the 

distribution of streamflow between surface runoff and baseflow. To calculate annual 

streamflow volumes for the study area, long-term monitoring data from USGS gages and 

local raingages were analyzed. In addition to analyzing average annual conditions, rainfall 

and streamflow volumes for the wet season (June - September) and the dry season 

(October - May) that comprise the average annual conditions were also determined. 

Finally, the data were also analyzed to establish a wet year and a dry year scenario. 

3.2.1 RAWFALL AND STREAMFLOW DATA 

Table 3-1 shows rainfall data for two long-term gages in the vicinity of the study area. 

The average year totals for the Bradenton and Myakka gages were determined by 

averaging the annual totals over the entire period of record. Similarly, the wet season and 

dry season volumes were calculated by averaging the total rainfall during each season 

over the period of record. The wet year and dry year values were established by ranking 

the annual rainfall totals from lowest to highest, selecting the 10th percentile value as the 

dry year total and the 90th percentile value as the wet year total. 
TABLE 3-1 

PRECIPITATION DATA FOR GAGES IN 

SARASOTA BAY NEP STUDY AREA 

PRECIPITATION (INCHES) 

BRADENTON GAGE (104 YEARS OF RECORD1 

AVERAGE YEAR 54.6 

- WET SEASON 33.7 

- DRY SEASON 20.9 

WET YEAR 69.7 

DRY YEAR 41.7 

MYAKKA GAGE (44 YEARS OF RECORD) 

AVERAGE YEAR 54.3 

-WET SEASON 32.5 

- DRY SEASON 21.8 

WET YEAR 69.6 

DRY YEAR 40.4 

BRADENTON GAGE (44 YEARS OF RECORD1 

AVERAGE YEAR 53.3 

- WET SEASON 31.8 

- DRY SEASON 21.5 

WET YEAR 69.7 

DRY YEAR 42.6 
The values generated for the Bradenton gage were selected for the runoff analysis. As 

shown in Table 3-1, the results for the same 44 years of record at the Bradenton and 

Myakka gages were comparable. Because the rainfall values for the kvo gages are 

similar, it is appropriate to use the values for the gage with the longest period of record. 

Thus, an average annual rainfall total of 54.6 inches was assumed for the study area. Of 

the 54.6 inches, 33.7 inches (62 percent) occurs during the months of June through 

September, and the remaining 20.9 inches (38 percent) occurs during the other eight 

months. The annual values for a wet year and a dry year are 69.7 and 41.7 inches, 

respectively. These values are 28 percent higher and 24 percent lower than the average 

annual rainfall total. 

Streamflow data for two USGS gages in the vicinity of the study area are presented in 

Table 3-2. For the Lake Manatee and Myakka River gages, streamflow values for average 

annual, wet season, dry season, wet year and dry year scenarios were established in the 

same manner as the rainfall values. 

Because the two gages had comparable periods of record, the streamflow values used in 

the runoff analysis were the average of the values for the two gages. As shown in Table 

3-2, the average annual streamflow volume is 14.8 inches, with 9.9 inches (67 percent) 

occumng during the wet season and 4.9 inches occumng during the dry season. The wet 

year and dry year streamflow values are 22.9 and 7.6 inches, respectively. Both values 

vary by roughly 50 percent from the average annual value. 

3.2.2 PERVIOUS AREA RUNOFF 

The streamflow volume due to surface runoff from pervious areas was estimated by 

assigning a runoff coefficient. The runoff coefficient was multiplied by the rainfall 

volume to calculate the surface runoff volume from pervious areas. A single value was 

established to estimate the average annual runoff, given the average rainfall total. 
TABLE 3-2 

STREAMFLOW DATA FOR GAGES IN 

SARASOTA BAY NEP STUDY AREA 

STREAMFLOW (INCHES) - 

LAKE MANATEE GAGE (50 YEARS OF RECORD1 

AVERAGE YEAR 15.0 

- WET SEASON 10.3 

- DRY SEASON 4.7 

WET YEAR 22.6 

DRY YEAR 9.0 

MYAKKA GAGE (50 YEARS OF RECORD) 

AVERAGE YEAR 14.5 

- WET SEASON 9.5 

- DRY SEASON 5.0 

WET YEAR 23.2 

DRY YEAR 6.1 

AVERAGE OF LAKE MANATEE AND MYAKKA GAGES 

AVERAGE YEAR 14.8 

-WET SEASON 9.9 

- DRY SEASON 4.9 

WET YEAR 22.9 

DRY YEAR 7.6 
An average annual runoff coefficient of 0.15 was established for pervious areas. In this 

analysis, the runoff coefficient of 0.15 combined with an annual rainfall total of 54.6 

inches results in an annual runoff of 8.2 inches. By subtracting the 8.2 inches from 

average annual streamflow volume of 14.8 inches, an annual groundwater baseflow 

volume of 6.6 inches is calculated. Thus, based on a runoff coefficient of 0.15, about 15 

percent of rainfall becomes surface runoff, 12 percent contributes to stream baseflow, and 

the remaining 73 percent (39.8 inches) is lost via evapotranspiration. 

3.2.3 IMPERVIOUS AREA RUNOFF 

The streamflow volume due to surface runoff from impervious areas was also determined 

using runoff coefficients. For all impervious area, a runoff coefficient of 0.95 was 

assumed. 

3.2.4 BASEFLOW 

The baseflow volume at the USGS gages was calculated as the difference between 

streamflow volume and surface runoff, with gage drainage areas assumed to be rural (i.e., 

pervious). For undeveloped areas, baseflow was calculated to contribute about 45 percent 

of the streamflow. 

For individual watersheds in the study area, the baseflow volume was reduced as a direct 

function of the percentage of urban impervious area. For example, if the drainage area 

was 50 percent impervious due to residential and commercial development, then the 

baseflow volume was reduced by 50 percent. Thus, the percentage of streamflow due to 

baseflow diminishes as development occurs in the watershed. However, it is not only 

because baseflow diminishes, but also because surface runoff increases. 
3.3 NONPOINT POLLUTION LOADING FACTORS 

For both surface runoff and baseflow, the pollution loadings were calculated by 

multiplying the flow volume by an appropriate pollutant concentration. In the case of 

surface runoff, event mean concentration (EMC) data developed through studies such as 

EPA's Nationwide Urban Runoff Program m) were used to characterize runoff 

concentrations. For baseflow, local ambient monitoring data were used to develop the 

concentrations. 

3.3.1 SURFACE RUNOFF 

Since the completion of EPA's Nationwide Runoff Program (NURP) in the early 1980's 

(USEPA, 1983), there is a general consensus in the field of nonpoint pollution 

management that local monitoring studies of single land use watersheds are no longer 

required to characterize urban nonpoint pollution loadings for management studies. In 

place of an expensive local monitoring program, available literature values for nonpoint 

pollution Ioading factors can be used to formulate the watershed management plan. In 

addition to the transferability evaluations in the EPA NCTRP study, this approach has 

worked out well in previous watershed management studies where mixed land use 

monitoring data were available for comparison. 

In the Sarasota Bay NEP study, pollutant loading analyses were limited to the constituents 

for which considerable loading data are reported in the literature. The following four 

pollutants were included: total phosphorus (total P), total nitrogen (total N), lead, and 

zinc. Total P and total N are included because they may be responsible for adverse 

eutrophication impacts. Lead and zinc are heavy metals which typically exhibit higher 

nonpoint pollution loadings than other metals found in urban runoff, and therefore, 

transferable loading factors are available in the literature. These heavy metals may be 

viewed as surrogates for a wide range of toxicants that have been identified in previous 

field monitoring studies of urban runoff pollution (USEPA, 1983). 
The EMC values to estimate surface runoff loadings are presented in Table 3-3. Again, 

the values are based primarily on data from the EPA NURP study, although values for 

land uses such as cropland (which tend to vary substantially from one location to another) 

are also based on local data. These same values have been applied successfully in several 

other studies in the State of Florida (CDM, 1990). 

For nutrients (total N and total P), the EMC values are highest for cropland, citrus, and 

low and medium density single family residential land uses. This is due to fertilization of 

the cropland and the lawns of the residential areas. Commercial, industrial and 

unimproved areas have the lowest EMC values for nutrients, less than half as large as the 

agricultural and residential values. 

It should be noted, however, that the pollutant loading depends on the EMC value the 

volume of surface mnoff for a particular land use. Because commercial and industrial 

land uses have a much greater percentage of impervious area than residential land use, 

they tend to produce greater loadings in terms of lbdaclyear, even though they are 

characterized by lower EMC values. For example, the average annual surface runoff 

loads for commercial and medium density single family residential land uses are relatively 

similar (1.6 lb/ac/yr for commercial and 2.1 lb/ac/yr for residential) for total P, even 

though the EMC is much higher for the residential area. The loadings for total N are 12.3 

and 10.1 lb/ac/yr for commercial and medium density residential land uses, respectively. 

Thus, EMC values alone cannot be used to determine the relative loading impacts of 

different land uses. 

For lead and zinc, Table 3-3 shows that unimproved and agricultural land uses have 

EMCs of zero, whereas residential, commercial, industrial and other urban land uses are 

shown to generate loadings of metals. The EMC values increase as the percent 

imperviousness of the land use increases. Because more impervious areas will also 

generate more runoff, the loadings from commercial and industrial areas will be 

substantially higher than the loadings from the residential areas. 
TABLE 3-3 

EVENT MEAN CONCENTRATION VALUES 

FOR SARASOTA BAY NEP STUDY 

Land Use 

Cropland 

Forested Uplands e 

Rangeland/Woodlands 

OpenIRecreation 

Wetland 

Citrus 

Low Density Single Family Residential 

Medium Density Single Family Residential 

High Density Single Family Residential 

Multi-family Building 

Mobile Home 

CommerciallSe~ices 

Institutional 

Industrial 

Transporlation 

Waterbody 

STP and Power Plants . 

Event Mean 

Total 

Phosphorus 

1.13 

0.16 

0.16 

0.16 

0.03 

0.41 

0.39 

0.39 

0.33 

0.33 

0.33 

0.15 

0.15 

0.15 

0.15 

0.15 

0.15 

Values 

Total 

Lead 

0.000 

0.000 

0.000 

0.000 

0.000 

0.000 

0.049 

0.049 

0.076 

0.076 

0.076 

0.235 

0.235 

0.235 

0.235 

0.006 

0.235 

Concentration 

Total 

Nitrogen 

3.74 

1.02 

1.02 

1.02 

0.25 

0.92 

1.87 

1.87 

1.65 

1.65 

1.65 

1.18 

1.18 

1.18 

1.18 

0.82 

1.18 

in mgll 

Total 

Zinc 

0.000 

0.000 

0.000 

0.000 

0.000 

0.000 

0.054 

0.054 

0.060 

0.060 

0.060 

0.120 

0.120 

0.120 

0.120 

0.146 

0.120 
Baseflow loadings, like surface runoff loadings, were calculated by determining the flow 

volume and the flow concentration. Based on analysis of existing water quality data, the 

following values were selected for baseflow concentrations: 

. Total P: 0.30 mg/L 

. Total N: 1.00 mg/L 

. Lead: 0.003 mg/L 

. Zinc: 0.05 mg/L 

The total N and total P values were selected after analyzing concentration-frequency 

curves for several STORET stations. It was assumed that the median concentrations for 

these stations were representative of dry weather conditions where baseflow was 

predominant, and that the stations were located in areas where dry weather flows are not 

affected by point source discharges or septic tank impacts. Unfortunately, the same 

stations did not include metals data. The values shown above for metals were selected 

based on a limited number of STORET values for raw water monitoring at various water 

treatment plants in and around the study area. 

A summary of available STORET data is presented in Appendix A. 

3.4 POINT SOURCE LOADINGS 

Table 3-4 lists the point source discharges included in the loading analysis. These 

represent all of the discharges located in the study area that have a daily average flow rate 

exceeding 0.1 mgd. Together, these plants account for over 95 percent of the total point 

source discharge in the study area. In all, the total discharge from these treatment plants 

is roughly 27 mgd. 

Data shown in the table include the watershed, average flow, level of treatment and 

method(s) of disposal. In Sarasota County and the City of Sarasota, discharges occur in 
TABLE 3-4 

POINT SOURCE DISCHARGES IN SARASOTA BAY NEP STUDY AREA 

Disposal 

Method(s) 

Irrigation 

Deep well Injection 

Percolation ponds 

lrrigation 

Irrigation 

Percolation ponds 

percolation ponds 

Surface water 

Percolation ponds 

irrigation 

Percolation ponds 

lrrigation 

Percolation ponds 

lrrigation 

Percolation ponds 

Percolation ponds 

Percolation ponds , 

Surface water 

Irrigation 

Surface water 

Percolation ponds 

irrigation 

Permiation ponds 

Irrigation 

Surface water 

Drainfieids 

Discharge 

Southwest Regional WWTP - 

Manatee County 

Southeastern-Bent Tree 

Atlantic Utilities 

Dolomite - Fruitville 

Florida Cities - Southgate 

Kensington Park 

Utilities - 27th St. 

Meadowood Utilities 

Tamaron Utility Authority 

Camelot 

Sorrento 

Central County Utilities 

Florida Cities - 

Gulf Gate 

Sarasota City WWTP 

Dolomite - Tri Par 

Kensington Park Util.. 

Monica Pkwy. 

Siesta Key Utilities 

Southbay Utilities 

Watershed 

South Bradenton 

Phillippi Creek 

Philiippi Creek 

Phillippi Creek 

Philiippi Creek 

Philiippi Creek 

Phiilippi Creek 

Phiilippi Creek 

Phiilippi Creek 

Matheny Creek 

Matheny Creek 

Matheny Creek 

Whitaker Bayou 

Whitaker Bayou 

Whitaker Bayou 

Siesta Key 

Direct to Bay 

Average 

Flow (mgd) 

12.80 

0.26 

0.77 

0.14 

1.10 

0.09 

0.47 

0.11 

0.1 0 

0.25 

0.31 

1.37 

6.90 

0.23 

0.36 

1.82 

0.13 

Level of 

Treatment 

Secondary 

Secondary 

Secondary 

Secondary 

AWT . 

Secondary 

Secondary 

Secondary 

Secondary 

Secondary 

Secondary 

AWT 

AWT 

Secondary 

Secondary 

AWT 

Secondary 
the Phillippi Creek, Matheny Creek and Whitaker Bayou watersheds. Disposal methods 

include a combination of irrigation, percolation ponds, and surface water discharge. 

Manatee County, Anna Maria Island and Longboat Key are served by the Southwest 

Regional WWTP, which disposes of effluent through irrigation and deep well injection. 

Siesta Key is served by Siesta Key Utilities, which disposes of effluent via surface water 

discharge. 

To estimate the point source pollution loadings, a number of assumptions were made. 

Assumptions regarding effluent quality (i.e., concentrations of pollutants in the effluent) 

were made based on limited monitoring data supplemented by literature values. For 

disposal methods other than surface water discharge, the fraction of effluent load reaching 

Bay tributaries was assumed. 

The effluent concentrations assumed for the point source analysis are presented below: 

Secondary treatment: 

Advanced waste treatment: 

Total N = 20 mg/L 

Total P = 4 mgL 

Lead = 25 ugL 

zinc = 100 ugn 

Total N = 3 mg/l 

Total P = l mgL 

Lead = 25 ugL 

Zinc = 100 ugL 

Nutrient values for secondary treatment were established based on monthly operating 

report (MOR) data from several of the Sarasota County dischargers, and the nutrient 

values for AWT were based on the typical standards included in NPDES permits. The 

reviewed data did not include any lead or zinc concentrations, so these values are strictly 

based on literature data. The values are in the mid-range of concentrations that may be 

expected, based on effluent data from 12 California WWTPs that use effluent for 
irrigation (Lewis, 1985) as well as typical influent concentration and removal efficiency 

data developed in an EPA study of priority pollutants in publicly owned treatment works 

@PA, 1982). 

For land disposal of wastewater, the fraction of the mass load reaching Bay tributaries 

depends upon the type of disposal. It was assumed that no load would reach the Bay for 

deep well injection. For percolation ponds and drainfields, a removal rate of 90 percent 

was assumed for all pollutants. A previous CDM study (CDM, 1985) found that the 90 

percent value was appropriate for retention and exf~ltration best management practices 

(BMPs). A slightly higher removal rate of 95 percent was assumed for irrigation 

practices, with the higher efficiency attributed to plant uptake. 

Based on the assumptions above, the point source loadings shown in Table 3-5 were 

developed for existing conditions. The flow rates represent only the surface water 

discharges plus a fraction of the discharge to percolation ponds and drainfield. This 

fraction was set equal to 0.27, which is the ratio of streamflow to precipitation. The 

concentrations of nutrients and metals were established such that the product of flow and 

concentration was equal to the combined mass of pollutant from surface water and 

percolation pondldrainfield discharges. Surface discharges for annual, wet season and 

dry season are constant for all watersheds except Whitaker Bayou, which receives 

discharge from the City of Sarasota. The City of Sarasota discharge of 3.30 mgd 

represents a projected discharge of 7 mgd for a total of 172 days during 1991, based on a 

mid-year estimate (CDM, 1991). Of the 172 days, 75 are during the wet season (June 

through September) and 97 are during the dry season (October through May). 

Consequently, the average flow rates during the wet and dry season are 4.30 and 2.79 

mgd, respectively. The remaining 0.17 mgd attributed to Whitaker Bayou comes from 

two small package plants. For the South Bradenton watershed, the flow value of 2.08 

mgd represents 27 percent of 7.7 mgd attributed to irrigation. Recent data indicate that 

about 60 percent of the SWWWTP effluent is used for irrigation, and the remaining 40 

percent is discharged via deep well injection. 
TABLE 3-5 

POINT SOURCE FLOWS AND CONCENTRATIONS FOR EXISTING CONDITIONS 

WATERSHED 

Phillippi Creek 

Matheny Creek 

Whitaker Bayou 

Direct to Bay 

Siesta Key 

South Bradenton 

DISCHARGE (rngd) 

ANNUAL 

1.63 

1.52 

3.47 

0.04 

1.82 

2.08 

CONCENTRATION (rng/L) 

TOTAL P 

1.03 

1.05 

1.02 

1.48 

1 .OO 

0.74 

WET 

SEASON 

1.63 

1.52 

4.47 

0.04 

1.82 

2.08 

DRY 

SEASON 

1.63 

1.52 

2.96 

0.04 

1.82 

2.08 

TOTAL N 

3.78 

3.44 

3.20 

7.38 

3.00 

3.69 

LEAD 

0.015 

0.019 

0.019 

0.007 

0.020 

0.004 

ZINC 

0.076 

0.094 

0.097 

0.037 

0.100 

0.018 
Point source flows for the 5-year future scenario were estimated using available data. In 

Manatee County, linear interpolation was used to estimate a 5-year flow rate of 14.1 mgd, 

given the existing rate of 12.8 rngd and the 20-year projected rate of 18 mgd. In addition, 

it was estimated that 9.8 mgd out of the 14.1 rngd would be used for irrigation. For the 

othw localities, the total flow rate was estimated by evaluating changes in residential land 

use, The total number of dwelling units in the localities for the existing and 5-year 

scenarios was calculated by assigning densities (dwellings per acre) to each residential 

land use. The ratio of future to existing dwelling units was then multiplied by the 

existing total point source flow rate in order to estimate the 5-year total flow ;ate. Based 

on this method, a value of 15.9 mgd was calculated. 

The 5-year future point source flow rate was then distributed between the City of Sarasota 

plant and the other existing utilities and package plants. The City was assigned a flow 

rate of 9.0 mgd, based on available flow projections. The remaining 6.9 rngd was 

equivalent to the sum of the existing discharges in the Phillippi Creek, Matheny Creek, 

Direct to Bay and Siesta Key watersheds. Consequently, the 5-year future surface water 

discharge values for facilities other than the City of Sarasota WWTP were set equal to the 

existing discharge values. 

. 

The same methodology was used to estimate the buildout future total point source flow 

rate in Sarasota County and the City of Sarasota. As a result, a total flow of 23.0 mgd 

was estimated. The City of Sarasota was assigned a flow rate of 13.0 mgd, the current 

design capacity of the plant. The remaining 10.0 rngd was divided between utilities and 

package plants, with the increase in flow applied mainly to the Manatee and Phillippi 

Creek watersheds. The relative proportions of disposal via surface discharge, percolation 

ponds, drainfields, and irrigation were consistent with existing conditions. For Manatee 

County, the 20-year estimates of 18 mgd (16 rngd irrigation and 2 rngd deep well 

injection) were used for the buildout scenario. 
Table 3-6 presents the point source loading data for the 5-year future scenario. The value 

of 1.61 rngd for the City of Sarasota WWTP is based on a flow of 9.0 rngd discharging to 

surface water for a total of 59 days per year plus package plant discharge of 0.17 mgd. 

The 59 days of discharge is consistent with an NPDES permit that is currently being 

sought by the City WWTP. The Southwest WWTP discharge of 2.66 rngd reflects 27 

percent of the 9.8 rngd attributed to irrigation. The other surface water discharge rates are 

equal to the rates for existing conditions. 

The point source data for the buildout future scenario are presented in Table 3-7. The 

value of 2.10 rngd for the City of Sarasota WWTP reflects a 13 rngd surface water 

discharge for a total of 59 days per year. In the other watersheds, the surface water 

discharges are higher than the 5-year future discharges. Futwe package plants and 

utilities are assumed to operate similarly to existing facilities. In other words, discharge is 

distributed between surface waters, percolation ponds, draifields, and irrigation. 

3.5 SEPTIC TANK LOADINGS 

Some of the existing developments in Sarasota County are serviced by septic tanks. The 

Sarasota County Health Department estimates that there are approximately 45,000 septic 

tanks within the County, and the majority of these are probably within the NEP study 

area. On Casey Key, all of the residential area is served by septic tanks, and the number 

of permitted tanks is 330. 

The assessment of septic tank loadings included a literature review designed to better 

quantify the impacts of failing and working septic tanks. Previous CDM studies have 

focused on the impacts of failing septic tanks. Reasons for septic tank failure include high 

water table, structural failure, and direct connection between septic tank and receiving 

water. These failing septic tanks are expected to discharge high concentrations of 
TABLE 3-6 

POINT SOURCE FLOWS AND CONCENTRATIONS FOR FIVE-YEAR FUTURE CONDITIONS 

WATERSHED 

Phillippi Creek 

Matheny Creek 

Whitaker Bayou 

Direct to Bay 

Siesta Key 

South Bradenton 

DISCHARGE(rngd) 

ANNUAL 

1.63 

1.52 

1.61 

0.04 

1.82 

2.66 

CONCENTRATION (rnglL) 

TOTAL P 

1.03 

1.05 

1.05 

1.48 

1 .OO 

0.74 

WET 

SEASON 

1.63 

1.52 

2.19 

0.04 

1.82 

2.66 

DRY 

SEASON 

1.63 

1.52 

1.33 

0.04 

1.82 

2.66 

TOTAL N 

3.78 

3.44 

3.43 

7.38 

3.00 

3.69 

LEAD 

0.015 

0.019 

0.019 

0.007 

0.020 

0.004 

ZINC 

0.076 

0.094 

0.094 

0.037 

0.100 

0.018 
TABLE 3-7 

POINT SOURCE FLOWS AND CONCENTRATIONS FOR BUILDOUT FUTURE CONDITIONS 

WATERSHED 

Phillippi Creek 

Matheny Creek 

Whitaker Bayou 

Direct to Bay 

Siesta Key 

South Bradenton 

DISCHARGE (mgd) CONCENTRATION (mg/L) 

TOTAL P 

1.03 

1.05 

1 .OO 

1.48 

1 .OO 

0.74 

ANNUAL 

2.62 

2.45 

2.10 

0.04 

1.82 

4.34 

LEAD 

0.015 

0.019 

0.020 

0.007 

0.020 

0.004 

TOTAL N 

3.78 

3.44 

3.00 

7.38 

3.00 

3.69 

ZINC 

0.076 

0.094 

0.100 

0.037 

0.100 

0.018 

WET 

SEASON 

2.62 rir 

2.45 

2.94 

0.04 

1.82 

4.34 

DRY 

SEASON 

2.62 

2.45 

1.68 

0.04 

1.82 

4.34 
nutrients. For working septic tanks, a methodology was established to estimate pollutant 

loadings based on of the literature review. Appendix B contain a technical memorandum 

summarizing the findings of the literature review. 

The septic tank evaluation conducted for purposes of this study was limited to the 

contaminants identified in section 1.2; namely total nitrogen, total phosphorus, lead and 

zinc. The evaluation focused on relative (to other sources evaluated) loading contribution 

from on-site disposal systems. As such, human health issues (such as proximity to 

drinking water sources, bacterial and viral issues) of septic systems were not explicitly 

evaluated. 

3.5.1 FAILING SEPTIC TANKS 

Nutrient concentrations for failing septic tanks were developed from a review of septic 

tank leachate monitoring studies. The typical concentrations established based on the 

literature values are as follows: 

TotalN 30 mglL 

Total P 2 mglL 

Generally, these values are in the range of groundwater concentrations measured at or 

near septic tank discharges. The values reflect pollutant removal within the soil of 

roughly 50 percent for total N and 90 percent for total P, based on average effluent 

concentrations cited in the literature. 

Nutrient loadings for specific land uses were calculated by multiplying the concentrations 

by a flow rate. The flow rate for a particular land use depended upon the number of 

residents per acre, and the per capita flow rate. 

Table 3-8 shows the septic tank flow rates developed for various land uses. For all land 

uses, a per capita flow rate of 75 gallons per day was established. This value is at the 
TABLE 3-8 

SEPTIC TANKS FLOW RATES FOR VARIOUS LAND USES 

Flow Rate 

(gallacrelday) 

150 

675 

1125 

1350 

1350 

1350 

1350 

1350 

Per Capita 

Wastewater 

Flow Rate 

(gaVdaylperson) 

75 

75 

75 

75 

75 

- 

Persons Per 

Dwelling unit 

2 

2 

2 

2 

2 

Land Use 

LDSF Residential 

MDSF Residential 

HDSF Residential 

Multi-family Bldg 

Mobile Home 

CommerciallSe~ices 

Institutional 

Industrial 

Dwelling Units 

Per Acre 

1 

4.5 

7.5 

9 

9 

- 

- 
high end of the range of flow rates documented in the literature, and reflects local water 

use data. For the residential areas, values of dwelling units per acre were determined 

based on the descriptions of various land uses in the Sarasota County Comprehensive Plan 

and densities observed in aerial photographs. Each dwelling unit is assumed to have 2.0 

people, based on household size estimates for Sarasota and Charlotte Counties and the 

City of North Port as cited in the City of North Port Comprehensive Plan. This value is 

somewhat lower than the national average, because some of the population is seasonal and 

because the population distribution is weighted more heavily toward retired persons who 

would tend to have a smaller household size. For commercial and industrial areas, the 

flow values were assumed equal. to the highest residential value. 

A final consideration in the loading analysis for failing septic tanks is the failure rate - 

that is, what percentage of the septic tanks are failing. These data were established based 

on permitting data from the Sarasota County Health Department. The Health Department 

requires a permit for repair or replacement of septic tanks. During the period 1980 

through 1990, an average of 740 repairs per year was recorded. Based on the estimate of 

45,000 septic tanks in the County, the annual repair rate is 1.6 percent. Recognizing that 

failure may occur for a number of years before repair is initiated, the failure rate at any 

time is likely to be higher than 1.6 percent. In previous studies, the annual failure rate 

has been multiplied by a factor of 5, which implies that septic systems on average fail for 

5 years before repairs are made. For Sarasota County, the resulting failure rate is 

8 percent. This value compared favorably with the results of a septic tank survey 

conducted in Jacksonville, FL by the Department of Health and Rehabilitative Services. 

In the study, an inspection of more than 800 sites revealed about 90 violations, or a ' 

failure rate of 12 percent Failure calculations also were done specifically for Casey Key, 

which had a higher annual repair rate. The failure rate established for Casey Key was 20 

percent. 
3.5.2 WORKING SEPTIC TANKS 

Based on the literature review of septic tanks, a methodology for assessing impacts of 

working septic tanks was developed. The methodology uses data such as suIficial aquifer 

transmissivity, recharge volume, typical distances from septic tanks to receiving waters, 

and pollutant decay rates to determine how much of the total N and total P discharged 

from septic tanks will reach the Bay tributaries. A detailed description of the calculations 

is provided in a technical memorandum developed in Phase 2 of the Sarasota Bay NEP 

study (see Appendix B). An overview of the methodology is presented below. 

The methodology is based on equations developed by Dupuit (Todd, 1985) to define 

baseflow to a stream. In the septic tank analysis, the baseflow was set equal to the 

recharge rate, which was defmed as the sum of natural baseflow calculated during the 

hydrologic analysis plus drainfield flow contributed by septic tanks. It was assumed that 

the recharge to the surficial aquifer occurred uniformly between the stream and the most 

distant recharge point. By inspection of USGS quadrangle maps, the typical maximum 

recharge distance (i.e., typical maximum distance to a perennial stream) was set equal to 

3,000 feet. Thus, at any distance from the stream, the flow could be calculated. 

Furthermore, by establishing typical values of aquifer thickness, transmissivity and 

porosity, the velocity at any distance from the stream could also be determined. These 

calculations were performed at 10-foot increments to establish cumulative travel times 

from the septic tank to the receiving stream as a function of distance. By applying a fvst 

order decay coefficient uniformly between the minimum distance and the maximum 

distance between septic tank and receiving stream, an overall delivery ratio was 

established. The minimum distance was set at 75 feet, which is the DER requirement for 

septic tank implementation, and the maximum distance was set at 3000 feet, the maximum 

recharge distance. 

Initially, the first order decay rate was established based on work done by Ostendorf 

(1986). Ostendorf developed a model to simulate total N, chloride, boron and methylene 
blue active substances WAS) downgradient from the Otis Air Force Base in 

Massachusetts. The base has discharged an average flow of 0.53 mgd since 1941. 

Samples were taken at various distances downgradient from the inf~ltration beds in order 

to test the model's predictive ability. Chloride, a conservative substance, was simulated 

fust to test the model accuracy. Results showed a very good agreement between 

predicted and measured concentrations. When total N and MBAS were modeled as 

conservative substances, the model overpredicted the concentrations. Consequently, first 

order decay rates were established for these constituents. For total N, a rate of 1.69 X 

10-g/second was established. At this rate, the total N concentration would be reduced by 

5 percent after a travel time of 1 year, 41 percent after 10 years, and 93 percent after 50 

years. 

Other parameter values used in the septic tank analysis are presented in Table 3-9. 

Values for aquifer thickness, porosity and conductivity were based on review of the soil 

survey for Sarasota County and a Southwest Florida Water Management District report 

on groundwater resource availability in Sarasota County (SWWMD, 1988). In addition, 

the minimum travel distance of 75 feet was based on septic tank regulations, and the 

maximum distance of 3,000 feet was based on inspection of USGS maps. 

To evaluate the methodology and the fust-order decay rate, the septic tank analysis 

results were applied to the drainage areas of two STORET stations in the Phillippi Creek 

watershed. These include the Main A Canal at Palmers Boulevard Bridge (Station 

24010630) and Phillippi Creek at Bahia Vista Street Bridge (Station 24010625). The 

Palmers Boulevard station has a drainage area of about 8 square miles, which consists 

primarily of cropland, rangeland, and low and medium density single family residential 

land uses. 89 percent of the low density and 66 percent of the medium density residential 

areas are served by septic tanks, and there are no substantial point source loads. The 

Bahia Vista Street station has a drainage area of 44 square miles, comprised mainly of 

cropland, rangelands, open space, and low and medium density residential land uses. 53 

percent of the low density, 41 percent of the medium density, and 37 percent of the high 
TABLE 3-9 

PARAMETERS FOR ANALYSIS OF WORKING SEPTIC TANKS 

Source 

SWFWMD, 1988 

SWFWMD, 1988 

SWFWMD, 1988 

FDER 

USGS maps 

Parameter 

Aquifer Conductivity 

Aquifer Thickness 

Porosity 

Minimum Travel Length 

Maximum Travel Length 

Value 

8.0 inlhr 

60.0 ft 

0.3 

75 ft 

3000 ft 
density residential areas are served by septic tanks. Unlike the Palmers Boulevard station, 

the water quality at Bahia Vista Street has been impacted by the Atlantic Utilities and 

Kensington Park Utilities point sources, which had been directly discharging secondary 

effluent to Phillippi Creek during the STORET period of record. It is possible that other 

smaller point sources may have also been discharging directly, rather than through 

percolation ponds and irrigation, during the period of record. 

The results of the analysis indicated that nitrogen concentrations were oversimulated when 

the low first-order decay rate was used. At the Palmers Boulevard station, the average 

annual total N concentration was oversimulated by 49 percent. The oversimulation of total 

N at the Bahia Vista Street station was only 11 percent, but sensitivity analyses indicated 

that significantly higher first-order decay rates would still result in reasonable total N 

predictions. Consequently, higher decay rates (which would result in a lower delivery 

ratio) were investigated. 

After analyzing various first order decay rates, a conservative value of 0.00055lday was 

established. The value was selected such that the observed and simulated total N 

concentrations at the Bahia Vista Street station matched. When the same value was used 

for the Palmer Street station, the simulated total N concentration exceeded the observed 

concentration by about 20 percent. This may indicate that the analysis methodology is 

better suited for stations with larger drainage areas like the Bahia Vista Street station. It is 

also possible that the septic tank loading is conservatively high, and that additional point 

source loadings (rather than septic tank loading) are responsible for the high total N 

concentrations at the Bahia Vista Bridge station. 

Figure 3-1 shows total N concentration of septic tank effluent as a function of distance 

from the receiving water, based on the conservative first order decay rate of 0.00055lday. 

As shown in the figure, most of the total N is predicted to reach the receiving water at a 

distance of 75 feet. AWT conditions (total N = 3 mg/L) are not met unless the distance 

from the septic tank to the receiving water is almost 1000 feet. Consequently, a setback 

distance in excess of 75 feet would appear to be appropriate for water quality protection 

in the study area. 

3.6 RAWFALL LOADINGS 

In order to account for rainfall loadings on the Bay water surface, concentrations 

representing rainfall loadings were included in the analysis. The Bay surface area is about 

52 square miles. For the average annual precipitation, the rainfall total of 54.6 inches per 

year spread over the Bay surface results in an equivalent flow rate of 135 mgd. . The pollutant concentrations selected for rainfall were based on monitoring data from the 

Tampa Bay NURP study (Priede-Sedgwick, Inc., 1982). The values are listed below: 

Total N 0.82 mgL 

Total P 0.15 mgL 

Lead 6 u& 

Zinc 146 ugL 

* The nitrogen concentrations from the Tampa Bay NURP Study are consistent with other 

local data, based on a review of other Tampa Bay loading studies and of rainfall 

monitoring data from Pinellas County. Unfortunately, the studies and monitoring data did 

not include concentrations for total P, lead or zinc. 

For the average rainfall year, the calculated rainfall loads in lblyr are 337,400 for total N, 

61,700 for total P, 2,500 for lead, and 60,100 for zinc. The significance of these loads 

with respect to the other sources discussed previously in this section will be evaluated in 

subsequent sections of this report. 
4.0 LOADING TRANSPORT TO BAY 

The surface runoff pollutant loadings described in Section 3.3 represent estimates of 

loadings which have been discharged into a storm sewer, swale or stream channel. 

However, it is possible that the loading to these conveyance systems (particularly stream 

channels) will be reduced enroute to the Bay by processes such as settling. Because many 

of the watersheds are relatively small, and storm flows will typically result in turbulent 

stream conditions, sign5cant load reductions are not likely to occur in the free-flowing 

sections of the Bay tributaries. On the other hand, some removal may occur in the tidal 

portions of the tributaries, where flows may tend to be more quiescent. 

Computer modeling using the EPA's WASP4 model (EPA, 1990) was conducted to 

evaluate the potential for pollution removal within the major tidal tributaries. The 

following tributaries were included in the analyses: 

. Phillippi Creek 

. Whitaker Bayou 

. Hudson Bayou 

. Bowlees Creek 

. Cedar Hammock 

The watersheds of these tributaries account for 55 percent of the total land area, and 64 

percent of the total improved (e.g., urban and agricultural) land area. Most of the 

remaining improved area drains directly to the Bay instead of discharging to a major 

tributary. 

The analysis focused strictly on the removal of suspended pollutants by means of settling. 

Thus, dissolved pollutants were assumed to exhibit 100 percent delivery to the Bay. 

Settling of suspended pollutants was influenced by the assumed settling rate, water depth, 

and tidal flushing effects. 
4.1 MODEL DESCRIPTION 

The Water Quality Analysis Simulation program -4 (WASP4) consists of two separate 

subprograms - the hydrodynamics program DYNHYDS and the water quality program 

WASP4. DYNHYDS solves the one - dimensional equations of continuity and 

momentum in order to calculate water surface elevations, velocities and flow rates at 

various points within the modeling system. WASP4 solves the conservation of mass 

equation in order to calculate pollutant concentrations within the system. 

Both programs use the "link-node" method of simulation. The water body is represented 

in the model as a series of storage points (nodes) interconnected with a series of channels 

(links). For each time step during the simulation, the model solves the equation of 

momentum for each link to calculate flow between connected nodes, and solves the 

equation of continuity to determine water surface elevations at each node. Water quality 

constituents are transported between nodes by the flows in each link. In addition, loads 

may be added to each node via point sources or nonpoint sources. For each node, the 

mass balance equation is solved to calculate the mass of pollutant, and this value is 

divided by the node volume (a function of the node water surface elevation) to determine 

the pollutant concentration. 

Figures 4-1 through 4-5 show the model segmentation developed for each major tributary. 

The model segmentation was designed to extend upstream from the mouth to most distant 

extent of tidal influence. Segments in individual systems were sized according to 

recommendations in the WASP4 User's Manual (EPA, 1990). Generally, it is best to 

segment the system so that the water volumes are roughly the same. This guideline 

allows larger time steps and results in greater numerical accuracy. 
FRUITVILLE ROAD 

SARASO 

BAY 

3 

-I CITY LIMIT 

Q 

Cti 

Ia c-. 

a 

C 

r--- -- 7 

0 

- 

1000 2000 3000 

DISTANCE IN FEET 

Figure 4-1. WASP4 . . Model Segmentation for Phillippi Creek. 
Figure 4-2. WASP4 Model Segmentation for Whitaker Bayou. 
Figure 4-3. WASP4 Model Segmentation for Hudson Bayou. 
Figure 4-4. WASP4 Model Segmentation for Bowlees Creek. 
BAYSHORE GARDENS PARKWAY 

/ 

DISTANCE IN FEET 

SARASOTA BAY 

Figure 4-5. WASP4 Model Segmentation for Cedar Hammock. 
4.2 MODEL PARAMETERS 

In modeling the Bay tributaries a number of parameters were evaluated. These included 

tidal influence, channel geometry, suspended pollutant settling rate and surface runoff 

loadings and flows. The evaluation of these parameters is discussed below. 

4.2.1 TIDAL INFLUENCE 

One of the major forcing functions in each of the tributaries is the inflow and outflow of 

the tide. In the model, a lunar semidiurnal tide was specified, with a period of 12.5 

hours. Key values that were established are the water elevation at mean tide, and the 

mean tidal range. 

Based on data from the 1991 Tide Tables, the selected parameter values for tidal effects 

were 1.0 ft National Geodetic Vertical Datum (NGVD) as the mean tide elevation, and 2.0 

feet as the mean tidal range. The mean water elevation is consistent with a review of 

Phillippi Creek cross-sectional data developed for the Sarasota County Stormwater Master 

Plan report (CDM, 1987). The value of 1.0 ft NGVD was the mid-range of water 

elevations measured at the cross-sections in the tidal part of the creek. 

The other major forcing function in each tributary is the flow and associated surface 

runoff pollutant loading from the watershed. Generally, because runoff from the large 

majority of the drainage area enters the stream above the first model segment, the total 

flow and load was assigned to the fist segment. The exception was Hudson Bayou, 

where 60 percent of the flow and load was assigned to Segment 1 and 40 percent was 

assigned to Segment 3, based on drainage area to each segment. 
The loadings and flows were simulated as steady input values, calculated from the average 

annual results generated by the spreadsheet model. Streamflow values, calculated by the 

model in terms of inches per year over the drainage area, were converted to a steady 

inflow in units of cubic meters per second. Similarly, the annual loading values in pounds 

per year were converted to a steady loading in kilograms per day. 

Sensitivity runs were conducted to determine whether wet year, dry year, and seasonal 

flows and loadings would generate substantially different results then the average year 

flows and loadings. Results showed that the delivery ratios (the percentage of surface 

runoff loading that reaches the Bay) were not substantially different for any of these 

cases. Consequently, the delivery ratios determined for average annual conditions were 

used for all rainfall scenarios. 

4.2.3 TRIBUTARY GEOMETRIES 

The depth, width, length and cross-sectional geometries of the tributaries were established 

based on existing data. Bowlees Creek and Phillippi Creek were the tributaries having the 

most detailed data. Cross-sections were developed for Phillippi Creek as part of the 

Sarasota County Stormwater Master Plan (CDM, 1987), and Bowlees Creek had 

previously been modeled using the U.S. Corps of Engineers HEC-2 model. Data for the 

other tributaries were developed based on USGS quadrangle maps, NOAA nautical charts 

and Florida Department of Transportation bridge records and field inspections. 

The cross-sectional data were used to develop HEC-2 datasets for each tributary, and these 

datasets were used to develop the input data required by the WASP4 model. For each 

tributary, the HEC-2 model was run at the average stream flow and mean tidal height, 

producing values of depth, volume and surface area This output was used to specify the 

depth, volume and surface area of each of the segments in the WASP4 model. 
4.2.4 POLLUTANT SE'I%ING PARAMETERS 

The WASP4 model was run for each tributary assuming that settling of suspended 

pollutants was the only pollutant removal process. Two key factors in determining how 

much removal will occur are the pollutant settling rate, and the fraction of total pollutant 

that is in the suspended form. 

A settling rate of 1.0 foot per day was selected based on a review of the literature. A 

review of previous modeling studies indicated that calculated settling velocities for 

suspended solids in quiescent waters have ranged from 0.1 to 5.0 meters per day (Delos 

et al, 1983). In studies of the Flint River (Richardson et al, 1983) and the Deep River 

(Metcalfe et al, 1984), calibrated settling rates ranged from 0.2 to 0.65 meters per day (0.7 

to 2.1 feet per day). In contrast, literature settling rates for phytoplankton range from 0.0 

to 30.0 meters per day, with most in the rage of 0.02 to 0.6 meters per day (EPA, 1985). 

Consequently, a settling rate of 1.0 feet per day (0.30 meters per day) is in the mid-range 

of typical settling values. 

Sensitivity runs were conducted to determine the effect of increasing or decreasing the 

settling rate. The analysis evaluated settling rates of 0.5 and 1.5 feet per day, which 

represent a 50 percent variation from the selected rate of 1.0 feet per day. Results 

indicated that the change in delivery ratio ranged from 2 to 20 percent, and was generally 

less than 10 percent. Thus, it appears that the delivery ratio results are not sensitive to 

these changes in the settling rate. 

The settling rate was applied to the suspended fraction of each pollutant, which was also 

established based on a literature review. The selected values for the constituents analyzed 

in the study area, based on typical values for urbanized areas, are as follows: 

. Total N 25% suspended 

. Total P 40% suspended 
. Lead 90% suspended 

. Zinc ' 40% suspended 

These values are typical of monitoring data from the NURe study. Based on these values 

lead is the pollutant that will exhibit the highest degree of settling in the tributaries, and 

total N will exhibit the lowest degree of settling. 

4.3 MODELING PROCEDURE 

The data described in Section 4.2 were used in the WASP4 model to determine the 

delivery ratio for each of the major tributaries. The model was run for a simulation 

period of 30 days, with a repeated semi-diurnal tide and constant streamflow and pollutant 

loading, to establish steady-state conditions (i.e., concentrations did not vary from one day 

to the next). The WASP4 model output included a mass balance file that summarized the 

outflow of pollutant from the tributary. The pollutant outflow was compared to the 

constant loading to establish the delivery ratio for each polIutant. 

A slightly different procedure was used for Cedar Hammock, due to the presence of two 

sediment traps in the non-tidal part of the tributary. Figure 4-2 shows the location of 

these traps at Florida Boulevard and Harvard Avenue. The areas upstream of the traps 

could not be modeled within the WASP4 framework. City records regarding sediment 

accumulation in these traps suggest that substantial pollutant removal may be occurring 

there, unlike salinity barriers or other minor structures that are unlikely to promote 

significant settling. 

The pollutant reduction due to the sediment traps was estimated in a spreadsheet. Based 

on ihe average flow rate and the weir length and elevation, the water elevation behind 

each weir was determined. Cross-sectional data from a survey of the area were entered 

into a HEC-2 model dataset to determine water depth, volume and surface area. Using 
these data along with the values for settling rate and suspended fraction of pollutant, 

removal due to settling was calculated. 

To run the WASP4 model for Cedar Hammock the pollutant parameters were adjusted to 

account for the settling in the sediment traps. The average pollutant loading was reduced 

to reflect the settling. In addition, the suspended fraction of each pollutant had to be 

changed, because the suspended fraction to the Bay was lower than the suspended fraction 

to the sediment traps. For example, if a 100 pound per day load to the sediment traps 

contains 40 pounds of suspended pollutant (i.e., suspended fraction is 40 percent), and 20 

pounds per days of suspended pollutant are removed in the sediment traps, then 20 pounds 

per day of suspended pollutant remains in the 80 pounds per day of pollutant reaching the 

Bay, resulting in a suspended fraction of 25 percent, rather than 40 percent. 

4.4 MODEL RESULTS 

The delivery ratios developed for each tidal tributary are presented in Table 4-1. As one 

would expect, the delivery ratios are highest for total N, which had the lowest suspended 

fraction, and lowest for lead, which had the highest suspended fraction. Bowlees Creek, 

Hudson Bayou and Whitaker Bayou exhibit little reduction in loadings to the Bay, with 

delivery ratios of over 90 percent for all pollutants. PhiUippi Creek provided 30 percent 

removal of lead in the tributary, but delivery of zinc, total N and total P are still in the 

neighb&hood of 90 percent. Cedar Hammock exhibits the most pollutant removal, chiefly 

because of the sediment traps in the non-tidal tributary. 

As presented in Chapter 5.0, these delivery ratios were applied to the surface runoff loads 

for the major tributaries. For the areas draining directly to the Bay, and for the other 

tributaries that were not modeled, 100 percent delivery was assumed. The ratios were not 

applied to point source discharges, septic tank discharges, baseflow and rainfall, because 

the pollutants for these type of discharges are expected to be primarily in the dissolved 

form. 
TABLE 4-1 

DELIVERY RATIOS FOR MAJOR TIDAL TRIBUTARIES 

Tributary 

Phillippi Creek 

Whitaker Bayou 

Hudson Bayou 

Bowlees Creek 

Cedar Hammock 

Delivery Ratio (%) 

Zinc 

87 

97 

92 

96 

82 

Total P 

87 

97 

92 

96 

82 

Total N 

93 

100 

96 

99 

90 

Lead 

71 

92 

82 

89 

58 
5.0 POLLUTION LOADING PROJECTIONS FOR 

EXISTING LAND USE CONDITIONS 

The data presented in the previous chapters were used as input to a spreadsheet model 

which calculated pollutant loadings to Sarasota Bay for existing land use conditions. The 

results were analyzed in a number of ways. Total loadings to the Bay were analyzed to 

determine the relative impacts of surface runoff, point sources, septic tanks, baseflow and 

rainfall. In addition, the loadings attributed to each of the study area jurisdictions, and 

each of the watersheds in the study area were calculated. For the average annual rainfall, 

the percentage of the total loading occuning in the wet and dry seasons was also 

established. Finally, the analysis considered the loadings for a wet year and a dry year, 

comparing the loadings with those of an average year. 

5. I AVERAGE' ANNUAL LOADING RESULTS 

5.1.1 LOADINGS BY WATERSHED 

Table 5-1 presents the average annual loading results by watershed for existing land use 

conditions. The results include the impacts of surface runoff, baseflow, septic tanks, point 

source discharges and rainfall. Values in the table include total area, total runoff, and 

loadig data for total P, total N, lead and zinc. The loadig data include the total mass in 

pounds; the unit load in pounds per acre, and the flow-weighted concentration in 

milligrams per liter. The last line in the table represents the values for the entire study 

area, including the land areas and the Bay water surface. 

For total P, the watersheds that exhibit the highest to@ loadings, unit loadings and 

concentrations include Phillippi Creek, Whitaker Bayou, South Bradenton and Siesta Key. 

Phillippi Creek, South Bradenton and Whitaker Bayou account for 40 percent of the total 

P load to the Bay. Though the total load from Siesta Key is not nearly as high as for 

Phillippi Creek and Whitaker Bayou, the unit loading and concentration from the 

watershed is higher than any other watershed. Surface water discharge of point sources is 

the major reason for high total P concentrations in the Siesta Key watershed. 

The same four watersheds, along with Matheny Creek and the areas in western Sarasota 

County that drain directly to the Bay, also have high loading and concentration values for 

total N. About 36 percent of the total N loading to the Bay comes from Phillippi Creek 

and Whitaker Bayou. Surface water discharge of point sources is the major reason for 

high total N concentrations in the Matheny Creek and Siesta Key watersheds. 

Phillippi Creek, Bowlees Creek and Whitaker Bayou are the largest loading sources for 

lead. These three watersheds account for 54 percent of the total Bay loading. Surface 

runoff is the major source of lead in Bowlees Creek, whereas a combination of surface 

runoff and point source discharge are the major contributors in Phillippi Creek and 

Whitaker Bayou. Relatively high lead concentrations were calculated for Hudson Bayou 

and Cedar Hammock. This can be attributed to surface runoff in these two watersheds. 

Other than rainfall, which accounts for 65 percent of the zinc loading to the Bay, major 

zinc loadings are attributed to Phiippi Creek, Bowlees Creek and Whitaker Bayou. The 

high zinc concentration in the rainfall, which was assumed based on Tampa Bay NLTRP 

data, is responsible for the large rainfall loading. Surface runoff and point source 

discharge are the major factors in Phillippi Creek and Whitaker Bayou, and surface runoff 

is the major factor in Hudson Bayou. 

The values in Table 5-1 do not include the effects of pollutant settling in the tidal 

tributaries. Pollutant settling was assumed to apply only to the loading from surface 

runoff. When the delivery ratios presented in Table 4-1 were applied to the tributaries, 

the reduction in surface runoff loadings was only 6 percent for total P and zinc, 3 percent 

for total N, and 13 percent for lead. When considering the total loading from all sources, 
the loading reductions due to settling were 3 percent for total P, 12 percent for lead, and 2 

percent for total N and zinc. 

5.1.2 LOADINGS BY SOURCE 

Overall loading statistics as a function of pollution source are presented in Table 5-2. The 

sources included in the table are surface runoff, baseflow, septic tanks, point sources, and 

rainfall. The total runoff and loadings of total P, total N, lead and zinc are presented, as 

is the percentage of the total loading attributed to each source. Generally, the results 

indicate that surface runoff and rainfall are the two largest sources of pollutant loadings to 

the Bay. Together, these two sources account for 71 percent of the total P, 73 percent of 

the total N, 98 percent of the lead and 91 percent of the zinc loadings. Surface runoff is 

the major source of total P, total N and lead, while rainfall is the major source of zinc; 

These are several reasons why these two sources dominate the total loadings. One is that 

the two sources account for 84 percent of the total flow that reaches the Bay. Another is 

that the remaining 16 percent of the total runoff is primarily attributed to baseflow, which 

has rather low pollutant concentrations. 

Septic tanks and point source discharges contribute a relatively small percentage of total 

loadings. The values in the table show that the combined loadings of septic tanks and 

point sources are 16 to 18 percent of the total loadings for the nutrients, and 1 to 3 

percent for the metals. Point source loadings are limited by the implementation of AWT 

standards at wastewater treatment plants, along with a shift from surface water discharge 

to land application of wastewater. Septic tank loadings are limited by the relatively low 

failure rate of 8 percent. In addition, as a result of the substantial travel time from septic 

tanks to the Bay and its tributaries, a relatively small fraction of septic tank effluent 

loading reaches the Bay. 
TABLE 5-2 

AVERAGE ANNUAL LOADINGS BY SOURCE 

FOR EXISTING LAND USE CONDITIONS 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of settling in tributaries. 

SOURCE 

SURFACE 

BASEFLOW 

SEPTIC 

POINT SOURCE 

RAINFALL 

TOTAL 

RUNOFF %OF 

(in) TOTAL 

14.58 42.6% 

3.84 11.2% 

0.70 2.0% 

1.10 3.2% 

14.04 41.0% 

34.26 

AVERAGE YEAR 

POLLUTANT LOADING 

TOTAL P %OF 

(Ib) TOTAL 

117,830 46.6% 

33,810 13.4% 

8,230 3.3% 

31,110 12.3% 

61,730 24.4% 

252.710 

TOTALN %OF 

(Ib) TOTAL 

609,990 47.2% 

112,700 8.7% 

123,520 9.6% 

109,230 8.4% 

337,460 26.1% 

1,292,900 

LEAD %OF 

(Ib) TOTAL 

33,790 91.2% 

300 0.8% 

0 0.0% 

490 1.3% 

2,470 6.7% 

37,050 

ZINC %OF 

(Ib) TOTAL 

24,890 26.7% 

5,620 6.0% 

0 0.0% 

2,510 2.7% 

60,080 64.5% 

93,100 
The values in Table 5-2 do not include the effects of pollutant settling in the tidal 

tributaries. When the delivery ratios presented in Table 4-1 were applied to the tidal 

tributaries to assess the impact on the source distribution percentage, the results were only 

slightly different than the results generated without considering the delivery ratios. The 

percentage of loading due to surface runoff changed by 2 percentage points or less, and 

the percentages for the other sources changed by no more than 1 percentage point. Thus, 

the relative distribution of the sources remained essentially the same, with surface runoff, 

baseflow and rainfall providing the highest percentages of the total pollutant loadings. 

5.1.3 LOADINGS BY JURISDICTION 

Loading totals were also calculated by jurisdiction, and these totals are presented in Table 

5-3. The jurisdiction list includes Sarasota County, Manatee County, the City of Sarasota, 

the Barrier Islands, and the Bay Water Surface. The Barrier Islands were initially 

included in a separate group because it was speculated that rainfall totals for the barrier 

islands could be significantly different than mainland rainfall totals. However, a statistical 

analysis of rainfall data indicated that the barrier island and inland rainfall data were very 

similar (See Appendix C). For each jurisdiction, the table lists the area, the runoff 

volume, and loadings of total P, total N, lead and zinc. The percentage of the total runoff 

and loadings attributed to each jurisdiction are also included. 

Table 5-3 shows that the largest areas generally produce the highest percentage of runoff 

volume and pollutant loading. Sarasota County is the largest jurisdiction, comprising 49 

percent of the total land and water surface area. The Bay Surface is next largest at 26 

percent, followed by Manatee County at 16 percent. Together, these three jurisdictions 

account for 89 percent of the total runoff, and 78 to 92 percent of the pollutants listed in 

the table. 

For total P and total N, Sarasota County accounts for about 44 percent of the loading to 

the Bay, and Manatee County and the Bay surface account for another 40 to 42 percent. 
TABLE 5-3 

AVERAGE ANNUAL LOADINGS BY JURISDICTION 

FOR EXISTING LAND USE CONDITIONS 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of settling in tributaries. 

JURISDICTION 

SARASOTA CO. 

SARASOTA CITY 

MANATEE CO. 

ISLANDS 

BAY SURFACE 

TOTAL 

AREA 

(ac) 

62,301 

7,844 

20,226 

5,761 

33,280 

129,412 

RUNOFF %OF 

(in) TOTAL 

11.94 34.8% 

2.20 6.4% 

4.66 13.6% 

1.42 4.1% 

14.04 41.0% 

34.26 

AVERAGE YEAR 

POLLUTANT LOADING 

Total P %OF 

(Ib) TOTAL 

108,900 43.1% 

25,230 10.0% 

41,240 16.3% 

15,610 6.2% 

61,730 24.4% 

252,710 

LEAD %OF 

(Ib) TOTAL 

14,720 39.7% 

6,170 16.7% 

11.830 31.9% 

1,860 5.0% 

2,470 6.7% 

37,050 

Total N %OF 

(Ib) TOTAL 

575.960 44.5% 

109.970 8.5% 

204.350 15.8% 

65,160 5.0% 

337,460 26.1% 

1,292.900 

ZINC %OF 

(Ib) TOTAL 

16,790 18.0% 

5,170 5.6% 

8,820 9.5% 

2,240 2.4% 

60,080 64.5% 

93,100 
Sources of nutrients in Sarasota County include surface runoff, baseflow, septic tanks and 

point sources. Of these, surface runoff is the major contributor. Surface runoff is also the 

key factor for Manatee County. Loading to the Bay surface occurs through rainfall. 

In comparison to nutrients, the values for lead indicate a higher percentage contribution 

from the City of Sarasota and Manatee County. This is primarily due to the surface 

runoff from the highly developed areas within the City and parts of the County. In 

contrast, rainfall has a much lower relative contribution due to the low lead concentration 

assumed, based on Tampa NURP data. 

Due to the high concentration of zinc in rainfall (based on the Tampa NURP study), the 

Bay surface loading accounts for 65 percent of the zinc loading to the Bay. As with lead, 

the zinc loadings from the City of Sarasota and Manatee County tend to be more 

significant with respect to the loading from Sarasota County, due to urban surface runoff. 

For all of the pollutants, the islands are responsible for 7 percent or less of the total 

loadings. The relative contribution of nutrients is higher than the relative contribution of 

metals for the islands. This can be attributed to the somewhat higher rate of septic tank 

- 

failure assumed for Casey Key (20 percent compared to 8 percent for other watersheds), 

and the surface water discharge from Siesta Key, in addition to surface runoff and 

baseflow loadings. 

The values in Table 5-3 do not include the effects of pollutant settling in the tidal 

tributaries. When the delivery ratios presented in Table 4-1 were applied to the tidal 

tributaries to assess the impact on the loading distribution between jurisdictions, the 

results were only slightly different than the results generated without considering the 

delivery ratios. The relative percentages of loading attributed to the various jurisdictions 

did not change by more than 1 percentage point. 
5.2 WET SEASON AND DRY SEASON RESULTS 

Separate analyses were conducted to evaluate loadings during the wet season (June 

through September) and the dry season (October through May) of an average year. The 

results were used to determine how the total annual load was distributed between the two 

seasons. In addition, the results for each season were compared to the annual results to 

identify changes in loading distributions by source and by jurisdiction. 

Tables 5-4 through 5-7 present the results of the seasonal loading analysis. Loadings by 

watershed for the wet season and dry season are listed in Tables 5-4 and 5-5, respectively. 

Table 5-6 includes loading values by pollutant source for both the wet and the dry season. 

Loading values by jurisdiction for both seasons are shown in Table 5-7. 

Based on the data in Tables 5-4 and 5-5, about 60 percent of the annual loading occurs 

during the wet season, and 40 percent occurs during the dry season, for all four of the 

analyzed pollutants. Because surface runoff and rainfall are major load contributors, one 

would expect that the loading distribution would reflect the precipitation distribution 

between wet and dry season. As shown previously in Table 3-1, 20.9 inches of rain was 

established for the dry season and 33.9 inches of rain was established for the wet season. 

This results in a rainfall distribution of 38 percent in the dry season and 62 percent in the 

wet season, which is almost identical to the loading distribution. The runoff distribution 

is also very similar, with 62 percent of the runoff attributed to the wet season and 38 

percent of the runoff attributed to the dry season. 

A review of the values in Table 5-6 indicates that the distributions of loadings between 

the various pollutant sources during the wet and dry season are very similar to the average 

annual distribution. The largest changes occur for point sources loadings. Unlike the 

other sources of pollution, point sources and septic tanks actually have a greater total flow 

volume during the 8-month dry season than during the 4-month wet season. 
WET SEASON LOADINGS BY WATERSHED 

FOR EXISTING LAND USE CONDlTiONS 

WATERSHED 

Note: Loading totals do not include pollutant senling in tidal tributaries. See text for discussion of settling in tributaries. 
TABLE 5-5 

DRY SEASON LOADINGS BY WATERSHED 

FOR EXISTING LAND USE CONDITIONS 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of settling in tributaries. 
TABLE 5-6 

WET SEASON AND DRY SEASON LOADINGS BY SOURCE 

FOR EXISTING LAND USE CONDITIONS 

SOURCE 

SURFACE 

BASEFLOW 

SEPTIC 

POINT SOURCE 

RAINFALL 

TOTAL 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of settling in tributaries. 

SOURCE 

SURFACE 

BASEFLOW 

SEPTIC 

POINT SOURCE 

RAINFALL 

TOTAL 

RUNOFF %OF 

(in) TOTAL 

9.00 42.6% 

2.82 13.4% 

0.23 1.1% 

0.40 1.9% 

8.67 41.1% 

21.12 

RUNOFF %OF 

(in) TOTAL 

5.58 42.5% 

1.03 7.8% 

0.47 3.6% 

0.69 5.3% 

5.37 40.9% 

13.14 

WET SEASON 

POLLUTANT LOADING 

TOTAL P %OF 

(Ib) TOTAL 

72,640 48.1% 

24,790 16.4% 

4.120 2.7% 

11,450 7.6% 

38,100 25.2% 

151.100 

DRY SEASON 

POLLUTANT LOADING 

TOTAL P %OF 

(lb) TOTAL 

45,050 44.4% 

9,030 8.9% 

4,130 4.1% 

19,660 19.4% 

23.630 23.3% 

101.500 

TOTALN %OF 

(lb) TOTAL 

376,520 49.0% 

82,600 10.7% 

61,740 8.0% 

39,780 5.2% 

208,290 27.1% 

768,930 

TOTAL N %OF 

(Ib) TOTAL 

233.420 44.6% 

30,090 5.7% 

61.820 11.8% 

69.440 13.3% 

129.1 70 24.7% 

523,940 

LEAD %OF 

(Ib) TOTAL 

20.880 91.6% 

200 0.9% 

0 0.0% 

190 0.8% 

1,520 6.7% 

22,790 

ZINC %OF 

(Ib) TOTAL 

15,360 26.7% 

4,110 7.1% 

0 0.0% 

950 1.7% 

37,090 64.5% 

57,510 

LEAD %OF 

(Ib) TOTAL 

12,970 90.8% 

50 0.4% 

0 0.0% 

310 2.2% 

950 6.7% 

14.280 

ZINC %OF 

(Ib) TOTAL 

9.520 26.7% 

1,510 4.2% 

0 0.0% 

1,570 4.4% 

23,000 64.6% 

35,600 
TABLE 5-7 

WET SEASON AND DRY SEASON LOADINGS BY JURISDICTION 

.FOR EXISTING LAND USE CONDITIONS 

JURISDICTION 

SARASOTA CO. 

SARASOTA CITY 

MANATEE CO. 

ISLANDS 

BAY SURFACE 

TOTAL 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of settling in tributaries. 

JURISDICTION 

SARASQTA CO. 

SARASOTA CITY 

MANATEE CO. 

ISLANDS 

BAY SURFACE 

TOTAL 

AREA 

(ac) 

62,301 

7,844 

20,226 

5,761 

33.280 

129,412 

AREA 

(ac) 

62,301 

7,844 

20,226 

5,761 

33,280 

129.41 2 

RUNOFF %OF 

(in) TOTAL 

7.39 35.0% 

1.32 6.3% 

2.90 13.7% 

0.85 4.0% 

8.67 41.0% 

21.12 

RUNOFF %OF 

(in) TOTAL 

4.55 34.6% 

0.88 6.7% 

1.77 13.4% 

0.57 4.3% 

5.37 40.9% 

13.14 

WET SEASON 

POLLUTANT LOADING 

DRY SEASON 

POLLUTANT LOADING 

Total P %OF 

(Ib) TOTAL 

66.080 43.7% 

13.790 9.1% 

24,850 16.4% 

8,280 5.5% 

38,100 25.2% 

151,100 

Total P %OF 

(Ib) TOTAL 

42.760 42.1% 

1 1,360 11.2% 

16.410 16.2% 

7,340 7.2% 

23,630 23.3% 

101.500 

LEAD %OF 

(Ib) TOTAL 

9,070 39.8% 

3,760 16.5% 

7,330 32.2% 

1,110 4.9% 

1,520 6.7% 

22,790 

Total N %OF 

(Ib) TOTAL 

340.160 44.2% 

62,460 8.1% 

121,860 15.8% 

36,160 4.7% 

208,290 27.1% 

768,930 

ZINC %OF 

(Ib) TOTAL 

10,600 18.4% 

3,030 5.3% 

5,530 9.6% 

1,260 2.2% 

37,090 64.5% 

57,510 

Total N %OF 

(Ib) TOTAL 

235.840 45.0% 

47,440 9.1% 

82,490 15.7% 

29,000 5.5% 

129,170 24.7% 

523,940 

LEAD %OF 

(Ib) TOTAL 

5,660 39.6% 

2,390 16.7% 

4,550 31.9% 

730 5.1% 

950 6.7% 

14,280 

ZINC %OF 

(Ib) TOTAL 

6,190 17.4% 

2,140 6.0% 

3,290 9.2% 

980 2.8% 

23,000 64.6% 

35.600 
Consequently, the percentage of the total loading due to point sources is substantially 

higher during the dry season, and lower during the wet season. Even during the dry 

season, however, the point source loadings for total P, total N and zinc are less than the 

loadings due to surface runoff or rainfall. 

A comparison between Tables 5-3 and 5-7 reveals that there is little difference in the 

distribution of runoff and loadings between the various jurisdictions during the wet season, 

dry season and average year. In almost all cases, the change in the distribution values 

were less than one percentage point. The values for the wet season tended to be more 

similar to the average annual vaes than were the dry season values. 

The values in Tables 5-4 through 5-7 do not include the effects of pollutant settling in the 

tidal tributaries. As discussed in Section 5.1, the application of the delivery ratios 

determined for the tributaries did not produce substantial changes in total loadings or the 

distribution of loadings between sources or jurisdictions. 

5.3 WET YEAR AND DRY YEAR RESULTS 

Analyses were also conducted to evaluate loadings during a wet year and a dry year. As 

discussed in Section 3.2, the 10th percentile and 90th percentile value for annual 

precipitation and annual streamflow were used to represent the wet and dry years. Results 

were used to determine the changes in total loadings to the Bay, and to assess changes in 

load distribution by source and by jurisdiction. 

Tables 5-8 through 5-11 present the results of the wet and dry year loading analyses. 

Loadings by watershed for the wet year and the dry year are listed in Tables 5-8 and 5-9, 

respectively. Table 5-10 includes loading values by pollutant source for both the wet and 

the dry year. Loading values by jurisdiction for both wet and dry years are shown in 

Table 5-11. 
TABLE 5-8 

WET YEAR LOADINGS BY WATERSHED 

FOR EXISTING LAND USE CONDITIONS 

BAY SURFACE 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of settling in tributaries. 
TABLE 5-9 

DRY YEAR LOADINGS BY WATERSHED 

FOR EXISTING LAND USE CONDITIONS 
TABLE 5-10 

WET YEAR AND DRY YEAR LOADINGS BY SOURCE 

FOR EXISTING LAND USE CONDITIONS 

SOURCE 

SURFACE 

BASEFLOW 

SEPTIC 

POINT SOURCE 

RAINFALL 

TOTAL 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of settling in tributaries. 

RUNOFF %OF 

(in) TOTAL 

18.62 40.9% 

7.23 15.9% 

0.70 1.5% 

1.10 2.4% 

17.92 39.3% 

45.57 

SOURCE 

SURFACE 

BASEFLOW 

SEPTIC 

POINT SOURCE 

RAINFALL 

TOTAL 

RUNOFF %OF 

(in) TOTAL 

11.14 45.6% 

0.78 3.2% 

0.70 2.9% 

1.10 4.5% 

10.72 43.9% 

. 24.44 

DRY YEAR 

POLLUTANT LOADING 

WET YEAR 

POLLUTANT LOADING 

TOTAL P %OF 

(Ib) TOTAL 

89,990 49.1% 

6,870 3.7% 

8.230 4.5'/0 

31.110 17.0% 

47,150 25.7% 

183,350 

ZINC %OF 

(Ib) TOTAL 

31,750 26.1% 

10,620 8.7% 

0 0.0% 

2,510 2.1% 

76.700 63.1% 

121,580 

TOTAL P %OF 

(Ib) TOTAL 

150,380 45.3% 

63.640 19.2% 

8,230 2.5% 

31,110 9.4% 

78,800 23.7% 

332.1 60 

LEAD %OF 

(Ib) TOTAL 

25,820 91.4% 

40 0.1% 

0 O.OO/o 

490 1.7% 

1,890 6.7% 

28,240 

TOTAL N %OF 

(Ib) TOTAL 

465.91 0 47.6% 

22,920 2.3% 

123,520 12.6% 

109,230 11.2% 

257.730 26.3% 

979,310 

ZINC %OF 

(lb) TOTAL 

18,980 27.7% 

1,150 1.7% 

0 O.OO/o 

2,510 3.7% 

45,890 67.0% 

68,530 

TOTAL N %OF 

(Ib) TOTAL 

778,600 47.1% 

212,200 12.8% 

123.520 7.5% 

109,230 6.6% 

430,790 26.0% 

1,654,340 

LEAD %OF 

(Ib) TOTAL 

43.140 91.0% 

640 1.3% 

0 0.0% 

490 1.0% 

3.150 6.6% 

47,420 
WET YEAR AND DRY YEAR LOADINGS BY JURISDICTION 

FOR EXISTING LAND USE CONDITIONS 

JURISDICTION 

SARASOTA CO. 

SARASOTA CITY 

MANATEE CO. 

ISLANDS 

BAY SURFACE 

TOTAL 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of settling in tributaries. 

AREA 

(ac) 

62,301 

7,844 

20,226 

5.761 

33,280 

1'29.41 2 

JURISDICTION 

SARASOTA CO. 

SARASOTA CITY 

MANATEE CO. 

ISLANDS 

BAY SURFACE 

TOTAL 

DRY YEAR 

POLLUTANT LOADING 

RUNOFF %OF 

(in) TOTAL 

16.59 36.4% 

2.86 6.3% 

6.31 13.9% 

1.89 4.1% 

17.92 39.3% 

45.57 

AREA 

(ac) 

62,301 

7,844 

20,226 

5,761 

33.280 

129,412 

RUNOFF % OF 

(in) TOTAL 

7.86 32.1% 

1.63 6.7% 

3.22 13.2% 

1.00 4.1% 

10.72 43.9% 

24.44 

Total P %OF 

(Ib) TOTAL 

74,270 40.5% 

20.630 11.3% 

29,140 15.9% 

12.160 6.6% 

47,150 25.7% 

183,350 

WET YEAR 

POLLUTANT LOADING 

LEAD %OF 

(Ib) TOTAL 

11,140 39.4% 

4.750 16.8% 

9,020 31.9% 

1,440 5.1% 

1,890 6.7% 

28,240 

Total N %OF 

(Ib) TOTAL 

433,080 44.Zo/o 

88.060 9.0% 

150,150 15.3% 

50,290 5.1% 

257,730 26.3% 

979,310 

ZINC %OF 

(Ib) TOTAL 

10,810 15.8% 

3.980 5.8% 

6,200 9.0% 

1,650 2.4% 

45,890 67.0% 

68,530 

ZINC %OF 

(Ib) TOTAL 

23,570 19.4% 

6,540 5.4% 

11,870 9.8% 

2,900 2.4% 

76.700 63.1% 

121,580 

LEAD %OF 

(Ib) TOTAL 

18,890 39.8% 

7,840 16.5% 

15,180 32.0% 

2,360 5.0°/0 

3.150 6.6% 

47,420 

Total P %OF 

(Ib) TOTAL 

148,220 44.6% 

30.440 9.2% 

55,100 16.6% 

19,600 5.9% 

78,800 23.7% 

332.1 60 

Total N %OF 

(Ib) TOTAL 

739,160 44.7% 

135,280 8.2% 

266,780 16.1% 

82.330 5.0% 

430,790 26.0% 

1,654,340 
Based on a comparison with the average year loadings, the values in Tables 5-8 and 5-9 

show that loadings for all pollutants will be 28 to 31 percent higher during the wet year, 

and 24 to 27 percent lower during the dry year. These values correspond closely to the 

differences in rainfall and runoff values between the wet year, dry year, and average year. 

During the wet year, rainfall was calculated to be 28 percent higher than average, and the 

runoff total shown in Table 5-8 is 31 percent higher than the average year. For the dry 

year, rainfall was calculated to be 24 percent lower than average, and the moff total 

shown in Table 5-9 is 29 percent lower than the average year. The results presented in 

Table 5-10 indicate that the wet year and dry year load distributions by source are very 

similar to the average year distribution. The largest change occurs for point source 

discharges, because the total loading is assumed to be the same regardless of the annual 

precipitation volume. Thus, the percentage of loadings due to point sources are noticeably 

higher during the dry year and lower during the wet year. Even during the dry year, 

however, the point source loadings for total P, total N and zinc are less than the loadings 

due to surface runoff and rainfall. 

A comparison between Tables 5-3 and 5-1 1 reveals that there is little difference in the 

distribution of runoff and loadings among the wet, dry and average years. For virtually 

all jurisdictions and pollutants, the change in the distribution values were less than one 

percentage point. Values for the wet year tended to be more similar to the average year 

than were the dry year values. 

The values in Tables 5-8 though 5-11 do not include the effects of pollutant settling in the 

tidal tributaries. As discussed in Section 5.1, the application of the delivery ratios 

determined for the tributaries did not produce substantial changes in total loadings or the 

distribution of loadings between sources or jurisdictions. 
5.4 SUMMARY 

The results from the analysis of existing loads indicates that, as one would expect, the 

largest runoff volumes and pollutant loadings are generated within the largest watersheds. 

However, small but highly urbanized watersheds can also contribute substantial loads from 

surface water runoff and point sources. Overall, surface runoff and rainfall account for 

the majority of the pollutant loading, to the Bay. Point sources and septic tanks combined 

account for 16 to 18 percent of the total nutrient loadings and 1 to 3 percent of the metals 

loadings, and baseflow provides an even lower percentage of the total loading. 

Analyses of wet and dry seasons, and wet and dry years, revealed that the distribution of 

loadings between jurisdictions and between sources of pollution remain relatively similar 

to the average year distributions. Point source loadings tend to be more prominent during 

dry periods, but are still lower than surface runoff and rainfall loadings. 
6.0 POLLUTION LOADING PROJECTIONS FOR FUTURE 

LAND USE CONDITIONS 

Preliminary loading estimates were also developed for two future land use scenarios. The 

scenarios included the five-year future and the buildout future land uses. The methods 

used to develop these land use scenarios were presented in Section 2.4 of this report. 

The results of the future land use analyses are presented in this section. Similar to the 

existing land use analysis, results are presented for average annual, wet and dry season, 

and wet and dry year rainfall conditions. Tables are presented which quantify loadings by 

watershed, by source and by jurisdiction. The values are compared to the existing land 

use results to identify local and Bay-wide changes. 

6.1 FIVE-YEAR FUTURE LAND USE 

Tables 6-1 through 6-11 present the loading results for the five-year future land use 

analysis. The results for the average annual rainfall condition are shown in Tables 6-1 

through 6-3. Wet season and dry season results are listed in Tables 6-4 through 6-7, and 

wet year and dry year results are presented in Tables 6-8 through 6-1 1. 

Table 6-1 shows the average annual loadings by watershed. A comparison of Tables 5-1 

(existing land use conditions) and 6-1 reveals a small decrease in overall total P loadings, 

and a small increase in total N and metals loadings. The projected shift in point sources 

from surface water discharge to land application accounts for reductions in pollutants, 

particularly nutrients, in point source loadings. This reduction is offset by an increase in 

surface runoff loadings due to new development. Phillippi Creek and Whitaker Bayou are 

still the major contributors of all four pollutants, and Bowlees Creek is a major 

contributor of metals loadings. 
TABLE 6-1 

AVERAGE ANNUAL LOADINGS BY WATERSHED 

FOR FIVE -YEAR FUTURE LAND USE CONDITIONS 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of senling in tributaries. 

OTHER ISLANDS 

DIRECT TO BAY 

BAY SURFACE 

TOTAL 

900 

4,241 

33,280 

129,412 

27.94 

31.85 

54.60 

34.32 

1.640 

8,850 

61,730 

250,600 

1.82 

2.09 

1.85 

2.12 

0.29 

0.29 

0.15 

0.30 

8.360 

51,430 

337,460 

1,294,680 

9.29 

12.13 

10.14 

10.22 

1.47 

1.68 

0.82 

1.47 

310 

2,340 

2,470 

38,090 

0.34 

0.55 

0.07 

0.42 

0.054 

0.076 

0.006 

0.060 

290 

1,880 

60.080 

93.290 

0.32 

0.44 

1.81 

0.46 

0.051 

0.061 

0.146 

0.061 
TABLE 6-2 

AVERAGE ANNUAL LOADINGS BY SOURCE 

FOR FIVE - YEAR FUTURE LAND USE CONDITIONS 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of settling in tributaries. 

SOURCE 

SURFACE 

BASEFLOW 

SEPTIC 

POINT SOURCE 

RAINFALL 

TOTAL 

RUNOFF %OF 

(in) TOTAL 

14.81 43.1% 

3.81 11.1% 

0.70 2.0% 

0.96 2.8% 

14.04 40.9% 

34.32 

AVERAGE YEAR 

POLLUTANT LOADING 

TOTAL P %OF 

(Ib) TOTAL 

120,300 48.0% 

33,490 13.4% 

8,260 3.3% 

26,820 10.7% 

61,730 24.6% 

250,600 

LEAD %OF 

(Ib) TOTAL 

34,930 91.7% 

300 0.8% 

0 0.0% 

390 1.0% 

2,470 6.5% 

38,090 

TOTAL N %OF 

(Ib) TOTAL 

622,830 48.1% 

11 1,660 8.6% 

123,890 9.6% 

98,840 7.6% 

337,460 26.1% 

1,294,680 

ZINC %OF 

(Ib) TOTAL 

25.660 27.5% 

5,570 6.0% 

0 0.0% 

1,980 2.1% 

60,080 64.4% 

93,290 
. 

TABLE 6-3 

AVERAGE ANNUAL LOADINGS BY JURISDICTION 

FOR FIVE - YEAR FUTURE LAND USE CONDITIONS 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of senling in tributaries. 

- 

JURISDICTION 

SARASOTA CO. 

SARASOTA CITY 

MANATEE CO. 

ISLANDS 

BAY SURFACE 

TOTAL 

AREA 

(ac) 

62,301 

7,844 

20,226 

5,761 

33,280 

129,412 

RUNOFF %OF 

(in) TOTAL 

12.13 35.3% 

2.01 5.9% 

4.72 13.8% 

1.42 4.1% 

14.04 40.9% 

34.32 

AVERAGE YEAR 

POLLUTANT LOADING 

Total P %OF 

(Ib) TOTAL 

11 1,080 44.3% 

19,630 7.8% 

42,550 17.0% 

15,610 6.2% 

61,730 24.656 

250,600 

LEAD %OF 

(Ib) TOTAL 

15,860 41.6% 

6,060 15.9% 

. 11,840 31.1% 

1,860 4.9% 

2,470 6.5% 

38,090 

Total N %OF 

(Ib) TOTAL 

588,130 45.4% 

93,060 7.2% 

210,870 16.3% 

65,160 5.0% 

337,460 26.1% 

1,294,680 

ZINC %OF 

(Ib) TOTAL 

17,510 18.8% 

4,610 4.9% 

8,850 9.5O/o 

2,240 2.4% 

60,080 64.4% 

93,290 
TABLE 64 

WET SEASON LOADINGS BY WATERSHED 

FOR FIVE-YEAR LAND USE CONDITIONS 

I 

I Note: Loading totals do not include pollutant senling in tidal tributaries. See text for discussion of settling In tributaries. 
TABLE 6-5 

DRY SEASON LOADINGS BY WATERSHED 

FOR FIVE-YEAR LAND USE CONDITIONS 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of senllng in tributaries. 
TABLE 6-6 

WET SEASON AND DRY SEASON LOADINGS BY SOURCE 

FOR FIVE - YEAR FUTURE LAND USE CONDITIONS 

SOURCE 

SURFACE 

BASEFLOW 

SEPTIC 

POINT SOURCE 

RAINFALL 

TOTAL 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of settling in tributaries. 

. ,..: .. ,,.,<\,cs.; . .. 

SOURCE 

SURFACE 

BASEFLOW 

SEPTIC 

POINT SOURCE 

RAINFALL 

TOTAL 

RUNOFF %OF 

(in) TOTAL 

9.14 43.2% 

2.79 13.2% 

0.23 1.1% 

0.34 1.6% 

8.67 40.9% 

21.17 

RUNOFF %OF 

(in) TOTAL 

5.67 43.1% 

1.02 7.7% 

0.47 3.6% 

0.62 4.7% 

5.37 40.9% 

13.15 

WET SEASON 

POLLUTANT LOADING 

DRY SEASON 

POLLUTANT LOADING 

TOTAL P %OF 

(Ib) TOTAL 

74,170 49.3% 

24,550 16.3% 

4130 2.7% 

9.610 6.4% 

38.100 25.3% 

150,560 

LEAD %OF 

(Ib) TOTAL 

21,590 92.1% 

200 0.9% 

0 0.0% 

140 0.6% 

1,520 6.5% 

23,450 

TOTAL N %OF 

(lb) TOTAL 

384,470 49.8% 

81,840 10.6% 

61,900 8.0% 

35,130 4.6% 

208,290 27.0% 

771.630 

TOTAL P %OF 

(Ib) TOTAL 

46,000 46.0% 

8,940 8.9% 

4,140 4.1% 

17,210 17.2% 

23,630 23.6% 

99,920 

ZINC %OF 

(Ib) TOTAL 

15,830 27.4% 

4,070 7.1% 

0 0.0% 

730 1.3% 

37,090 64.3% 

57,720 

LEAD %OF 

(Ib) TOTAL 

13,410 91.5% 

50 0.3% 

0 0.0% 

250 1.7% 

950 6.5% 

14,660 

TOTALN %OF 

(lb) TOTAL 

238,330 45.6% 

29,810 5.7% 

61,980 11.9% 

63.700 12.2% 

129,170 24.7% 

522,990 

ZINC %OF 

(Ib) TOTAL 

9,820 27.6% 

1,510 4.2% 

0 0.0% 

1,260 3.5% 

23.000 64.6% 

35,590 
1 TABLE 6-7 

WET SEASON AND DRY SEASON LOADINGS BY JURISDICTION 

FOR.FIVE -YEAR FUTURE LAND USE CONDITIONS 

JURISDICTION 

SARASOTA CO. 

SARASOTA CITY 

MANATEE CO. 

ISLANDS 

BAY SURFACE 

TOTAL 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of settling in tributaries. 

- 

JURISDICTION 

SARASOTA CO. 

SARASOTA CITY 

MANATEE GO. 

ISLANDS 

BAY SURFACE 

TOTAL 

AREA 

(ac) 

62,301 

7,844 

20,226 

5,761 

33.280 

129,412 

DRY SEASON 

POLLUTANT LOADING 

RUNOFF %OF 

(in) TOTAL 

7.50 35.4% 

1.24 5.9% 

2.92 13.8% 

0.85 4.0% 

8.67 40.9% 

21.17 

WET SEASON 

POLLUTANT LOADING 

AREA 

(ac) 

62,301 

7,844 

20,226 

5,761 

33,280 

129,412 

RUNOFF %OF 

(in) TOTAL 

4.63 35.2% 

0.77 5.8% 

1.80 13.7% 

0.57 4.3% 

5.37 4O.go/o 

13.15 

ZINC %OF 

(Ib) TOTAL 

6,490 18.2% 

1,810 5.1% 

3,310 9.3% 

980 2.8% 

23,000 64.6% 

35,590 

Total P %OF 

(Ib) TOTAL 

43,630 43.7% 

8,050 8.1% 

17,270 17.3% 

7,340 7.3% 

23,630 23.6O/. 

99.920 

ZINC %OF 

(Ib) TOTAL 

11,030 19.1% 

2,800 4.9% 

5.540 9.6% 

1,260 2.2% 

37,090 64.3% 

57,720 

LEAD %OF 

(Ib) TOTAL 

9,780 41.7% 

3,710 15.8% 

7,330 31.3% 

1,110 4.7% 

1.520 6.5% 

23.450 

Total P %OF 

(Ib) TOTAL 

67,380 44.8% 

11,500 7.636 

25,300 16.8% 

8,280 5.5% 

38,100 25.3% 

150,560 

Total N % 0c 

(Ib) TOTAL 

240,630 46.0% 

37,420 7.2% 

86,770 16.6% 

29,000 5.5% 

129,170 24.7O/o 

522.990 

Total N %OF 

(Ib) TOTAL 

347,510 45.0% 

55,570 7.2% 

124,100 16.1% 

36,160 4.7% 

208,290 27.0% 

771,630 

LEAD %OF 

(Ib) TOTAL 

6,100 41.6% 

2,330 15.9% 

4,550 31.0% 

730 5.0% 

950 6.5% 

14,660 
TABLE 6-8 

WET YEAR LOADINGS BY WATERSHED 

FOR FIVE -YEAR FUTURE LAND USE CONDITIONS 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of senling in tributaries. 
. TABLE 6-9 

DRY YEAR LOADINGS BY WATERSHED 

FOR FIVE - YEAR FUTURE LAND USE CONDiTlONS 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of sealing in tributaries. 
TABLE 6-1 0 

WET YEAR AND DRY YEAR LOADINGS BY SOURCE 

FOR FIVE -YEAR FUTURE LAND USE CONDITIONS 

SOURCE 

SURFACE 

BASEFLOW 

SEPTIC 

POINT SOURCE 

RAINFALL 

TOTAL 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of settling in tributaries. 

SOURCE 

SURFACE 

BASEFLOW 

SEPTIC 

POINT SOURCE 

RAINFALL 

TOTAL 

RUNOFF %OF 

(in) TOTAL 

18.91 41.4% 

7.17 15.7% 

0.70 1.5% 

0.96 2.1% 

17.92 39.3% 

45.66 

RUNOFF %OF 

(in) TOTAL 

11.31 46.2% 

0.77 3.2% 

0.70 2.9% 

0.96 3.9% 

10.72 43.8% 

24.48 

WET YEAR 

POLLUTANT LOADING 

TOTAL P %OF 

(16) TOTAL 

153,500 46.5% 

63,060 19.1% 

8,260 2.5% 

26,820 8.1% 

78.800 23.8% 

330,440 

DRY YEAR 

POLLUTANT LOADING 

TOTAL P %OF 

(Ib) TOTAL 

91,860 50.8% 

6,800 3.8% 

8,260 4.6% 

26.820 14.8% 

47,150 26.1% 

180,890 

TOTALN %OF 

(Ib) TOTAL 

795,000 47.9% 

210,260 12.7% 

123,890 7.5% 

98.840 6.0% 

430,790 26.0% 

1,658,780 

TOTAL N %OF 

(Ib) TOTAL 

475,710 48.6% 

22,710 2.3% 

123.890 12.7% 

98,840 10.1% 

257.730 26.3% 

978.880 

LEAD %OF 

(Ib) TOTAL 

44,600 91.4% 

630 1.3% 

0 0.0% 

390 0.8% 

3,150 6.5% 

48.770 

ZINC %OF 

(Ib) TOTAL 

32,740 26.8% 

10,520 8.6% 

0 0.0% 

1,980 1.6% 

76,700 62.9% 

121,940 

LEAD %OF 

(Ib) TOTAL 

26.690 92.0% 

40 0.1% 

0 0.0% 

390 1.3% 

1,890 6.5% 

29,010 

ZINC %OF 

(Ib) TOTAL 

19,570 28.5% 

1,140 1.7% 

0 0.0% 

1,980 2.9% 

45,890 66.9% 

68,580 
TABLE 6-1 1 

WET YEAR AND DRY YEAR LOADINGS BY JURISDICTION 

FOR FIVE -YEAR FUTURE LAND USE CONDITIONS 

JURISDICTION 

SARASOTA CO. 

SARASOTA CITY 

MANATEE CO. 

ISLANDS 

BAY SURFACE 

TOTAL 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of settling in tributaries. 

JURISDICTION 

SARASOTA CO. 

SARASOTA CITY 

MANATEE CO. 

ISLANDS 

BAY SURFACE 

TOTAL 

AREA 

(ac) 

62,301 

7,844 

20,226 

5,761 

33,280 

129.412 

AREA 

(ac) 

62,301 

7,844 

20,226 

5,761 

33,280 

129.412 

RUNOFF %OF 

(in) TOTAL 

16.81 36.8% 

2.66 5.8% 

6.37 14.0% 

1.89 4.1% 

17.92 39.3% 

45.66 

RUNOFF %OF 

(in) TOTAL 

8.02 32.8% 

1.44 5.9% 

3.28 13.456 

1.00 4.1% 

10.72 43.8% 

24.48 

WET YEAR 

POLLUTANT LOADING 

DRY YEAR 

POLLUTANT LOADING 

ZINC %OF 

(Ib) TOTAL 

24,460 20.1% 

5,980 4.9% 

11,900 9.8% 

2,900 2.4% 

76,700 62.9% 

121,940 

LEAD %OF 

(Ib) TOTAL 

20,340 41.7% 

7,730 15.8% 

15,190 31.1% 

2,360 4.8% 

3,150 6.5% 

48,770 

Total P %OF 

(Ib) TOTAL 

150,790 45.6% 

24,840 7.5% 

56,410 17.1% 

19,600 5.9% 

78,800 23.8% 

330.440 

Total P %OF 

(Ib) TOTAL 

76,100 42.1% 

15,030 8.3% 

30.450 16.8% 

12,160 6.7% 

47,150 26.1% 

180,890 

Total N %OF 

(Ib) TOTAL 

753.990 45.5% 

118,370 7.1% 

273,300 16.5% 

82,330 5.0% 

430,790 26.0% 

1,658,780 

LEAD %OF 

(Ib) TOTAL 

12,010 41.4% 

4,640 16.0% 

9,030 31.1% 

1,440 5.0% 

1.890 6.5% 

29,010 

Total N %OF 

(Ib) TOTAL 

443,040 45.3% 

71,150 7.3% 

156,670 16.0% 

50.290 5.1% 

257,730 26.3% 

978,880 

ZINC %OF 

(Ib) TOTAL 

11,390 16.6% 

3,420 5.0% 

6,230 9.1% 

1,650 2.4% 

45,890 66.9% 

68,580 - 
The average annual loadings by source are presented in Table 6-2. The results for the 

five-year future land use are similar to the results for the existing land use. The total 

runoff for the study area increased slightly, from 34.26 to 34.32 inches per year. The 

percentage of the total runoff due to surface runoff increased slightly (from 42.6% to 

43.1%), while the percentages for the other sources were the same or slightly lower. For 

pollution loadings, the percentage of total loading due to point sources decreased 

noticeably, whereas the nonpoint source percentage exhibited the largest increase. Again, 

a decrease in the amount of pollution due to point sources is projected based on a shift 

from surface water discharge to land application. Increases in nonpoint source loadings 

are due to future development. Baseflow loadings decrease slightly, because new 

development results in less groundwater recharge and a corresponding reduction in 

baseflow quantity. Septic tank and rainfall loadings are assumed to be the same as for 

existing conditions. Surface runoff is still the major source of total N, total P and lead, 

whereas rainfall is the major source of zinc. 

Table 6-3 lists the average annual loadings by jurisdiction. A comparison to the results 

for the existing land use condition (see Table 5-3) shows an increase in loadings for 

Sarasota County and Manatee County, a decrease in loadings for the City of Sarasota, and 

the same loadings for the Barrier Islands. No changes were projected for the Barrier 

Islands during the 5-year planning horizon because no DRIs were identified in the study 

area, and the load to the Bay surface due to rainfall was not expected to change. In 

Sarasota County, the new development and corresponding increase in surface runoff is the 

reason for the loading increases. Transition from surface water discharge to land 

application for the City of Sarasota wastewater treatment plant is the reason for the 

reduced loadings from the City. In Manatee County, increased wastewater flows account 

.for the loading increases. Similar to existing conditions, the jurisdictions of Sarasota 

County, Manatee County and the Bay Surface together account for 90 percent of the total 

runoff and 79 to 93 percent of the pollutants analyzed. 
The wet season and dry season results for the five-year future land use scenario, as 

presented in Tables 6-4 through 6-7, show the same trends that were identified in the 

analysis for existing land use conditions. Based on the data in Tables 6-4 and 6-5, about 

60 percent of the annual loading occurs during the wet season, and 40 percent occurs 

during the dry season, for sunoff, nutrients and metals. Surface runoff and rainfall are the 

major loading sources for all of the pollutants, as shown in Table 6-6. Surface runoff 

tends to provide a larger percentage of loading during the wet season than during the dry 

season, whereas point sources and septic tanks provide a larger percentage of the total 

loading during the dry season than during the wet season. However, the total contribution 

due to point sources and septic tanks is less than contributions due to surface runoff and 

rainfall. In Table 6-7, the distributions of runoff and loadings by jurisdiction for the wet 

and the dry seasons are only slightly different than the distributions for the average annual 

loadings, shown in Table 6-3. 

Tables 6-8 through 6-11 present the five-year future land use scenario results for the wet 

year and dry year analysis. In comparison with the average annual five-year future 

loadings, the values in Tables 6-8 and 6-9 show that loadings for all pollutants will be 28 

to 32 percent higher during the wet year, and 24 to 28 percent lower during the dry year. 

Results in Table 6-10 indicate that the wet year and dry year load distribution by source 

are similar to the distribution for the average rainfall year. Similarly, the results in Table 

6-1 1 indicate that the wet year and dry year distributions by jurisdiction are virtually the 

same as for the average rainfall year. 

The values in Tables 6-1 through 6-11 do not include the effects of pollutant settling in 

the tidal tributaries. As discussed in Section 5.1, the application of the delivery ratios 

established for the tidal tributaries did not produce substantial changes in total loadings or 

the distribution of loadings between sources or jurisdictions. The reduction in surface 

loadings due to the delivery ratios was only 6 percent for total P and zinc, 3 percent for 

total N and 13 percent for lead. When considering the loadings from all sources, the 

loading reductions due to settling were 3 percent for total P, 12 percent for lead, and 1 
percent for total N and zinc. With regard to source distribution, the percentage of total 

loading due to surface runoff changed by 2 percentage points or less, and the percentages 

for the other sources changed by no more than 1 percentage point. Distribution by 

jurisdiction remained essentially unchanged, with differences always less than 1 

percentage point. 

6.2 BUILDOUT FUTURE LAND USE 

Tables 6-12 through 6-22 present the loading results for the buildout future land use 

analysis. The buildout future land use includes development in all areas identified in the 

Comprehensive Plans for Manatee and Sarasota Counties. The results for the average 

annual rainfall condition are shown in Tables 6-12 through 6-14. Wet season and dry 

season results are listed in Tables 6-15 through 6-18, and wet year and dry year results are 

presented in Tables 6-19 through 6-22. 

Table 6-12 shows the average annual loading results by watershed. A comparison of 

Tables 5-1 (existing land use conditions) and 6-12 show an overall increase from existing 

loadings for all pollutants. The increase in loading for total P, total N and zinc ranges 

from 10 to 15 percent, whereas the increase for lead is 35 percent. The percent increase 

in loadings for zinc and the nutrients is relatively small because of the substantial loadings 

attributed to rainfall and baseflow. Lead, on the other hand, has very low concentrations 

in baseflow and rainfall but moderate to high values in runoff from urbanized areas. 

Consequently, projected increases in urban area result in a substantially larger annual 

loading of lead from the study area. 

In some cases, watersheds that had minor impacts under existing land use conditions have 

a substantially larger contribution in the buildout future scenario. For total P, Phillippi 

Creek and South Bradenton are still the largest watershed contributors, but the loading 

from South Creek has increased 46 percent, such that the total loading from South Creek 

is greater than the Whitaker Bayou loading. The Phillippi Creek, South Bradenton, South 
. TABLE 6-12 

AVERAGE ANNUAL LOADINGS BY WATERSHED 

FOR BUILDOUT FUTURE LAND USE CONDITIONS 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of settling in tributaries. 
TABLE 6-13 

AVERAGE ANNUAL LOADINGS BY SOURCE 

FOR BUILDOUT FUTURE LAND USE CONDITIONS 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of settling in tributaries. 

SOURCE 

SURFACE 

BASEFLOW 

SEPTIC 

POINT SOURCE 

RAINFALL 

TOTAL 

RUNOFF %OF 

(in) TOTAL 

17.79 47.7% 

3.34 9.0% 

0.70 1.9% 

1.39 3.7% 

14.04 37.7% 

37.26 

AVERAGE YEAR 

POLLUTANT LOADING 

TOTAL P %OF 

(Ib) TOTAL 

151,340 52.4% 

29,380 10.2% 

8,270 2.9% 

37,940 13.1% 

61,730 21.4% 

288,660 

TOTAL N %OF 

(Ib) TOTAL 

790,360 53.0% 

98,030 6.6% 

124,060 8.3% 

141,230 9.5% 

337.460 22.6% 

1,491,140 

LEAD %OF 

(Ib) TOTAL 

46,900 93.5% 

260 0.5% 

0 0.0% 

550 1.1% 

' 2,470 4.9% 

50,180 

ZINC %OF 

(Ib) TOTAL 

34,990 34.1% 

4,910 4.8% 

0 0.0% 

2,740 2.7% 

60,080 58.5% 

102,720 
TABLE 6-14 

AVERAGE ANNUAL LOADINGS BY JURISDICTION 

FOR BUILDOUT FUTURE LAND USE CONDITIONS 

Note: Loading lotals do not include pollutant settling in tidal tributaries. See text for discussion of settling in tributaries. 

JURISDICTION 

SARASOTA CO. 

SARASOTA CITY 

MANATEE CO. 

ISLANDS 

BAY SURFACE 

TOTAL 

AREA 

(ac) 

62,301 

7,844 

20,226 

5,761 

33,280 

129,412 

RUNOFF %OF 

(in) TOTAL 

14.17 38.0% 

2.06 5.5% 

5.57 14.9% 

1.42 3.8% 

14.04 37.7% 

37.26 

AVERAGE YEAR 

POLLUTANT LOADING 

Total P %OF 

(Ib) TOTAL 

139.670 48.4% 

20,880 7.2% 

50,770 17.6% 

15,610 5.4% 

61.730 21.4% 

288.660 

Total N %OF 

(Ib) TOTAL 

731,660 49.1% 

95,430 6.4% 

261,430 17.5% 

65,160 4.4% 

337,460 22.6% 

1,491,140 

LEAD %OF 

(Ib) TOTAL 

24,440 48.7% 

6,100 12.2% 

15,310 30.5% 

1,860 3.7% 

2,470 4.9% 

50,180 

ZINC %OF 

(Ib) TOTAL 

24,380 23.7% 

4,790 4.7% 

11,230 10.9% 

2,240 2.2% 

60.080 58.5% 

102,720 
TABLE 6-15 

WET SEASON LOADINGS BY WATERSHED 

FOR EUlLDOUT FUTURE LAND USE CONDiTlONS 

Note: Loading totals do not include pollutant seniing in tidal tributaries. See text for discussion of seniing In tributaries. 

TABLE 6-1 7 

WET SEASON AND DRY SEASON LOADINGS BY SOURCE 

FOR BUILDOUT FUTURE LAND USE CONDITIONS 

SOURCE 

SURFACE 

BASEFLOW 

SEPTIC 

POINT SOURCE 

RAINFALL 

TOTAL 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of settling in tributaries. 

SOURCE 

SURFACE 

BASEFLOW 

SEPTIC 

POINT SOURCE 

RAINFALL 

TOTAL 

RUNOFF %OF 

(in) TOTAL 

10.98 48.1% 

2.45 10.7% 

0.23 1.0% 

0.49 2.2% 

8.67 38.0% 

22.82 

RUNOFF %OF 

(in) TOTAL 

6.81 47.2% 

0.89 6.2% 

0.47 3.2% 

0.90 6.2% 

5.37 37.2% 

14.44 

WET SEASON 

POLLUTANT LOADING 

TOTAL P %OF 

(Ib) TOTAL 

93,290 54.7% 

21.550 12.6% 

4,130 2.4% 

13,520 7.9% 

38,100 22.3% 

170,590 

DRY SEASON 

POLLUTANT LOADING 

TOTAL P %OF 

(Ib) TOTAL 

57,880 49.1% 

7,830 6.6% 

4,140 3.5% 

24,410 20.7% 

23,630 20.0% 

117,890 

TOTALN %OF 

(Ib) TOTAL 

487,860 55.5% 

71,860 8.2% 

61,990 7.0% 

49,710 5.7% 

208.290 23.7% 

879,710 

TOTAL N %OF 

(Ib) TOTAL 

302,480 49.5% 

26,170 4.3% 

62,070 10.2% 

91,540 15.0% 

129.170 21.1% 

61 1,430 

LEAD %OF 

(Ib) TOTAL 

28,950 93.8% 

180 0.6% 

0 0.0% 

210 0.7% 

1,520 4.9% 

30.860 

ZINC 'OF 

(Ib) TOTAL 

21,580 34.1% 

3,580 5.7% 

0 0.0% 

1,000 1.6% 

37,090 58.6% 

63,250 

LEAD %OF 

(Ib) TOTAL 

17,980 93.1% 

40 0.2% 

0 0.0% 

340 1.8% 

950 4.9% 

19,310 

ZINC %OF 

(Ib) TOTAL 

13,390 33.9% 

1,330 3.4% 

0 0.0% 

1,750 4.4% 

23,000 58.3% 

39,470 
.. TABLE 6-1 8 

WET SEASON AND DRY SEASON LOADINGS BY JURISDICTION 

FOR BUILDOUT FUTURE LAND USE CONDITIONS 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of settling in tributaries. 

JURISDICTION 

SARASOTA CO. 

SARASOTA CtTY 

MANATEE CO. 

ISLANDS 

BAY SURFACE 

TOTAL 

WET SEASON 

POLLUTANT LOADING 

JURISDICTION 

SARASOTA CO. 

SARASOTA CITY 

MANATEE CO. 

ISLANDS 

BAY SURFACE 

TOTAL 

AREA 

(ac) 

62,301 

7.844 

20,226 

5.761 

33,280 

129.41 2 

RUNOFF %OF 

(in) TOTAL 

5.51 38.1% 

0.79 5.5% 

2.19 15.2% 

0.57 4.0% 

5.37 37.2% 

14.44 

AREA 

(ac) 

62,301 

7,844 

20,226 

5,761 

33,280 

129,412 

RUNOFF %OF 

(in) TOTAL 

8.67 38.0% 

1.27 5.6O/~ 

3.38 14.8% 

0.85 3.7% 

8.67 38.0% 

22.82 

ZINC %OF 

(Ib) TOTAL 

15,060 23.8% 

2,890 4.6O/o 

6,950 11.0% 

1,260 2.0% 

37,090 58.6% 

63.250 

LEAD %OF 

(Ib) TOTAL 

15,050 48.8% 

3,730 12.1% 

. 9,450 30.6% 

1.110 3.6% 

1,520 4.9% 

30,860 

Total P %OF 

(Ib) TOTAL 

82,960 48.6% 

12,120 7.1% 

29,130 17.1% 

8.280 4.9% 

38.100 22.3% 

170,590 

DRY SEASON 

POLLUTANT LOADING 

Total N %OF 

(Ib) TOTAL 

428,910 48.8% 

56,800 6.5% 

149,550 17.0% 

36.160 4.1% 

208,290 23.7% 

879,710 

ZINC %OF 

(Ib) TOTAL 

9,310 23.6% 

1,900 4.8% 

4.280 10.8% 

980 2.5% 

23,000 58.3% 

39,470 

LEAD %OF 

(Ib) TOTAL 

9,410 48.7% 

2,350 12.2% 

5,870 30.4% 

730 3.8% 

950 4.9% 

19,310 

Total P %OF 

(Ib) TOT& 

56,620 48.0% 

8,680 7.4% 

21,620 18.3% 

7,340 6.2% 

23,630 20.0% 

1 17.890 

Total N %OF 

(Ib) TOTAL 

302,740 49.5% 

38,570 6.3% 

11 1,950 18.3% 

29,000 4.7% 

129.170 21.1% 

61 1,430 
TABLE 6-19 

WETYEAR LOADINGS BY WATERSHED 

FOR BUiLDOUT FUTURE LAND USE CONDITIONS 

Note: Loading totals do not include pollutant senling in tidal tributaries. See text for disarssion of senllng In tributaries. 

OTHER ISLANDS 

DIRECT TO BAY 

BAY SURFACE 

TOTAL 

38.53 

43.93 

69.70 

49.02 

900 

4.241 

33,280 

129,412 

2.260 

12,210 

78,800 

373,540 

2.51 

2.88 

2.37 

3.1 1 

0.29 

0.29 

0.15 

0.31 

11,090 

67,510 

430,790 

1,889,530 

12.32 

15.92 

12.94 

14.96 

1.41 

1.60 

0.82 

1.47 

400 

3.250 

3.150 

64,060 

0.44 

0.77 

0.09 

0.65 

0.051 

0.077 

0.006 

0.065 

400 

2.670 

76.700 

133,310 

0.44 

0.63 

2.30 

0.69 

0.051 

0.063 

0.146 

0.065 

TABLE 6-21 

WETYEAR AND DRY YEAR LOADINGS BY SOURCE 

FOR BUILDOUT FUTURE LAND USE CONDITIONS 

- 

SOURCE 

SURFACE 

BASEFLOW 

SEPTIC 

POINT SOURCE 

RAINFALL 

TOTAL 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of settling in tributaries. 

SOURCE 

SURFACE 

BASEFLOW 

SEPTIC 

POINT SOURCE 

RAINFALL 

TOTAL 

RUNOFF %OF 

(in) TOTAL 

22.71 46.3% 

6.29 12.8% 

0.70 1.4% 

1.39 2.8% 

17.92 36.Vh 

49.02 

RUNOFF %OF 

(in) TOTAL 

13.59 50.2% 

0.68 2.5% 

0.70 2.6% 

1.39 5.1% 

10.72 39.6% 

27.08 

WET YEAR 

POLLUTANT LOADING 

TOTALP %OF 

(Ib) TOTAL 

193,150 51.7% 

55,380 14.8% 

8,270 2.2% 

37,940 10.2% 

78,800 21 .I% 

373,540 

DRY YEAR 

POLLUTANT LOADING 

TOTAL P %OF 

(Ib) TOTAL 

115,620 54.8% 

5,990 2.8% 

4,140 2.0% 

37,940 18.0% 

47,150 22.4% 

210,840 

TOTAL N %OF 

(Ib) TOTAL 

1,008,890 53.4% 

184,560 9.8% 

124,060 6.6% 

141,230 7.5% 

430.790 22.8% 

1,889,530 

TOTALN %OF 

(lb) TOTAL 

603,760 52.7% 

19,950 1.7% 

124,060 10.8% 

141,230 12.3% 

257,730 22.5% 

1 ,I 46.730 

LEAD %OF 

(Ib) TOTAL 

59,830 93.4% 

530 0.8% 

0 0.0% 

550 0.9% 

3.150 4.9% 

64.060 

ZINC %OF 

(Ib) TOTAL 

44,660 33.5% 

9.210 6.9% 

0 0.0% 

2,740 2.1% 

76.700 57.5% 

133.310 

LEAD %OF 

(Ib) TOTAL 

35,860 93.6% 

30 0.1% 

0 0.0% 

550 1.4% 

1,890 4.9% 

38,330 

ZINC %OF 

(Ib) TOTAL 

26,720 35.0% 

1,000 1.3% 

0 0.0% 

2,740 3.6% 

45.890 60.1% 

76.350 
TABLE 6-22 

WET YEAR AND DRY YEAR LOADINGS BY JURISDICTION 

FOR BUILDOUT FUTURE LAND USE CONDITIONS 

Note: Loading totals do not include pollutant settling in tidal tributaries. See text for discussion of settling in tributaries. 

JURISDICTION 

SARASOTA CO. 

SARASOTA CITY 

MANATEE CO. 

ISLANDS 

BAY SURFACE 

TOTAL 

JURISDICTION 

SARASOTA CO. 

SARASOTA CITY 

MANATEE GO. 

ISLANDS 

BAY SURFACE 

TOTAL 

AREA 

(ac) 

62,301 

7,844 

20,226 

5,761 

33,280 

129.41 2 

RUNOFF '10 OF 

(in) TOTAL 

19.15 39.1% 

2.72 5.5% 

7.33 15.0% 

1.89 3.9% 

17.92 36.6% 

49.02 

AREA 

(ac) 

62,301 

7,844 

20,226 

5,761 

33,280 

129,412 

WET YEAR 

POLLUTANT LOADING 

Total P %OF 

(Ib) TOTAL 

183,840 49.2% 

26,090 7.0% 

65,210 17.5% 

19.600 5.2% 

78.800 21.1% 

373,540 

RUNOFF %OF 

(in) TOTAL 

9.82 36.3% 

1.50 5.5% 

4.04 14.9% 

1.00 3.7% 

10.72 39.6% 

27.08 

Total N %OF 

(Ib) TOTAL 

925,240 49.0% 

120.740 6.4% 

330,430 17.5% 

82,330 4.4% 

430,790 22.8% 

1,889.530 

DRY YEAR 

POLLUTANT LOADING 

LEAD %OF 

(Ib) TOTAL 

31,230 48.8% 

7,770 12.1% 

19,550 30.5% 

2,360 3.7% 

3,150 4.9% 

64.060 

Total P %OF 

(Ib) TOTAL 

101,110 47.0% 

. 16,290 7.6% 

38,260 17.8% 

12,160 5.7% 

47.150 21.9% 

21 4.970 

ZINC %OF 

(Ib) TOTAL 

32,790 24.6% 

6,160 4.6% 

14,760 11.1% 

2,900 2.2% 

76,700 57.5% 

133,310 

TotalN %OF 

(Ib) TOTAL 

563,320 49.1% 

73,600 6.4% 

201,790 17.6% 

50,290 4.4% 

257,730 22.5% 

1,146,730 

LEAD %OF 

(Ib) TOTAL 

18,610 48.6% 

4,700 12.3% 

11,690 30.5% 

1,440 3.8% 

1,890 4.9% 

38,330 

ZINC %OF 

(Ib) TOTAL 

17,040 22.3% 

3,610 4.7% 

8,160 10.7% 

1,650 2.2% 

45,890 60.1% 

76,350 
Creek and Whitaker Bayou watersheds are also the largest contributors of total N in the 

study area. Phillippi Creek, Whitaker Bayou and Bowlees Creek are still the major 

contributors of lead and zinc loadings. Like South Creek, Matheny Creek also exhibits 

substantial loading increases, such that the loading from Matheny Creek is comparable to 

the Whitaker Bayou loadings for all of the analyzed pollutants. 

The average annual loadings by source are presented in Table 6-13. A comparison of 

Tables 5-2 and 6-13 shows that the total study area flow increased from 34.26 inches to 

37.26 inches. Surface runoff is the major factor, increasing from 14.58 inches to 17.79 

inches. Point source flows also increase, whereas baseflow is lower, and septic tank and 

rainfall flows are the same. As expected, the increase in surface runoff also generates 

increased surface runoff loadings, with increases ranging from 28 percent for total P to 41 

percent for zinc. The increase in surface runoff due to urban development also results in 

a decrease in baseflow quantity, so baseflow loading is less for the buildout scenario. 

Point source loading is greater than existing conditions for the buildout scenario, due to 

the increase in wastewater flows generated by the buildout population. Rainfall and septic 

tank loadings are assumed to be the same for the existing and buildout scenarios, the lam 

because all future development was assumed to be sewered rather than served by septic 

tanks. Overall, the major sources of nutrients and metals are surface runoff and rainfall. 

Table 6-14 lists the average annual loadings by jurisdiction. A comparison of these 

results with the results for the existing land use condition (see Table 5-3) shows an 

increase in loadings for Sarasota and Manatee Counties, a decrease in loadings for the 

City of Sarasota, and the same loadings for the Barrier Islands and the Bay Surface. 

Future land use changes for the islands were not considered because comprehensive 

planning documents were not available. In Sarasota County, loading increases ranged 

from 27 percent for total N to 66 percent for lead, primarily due to surface runoff. In 

Manatee County, future urbanization accounted for loading increases ranging from 23 

percent for total P to 29 percent for lead. Reductions in City of Sarasota loadings ranged 

from 1 percent for lead to 17 percent for total P, due to transition from surface water 
discharge to land application of wastewater. Because most of the future development is in 

Sarasota County, the relative percentage of total loading contributed by Sarasota County 

increased, whereas the percentage contributed by each of the other jurisdictions typically 

either decreased or remained the same. 

The wet season and dry season results for the buildout future land use scenario, as 

presented in Tables 6-15 through 6-18, show trends similar to those identifed in the 

analysis of existing land use conditions. Based on the data in Tables 6-15 and 6-16, about 

60 percent of the annual loading occurs during the wet season, and 40 percent occurs 

during the dry season, for runoff, nutrients and metals. Surface runoff and rainfall are the 

major loading sources for all of the pollutants, as shown in Table 6-17. Surface runoff 

tends to provide a larger percentage of loading during the wet season than during the dry 

season, whereas point sources and septic tanks provide a larger percentage of the total 

load during the dry season than during the wet season. In fact, the combination of point 

source and septic tank nutrient loadings actually exceed the rainfall nutrient loadings 

during the dry season. In Table 6-18, the distributions of runoff and loadings by 

jurisdiction for the wet and dry seasons are only slightly different than the distributions 

for the average annual loadings, shown in Table 6-14. . 

. 

Tables 6-19 through 6-22 present the buildout future land use scenario results for the wet 

year and dry year analysis. In comparison with the average ye& loadings, the values in 

Tables 6-19 and 6-20 show that loadings for all pollutants are 27 to 30 percent higher 

during the wet year and 23 to 26 percent lower during the dry year. Results in Table 6-21 

indicate that the wet year and dry year loading distributions by source are essentially the 

same as the distribution for the average rainfall year. Similarly, the results in Table 6-22 

indicate that the wet year and dry year distributions by jurisdiction are virtually the same 

as for the average rainfall year. 

The values in Tables 6-12 through 6-22 do not include the effects of pollutant settling in 

the tidal tributaries. As discussed in Section 5.1, the application of the delivery ratios 
established for the tidal tributaries did not produce substantial changes in total loadings or 

the distribution of loadings between sources or jurisdictions. The reduction in surface 

loadings due to the delivery ratios was only 6 percent for total P and zinc, 3 percent for 

total N and 13 percent for lead. When considering the loadings from all sources, the 

loading reductions due to settling were 3 percent for total P, 12 percent for lead, and 1 

percent for total N and zinc. With regard to source distribution, the percentage of total 

loading due to surface runoff changed by 2 percentage points or less, and the percentages 

for the other sources changed by no more than 1 percentage point. Distribution by 

jurisdiction remained essentially unchanged, with differences always less than 1 

percentage point. 
7.0 SUMMARY 

In Phase I of the Sarasota Bay National Estuary Program Point and Nonpoint Source Loading 

Assessment, existing data were used to quantify the loadings of nutrients and metals to 

Sarasota Bay. Loadings were estimated for three different land use scenarios: existing, fiveyear future and buildout future. The three scenarios were investigated to establish current 

loading trends and to project how these trends may change in the future. 

The results of the analysis for the average rainfall year are presented graphically in Figures 7- 

1 through 7-10. Figures 7-1 through 7-5 are stacked bar graphs which show the distribution 

of average annual loadings between the watersheds that comprise the study area. Results are 

presented for total runoff, total P, total N, lead and zinc. In Figures 7-6 through 7-10, the bar 

graphs show the distribution by source for the same five constituents. In all of the figures, 

values are presented for all three land use scenarios. 

Figures 7-1 through 7-5 show that the Phillippi Creek, Whitaker Bayou, Bowlees Creek and 

South Creek watersheds account for over half of the runoff and total loadings to the Bay, 

excluding rainfall. Phillippi Creek and Whitaker Bayou contribute a large percentage of the 

total loadings for both the nutrients and the metals. In contrast, Bowlees Creek has a greater 

relative impact on metals loadings than on nutrient loadings, and South Creek has a greater 

relative impact on nutrient loadings than on metals loadings. This is because Bowlees Creek 

is highly industrialized relative to the other watersheds, whereas South Creek is primarily 

rural. 

The figures also show that five-year future loadings will be similar to existing loadings, but 

that the buildout future loadings will be considerably higher. The results show a very small 

increase in total runoff volume, a small decrease in total P loadings, and a small increase in 

total N and metals loadings for the five-year scenario. Loading increases due to increased 

urbanization are offset by loading reductions due to changes in wastewater disposal methods. 
EXISTING 5-YR FUTURE BUllDOUl 

Land Use Scenario 

Rainfall Philippi Creek Whitoker Bayou Bowlees Creek 1223 South Creek 

a Other 

Figure 7-1. Flow Distribution by Watershed in Sarasota Bay NEP Study Area. 
EXISTING 5-YR FUTURE BUllDOUI 

Land Use Scenorio 

@ Rainloll Philippi Creek Whiloker Boyou Bowlees Creek [Za South Creek 

a Other 

~i~ure 7-2. Total P Distribution by Watershed in Sarasota Bay NEP Study Area. 
EXISTING 5-YR FUTURE BUllOOUT 

Land Use Scenario 

Rainfall philippi Creek Whitaker Bayou Bowlees Creek jZS] Saulh Creek 

Olher 

Figure 7-3. Total N Distribution by Watershed in Sarasota Bay NEP Study Area. 
EXISTING 5-YR FUTURE BUllOOUT 

Lond Use Scenario 

Rainfall Philippi Creek a Wnitoker Bayou 618] Bowlees Creek [m South Creek 

Other 

Figure 7-4. Lead Distribution by Watershed in Sarasota Bay NEP Study Area. 
EXISTING 5-YR FUTURE BUILDOUT 

Lond Use Scenario 

Roinfoll ~hlippi Creek Whitaker Boyou Bowlees Creek IZZ] Soulh Creek 

Other 

Figure 7-5. Zinc Distribution by Watershed in Sarasota Bay NEP Study Area. 
EXISTING 5-YR FUTURE BUILDOUT 

Lond Use Scenario 

Roinfoll Point Source Septic Tanks Bosefiow Surfoce Runoff 

Figure 7-6. Flow Distribution by Source in Sarasota Bay NEP Study Area. 
EXISTING 5-YR FUTURE BUILDOUT 

Land Use Scenario 

Rainfall Point Source Septic Tanks [aSI Boseflow 1ZZ] Surfoce Runoff 

Figure 7-7. Total P Distribution by Source in Sarasota Bay NEP Study Area. 
. 

EXISTING 5-YR FUTURE BUllDOUT 

Land Use Scenario 

@ Rainfall Painl Source Seplit Tonks 188] Boseflow a Sutfoce Runoff 

Figure 7-8. Total N Distribution by Source in Sarasota Bay NEP Study Area. 
" 

EXISTING 5-YR FUTURE BUILDOUT 

Land Use.Scenario 

Rainfall a Point Source Septic Tonks Boseflow Surfoce Runoff 

Figure 7-9. Lead Distribution by Source in Sarasota Bay NEP Study Area. 
EXISTING 5-YR FUTURE BUllOOUT 

Lond Use Scenaria 

Rainfall Point Source a Seplic Tanks Baseflow Surloce Runoff 

Figure 7-10. Zinc Distribution by Source in Sarasdta Bay NEP Study Area. 
For the buildout scenario, however, loadings are higher for all constituents. The figures 

generally show increased loadings for all of the watersheds, due to impacts of increasing 

urbanization. 

Figures 7-6 through 7-10 indicate that surface runoff and rainfall are currently the most 

important factors in determining total loadings to the Bay. The two major sources account 

for 84 percent of the total runoff, and 71 to 98 percent of the nutrient and metal loadings. Of 

the two sources, surface runoff is the largest source of total P, total N and lead, whereas 

rainfalI is the largest source of zinc. 

. As shown in the figures, point sources and septic tanks are less significant than surface runoff 

and rainfall. Point source loadings are relatively low because most of the treatment facilities 

achieve Advanced Wastewater Treatment (AWT) standards. Most of the facilities also 

discharge via land application (irrigation, percolation ponds, drainfields) which removes much 

or all of the effluent pollutant load via plant uptake or soil removal mechanisms. Septic tank 

loadings are not a major contributor for several reasons. One is that Sarasota County and 

Casey Key are the only parts of the study area in which septic tanks are in use. In addition, 

the failure rate of septic tanks in Sarasota County is believed to be low, based on repair 

permit data from the County Health Department Finally, because of the low hydraulic 

conductivity and hydraulic gradients in the surficial aquifer, the travel time from any septic 

tank to the Bay or one of its tributaries is probably on the order of months or years. With 

some pollutant removal occurring as the waste travels through the aquifer, it is likely that 

most of the septic tank effluent loading does not reach the Bay. However, the septic tank 

loadings may result in localized water quality impacts, particularly in areas of high septic tank 

densities, relatively steep hydraulic gradients, and relatively short travel distances from the 

septic tanks to the receiving waters. 

A comparison of existing and future distributions indicates that surface runoff wiU have an 

increasing impact on Bay loadings, while the impact of other sources will decrease or remain 

the same. With increased urban development, more surface runoff is generated and less 
recharge occurs, resulting in lower baseflow quantities. Because new development is 

typically served by collection systems rather than septic tanks, the septic tank loading is not 

expected to increase in the future. Wastewater quantities will increase in the future as the 

population increases, but the combination of AWT effluent quality and land application of the 

effluent results in a relatively low contribution to Bay loading. Rainfall loadings are assumed 

to be constant 

The estimates presented here will be used as input to Phase 3 of the Sarasota Bay NEP Point 

and Nonpoint Source Loading Assessment. In Phase 3, alternative management strategies for 

control of Bay loadings will be identified and evaluated. These strategies may include 

nonstructural controls (e.g., land use controls), structural stormwater controls (e.g., wet 

detention basins, infiltration basins) and point source controls (e.g., increased use of effluent 

for irrigation). Both the water quality benefits and the costs of alternative strategies will be 

assessed. Based on the analysis, selected management strategies will be recommended. 
8.0 REFERENCES 

Ambrose, R.B., T.A. Wool, J.P. Connolly, and R.W. Schanz, "WASP4, A Hydrodynamic 

and Water Quality Model - Model Theory, User's Manual and Programmer's Guide 

(Draft)," U.S. Environmental Protection Agency, Office of Research and Development, 

Environmental Research Laboratory, Athens, Georgia, August 1990. 

Bowie, G.L., W.B. Mills, D.B. Porcella, C.L. Campbell, J.R. Pagenkopf, G.L. Rupp, K.M. 

Johnson, P.W.H. Chan, S.A. Gherini and C.E. Chamberlin, "Rates, Constants, and Kinetics 

Formulations in Surface Water Quality Modeling (Second Edition)," EPAl60013-851040, 

U.S. Environmental Protection Agency, Office of Research and Development, 

Environmental Research Laboratory, Athens, Georgia, June 1985. 

Briley , Wild & Associates, Inc., "Master Stormwater Drainage Plan Areas "A" and "B"," 

prepared for Manatee County Public Works Department, 1984. 

Camp Dresser & McKee, "Southeast Area Study Model Update," prepared for Manatee 

County Planning and Development Department, March 1987. 

Camp Dresser & McKee, "Sarasota Bay Water Quality Trend Evaluation, Progress Report 

No. 1," prepared for Mote Marine Laboratory, November 1990. 

Camp Dresser & McKee, "Lake Manatee Watershed Water Resources Development 

Report (Final Draft)," prepared for the Manatee County Public Works Department, May 

1991. 

Camp Dresser & McKee, "Sarasota County Stormwater ~&ter Plan Final Report," 

prepared for the Sarasota County Commission and the Sarasota County Transportation 

Department, March 1987. 

Camp Dresser & McKee, "An Assessment of Stormwater Management Programs," 

prepared for the Florida Department of Environmental Regulation, December 1985. 

Delos, C.G., W.L. Richardson, J.V. DePinto, P.W. Rogers, K. Rygwelski, J.P. St. John, 

W.L. Shaughnessy, T.A. Faha, and W. N. Christie, "Technical Guidance Manual for 

Performing Wasteload Allocations, Book 11: Streams and Rivers, Chapter 3, Toxic 

Substances," EPA-44014-84-022, U.S. Environmental Protection Agency, Office of Water 

Regulations and Standards, Washington, DC, 1984. 

Hand, J., V. Tauxe, M. Friedemann and L. Smith, "1990 Florida Water Quality 

Assessment 305(b) Technical Appendix," Florida Department of Environmental 

Regulation, Division of Water Management, Bureau of Surface Water Management, 

Standards and Monitoring Section, June 1990. 
Hand, J., V. Tauxe, and M. Friedemann, "Sarasota Bay Basin Technical Report: An 

Appendix of the 305(b) Water Quality Assessment for the State of Florida," Florida 

Department of Environmental Regulation, Division of Water Managemenf Bureau of 

Surface Water Management, Standards and Monitoring Section, July 1988. 

Heyl, M.G. and L.K. Dixon,"Water Quality Status and Trends in Sarasota Bay (1966- 

1986)," presented at the Sarasota Bay Area Scientific Information Symposium, April 1987. 

Metcalfe, J., R. Wagner and A. Stoddard, "Development of Heavy Metal Wasteload 

Allocations for the State of North Carolina," prepared for the U.S. Environmental 

Protection Agency, Washington, DC, 1984. 

Ostendorf, D.W., Modeling Contamination of Shallow Unconfined Aquifers through 

Infitration Beds," Water Resources Research, Vol. 22, No. 3, March 1986, pp. 375-382. 

Pettygrove, G.S. and T. Asano (editors), "Irrigation with Reclaimed Municipal Wastewater 

- A Guidance Manual," prepared by the Department of Land, Air and Water Resources, 

University of California, Davis, prepared for the California State Water Resources Control 

Board, Lewis Publishers, Inc., 1985. 

Priede-Sedgwick, Inc., "Tampa Bay Nationwide Urban Runoff Program Study, Phase 11, 

Task II.3: Precipitation Quantity and Quality," February 1983. 

Richardson, W.L., R. Brown, F. Gawronski, K. McGunagle, and R. Wethington, "User's 

Manual for the Transport and Fate Model MICHRN," U.S. Environmental Protection 

Agency, Large Lake Research Station, Grosse nle, Michigan, 1983. 

Southwest Florida Water Management District, "Groundwater Resource Availability: 

Sarasota County, Florida," Resource Management and Planning Department, March 1988. 

U.S. Environmental Protection Agency, "Fate of Priority Pollutants in Publicly Owned 

Treatment Works, Final Report, Volume I," EPA 44011-821303, Effluent Guidelines 

Division, Washington, DC, September 1982. 

U.S. Environmental Protection Agency, "Results of the Nationwide Urban Runoff . 

Program: Volume I Final Report," Water Planning Division, Washington, DC, December 

1983. 

Wanielista, M.P., "Evers Reservoir Hydrologic Study," University of Central Florida, 

Orlando, Florida, September 1989. 
APPENDIX A 

STORET WATER QUALITY DATA FOR TRIBUTARIES TO SARASOTA BAY 
WHITAKER BAYOU AT TRI-PAR DR - BRK 

TOTAL P 

(MGIL) 

0.170 

0.298 

0.234 

0.325 

0.609 

0.762 

1.092 

0.398 

0.944 

0.404 

0.193 

0.132 

0.351 

0.320 

0.554 

0.406 

0.1 88 

0.226 

0.307 

0.300 

0.445 

0.551 

0.354 

0.353 

0.386 

0.425 

0.431 

0.282 

0.444 

0.246 

0.378 

0.729 

0.318 

0.456 

0.480 

YEAR 

75 

n 

78 

78 

80 

80 

80 

81 

82 

82 

83 

83 

83 

83 

83 

84 

84 

84 

84 

84 

85 

85 

85 

85 

85 

86 

86 

86 

87 

87 

87 

87 

88 

88 

88 

88 

89 

89 

MONTH 

1 

11 

1 

3 

4 

9 

10 

2 

. 6 

1 

4 

2 

3 

4 

5 

10 

1 

2 

4 

7 

11 

1 

3 

8 

10 

11 

1 

3 

6 

1 

3 

6 

9 

1 

5 

9 

11 

1 

4 

DAY 

27 

15 

17 

27 

28 

15 

13 

2 

15 

5 

5 

8 

28 

25 

31 

24 

23 

27 

2 

9 

26 

21 

25 

20 

28 

18 

6 

31 

16 

20 

23 

8 

22 

4 

2 

26 

14 

17 

24 

STATION 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

24010558 

TOTAL N 

(MGIL) 

0.63 

1.22 

1.66 

1.07 

1.32 

1.01 

0.87 

0.93 

0.92 

0.76 

0.67 

0.82 

0.82 

0.91 

0.69 

0.73 

0.80 

0.82 

1.25 

0.69 

1.54 

0.82 

0.63 

0.66 

0.86 

0.94 

0.98 

1.51 

0.98 

0.80 

0.80 
WHITAKER BAYOU ON 27TH STREET 

YEAR 

75 

75 

77 

78 

78 

80 

80 

80 

81 

81 

82 

82 

83 

83 

83 

83 

83 

84 

84 

84 

84 

84 

85 

85 

85 

85 

85 

86 

86 

86 

86 

87 

87 

87 

87 

88 

88 

88 

88 

89 

89 

TOTAL P 

(MGIL) 

1.300 

1.300 

1.328 

0.865 

1.358 

1.023 

0.737 

1.768 

1.295 

1.466 

1.387 

0.764 

0.323 

0.559 

0.776 

0.895 

1.118 

1.230 

1.330 

1.722 

1.982 

1.062 

1.850 

1.492 

1.736 

1.810 

0.517 

1.139 

1.371 

0.867 

0.706 

0.389 

0.358 

0.394 

0.358 

0.595 

0.960 

MONTH 

1 

1 

11 

1 

3 

4 

9 

10 

2 

6 

1 

4 

2 

3 

4 

5 

10 

1 

2 

4 

7 

11 

1 

3 

8 

10 

11 

1 

3 

6 

12 

1 

3 

6 

9 

1 

5 

9 

11 

1 

4 

STATION 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

24010553 

DAY 

27 

27 

15 

17 

27 

28 

15 

13 

2 

15 

5 

5 

8 

28 

25 

31 

24 

23 

27 

2 

9 

26 

21 

25 

20 

28 

18 

6 

31 

16 

1 

20 

23 

8 

22 

4 

2 

26 

14 

17 

24 

TOTAL N 

(MGIL) 

5.77 

3.08 

3.21 

6.13 

2.39 

5.68 

3.30 

5.25 

3.42 

1.88 

1.46 

4.50 

2.20 

1.70 

2.73 

5.09 

6.35 

5.64 

6.56 

3.03 

2.93 

7.00 

5.83 

2.02 

3.40 

1.65 

2.10 

1.99 

1.06 

1.83 

1.78 

1.52 

1.86 

1.58 

2.30 

2.18 
PHILLIPPI CREEK AT FRUlNlLLE ROAD 

TOTAL P 

(MGIL) 

1.662 

1.634 

1.177 

1.733 

0.900 

1.861 

0.551 

1.007 

1.656 

0.561 

0.400 

0.520 

0 847 

0 513 

0.591 

0.555 

0.673 

2.071 

1.509 

1.535 

0.393 

0.696 

0.569 

1.002 

0.821 

1.298 

1.539 

0.728 

0 460 

0.462 

0.401 

0.307 

0.542 

YEAR 

75 

77 

77 

78 

78 

79 

80 

80 

81 

81 

82 

82 

82 

83 

83 

83 

83 

83 

84 

84 

84 

84 

84 

85 

85 

85 

85 

86 

86 

86 

86 

87 

87 

87 

87 

88 

88 

88 

88 

88 

89 

89 

STATION 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

24010626 

TOTAL N 

(MGIL) 

5.20 

4.38 

4.03 

4.54 

5.79 

9.04 

1.22 

7.34 

4.36 

1.63 

6.28 

1.14 

5.97 

3.64 

2.59 

3.46 

4.81 

8.64 

6.51 

9.71 

2.72 

10.54 

7.10 

1.14 

2.04 

2.09 

3.68 

4.22 

3.30 

4.31 

1.73 

1.36 

1.18 

0.99 

1 .OO 

0.69 

MONTH 

1 

11 

12 

2 

4 

10 

3 

12 

5 

9 

3 

6 

10 

1 

3 

5 

7 

9 

1 

3 

4 

8 

10 

1 

3 

5 

8 

1 

4 

7 

10 

1 

3 

8 

11 

5 

7 

10 

11 

12 

2 

5 

DAY 

10 

1 

6 

7 

11 

9 

10 

1 

18 

8 

1 

7 

5 

18 

7 

16 

18 

27 

3 

5 

9 

27 

15 

14 

18 

20 

12 

20 

21 

28 

20 

26 

2 

24 

2 

31 

26 

17 

29 

27 

27 

22 
PHILLIPPI CREEK AT 17TH STREET BRIDGE 

YEAR 

75 

77 

77 

78 

78 

79 

79 

79 

79 

80 

80 

80 

81 

81 

82 

82 

82 

83 

83 

83 

83 

83 

84 

84 

84 

84 

84 

85 

85 

85 

85 

86 

86 

86 

86 

87 

87 

87 

87 

88 

88 

88 

88 

88 

88 

89 

89 

MONTH 

1 

11 

12 

2 

4 

1 

3 

4 

10 

1 

3 

12 

5 

9 

3 

6 

10 

1 

3 

5 

7 

9 

1 

3 

4 

8 

10 

1 

3 

5 

8 

1 

4 

7 

10 

1 

3 

8 

11 

2 

5 

7 

10 

11 

12 

2 

5 

DAY 

10 

1 

6 

7 

11 

24 

5 

24 

9 

21 

10 

1 

18 

8 

1 

7 

5 

18 

7 

16 

18 

- 27 

3 

5 

9 

27 

15 

14 

18 

20 

12 

20 

21 

28 

20 

26 

2 

24 

2 

16 

31 

26 

17 

29 

27 

27 

22 

TOTAL N 

(MGIL) 

0.79 

1.01 

1.44 

0.59 

0.53 

1.97 

2.13 

1.29 

1.01 

1.25 

1.15 

1.40 

1.53 

3.29 

0.74 

0.83 

1.15 

1 .OO 

1.30 

0.83 

0.65 

0.74 

1.01 

1.23 

1.11 

0.96 

0.95 

1.33 

1.16 

3.19 

0.96 

0.96 

0.56 

1.13 

1.11 

1.22 

1.05 

0.94 

0.82 

------ STATION 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

24010627 

TOTAL P 

(MGIL) 

0.468 

0.51 1 

0.438 

0.498 

0.585 

0.532 

1.193 

0.579 

0.985 

0.420 

0.428 

0.451 

0.549 

0.791 

0.357 

0.208 

0.750 

1.426 

0.402 

0.306 

0.329 

0.416 

0.353 

0.418 

0.290 

0.350 

0.270 

0.672 

0.432 

0.445 

0.427 

0.681 

0.450 

0.440 

0.279 

0.480 

0.413 

0.420 

0.362 

0.421 

0.348 

0.659 
PHILLIPPI CREEK AT l7TH ST WEST BRIDGE 

TOTAL P 

(MGIL) 

0.976 

1.101 

0.945 

1.187 

1.568 

3.664 

1.489 

1.920 

0.845 

1.161 

0.874 

1.783 

1.447 

5.969 

0.716 

1.299 

0.661 

0.603 

0.562 

0.613 

0.794 

STATION 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

24010670 

DAY 

1 

21 

10 

3 

5 

9 

27 

15 

14 

18 

20 

12 

20 

21 

28 

20 

26 

2 

24 

2 

16 

26 

17 

29 

27 

27 

22 

YEAR 

79 

80 

80 

83 

84 

84 

84 

84 

85 

85 

85 

85 

86 

86 

86 

86 

87 

87 

87 

87 

88 

88 

88 

88 

88 

89 

89 

TOTAL N 

(MGIL) 

1.88 

9.71 

8.55 

8.34 

7.67 

16.32 

18.59 

13.14 

19.23 

12.82 

0.88 

3.06 

3.57 

6.64 

2.56 

5.61 

3.30 

2.20 

0.93 

0.91 

0.80 

1.31 

0.81 

MONTH 

10 

1 

3 

1 

3 

4 

8 

10 

1 

3 

5 

8 

1 

4 

7 

10 

1 

3 

8 

11 

2 

7 

10 

11 

12 

2 

5 
CLARKS LAKE DRAINAGE AT WlLKlNSON ROAD 

YEAR 

77 

79 

80 

80 

80 

81 

82 

82 

82 

83 

83 

83 

83 

83 

84 

84 

84 

84 

84 

85 

85 

85 

85 

86 

86 

86 

86 

87 

87 

87 

87 

88 

88 

88 

88 

88 

88 

89 

89 

TOTAL N 

(MGIL) 

1 .Ol 

1 .OO 

3.35 

2.70 

1.13 

1.40 

1.85 

2.38 

1.50 

2.13 

0.82 

0.93 

1.59 

1.36 

1.84 

0.69 

1.28 

1.29 

1.10 

1.18 

0.76 

1.26 

1.16 

1.39 

1.56 

1.46 

1.54 

1.39 

0.81 

0.95 

1.34 

1.28 

1.32 

1.38 

1.09 

MONTH 

10 

9 

1 

3 

12 

9 

3 

6 

10 

1 

3 

5 

7 

9 

1 

3 

4 

8 

10 

1 

3 

5 

8 

1 

4 

7 

10 

1 

3 

8 

11 

2 

5 

7 

10 

11 

12 

2 

5 

TOTAL P 

(MGIL) 

0.415 

0.414 

0.608 

0.451 

0.349 

0.617 

0.485 

0.256 

0.358 

0.492 

0.718 

0.265 

0.267 

0.297 

0.235 

0.438 

1.276 

0.214 

0.423 

0.413 

0.232 

0.305 

0.441 

0.450 

0.398 

0.199 

0.440 

0.433 

0.280 

0.321 

0.421 

0.286 

0.736 

DAY 

25 

24 

21 

10 

1 

8 

1 

7 

5 

18 

7 

16 

18 

27 

3 

5 

9 

27 

15 

14 

18 

20 

12 

20 

21 

28 

20 

26 

2 

24 

2 

16 

31 

26 

17 

29 

27 

27 

22 

STATION 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 

24010642 
MAIN A CNL AT CATTLEMANS ROAD BRIDGE 

YEAR 

74 

78 

78 

79 

80 

80 

80 

81 

81 

82 

82 

82 

83 

83 

83 

83 

83 

84 

84 

84 

64 

84 

85 

85 

85 

85 

86 

86 

87 

87 

87 

87 

88 

88 

88 

88 

88 

88 

89 

89 

MONTH 

11 

1 

4 

10 

1 

3 

12 

5 

. 9 

3 

6 

10 

1 

3 

5 

7 

9 

1 

3 

4 

8 

10 

1 

3 

5 

8 

7 

10 

1 

3 

8 

11 

2 

5 

7 

10 

11 

12 

2 

5 

DAY 

19 

24 

4 

1 

21 

10 

1 

18 

8 

1 

7 

5 

18 

7 

16 

18 

27 

3 

5 

9 

27 

15 

14 

18 

20 

12 

28 

20 

26 

2 

24 

2 

16 

31 

26 

17 

29 

27 

27 

22 

STATION 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

24010629 

TOTAL N 

(MGR) 

1.31 

2.04 

2.86 

1.53 

1.79 

1.52 

1.46 

1.65 

1.62 

0.87 

1.12 

0.81 

0.95 

1.59 

1.15 

1.39 

1.29 

2.72 

0.67 

1.52 

2.37 

1.30 

2.43 

2.29 

1.72 

1.49 

1.57 

0.67 

1.04 

1.20 

1.42 

1.33 

0.89 

2.71 

TOTAL P 

(MGR) 

0.150 

0.328 

0.310 

0.965 

0.606 

0.210 

0.553 

1.027 

0.458 

0.791 

0.602 

0.250 

0.177 

0.323 

1.273 

0.286 

0.185 

0.297 

0.510 

0.81 7 

0.619 

0.529 

1.022 

0.469 

0.628 

0.358 

0.820 

0.787 

0.240 

0.562 

0.341 

0.307 

0.446 
PHILLIPPI CREEK AT BAHlA VISTA STREET BRIDGE 

TOTAL P 

(MGIL) 

1.041 

1.109 

0.492 

1.520 

0.772 

1.888 

0.695 

0.420 

2.188 

1.111 

1.114 

0.642 

0.944 

0.583 

1 .I 74 

0.985 

0.653 

0.637 

0.673 

1.215 

1.857 

0.413 

0.754 

0.590 

1.002 

0.704 

0.942 

0.537 

1.099 

0.846 

0.801 

0.663 

0.822 

1.472 

1.646 

TOTAL N 

(MGIL) 

2.62 

2.73 

2.18 

2.78 

2.37 

1.70 

2.20 

0.91 

2.88 

5.03 

3.73 

2.57 

1.89 

4.38 

2.57 

3.47 

1.76 

2.43 

2.52 

2.84 

4.70 

2.93 

5.07 

1.53 

5.94 

4.60 

1.42 

2.36 

2.16 

3.55 

2.39 

2.77 

2.06 

2.63 

2.62 

3.23 

2.77 

3.16 

4.84 

6.19 

STATION 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

24010625 

DAY 

10 

1 

6 

7 

11 

24 

5 

24 

24 

10 

1 

18 

1 

7 

5 

18 

7 

16 

18 

27 

3 

5 

9 

27 

15 

14 

18 

20 

12 

20 

21 

28 

20 

26 

2 

24 

2 

16 

31 

26 

17 

29 

27 

27 

22 

YEAR 

75 

n 

77 

78 

78 

79 

79 

79 

79 

80 

80 

81 

82 

82 

82 

83 

83 

83 

83 

83 

84 

84 

84 

84 

84 

85 

85 

85 

85 

86 

86 

86 

86 

87 

87 

87 

87 

88 

88 

88 

88 

88 

88 

89 

89 

MONTH 

1 

11 

12 

2 

4 

1 

3 

4 

9 

3 

12 

5 

3 

6 

10 

1 

3 

5 

7 

9 

1 

3 

4 

8 

10 

1 

3 

5 

8 

1 

4 

7 

10 

1 

3 

8 

11 

2 

5 

7 

10 

11 

12 

2 

5 
MAIN A CNL AT BAHlA VISTA ST BRIDGE 

YEAR 

75 

79 

79 

79 

79 

80 

80 

80 

80 

80 

81 

82 

82 

82 

82 

83 

83 

83 

83 

84 

84 

84 

84 

84 

85 

85 

85 

85 

86 

86 

86 

86 

87 

87 

87 

87 

88 

88 

88 

88 

88 

88 

89 

89 

DAY 

10 

24 

5 

24 

24 

21 

21 

10 

10 

1 

18 

18 

1 

7 

5 

7 

16 

18 

27 

3 

5 

9 

27 

15 

14 

18 

20 

12 

20 

21 

28 

20 

26 

2 

24 

2 

16 

31 

26 

17 

29 

27 

27 

22 

MONTH 

1 

1 

3 

4 

9 

1 

1 

3 

3 

12 

5 

I 

3 

6 

10 

3 

5 

7 

9 

1 

3 

4 

8 

10 

1 

3 

5 

8 

1 

4 

7 

10 

1 

3 

8 

11 

2 

5 

7 

10 

11 

12 

2 

5 

STATION 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

24010628 

TOTAL N 

(MGIL) 

1.87 

1.51 

0.94 

0.29 

3.95 

4.41 

2.65 

2.54 

5.03 

2.14 

2.96 

2.00 

1.84 

2.39 

3.28 

1.18 

2.01 

2.73 

1.95 

3.55 

3.08 

5.89 

1.10 

4.89 

9.61 

1.70 

2.12 

2.34 

4.76 

2.48 

2.33 

2 13 

4.93 

2.24 

2.26 

3.22 

2.1 1 

2.77 

5.15 

TOTAL P 

(MGIL) 

0.632 

0.691 

0.705 

1.061 

1.061 

0.420 

0.420 

3.043 

0.761 

1.385 

0.894 

0.702 

0.832 

1.040 

2.874 

0.367 

0.435 

1.293 

1.374 

1.934 

0.948 

0.569 

1.282 

0.704 

0.754 

0.438 

1.959 

0.866 

0.380 

0.723 

0.421 

0.532 

1.937 
PHILLIPPI CREEK AT SARASOTA, FL 

TOTAL P 

(MGIL) 

2.500 

1 .OOO 

0.880 

2.600 

0.350 

1.600 

2.400 

0.700 

3.200 

0.510 

0.940 

0.430 

1.160 

0.750 

STATION 

2299800 

2299800 

2299800 

2299800 

2299800 

2299800 

2299800 

2299800 

2299800 

2299800 

2299800 

2299800 

2299800 

2299800 

2299800 

DAY 

14 

11 

6 

21 

13 

27 

2 

15 

14 

20 

1 

16 

2 

12 

8 

YEAR 

68 

70 

70 

71 

71 

72 

74 

74 

75 

75 

76 

76 

77 

77 

78 

TOTAL N 

(MGIL) 

3.04 

3.09 

1.50 

3.10 

1.70 

MONTH 

5 

6 

10 

4 

10 

4 

5 

10 

4 

10 

6 

9 

5 

10 

5 
MAIN A CNL AT PALMERS BLVD BRIDGE 

YEAR 

74 

77 

77 

78 

78 

79 

79 

79 

79 

80 

80 

80 

81 

81 

82 

82 

83 

83 

83 

83 

83 

84 

84 

84 

84 

84 

85 

85 

85 

85 

86 

86 

86 

86 

87 

87 

87 

87 

88 

88 

88 

88 

88 

88 

89 

89 

MONTH 

11 

11 

12 

1 

4 

1 

3 

4 

10 

1 

3 

12 

5 

9 

3 

6 

1 

3 

5 

7 

9 

1 

3 

4 

8 

10 

1 

3 

5 

8 

1 

4 

7 

10 

1 

3 

8 

11 

2 

5 

7 

10 

11 

12 

2 

5 

DAY 

19 

1 

6 

24 

4 

24 

5 

24 

1 

21 

10 

1 

18 

8 

1 

7 

18 

7 

16 

18 

27 

3 

5 

9 

27 

15 

14 

18 

20 

12 

20 

21 

28 

20 

26 

2 

24 

2 

16 

31 

26 

17 

29 

27 

27 

22 

STATION 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

24010630 

TOTAL N 

(MGIL) 

0.88 

1.90 

1.33 

0.95 

1.99 

0.54 

1.61 

2.51 

3.05 

0.75 

1.46 

1.51 

1.56 

1.03 

1.28 

0.89 

1.28 

1.61 

1.50 

1.29 

0.69 

0.57 

1.72 

0.84 

0.74 

1.10 

1.42 

3.07 

1.52 

1.73 

1.12 

4.35 

0.69 

1.07 

2.27 

1.60 

1.06 

0.48 

4.10 

TOTAL P 

(MGIL) 

0.160 

0.194 

0.527 

0.270 

0.417 

1.652 

0.681 

0.658 

0.886 

0.606 

0.252 

0.877 

0.291 

0.820 

0.148 

0.225 

0.209 

0.965 

0.224 

0.137 

0.697 

0.058 

0.389 

0.384 

0.155 

0.285 

0.461 

0.391 

0.251 

0.318 

0.240 

0.472 

0.1 60 

0.362 

0.140 

0.061 

0.484 
APPENDIX B 

TECHNICAL MEMORANDUM ON SEPTIC TANK LITERATURE REVIEW AND ANALYSIS 
MEMORANDUM 

TO : Mike Heyl, CDM/Sarasota 

FROM : Rich Wagner, CDM/Annandale 

SUBJECT: Literature Review of Septic Tank Loadings 

DATE : January 27, 1992 

INTRODUCTION 

CDM conducted a literature review to evaluate the pollutant 

loadings contributed by septic tank systems. Data gathered as part 

of this review include septic tank wastewater flow rates and 

effluent characteristics, pollutant removal mechanisms and 

efficiencies in drainfields, and delivery of loadings to surface 

waters. These data were used to assess the pollution impacts of 

septic tanks in the Sarasota Bay NEP study area. 

LITERATURE SEARCH 

Sources of information were obtained through a computerized 

literature review in conjunction with manual research of sources 

based on previous work. The initial computer search was conducted 

through the United States Geological Survey (USGS) on-line library 

system. Key words such as "septic tank","onsite disposal system", 

and "groundwater contamination" were used to identify potential 

sources of data in technical journals, textbooks, and conference 

proceedings. In addition, sources were identified by examining the 

literature review that is published annually by the Journal of the 

Water Pollution Control Federation (WPCF). The review includes a 

segment devoted specifically to on-site alternatives for treatment 

and disposal. 

After the USGS and WPCF listings had been reviewed, specific 

journal articles, reports and conference proceedings were obtained. 

These sources were reviewed to provide data for the septic tank 

loading assessment. In addition, the sources included other 

references that were reviewed in order to identify other apparent 

data sources, which were then obtained and reviewed. 

In all, over 50 articles and reports were reviewed as part of the 

septic tank loading evaluation. Those that were used in the 

evaluation are identified in the bibliography of this memorandum. 

DEFINITION OF LOADING PARAMETERS 

The data found during the literature was used to define a nuniber of 

loading parameters for the analysis. These parameters include the 

following: 
0 Typical effluent flow rate (gal/capita/day) 

o Typical effluent water quality characteristics (e.g., 

concentrations of various pollutants) 

o Typical removal processes and efficiencies that 

characterize effluent percolation through the soil 

o Processes occurring after the effluent reaches the 

groundwater and is ultimately transported to surface 

waters 

Of these parameters, effluent quantity and quality were documented 

in many studies. Removal processes during percolation are also 

well-documented, and efficiencies are listed or can be estimatedby 

analysis of monitoring data in the literature. Unfortunately, 

processes occuring between the drainfields and surface waters and 

estimation of surface water loadings due to septic tanks are not 

well-documented. However, several articles have considered the 

far-field impacts of septic tank loadings, and these were used to 

develop a methodology for estimating septic tank impacts for 

Sarasota Bay. 

Effluent Flow Rate 

Table 1 presents a listing of effluent flow rates in 

gallons/capita/day (gpcd) that were either measured or assumed by 

various authors. Typically, the values range between 40 and 80 

gpcd. Local water usage data for the study area indicate that the 

per capita wastewater flow rate for the study area is expected to 

be about 75 gpcd, which is at the high end of the identified range. 

Based on the local data, a value of 75 gpcd was selected as a 

typical flow rate for septic tank systems in the study area. 

Effluent Oualitv 

Water quality characteristics for septic tank effluent are 

presented in Table 2. These values were either measured or assumed 

by various authors. The table focuses on total nitrogen (total N) 

and total phosphorus (total P), because nutrient impacts are 

typically the major concern for septic tanks. For each source, the 

table indicates the number of samples and the average concentration 

for total N and total P. 

As shown in Table 2, both the concentration and the range of 

concentrations for total N is larger than for total P. Total N 

values in the literature ranged from 40 to 130 mg/L, with most of 

the values in the 40 to 80 mg/L range. For the purposes of this 

study, a mid-range value of 60 mg/L was selected for the total N 

effluent concentration. In contrast, the total P effluent 

concentratins ranged from 4 to 16 mg/L. A number of the 

observations fall between 13 and 16 mg/L, so a value of 15 mg/L was 
selected for the purposes of this study. 

Removal Processes and Efficiencies in Unsaturated Zone 

In the case of total PI the main removal process in the unsaturated 

soil zone between the septic tank and the water table is 

adsorption. Many articles have concluded that most or virtually 

all of the total P in the effluent will be captured in the 

drainfield, such that adverse water quality impacts on groundwater 

or surface waters are unlikely. Several articles have indicated 

that pollution due to total P may ultimately occur when the 

adsorptive capacity of the soil is exceeded, but this would require 

a substantial, long-term discharge. 

Removal of total N is not as complete nor as simple as that of 

total P. Of the total N, typically about 20 percent is in the 

organic form, and the remaining 80 percent is present as ammonia 

nitrogen. The organic nitrogen is likely to be removed during 

effluent percolation via filtration. The ammonia nitrogen, on the 

other hand, will primarily remain in solution rather than being 

removed through adsorption. In most cases, the ammonia nitrogen is 

completely converted to nitrate nitrogen in the drainfield, if 

aerobic conditions are present. Nitrate nitrogen is very mobile in 

solution, and very little nitrate removal would be expected in the 

drainfield under aerobic conditions. If part of the drainfield is 

anaerobic, however, and a sufficient carbon source is present, 

denitrification may occur, converting nitrate nitrogen to nitrogen 

gas. This would reduce the amount of nitrogen entering the 

groundwater and, ultimately, surface waters. 

Based on the literature data, it is difficult to precisely quantify 

the removal of total N and total P between the septic tank and the 

water table. In some cases, nutrient concentrations are reported 

in the soil near the septic tank, but the septic tank effluent 

concentrations are not specified. In other cases, a removal 

percentage is defined without specifying the effluent and soil 

concentrations. Also, the removal efficiency may vary at a 

particular site depending upon the conditions at the time of the 

monitoring. 

After a review of the data, it was assumed that the concentrations 

of total N and total P after percolation to the water table are 30 

mg/L and 2 mg/L, respectively. Given the assumed total N effluent 

concentration of 60 mg/L, the water table concentration of 30 mg/L 

presumes that 50 percent of the total N is removed as the effluent 

percolates through the soil. This concentration and percent 

removal are consistent with the results presented by Andreoli & 

- al. (1980), Walker et a1. (1973a), Ellis (1983), and Cogger et al. 

(1988). For total PI an assumed water table concentration of 2 

mg/L implies a removal of about 90 percent during the effluent 
percolation, based on the assumed effluent concentration of 15 

mg/L. Numerous studies, such as Jones and Lee (19791, Gilliam and 

Patmont (1983), Reneau et al. (1989) and Sawhney and Starr (1977) 

have concluded that 90 percent or more of the total P remains in 

the soil. Thus, both of these assumed values are reasonable with 

respect to findings of other studies. 

Surface Water Impacts 

Though most of the focus in the literature has been on effluent 

quality and groundwater concentration in the immediate vicinity of 

individual septic tanks, several studies have analyzed the 

potential extent of large-scale septic t'ank impacts on groundwater 

quality and surface water quality. Surface water studies typically 

used monitoring data to identify areas with septic tank loadings, 

and to quantify the loadings, whereas groundwater studies typically 

developed a modeling framework to assess loadings. Several of the 

modeling studies are described below. 

Bauman and Schafer (1984) have developed a method of estimating 

groundwater impacts for nitrogen based on a mass balance approach. 

The approach requires an evaluation of the water budget and the 

nitrogen budget within the surficial aquifer. Water budget and 

nitrogen budget components include groundwater flow, natural 

recharge and septic system effluent. By determining the volume of 

water from each source and defining a nitrogen concentration for 

each source, the concentration of the combined sources can be 

calculated. Groundwater flow is calculated by determining aquifer 

thickness, width, hydraulic conductivity and hydraulic gradient, 

and groundwater concentration is based on local monitoring data. 

Similarly, recharge volume and concentration is determined from 

local data, and septic tank data are estimated based on population 

density and typical effluent characteristics. 

A transport model for pollutants in the water table aquifer has 

been developed by Anderson et a1 (1987) as part of the Florida 

Onsite Disposal System (OSDS) project. The model is more complex 

than that of Bauman and Schafer, simulating transport as a steady, 

one-dimensional flow field with three-dimensional dispersion, 

retardation, and first-order decay. Though more complex, required 

data for this model are similar to those required for the mass 

balance model, including septic tank flows and .loadings, as well as 

aquifer hydraulic conductivity and hydr'aulic gradient. 

Dispersivity factors are also required to quantify the dispersion 

of the effluent down gradient of the source. 

A third approach to modeling surficial aquifers affected by septic 

tank effluent was presented by Ostendorf (1986). This model 

simulates the near field mixing and- routing of effluent and 

groundwater as a linear reservoir, and simulates far field 

pollutant concentrations based on a first order decay rate. As 

with the other models, the essential data include effluent flow, 
effluent water quality characteristics, and aquifer characteristics 

(e.g., hydraulic conductivity, hydraulic.gradient, thickness). 

The Ostendorf model was applied at the Otis Air Force Base in 

Massachusetts, which has discharged an average effluent flow rate 

of 0.53 mgd to its drainfields since 1941. Concentrations of 

chloride, boron, total N and methylene blue active substances 

(MBAS) were measured in the groundwater at various distances 

downgradient of the drainfields. Using the input data described 

above, the model was tested first for chloride, a conservative 

constituent, to determine if the model transport simulation was 

accurate. The results showed an excellent agreement between 

predicted and measured concentrations downgradient of the 

drainfields. Then, the model was applied for the other 

constituents, including the calibration of total N and MBAS by the 

evaluation of a first order decay rate. 

After reviewing these modeling approaches, a methodology for 

estimating septic loads that reach receiving waters was developed. 

The methodology uses some of the components of these models. The 

development of the methodology is discussed in the next section. 

MODELING METHODOLOGY 

Calculation of septic tank loadings to surface waters is performed 

within the NPS spreadsheet model. The existing model already 

includes a method for calculating loads due to failing septic 

tanks. As part of the Sarasota Bay NEP Study, the model has been 

modified to include the calculation of loads due to septic tanks 

that are not failing. The calculation method for both failing and 

working septic tanks are presented below. 

Failina Septic Tanks 

Nutrient concentrations for failing septic tanks were established 

based on the literature review. It was assumed that the 

concentrations for a failing septic tank are equivalent to the 

concentrations established for the effluent at the water table. 

These were as follows: 

o Total N 30 mg/L 

o Total P 2 mg/L 

The values reflect pollutant removal within the soil of roughly 50 

percent for total N and 90 percent for total P. 

Nutrient loadings for specific land uses were calculated by 

multiplying the concentrations by a flow rate. The flow rate for 

a particular land use depended upon the number of residents per 

acre, and the per capita flow rate. 
Table 3 shows the septic tank flow rates developed for various land 

uses. For all land uses, a per capita flow rate of 75 gallons per 

day was established. For the residential areas, values of dwelling 

units per acre were determined based on the descriptions of various 

land uses in the Sarasota County Comprehensive Plan and densities 

observed in aerial photographs. Each dwelling unit is assumed to, 

have 2.0 people, based on household size estimates for Sarasota and 

Charlotte Counties and the City of North Port, as cited in the City 

of North Port Comprehensive Plan. This value is somewhat lower 

than the national average, because some of the population is 

seasonal and because the population distribution is weighted more 

heavily toward retired persons who would tend to have a smaller 

household size. For commercial and industrial areas, the flow 

values were assumed equal to the highest residential value. The 

literature indicates that commercial and industrial flows can vary 

widely, but it was assumed that any large industrial or commercial 

wastewater source would be served by sewers. 

A final consideration in the loading analysis for failing septic 

tanks is the failure rate - that is, what percentage of the tanks 

are failing. These data were established based on permitting data 

from the Sarasota County Health Department. The Health Department 

requires a permit for repair or replacement of septic tanks. 

During the period 1980 through 1990, an average of 740 repairs per 

year was recorded. Based on the estimate of 45,000 septic tanks in 

the County, the annual repair rate is 1.6 percent. Recognizing 

that failure may occur for a number of years before repairs are 

initiated, the failure rate at any time is likely to be higher than 

1.6 percent. In previous studies, the annual failure rate has been 

multiplied by a factor of 5, which implies that septic systems on 

average fail for 5 years before repairs are made. For Sarasota 

*County, the resulting failure rate is 8 percent. This value 

compares favorably with the results of a septic tank survey 

conducted in Jacksonville, FL by the Department of Health and 

Rehabilitative Services. In the study, an inspection of more than 

800 sites revealed about 90 violations, or a failure rate of 12 

percent. Failure calculations also were done specifically for 

Casey Key, which had a higher annual repair rate. The failure rate 

established for Casey Key was 20 percent. 

Workins Septic Tanks 

The surface water loading due to effluent from working septic tanks 

is calculated based on the modeling frameworks that were found in 

the literature (Bauman and Schafer, 1984; Ostendorf, 1986) . The 

model assumes that a load of total N and total P enters the 

surficial aquifer below the septic tank after percolating through 

the unsaturated soil zone. The load then travels along with the 

surficial aquifer flow, subject to a first order decay rate that 

reduces the mass over time. The load that is left when the 

surficial aquifer reaches a permanent water body is assumed to 

discharge to the water body. 
Figure 1 shows the theoretical basis of the septic tank analysis. 

It is assumed that recharge of the surficial aquifer occurs at a 

constant rate (W) . Under steady flow conditions, this recharge 

rate is equivalent to the discharge rate in the receiving stream, 

assuming no leakage to the lower aquifer layers. By determining 

factors such as the length (L), the aquifer thickness at the 

receiving stream ha, and the hydraulic conductivity of the 

aquifer, groundwater flow equations can be used to calculate the 

water table height and flow velocity at various distances from the 

receiving stream. The velocity values can then be used to 

determine travel times to the receiving stream. The travel time, 

combined with the first order decay rate, determines the fraction 

of septic tank loading that reaches the receiving stream. 

Equations developed by Dupuit (Todd, 1985) were used to perform the 

groundwater flow and velocity calculations. These equations have 

been used in models such as DRAINMOD to simulate subsurface water 

movement into drain tubes or ditches. One of the basic equations 

used in the analysis is 

where 

x = distance from the point of maximum water table 

elevation (ft) 

q, = groundwater flow per unit of stream length 

(ft2/day) 

W = groundwater recharge (ft/day) 

In the septic tank analysis, the value of W was calculated as the 

sum of the natural recharge determined by the hydrologic analysis 

plus the flow contributed by septic tanks. For the areas with 

septic tanks, an average value of 7 inches/year (0.0016 ft/day) was 

calculated. Using equation (I), the flow rate at any distance from 

the point of maximum water table elevation can be calculated. The 

other basic equation is 

where 

h, = water table height at distance x from maximum 

- water table elevation (ft) 

ha = water table height at receiving stream (ft) 

K = hydraulic conductivity of aquifer (ft/day) 
L = distance from point of maximum water table 

elevation to centerline of stream (ft) 

This equation can be used to calculate water table height at any 

point between the stream and the location of the maximum water 

table elevation. These two equations were used to develop a third 

equation, that determines the velocity at various distances from 

the receiving stream. The equation is as follows: 

where 

V, = velocity at distance x from location of maximum 

water table elevation (ft/day) 

P = porosity of surficial aquifer 

Velocity is an important parameter in the analysis because it 

influences the travel time from the septic tank to the surface 

water, which in turn affects the extent of pollutant decay en route 

to the surface water. 

The aquifer values established for the Sarasota Bay study area are 

presented in Table 4. The values represent an average of the data 

obtained at a number of monitoring sites in the vicinity of the 

study area. Values for aquifer thickness ha, porosity (P) and 

conductivity (K) are based on review of the Soil Survey for 

Sarasota County and a Southwest Florida Water Management District 

(SWFWMD) report on groundwater availability in Sarasota County 

(SWFWMD, 1988) . The maximum distance value (L) of 3,000 feet was 

selected based on inspection of USGS quadrangle sheets covering the 

study area, and the recharge value of 7 inches/year was based on 

the hydrologic simulation. 

Travel times at distances up to 1000 feet from the receiving stream 

are shown in Figure 2. The cumulative travel times shown in the 

figure were developed in a spreadsheet which generates values at 

points established at 10 foot increments, from 0 to 3000 feet from 

the stream. At each point, the spreadsheet calculates the 

velocity, and then determines the travel time across the 10-foot 

distance between that point and the previous point. By summing 

travel times from consecutive 10-foot intervals, cumulative travel 

times are also calculated. Thus, at any point between 0 and 3000 

feet from the stream, the total travel time to the stream is 

determined. Figure 2 shows that the travel distance in one year 

ranges from 60 to 110 feet, with higher velocities in the areas 

closest to the stream. These travel times translate into 

velocities of 0.16 to 0.30 feet per day. 
Assuming a first order decay rate, the reduction in pollutant mass 

as a function of travel time is determined by the following 

equation : 

where 

f = fraction of pollutant removed after time t 

k = first order decay coefficient (l/days) 

t = time since the effluent entered the surficial 

aquifer (days) 

This equation was also included in the spreadsheet to determine the 

pollutant reduction associated with each 10-foot increment. In 

this way, the fraction of pollutant delivered at various distances 

from the stream were determined. In addition, an overall average 

delivery ratio was established by averaging the delivery ratios at 

each 10-foot increment. 

APPLICATION OF MODEL 

The methodology presented above was incorporatedinto a spreadsheet 

model that calculates pollutant loadings from surface runoff, 

baseflow and point sources as well as septic tank loadings. This 

model was used to estimate loadings at two water quality stations 

located on Phillippi Creek. In estimating septic tank loadings, 

initial model runs were conducted assuming the first order decay 

rate of 0.00015/day that was established in the Ostendorf study. 

The results indicated a very high delivery of working septic tank 

loadings to the receiving streams, which could not be verified by 

analysis of stream water quality monitoring data in the study area. 

Subsequently, the first order decay rate was adjusted upward so 

t h a t t h e loadings were more consistent with observed instream water 

quality. Once an appropriate decay rate was established, the model 

was applied to each watershed in the study area to determine the 

relative significance of septic tank loadings. 

Comparison of Observed and Simulated Instream Water Oualitv 

The methodology and first-order decay rate were evaluated by 

comparing observed and simulated water quality data at two 

monitoring stations in the Phillippi Creek watershed. The STORET 

stations used in the analysis were the Main A Canal at Palmers 

Boulevard Bridge (Station 24010 630) and Philrippi Creek at Bahia 

Vista Street Bridge (Station 24010625). The Palmers Boulevard 

Bridge station has a drainage area of about 8 square miles, which 

consists primarily of cropland, rangeland, and low and medium 

density single family residential land uses. 89 percent of the low 
density and 66 percent of the medium density residential areas are 

served by septic tanks, and there are no substantial point source 

loads. The Bahia Vista Street station has a drainage area of 44 

square miles, comprised mainly of cropland, rangeland, open space, 

and low and medium density residential land uses. 53 percent of 

the low density, 41 percent of the medium density and 37 percent of 

the high density residential areas are served by septic tanks. 

Unlike the Palmers Boulevard station, the water quality at the 

Bahia Vista Street station has been impacted by the Atlantic 

Utilities and Kensington Park Utilities point sources, which had 

been directly discharging secondary effluent to Phillippi Creek 

during the STORET period of record. It is possible that other 

smaller point sources may also have been discharging directly, 

rather than through percolation ponds and irrigation, during the 

period of record. 

The results of the water quality analyses for the two monitoring 

stations are presented in Table 5. The table includes results for 

three different first-order decay rates, which are as follows: 

o The initial Ostendorf decay rate of 0.00015/day 

o Decay rate of 0.00170/day based on assumed removal via 

percolation ponds and irrigation 

o Decay rate of 0.00055/day based on calibration of Bahia 

Vista Street station loadings 

For each decay rate, the results include the overall percent 

delivery from septic tanks, and a comparison of observed and 

simulated instream concentrations on an annual and seasonal basis. 

The overall percent delivery includes the impacts of both failing 

and working septic tanks, and reflects the fraction of nitrogen 

loading that is transported from the drainfield to the receiving 

water. The methodology used to determine the simulated and 

observed instream concentrations are described below. 

The simulated instream concentrations were determined by 

calculating loadings from septic tanks, surface runoff, point 

sources and baseflow for an averase rainfall year, divided into a 

wet season (June through September) and a dry season (October 

through May) . The calculations were conducted by using a 

spreadsheet model developed by CDM, which has been applied in a 

number of water quality studies in Florida and other states. The 

model uses long-term precipitation and streamflow data, along with 

runoff coefficients for pervious and impervious area, to determine 

the relationship between rainfall, surface runoff and baseflow. 

Loadings associated with surface runoff are calculated by using 

event mean concentration (EMC) values. The EMC represents the 

flow-weighted concentration of pollutant in surface runoff during 

a typical storm event. Distinct EMC values are specified for 

different types of land use (e.g., residential, commercial, 
agricultural) . Baseflow loadings are calculated assuming a 

constant baseflow concentration. Point source loadings are 

included by specifying wastewater flows and concentrations, and 

septic tank loadings are calculated using the newly developed 

methodology described in this memorandum. The results of the 

modeling include total loading (e.g., pounds of nitrogen) and the 

total runoff depth (e-g., inches of flow distributed over the 

drainage area) for the modeling period. Given these output values 

and the size of the drainage area, a flow-weighted instream 

concentration can be calculated. 

Because the simulated results include a flow-weighted instream 

concentration, the monitoring data were also analyzed to provide 

flow-weightedinstream concentrations. The concentration data were 

sorted by month, and an average concentration was calculated for 

each month. Monthly flows were estimated using the same long-term 

streamflow records used in the spreadsheet model, and the 

percentage of annual flow attributedto each month was established. 

Usingthis information, a flow-weighted concentration was developed 

on an annual and a seasonal basis. 

As shown in Table 5, nitrogen concentrations are substantially 

oversimulated when the Ostendorf decay rate was used. At the 

Palmers Boulevard station, the average annual total N concentration 

was oversimulated by 49 percent, and both wet season and dry season 

concentrations were oversimulated by more than 40 percent. The 

oversimulation at the Bahia Vista Street station was only 11 

percent for the annual average analysis, but was 23 percent for the 

wet season analysis. Sensitivity analyses indicated that higher 

first-order decay rates would still result in reasonable total N 

predictions at the Bahia Vista Street station, while improving the 

predictions at the Palmers Boulevard station. Consequently, higher 

decay rates (which would result in a lower delivery ratio) were 

investigated. 

Using a decay rate of 0.00170/day produces results that appear to 

be more reasonable that those developed using the Ostendorf rate. 

Based on the typical surficial aquifer velocity, this decay rate 

translates into 20 percent delivery (i.e., 80 percent removal) for 

sources at a distance of 300 feet from the receiving water. 

Together with the previous assumption of 50 percent total N removal 

in the unsaturated soil above the water table, the overall delivery 

is reduced to 10 percent. This value is consistent with the 90 

percent removal value assumed for percolation ponds, which in many 

cases are located only a few hundred feet from receiving waters. 

The simulation results indicate an oversimulation of total N at the 

Palmers Boulevard station and an undersimulation of total N at the 

Bahia Vista Street station for the annual average analysis. 

However, both sets of simulated results are within 20 percent of 

the observed values. Given the uncertainty associated with the 

monitoring data and the calculation of the surface runoff, baseflow 
and point source loads, these results appear to be reasonable. The 

largest discrepancy between simulated and observedvalues occurs in 

the dry season analysis for the Bahia Vista Street station. During 

the dry season, the loading due to surface runoff is less 

significant, whereas point sources and septic tanks account for a 

greater percentage of the overall loading. Thus, one possible 

explanation of the total N undersimulation is that the point source 

loading during the monitoring period was underestimated. As 

mentioned earlier, two relatively large plants were discharging 

directly to Phillippi Creek, and it is possible that some of the 

smaller plants were also discharging directly to the creek at that 

time . In addition, the Florida South 'Gate plant discharges 

directly to the creek downstream of the Bahia Vista Street 

station, and some of that loading may influence the water quality 

at the station due to dispersion and tidal transport. 

Alternatively, the undersimulation may indicate that the loading 

due to septic tanks has been underestimated. 

A third set of results was generated in which the decay rate was 

established such that the simulated and observed total N 

concentration at the Bahia Vista Street station matched for the 

average annual simulation. Through a calibration process, a decay 

rate value of 0.00055/day was established. The corresponding 

overall delivery from the drainfield to the receiving water is 20 

percent, which is one-half of the delivery ratio calculated using 

the Ostendorf decay rate and twice the delivery ratio calculated 

using the decay rate of 0.00170/day. 

The results show that there is good agreement between simulated and 

observed values at the Bahia Vista Street station, but that total 

N is oversimulated at the Palmers Boulevard station. As discussed 

above, the annual average simulated results match the observed 

values because that was the basis of the decay rate calibration. 

The results from the seasonal analyses for the Bahia Vista Street 

station shgw simulated total N concentrations that are somewhat 

higher than observed values during the wet season, and somewhat 

lower than observed values during the dry season. However, in both 

cases the difference between simulated and observed values is less 

than 20 percent. Given the uncertainty associated with the 

monitoring data and the calculation of the surface runoff, baseflow 

and point source loads, these results appear to be reasonable. For 

the Palmers Boulevard station, there is an oversimulation of total 

N for the annual and seasonal analyses. In all cases, the 

simulated loads are about 20 percent higher than the observed 

loads. 

Due to the uncertainty in estimating loads and measuring instream 

concentrations, it is impossible to determine the loading due to 

septic tanks with any certainty. The results using the Ostendorf 

decay rate appear to generate excessive total N loadings from 

septic tanks. The results based on the other two decay rates both 

appear to give a reasonable estimate of septic tank loadings. The 
rate of 0.00170/day seems reasonable for the annual and seasonal 

analyses, with the percent difference between simulated and 

observed total N concentrations ranging from +14 to -20. In 

comparison, the results for the decay rate of 0.00055/day show 

percent differences ranging from +26 to -16. The overall septic 

tank delivery ratio for the two alternative decay rates are 10 

percent and 20 percent, respectively. Thus, based on the analysis 

described above, a delivery ratio of 10 to 20 percent is assumed to 

be representative for septic tank loadings in the study area. 

In the final calculation of study area loadings, the more 

conservative delivery ratio value of 20 percent was selected. 

Because the relatively high delivery ratio was selected, it is 

likely that the impacts of septic tank loadings will not be 

underestimated, and in fact may be overestimated. 

Average Annual Loadings for Studv Area Watersheds 

The average annual loading results for the watersheds in the study 

area are shown in Table 6. Because nutrients are the major concern 

for septic tanks, total P and total N results are included in the 

table. The results are shown separately for those watersheds that 

do have septic tanks, whereas the watersheds without septic tanks 

have been combined into one category. For each watershed, the 

table lists the total loading in pounds attributed to septic tanks, 

the total loading from all sources, and the percentage of the total 

loading attributable to septic tanks. The total loading includes 

septic tanks plus surface runoff, baseflow, point sources, and 

precipitation falling directly onto the Bay surface. 

The results presented in Table 6 indicate that septic tanks account 

for only a small fraction of the total study area loadings, but 

that the impact of septic tanks varies between watersheds. 

Overall, septic tanks account for 3.3 percent of the total P 

loading and 9.6 percent of the total N loading to the Bay. For 

total P, the septic tank contribution is less than the contribution 

of any of the other four pollution sources (surface runoff, 

baseflow, point sources and rainfall). For total N, the septic 

tank contribution is less than that of surface runoff and rainfall, 

but is slightly higher than the contribution due to point sources 

or baseflow. Watersheds that have a relatively high percentage of 

septic tank contribution include Phillippi Creek, Matheny Creek and 

the areas along the west coast that drain directly to the Bay. 

Phillippi Creek alone accounts for 71 percent of the septic tank 

loading for total N. 

SUMMARY 

CDM conducted a literature review to evaluate the pollutant 

loadings contributed by septic tank systems. Data gathered as part of this review included septic tank wastewater flow rates and 
effluent characteristics, pollutant removal mechanisms and 

efficiencies in drainfields, and delivery of loadings to surface 

waters. Methods for estimating pollution loadings due to working 

and failing septic tanks were developed, based on previous studies 

and the literature review. When these methods were applied to the 

Sarasota Bay study area, the results indicated that the septic tank 

loadings may have significant local water quality impacts, but do 

not appear to be a major factor i n t o t a l pollution loadings to the 

Bay. 
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16 
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TABLE 1 

FLOW RATES FOR SEPTIC TANK EFFLUENT 

TABLE 2 

NUTRIENT CONCENTRATIONS IN SEPTIC TANK EFFLUENT 

Reference 

Waller et al., 1987 

Sawnhey and Starr, 1977 

Cogger and Carlile, 1984 

Starr and Sawhney, 1979 

Alhaggar et al., 1989 

Andreoli et al.. 1979 

Piluk and Hao, 1989 

Reneau et al., 1989 

Brandes, 1978 

Reddy and Dunn, 1984 

Flow Rate 

(gal/capita/day) 

75 

50 

50 

50 

36-47 

60 

66 

45 

41 -45 

40-80 

Notes: 

1. n = number of samples 

2. NIA = not available 

Reference 

Waller et al., 1987 

Walker et al., 1973 

Whelan and Titmanis, 1981 

Gilliam and Patmont, 1983 

Alhaggar et al., 1989 

Andreoli et al., 1979 

Piluk and Hao, 1989 

USEPA, 1978 

Reneau et al., 1989 

Cogger et al., 1988 

Total N Total P 

n 

5 

NIA 

NIA 

54 

357 

N/A 

NIA 

NIA 

N/A 

32 

n 

5 

NIA 

NIA 

NIA 

356 

NIA 

NIA 

NIA 

NIA 

32 

(mg/l) , 

4.1 

NIA 

15 

15 

7.9-14 

NIA 

8.3 

13 

16 

6.2 

(mgll) 

39 

80 

100 

NIA 

73 

61.3 

69.4 

45 

40-80 

28 
TABLE 3 

SEPTIC TANKS FLOW RATES FOR VARIOUS LAND USES 

TABLE 4 

PARAMETERS FOR ANALYSIS OF WORKING SEPTIC TANKS 

Land Use 

LDSF Residential 

MDSF Residential 

HDSF Residential 

Multi-family Bldg 

Mobile Home 

CommercialIServices 

Institutional 

Industrial 

Dwelling Units 

Per Acre 

1 

4.5 

7.5 

9 

9 

- 

Source 

SWFWMD. 1988 

SWFWMD, 1988 

SWFWMD, 1988 

FDER 

USGS maps 

Parameter 

Aquifer Conductivity 

Aquifer Thickness 

Porosity 

Minimum Travel Length 

Maximum Travel Length 

Value 

8.0 inlhr 

60.0 ft 

0.3 

75 ft 

3000 ft 

Flow Rate 

(gavacrelday) 

150 

675 

1125 

1350 

1350 

1350 

1350 

1350 

Persons Per 

Dwelling unit 

2 

2 

2 

2 

2 

- 

- 

Per Capita 

Wastewater 

Flow Rate 

(gaVday1person) 

75 

75 

75 

75 

75 

- 

- 
TABLE 5 

SIMULATED AND OBSERVED TOTAL N DATA 

PALMERS BOULEVARD AND BAHlA VISTA STREET STATIONS 
TABLE 6 

AVERAGE ANNUAL LOADINGS FOR STUDY AREA WATERSHEDS 

EXISTING LAND USE CONDITIONS 


APPENDIX C 

STATISTICAL ANALYSIS OF BARRIER ISLAND AND MAINLAND RAINFALL DATA 
APPENDIX C 

STATISTICAL ANALYSIS OF BARRIER ISLAND AND MAINLAND RAINFALL DATA 

The climate in the Sarasota Bay National Estuary Program study area is 

subtropical, with a mean daily temperature of about 78 degrees. The weather 

is influenced by latitude, low elevation, wind patterns and proximity to the 

Gulf of Mexico and to Sarasota Bay. Average annual rainfall for the study 

area is about 54 inches, with approximately 60 percent of the total 

precipitation falling during the summer months of June through September. 

The summer rainfall patterns typically consist of short duration, intense 

convective thunderstorms occurring in the late afternoon. Rainfall in the 

winter months is generally associated with cold fronts moving across the 

region. Compared to summer rainfall events, winter rainfall events usually 

have a longer duration and lower intensity. 

Typically, continuous monthly rainfall totals in excess of 25-30 years period 

of record are used in estimating annual, or seasonal non-point source 

pollutant loadings. Of the local rain gage data available, data from only 

one station (Bradenton ESE) meets the criteria, although except for a 15 

month gap in record keeping, the Myakka River rainfall record meets the 

duration criteria. The Bradenton station is located in the upper 

northeastern portion of the study area. It was felt that use of this gage 

alone was insufficient to represent the rainfall over the entire study area, 

and the Myakka gage record was extrapolated to provide gage data in the 

southern portion of the study area. 

There are two rain gages within the Myakka River State Park. The Myakka River 

NWS (National Weather Station # 6065-04) rainfall record is lacking three 

consecutive months in 1966, and all of 1967. In order to obtain a long-term 

rainfall record in the southern portion of the NEP study area, the 1966 

rainfall records for the two stations for the nine months were.compared, and 

deemed sufficiently close to allow substitution of the missing three months 

in the NWS station. 

Except for 1967, the Bradenton ESE and Myakka NWS station data were averaged 

for each month of the period of record, and annual totals derived. The 

Bradenton ESE rainfall totals were used for the year of 1967. 

The question of dissimilar distribution, and rainfall totals between the 

seaward barrier islands and the inland rainfall totals was investigated. 

Monthly, and annual rainfall totals from several local rain gages, including 

three stations located on barrier islands were compiled and statistically 

compared with four inland gages and one composite rainfall record. Rainfall 

totals were obtained as hard copy from the South West Florida Water 

Management District for the following stations : 

Station Name 

Sarasota Bradenton 

Cutter 

Siesta Key 

Tallevast 

Approximate Location 

Airport 

South Longboat Key (barrier island) 

North Siesta Key (barrier island) 

US 301 North of Airport 
Rainfall data for City Island was obtained from Mote Marine Laboratory as 

daily digital data, and was totalled by month and year. Myakka River State 

Park was obtained as daily totals from National Climatic Data Center (NOAA) 

in digital form from 1957-1985, and totalled by month and year. This data was 

supplemented with climatological bulletins published by National Climatic 

Data Center (NCDC) . Bradenton ESE (NWS station 0945-04) rainfall totals were 

also taken from climatological bulletins. Data prior to 1981 were taken from 

microfiche of earlier bulletins. 

Arithmetic averages of monthly totals (except as previously noted) were 

derived from the Myakka NWS and Bradenton ESE stations, and are designated as 

- BRAdenton-Easka (BRAMYK) . 

The descriptive statistics of each rainfall record are given below in Box 1. 

In accordance with USEPA (EPA-600/4-82-029) guidelines, all stations except 

Tallevast and Bradenton appear to be normally distributed (coefficient of 

skewness < 0.7). These two stations were deleted from further parametric 

evaluation and subjected to non-parametric tests. 

Box 1 

years df mean s.d. var . skew kurt. 

CITY ISL. 9 8 48.20 10.20 104.05 0.6361 2.987 

MYAKKA 4 6 45 55.70 11.56 133.65 0.2009 2.238 

CUTTER 10 9 50.14 11.04 121.98 0.2345 2.026 

SARBRA 10 9 49.52 9.40 88.41 -0.5432 2.692 

SIESTA 12 11 49.31 8.78 77.12 0.3334 2.721 

TALLY 39 38 58.04 15.25 232.64 0.8702 4.479 

BRADENTON 51 50 54.54 11.30 127.76 0.7224 4.314 

BRADtMYAK 4 6 45 55.48 7.95 63.28 0.4167 2.299 

Prior to using parametric means tests (Student's 't' test), the equality of 

variance must be established. The 'F' ratio was used to determine if the 

variance of the station pairs were statistically similar. The results, 

given in Box 2, indicate that the following station pairs had statistically 

different variances (two-tailed significance < = 0.05): Myakka/Tallevast, 

Siesta/Tallevast, Bradenton/Tallevast, BRAMYK/Tallevast, BRAMYK/Bradenton 

and BRAMYK/Myakka. Non-parametric tests were conducted for these station 

pairs. 
Box 2 

two-tailed 

Stations Compared F df/df signif. 

City Is1 vs. Myakka 1.2845 45/8 0.3764 

City Is1 vs. Cutter 1.1723 9/8 0.4166 

City Is1 vs. SarBra 1.1769 8/9 0.4036 

City Is1 vs. Siesta 1.3492 8/11 0.3151 

City Is1 vs. Tally 2.2358 38/8 0.1154 

City Is1 vs. Bradenton 1.2279 50/8 0.4066 

City Is1 vs. BRAMYK 1.6443 8/45 0.1392 

Myakka vs. Cutter 1.0957 45/9 0.4771 

Myakka vs. SarBra 1.5117 45/9 0.2614 

Myakka vs. Siesta 1.7330 45/11 0.1629 

Myakka vs. Tally 1.7407 38/45 0.0376 

Myakka vs. Bradenton 1.0461 45/50 0.4366 

Myakka vs. BRAMYK 2.1120 45/45 0.0068 

Cutter vs. SarBra 1.3797 9/19 0.2642 

Cutter vs. Siesta 1.5817 9/11 0.2335 

Cutter vs. Tally 1.9072 38/9 0.1524 

Cutter vs. Bradenton 1.0474 50/9 0.5123 

Cutter vs. BRAMYK 1.9276 9/45 0.0720 

SarBra vs. Siesta 1.1464 9/11 0.4085 

SarBra vs. Tally 2.6314 38/9 0.0625 

SarBra vs. Bradenton 1.4451 50/9 0.2873 

SarBra vs. BRAMYK 1.3971 9/45 0.2180 

Siesta vs. Tally 3.0166 38/11 0.0267 

Siesta vs. Bradenton 1.6566 50/11 0.1833 

Siesta vs. BRAMYK 1.2187 11/45 0.3028 

Tally vs. Bradenton 1.8209 38/50 0.0236 

Tally vs. BRAMYK 3.6764 38/45 0.0000 

Bradenton vs. BRAMYK 2.0190 50/45 0.0091 

Station pairs consisting of normally distributed data and equivalent 

variances were then evaluated for statistical similarity of annual means, 

without pairing of occurrence. The null hypothesis evaluated was that 

there is no difference in the annual mean rainfall totals. The results are 

given in the top half of Box 3, and indicate that twelve pairs were 

statistically equivalent at, or above the 0.05 significance level, and two 

pairs were not equivalent at the 0.05 level. 

Next, the station pairs were subjected to a 't' test of paired annual 

means. The results are illustrated in the bottom half of the previous 

box. Station pairs with significance levels less than 0.05 are considered 

statistically different. Eight were found to be different, and six pairs 

were found to be statistically equivalent at the 0.05 significance level. 

For station pairs that could not be evaluated with parametric techniques 

due to non-normality, or unequal variances, a Mann-Whitney test of 

similarity was employed. The results for annual, unpaired rainfall totals 

are given in the lower portion of Box 3. The results indicate that none of 
the annual mean comparisons excluded from parametric testing were 

significantly different (at the 0.05 significance level). 

Box 3 

two-sided significance level: unpaired t-test of annual means 

2 3 4 5 6 7 8 

0.0568 0.6956 0.7745 0.7970 /<non-normal>/ 0.0566 

0.1620 0.0787 0.0417 /<non-normal>/ (F-test) 

It' 3 0.2735 0.0012~ , 0.8930 0.8490 Icnon-normal>/ 0.1631 

test 4 0.6794 0.0062 0.6408 0.9581 1cnon-normal>/ 0.0740 

(two- 5 0.4841 0.0086 0.1767 0.9225l /<non-normal>/ 0.0345 

sided) 6 I<----------- non-normal-------------> I f-) (F-test) 1 

7 I<----------- non-normal--------------- I (F-test) 

8 0.0021 (F-test) 0.0012 0.0020 0.0032 (F-test) (F-test) 

Non-Parametric;two-sided significance level: Mann- Whitney test 

1 2 3 4 5 6 7 8 

l1 0.0684 0.1086 

2 I 0.6753 0.5463 0.9906 

3 0 .I538 0.2973 

0.1467 0.3252 

Mann - Whitney Comparison for 5l 10.0637 0.1132 

non-normal stations, or unequal variances. 6 0.2509 0.7276 

7[ -- 0.4902 

1 = City Island, 2 = Myakka, 3 = Cutter, 4 = Sarasota Bradenton AOP 

5 = Siesta, 6 = Tallevast, 7 = Bradenton ESE, 8 = (Bradenton+Myakka)/2 

Italics type indicates either non-normal distributions based on the 

coefficient of skewness, or unequal variances as determined by the 

F'test. Bold faced type are significance levels <= 0.05. 

The monthly totals were compared using a linear least squares regression. 

The regression test, by definition, is a 'paired' comparison. The following 

illustration (Box 4) indicates that there is no significant difference in 

the seasonal distribution of any station pair, suggesting that monthly 

variationbetween stations is consistent. In other words, when the monthly 

rainfall at one station is higher, it is likely to be relatively higher at 

all stations, although the magnitude of increase may vary between stations. 

In essence, the 'wet season' occurs at all stations during the same time of 

the year. All pairs were significant at P < = 0.0000, although it should 

be noted that station pairs including Tallevast and Bradenton are not 

normally distributed and thus may not be accurately represented by a least 

squares evaluation based on a normal.distribution assumption. 
Box 4 

rA2 of linear regression (monthly totals) 

1 2 3 4 5 6 7 8 

1 - - - - - - - - 0.7444 0.8831 0.8466 0.8574 0.7691 0.5900 0.7317 

2 - - - - - - - - 0.7331 0.8029 0.7584 0.7461 0.7513 0.9322 

3 - - - - - - - - 0.8582 0.8815 0.7881 0.8269 0.8089 

4 - -- - - - - - 0.8744 0.9478 0.8969 0.8870 

5 (Note : Significance of all - - - - - - - - 0.8350 0.8491 0.8387 

6 Comparisons < = 0.0000) - - - - - - - - 0.8668 0.8451 

7 - - -- - - - - 0.9350 

8 - - - - - - - - 

The results of the statistical evaluations indicate the following points: 

1) The wet and dry seasons occur synchronously at all stations evaluated. 

2) Taken as pairs of annual rainfall totals, there are significant 

differences among the stations evaluated. When the station locations are 

considered, the differences appear to be related to distance from the 

coast. For example, the results from the Myakka (and BRAMYK, which 

includes the Myakka results) were statistically different from the coastal 

stations, but not from the other inland stations. Station pairs exhibiting 

differences in paired annual means include the following: 

Myakka and a) Cutter, b) Sarasota-Bradenton, c) Siesta, d) City Island. 

BRAMYK and a) Cutter, b) Sarasota-Bradenton, c) Siesta, d) City Island. 

3) When evaluated as unpaired annual totals, the number of significant 

differences were minimal, with the Siesta Key annual rainfall differing 

from both the Myakka results and the BRAMYK results. 

For the present annual NPS loading evaluation, 'whenf an annual rainfall 

total occurs is immaterial, and the unpaired comparisons are more 

appropriate than the paired comparisons. Considering the relatively short 

(10-12 years) period of the coastal stations, use of the longer records of 

the inland stations is appropriate.