Streamflow and Water-Quality Characteristics

at Selected Sites of the St. Johns River 

in Central Florida, 1933 to 2002

By Sharon E. Kroening

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the

St. Johns River Water Management District

Scientific Investigations Report 2004-5177
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted materials contained within this report.

Suggested citation: Kroening, S.E., 2004, Streamflow and Water-Quality Characteristics at 

Selected Sites of the St. Johns River in Central Florida, 1933 to 2002: U.S. Geological Survey 

Scientific Investigations Report 2004-5177, 102 p.
 iii

Contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Previous Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Purpose and Scope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Description of the Study Area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Climate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Hydrogeologic Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Hydrologic Setting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Land Use and Land Cover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Approach and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Streamflow Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Retrospective Water-Quality Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Data Collected for this Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Sample Collection and Analysis Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Quality Assurance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Continuous Monitoring Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Data Analysis Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Streamflow Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Temporal Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Seasonal Variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Streamflow Reversals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Flow Duration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Low-Flow Frequency Statistics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Water-Quality Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Water Temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Dissolved Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Water pH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Specific Conductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Total Dissolved Solids. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Major Ions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Silica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Minor Inorganic Constituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Sulfide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Iron. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Bromide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Water Color and Organic Carbon. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

Nutrients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
iv 

Total Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Ammonia Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

Organic Plus Ammonia Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

Nitrate Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

Total Phosphorus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

Orthophosphate Phosphorus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

Chlorophyll-a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

Turbidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

Total Suspended Solids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

Human and Veterinary Antibiotics, Other Human Prescription and Nonprescription Drugs, Pesticides, and Organic and Wastewater Constituents. . . . . . . . 81

Temporal Trends (1991-99). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

Appendix A. Sites with retrospective water-quality data compiled for this report. . . . . . 96

Appendix B. Water-quality constituents analyzed in stream water collected 

during this study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

Appendix C. Streamflow duration statistics at selected sites on the St. Johns, 

Econlockhatchee, and Wekiva Rivers and Blackwater Creek. . . . . . . . . . . . . . . . 102

Figures

1. Map showing location of the St. Johns River Basin, central Florida. . . . . . . . . . . . 3

2. Graph showing mean monthly temperature and rainfall at the Orlando airport, 

1961-90. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3. Map showing generalized land use and land cover in the study area, 1999 . . . . . 9

4. Map showing location of study area and stream sites . . . . . . . . . . . . . . . . . . . . . . 13

5-11. Graphs showing:

5. Seven-day annual minimum low-flow values plus 4,000, and the 

corresponding fit by the Pearson Type III distribution for data from the 

St. Johns River near DeLand, Florida. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

6. Annual minimum streamflow averaged over 7 consecutive days at the 

Econlockhatchee River near Chuluota, Florida . . . . . . . . . . . . . . . . . . . . . . . . 24

7. Annual minimum streamflow averaged over 7 consecutive days at 

the Wekiva River near Sanford, Florida, 1935-2000 . . . . . . . . . . . . . . . . . . . . . 24

8. Streamflow-duration hydrographs for sites on the St. Johns River and 

Blackwater Creek, water years 1985-2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

9. Flow-duration curves for St. Johns River sites, water years 1957-2000 . . . 27

10. Flow-duration curves for sites on the St. Johns River, Econlockhatchee 

River, Wekiva River, and Blackwater Creek, water years 1985-2000. . . . . . 28

11. Streamflow characteristics and precipitation at St. Johns River sites, 

2000-02 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

12. Boxplots showing seasonal variations in water temperature in the St. Johns 

River upstream of Lake Harney near Geneva, Florida, 1991-99 . . . . . . . . . . . . . . . 34
 v

13. Boxplots showing dissolved oxygen concentrations in the St. Johns River and

near the mouths of the Econlockhatchee and Wekiva Rivers, 1991-1999. . . . . . . 34

14. Graph showing dissolved oxygen concentration and streamflow at the 

St. Johns River near Christmas, Florida, 2000-02. . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

15. Boxplots showing water pH in the St. Johns River and near the mouths of 

the Econlockhatchee and Wekiva Rivers, 1991-1999 . . . . . . . . . . . . . . . . . . . . . . . . 36

16. Graphs showing water pH and streamflow at St. Johns River sites, 2000-02 . . . 37

17. Graphs showing relation between specific conductance and streamflow at 

sites on the St. Johns River, 1991-99 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

18. Boxplots showing specific conductance values at St. Johns River sites and

near the mouths of the Econlockhatchee and Wekiva Rivers, May 1991-99 . . . . 40

19. Graphs showing continuous specific conductance and stream discharge 

at St. Johns River sites, 2000-02 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

20. Graphs showing total dissolved solids concentration and streamflow at 

St. Johns River sites, 2000-02 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43-44

21. Trilinear diagrams for water samples collected from St. Johns River 

sites, Wekiva River, Blackwater Creek, and Blue Springs, 2000-02. . . . . . . . . 45-48

22-24. Graphs showing:

22. Sulfate concentration and streamflow at St. Johns River sites, 

2000-02 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

23. Chloride concentration and streamflow at St. Johns River sites, 

2000-02 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50-51

24. Dissolved silica concentration and streamflow at St. Johns River sites, 

2000-02 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

25. Map showing silica concentrations in the Floridan aquifer system, 

1990-2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

26-28. Graphs showing:

26. Total iron concentrations and streamflow at St. Johns River sites, 

2000-02 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

27. Percent of dissolved iron comprising the total iron concentration and 

streamflow at the St. Johns River near Sanford, Florida, 2000-02 . . . . . . . . 58

28. Bromide concentration and streamflow at St. Johns River sites, 

2000-02 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59-60

29. Map showing bromide concentrations in the Floridan aquifer system, 

1990-2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

30. Graphs showing water color values and streamflow at St. Johns River sites, 

2000-02 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

31. Graphs showing dissolved organic carbon concentration, specific ultraviolet light absorbance, and streamflow at St. Johns River sites, 2000-02. . . 65-66

32. Plots showing total nitrogen concentration and nitrogen speciation in 

the St. Johns River and near the mouths of the Econlockhatchee and 

Wekiva Rivers, 1991-99. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

33. Graphs showing dissolved ammonia nitrogen concentration and streamflow 

at St. Johns River sites, 2000-02 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

34. Boxplots showing nitrate nitrogen concentrations by seasonal period near 

the mouths of the Econlockhatchee and Wekiva Rivers, 1991-99 . . . . . . . . . . . . . 71
vi 

35. Graphs showing dissolved nitrite plus nitrate nitrogen concentration and 

water temperature at St. Johns River sites, 2002-02 . . . . . . . . . . . . . . . . . . . . . 72-73

36. Graphs showing total phosphorus concentration and streamflow at 

St. Johns River sites, 2002-02 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

37. Boxplots showing chlorophyll-a concentrations in the St. Johns River and 

near the mouths of the Econlockhatchee and Wekiva Rivers, 1991-99 . . . . . . . . 78

38. Boxplots showing seasonal variations in chlorophyll-a concentrations in 

the St. Johns River downstream of Blue Springs, Florida,1991-99. . . . . . . . . . . . . 79

39. Graph showing chlorophyll-a concentrations and streamflow at the 

St. Johns River site near Cocoa, Florida, 2000-02 . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

40. Boxplots showing seasonal variations in total suspended solids 

concentration in the St. Johns River near Hontoon Marina, Florida, 1991-99 . . . 82

41-43. Graphs showing:

41. Total suspended solids concentration and streamflow at the 

 St. Johns River site near Christmas, Florida, 2000-02 . . . . . . . . . . . . . . . . . . 82

42. Frequency of detection of human and veterinary antibiotics, other 

human prescription and nonprescription drugs, pesticides, and organic

wastewater constituents at the St. Johns River sites near Cocoa and 

Sanford, Florida, water years 2002-03 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

43. Number of human and veterinary antibiotics, other human prescription 

and nonprescription drugs, pesticides, and organic wastewater 

constituents detected at selected St. Johns River sites near Cocoa 

and Sanford, Florida, water years 2002-03 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

Tables

1. Locations of sites with streamflow and water-quality data analyzed in 

this study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2. Retrospective water-quality sites with paired field- and laboratorydetermined water pH values and sign test results . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3. Remark codes associated with retrospective water-quality data collected 

by Florida Department of Environmental Protection, Orange County, 

Volusia County, and the St. Johns River Water Management District . . . . . . . . . 14

4. Censoring limits applied to retrospective water-quality data. . . . . . . . . . . . . . . . . 15

5. Land-use and land-cover percentages of the drainage area upstream of 

the sites sampled for this study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

6. Statistical summary of field-blank samples for water-quality data collected 

for this study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

7. Variability in constituent values for water-quality data collected for this study. . 19

8. Temporal trends in daily-mean and N-day consecutive low-streamflow data 

from sites with more than 10 years of data on the St. Johns River, 

Econlockhatchee River, Wekiva River, and Blackwater Creek . . . . . . . . . . . . . . . 23

9. Low-flow frequency statistics for the St. Johns River near Cocoa, Florida. . . . . . 29

10. Low-flow frequency statistics for the St. Johns River near Christmas, Florida . . 29

11. Low-flow frequency statistics for the Econlockhatchee River near 

Chuluota, Florida . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
 vii

12. Low-flow frequency statistics for the St. Johns River upstream of Lake 

Harney near Geneva, Florida. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

13. Low-flow frequency statistics for the St. Johns River near Sanford, Florida. . . . 30

14. Low-flow frequency statistics for the Wekiva River near Sanford, Florida . . . . . 30

15. Low-flow frequency statistics for Blackwater Creek near Cassia, Florida . . . . . 31

16. Low-flow frequency statistics for the St. Johns River near DeLand, Florida . . . . 31

17. Mean annual streamflow for sites in the study area . . . . . . . . . . . . . . . . . . . . . . . . 32

18. Relations between dissolved oxygen concentration and water pH and 

streamflow at St. Johns River sites, 1991-2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

19. Relations between specific conductance values and total dissolved solids 

concentration and streamflow at St. Johns River sites, 1991-2002 . . . . . . . . . . . . 38

20. Relations between major dissolved inorganic constituent concentrations 

and streamflow at St. Johns River sites, 2000-2002 . . . . . . . . . . . . . . . . . . . . . . . . . 52

21. Relations between dissolved silica, total sulfide, total iron, dissolved iron, 

and bromide concentrations and streamflow at St. Johns River sites, 

2000-02 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

22. Descriptive statistics of total sulfide concentrations at St. Johns River, 

Wekiva River, and Blackwater Creek sites, 2000-02. . . . . . . . . . . . . . . . . . . . . . . . . 55

23. Relations between water color values and dissolved organic carbon 

concentrations and streamflow at St. Johns River sites, 2000-2002 . . . . . . . . . . . 64

24. Relations between total nitrogen, ammonia nitrogen, and organic plus 

ammonia nitrogen concentrations and streamflow at St. Johns River sites, 

1991-2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

25. Descriptive statistics of nitrate nitrogen concentrations in the St. Johns, 

Econlockhatchee, and Wekiva Rivers, 1991-99 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

26. Descriptive statistics of total phosphorus concentrations in the St. Johns, 

Econlockhatchee, and Wekiva Rivers, 1991-99 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 

27. Relations between total phosphorus and orthophosphate concentrations 

and streamflow at St. Johns River sites, 1991-2002 . . . . . . . . . . . . . . . . . . . . . . . . . 77

28. Descriptive statistics of orthophosphate concentrations in the St. Johns, 

Econlockhatchee, and Wekiva Rivers, 1991-99 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

29. Relations between turbidity values and total suspended solids 

concentrations and streamflow at St. Johns River sites, 1991-2002 . . . . . . . . . . . 80

30. Descriptive statistics of total suspended solids concentrations in the 

St. Johns, Econlockhatchee, and Wekiva Rivers, 1991-99. . . . . . . . . . . . . . . . . . . . 81

31. Kendall’s tau correlation coefficients describing the relation between 

chlorophyll-a and total suspended solids concentration at sites on the 

St. Johns, Econlockhatchee, and Wekiva Rivers, 1991-99. . . . . . . . . . . . . . . . . . . . 83

32. Temporal trends in selected physical properties, nutrients, and 

chlorophyll-a at St. Johns River sites, 1991-99. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
viii

Conversion Factors and Verical Datum

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) 

as follows: °C = (°F - 32) / 1.8

Vertical coordinate information is referenced to the National Geodetic Vertical 

Datum of 1929 (NGVD 29).

Horizontal coordinate information (latitude-longitude) is referenced to the North 

American Datum of 1927 (NAD 27).

Acronyms and Abbreviations

Multiply By To obtain

Length

inch (in.) 2.54 centimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Area

square mile (mi2) 2.59 square kilometer

Mass

ton, long (2,240 lb) 1.016 megagram

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second

million gallons per day (Mgal/d) 0.04381 cubic meter per second

ET evapotranspiration

FDEP Florida Department of Environmental Protection

FFWCC Florida Fish and Wildlife Conservation Commission

LOWESS locally-weighted scatterplot

MCL maximum contaminant level

μg/L micrograms per liter

μ micron

μS/cm microsiemens per centimeter

mg/L milligrams per liter

mL milliliter

nm nanometer

NWIS National Water Information System

NTU nephelometric turbidity units

SJRWMD St. Johns River Water Management District

SWIM Surface Water Improvement and Management

TMDL total maximum daily load

UV ultraviolet light

USACE U.S. Army Corps of Engineers

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey
Streamflow and Water-Quality Characteristics at 

Selected Sites of the St. Johns River in Central 

Florida, 1933 to 2002

By Sharon E. Kroening

Abstract

To meet water-supply needs in central Florida for 

2020, the St. Johns River is being considered as a source of 

water supply to augment ground water from the Floridan 

aquifer system. Current (2004) information on streamflow 

and water-quality characteristics of the St. Johns River in 

east-central Florida is needed by water resources planners 

to assess the feasibility of using the river as an alternate 

source of water supply and to design water treatment facilities. To address this need, streamflow and water quality of 

the 90-mile-long middle reach of the St. Johns River, Florida, from downstream of Lake Poinsett to near DeLand, 

were characterized by using retrospective (1991-99) and 

recently collected data (2000-02). Streamflow characteristics were determined by using data from water years 1933-

2000. Water-quality characteristics were described using 

data from 1991-99 at 15 sites on the St. Johns River and 

1 site each near the mouths of the Econlockhatchee and 

Wekiva Rivers. Data were augmented with biweekly 

water-quality data and continuous physical properties data 

at four St. Johns River sites and quarterly data from sites 

on the Wekiva River, Blackwater Creek, and downstream 

of Blue Springs from 2000-02. Water-quality constituents 

described were limited to information on physical properties, major ions and other inorganic constituents, nutrients, 

organic carbon, suspended solids, and phytoplankton chlorophyll-a. The occurrence of antibiotics, human prescription and nonprescription drugs, pesticides, and a suite of 

organic constituents, which may indicate domestic or 

industrial waste, were described at two St. Johns River 

sites using limited data collected in water years 2002-03. 

The occurrence of these same constituents in water from 

a pilot water treatment facility on Lake Monroe also was 

described using data from one sampling event conducted 

in March 2003. 

Dissolved oxygen concentration and water pH values 

in the St. Johns River were significantly lower during 

high-flow conditions than during low-flow conditions. 

Low dissolved oxygen concentrations may have resulted 

from the input of water from marsh areas or the subsequent decomposition of organic matter transported to the 

river during high-flow events. Low water pH values 

during high-flow conditions likely resulted from the 

increased dissolved organic carbon concentrations in the 

river.

Concentrations of total dissolved solids and other 

inorganic constituents in the St. Johns River were 

inversely related with streamflow. Most major ion 

concentrations, total dissolved solids concentrations, and 

specific conductance values varied substantially at the 

Christmas, Sanford, and DeLand sites during low-flow 

periods in 2000-01 probably reflecting wind and tidal 

effects.

Sulfide concentrations as high as 6 milligrams per 

liter (mg/L) were measured in the St. Johns River during 

high-flow periods. Increased sulfide concentrations likely 

resulted from the decomposition of organic matter or the 

reduction of sulfate. Bromide concentrations as high as 

17 mg/L were measured at the most upstream site on the 

St. Johns River during 2000-02. Temporal variations in 
2 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

bromide were characterized by sharp peaks in concentration during low-flow periods. Peaks in bromide 

concentrations tended to coincide with peaks in chloride 

concentrations because the likely source of both constituents is ground water affected by relict seawater.

Median dissolved organic carbon concentrations 

ranged from 15 to 26 mg/L during 2000-02, and concentrations as high as 42 mg/L were measured. Water color 

values and dissolved organic carbon concentrations 

generally were significantly greater during high-flow 

conditions than during low-flow conditions. Specific 

ultraviolet light absorbance data indicated the organic 

carbon during high-flow events was more aromatic in 

composition and likely originated from terrestrially 

derived sources compared to organic carbon in the river 

during other times of the year.

Detections of ammonia nitrogen and orthophosphate 

phosphorus in the St. Johns River indicated that bioavailable forms of nutrients generally are present. Ammonia 

nitrogen concentrations in the river generally were greater 

during high-flow conditions than during low-flow conditions. Nitrate nitrogen concentrations during 1991-99 

were significantly lower in the upstream part of the river 

compared to other sites, which may be related to nitrate 

being used as a nutrient by terrestrial or aquatic plants or 

denitrification in wetland areas. High nitrate concentrations at sites downstream of the confluence with the 

Wekiva River may have been due to nitrogen-enriched 

ground-water inflow. Seasonal variations in nitrate 

concentrations from 2000-02 were characterized by sharp 

peaks in concentration from about October through 

February, which probably resulted from nitrification.

Chlorophyll-a and total suspended solids concentrations generally were greatest in May and June compared 

to other times of the year. Low chlorophyll-a concentrations in July and August may have resulted from light 

limitation due to increased concentrations of highly 

colored organic matter in the river. High total suspended 

solids concentrations during the May-June low-flow 

period indicate that runoff of soil eroded from the land 

surface is not the primary mechanism by which 

suspended solids are contributed to the St. Johns River, 

but rather that suspended solids are contributed by algal 

production or the resuspension of bottom sediments.

Pesticides, antibiotics, plasticizers, and detergent 

metabolites were detected in water from the St. Johns 

River. No constituent concentrations exceeded applicable 

Maximum Contaminant Levels established by the U.S. 

Environmental Protection Agency. Atrazine, metolachlor, and cholesterol were the most frequently detected 

compounds. More constituents were detected in water 

from the Sanford site (20) compared to the Cocoa site (5), 

which likely resulted because the Sanford site is downstream of point sources of municipal wastewater effluent 

to streams in the study area and urban-residential areas on 

septic systems. Limited data suggested these constituents 

are derived from runoff to the Cocoa site and from pointsource discharges or septic tank leachate at the Sanford 

site.

The most notable temporal trends in water-quality 

constituents during 1991-99 were increased concentrations of total phosphorus and dissolved orthophosphate at 

three sites on the St. Johns River upstream of Lake 

Harney. Increased fertilizer sales, increased livestock 

populations, or increased phosphorus concentrations in 

recently restored wetlands do not account for this trend.

Introduction 

The St. Johns River Basin encompasses about 9,400 

square miles (mi2

) in east-central Florida (fig. 1). The 

river originates in the St. Johns River marsh, a vast 

wetlands area near Vero Beach, and flows northward 

about 275 miles (mi) , where it discharges to the Atlantic 

Ocean near Jacksonville. The St. Johns River Water 

Management District (SJRWMD) encompasses about 

12,400 mi2

 in east-central Florida—about 21 percent of 

the land area in the State of Florida. The SJRWMD is 

responsible for managing the ground- and surface-water 

supplies in this region.

Sites along the St. Johns River are being considered 

as potential intake points for surface water to augment 

ground water from the Floridan aquifer system to meet 

future water-supply needs in central Florida (St. Johns 

River Water Management District and CH2MHILL, 

1998). The population of the area managed by the 

SJRWMD is projected to increase from about 3.7 million 

in 1998 to 5.2 million in 2020 (St. Johns River Water 

Management District, 2000). With this projected population increase, there may not be sufficient water available 

from the Floridan aquifer system to supply all existing and 

projected water uses and to sustain the aquifer system and 

related natural systems, such as springs and lakes. 

Approximately 40 percent of the area managed by the 

SJRWMD was classified as Water Resource Caution 

Areas (St. Johns River Water Management District, 2000) 

as part of the 1998 water-supply assessment. Water 

Resource Caution Areas are places where water-supply 

problems exist or where proposed ground-water withdrawals have been projected to result in substantial harm 

to surface- or ground-water resources.
Introduction 3

Figure 1. Location of the St. Johns River Basin, central Florida.
4 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

The middle St. Johns River Basin (defined as the part 

of the basin from the confluence with the Econlockhatchee River to the confluence with the Wekiva River, 

fig. 1) is being preserved and restored through the State of 

Florida's Surface Water Improvement and Management 

(SWIM) Act. The Florida Legislature enacted the SWIM 

Act in 1987 to preserve water bodies that were in good 

condition and to restore some of the State's most important water bodies. The SJRWMD developed a SWIM plan 

for the middle St. Johns River Basin that focuses on nutrient load reduction, lake protection, and stormwater 

management (St. Johns River Water Management 

District, 2002).

Under section 303(d) of the Clean Water Act, every 

2 years the State of Florida must identify water bodies 

that do not meet applicable water-quality standards by 

developing an impaired waters or 303(d) list. The 

impaired waters list is developed using a detailed methodology described in section 62-303.300 of the Florida 

Administrative Code. In general, a water body is placed 

on the impaired waters list if: (1) water-quality criteria 

established to maintain the water body’s designated use, 

such as drinking water supply or recreation, propagation 

and maintenance of a healthy and well-balanced population of fish and wildlife, are not met; (2) the water body 

does not meet biological assessment thresholds; (3) the 

water body is acutely or chronically toxic; or (4) the water 

body exceeds narrative nutrient thresholds established by 

the State of Florida. Within the study area, parts of the 

St. Johns River, Econlockhatchee River, Blackwater 

Creek, Wekiva Springs, and Lake Monroe are identified 

as impaired waters on the Florida Department of Environmental Protection’s (FDEP) 1998 303(d) list (St. Johns 

River Water Management District, 2002).

A total maximum daily load (TMDL) may be developed for streams, springs, and lakes in the study area that 

are on the verified impaired waters list developed by the 

FDEP. A TMDL is defined as the maximum amount of a 

constituent that a water body can absorb and still maintain 

its designated uses, such as drinking, fishing, or swimming. TMDLs include all existing and future loads from 

point sources such as municipal wastewater treatment 

facilities and industry; nonpoint sources such as runoff 

from agricultural areas, urban areas, and forests; and 

natural sources such as decaying organic matter and nutrients in the soil. The verified list of impaired waters is used 

to prioritize development of TMDLs in the State of Florida. The verified list of impaired waters is different from 

the impaired waters list described in the previous paragraph. Some water bodies may be included on the 

impaired waters list because water-quality criteria are 

exceeded due to natural conditions or physical alterations 

of the water body not related to contaminants. These water 

bodies are not included on the verified list of impaired 

waters. The methodology used to develop the verified list 

of impaired waters is described in detail in section 62-

303.400 of the Florida Administrative Code. More than 80 

water bodies were included in the draft verified impaired 

waters list released by the FDEP in June 2003 (Florida 

Department of Environmental Protection, 2003a) for the 

middle St. Johns River Basin, including the St. Johns 

River, Econlockhatchee River, Wekiva River, Blackwater 

Creek, Lake Harney, Lake Monroe, Lake Jesup, and Blue 

Springs. 

In 2000, the U.S. Geological Survey (USGS) and the 

SJRWMD began a 4-year study to characterize the 

streamflow and water quality of the St. Johns River in 

east-central Florida. Current (2004) information on 

streamflow and water-quality characteristics of the 

St. Johns River is needed by water resources planners to 

assess the feasibility of using the river as an alternate 

source of water supply and to design water treatment 

facilities. This information also may be used by water 

resources planners to conduct water-quality monitoring or 

modeling studies associated with other activities, such as 

implementing the middle St. Johns River SWIM Plan or 

the development of TMDLs. 

Previous Studies

Several reports have presented information on lowflow characteristics of the St. Johns River and its tributaries. Lichtler and others (1968) presented low-flow 

frequency curves and flow-duration curves for sites on the 

St. Johns and Econlockhatchee Rivers based on data 

collected from 1934-62. Snell and Anderson (1970) 

presented flow-duration curves for sites on the St. Johns 

and Wekiva Rivers based on data collected from 1934-65. 

Heath and Wimberly (1971) determined flow-duration 

and low-flow frequency statistics based on data collected 

from 1933-65. Hughes (1981) presented low-flow 

frequency statistics based on data collected from 1933-77. 

Rumenik and Grubbs (1996) presented low-flow 

frequency statistics based on data collected from 1933-87. 

These previously published low-flow frequency and flowduration curves can be improved by incorporating additional streamflow data that were collected as part of 

USGS stream-gaging activities since 1987. In addition, 

previously published low-flow frequency and flow-duration statistics may not represent current (2004) conditions 

due to the rapid urbanization in the middle St. Johns River 

Basin.
Introduction 5

Information has been published on the water quality 

of the St. Johns River and its tributaries. Lichtler and 

others (1968) presented the range of major ion and nitrate 

concentrations near Cocoa and Christmas from 1953-63. 

Goolsby and McPherson (1970) described the chemical 

and biological characteristics in the St. Johns River south 

of Lake Harney, based on 1969-70 data. DeMort (1990) 

presented summary statistics of selected water-quality 

constituents, based on 1974-79 data.

Purpose and Scope

The purpose of this report is to describe the streamflow and water-quality characteristics of the St. Johns 

River from downstream of Lake Poinsett to near DeLand. 

Streamflow characteristics were determined by using data 

collected by the USGS. Streamflow characteristics 

described include temporal trends and seasonal variations 

in streamflow. Flow-duration curves and low-flow 

frequency statistics also are presented. Temporal trends in 

streamflow were determined at sites with at least 10 years 

of data using the longest period of record available, which 

ranged from 16 years (water years 1985-2000) to 68 years 

(water years 1933-2000). The water year is the 12-month 

period from October 1 to September 30 and is designated 

by the calendar year in which it ends. Thus, the water year 

ending on September 30, 2000, is called water year 2000. 

Low-flow frequency statistics were computed using the 

longest period of trend-free record. Seasonal variations in 

streamflow were determined using data from water years 

1985-2000. Flow-duration curves for St. Johns River 

sites were determined using data from water years 1957-

2000. Flow-duration curves also were determined using 

data from water years 1985-2000 to compare streamflow 

characteristics among sites on the St. Johns River, Econlockhatchee River, Wekiva River, and Blackwater Creek.

Water-quality characteristics are described using 

retrospective information from 1991-99 at 15 sites on the 

St. Johns River and 1 site each near the mouths of the 

Econlockhatchee and Wekiva Rivers. Water-quality 

constituents described in this report include physical 

properties (water temperature, specific conductance, 

turbidity, and water color); dissolved oxygen concentration and water pH; major ions and other inorganic 

constituents such as silica, bromide, and sulfide; nutrients; organic carbon and ultraviolet light absorbance; 

phytoplankton chlorophyll-a; and total suspended solids. 

The retrospective data were augmented with biweekly 

data collected by the USGS during water years 2000-02 

at four St. Johns River sites; continuous field properties 

data collected primarily during water years 2001-02 at 

four St. Johns River sites; and quarterly data collected 

during water years 2000-02 from the Wekiva River, 

Blackwater Creek, and downstream of Blue Springs (in 

the spring run). Retrospective water-quality data included 

information from the FDEP, Florida Fish and Wildlife 

Conservation Commission (formerly the Florida Game 

and Freshwater Fish Commission), Orange County 

Department of Environmental Protection, SJRWMD, 

Volusia County Health Department, and the USGS. The 

occurrence of antibiotics, human prescription and nonprescription drugs, pesticides, and organic wastewater 

constituents were described at two St. Johns River sites 

using limited data collected in water years 2002-03. The 

occurrence of antibiotics, prescription and nonprescription drugs, pesticides, and a suite of organic constituents 

that may indicate industrial or domestic waste in water 

from a pilot water treatment facility on Lake Monroe was 

described using data from one sampling event conducted 

in March 2003. Variations in water-quality constituent 

values among sites are presented by comparing boxplots 

and summary statistics. Seasonal variations in water-quality constituent values are presented by comparing 

boxplots and plots of constituent values over time. Correlation coefficients are presented to quantify the relation 

between water-quality constituent values and streamflow. 

Temporal trend test results are presented for water-quality 

constituents for 1991-99. Water-quality constituent 

values are compared to 2000 Federal and State of Florida 

regulations.

Description of the Study Area

The study area includes about 90 mi of the St. Johns 

River from downstream of Lake Poinsett to near DeLand 

(fig. 1). The St. Johns River Basin within and upstream of 

the study area encompasses about 3,000 mi2

 and includes 

all or part of Brevard, Indian River, Lake, Okeechobee, 

Orange, Osceola, Seminole, and Volusia Counties. In 2000, 

the population of these counties was about 2.7 million 

(U.S. Bureau of Census, 2004).

Climate

The climate of the study area is humid subtropical. 

The study area normally (1961-90) receives about 50 

inches (in.) of rainfall per year at the Orlando airport 

(Southeast Regional Climate Center, 2001). Over onehalf of the rainfall occurs from June through September 

(fig. 2), typically as localized, brief afternoon thunderstorms. The June through September rainy period 

commonly is referred to as the wet season. Tropical storms 

and hurricanes also periodically contribute rainfall during 
6 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

the summer. The air temperature in the study area 

normally (1961-90) is highest in July and August and 

lowest in January and February (fig. 2). The normal maximum air temperature ranges from about 92 degrees Fahrenheit (o

F) in July and August to about 72 oF in January 

and February. The normal minimum air temperature 

ranges from about 50 o

F in February to 74 oF in August. 

Evapotranspiration (ET) generally is about the same 

magnitude as rainfall, averaging from 27 to 57 in. per 

year, depending on the land cover (Sumner, 1996; Swancar 

and others, 2000). ET in central Florida generally is highest in July and August and lowest in January and February 

(D. Sumner, U.S. Geological Survey, written commun., 

2003).

Hydrogeologic Setting

The study area is underlain by a thick sequence 

(greater than 5,000 feet (ft)) of sedimentary rocks (Lichtler 

and others, 1968). The principal water-bearing units in the 

study area are the surficial and Floridan aquifer systems. 

These two units are separated by an intermediate confining 

unit.

The surficial aquifer system generally consists of 

fine-to-medium grained quartz sands that contain varying 

amounts of silt, clay, and loose shell fragments (Tibbals, 

1990). Recharge to the surficial aquifer system primarily 

occurs by the infiltration of rainfall. Other sources of 

recharge to the surficial aquifer system include the land 

85

80

75

70

65

60

AIR TEMPERATURE,

IN DEGREES FAHRENHEIT

90

MONTH

JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC.

8

7

6

5

4

3

AVERAGE PRECIPITATION,

IN INCHES

9

2

1

0

Source of data:

Southeast Regional Climate Center, 2001.

Figure 2. Mean monthly temperature and rainfall at the Orlando airport, 1961-90.
Introduction 7

application of wastewater and reclaimed water, irrigation 

water, lateral ground-water inflow from adjacent areas, 

and upward leakage from the Floridan aquifer system. 

Discharge from the surficial aquifer system occurs by 

seepage to lakes, canals, and streams, by evapotranspiration where the water table is near the land surface, by 

pumping, and by downward leakage to the Floridan 

aquifer system.

The Floridan aquifer system is composed primarily 

of calcite and dolomite, with minor amounts of gypsum, 

apatite, glauconite, quartz (or chert), clay minerals, and 

trace amounts of metallic oxides and sulfide (Sprinkle, 

1989). The uppermost stratigraphic units in the aquifer 

system are characterized by fractures and vuggy and 

cavernous porosity. In central Florida, the Floridan aquifer system consists of the Upper Floridan aquifer and the 

Lower Floridan aquifer, separated by a middle semiconfining unit. Both recharge and discharge areas to the 

Floridan aquifer system are present within the study area 

(Tibbals, 1990). Discharge areas generally are in the 

vicinity of the St. Johns and Wekiva Rivers and along the 

lower part of the Econlockhatchee River. The remainder 

of the study area consists of recharge areas to the Floridan 

aquifer system. Recharge to the Floridan aquifer system 

occurs by downward leakage from the surficial aquifer 

system, by lateral inflow of ground water from adjacent 

areas, and through drainage wells. Drainage wells primarily are used for control of lake levels and disposal of 

storm water by emplacing surface water into the aquifer. 

Spring flow, pumpage, and upward leakage to the surficial aquifer system are additional sources of discharge 

from the Floridan aquifer.

The vicinity of the St. Johns River generally is 

classified as a discharge area from the Floridan aquifer 

system. However, the intermediate confining unit generally varies from less than 50 to 150 ft thick in the vicinity 

of the St. Johns River (Murray and Halford, 1996); the 

confining unit probably retards movement of water from 

the Upper Floridan aquifer to the surficial aquifer system. 

As a result, discharge of water from the Upper Floridan 

aquifer to the river probably occurs through fractures or 

other karstic features. The presence of faults parallel to 

the St. Johns River from the headwaters to near the northern border of Orange County, from Lake Harney to Lake 

Monroe, and from Lake Monroe northward to Lake 

George were reported by Pirkle (1971).

The quality of water from the Upper Floridan aquifer 

varies greatly in central Florida. Total dissolved solids 

concentrations are used as an indicator of the mineral 

content of water. In the Upper Floridan aquifer in central 

Florida, total dissolved solids concentrations were 

reported to vary from less than 150 to greater than 

2,000 mg/L (Lichtler and others, 1968; Tibbals, 1990; 

Adamski and German, 2004). Chloride concentrations in 

the Upper Floridan aquifer in central Florida were 

reported to vary from less than 10 to greater than 5,000 

mg/L (Lichtler and others, 1968; Tibbals, 1990; Spechler 

and Halford, 2001; Adamski and German, 2004). The 

highest chloride and total dissolved solids concentrations 

in the Upper Floridan aquifer occur along the St. Johns 

River, especially south of Lake Harney (Spechler and 

Halford, 2001). The higher mineral content of water in 

this part of the Upper Floridan aquifer generally is attributed to incomplete flushing of seawater that entered the 

aquifer in the geologic past (Lichtler and others, 1968; 

Tibbals, 1990; Boniol, 1996).

Hydrologic Setting

The St. Johns River originates in the St. Johns River 

marsh (fig. 1). The river channel is poorly defined in the 

headwaters of the St. Johns River. Water from the marsh 

sheetflows north about 30 mi before the St. Johns River 

forms a distinct channel in the vicinity of Lake Hellen 

Blazes. The St. Johns River flows northward through 

Lake Hellen Blazes, Sawgrass Lake, Lake Washington, 

Lake Winder, and Lake Poinsett and then northwestward 

through Lake Harney and Lake Monroe. These lakes 

likely are relicts of an ancient estuary (White, 1970) rather 

than depressions formed by the dissolution of limestone, 

which generally is how most of the lakes in Florida were 

formed (Schiffer, 1998).

The St. Johns River has a low gradient—the river 

falls only about 30 ft from the source to the mouth 

(DeMort, 1990). Within the study area, the river does not 

have daily streamflow reversals due to tides. Backwater 

from tides within the study area, however, affects streamflow in the St. Johns River, especially at sites downstream 

from Lake Monroe. As a result of the very low gradient, a 

strong tide combined with low-flow conditions can cause 

periodic streamflow reversals in the St. Johns River as far 

upstream as Lake Monroe (DeMort, 1990). A strong 

northerly wind during low-streamflow conditions also 

may cause flow reversals in the river.

The Econlockhatchee and Wekiva Rivers are the two 

largest tributaries to the St. Johns River within the study 

area. The Econlockhatchee River is 35.8 mi long and has 

a drainage area of 270 mi2

 (St. Johns River Water 

Management District, 2002). The Econlockhatchee River 

originates in a wetlands area in northern Osceola County. 

The drainage basin of the Econlockhatchee River is 

composed of poorly drained soils and a high water table 
8 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

with few areas of high ground-water recharge. The Econlockhatchee River is characterized by highly colored, 

acidic, slow-moving water containing little sediment 

(St. Johns River Water Management District, 2002). In 

1992, the Florida Legislature designated the Econlockhatchee River an Outstanding Florida Water. Generally, 

permits cannot be issued for new direct discharges of 

contaminants into an Outstanding Florida Water that 

would lower the existing water quality (Florida Department of Environmental Protection, 2002). Contaminant 

discharges from tributaries or adjacent waters also must 

not significantly degrade an Outstanding Florida Water 

(Florida Department of Environmental Protection, 2002).

The Little Econlockhatchee River is the largest 

stream tributary to the Econlockhatchee River. Many 

miles of the Little Econlockhatchee River and its tributaries have been channelized to lower ground-water levels, 

resulting in a network of drainage ditches (St. Johns River 

Water Management District, 2002). These ditches also 

convey stormwater runoff from the Orlando metropolitan 

area to the Little Econlockhatchee River. Prior to 1983, 

the Little Econlockhatchee River received substantial 

amounts of municipal wastewater effluent from up to 

12 facilities (St. Johns River Water Management District, 

2002). By 1984, the effluent from most of these facilities 

was diverted to regional municipal wastewater facilities, 

which currently (2004) provide tertiary treatment.

The Wekiva River forms at the confluence of 

Wekiwa Springs Run and Rock Springs Run. The river 

flows approximately 14.2 mi to the St. Johns River. The 

drainage basin of the Wekiva River contains both springfed and blackwater streams. The two major tributaries to 

the Wekiva River are the Little Wekiva River and Blackwater Creek. The Wekiva River drains an area of rolling 

hills interspersed with lakes and sinks. Soils in the drainage basin generally are very sandy; a large portion of the 

basin is an area of high ground-water recharge (St. Johns 

River Water Management District, 2002). The flow of the 

Wekiva River is maintained by springs that discharge 

from the Upper Floridan aquifer. The Wekiva River was 

designated as an Outstanding Florida Water. Part of the 

Wekiva River was designated as a Florida Scenic and 

Wild River in 1982, and as a National Wild and Scenic 

River by the Federal Government in 2000.

Blue Springs is located to the east of the St. Johns 

River in Volusia County. The spring does not flow 

directly into the river, but vents into a 100-ft-diameter 

spring pool. From the spring pool, the water flows about 

0.4 mi in a spring run to the river (Rosenau and others, 

1977).

Modifications were made to the St. Johns River 

upstream and within the study area. The U.S. Army Corps 

of Engineers (USACE) maintains a navigation channel on 

part of the river. The river channel is maintained at a 5-ft 

depth from Lake Harney to Sanford, and at an 8-ft depth 

from Sanford to DeLand (Tibbals, 1990). Prior to the 

1930s, Lake Jesup was hydraulically connected to the 

St. Johns River by at least four channels. In the 1930s, 

the USACE replaced these channels with one navigation 

channel to improve accessibility to the lake by barge 

traffic (Belleville, 2000).

Historically, most of the floodplain of the upper 

St. Johns River (the reach from the headwaters to the 

confluence with the Econlockhatchee River) was a broad, 

shallow marsh (Sterling and Padera, 1998). By about 

1970, approximately 70 percent of the floodplain marsh of 

the upper St. Johns River was drained by an extensive 

canal system to support the production of citrus, row 

crops, and beef cattle (Sterling and Padera, 1998). Some 

of these canals diverted water outside of the St. Johns 

River Basin to the Indian River Lagoon. In addition, there 

were more than 40 private pumping stations throughout 

the upper St. Johns River Basin that discharged agricultural runoff directly into the river (U.S. Environmental 

Protection Agency, 2002). These modifications resulted 

in a reduction of water storage areas, increased nutrient 

concentrations in water and bottom sediments, altered 

hydroperiods, and excessive amounts of freshwater 

discharging to the Indian River Lagoon (St. Johns River 

Water Management District, 2003). In 1988, a construction project co-sponsored by the SJRWMD and the 

USACE began to restore part of the upper St. Johns River 

Basin—about 240 mi2

 of wetlands were enhanced or 

restored, nine canals were plugged (Sterling and Padera, 

1998), and some of the agricultural pumping stations were 

eliminated (Fall, 1990). Restored wetlands consist of 

marsh conservation areas and water management areas. 

Marsh conservation areas provide temporary storage of 

floodwaters and reduce the need to discharge freshwater 

to the Indian River Lagoon (Sterling and Padera, 1998). 

Water management areas store and treat agricultural 

drainage and provide water supply for agricultural 

purposes (Sterling and Padera, 1998).

Land Use and Land Cover

Wetlands, agriculture, forests, and urban are the 

predominant land use and land covers in the study area 

(fig. 3). Wetlands make up about 31 percent of the study 

area and generally border the St. Johns River, especially 

in the upstream reaches. About 25 percent of the study
Introduction 9

Figure 3. Generalized land use and land cover in the study area, 1999.
10 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

area is agricultural land, mostly concentrated in the southern 

part of the area. The primary agricultural products are 

beef cattle, citrus, and vegetables. About 70 percent of the 

agricultural land is pasture—most of which is unimproved (according to 1995 land-use data from the 

St. Johns River Water Management District). In some 

areas of Florida, pastures are planted with grasses, such as 

Bahia grass, and are referred to as improved pasture. 

Citrus is the most important crop in the study area; citrus 

groves comprise about 13 percent of the agricultural land. 

About 70 percent of the citrus groves are located in the 

extreme southeastern part of Indian River County. 

Surface water primarily is used to irrigate citrus in Indian 

River County. In 1996, 158 million gallons per day 

(Mgal/d) was used for citrus irrigation in this county; 

75 percent of the water was from surface-water sources 

(Florence and Moore, 1999). Citrus crops typically are 

fertilized one to two times per year using slow-release 

fertilizers (Ferguson, 1995). In 1992, approximately 80 

percent of the growers surveyed reported that their crops 

were irrigated with microsprinklers or drip systems and 

60 percent of these systems also were designed for fertigation (Ferguson, 1995). Fertigation systems deliver both 

irrigation water and liquid fertilizer.

Approximately 30,000 tons of nitrogen fertilizer and 

about 5,200 tons of phosphorus fertilizer were sold in 

1997 in the counties within and upstream of the study area 

(D. Lorenz, U.S. Geological Survey, written commun., 

1999). Approximately 18,700 tons of nitrogen and 

5,400 tons of phosphorus in these counties were generated each year as part of livestock manure, based on 1997 

data (B. Ruddy, U.S. Geological Survey, written commun., 

1999). Fertilizer sales data were compiled based on State 

sales totals. County-level fertilizer sales data were estimated by disaggregating the State fertilizer sales totals 

based on agricultural land cover. As a result, these data 

underestimate fertilizer use in areas where non-agricultural fertilizer use may be substantial, such as the Orlando 

metropolitan area. The largest amounts of fertilizer sold, 

in terms of total amounts and amount per area, were in 

Indian River and Orange Counties. The largest amount of 

nitrogen and phosphorus generated as part of livestock 

manure, again in terms of total amounts and amount per 

area, were in Okeechobee and Orange Counties; however, 

Okeechobee County accounts for a small part of the study 

area (about 3 percent).

Forests cover about 16 percent of the study area, and 

are concentrated in the northern part of the study area. 

Urban land comprises about 14 percent of the study area, 

and is concentrated in the Orlando metropolitan area and 

in the cities of Cocoa and Titusville. St. Augustine grass 

is the most commonly planted turfgrass in the urban areas 

in Florida (Trenholm and others, 2000). Two to six fertilizer applications from March-October are recommended 

for St. Augustine grass (Trenholm and others, 2000). 

Fertilizer containing nitrogen (50 percent soluble and 

50 percent slow release), phosphorus, and potassium is 

recommended to be applied in March and October.

Wastewater

In 1995, 240 Mgal/d of municipal wastewater was 

discharged in Brevard, Indian River, Lake, Okeechobee, 

Orange, Osceola, Seminole, and Volusia Counties 

(Marella, 1999). Approximately 65 percent of this wastewater was treated and discharged to the ground. Municipal wastewater is discharged to streams by a few facilities 

in the study area. The largest of these is a tertiary treatment facility located on the Little Econlockhatchee River. 

In 2003, this facility had a total permitted capacity of 

40 Mgal/d (Florida Department of Environmental Protection, 2003b). About one-half of the treated wastewater 

from this facility is discharged to the Little Econlockhatchee River; the remaining effluent is discharged to a 

constructed wetlands area where it undergoes further 

nitrogen and phosphorus removal (City of Orlando, 

2002). The effluent from the constructed wetlands flows 

through a State-owned reserve and discharges into the 

St. Johns River.

Septic tanks are prevalent in the study area (St. Johns 

River Water Management District, 2002) and also may be 

a source of contaminants to the St. Johns River. Within 

the middle St. Johns River Basin, there are approximately 

176,000 septic systems (Florida Department of Environmental Protection, 2003c). When septic systems fail, 

inadequately treated wastewater may be transported to 

streams by runoff or as ground-water inflow.

Approach and Methods

Streamflow characteristics are described in this 

report by using flow-duration hydrographs, flow-duration 

statistics and curves, and low-flow frequency statistics. 

Flow-duration hydrographs show the general pattern and 

variability in streamflow that occurs during the year, 

based on a given period of record.

Flow-duration curves show the percent of time 

streamflows are equaled or exceeded during a given 

period (Searcy, 1959). The shape of the flow-duration 

curve provides information about the hydrologic characteristics and variability of streamflow in a drainage basin. 

A flow-duration curve with a steep slope results from 
Introduction 11

a stream with highly variable flow, whereas a flowduration curve with a flat slope indicates the presence of 

surface- or ground-water storage that tends to minimize 

streamflow variations. The shape of the flow-duration 

curve may vary depending upon the period of record 

selected for analysis, particularly at the low-flow end. 

The longer the period of record selected for analysis, the 

more the flow-duration curve will represent long-term 

conditions.

Low-flow frequency statistics relate the magnitude 

of a low-flow event to its average frequency of occurrence. Low-flow frequency statistics commonly are used 

to evaluate the water-supply and waste-dilution potential 

of streams, and to establish minimum streamflows for 

regulatory purposes.

The water quality of the St. Johns River and selected 

tributaries was characterized by using several sources of 

information, including retrospective data, biweekly data 

at selected St. Johns River sites from 2000-02, and 

continuous physical properties data. The primary sources 

of water-quality data used in this report were the retrospective and biweekly data. Retrospective data from 

1991-99 were used to document water-quality conditions 

at a greater number of sites than could be sampled on a 

biweekly basis from 2000-02, to characterize water-quality 

conditions over a wider range of hydrologic conditions 

compared to 2000-02, and to assess temporal trends in 

constituent values. Biweekly data from 2000-02 at 

selected St. Johns River sites were used to provide more 

frequent information to augment the retrospective data 

(which generally were collected monthly or bimonthly), 

and to provide information on a wider range of constituents compared to the retrospective data. Quarterly waterquality data also were collected from the Wekiva River 

and Blackwater Creek from 2000-02 to provide information on the quality of tributary inflow to the St. Johns 

River. Water-quality data were collected quarterly downstream of Blue Springs (in the spring run) from 2000-02 

as part of USGS data collection activities (U.S. Geological Survey, 2000; 2001; 2002). These data were compiled 

to document the quality of springflow to the St. Johns 

River and to better explain the spatial variation in constituent values along the river.

Continuous specific conductance data were collected 

primarily in water years 2001 and 2002 at four sites to 

provide more detailed information on the variability of 

dissolved inorganic constituents in the St. Johns River. 

Specific conductance is an indicator of the concentration 

of dissolved inorganic constituents in the water. In the 

upstream part of the study area, highly mineralized water 

was reported in the river during drought conditions 

(Lichtler and others, 1968), which would influence water 

treatment strategies. Streamflow reversals occur in the 

St. Johns River as a result of wind and tidal effects, which 

also may influence inorganic constituent concentrations.

Streamflow Data

Streamflow data were compiled from the USGS 

National Water Information System (NWIS). Continuous 

data were obtained from eight sites on the St. Johns River, 

the Econlockhatchee River, Wekiva River, and Blackwater 

Creek (sites 1, 3, 6, 8, 12, 14, 15, and 21; fig. 4, table 1).

Data were not compiled from other sites with continuous 

streamflow data because these sites generally had 2 years 

or less of data. This period is too short to develop flowduration curves, which are representative of long-term 

conditions, or accurate low-flow frequency statistics. 

Although there was less than 10 years of streamflow data 

from the Sanford site (site 12, fig. 4, table 1), these data 

were compiled and analyzed because the St. Johns River 

in the vicinity of the Sanford site may be used as an alternate source of water supply in central Florida.

Retrospective Water-Quality Data

 Water-quality data were available from approximately 100 sites on the St. Johns River and its direct 

tributaries within the study area from the U.S. Environmental Protection Agency's STORET Legacy Data 

Center. At least 20 years of water-quality data collected 

on a regular frequency were available from some sites. 

Water-quality data also were available for periods spanning 20 to 30 years at other sites, but with large gaps in 

the period of record. Data were reported from only one 

sampling event at many sites. At other sites, no waterquality data had been collected recently; the most recent 

data were about 20 to 30 years old.

The criterion for inclusion of retrospective waterquality data for this study was that a minimum of 4 years 

of data collected at least quarterly were available at a site 

from 1991-99. This criterion was established to obtain 

data representing more recent conditions and to reduce the 

amount of data to be analyzed to a size manageable within 

the timeframe of this study. Thirty water-quality sites 

sampled by the FDEP, Florida Fish and Wildlife Conservation Commission (FFWCC), Orange County Department of Environmental Protection (hereinafter referred to 

as Orange County), SJRWMD, Volusia County Department of Environmental Health (hereinafter referred to as 

Volusia County), and USGS met this criterion (app. A). 

Data from Orange County, the SJRWMD, and Volusia 
12 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

County were available from both the U.S. Environmental 

Protection Agency's STORET Legacy Data Center and 

the agency's internal database. Data from these three 

agencies were obtained from the internal databases 

instead of from the STORET Legacy Data Center because 

personnel of these agencies indicated their databases were 

the best sources of data.

Data were compiled on water pH, specific conductance, water temperature, and concentrations of nutrients, 

dissolved oxygen, phytoplankton chlorophyll-a, and total 

suspended solids because information on these constituents is necessary to assess the feasibility of using the 

St. Johns River as an alternate source of water supply. 

Information on water color and concentrations of major 

ions, organic carbon, and total dissolved solids also is 

needed to assess the feasibility of using the river as a 

source of water supply. Information on these constituents 

was not compiled, however, because the period of record 

for these data was limited.

Water-quality samples collected by the FFWCC, 

Orange County, USGS, and Volusia County were 

analyzed by the respective agency’s laboratory. Some 

samples collected by the SJRWMD were analyzed by the 

agency's laboratory. When the sample backlog at the 

SJRWMD laboratory became too great, however, waterquality samples were sent to several contract laboratories 

for analysis (S. Connors, Database Manager, St. Johns 

River Water Management District, written commun., 

2001).

Specific conductance and water pH data were 

reported as determined in the field or laboratory. In this 

report, specific conductance and water pH values determined in the field were used as the primary source of 

information. For dates when no field-determined values 

were available, laboratory-determined values were used 

to supplement the record. There may be substantial differences between field- and laboratory-determined specific 

conductance and water pH values, especially for water 

pH values because biological processes can influence 

Table 1. Locations of sites with streamflow and water-quality data analyzed in this study.

Map ID

(fig. 4) Station name

Period of record for 

streamflow data 

(water years)

Streamflow 

data

Period of record for 

retrospective data 

(calendar years)

Retrospective 

water-quality 

data

2000-02 

water-quality 

data

1 St. Johns River near Cocoa 1953-2000 x 1979-99 x x

2 St. Johns River at Beeline Highway 1988-99 x

3 St. Johns River near Christmas 1933-2000 x 1980-99 x x

4 St. Johns River at Orange Mound 1981-99 x

5 St. Johns River at Puzzle Lake 1980-81, 1983-99 x

6 Econlockhatchee River near Chuluota 1935-2000 x

7 Econlockhatchee River near the mouth 1973-99 x

8 St. Johns River above Lake Harney near Geneva 1982-2000 x 1980-99 x

9 St. Johns River at north end of Lake Harney 1981-99 x

10 St. Johns River at Lemon Bluff 1991-99 x

11 St. Johns River at State Road 415 bridge 1991-99 x

12 St. Johns River near Sanford 1987-1989, 1996-2000 x 1968-99 x x

13 St. Johns River south of the confluence 1987-99 x

 with the Wekiva River

14 Wekiva River near Sanford 1935-2000 x x

15 Blackwater Creek near Cassia 1968-1969, 1985-2000 x x

16 Wekiva River near the mouth 1987-99 x

17 St. Johns River north of the confluence 1987-99 x

 with the Wekiva River

18 St. Johns River downstream of Blue Springs 1991-99 x

19 Blue Springs near Orange City x

20 St. Johns River near Hontoon Marina 1991-99 x

21 St. Johns River near DeLand 1933-2000 x 1948-95 x x
Introduction 13

streamwater pH. Paired field- and laboratory-determined 

water pH values were compiled and analyzed from the 

retrospective data to determine whether there was any 

significant difference between field- and laboratorydetermined values. Paired field- and laboratory-determined water pH values were available at three sites with 

retrospective data (table 2). The sign test (Helsel and 

Hirsch, 1992) was used to quantify differences between 

field- and laboratory-determined water pH values. The 

exact form of the test was used when comparing sample 

sizes less than 20. No statistically significant differences 

were found at the 0.05 significance level.

Nutrient data compiled for this report consisted of 

ammonia nitrogen, nitrate nitrogen, organic plus ammonia 

nitrogen (Kjeldahl), total phosphorus, and orthophosphate 

concentrations. Ammonia data were reported as “total” or 

“dissolved” concentrations. Throughout this report, the 

term “total” refers to a concentration determined on an 

Figure 4. Location of study area and stream sites.
14 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

unfiltered sample, and the term “dissolved” refers to a 

concentration determined on a filtered sample. Total 

ammonia concentration data were used as the primary 

source of retrospective data because most agencies 

reported total ammonia concentrations. Dissolved 

ammonia concentration data were used to supplement the 

record when total ammonia concentration data were not 

available.

Total nitrite-plus-nitrate, dissolved nitrite-plusnitrate, or total nitrate concentrations were reported by 

agencies having retrospective information. These three 

constituents were combined and are referred to collectively as nitrate concentrations throughout the remainder 

of this report. Aggregating these three constituents was 

considered valid because nitrite concentrations generally 

are small compared to nitrate, and nitrate generally occurs 

in the dissolved phase because nitrate as a negatively 

charged species does not sorb to sediment particles.

Total organic-plus-ammonia nitrogen (Kjeldahl) 

concentration data generally were available from all of the 

agencies having retrospective data, except the FFWCC. 

Water-quality data obtained from the FFWCC reported 

total ammonia nitrogen and total organic nitrogen concentrations separately. Total organic-plus-ammonia nitrogen 

concentrations were calculated at sites sampled by the 

FFWCC by summing total organic nitrogen and total 

ammonia nitrogen concentration data. In this calculation, 

censored values (values below the method reporting limit) 

were set to one-half the reporting limit.

Orthophosphate concentration data were reported as 

determined on filtered or unfiltered samples, depending 

on the agency. Total and dissolved concentrations of 

these two constituents were combined and are referred to 

as orthophosphate concentrations throughout this report. 

Preliminary analyses of the data indicated little difference 

between the total and dissolved forms of orthophosphate.

Chlorophyll-a concentration data were reported as 

determined by using the spectrophotometric method or 

the trichromatic method. Only chlorophyll-a concentration 

data reported as determined by the spectrophotometric 

method were compiled because the trichromatic procedure does not correct measured concentrations for interferences caused by the presence of pheophytin-a, a 

chlorophyll degradation product (American Public Health 

Association, 1986).

Water-quality data were qualified by using several 

types of remark codes. USGS concentration data below 

the method reporting limit were qualified using a “<” 

(less-than) remark code. Water-quality data from the 

FDEP, Orange County, SJRWMD, and Volusia County 

were qualified by several remark codes (table 3). Data 

values with a remark code of “J,” which indicated the 

value was estimated and not the result of a laboratory or 

field analysis, were not compiled in this report. Method 

detection limits associated with the remark codes “I,” “T,” 

“U,” and “W” were obtained from the FDEP, FFWCC, 

Orange County, and Volusia County. Method detection 

limits associated with the SJRWMD data were not readily 

available. Method detection limits for SJRWMD data 

were assumed to be the highest concentration reported 

with the remark codes “K,” “U,” “I,” “T,” or “W.” 

Table 2. Retrospective water-quality sites with paired field- and 

laboratory-determined water pH values and sign test results.

[FDEP, Florida Department of Environmental Protection; USGS, U.S. 

Geological Survey; p, p-value; n, number of samples]

Map ID

(fig. 4) Station name Agency Sign test results

8 St. Johns River upstream 

of Lake Harney, near 

Geneva

FDEP p = 0.3036, n = 15

12 St. Johns River near 

Sanford

FDEP p = 0.4018, n = 16

21 St. Johns River near 

DeLand

USGS p = 0.4207, n = 24

Table 3. Remark codes associated with retrospective water-quality 

data collected by Florida Department of Environmental Protection, 

Orange County, Volusia County, and the St. Johns River Water 

Management District.

Remark 

code Explanation

I The value reported is less than the practical quantification limit and higher than or equal to the method 

detection limit.

J Estimated. Value shown is not a result of analytical 

measurement.

K Off-scale low. Actual value not known, but known to be 

lower than value shown.

L Off-scale high. Actual value not known, but known to be 

higher than value shown.

Q Sample held beyond normal holding time.

T Value reported is less than the criterion of detection.

U Material was analyzed, but not detected. Value stored is 

the limit of detection for the process in use.

W Value observed is less than the lowest value reportable 

under remark “T.”
Introduction 15

Water-quality data from agencies other than the 

USGS were censored at twice the method detection limit 

to minimize the risk of utilizing false negative measurements. Table 4 shows the censoring limits used in this 

report. A false negative is defined as a constituent 

reported as not being present in the sample when it is 

present at a concentration greater than the method detection limit. Childress and others (1999) reported that a 

sample with a true concentration at the method detection 

limit calculated according to U.S. Environmental Protection Agency (USEPA) procedures has a 50-percent chance 

of being reported as a false negative. Chlorophyll-a data 

were analyzed as reported and not censored because no 

information was available on method detection limits.

Retrospective water-quality data were available from 

two or more agencies at several St. Johns River sites and 

near the mouths of the Econlockhatchee and Wekiva 

Rivers. Water-quality data from these sites were pooled; 

this resulted in data from a total of 17 sites (table 1, fig. 4). 

There may be differences in constituent values reported 

by the agencies as a result of differences in sampling, 

processing, and analytical methods. Differences in 

constituent concentrations reported among the various 

agencies were not assessed due to insufficient data.

Data Collected for this Study

From January 2000 to September 2002, biweekly 

water-quality data were collected at four St. Johns River 

sites (sites 1, 3, 12, and 21), and quarterly data were 

collected from sites on the Wekiva River near Sanford 

and Blackwater Creek sites (sites 14 and 15) (fig. 4). All 

sites sampled from 2000-02 were co-located with existing 

USGS stream-gaging sites because water-quality constituent concentrations often vary with streamflow. Waterquality samples at each of these sites were analyzed to 

determine concentrations of about 40 inorganic constituents (app. B). Total calcium, total magnesium, total 

barium, total strontium, total nitrite-plus-nitrate, total 

nitrite, and total organic carbon concentrations were 

determined only during water years 2000-01. Total 

organic-plus-ammonia nitrogen concentrations were 

determined only in water year 2002. In addition, quarterly 

major ion, nutrient, and trace constituent data collected as 

part of the USGS network (U.S. Geological Survey, 2000; 

2001; 2002) at Blue Springs (site 19, fig. 4) were included 

in some analyses. That site is in the spring run, downstream of the spring pool but upstream of backwater from 

the St. Johns River.

Limited sampling was conducted at two St. Johns 

River sites (sites 1 and 12, fig. 4) in water years 2002 and 

2003 to determine the occurrence of human and veterinary 

antibiotics, other human prescription and nonprescription 

drugs, and a suite of organic constituents that may indicate 

domestic or industrial waste (app. B). More than 70 

constituents were included in this suite of organic constituents, such as, pesticides, plasticizers, fragrances, fire 

retardants, detergent metabolites, and disinfectants. Site 1 

was sampled three times from December 2001 to August 

2002. Human and veterinary antibiotics and the suite of 

organic constituents that may indicate wastewater were 

analyzed at this site. Site 12 was sampled three times from 

June 2002 to March 2003. One set of samples also was 

collected in March 2003 at a pilot water treatment facility 

on Lake Monroe to determine the occurrence of these 

organic constituents: (1) in the influent water to the pilot 

Table 4. Censoring limits applied to retrospective water-quality data.

[FDEP, Florida Department of Environmental Protection; FFWCC, Florida Fish and Wildlife Conservation Commission; Orange County,

Orange County Department of Environmental Protection; SJRWMD, St. Johns River Water Management District; Volusia County, Volusia 

County Department of Environmental Health; USGS, U.S. Geological Survey. All units in milligrams per liter; N, nitrogen; P, phosphorus]

Agency Nitrate

(as N)

Organic 

plus 

ammonia 

nitrogen

(as N)

Total 

phosphorus

Orthophosphate

(as P)

Total 

suspended 

solids

Ammonia

(as N)

FDEP 0.01 0.12 0.01 Not compiled Not compiled 0.02

FFWCC .06 .08 No data Not compiled No data .08

Orange County .02 .4 .01 0.01 2 .02

SJRWMD .02 .1 .01 .02 10 .01

Volusia County .02 .3 .06 .006 4 Not analyzed 

USGS .05 No censored data .01 .01 Not compiled .01
16 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

plant; (2) at the point in the treatment process immediately before reverse osmosis; and (3) in the finished 

water. Human and veterinary antibiotics were not 

analyzed in these samples because none were detected at 

sites 1 and 12. The March low-flow period was selected 

because preliminary results from sampling at sites 1 and 

12 indicated more constituents generally were detected at 

greater concentrations during low-flow periods.

Land use and land cover varied among the sites 

sampled, based on 1995 data from the SJRWMD. The 

drainage areas of the Cocoa and Christmas sites (sites 1 

and 3, fig. 4) contained the greatest percentage of agricultural land, about 40 percent (table 5). The area surrounding the Wekiva River near Sanford site (site 14) had the 

greatest percentage of urban land of the sites sampled; 

almost one-half of the drainage area of this site was 

composed of urban land. 

Sample Collection and Analysis Methods

For this study, most water-quality data were 

collected in accordance with USGS protocol (U.S. 

Geological Survey, 1997-99). Stream velocities generally 

were too low to use isokinetic depth-integrating samplers 

(U.S. Geological Survey, 1997-99); water samples were 

collected at three points across the stream using a 

weighted glass bottle. The glass bottle was lowered from 

the surface to the bottom of the water column at each 

point to obtain a depth-integrated sample. Field measurements of water temperature, specific conductance, water 

pH, and dissolved oxygen concentration also were made 

at the time of sampling. Prior to sampling, water-quality 

equipment was decontaminated by soaking in a 2 percent 

solution of LiquinoxTM and tap water, rinsed with tap 

water, then rinsed with deionized water, and allowed to 

air dry. Equipment used to collect and process samples 

that were analyzed to determine concentrations of human 

and veterinary antibiotics, other human prescription and 

nonprescription drugs, pesticides, and the suite of organic 

constituents that may indicate domestic or industrial 

waste also were rinsed with pesticide-grade methanol to 

remove any trace amounts of these constituents prior to 

sampling. Decontaminated sampling equipment was 

stored in clean plastic bags prior to use. Prior to sample 

collection, all equipment was rinsed with native stream 

water. Samples analyzed for dissolved inorganic constituents were filtered through a 0.45-micron (μ) encapsulated 

filter. Samples analyzed for human and veterinary antibiotics and other human prescription and nonprescription 

drugs were filtered through a 0.7-μ nominal pore size 

glass-fiber filter (precombusted at 450 degrees Celsius 

(

o

C)). Dissolved and total organic carbon samples were 

collected at the center of the stream by using a weighted 

glass bottle (precombusted at 450 o

C). The protocol for 

processing dissolved organic carbon samples changed 

during this study from filtering through a 0.45-μ silver 

filter to filtering through a 0.45-μ encapsulated filter due 

to the decreasing availability of silver filters. There is no 

significant difference between the results obtained using 

silver filters and encapsulated filters (U.S. Geological 

Survey Office of Water Quality Technical Memorandum 

2000.08). Dissolved organic carbon samples collected 

from January 2000 through February 2001 were filtered 

through a 0.45-μ silver filter. From March 2001 to 

September 2002, dissolved organic carbon samples were 

filtered through a 0.45-μ encapsulated filter and preserved 

with 1 milliliter (mL) of 4.5N sulfuric acid. Equipment 

used to filter dissolved organic carbon samples was rinsed 

with water certified to be free of carbon and wrapped in 

aluminum foil in accordance with USGS protocol 

(U.S. Geological Survey, 1997-99). Sulfide samples were 

collected as surface grab samples at the center of the 

Table 5. Land-use and land-cover percentages of the drainage area upstream of the sites sampled for this study.

[Percentages may not total 100 due to rounding]

Map ID

(fig. 4) Station name Urban Agriculture Rangeland

Upland 

forests Water Wetlands Barren land

Transportation, 

communication, 

and utilities

Not 

classified

 1 St. Johns River near Cocoa 5.3 44.5 5.2 8.7 3.6 29.3 0.7 0.7 1.1

 3 St. Johns River near Christmas 6.2 39.7 6.9 10.4 3.4 30.9 .6 .9 .9

12 St. Johns River near Sanford 11.0 27.5 7.1 14.7 5.2 32.2 .5 1.3 .5

14 Wekiva River near Sanford 45.5 9.0 6.2 15.0 4.4 18.0 .3 1.6 0.

15 Blackwater Creek near Cassia 11.1 17.0 6.0 32.5 6.1 26.6 .4 .1 0.

21 St. Johns River near DeLand 13.8 25.4 6.9 16.0 5.1 30.6 .5 1.2 .5
Introduction 17

stream and were preserved immediately with sodium 

hydroxide and 0.5 gram of zinc acetate. Samples analyzed 

for major cations, such as calcium and magnesium, were 

preserved with 2 mL of 6N nitric acid. Samples analyzed 

to determine total phosphorus concentrations were 

preserved with 1 mL of 4.5N sulfuric acid. Chlorophyll-a

samples were filtered through a 0.7-μ glass-fiber filter 

which was wrapped in aluminum foil and kept frozen 

until shipped to the laboratory.

Most water-quality samples were analyzed at the 

USGS Water Quality and Research Laboratory in Ocala, 

Florida. Samples were analyzed at the USGS National 

Water-Quality Laboratory in Denver, Colorado, to determine human prescription and nonprescription drugs and 

the suite of organic constituents that may indicate domestic or industrial waste. Analytical methods used to determine constituent concentrations are listed in appendix B.

Quality Assurance

Information on the bias and variability in a data set is 

necessary to correctly interpret water-quality data. Bias 

and variability may exist in a data set as a result of sample 

collection, processing, or analytical procedures. Bias is 

the systematic error inherent in a method and may be 

either positive or negative. Positive bias may result from 

the introduction of contaminants into the sample during 

sampling or processing procedures, laboratory analysis, 

exposure to airborne gases and particulates, or due to 

inadequately cleaned sample collection or analytical 

equipment. Variability is the random error in independent 

measures of the same quantity.

Contamination bias was assessed in the data 

collected as part of this study by collecting 16 field blank 

samples. A field blank is a water sample that is intended 

to be free of the analytes of interest and is prepared at the 

sampling location. No field blank samples were analyzed 

to determine concentrations of human and veterinary 

antibiotics, other human prescription and nonprescription 

drugs, pesticides, or the suite of organic constituents that 

may indicate domestic or industrial waste. Field blank 

samples were prepared as described by Mueller and 

others (1997). Briefly, field blank samples were prepared 

by pouring water, certified by the USGS to be free of the 

constituents listed in table 6, through all of the sample 

collection and splitting equipment as if an environmental 

sample were being processed. These samples were 

filtered, preserved, and shipped to the laboratory according to USGS protocol (U.S. Geological Survey, 1997-99).

Contamination bias in the water-quality data was 

quantified by constructing an upper tolerance bound on 

the field-blank concentration data. A tolerance bound is a 

statistical interval that contains a specified proportion of a 

sampled population. In analyses of water-quality data, an 

upper tolerance bound is constructed to determine a limiting concentration that most values do not exceed. Because 

the distributions of the field-blank concentration data 

were highly skewed, distribution-free methods (Hahn and 

Meeker, 1991) were used to calculate upper tolerance 

bounds. Upper tolerance bounds, which 85 percent of the 

concentrations generally did not exceed, were calculated 

with a 70-percent confidence level (table 6). The 70-

percent confidence level was selected because there were 

not sufficient field-blank data to calculate upper tolerance 

bounds with a greater confidence level.

Variability was assessed in the data collected as part of 

this study by collecting 12 concurrent replicate samples. 

Concurrent replicate samples are used to assess the variability introduced by sample collection, processing, handling, and laboratory analysis procedures. Concurrent 

replicate samples were prepared by collecting two waterquality samples at approximately the same time. Each of 

the samples was processed and analyzed separately. No 

concurrent replicate samples were analyzed to determine 

concentrations of human and veterinary antibiotics, other 

human prescription and nonprescription drugs, pesticides, 

or the suite of organic constituents that may indicate 

domestic or industrial waste.

The variability in constituent concentrations was quantified by calculating the mean relative standard deviation 

for each of the constituents. The relative standard deviation is defined as the standard deviation divided by the 

mean concentration. Censored values were handled in the 

calculation by arbitrarily setting each censored value to 

one-half the method reporting limit.

The constituents with the widest variability were 

chlorophyll-a, turbidity, total suspended solids, total 

ammonia nitrogen, and sulfide (table 7). The mean relative standard deviation was greater than 10 percent for 

these constituents. The large variability in chlorophyll-a

concentrations (38 percent) primarily resulted from four 

pairs of replicate samples in which chlorophyll-a was 

detected in one sample but not in the other. When these 

four pairs were eliminated from the analysis, the variability in chlorophyll-a concentrations was 11.1 percent for 

the remaining eight pairs. Wider variability in chlorophyll-a, turbidity, total dissolved solids, total ammonia 

nitrogen, and sulfide concentrations was consistent 

with the precision of the laboratory analytical methods.
18 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Table 6. Statistical summary of field-blank samples for water-quality data collected for this study.

[mg/L, milligrams per liter; <, less than; μg/L, micrograms per liter; NTU, nephelometric turbidity units; μS/cm, microsiemen per centimeter; 

°C, degrees Celcius; nm, nanometers; cm, centimeters]

Constituent Number of fieldblank samples Method reporting limit

Upper tolerance 

bound for 85 percent 

of the samples

Total sulfide, mg/L 13 1 <1*

Dissolved calcium, mg/L 16 .02 .06

Dissolved magnesium, mg/L 16 .03 <.03

Dissolved sodium, mg/L 16 .1 <.1

Dissolved potassium, mg/L 16 .1 <.1

Dissolved chloride, mg/L 16 .05 .06

Dissolved sulfate, mg/L 16 .07 <.07

Dissolved silica, mg/L 16 .01 .04

Dissolved barium, μg/L 16 .5 <.5 

Dissolved iron, μg/L 16 2 <2.0

Total iron, μg/L 16 2 2.9

Dissolved strontium, μg/L 16 .5 <.5

Dissolved bromide, mg/L 16 .05 <.05

Dissolved ammonia nitrogen, mg/L as nitrogen 16 .01 <.01

Total ammonia nitrogen, mg/L as nitrogen 16 .01 .01

Dissolved nitrite nitrogen, mg/L as nitrogen 16 .01 <.01

Dissolved nitrite plus nitrate nitrogen, mg/L as nitrogen 16 .02 <.02

Total phosphorus, mg/L 16 .02 <.02

Dissolved orthophosphate phosphorus, mg/L as phosphorus 16 .01 .02

Total orthophosphate phosphorus, mg/L as phosphorus 16 .01 .02

Dissolved organic carbon, mg/L 16 .1 3.2

Turbidity, NTU 16 .05 .40

Color, platinum-cobalt units 16 5 <5

Laboratory specific conductance, μS/cm 16 1 2

Total nonfilterable residue at 105 °C

(total suspended solids), mg/L

16 1 1

Dissolved residue at 180 °C

(total dissolved solids), mg/L

16 1 6

Phytoplankton chlorophyll a, μg/L 13 .1 <.1*

Acid-neutralizing capacity, mg/L 16 1 5.50

Ultraviolet absorbance at 254 nanometers, cm-1 16 .001 .004

*70-percent upper tolerance bound for 60 percent of the samples.
Introduction 19

The laboratory reported a relative standard deviation of 

13.9 percent for chlorophyll-a, 15.8 percent for turbidity, 

9.2 percent for total suspended solids, 8.7 to 12.2 percent 

for total ammonia nitrogen depending upon the concentration range measured, and 18.6 percent for sulfide (U.S. 

Geological Survey, 1999). The variability in the other 

constituents was reported to range from 0.5 to 8.7 percent 

(U.S. Geological Survey, 1999).

Continuous Monitoring Data

Four St. Johns River sites (sites 1, 3, 12, and 21) were 

monitored continuously primarily in water years 2001-02 

using a YSI model 600XL sonde to determine water temperature and specific conductance. Fouling and calibration drift 

affects data from continuous monitors. Continuous monitors 

at all sites were inspected, cleaned, and calibrated according 

to USGS protocol (Wagner and others, 2000) to ensure verifiable results. Sensors on the YSI model 600XL sondes 

generally were cleaned biweekly.

Data Analysis Methods

Low-flow frequency statistics and flow-duration 

curves determined from data with significant temporal 

trends do not represent current conditions. Temporal 

trends in streamflow were determined using Kendall’s tau 

(Helsel and Hirsch, 1992) to select the longest period of 

record of trend-free data. The 0.05 significance level was 

used as the criterion for statistical significance. Locallyweighted scatterplot (LOWESS) smoothing, a robust 

smoothing technique (Helsel and Hirsch, 1992), also 

provided information on significant trends in streamflows.

Flow-duration curves were constructed as described 

by Searcy (1959). Briefly, daily-mean streamflows were 

ranked according to magnitude and placed into about 30 

class intervals. Each class interval represented an incremental range in daily-mean streamflow. The percentage 

of time daily-mean streamflows in each class interval 

were equaled or exceeded was computed. The lower limit 

of each class interval was plotted on the y-axis, and the 

percentage of time daily-mean streamflows in each class 

interval were equaled or exceeded was plotted on the 

x-axis (on a probability scale), and a smooth curve was 

drawn through the data. From this smooth curve, dailymean streamflows that were equaled or exceeded 99, 98, 

95, 90, 85, 80, 75, 70, 65, 60, 55, 50, 45, 40, 35, 30, 25, 

20, 15, 10, 5, 2, and 1 percent of the time were determined.

To compare streamflow characteristics among sites, 

flow-duration curves were constructed for the same time 

period. Flow-duration curves constructed with data from 

different time periods may be substantially different 

because one time period used in the analyses may not have 

included an important high- or low-flow event. At least 

40 years of daily-mean streamflow data were available at 

most of the St. Johns River sites (table 1). A shorter 

period of record (less than 20 years) was available at two 

St. Johns River sites (sites 8 and 12) and at the Blackwater 

Creek near Cassia (site 15). For the two St. Johns River 

sites, flow-duration curves were adjusted to longer time 

periods by using the graphical-correlation method 

described by Searcy (1959). In this method, a relation was 

Table 7. Variability in constituent values for water-quality data 

collected for this study.

[°C, degrees Celcius; nm, nanometers]

Constituent

Mean relative 

standard 

deviation 

(in percent)

Total sulfide 10.2 

Dissolved calcium .4

Dissolved magnesium 1.1

Dissolved sodium 1.6

Dissolved potassium .8

Dissolved chloride 1.2

Dissolved sulfate 1.0

Dissolved silica .3 

Dissolved barium 1.4 

Dissolved iron 4.6 

Total iron 4.3

Dissolved strontium 1.2

Dissolved bromide 9.7

Dissolved ammonia nitrogen 4.9

Total ammonia nitrogen 11.9

Dissolved nitrite nitrogen 3.9

Dissolved nitrite plus nitrate nitrogen 4.9

Total phosphorus 6.3

Dissolved orthophosphate phosphorus 5.7

Total orthophosphate phosphorus 7.8

Dissolved organic carbon 4.5

Turbidity 12.2

Color 8.3

Laboratory specific conductance .6 

Total nonfilterable residue at 105 °C 

(total suspended solids)

11.7

Dissolved residue at 180 °C

(total dissolved solids)

1.1

Phytoplankton chlorophyll-a 38.0*

Acid-neutralizing capacity .5

Ultraviolet absorbance at 254 nm 0

*Mean relative standard deviation was 11.1 percent when four pairs of 

replicate samples were eliminated from the analysis.
20 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

established between the flow-duration curve for the site 

with the shorter record and the site with the longer record 

(termed an index site). This relation was used to estimate 

the duration curve (based on the longer period of record) 

at the site with the shorter streamflow record. In this 

report, the 1-, 2-, 5-, 10-, 15-, 20-, 25-, 30-, 35-, 40-, 45-, 

50-, 55-, 60-, 65-, 70-, 75-, 80-, 85-, 90-, 95-, 98-, and 

99-percent durations were used to establish the relation 

between the short-term and index sites. The smooth line 

establishing the relation was computed using locallyweighted regression (Helsel and Hirsch, 1992).

Low-flow frequency statistics were determined by 

fitting low-flow data to a Pearson Type III distribution 

(Riggs, 1972). In cases where the data did not fit a Pearson 

Type III distribution, low-flow frequency statistics were 

determined using graphical methods. Time series of annual 

minimum low flows over 1, 2, 3, 7, 10, 30, 60, 90, 183, and 

365 consecutive-day periods, commonly referred to as 

N-day annual minimum low-flow values, were used to 

determine low-flow frequency statistics. N-day annual 

minimum low flows typically are determined using a 

climatic year, which usually begins on April 1 or October 

1, to ensure all low-flow data related to a particular drought 

are contained in the same climatic year. In this report, Nday annual minimum low flows were computed by using a 

climatic year beginning on October 1 because the lowest 

streamflows in the study area typically occur from April 

through June. N-day annual low-flow values were fit to a 

Pearson Type III distribution by solving the equation

QN,T = QN + SN x KT,

where,

QN,T is an estimate of the N-day annual minimum 

low-flow at the T-year recurrence interval;

QN is the mean of the N-day annual minimum low 

flows;

SN is the standard deviation of the N-day annual 

minimum low flows; and

KT is a frequency factor that is a function of the skewness of the data and recurrence interval.

Typically, the logarithms of the N-day annual minimum low flow are fit to a Pearson Type III distribution. 

A slight variation of this procedure was used to determine 

low-flow frequency statistics at selected sites where 

streamflow reversals, which are reported as negative 

numbers, occur as a result of tidal and wind effects. Fitting 

logarithms of the N-day annual minimum low flows to a 

Pearson Type III distribution could not be done at these 

sites because the logarithm of a negative number is not 

defined. At these sites, a factor was added to all N-day 

annual minimum low flows to make all values positive. 

The resulting time series of positive numbers were fit to a 

Pearson Type III distribution. All N-day annual minimum 

low flows calculated in this manner fit the Pearson Type 

III distribution well as shown with 7-day annual minimum 

low flows from the St. Johns River near DeLand (fig. 5).

Figure 5. Seven-day annual minimum low-flow values plus 4,000, and the corresponding fit by 

the Pearson Type III distribution for data from the St. Johns River near DeLand, Florida.
Introduction 21

Low-flow frequency statistics for the original time series 

were determined by subtracting the factor from the lowflow frequency statistics computed by using the modified 

time series. In the graphical technique, N-day annual 

minimum low-flow values were ranked and assigned a 

recurrence interval by using the Weibull plotting position 

(Riggs, 1972). These data were plotted and a smooth 

curve was fit through the plotted values. The resulting 

graph was used to determine low-flow frequency values 

at selected recurrence intervals.

The accuracy of low-flow frequency statistics is 

dependent upon the period of record used in the analysis, 

the variability and skewness in the data, and the recurrence interval. Standard error was calculated according to 

Hardison (1969) for selected low-flow frequency statistics when the statistics were determined using a Pearson 

Type III distribution. This procedure assumes the error in 

the estimation of frequency statistics is a function of the 

uncertainty in estimating the mean, standard deviation, 

and skewness of the data.

Variations in water-quality constituent values among 

sites were shown by using boxplots or summary statistics. 

Seasonal variations in constituent values in the retrospective data were shown by using boxplots for six periods (January-February, March-April, May-June, JulyAugust, September-October, and November-December). 

Data were divided into six periods because some sites 

were sampled on a bimonthly frequency from 1991-99. 

Variations in water-quality constituent values among 

sites or by seasonal period were quantified by using a 

one-way analysis of variance (Devore and Peck, 1986, 

chapter 13) and Tukey multiple comparison (Tukey, 

1953, unpublished report, Princeton University). Data 

were transformed when necessary to make the values 

more normally distributed and variances more homogeneous among groups. Relations between water-quality 

constituent values and streamflow were quantified using 

Kendall’s tau (Helsel and Hirsch, 1992) at selected sites 

where both water-quality and continuous streamflow data 

were available (fig. 4). For all statistical tests in this 

report, the criterion for statistical significance was the 

0.05 significance level.

Censored values were present in some of the waterquality data analyzed. Aggregating data from multiple 

agencies also resulted in water-quality data with multiple 

reporting limits. One common approach to handling 

censored water-quality data in statistical analyses is to 

arbitrarily substitute a value, such as one-half the reporting limit, zero, or the reporting limit, for each censored 

value. This method has no theoretical basis and produced 

biased estimates of summary statistics (Helsel, 1990). 

In this report, summary statistics for data with greater than 

5 percent censored values were estimated by using a probability plotting procedure adapted for data with multiple 

reporting limits (Helsel and Cohn, 1988).

Temporal trends in water-quality constituent values 

were determined by using the Seasonal-Kendall test or 

Tobit regression (Helsel and Hirsch, 1992). The SeasonalKendall test is a nonparametric test for monotonic trend. 

This test reduces seasonal variability by making comparisons of data only from similar seasons. In this report, six 

seasonal periods were used when applying the SeasonalKendall test because some sites were sampled only 

bimonthly during part of the 1991-99 period. Streamflowrelated variability in the water-quality data was removed 

by first determining the relation between water-quality 

constituent values and streamflow by using locallyweighted regression. The residuals (predicted minus the 

observed values) from this regression were tested for 

temporal trend by using the Seasonal-Kendall test. 

Locally-weighted regression was selected because this is 

a more robust method of flow adjustment compared to 

simple linear regression.

Tobit regression (Helsel and Hirsch, 1992) was used 

to determine temporal trends in water-quality values when 

more than 5 percent of the observed values were censored. 

Tobit regression is a parametric approach that assumes 

normally distributed errors. This technique also may be 

used to determine temporal trends when there are several 

reporting limits in the data. Flow- and seasonal-variability 

in water-quality constituent values were taken into 

account by including flow and cyclical terms in the 

regression model. In this report, temporal trends in both 

streamflow and water-quality data were considered statistically significant at the 0.05 significance level.

Acknowledgments

The author thanks Mike Gately from the Volusia 

County Department of Environmental Health; Sue 

Connors and Steve Winkler from the St. Johns River 

Water Management District; Ken Cossin from the Orange 

County Department of Environmental Protection; Ted 

Lange from the Florida Fish and Wildlife Conservation 

Commission; and Patrick Detscher, Eric Pluchino, and 

Donald Smith from the Florida Department of Environmental Protection for providing water-quality data or 

information on method detection limits. The author 

expresses appreciation to Bill Kirby of the U.S. Geological Survey for his advice on the determination of lowstreamflow characteristics at sites with reverse streamflows. The author also expresses appreciation to 
22 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Ed Furlong of the U.S. Geological Survey for providing 

pharmaceuticals analyses and Mike Meyer of the U.S. 

Geological Survey for providing antibiotics analyses and 

for their advice on interpretation of those respective data.

Streamflow Characteristics 

The streamflow characteristics of the St. Johns River 

will affect the amount of water that can be withdrawn for 

public supply and the amount of storage necessary to 

maintain the water demand throughout the year. Mean 

annual streamflow in the St. Johns River increases from 

1,030 cubic feet per second (ft3

/s) downstream of Lake 

Poinsett (site 1, fig. 4) to 2,850 ft3

/s near DeLand (site 21, 

fig. 4), based on data from water years 1985-2000. The 

greatest increase in mean annual streamflow occurs 

between Lake Harney and DeLand. Mean annual streamflow increases by 940 ft3

/s along this reach; the majority 

of this increase can be attributed to inflow from the 

Wekiva River, Blackwater Creek, and Blue Springs.

Temporal Trends

Temporal trends in streamflow were determined at 

seven sites that had at least 10 years of data (sites 1, 3, 6, 

8, 14, 15, and 21) on the St. Johns River, Econlockhatchee River, Wekiva River, and Blackwater Creek 

(table 8, fig. 4). Only data from water years 1957-2000 

were analyzed to determine temporal trends at the 

DeLand site (site 21) on the St. Johns River because data 

collected prior to water year 1957 were not considered 

representative for use in developing flow-duration curves 

or low-flow frequency statistics. Daily-mean streamflow 

data at the DeLand site begin in 1933; streamflow at this 

site, however, often is reversed as a result of wind and 

tidal effects. Examination of paper records archived in the 

USGS Altamonte Springs office showed that instrumentation capable of measuring reverse streamflows was not 

installed at this site until 1956. As a result, all streamflow 

data collected at this site prior to water year 1957 is biased 

toward positive values and probably did not accurately 

represent streamflow conditions.

No significant temporal trends were detected in the 

daily-mean or N-day low-flow time series from the 

St. Johns River or Blackwater Creek sites (table 8, fig. 4). 

These results generally were consistent with those 

reported by Rumenik and Grubbs (1996), who determined temporal trends in N-day low flows at most of 

these sites from the beginning of record to 1987.

Significant temporal trends were detected in most of 

the N-day annual minimum low flows from the Econlockhatchee River. Significant increases in N-day annual 

minimum low flows in the Econlockhatchee River are 

consistent with a study by Wanielista and others (1992) 

that determined streamflow volumes in the Econlockhatchee River increased during 1970-90. Increased 7-day 

annual minimum low flows in the Econlockhatchee River 

primarily occurred from about 1950-84 (fig. 6). From 

about 1985-2000, there were no readily apparent increases 

in the 7-day annual minimum low streamflows at this site. 

Similar temporal patterns were found in most of the other 

N-day annual minimum low flows at this site.

Increased low flows at the Econlockhatchee River 

site from about 1950-84 probably resulted from wastewater discharges to the Little Econlockhatchee River or 

modifications to the Little Econlockhatchee River Basin. 

Residential development in the Little Econlockhatchee 

River Basin increased substantially in the 1950’s 

(Michaels and others, 1960). Prior to 1940, most development in the Little Econlockhatchee River Basin was for 

agricultural purposes and consisted primarily of improved 

pasture (Michaels and others, 1960). By 1960, there were 

about six municipal wastewater treatment plants discharging to the Little Econlockhatchee River (Michaels and 

others, 1960). The Little Econlockhatchee River received 

substantial amounts of municipal wastewater effluent 

from about 12 facilities by 1983 (St. Johns River Water 

Management District, 2002). In 1984, wastewater effluent 

from most of the municipal wastewater treatment facilities on the Little Econlockhatchee River were diverted to 

regional municipal wastewater facilities (Miller & Miller, 

1984). Major modifications to Little Econlockhatchee 

River Basin also have occurred since the 1960’s. Many 

reaches of the Little Econlockhatchee River and its tributaries were deepened and channelized to lower the water 

table (St. Johns River Water Management District, 2002), 

creating a network of drainage ditches in the basin. 

Streamflow in the Little Econlockhatchee River also is 

controlled by 13 major water control structures (Miller & 

Miller, 1984). One purpose for installing these structures 

was to prevent water loss from the river system during dry 

periods (Michaels and others, 1960). Changes in evapotranspiration rates resulting from increased urban land 

use also may have affected low streamflows in the Econlockhatchee River Basin. Evapotranspiration rates may 

change with increasing urban land use as a result of a 

loss of vegetation as impervious areas increase. In addition, different types of vegetation may be planted by property owners in newly developed urban areas, which may 
Table 8. Temporal trends in daily-mean and N-day consecutive low-streamflow data from sites with more than 10 years of data on the St. Johns R

River, Wekiva River, and Blackwater Creek.

[Top number, Kendall’s tau; number in parenthesis, p-value; trends significant at the 0.05 significance level are shown in bold]

Map ID

(fig. 4) Station name

Period of 

record 

(water years)

N-day low-streamflow 

1 2 3 7 10 30 60 90 183

 1 St. Johns River near Cocoa 1953-2000 -0.124

(.223)

-0.119

(.240)

-0.118

(.244)

-0.114

(.263)

-0.117

(.248)

-0.112

(.271)

-0.099

(.331)

-0.095

(.350)

-0.084

(.409)

 3 St. Johns River near Christmas 1933-2000 -.026

(.762)

-.026(.762)

-.022(.795)

-.019(.820)

-.014(.875)

-.013(.880)

-.019(.829)

-.009(.922)

-.013(.880)

 6 Econlockhatchee River near 

Chuluota

1935-2000 .593

(0)

.603

(0)

.609

(0)

.618

(0)

.620

(0)

.584

(0)

.469

(0)

.364

(0)

.205

(.016)

 8 St. Johns River upstream of 

Lake Harney, near Geneva

1982-2000 -.135

(.441)

-.099(.576)

-.088(.624)

-.076(.675)

-.053(.780)

-.029(.889)

-.064(.726)

-.076(.675)

-.099(.576)

14 Wekiva River near Sanford 1935-2000 .337

(0)

.329

(0)

.324

(0)

.298

(0)

.287

(.001)

.267

(.002)

.185

(.030)

.155

(.069)

.093

(.284)

15 Blackwater Creek near Cassia 1986-2000 -.200

(.321)

-.219(.276)

-.210(.298)

-.200(.322)

-.219(.276)

-.200(.322)

-.200(.322)

-.181(.373)

-.162(.428)

21 St. Johns River near DeLand 1957-2000 .118

(.256)

.128

(.218)

.103

(.323)

.026

(.807)

.046

(.660)

.018

(.868)

-.018(.868)

-.055(.604)

-.022(.837)
24 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

influence evapotranspiration rates. Water not evaporated 

from the land surface may seep into the ground and eventually be returned to streams as baseflow. In the eastern 

United States, decreasing trends in evaporation were 

found in watersheds with increased urban land use (Dow 

and DeWalle, 2000).

A LOWESS smooth line through the 7-day annual 

minimum low streamflows indicates low streamflows in 

the Wekiva River near Sanford generally increased 

between 1935-68 (fig. 7). There did not appear to be any 

increase in 7-day annual minimum low flows at this site 

from 1968-2000. Subsequent temporal trend analyses on 

Figure 7. Annual minimum streamflow averaged over 7 consecutive days at the Wekiva River near 

Sanford, Florida, 1935-2000.

Figure 6. Annual minimum streamflow averaged over 7 consecutive days at the Econlockhatchee 

River near Chuluota, Florida.
Streamflow Characteristics 25

the 1968-2000 portion of the 7-day annual minimum 

low-flow time series did not detect any significant trend 

(Kendall’s tau = -0.1458, p = 0.2389). Similar results 

were noted in the other N-day low-flow time series at this 

site.

Increased low streamflows at this site may be attributed to basin modifications that began around 1926, especially in the Little Wekiva River Basin. The headwaters 

of the Little Wekiva River originally were near the Seminole/Orange County border, but were extended farther 

south by the construction of ditches to Lake Lawne 

(CDM, 2003) (fig. 4). Originally, the land cover in the 

Little Wekiva River Basin was subtropical forest (Woodward-Clyde and others, 1998). Around 1926, the basin 

began to be developed for citrus farming. By the 1950’s, 

most of the subtropical forest in the basin was replaced by 

orange groves (Woodward-Clyde and others, 1998). To 

promote agriculture, reaches of the Little Wekiva River 

were ditched to lower the water table (Woodward-Clyde 

and others, 1998). Ditches drain ground water into 

streams at a faster rate than under undisturbed conditions 

and act to increase low streamflows for a short period of 

time. The Little Wekiva River Basin began to be urbanized in the 1950’s (Woodward-Clyde and others, 1998). 

Much of the development was done prior to the enactment 

of stormwater management regulations, which resulted in 

periodic flooding problems (CDM, 2003). To alleviate 

flooding, water control structures were placed on the 

outlets of several lakes within and outside of the basin to 

transport water from otherwise closed basins to the Little 

Wekiva River (CDM, 2003). In addition, a canal was 

constructed in the mid-1960’s to connect Lake Lawne to 

Lake Orlando.

Increased low streamflows in the Wekiva River also 

could have resulted from increased springflow; however, 

the data indicate that springflows have not increased. On 

the contrary, flow from Wekiva Springs, which contributes 

about one-third of the baseflow to the Wekiva River, has 

decreased from 1970-2000 (Spechler and Halford, 2001). 

Springflow from Palm Springs, which also discharges to 

the Wekiva River, has decreased from 1970-2000 

(Spechler and Halford, 2001).

Seasonal Variation

Streamflows in the St. Johns River near Christmas 

and Blackwater Creek generally are lowest in May 

(fig. 8). Streamflows also were lowest in May at the other 

St. Johns River sites and in the Econlockhatchee and 

Wekiva Rivers. Low streamflows in May probably result 

from several factors. There is less antecedent rainfall in 

May compared to the June-September wet season, resulting in decreased water storage in lakes and wetlands. 

Also, evapotranspiration rates are greater in May 

compared to January-April due to the high air temperatures and high solar radiation. Streamflows in the 

St. Johns, Econlockhatchee, and Wekiva Rivers generally 

were highest in September and October. Higher streamflows in September and October probably result from 

increased rainfall combined with less available storage in 

the lakes and wetlands due to increased rainfall during the 

wet season. In contrast to the results for the St. Johns, 

Econlockhatchee, and Wekiva Rivers, median streamflows in Blackwater Creek generally were highest in 

February and March, a period when rainfall amounts typically are about 2 to 3 in. lower per month compared to 

June through September (fig. 2). High median streamflows 

in Blackwater Creek during February and March may have 

resulted from differences in water storage between this 

basin and the other basins described in this report.

Streamflow Reversals

Streamflow reversals were observed at all St. Johns 

River sites in the study area. At the Sanford and DeLand 

sites (sites 12 and 21, fig. 4), reverse streamflows typically 

are measured at least once every year and result from wind 

and tidal effects. Examination of hourly streamflow and 

stage data from these two sites showed no tidal signal was 

present at streamflows greater than 3,000 ft3

/s at the 

Sanford site and at streamflows greater than 4,000 ft3

/s at 

the DeLand site. Reverse streamflows at the DeLand site 

occurred, on average, about 4 percent of the time based on 

streamflow data from water years 1957-2000. Reverse 

streamflows occurred at the Sanford site, on average, about 

13 percent of the time, but this percentage is based on a 

shorter period of record (water years 1996-2000). The 

percentage of time reverse flows occurred in an individual 

year at the DeLand site ranged from zero to about 18 

percent. At the Sanford site, the percentage of time reverse 

flows occurred in an individual year ranged from 5 to 22 

percent. The greatest percentage of zero or reverse daily 

mean streamflow observations at the DeLand site tended to 

occur during years with less than average mean annual 

streamflow. There is a significant correlation between the 

annual percentage of zero or reverse streamflow and mean 

annual streamflow (Kendall’s tau = -0.36, p = 0.001).

Streamflow reversals were measured sporadically 

during periods of extreme low flow at the Cocoa, Christmas, and upstream of Lake Harney sites (sites 1, 3, and 8, 

respectively; fig. 4). Instrumentation capable of measuring 

reverse streamflows was installed at these three sites in 
26 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

1993 and 1994. Prior to 1993, it is likely that reverse 

streamflows occurred but were not reported. Streamflow 

reversals at these sites resulted from wind moving the 

water upstream because this reach of the river is not 

affected by tides from the Atlantic Ocean. At the Cocoa 

and upstream of Lake Harney sites, streamflow reversals 

were reported only in water year 2000. At the Christmas 

site, streamflow reversals were reported in water years 

1997, 1999, and 2000. Daily-mean streamflows were zero 

or reverse flow about 9 percent of the time at the Cocoa 

site and about 6 percent of the time at the upstream of 

Lake Harney site in water year 2000. At the Christmas 

site, daily-mean streamflows were zero or reverse flow 

about 3 percent of the time in water year 1997; about 

9 percent of the time in water year 1999; and about 

6 percent of the time in water year 2000.

ST. JOHNS RIVER NEAR CHRISTMAS, FLORIDA

BLACKWATER CREEK NEAR CASSIA, FLORIDA

60

70

80

90

100

200

300

400

500

600

700

800

900

1,000

2,000

3,000

4,000

5,000

7

8

9

10

20

30

40

50

60

70

80

90

100

200

STREAMFLOW, IN CUBIC FEET PER SECOND

OCT. NOV. DEC. JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT.

EXPLANATION

25th PERCENTILE MEDIAN 75th PERCENTILE

Figure 8. Streamflow-duration hydrographs for sites on the St. Johns River and Blackwater 

Creek, water years 1985-2000.
Streamflow Characteristics 27

Flow Duration

Flow-duration statistics were developed using data 

from water years 1957-2000 for the St. Johns River, and 

the longest period of trend-free data for the Econlockhatchee River, Wekiva River, and Blackwater Creek. 

Flow-duration curves also were developed using data 

from water years 1985-2000 to compare streamflow characteristics among these four streams. Flow-duration statistics for both of these periods are listed in appendix C. The 

longest common period of record of trend-free data for 

most of the St. Johns River sites was from water years 

1957-2000. Streamflow records from the site upstream of 

Lake Harney and the Sanford site (sites 8 and 12, respectively, fig. 4) exist only from water year 1982 to the 

present (2004). At these two sites, flow-duration curves 

were extended to the water year 1957-2000 period. The 

Christmas site (site 3, fig. 4) was used as an index site to 

extend the flow-duration curve for the site upstream of 

Lake Harney. The DeLand site (site 21, fig. 4) was used 

as an index site to extend the flow-duration curve for the 

Sanford site. For tributaries to the St. Johns River, the 

longest period of record of trend-free data ranged from 

water years 1968-2000 for the Wekiva River site to water 

years 1985-2000 for the Econlockhatchee River site. 

Flow-duration curves for the Econlockhatchee River, 

Wekiva River, and Blackwater Creek were not extended 

to water years 1957-2000 because suitable index sites 

were not available.

 Flow-duration curves for the St. Johns River sites 

show that streamflows at the Sanford and DeLand sites 

decreased rapidly to below zero during low-flow conditions because of streamflow reversals (fig. 9). Streamflows also were expressed on a unit area basis (fig. 10) to 

eliminate drainage-area size effects in order to better 

compare the flow characteristics at selected sites. Flowduration curves for the Cocoa, Christmas, and upstream of 

Lake Harney sites have the same general shape. The flowduration curve for the Econlockhatchee River site flattens 

during low-flow conditions due to the effects of upstream 

wastewater discharges. For this same reason, streamflows 

at the upstream of Lake Harney site did not decrease as 

rapidly during low-flow conditions compared to the 

Cocoa and Christmas sites. At all St. Johns River sites, the 

slope of the flow-duration curves flattens slightly during 

high-flow conditions because of storage in lakes and 

wetlands. Streamflows in the Wekiva River are substantially less variable compared to those in the St. Johns and 

Econlockhatchee Rivers and Blackwater Creek because 

flow in the Wekiva River depends substantially upon 

springflow from the Floridan aquifer system.

Figure 9. Flow-duration curves for St. Johns River sites, water years 1957-2000.
28 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Low-Flow Frequency Statistics

Low-flow frequency statistics are presented in 

tables 9 to 16. Statistics for the Econlockhatchee and the 

Wekiva River sites were not determined using the entire 

period of record because there were significant temporal 

trends in low streamflows at these two sites. Water years 

1957-2000 were used to determine low-flow frequency 

statistics for the DeLand site because of the change in 

streamflow-computation methods in 1956, which resulted 

in the reporting of reverse flows at this site.

Low-flow frequency statistics at the St. Johns River 

sites should be interpreted with caution, especially at the 

Sanford and DeLand sites, because N-day annual minimum low flows may represent the sum of positive and 

reverse flows. The low-flow frequency statistics presented 

in this report for the Cocoa and Christmas sites on the 

St. Johns River differ substantially from those reported by 

Rumenik and Grubbs (1996). Low-flow frequency statistics at these sites were greatly influenced by reverse 

streamflows. Rumenik and Grubbs (1996) used the streamflow data collected from 1933-87 to determine low-flow 

frequency statistics at these sites; however, equipment 

capable of measuring reverse streamflows was not installed 

at these locations until 1994.

Water-Quality Characteristics 

The 1991-99 data documented a wide range of 

hydrologic conditions from below to above average 

streamflow (table 17). The 2000-02 period documented 

extreme high- and low-flow conditions (fig. 11). Yearly 

rainfall totals measured at the Orlando airport were 

substantially below the long-term average (about 50 in.) 

in water year 2000 (38.99 in.). Totals were near average 

in water year 2001 (55.76 in.) and water year 2002 

(54.35 in.), although more than one-half of the months 

during water years 2001-02 had a rainfall deficit. This 

deficit resulted in streamflows in the St. Johns River at or 

below the 25th percentile during parts of water years 

2000, 2001, and 2002, based on a 43-year period of 

record. Rainfall totals were substantially above average 

in July 2001, September 2001, and June through September 2002. During these months, rainfall totals were 2.7 to 

10.8 in. above the 30-year average, based on the 1972-

2001 period of record. This increased rainfall resulted in 

streamflows greater than the 75th percentile in the latter 

parts of 2001 and 2002, based on a 43-year period of 

record.

Figure 10. Flow-duration curves (expressed on a unit area basis) for sites on the St. Johns River, Econlockhatchee 

River, Wekiva River, and Blackwater Creek, water years 1985-2000.
Water-Quality Characteristics 29

Table 9. Low-flow frequency statistics for the St. Johns River near Cocoa, Florida.

[Site 1, fig. 4. Period of record analyzed was from water years 1953 to 2000; NA, not available. Accuracy not determined 

because statistics determined graphically]

Recurrence 

interval

(in years)

Annual minimum low flow, in cubic feet per second, 

for indicated number of consecutive days

1 2 3 7 10 30 60 90 183

365

(annual 

mean)

 2 95 96 97.5 99.9 112.5 124 159.2 187.2 327.0 995.5

 5 34 37 34.5 38.8 41 45.5 54.8 74.7 126.9 563.9

10 13 17.5 12.5 18.7 18 23.2 30.2 44.0 84.9 375.1

20 3 8 2 8.5 6 10.3 13.1 21.6 53.4 193.4

50 -63 -41 -30 -17.5 -7.5 1.06 NA NA NA NA

Table 10. Low-flow frequency statistics for the St. Johns River near Christmas, Florida.

[Site 3, fig. 4. Period of record analyzed was from water years 1933 to 2000. Accuracy not determined because statistics 

determined graphically]

Recurrence 

interval

(in years)

Annual minimum low flow, in cubic feet per second, 

for indicated number of consecutive days

1 2 3 7 10 30 60 90 183

365

(annual 

mean)

 2 112 115 113.7 122 121.2 158 192.6 229.2 427.8 1,281.3

 5 35.5 37.5 36.9 37 40.5 59 62.2 85.8 175.1 751.8

10 9 11 11.9 11 15.2 29 37.2 49.1 110.5 566.7

20 -27 -24 -20 0 -7 7 17.9 33.9 77.5 358.6

50 -125 -107.5 -84 -77 -66.5 -17 5.2 25.7 57.4 164.0

Table 11. Low-flow frequency statistics for the Econlockhatchee River near Chuluota, Florida.

[Site 6, fig. 4. Period of record analyzed was from water years 1985 to 2000; SEx,y, standard error of the x-day annual minimum 

flow with a recurrence interval of y years. Accuracy, SE7,2 = 2.1 ft3

/s; SE7,10 = 2.0 ft3/s; SE30,2 = 3.5 ft3/s; SE30,10 = 2.9 ft3/s]

Recurrence 

interval

(in years)

Annual minimum low flow, in cubic feet per second, 

for indicated number of consecutive days

1 2 3 7 10 30 60 90 183

365

(annual 

mean)

 2 39.9 40.7 41.3 42.8 44.3 50.4 58.1 69.0 121 304

 5 33.4 34.6 35.1 36.2 37.3 40.5 44.5 50.2 79.3 217

10 30.6 32.0 32.5 33.7 34.5 36.7 39.6 44.6 64.8 178

20 28.5 30.2 30.6 32.0 32.6 34.1 36.4 41.2 55.3 149

50 26.4 28.3 28.8 30.3 30.8 31.6 33.5 38.5 46.8 120
30 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Table 12. Low-flow frequency statistics for the St. Johns River upstream of Lake Harney near Geneva, Florida.

[Site 8, fig. 4. Period of record analyzed was from water years 1982 to 2000; NA, not available. Accuracy not determined because 

statistics determined graphically]

Recurrence 

interval

(in years)

Annual minimum low flow, in cubic feet per second, 

for indicated number of consecutive days

1 2 3 7 10 30 60 90 183

365

(annual 

mean)

 2 235 225 234 254.5 298.8 366 463 533 728 1,846

 5 87.5 95 109 107.5 126.8 152 180 211 335 1,167

10 57.5 56.5 57 66 70 101 158 172 290 926

20 -77 -68 -63 -43 -35 -10 12.4 31.9 101 819

50 -171 -142 -146 -109.5 -98.2 -64 NA NA NA NA

Table 13. Low-flow frequency statistics for the St. Johns River near Sanford, Florida.

[Site 12, fig. 4. Period of record analyzed was from water years 1988 to 1989 and water years 1996 to 2000; SEx,y, standard 

error of the x-day annual minimum flow with a y-year recurrence interval. Accuracy, SE7,2 = 328 ft3

/s; SE7,10 = 341 ft3/s; 

SE30,2 = 237 ft3

/s; SE30,10 =301 ft3/s]

Recurrence 

interval

(in years)

Annual minimum low flow, in cubic feet per second, 

for indicated number of consecutive days

1 2 3 7 10 30 60 90 183

365

(annual 

mean)

 2 -1,100 -820 -658 -287 -174 186 392 518 670 1,910

 5 -1,750 -1,540 -1,410 -1,030 -812 -321 -140 -47 204 1,240

10 -2,010 -1,880 -1,780 -1,420 -1,140 -568 -385 -305 102 990

20 -2,200 -2,140 -2,080 -1,740 -1,400 -764 -570 -499 56 822

50 -2,370 -2,410 -2,420 -2,110 -1,690 -974 -761 -698 27 666

Table 14. Low-flow frequency statistics for the Wekiva River near Sanford, Florida. 

[Site 14, fig. 4. Period of record analyzed was from water years 1968 to 2000; SEx,y, standard error of the x-day annual minimum 

flow with a y-year recurrence interval. Accuracy, SE7,2 = 4.1 ft3

/s; SE7,10 = 5.3 ft3/s; SE30,2 = 6.3 ft3/s; SE30,10 = 7.2 ft3/s]

Recurrence 

interval

(in years)

Annual minimum low flow, in cubic feet per second, 

for indicated number of consecutive days

1 2 3 7 10 30 60 90 183

365

(annual 

mean)

 2 202 202 203 204 205 210 217 223 246 290

 5 182 182 183 184 185 189 194 199 215 255

10 172 173 173 175 176 180 184 188 201 239

20 165 165 166 167 168 173 176 180 190 227

50 156 157 157 159 160 165 168 172 179 214
Water-Quality Characteristics 31

Water Temperature

Water temperature affects many of the chemical and 

biological processes in streams. Growth rates of 

phytoplankton and other aquatic plants also are influenced 

by temperature and tend to increase with greater water 

temperatures. Oxygen, which is essential for the survival of 

fish and other aquatic life, is less soluble in warmer water. 

As a result, oxygen concentrations naturally are lower in 

warmer water compared to cooler water.

The water temperature generally ranged from about 

10 to 35 o

C in the St. Johns River and near the mouths of 

the Econlockhatchee and Wekiva Rivers, based on the 

1991-99 data. Data collected from the St. Johns River 

from 2000-02 showed a similar range in water temperatures. The greatest median water temperatures in the 

St. Johns River upstream of Lake Harney typically 

occurred during July-August; the lowest median temperatures typically occurred from December-January (fig. 12).

Similar seasonal variations were observed at the other 

sites on the St. Johns, Econlockhatchee, and Wekiva 

Rivers. The seasonal variations in water temperature coincide with the general seasonal distribution of air temperatures in the study area (fig. 2). Water temperatures in the 

St. Johns River and near the mouths of the Econlockhatchee and Wekiva Rivers were more variable from 

November through March compared to other times of the 

year, as shown by the wider interquartile range on the 

boxplots in figure 12. Wider variations in water temperatures from November through March resulted from the 

wider variations in air temperature that occurred during 

this period.

Table 15. Low-flow frequency statistics for Blackwater Creek near Cassia, Florida. 

[Site 15, fig. 4. Period of record analyzed was from water years 1968 to 1969 and water years 1985 to 2000; SEx,y, standard error 

of the x-day annual minimum flow with a y-year recurrence interval. Accuracy, SE7,2 = 1.3 ft3

/s; SE7,10 = 0.5 ft3/s; SE30,2 = 1.7 ft3/s; 

SE30,10 = 0.7 ft3

/s]

Recurrence 

interval

(in years)

Annual minimum low flow, in cubic feet per second, 

for indicated number of consecutive days

1 2 3 7 10 30 60 90 183

365

(annual 

mean)

 2 7.0 7.1 7.2 7.6 7.9 9.3 12 14 25 51

 5 3.8 3.8 3.9 4.1 4.2 4.9 5.9 6.9 13 30

10 2.7 2.7 2.7 2.8 3.0 3.4 3.9 4.6 8.5 22

20 2.0 2.0 2.0 2.1 2.2 2.5 2.7 3.3 6.0 17

50 1.4 1.4 1.4 1.4 1.5 1.7 1.8 2.1 3.9 13

Table 16. Low-flow frequency statistics for the St. Johns River near DeLand, Florida. 

[Site 21, fig. 4. Period of record analyzed was from water years 1957 to 2000; SEx,y, standard error of the x-day annual minimum 

flow with a y-year recurrence interval. Accuracy, SE7,2 = 138 ft3

/s; SE7,10 = 195 ft3/s; SE30,2 = 98 ft3/s; SE30,10 = 148 ft3/s]

Recurrence 

interval

(in years)

Annual minimum low flow, in cubic feet per second, 

for indicated number of consecutive days

1 2 3 7 10 30 60 90 183

365

(annual 

mean)

 2 -934 -757 -583 86 303 819 1,064 1,216 1,655 2,870

 5 -1,921 -1,741 -1,551 -733 -407 298 543 669 972 2,030

10 -2,454 -2,278 -2,083 -1,191 -787 44.3 315 439 702 1,645

20 -2,903 -2,733 -2,537 -1,585 -1,106 -156 148 275 519 1,353

50 -3,418 -3,259 -3,062 -2,046 -1,469 -371 -17 117 353 1,052
32 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Water temperatures did not vary as widely in the 

Wekiva River near Sanford (located about 7 mi upstream 

from the mouth), Blackwater Creek, and Blue Springs 

sites compared to the St. Johns River. At the Wekiva 

River near Sanford and Blackwater Creek, water temperatures ranged from about 16 to 26 o

C from 2000-02. At 

Blue Springs, the water temperature ranged from about 23 

to 24 o

C from 2000-02. Water temperatures in the Wekiva 

River near Sanford and Blue Springs probably were not as 

variable compared to those in the St. Johns River because 

a substantial amount of the flow at these sites is contributed by springflow from the Floridan aquifer system.

Dissolved Oxygen

Dissolved oxygen in the water is necessary for the 

survival of fish and other higher forms of aquatic life. The 

equilibrium concentration of dissolved oxygen in streams 

is a function of temperature and pressure, and to a lesser 

extent, the concentration of other solutes. Dissolved 

oxygen in streams may be depleted by processes that 

consume organic matter; values above equilibrium can be 

produced in streams containing actively photosynthesizing biota. The USEPA recommends that the 7-day average dissolved oxygen concentration in streams not be less 

than 5 mg/L to protect sensitive species of fish and other 

aquatic life (U.S. Environmental Protection Agency, 

1986). The State of Florida recommends that dissolved 

oxygen concentrations not be less than 5 mg/L for streams 

designated as potable water supplies, designated as shellfish propagation and harvesting areas, or designated for 

recreational use and the maintenance of healthy fish 

communities (Florida Administrative Code, 2003a).

Dissolved oxygen concentrations equal to or less than 

5 mg/L were reported at all sites during 1991-99 and 

2000-02. Concentrations equal to or less than 5 mg/L 

were reported most frequently at the Beeline Highway, 

Christmas, and Orange Mound sites on the St. Johns 

River, and near the mouth of the Wekiva River during 

1991-99 (fig. 13). Concentrations less than 5 mg/L were 

measured in Blackwater Creek and Blue Springs during 

2000-02; median concentrations were 4.8 and 1.1 mg/L, 

respectively.

Median dissolved oxygen concentrations in the 

St. Johns River during 1991-99 were slightly greater at the 

Cocoa, downstream of Lake Harney, Sanford, upstream of 

the Wekiva River, and downstream of the Wekiva River 

sites compared to other sites on the river (fig. 13). All of 

these sites are located downstream of large lakes on the 

St. Johns River—Lake Poinsett, Lake Harney, and Lake 

Monroe. Increased dissolved oxygen concentrations at 

these sites may have been the result of algal production in 

the lakes; the quiescent lake conditions may have 

provided more optimal conditions for algal growth.

Table 17. Mean annual streamflow for sites in the study area.

[Units are cubic feet per second; NA, not available]

Map 

ID

(fig. 4)

Station name

Water year Mean 

(water 

years 

1985-2000) 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

 1 St. Johns River near Cocoa 1,228 1,132 973 653 2,059 1,380 759 2,007 497 1,120 753 1,746 1,028

 3 St. Johns River near Christmas 1,685 1,603 1,479 891 2,319 1,847 710 2,221 621 1,174 780 2,012 1,261

 6 Econlockhatchee River near 

Chuluota

378 320 358 354 492 438 247 450 170 183 266 390 313

 8 St. Johns River upstream of 

Lake Harney, near Geneva

2,340 2,148 2,239 1,718 3,784 2,586 1,280 3,056 858 1,583 1,155 2,686 1,909

12 St. Johns River near Sanford NA NA NA NA NA 3,499 1,275 3,509 951 2,092 1,258 3,930 NA

14 Wekiva River near Sanford 274 260 285 310 376 382 271 368 254 246 228 293 290

15 Blackwater Creek near Cassia 48 16 56 53 118 102 29 100 31 27 37 65 54

21 St. Johns River near Deland 2,925 2,716 3,251 2,382 4,937 4,445 2,111 4,356 1,551 2,368 1,809 4,137 2,847
Water-Quality Characteristics 33

Dissolved oxygen concentrations had a significant 

inverse relation with streamflow based on Kendall’s tau 

at all St. Johns River sites (table 18). All correlations 

between dissolved oxygen concentration and discharge 

were significant at the 95-percent confidence level. Data 

collected during 2000-02 at the Christmas site show the 

relation between dissolved oxygen concentrations and 

streamflow (fig. 14); at this site, dissolved oxygen 

concentrations approached zero during high-flow periods 

in 2001 and 2002. One likely source of water with low 

dissolved oxygen concentrations to the St. Johns River is 

runoff from wetland areas. For 1989-98, limited 

SJRWMD data from wetlands in the upper St. Johns River 

Basin showed dissolved oxygen concentrations ranging 

from 0.70 to 6.3 mg/L, with a median concentration of 

3.1 mg/L. Lower dissolved oxygen concentrations during 

high-flow conditions also may have resulted from the 

decomposition of organic matter in the river. The distribution of dissolved organic carbon concentrations in the 

St. Johns River will be discussed in detail later in this 

report; however, dissolved organic carbon concentrations 

generally increased during high-streamflow conditions.

Figure 11. Streamflow characteristics and precipitation at St. Johns River sites, 2000-02.
34 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Figure 12. Seasonal variations in water temperature in the St. Johns River upstream of Lake 

Harney near Geneva, Florida, 1991-99.

Figure 13. Dissolved oxygen concentrations in the St. Johns River and near the mouths of the 

Econlockhatchee and Wekiva Rivers, 1991-99.
Water-Quality Characteristics 35

Table 18. Relations between dissolved oxygen concentration and water pH and streamflow at St. Johns River sites, 1991-2002.

[Explanation of values: tau, Kendall’s tau; p, p-value; n, number of observations. Retrospective data were compiled from the Florida Department of Environmental Protection, Florida Fish and Wildlife Conservation Commission, Orange County, St. Johns River Water Management District, Volusia County, and the 

U.S. Geological Survey]

Map ID

(fig. 4) Station name Explanation of values

Dissolved oxygen concentration Water pH values

Retrospective 

data 

(1991-99)

U.S. Geological 

Survey data 

(2000-02)

Retrospective 

data 

(1991-99)

U.S. Geological 

Survey data 

(2000-02)

 1 St. Johns River near Cocoa tau

p

n

-0.1835

.0269

68

-0.1709

.0335

72

-0.5020

0

64

-.4644

0

72

 3 St. Johns River near Christmas tau

p

n

-.4342

0

121

-.3292

0

72

-.4409

0

121

-.5184

0

72

 8 St. Johns River upstream of Lake 

Harney, near Geneva

tau

p

n

-.3196

0

245

Not sampled -.3442

0

240

Not sampled

12 St. Johns River near Sanford tau

p

n

Not determined -.1992

.0140

71

Not determined -.3662

0

72

21 St. Johns River near DeLand tau

p

n

-.3611

0

126

-.3199

.0001

71

-.4798

0

124

-.4941

0

72

Figure 14. Dissolved oxygen concentration and streamflow at the St. Johns River near Christmas, 

Florida, 2000-02 (reverse flow not shown).
36 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Water pH

The pH is the hydrogen ion activity in water and is an 

indicator of the acidity of the water. The pH of water is 

controlled by interrelated chemical and biological reactions that produce or consume hydrogen ions. The 

USEPA recommends a water pH between 6.5 and 9 to 

protect freshwater aquatic life, and a water pH between 5 

and 9 for waters to be used as domestic water supplies 

(U.S. Environmental Protection Agency, 1986). For 

waters designated as public water supplies, the State of 

Florida generally recommends a water pH between 6 and 

8.5 (Florida Administrative Code, 2003b).

Median water pH values in the St. Johns River 

ranged from 6.8 to 7.5 from 1991-99 (fig. 15). Water pH 

values were slightly greater at the Cocoa, downstream of 

Lake Harney, Sanford, upstream of Wekiva River, and 

downstream of Wekiva River sites compared to the other 

sites. Increased water pH values at these sites may have 

resulted from algal activity in Lake Poinsett, Lake 

Harney, and Lake Monroe. Carbon dioxide (CO2) in the 

water is utilized by algae during photosynthesis, which 

tends to reduce the CO2 concentration of the water and 

thus, shifts the carbonate equilibrium to more basic 

conditions (Wetzel, 1983, p. 211). 

Water pH values at the Cocoa and Sanford sites had 

a significant inverse relation with streamflow, based on 

1991-2002 data (table 18, fig. 16). Significant inverse 

relations between water pH and streamflow also were 

observed at other St. Johns River sites (table 18). All 

correlations between water pH values and streamflow 

were significant at the 95-percent confidence level. Lower 

water pH values during high-flow conditions resulted 

from the increased dissolved organic carbon concentrations that also occur in the river during high-flow 

conditions. Dissolved organic carbon in natural waters 

generally is composed of aquatic fulvic and hydrophilic 

acids (Aiken and Cotsaris, 1995), which lower the water 

pH. During high-flow events, the water pH typically 

decreased to less than 7.0.

The 2000-02 data showed a noticeable seasonality 

in the water pH values at the Cocoa and Sanford sites 

(fig. 16). Water pH values at these sites were greater than 

8.5 standard units from about May through June in water 

years 2000 and 2001. Higher water pH values at these 

two sites during these periods also probably resulted from 

algal activity in the St. Johns River or upstream lakes. The 

distribution of chlorophyll-a, an indicator of the amount

Figure 15. Water pH in the St. Johns River and near the mouths of the Econlockhatchee and Wekiva 

Rivers, 1991-99.
Water-Quality Characteristics 37

Figure 16. Water pH and streamflow at St. Johns River sites, 2000-02.
38 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

of phytoplankton, will be discussed in more detail later in 

the report; however, chlorophyll-a concentrations in the 

river generally were highest in May and June.

Specific Conductance

Specific conductance, or the ability of water to 

conduct an electric current, is related to the presence of 

charged ionic species in water, thus providing an indication of the ion concentration of water. The State of Florida 

has established criteria for the specific conductance 

values in surface waters based on designated uses. Stream 

water designated to be used as potable water supplies, 

for recreational purposes, or for the propagation and 

maintenance of healthy populations of fish and wildlife 

should not have specific conductance values increased 

by more than 50 percent above background levels or 

1,275 microsiemens per centimeter (μS/cm), whichever 

is greater (Florida Administrative Code, 2003a). Stream 

water designated for navigational or industrial use should 

not have specific conductance values greater 

than 4,000 μS/cm (Florida Administrative Code, 2003a).

Specific conductance values in the St. Johns River 

had an inverse relation with streamflow, based on both the 

1991-99 retrospective and 2000-02 data (table 19). 

All correlations were significant at the 95-percent confidence level; however, the relation between specific conductance values and streamflow varied among the sites. 

LOWESS smooth lines for the 1991-99 data show the relation between specific conductance values and streamflow 

at selected sites (fig. 17). Data points are not shown in 

figure 17 to reduce clutter and to make the smooth lines 

distinguishable. Although not shown, specific conductance 

values tracked the LOWESS smooth line fairly closely. The 

relation between specific conductance and streamflow 

based on the 2000-02 data (not shown) was similar to the 

1991-99 data. Specific conductance values from 1991-99 

varied considerably at the Cocoa, Christmas, and upstream 

of Lake Harney sites when streamflows were less than 

about 3,000 ft3

/s. Values at these three sites varied most 

dramatically when streamflows were less than about 1,000 

ft3

/s. During such conditions, specific conductance values 

at these three sites ranged from less than 600 μS/cm to 

greater than 2,000 μS/cm. In contrast, specific conductance 

values at the DeLand site did not vary as widely during lowflow conditions compared to the other three sites. At the 

DeLand site, specific conductance values generally ranged 

from about 1,200 to 1,500 μS/cm when streamflows were 

less than 1,000 ft3

/s.

Table 19. Relations between specific conductance values and total dissolved solids concentration and streamflow at 

St. Johns River sites, 1991-2002.

[Explanation of values: tau, Kendall’s tau; p, p-value; n, number of observations. Retrospective data were compiled from the Florida 

Department of Environmental Protection, Florida Fish and Wildlife Conservation Commission, Orange County, St. Johns River Water

Management District, Volusia County, and the U.S. Geological Survey]

Map ID

(fig. 4) Station name Explanation of values

Specific conductance Total dissolved 

solids

Retrospective 

data 

(1991-99)

U.S. Geological 

Survey data 

(2000-02)

U.S. Geological 

Survey data 

(2000-02)

 1 St. Johns River near Cocoa tau

p

n

-0.6370

0

68

-0.6698

0

72

-0.6616

0

72

 3 St. Johns River near Christmas tau

p

n

-.5563

0

123

-.7782

0

72

-.7782

0

72

 8 St. Johns River upstream of Lake 

Harney, near Geneva

tau

p

n

-.5795

0

246

Not sampled Not determined

12 St. Johns River near Sanford tau

p

n

Not determined -.3875

0

71

-.3928

0

72

21 St. Johns River near DeLand tau

p

n

-.5738

0

126

-.4413

0

72

-.3967

0

72
Water-Quality Characteristics 39

The LOWESS smooth lines also show that the spatial 

variation in specific conductance values among the 

St. Johns River sites was different under low- and highflow conditions. When streamflows were less than about 

1,000 ft3

/s, specific conductance values at the Christmas 

and upstream of Lake Harney sites generally were about 

300 to 800 μS/cm greater than values at the DeLand site. 

In contrast, when streamflows were greater than about 

1,000 ft3

/s, specific conductance values at the DeLand 

site were about 200 to 400 μS/cm greater than those at the 

Christmas and upstream of Lake Harney sites.

To further examine the influence that low-flow 

conditions had on specific conductance values at a greater 

number of sites along the St. Johns River, boxplots of 

retrospective data from May 1991-99 were constructed by 

site (fig. 18). Data for the Orange Mound site are not 

shown because only one measurement was reported for 

May 1991-99. During the May 1991-99 low-flow conditions, median specific conductance values were greater at 

the Puzzle Lake site and the sites from downstream of 

Lake Harney to State Road 415 compared to the other 

St. Johns River sites. Increased specific conductance 

values during the May low-flow period at sites from 

Puzzle Lake to State Road 415 are consistent with a study 

by DeMort (1990), which reported saline water and the 

presence of plants more tolerant of saline conditions in the 

Puzzle Lake and Mullet Lake areas of the St. Johns River. 

High specific conductance values at these locations on the 

river are due to the inflow of brackish water from the 

Floridan aquifer system. Specific conductance values in 

the Upper Floridan aquifer in the vicinity of the St. Johns 

River from upstream of Puzzle Lake to Lake Jesup ranged 

from about 2,000 to 15,000 μS/cm (Spechler and Halford, 

2001). The reach of the St. Johns River from Puzzle Lake 

to State Road 415 also coincides with areas where faults 

or subsidence features—which probably extend into the 

Floridan aquifer system—were reported, and known or 

suspected springflow from the Floridan aquifer system to 

the St. Johns River occurs (Tibbals, 1990).

Higher specific conductance values at the Cocoa 

and Christmas sites during the May low-flow period also 

indicate the Floridan aquifer system discharges to the 

St. Johns River in the vicinity of these two sites. The total 

amount of inflow from the Floridan aquifer system 

between the Cocoa and Christmas sites is not known 

(Tibbals, 1990).

Figure 17. Relation between specific conductance and streamflow at sites on the St. Johns River, 

1991-99.
40 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Continuous data from 2001-02 show the variations in 

specific conductance values with streamflow at the 

Christmas and Sanford sites (fig. 19). At the Christmas 

site, specific conductance had several large spikes in 

values during low-streamflow conditions in water year 

2001. It is unlikely that variations in ground-water quality 

caused these spikes in specific conductance values. One 

explanation, however, is that the wind pushed water with 

greater specific conductance values from the vicinity of 

the Puzzle Lake site upstream to the Christmas site. 

Specific conductance values fluctuated considerably at 

the Sanford and DeLand sites (not shown) during lowstreamflow conditions in water year 2001, which also 

probably resulted from wind and tidal effects.

Total Dissolved Solids

Total dissolved solids concentration (also termed 

residue on evaporation) is a measure of the dissolved 

material in water. Total dissolved solids concentrations 

are determined by measuring the weight of the dry residue 

remaining from an aliquot of sample after removing the 

water by evaporation. The results of total dissolved solids 

concentration analyses may not solely represent the 

amount of inorganic constituents in the water, which can 

make interpretation of the results difficult. Dissolved 

organic matter in the water can contribute to the total 

dissolved solids concentration because organic matter is 

not completely removed from the sample unless strongly 

ignited. Water is retained by some types of residues, such 

as sulfate, and therefore, may not be completely removed 

at the temperature used for evaporation (Hem, 1992). 

Other inorganic constituents, such as nitrate, bicarbonate, 

and boron, are partly volatile and may or may not be 

present as part of the total dissolved solids concentration 

(Hem, 1992). Despite these difficulties in interpreting the 

data, total dissolved solids concentrations are widely used 

in evaluating water quality. The USEPA (2000a) and the 

State of Florida (Florida Administrative Code, 2003b) 

have established a secondary drinking water regulation of 

500 mg/L for total dissolved solids concentrations.

MAP IDENTIFIER

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER CENTIMETER

AT 25 DEGREES CELSIUS

1 2 3 5 4 7 8 9 10 11 12 13 16 17 18 20 21

3,500

2,500

1,500

1,000

3,000

2,000

500

0

7

of

9

2

of

4

7

of

11

DATA NOT AVAILABLE

4

of

6

6

of

8

15

of

21

8

of

10

5

of

7

5

of

7

8

of

10

7

of

11

7

of

11

8

of

11

6

of

8

6

of

8 9

of

13

Data from the Florida Department of Environmental Protection, Florida Fish and Wildlife

Conservation Commission, Orange County, St. Johns River Water Management District,

U.S. Geological Survey, and Volusia County.

17

of

30

10th/90th and

total values

EXPLANATION

SCHEMATIC BOXPLOT

75th percentile

median

25th percentile

1. St. Johns River near Cocoa, Florida

2. St. Johns River at Beeline Highway

3. St. Johns River near Christmas, Florida

4. St. Johns River near Orange Mound

5. St. Johns River at Puzzle Lake

7. Econlockhatchee River at the mouth

8. St. Johns River upstream of Lake Harney

near Geneva, Florida

9. St. Johns River downstream of Lake Harney

10. St. Johns River at Lemon Bluff

11. St. Johns River at State Road 415

12. St. Johns River near Sanford, Florida

13. St. Johns River upstream of the Wekiva River

16. Wekiva River near the mouth

17. St. Johns River downstream of the Wekiva River

18. St. Johns River downstream of Blue Springs

20. St. Johns River near Hontoon Marina

21. St. Johns River near DeLand, Florida

Figure 18. Specific conductance values at St. Johns River sites and near the mouths of the 

Econlockhatchee and Wekiva Rivers, May 1991-99.
Water-Quality Characteristics 41

Figure 19. Continuous specific conductance and stream discharge at St. Johns River sites, 2000-02.
42 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Total dissolved solids concentrations greater than the 

secondary drinking water regulation of 500 mg/L were 

measured at all sites sampled during 2000-02, except the 

Wekiva River site near Sanford where the maximum 

concentration measured was 336 mg/L. In the St. Johns 

River, more than 50 percent of the samples collected at each 

of the sites had concentrations exceeding 500 mg/L. The 

Cocoa site had the lowest percentage of samples exceeding 

500 mg/L (58.3 percent); the DeLand site had the highest 

percentage of samples exceeding 500 mg/L (80.6 percent).

Total dissolved solids concentrations in the St. Johns 

River had a significant inverse relation with streamflow 

(table 19). All correlations were significant at the 95-percent confidence level. Relations between concentration 

and streamflow generally were stronger at the Cocoa and 

Christmas sites compared to the Sanford and DeLand 

sites, as indicated by the larger absolute value of Kendall’s tau (table 19). Variations in total dissolved solids 

concentrations at selected sites from 2000-02 are shown 

in figure 20. Concentrations greater than the secondary 

drinking water regulation of 500 mg/L occurred during 

low-flow periods at each of the sites.

Major Ions

The predominant inorganic species in stream water 

typically are calcium, magnesium, sodium, potassium, 

bicarbonate, chloride, and sulfate. Secondary drinking 

water regulations established by the USEPA and the State 

of Florida for chloride and sulfate concentrations in drinking water recommend that concentrations not exceed 

250 mg/L (U.S. Environmental Protection Agency, 2000a; 

Florida Administrative Code, 2003b). The major source of 

inorganic constituents to the streams in the study area is 

ground-water inflow from the Floridan aquifer system. 

Water from the surficial aquifer system typically is low in 

dissolved mineral content compared to the Floridan aquifer 

system (Lichtler and others, 1968; Boniol, 1996).

A specific water type is based on the types of cations 

or anions that predominate in the water (50 percent or 

greater). For example, a water type in which calcium is 

the dominant cation and bicarbonate is the dominate 

anion is called calcium-bicarbonate water. A water type 

in which neither cation or anion predominate is referred 

to as mixed-ion type water. The major ion composition at 

the Cocoa and Christmas sites was more variable 

compared to the Sanford and DeLand sites, as indicated 

by the greater scatter of values plotted on the trilinear 

diagrams in figure 21. At times, the water at the Cocoa 

and Christmas sites was a mixed-ion type; this occurred 

during periods of increased streamflow. Chloride was the 

predominant anion at the Sanford and DeLand sites and 

was the predominant anion at the Cocoa and Christmas 

sites during low-flow periods. The major ion composition 

at the Wekiva River near Sanford and Blackwater Creek 

sites was substantially different compared to the St. Johns 

River. The water from the Wekiva River near Sanford was 

a mixed-ion type; the water from Blackwater Creek generally was a calcium-sulfate type. The water from Blue 

Springs generally was a sodium-chloride type.

Chloride and sulfate concentrations greater than 

250 mg/L were measured in the St. Johns River. Chloride 

concentrations greater than 250 mg/L were measured 

most frequently at the Sanford site (62.5 percent) and least 

frequently at the Cocoa site (36.1 percent). About one-half 

of the chloride concentrations measured at the Christmas 

(54.2 percent) and DeLand sites (50 percent) were higher 

than 250 mg/L. Sulfate concentrations periodically 

exceeded 250 mg/L at the Cocoa and Christmas sites. 

Approximately 14 percent of the samples collected from 

the Cocoa site and about 20 percent of the samples 

collected from the Christmas site had sulfate concentrations exceeding 250 mg/L. No sulfate concentrations 

exceeded 250 mg/L at the Sanford and DeLand sites.

Figures 22 and 23 show variations in sulfate and 

chloride concentrations with streamflow at selected 

St. Johns River sites, respectively, during 2000-02.

Variations in calcium, magnesium, sodium, and potassium concentrations (not shown) generally were similar 

to those for chloride. Chloride concentrations generally 

exceeded 250 mg/L for a greater period of time in 

water years 2000-01, when rainfall amounts were below 

average, compared to water year 2002. Sulfate concentrations greater than 250 mg/L were measured at the Cocoa 

site during water year 2001. Sulfate concentrations 

greater than 250 mg/L were measured at the Christmas 

site during water years 2000-01.

During low-flow conditions, chloride concentrations, 

as well as concentrations of the other major ions, were 

greater at the Cocoa and Christmas sites compared to the 

Sanford and DeLand sites. Higher major ion concentrations 

at the Cocoa and Christmas sites during low-flow periods 

resulted from the inflow of mineralized water from the 

Floridan aquifer system underlying the St. Johns River in 

the vicinity of these two sites. The highest concentrations of 

chloride and the other major ions were measured at the 

Christmas site during 2000-02. Chloride concentrations of 

nearly 1,200 mg/L were measured at this site during lowflow conditions in 2000. The maximum chloride concentration measured at the Cocoa site was 647 mg/L.
Water-Quality Characteristics 43

Figure 20. Total dissolved solids concentration and streamflow at St. Johns River sites, 2000-02. 
44 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Figure 20. Total dissolved solids concentration and streamflow at St. Johns River sites, 2000-02. 

(Continued)
Water-Quality Characteristics 45

Figure 21. Trilinear diagrams for water samples collected from St. Johns River sites, Wekiva 

River, Blackwater Creek, and Blue Springs, 2000-02.
46 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Figure 21. Trilinear diagrams for water samples collected from St. Johns River sites, Wekiva 

River, Blackwater Creek, and Blue Springs, 2000-02. (Continued)
Water-Quality Characteristics 47

Figure 21. Trilinear diagrams for water samples collected from St. Johns River sites, Wekiva 

River, Blackwater Creek, and Blue Springs, 2000-02. (Continued)
48 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Chloride and sulfate concentrations at the Christmas 

site varied greatly from about March through June 2001. 

Variations in most major ion concentrations, total 

dissolved solids concentrations (fig. 20), and specific 

conductance values (fig. 19) also were measured during 

this same period. These variations in constituent concentrations likely resulted from periodic streamflow reversals, 

which may have moved more mineralized water from the 

vicinity of the Puzzle Lake site (fig. 4) upstream. Increased 

specific conductance values at the Puzzle Lake site indicated increased major ion concentrations at this site as well.

Slight variations in chloride concentrations (as well 

as most major ion concentrations, total dissolved solids 

concentrations, and specific conductance values) were 

observed at the Sanford and DeLand sites from about 

March 2000 to July 2001. Variations in chloride concentrations likely resulted from wind and tidal effects.

In the St. Johns River, major ion concentrations had 

a significant inverse relation with streamflow (table 20). 

Correlations between constituent concentrations and 

streamflow were stronger at the Cocoa and Christmas sites 

compared to the Sanford and DeLand sites, as indicated 

by the larger absolute value of Kendall’s tau (table 20). 

All of the correlations were significant at the 95-percent 

confidence level.

Figure 21. Trilinear diagrams for water samples collected from St. Johns River 

sites, Wekiva River, Blackwater Creek, and Blue Springs, 2000-02. (Continued)
Water-Quality Characteristics 49

Figure 22. Sulfate concentration and streamflow at St. Johns River sites, 2000-02.
50 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Figure 23. Chloride concentration and streamflow at St. Johns River sites, 2000-02.
Water-Quality Characteristics 51

Figure 23. Chloride concentration and streamflow at St. Johns River sites, 2000-02. (Continued)
52 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Silica

Unlike the major ions, silica concentrations in the 

St. Johns River were significantly greater during high-flow 

conditions compared to other times of the year (table 21; 

fig. 24). All correlation coefficients were significant at the 

95-percent confidence level. During high-flow conditions, 

silica concentrations in the river generally were at least 

10 mg/L. During low-flow conditions, however, silica concentrations generally were less than 5 mg/L. At the Cocoa, 

Christmas, and Sanford sites, silica concentrations generally were 1 mg/L or less during low-flow conditions. One 

possible explanation for increased silica concentrations 

during high-flow periods might be that water from the 

Floridan aquifer system, which contributes proportionally 

more inorganic constituents to the river during low-flow conditions than during high flow, contains little silica because 

the aquifer is composed primarily of limestone and dolomite. This explanation, however, is not consistent with 

silica concentration data for water from the Floridan aquifer 

system collected by the FDEP and the USGS from 1990-

2002. Silica concentrations in the Floridan aquifer system 

generally ranged from about 8 to 30 mg/L, which is substantially greater than concentrations generally measured in 

the St. Johns River during low-flow conditions (fig. 25).

Also, silica concentrations in water from the surficial

Table 20. Relations between major dissolved inorganic constituent concentrations and streamflow at St. Johns River sites, 2000-2002. 

[Explanation of values: tau, Kendall’s tau; p, p-value; n, number of observations]

Map ID

(fig. 4) Station name Explanation of values Calcium Magnesium Sodium Potassium Chloride Sulfate

 1 St. Johns River near Cocoa tau

p

n

-0.6804

0

72

-0.6667

0

66

-0.6820

0

61

-0.4770

0

61

-0.6831

0

72

-0.6663

0

72

 3 St. Johns River near Christmas tau

p

n

-.7715

0

72

-.7692

0

66

-.8055

0

61

-.7295

0

61

-.8032

0

72

-.7054

0

72

12 St. Johns River near Sanford tau

p

n

-.4781

0

72

-.3697

0

65

-.3825

0

61

-.2842

.0012

61

-.3854

0

72

-.3897

0

72

21 St. Johns River near DeLand tau

p

n

-.4570

0

72

-.3907

0

66

-.4093

0

61

-.3180

.0003

61

-.4264

0

72

-.3803

0

72

Table 21. Relations between dissolved silica, total sulfide, total iron, dissolved iron, and bromide concentrations and 

streamflow at St. Johns River sites, 2000-02. 

[Explanation of values: tau, Kendall’s tau; p, p-value; n, number of observations]

Map ID

(fig. 4) Station name Explanation of values Silica Sulfide Total iron

Dissolved 

iron Bromide

 1 St. Johns River near Cocoa tau

p

n

0.3779

0

72

0.3903

0

49

0.3889

0

72

0.6228

0

72

-0.6451

0

72

 3 St. Johns River near Christmas tau

p

n

.4593

0

72

.4762

0

49

.3811

0

72

.6052

0

72

-.7410

0

72

12 St. Johns River near Sanford tau

p

n

.3087

.0001

72

.3690

0

49

.5458

0

72

.5575

0

72

-.3744

0

72

21 St. Johns River near DeLand tau

p

n

.3772

.0006

72

.3644

0

48

.6123

0

72

.5806

0

72

-.3556

0

72
Water-Quality Characteristics 53

Figure 24. Dissolved silica concentration and streamflow at St. Johns River sites, 2000-02.
54 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Figure 25. Silica concentrations in the Floridan aquifer system, 1990-2002.
Water-Quality Characteristics 55

aquifer system generally were greater than concentrations 

in the river during low-flow periods. Data collected by the 

FDEP and USGS showed silica concentrations in the surficial aquifer system ranged from about 3 to 28 mg/L 

during 1990-2000. Another explanation for reduced silica 

concentrations during low-flow conditions is that silica 

was utilized by the biota. Some types of algae, primarily 

diatoms, use silicon to construct a frustule or shell that 

surrounds the organism (Wetzel, 1983). In eutrophic 

lakes, silica concentrations commonly are near method 

reporting limits due to intensive assimilation by diatoms 

(Wetzel, 1983). Silica concentrations are high during 

high-flow conditions due to less utilization by algae. 

Chlorophyll-a concentrations are discussed in detail 

later in this report. Concentrations typically are low 

during high-flow conditions, which indicates that algal 

abundances also are low during low-flow conditions.

Minor Inorganic Constituents

Sulfide, iron, and bromide typically are present at 

lower concentrations in stream water compared to the 

major inorganic constituents. Sulfide imparts an unpleasant 

odor in the water and is produced by the reduction of 

sulfate, which occurs under anaerobic conditions. 

Sources of iron in water include the oxidation of pyrite 

(which generally is not present in large quantities in the 

study area), the oxidation of humic materials, and the 

dissolution of iron oxides and silicate minerals. The 

reduction of the relatively insoluble ferric iron (Fe3+) 

compounds to the relatively soluble ferrous iron (Fe2+) 

compounds is dependent upon the pH and oxidationreduction potential of the water. Increased concentrations 

of iron may cause discoloration problems if the water is 

used for domestic purposes. To prevent discoloration 

problems, the USEPA (U.S. Environmental Protection 

Agency, 2000a) and the State of Florida (Florida Administrative Code, 2003b) established a secondary drinking 

water regulation of 300 micrograms per liter (μg/L) for 

the concentration of iron in water.

Bromide is an important constituent in seawater and 

occurs naturally in the Floridan aquifer system because 

the aquifer originally formed in a marine environment; 

bromide also could have been introduced during subsequent advances of the sea in the geologic past. Elevated 

concentrations of bromide in water may lead to the formation of bromate or other brominated disinfection byproducts if the water is disinfected using either ozone or 

chlorine. Some of these disinfection byproducts, such as 

bromate, bromodichloromethane, and bromoform, are 

known carcinogens (U.S. Environmental Protection 

Agency, 1998). A Maximum Contaminant Level (MCL) 

of 0.010 mg/L was established by the USEPA (U.S. Environmental Protection Agency, 1998) for the concentration 

of bromate in drinking water.

Sulfide

Sulfide concentrations in the St. Johns River, the 

Wekiva River near Sanford, and Blackwater Creek were 

determined from November 2000 to September 2002. 

Summary statistics of sulfide concentrations are presented 

in table 22. Sulfide concentrations were not analyzed in 

Blue Springs. In the St. Johns River, sulfide was detected 

most frequently in samples from the Cocoa site and least 

frequently in samples from the DeLand site.

Table 22. Descriptive statistics of total sulfide concentrations at St. Johns River, Wekiva River, and Blackwater Creek sites, 2000-02.

[Units are milligrams per liter; N, number of samples; <, less than. No data available for 02235500, Blue Springs near Orange City]

Map ID

(fig. 4) Station number Station name N

Percent of 

samples with 

detectable 

concentrations

Minimum 25th

percentile Median 75thpercentile Maximum

 1 02232400 St. Johns River near Cocoa 49 57.1 <1.0 1.0 1.0 2.0 4.0

 3 02232500 St. Johns River near Christmas 49 55.1 <1.0 1.0 1.0 2.0 6.0

12 02234500 St. Johns River near Sanford 49 51.0 <1.0 1.0 1.0 2.0 3.0

14 02235000 Wekiva River near Sanford 7 14.3 <1.0 <1.0 <1.0 <1.0 2.0

15 02235200 Blackwater Creek near Cassia 7 57.1 <1.0 1.0 3.0 3.5 5.0

21 02236000 St. Johns River near DeLand 48 41.7 <1.0 <1.0 <1.0 2.0 4.0
56 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

In the St. Johns River, sulfide concentrations 

increased during high-flow conditions. All correlations 

between sulfide concentrations and streamflow were 

significant and positive (table 21). Concentrations as 

great as 6 mg/L were measured during high-flow conditions. During low-flow conditions, sulfide concentrations 

generally were near method reporting limits (1 mg/L). 

One potential source of sulfide is ground-water inflow 

from the Floridan aquifer system. Sulfide was reported in 

water from wells in the Floridan aquifer system in Orange 

County (Lichtler and others, 1968), particularly in flowing wells in the eastern part of the county. Lower sulfide 

concentrations in the St. Johns River during low-flow 

conditions, however, is inconsistent with ground-water 

inflow as the predominant source of sulfide to the river. 

Sulfide also may be produced by the decomposition of 

organic matter (Wetzel, 1983, p. 319). Sulfate may be 

reduced to sulfide when the oxidation-reduction potential 

declines to less than 100 millivolts (Wetzel, 1983, p. 320). 

It is likely that these processes strongly influence sulfide 

concentrations in the St. Johns River.

Iron

Total iron concentrations greater than 300 μg/L were 

measured at all sites on the St. Johns River, Wekiva 

River, and Blackwater Creek. In the St. Johns River, 

concentrations exceeded 300 μg/L most frequently at the 

Christmas site (88.9 percent of the samples) and least 

frequently at the DeLand site (27.8 percent of the 

samples). Total iron concentrations in 75 percent of the 

samples from Blackwater Creek exceeded 300 μg/L. In 

contrast, concentrations in less than 10 percent of the 

samples from the Wekiva River near Sanford exceeded 

300 μg/L.

In the St. Johns River, total iron concentrations had a 

significant positive relation with streamflow (table 21, 

fig. 26). Total iron concentrations greater than 300 μg/L 

generally were measured at the Cocoa, Sanford, and 

DeLand sites only during periods of increased streamflow. Total iron concentrations at the Christmas site 

generally were greater than 300 μg/L at all times of the 

year.

Dissolved iron ranged from 5 to 83 percent of the 

total iron concentration. The percentage of dissolved iron 

in the river water was dependent upon streamflow conditions, as indicated by data from the Sanford site (fig. 27).

During low-flow conditions, dissolved iron generally 

was, at most, about 20 percent of the total iron concentration at each of the St. Johns River sites. The increased 

percentage of dissolved iron at the Sanford site from 

October 2000 through January 2001 resulted from 

upstream inputs. Concentrations were high from October 

through November 2001 at the Cocoa and Christmas sites 

due to increased streamflows. In contrast, dissolved iron 

accounted for 70 percent or more of the total iron concentration at each of the sites during high-flow conditions. 

Ferrous iron (Fe2+) is a soluble form of iron in water. 

Water pH, temperature, and oxidation-reduction potential 

influence the reduction of ferric iron (Fe3+) to ferrous iron. 

In the St. Johns River, water pH and dissolved oxygen 

concentrations are lower during high-flow conditions. It is 

possible that during high-flow events, iron was reduced to 

the ferrous form, resulting in a greater percentage of 

dissolved iron in the river.

Bromide

Bromide concentrations had temporal variations 

unlike any of the other inorganic constituents. Concentrations ranged from 0.19 to 17 mg/L during 2000-02 and 

had a significant inverse relation with streamflow (table 

21). All correlations were significant at the 95-percent 

confidence level. Temporal variations in bromide concentration were characterized by sharp peaks in concentration 

during low-flow periods at all St. Johns River sites 

(fig. 28), especially in water year 2001 when concentrations as great as 17 mg/L were measured at the Cocoa site.

Maximum bromide concentrations at the other sites were 

substantially lower than those at the Cocoa site; concentrations at these sites did not exceed 9.0 mg/L. Increased 

concentrations during water year 2001 may have resulted 

from less dilution due to the low-streamflow conditions. 

Mean annual streamflows were less in water year 2001 

compared to water years 2000 and 2002 (table 17). Periods 

of increased bromide concentrations generally were of a 

shorter duration compared to the other inorganic constituents, such as chloride (fig. 23), especially in water year 

2001. There also was an increase in bromide concentrations at the Cocoa and Christmas sites during water year 

2000 that was not observed at the Sanford and DeLand 

sites.

Like most of the inorganic constituents, maximum 

bromide concentrations in the St. Johns River occur during 

low-flow conditions because the source of bromide to the 

river is ground water affected by relict seawater. Limited 

data were available from USGS files on bromide concentrations in water from the Floridan or surficial aquifer systems 

within the study area; a majority of the data were collected 

in the late 1990's west of St. Johns River in the central part 

of the study area. Bromide concentrations in water from 

the Floridan aquifer system ranged from below the 

method reporting limit (0.05 mg/L) to 22 mg/L (fig. 29).
Water-Quality Characteristics 57

Figure 26. Total iron concentrations and streamflow at St. Johns River sites, 2000-02.
58 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Concentrations generally were lower in water from the 

surficial aquifer system compared to the Floridan aquifer 

system; concentrations ranged from below the method 

reporting limit (0.05 mg/L) to 3.2 mg/L. The highest 

concentrations in the surficial and Floridan aquifer systems 

generally were reported in the discharge areas for the Floridan aquifer system in the vicinity of the Econlockhatchee 

and St. Johns Rivers in the central part of the study area.

Peaks in bromide concentrations were expected to be 

concomitant with maximum chloride concentrations at 

the Cocoa and Christmas sites. Both chloride and bromide 

in ground water in the study area likely originated from 

relict seawater. The limited ground-water-quality data 

also showed significant correlations between bromide 

and chloride concentrations in water from the Floridan 

aquifer system (Kendall’s tau = 0.6990, p = 0). Peaks in 

bromide concentrations that were not coincident with 

maximum chloride concentrations measured during water 

year 2001 were unexpected, suggesting different sources 

of bromide and chloride to the St. Johns River. Agricultural canals in the Upper St. Johns River Basin may be 

a source of bromide to the Cocoa site. Goolsby and 

McPherson (1970) reported that an increase in dissolved 

solids concentrations in the St. Johns River downstream 

of Lake Washington was due to the inflow of mineralized 

water from agricultural drainage canals. Although chloride and bromide concentrations in agricultural canal 

water were not reported by Goolsby and McPherson 

(1970), it is possible that agricultural drainage water 

contains chloride and bromide.

Water Color and Organic Carbon

The yellowish-brown color of some Florida streams 

is due in part to organic matter in the water. Organic 

matter in streams serves many functions. Organic matter 

may serve as a carbon substrate in microbially mediated 

reactions and also may influence the water pH because 

organic acids makeup most of the organic matter in natural 

waters. Colored organic matter in the water absorbs light. 

As a result, the depth of the photic zone (area in which 

photosynthesis occurs) potentially may be controlled by 

the amount of organic matter present in highly colored 

waters. Organic matter in water also has some undesirable 

effects. If water is disinfected with chlorine during treatment, the presence of organic matter in the water may lead 

to the formation of disinfection byproducts, some of 

which are known carcinogens. Organic matter in the water 

also may transport contaminants. Contaminants such as 

hydrophobic organic compounds and metals, which are 

relatively immobile in the environment, may interact with 

organic matter and be transported considerable distances 

downstream.

Figure 27. Percent of dissolved iron comprising the total iron concentration and streamflow at the 

St. Johns River near Sanford, Florida, 2000-02.
Water-Quality Characteristics 59

Figure 28. Bromide concentration and streamflow at St. Johns River sites, 2000-02.
60 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Figure 28. Bromide concentration and streamflow at St. Johns River sites, 2000-02. (Continued)
Water-Quality Characteristics 61

Figure 29. Bromide concentrations in the Floridan aquifer system, 1990-2002.
62 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Sources of organic carbon can be categorized as 

entering the stream from a terrestrial watershed (allochthonous) or being derived from biota in the water body 

(autochthonous). Organic matter derived from different 

source materials has distinctive characteristics. Organic 

matter derived from lignin, a component of woody plants 

such as shrubs and trees, has relatively large amounts of 

aromatic carbon and is low in nitrogen content (Aiken and 

Cotsaris, 1995). In contrast, microbially derived organic 

matter has relatively low amounts of aromatic carbon and 

a high nitrogen content (Aiken and Cotsaris, 1995). The 

aromatic carbon content of organic matter also may influence water treatment strategies. In water treatment, there 

is a strong correlation between chlorine consumption and 

the aromatic carbon content of the source water (Reckhow and others, 1990), so the generation of disinfection 

byproducts may be greater during periods when organic 

carbon concentrations and the aromatic carbon content 

are high.

Water color is determined by matching dilutions of 

a standard solution of cobalt chloride and potassium chloroplatinate. The intensity of color is rated numerically. 

For example, a color of 5 is equal to a 1:100 dilution of 

the standard solution. The numerical value assigned to the 

color has no direct connection with the amount of organic 

material in the water. For aesthetic purposes, the USEPA 

and the State of Florida established a secondary drinking 

water regulation of 15 color units (U.S. Environmental 

Protection Agency, 2000a; Florida Administrative Code, 

2003b).

Ultraviolet light (UV) absorbance is a surrogate 

measure of the aromaticity of organic carbon. Specific 

UV absorbance is defined as the UV absorbance divided 

by the dissolved organic carbon concentration. Specific 

UV absorbance values are used to indicate the source of 

organic matter because algal-derived organic carbon has 

a lower molar absorptivity than terrestrially derived 

organic matter (McKnight and others, 1994). Specific 

UV absorbance also was proposed as a predictor of the 

disinfectant byproduct formation potential of the water 

(Edzwald and others, 1985).

The color of the St. Johns River water varied 

substantially. Water color values ranged from 10 to 400 

platinum-cobalt units. More than 95 percent of the 

samples had water color values exceeding 15 platinumcobalt units. Median water color values were greater at 

the Cocoa and Christmas sites (120 and 140 platinumcobalt units, respectively) than at the Sanford and DeLand 

sites (100 and 75 platinum-cobalt units, respectively).

Water color values generally increased substantially 

with increasing streamflow at each of the sites, as shown 

by data from the Christmas and DeLand sites (fig. 30).

Correlations between water color values and streamflow 

at each of the sites also were significant and positive 

(table 23). During low-flow conditions, water color 

values generally were greater at the Cocoa and Christmas 

sites (about 50 to 100 platinum-cobalt units) compared to 

the Sanford and DeLand sites (about 25 to 50 platinum 

cobalt units); these lower water color values may be due 

to inputs from spring-fed streams or ground-water inflow. 

Data from the Wekiva River near Sanford, which receives 

a substantial amount of flow from Wekiva Springs, and 

Blue Springs documented that water from these sources is 

less colored compared to the St. Johns River. Median 

water color values in the Wekiva River near Sanford and 

Blue Springs were 10 and less than 5 platinum-cobalt 

units, respectively. Water color values at each St. Johns 

River site remained high for about a month after high 

flows receded. These continued high water color values 

may be due to the storage of water in lakes and wetlands.

Dissolved organic carbon concentrations ranged 

from 1.6 to 42 mg/L during 2000-02. Median dissolved 

organic carbon concentrations ranged from 15.0 at the 

DeLand site to 26.0 mg/L at the Cocoa site. Median concentrations at the Christmas and Sanford sites were 25.0 

and 19.5 mg/L, respectively. Lower median dissolved 

organic carbon concentrations at the DeLand site compared to the Cocoa and Christmas sites probably result 

from dilution by inflow from the Wekiva River (which 

had a median dissolved organic carbon concentration of 

2.7 mg/L during 2000-02) and by ground-water inflow.

Dissolved organic carbon concentrations were not 

related to streamflow at the Cocoa site during 2000-02 

(fig. 31, table 23); however, specific UV absorbance 

values at this site increased substantially during high-flow 

conditions (fig. 31). High specific UV absorbance values 

indicated that the dissolved organic matter was more 

aromatic in composition and probably originated from 

terrestrially derived sources. Low specific UV absorbance 

values during low-flow conditions indicated the organic 

matter was less aromatic in composition and probably 

originated from algal-derived sources. One likely source 

of algal production in the vicinity of the Cocoa site is Lake 

Poinsett.

At the Christmas, Sanford, and DeLand sites, 

dissolved organic carbon concentrations were positively 

correlated with streamflow, as indicated by data from the 

Sanford site (fig. 31, table 23). Relations between 

dissolved organic carbon concentrations and streamflow 

were stronger at the Sanford and DeLand sites compared
Water-Quality Characteristics 63

Figure 30. Water color values and streamflow at St. Johns River sites, 2000-02.
64 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

to the Christmas site, as indicated by the larger absolute 

values of Kendall’s tau shown in table 23. Specific UV 

absorbance values also increased substantially at these 

three sites during high-flow periods, indicating that 

increased dissolved organic carbon concentrations resulted 

from inputs of organic matter from terrestrial sources.

Nutrients

Nutrients (defined in this report as nitrogen and 

phosphorus compounds) are essential for plant and 

animal growth. In streams and lakes, nitrogen primarily 

occurs as nitrite (NO2

-

), nitrate (NO3

-

), ammonium 

(NH4

+

), or as part of particulate or dissolved organic 

matter. Phosphorus in streams and lakes primarily occurs 

as orthophosphate (PO4

3-) or as part of particulate or 

dissolved organic matter or inorganic complexes. Nitrate 

nitrogen concentrations greater than 10 mg/L in drinking 

water can cause a potentially fatal condition, primarily in 

infants, called methemoglobinemia. Unionized ammonia 

in water can be toxic to fish and other aquatic life.

Elevated concentrations of nutrients in slower 

moving reaches of streams and lakes can result in 

eutrophication, which is the excessive growth of aquatic 

plants and algae. This excessive growth can result in the 

water smelling or tasting foul, which may ultimately 

increase water treatment costs if the water is used as a 

source of drinking water. In addition, when these plants 

and animals die, dissolved oxygen may be depleted in the 

water, ultimately resulting in fish kills. Within the study 

area, Lake Monroe is known to exhibit eutrophication and 

reaches of the St. Johns, Wekiva, and Econlockhatchee 

Rivers are listed by the State of Florida as being impacted 

by nutrients (St. Johns River Water Management District, 

2002).

Sources of nitrogen and phosphorus include fertilizer, animal manure, phosphatic rock, dishwashing detergents, and human waste. Nitrogen and phosphorus may 

enter streams and lakes through runoff from the land 

surface, inputs from springs or ground-water inflow, point 

discharges such as municipal wastewater effluent, or 

atmospheric deposition. To prevent methemoglobinemia, 

the USEPA and the State of Florida established a MCL for 

nitrate of 10 mg/L as nitrogen in drinking water (U.S. 

Environmental Protection Agency, 2000a; Florida 

Administrative Code, 2003b). The USEPA also recommends that total nitrogen concentrations not exceed 

0.9 mg/L and total phosphorus concentrations not exceed 

0.04 mg/L in aggregate ecoregion XII (which includes the 

State of Florida) to prevent eutrophication in streams 

(U.S. Environmental Protection Agency, 2000b).

Nitrogen is dynamic in the environment. Ammonium 

is the most readily available form of nitrogen to plants and 

is readily converted to organic nitrogen after assimilation. 

Organically bound nitrogen also may be converted to 

ammonium through a series of biological transformations 

as the organic matter is decomposed and degraded. Under 

high pH conditions (pH greater than 8), which can occur 

with excessive algal blooms, ammonium can be converted 

to ammonia (NH3) that is released to the atmosphere 

through volatilization. Ammonium also can be immobilized through ion exchange onto negatively charged soil 

particles, which makes ammonium less mobile in the 

environment compared to other forms of nitrogen.

Ammonium can be oxidized to nitrate in an aerobic 

environment through the microbially mediated process of 

nitrification. Nitrite is formed as an intermediate product 

in the nitrification process and typically is not found in 

large concentrations in the environment. Plants and 

microbes may assimilate nitrate through assimilatory 

nitrate reduction. Unlike ammonium, nitrate as a negatively charged compound is not subject to immobilization 

by negatively charged soil particles and is more mobile in 

the environment. Under anaerobic conditions, nitrate may 

be reduced to molecular nitrogen (N2) and nitrous oxide 

(N2O) by microorganisms. Denitrification is an important 

pathway for nitrogen loss from most kinds of wetlands 

(Mitsch and Gosselink, 2000). Molecular nitrogen also 

may be converted to organic nitrogen by certain aerobic 

and anaerobic bacteria and blue-green algae, a process 

known as nitrogen fixation.

Table 23. Relations between water color values and dissolved 

organic carbon concentrations and streamflow at St. Johns 

River sites, 2000-2002.

[Explanation of values: tau, Kendall’s tau; p, p-value; n, number of 

observations]

Map ID 

(fig. 4) Station name

Explanation 

of 

values

Water 

color

Dissolved 

organic 

carbon

 1 St. Johns River near 

Cocoa

tau

p

n

0.5900

0

72

0.0704

.3781

72

 3 St. Johns River near 

Christmas

tau

p

n

.7230

0

72

.3466

0

72

12 St. Johns River near 

Sanford

tau

p

n

.5595

0

72

.5008

0

72

21 St. Johns River near 

DeLand

tau

p

n

.5293

0

72

.5278

0

72
Water-Quality Characteristics 65

Figure 31. Dissolved organic carbon concentration, specific ultraviolet light absorbance, and 

streamflow at St. Johns River sites, 2000-02.
66 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Figure 31. Dissolved organic carbon concentration, specific ultraviolet light absorbance, and 

streamflow at St. Johns River sites, 2000-02. (Continued)
Water-Quality Characteristics 67

Phosphorus often is the most limiting nutrient in 

streams and lakes. Phosphorus occurs as dissolved and 

particulate inorganic and organic forms in water. Orthophosphate is the most common inorganic form of phosphorus in streams and is readily available to biota. 

Phosphorus has an affinity for calcium, iron, and aluminum and forms complexes with these elements. Phosphate also may sorb onto clay particles, organic matter, 

and ferric and aluminum oxides and hydroxides. Changes 

in environmental conditions may cause phosphate to be 

released from streambed sediments and soils. Under 

anaerobic conditions, ferric iron may be reduced to the 

more soluble ferrous iron, resulting in the release of phosphorus that was associated with the ferric iron. Phosphorus also can be released from insoluble salts when the pH 

of the water is lowered.

Total Nitrogen

Median total nitrogen concentrations in the St. Johns 

River ranged from 1.2 to 1.7 mg/L as nitrogen during 

1991-99 (fig. 32). Concentrations at most sites were not 

significantly related to streamflow (table 24). Median 

concentrations were slightly lower at sites 17-18 and 

20-21 (1.2-1.3 mg/L as nitrogen) compared to sites 1-5 

(1.6-1.7 mg/L as nitrogen). Low concentrations may have 

resulted from dilution by the Wekiva River and Blue 

Springs. Concentrations near the mouth of the Wekiva 

River (site 16) during 1991-99 were lower than concentrations at all of the St. Johns River sites. The USEPA's 

recommended ecoregional nutrient criterion for total 

nitrogen of 0.9 mg/L as nitrogen was exceeded in about 

90 percent or more of the samples collected from the 

Figure 32. Total nitrogen concentration and nitrogen speciation in the St. Johns River and near the mouths of the 

Econlockhatchee and Wekiva Rivers, 1991-99.
68 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

St. Johns River during 1991-99. Fewer samples (about 65 

percent) collected near the mouth of the Wekiva River 

exceeded 0.9 mg/L.

Ammonia Nitrogen

More than 75 percent of the ammonia concentrations 

reported at each of the St. Johns River sites during 1991-

99 were greater than 0.02 mg/L. Ammonia also generally 

was detected in the river during 2000-02. Ninety percent 

or more of the concentrations measured from 2000-02 

were at or greater than the method reporting limit for 

ammonia. Detections of ammonia nitrogen indicated that 

bioavailable nitrogen generally was present in the 

St. Johns River from 1991-2002. Ammonia concentrations generally increased during high-flow conditions at 

the Cocoa and Sanford sites, based on the 1991-99 data 

(table 24) and the 2000-02 data (table 24 and fig. 33).

Higher ammonia concentrations during high-flow conditions may have resulted from less uptake of ammonia by 

phytoplankton or from runoff of animal wastes. During 

high-flow conditions, chlorophyll-a concentrations 

rapidly decreased to below method reporting limits, indicating phytoplankton were less prevalent compared to 

other times of the year.

In contrast, ammonia concentrations generally were 

greater during low-flow conditions at the Christmas and 

above Lake Harney sites on the St. Johns River, based on 

the 1991-2002 data (fig. 33, table 24). One source of ammonia nitrogen to streams during low-flow conditions is 

ground-water inflow. The FDEP's data showed ammonia 

nitrogen concentrations as high as 1.2 mg/L in the Floridan 

aquifer system. There was a substantial increase in ammonia nitrogen concentrations at the Cocoa site during lowflow conditions in May 2000—values as high as 0.35 mg/L 

as nitrogen were measured. Increased concentrations 

during May 2000 likely did not result from ground-water 

inflow because ammonia nitrogen concentrations generally 

were not increased during low-flow conditions in water 

years 2001 and 2002 at this site. Another explanation for 

increased ammonia nitrogen concentrations is that decomposition of organic matter occurred, resulting in the production of ammonia.

Table 24. Relations between total nitrogen, ammonia nitrogen, and organic plus ammonia nitrogen concentrations and streamflow at 

St. Johns River sites, 1991-2002.

[Correlations statistically significant at the 0.05 significance level are shown in bold. Explanation of values: tau, Kendall’s tau; p, p-value; n, number of 

observations. Retrospective data were compiled from the Florida Department of Environmental Protection, Florida Fish and Wildlife Conservation Commission, 

Orange County, St. Johns River Water Management District, Volusia County, and the U.S. Geological Survey]

Map ID

(fig. 4) Station name Explanation of values

Total nitrogen Ammonia nitrogen Total organic plus 

ammonia nitrogen

Retrospective 

data 

(1991-99)

Retrospective 

data 

(1991-99)

U.S. Geological 

Survey data 

(2000-02)*

Retrospective 

data

(1991-99)

 1 St. Johns River near Cocoa tau

p

n

-0.0503

.5410

69

0.2420

.0038

67

0.2355

.0034

72

-0.0618

.4525

69

 3 St. Johns River near Christmas tau

p

n

-.2411

.0001

123

-.3900

0

123

-.2907

.0003

72

-.2088

.0005

126

 8 St. Johns River upstream of Lake Harney, 

near Geneva

tau

p

n

-.0543

.2082

242

-.3141

0

143

Not determined -.0274

.5190

250

12 St. Johns River near Sanford tau

p

n

Not determined Not determined .2879

.0003

72

Not determined

21 St. Johns River near DeLand tau

p

n

.0699

.2473

125

.1723

.2121

26

.0837

.2964

72

.0781

.1888

130

*Dissolved ammonia nitrogen data.
Water-Quality Characteristics 69

Figure 33. Dissolved ammonia nitrogen concentration and streamflow at St. Johns River sites, 

2000-02.
70 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Organic Plus Ammonia Nitrogen

Most of the nitrogen in the St. Johns and Econlockhatchee Rivers was in the form of organic plus ammonia 

nitrogen. In the St. Johns River, more than 90 percent of 

the nitrogen in the water during 1991-99 was in the form 

of organic plus ammonia nitrogen. More than 80 percent 

of the nitrogen near the mouth of the Econlockhatchee 

River was in the form of organic plus ammonia nitrogen. 

Near the mouth of the Wekiva River, the percentage of 

organic plus ammonia nitrogen in the water was less 

compared to the St. Johns and Econlockhatchee Rivers. 

At this site, generally about one-half of the nitrogen in the 

water during 1991-99 was in the form of organic plus 

ammonia nitrogen.

The distribution of total organic plus ammonia 

nitrogen concentrations in the St. Johns River and near 

the mouths of the Econlockhatchee and Wekiva Rivers 

during 1991-99 were similar to those for total nitrogen 

concentrations (fig. 32) because organic plus ammonia 

nitrogen made up more than one-half of the nitrogen in 

the streams. Total organic plus ammonia nitrogen concentrations generally were not significantly correlated with 

streamflow at the St. Johns River sites during 1991-99 

(table 24).

Nitrate Nitrogen

Nitrate concentrations in the St. Johns River and 

near the mouths of the Econlockhatchee and Wekiva 

Rivers generally were less than 0.5 mg/L during 1991-99 

(table 25). The distribution of nitrate concentrations in the 

St. Johns River was not similar to the distribution of total 

nitrogen. Nitrate generally comprised less than 10 percent 

of the total nitrogen concentration in the river. Nitrate 

concentrations in the St. Johns River were significantly 

greater at sites downstream of the Wekiva River 

compared to the other sites (table 25). High median nitrate 

concentrations at these sites may be due in part to nitrogen-enriched ground-water inflow. Nitrate concentrations 

exceeding 1.0 mg/L were reported in water from the 

Table 25. Descriptive statistics of nitrate nitrogen concentrations in the St. Johns, Econlockhatchee, and Wekiva Rivers, 1991-99.

[N, number of samples; IQR, interquartile range; the IQR may not equal the 75th percentile minus the 25th percentile due to rounding. The a, b, c, letters next to 

the IQR indicates whether concentrations were significantly different among sites at the 0.05 significance level. IQRs denoted with the same letter indicate concentrations were not significantly different]

Map ID

(fig. 4) Station name N Percent censored data

25th

percentile Median 75thpercentile IQR

 1 St. Johns River near Cocoa 69 49.3 0.010* 0.020 0.057 0.048 a

 2 St. Johns River at Beeline Highway 77 40.3 .014* .023 .056 .043 a

 3 St. Johns River near Christmas 123 41.5 .013* .027 .070 .057 ab

 4 St. Johns River at Orange Mound 30 53.3 .010* .024* .062 .052 a

 5 St. Johns River at Puzzle Lake 79 67.1 .010* .017* .030 .020 a

 7 Econlockhatchee River near the mouth 73 15.1 .052 .15 .23 .178 d

 8 St. Johns River upstream of Lake Harney, 

near Geneva

248 11.3 .030 .048 .075 .045 ab

 9 St. Johns River at north end of Lake Harney 127 47.2 .019* .030 .060 .041 ab

10 St. Johns River at Lemon Bluff 107 35.5 .023* .040 .060 .037 ab

11 St. Johns River at State Road 415 Bridge 107 38.3 .024* .040 .060 .036 ab

12 St. Johns River near Sanford 150 36.7 .020* .040 .097 .077 ab

13 St. Johns River south of the confluence with the 

Wekiva River

125 29.6 .020* .060 .10 .080 b

16 Wekiva River near the mouth 124 2.4 .245 .40 .500 .255 e

17 St. Johns River north of the confluence with the 

Wekiva River

124 15.3 .060 .091 .120 .060 c

18 St. Johns River downstream of Blue Springs 106 5.7 .079 .10 .200 .121 c

20 St. Johns River near Hontoon Marina 107 10.3 .070 .10 .147 .077 c

21 St. Johns River near DeLand 131 9.2 .062 .10 .170 .108 c

*Percentile estimated using the probability plotting procedure described by Helsel and Cohn, 1988.
Water-Quality Characteristics 71

Upper Floridan aquifer in the Wekiva Springs groundwater basin (Toth and Fortich, 2002). Wekiva Springs, 

with an average flow of about 56 ft3

/s, forms the headwaters of the Wekiva River. Median nitrate concentrations near the mouth of the Wekiva River during 1991-99 

were significantly greater during the January-June lowflow period compared to the September-October highflow period (fig. 34). These results indicate that nitrateenriched ground water affected the Wekiva River and 

probably affected the St. Johns River downstream of its 

confluence with the Wekiva River. Low median nitrate 

concentrations in the upstream part of the St. Johns River, 

may be related to: (1) nitrate being used as a nutrient by 

terrestrial and aquatic plants; (2) denitrification in wetland 

areas; or (3) the fact that much of the agricultural land is 

pasture, which generally is not fertilized.

Median nitrate concentrations near the mouth of the 

Econlockhatchee River were greater than those in the 

St. Johns River during 1991-99. At this site, nitrate 

concentrations were significantly greater during the 

January-February and May-June low-flow periods 

compared to the September-October high-low period 

(fig. 34). These results indicate that increased nitrate 

concentrations in the Econlockhatchee River were related 

to discharges of municipal wastewater or to ground-water 

inflow.

Seasonal variations in nitrate concentrations in the 

St. Johns River were characterized by sharp peaks in 

concentration from about October-February during 

2000-02, most notably at the Cocoa, Sanford, and DeLand 

sites (fig. 35). Nitrate concentrations from about OctoberFebruary at the Cocoa and DeLand sites increased to 

values as high as 0.44 mg/L. During most other times of

0.0

0.2

0.4

0.6

0.8

1.0

NITRATE NITROGEN CONCENTRATION,

IN MILLIGRAMS PER LITER AS NITROGEN

JAN /

FEB

MAR /

APR

MAY /

JUNE

JULY /

AUG

SEPT /

OCT

NOV /

DEC

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

23

ab

17 22 23 17 22

ac

c

bc

a

ab

15

a 10 15 14 8 11

ab

b

ac a

ab

WEKIVA RIVER

ECONLOCKHATCHEE RIVER

23 number of values

EXPLANATION

SCHEMATIC BOXPLOT

75th percentile

median

25th percentile

a letter indicates whether

concentrations from each

seasonal period were

significantly different

at the 0.05 significance level.

Boxplots denoted with the same

letter indicates concentrations

were not significantly different.

Figure 34. Nitrate nitrogen concentrations by seasonal period near the mouths of the Econlockhatchee 

and Wekiva Rivers, 1991-99.
72 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Figure 35. Dissolved nitrite plus nitrate nitrogen concentration and water temperature at 

St. Johns River sites, 2000-02.
Water-Quality Characteristics 73

Figure 35. Dissolved nitrite plus nitrate nitrogen concentration and water temperature at 

St. Johns River sites, 2000-02. (Continued)
74 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

the year, concentrations were at or below method reporting 

limits (0.02 mg/L) at the Cocoa and DeLand sites. 

Increased concentrations from October-February probably 

did not result from wastewater discharges or ground-water 

inflow. There are few wastewater discharges upstream of 

the Cocoa site. Increased concentrations from wastewater 

discharges or ground-water inflow also are expected to 

occur during low-flow conditions. The October-February 

period, however, encompassed both high- and low-flow 

conditions. If variations in concentrations were primarily 

the result of wastewater discharges or ground-water inflow, 

increased concentrations also would be expected during the 

March-June low-flow period, which did not occur. Nitrate 

can be used as a nutrient by plants, which could explain 

lower concentrations from March-June. Ammonium nitrogen, which generally is used preferentially by plants before 

nitrate is used, typically was detected at all St. Johns River 

sites; this indicates that nitrate may not have been assimilated by plants from March-June. Another explanation for 

increased nitrate concentrations from October-February is 

that conditions may have been more favorable for nitrification of ammonium in the St. Johns River or floodplain sediments. The main factors that limit nitrification include the 

availability of ammonium nitrogen, dissolved oxygen 

concentration, and water temperature (Schmidt, 1982). 

Nitrate nitrogen concentrations at the Cocoa, Christmas, 

Sanford, and DeLand sites were inversely correlated to 

water temperature (fig. 35). During periods of low water 

temperatures, conditions may have been favorable for nitrification because of reduced uptake of ammonium nitrogen 

by plants and the greater solubility of oxygen in the water.

At the Sanford and DeLand sites, increased nitrate 

concentrations also were measured during high-flow 

conditions in water years 2001 and 2002. Nitrate concentrations were as high as 0.27 mg/L and occurred on the 

rising limb of the streamflow hydrograph. Increased 

concentrations at these two sites may have resulted from 

the runoff of fertilizers or animal wastes from urban land.

Total Phosphorus

Median total phosphorus concentrations in the 

St. Johns River ranged from about 0.03 to 0.09 mg/L 

during 1991-99 (table 26). Increased total phosphorus 

concentrations at sites downstream of the Wekiva River 

may have resulted from phosphorus-enriched groundTable 26. Descriptive statistics of total phosphorus concentrations in the St. Johns, Econlockhatchee, and Wekiva Rivers, 1991-99.

[Map identifier refers to fig. 4. N, number of samples; IQR, interquartile range; the IQR may not equal the 75th percentile minus the 25th percentile in some 

cases due to rounding]

Map ID

(fig. 4) Station name N Percent censored data

25th

percentile Median 75thpercentile IQR

 1 St. Johns River near Cocoa 67 0 0.050 0.066 0.099 0.049

 2 St. Johns River at Beeline Highway 71 0 .058 .083 .118 .060

 3 St. Johns River near Christmas 122 1.6 .049 .072 .110 .062

 4 St. Johns River at Orange Mound 29 3.4 .036 .053 .091 .055

 5 St. Johns River at Puzzle Lake 78 2.6 .018 .032 .065 .047

 7 Econlockhatchee River near the mouth 52 0 .065 .083 .091 .029

 8 St. Johns River upstream of Lake Harney, 

near Geneva

250 52.0 .040* .060* .087 .047

 9 St. Johns River at north end of Lake Harney 102 65.0 .040* .053* .069 .029

10 St. Johns River at Lemon Bluff 103 53.4 .050* .062* .081 .032

11 St. Johns River at State Road 415 Bridge 104 41.3 .050* .069* .095 .045

12 St. Johns River near Sanford 146 31.0 .054* .080 .100 .046

13 St. Johns River south of the confluence with the 

Wekiva River

100 17.0 .068 .082 .107 .039

16 Wekiva River near the mouth 103 4.9 .076 .091 .113 .037

17 St. Johns River north of the confluence with the 

Wekiva River

104 15.4 .067 .085 .101 .035

18 St. Johns River downstream of Blue Springs 103 17.5 .066 .080 .097 .031

20 St. Johns River near Hontoon Marina 104 16.3 .067 .080 .101 .034

21 St. Johns River near DeLand 129 12.4 .065 .078 .095 .030

*Percentile estimated using the probability plotting procedure described by Helsel and Cohn, 1988.
Water-Quality Characteristics 75

water inflow. Phosphorus concentrations as high as 

0.35 mg/L were reported in water from the Upper Floridan aquifer in the Wekiva Springs ground-water basin 

(Toth and Fortich, 2002). Phosphorus concentrations in 

the Wekiva River were greater than those in the St. Johns 

River. The 1991-99 data showed a median total phosphorus concentration of 0.091 mg/L near the mouth of the 

Wekiva River; the 2000-02 data showed a median total 

phosphorus concentration of 0.10 mg/L at the Wekiva 

River near Sanford.

More than 80 percent of the total phosphorus concentrations from the St. Johns River, Econlockhatchee River, 

Wekiva River, and Blue Springs exceeded the recommended ecoregional nutrient criterion of 0.04 mg/L established by the USEPA, based on 1991-99 and 2000-02 data. 

In the St. Johns River, the fewest exceedances of the 

recommended ecoregional nutrient criterion were at the 

Orange Mound and Puzzle Lake sites (sites 4-5, fig. 4). 

At these two sites, 72 and 44 percent of the concentrations 

exceeded the recommended ecoregional nutrient criterion, respectively.

The 2000-02 data showed increased total phosphorus 

concentrations in the St. Johns River on the rising limb of 

the hydrograph, especially in water years 2001 and 2002 

(fig. 36). Increased phosphorus concentrations generally 

coincided with increased total iron concentration, 

suggesting that phosphorus was associated with iron. 

There was one occurrence of a pronounced increase in 

total phosphorus concentrations in the St. Johns River 

during low-flow conditions. At the Cocoa site, total phosphorus concentrations increased to 0.25 mg/L in MayJune 2000. This pattern is not consistent with inputs by 

ground water because concentrations did not increase to 

these levels during low-flow periods in 2001 and 2002. 

A major process in the cycling of phosphorus in lakes and 

rivers is the interaction of phosphorus with bottom sediments (Correll, 1998; House and Denison, 2002). One 

explanation for increased concentrations from May-July 

2000 is that conditions were conducive for phosphorus 

release from sediments in the St. Johns River or riverine 

lakes.

Relations between total phosphorus concentrations 

and streamflow were not consistent between the 1991-99 

and 2000-02 data at the St. Johns River sites (table 27), 

suggesting that relations determined using the 2000-02 

data may be due to the extreme high- and low-flow conditions of this period. The 1991-99 data showed total phosphorus concentrations generally increased during high 

streamflow conditions at the Cocoa site. Detection of a 

significant relation between total phosphorus concentration 

and streamflow at this site during 2000-02 may have been 

masked by the increased total phosphorus concentrations 

during May-July 2000, which did not occur during other 

low-flow periods. The 1991-99 data also showed that total 

phosphorus concentrations at the Christmas and upstream 

of Lake Harney sites generally decreased with increasing 

streamflow; the data showed no relation between total 

phosphorus concentration and streamflow at the DeLand 

site. The positive relation between total phosphorus 

concentrations and streamflow at the Christmas site 

(2000-02 data) may have resulted from increased phosphorus accumulation on the land surface during the drought 

conditions in 2000 and 2001 that washed off during the 

high-flow events.

Orthophosphate Phosphorus

At most of the sites on the St. Johns River and near 

the mouths of the Econlockhatchee and Wekiva Rivers, 

orthophosphate was reported in more than 90 percent of 

the samples from 1991-99 (table 28), indicating that 

bioavailable phosphorus generally is present in the 

St. Johns River. Orthophosphate made up about 35 to 66 

percent of the median total phosphorus concentration at 

the St. Johns River sites. Orthophosphate made up about 

69 percent of the median total phosphorus concentration 

near the mouth of the Econlockhatchee River, and about 

78 percent of the median total phosphorus concentration 

near the mouth of the Wekiva River.

The 1991-2002 data showed orthophosphate concentrations were significantly greater during high-flow 

conditions at the Cocoa, Sanford, and DeLand sites 

(table 27). Increased orthophosphate concentrations 

during high-flow periods may indicate the runoff of phosphorus fertilizer applied to the land surface. In contrast, 

little fertilizer is applied to the agricultural land in the 

vicinity of the Cocoa and Christmas sites because most of 

this land is unimproved pasture. It is likely that increased 

orthophosphate concentrations at these sites during highflow conditions resulted from the mineralization of 

livestock manures or organic soils, which may have 

occurred during drier conditions.

Relations between orthophosphate concentrations 

and streamflow determined using the 1991-99 and 

2000-02 data were not consistent at the Christmas site. 

The 1991-99 data showed orthophosphate concentrations 

at the Christmas site had no relation with streamflow. The 

2000-02 data, however, showed that orthophosphate 

concentrations significantly increased during high-flow 

conditions.
76 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Figure 36. Total phosphorus concentration and streamflow at St. Johns River sites, 2000-02.
Water-Quality Characteristics 77

Table 27. Relations between total phosphorus and orthophosphate concentrations and streamflow at St. Johns River sites, 

1991-2002.

[Correlations statistically significant at the 0.05 significance level are shown in bold. Explanation of values: tau, Kendall’s tau; p, p-value; n, number of 

observations. Retrospective data were compiled from the Florida Department of Environmental Protection, Florida Fish and Wildlife Conservation Commission, Orange County, St. Johns River Water Management District, Volusia County, and the U.S. Geological Survey]

Map ID

(fig. 4) Station name Explanation of values

Total phosphorus Orthophosphate phosphorus

Retrospective 

data 

(1991-99)

U.S. Geological 

Survey data 

(2000-02)

Retrospective 

data 

(1991-99)

U.S. Geological 

Survey data 

(2000-02)

 1 St. Johns River near Cocoa tau

p

n

0.2415

.0038

67

0.0395

.6218

72

0.3107

.0001

69

0.3760

0

72

 3 St. Johns River near Christmas tau

p

n

-.1600

.009

122

.1941

.0151

72

-.0569

.3478

123

.3772

0

72

 8 St. Johns River upstream of Lake 

Harney, near Geneva

tau

p

n

-.1002

.0177

250

Not determined -.0695

.1099

238

Not determined

12 St. Johns River near Sanford tau

p

n

Not determined .1342

.0926

72

Not determined .3290

0

72

21 St. Johns River near DeLand tau

p

n

.0329 

.579

129

.2516

.0016

72

.3544

0

131

.4433

0

72

Table 28. Descriptive statistics of orthophosphate concentrations in the St. Johns, Econlockhatchee, and Wekiva Rivers, 1991-99. 

[All concentrations are expressed as phosphorus; N, number of samples; IQR, interquartile range; the IQR may not equal the 75th percentile minus the 

25th percentile due to rounding. Data from the Florida Department of Environmental Protection, Florida Fish and Wildlife Conservation Commission, 

Orange County, St. Johns River Water Management District, Volusia County, and the U.S. Geological Survey]

Map ID

(fig. 4) Station name N

Percent 

censored 

data

Maximum 

reporting 

limit

25th

percentile Median 75thpercentile IQR

 1 St. Johns River near Cocoa 69 22 0.02 0.015* 0.030 0.059 0.044

 2 St. Johns River at Beeline Highway 75 2.7 .02 .034 .049 .087 .053

 3 St. Johns River near Christmas 123 22 .02 .023 .038 .059 .036

 4 St. Johns River at Orange Mound 27 18.5 .01 .015 .026 .060 .045

 5 St. Johns River at Puzzle Lake 81 35.8 .01 .006* .014 .027 .020

 7 Econlockhatchee River near the mouth 54 3.7 .01 .040 .057 .068 .028

 8 St. Johns River upstream of Lake Harney, 

near Geneva

238 28 .02 .020 .035 .049 .029

 9 St. Johns River at north end of Lake Harney 105 1.9 .006 .014 .023 .037 .023

10 St. Johns River at Lemon Bluff 107 1.9 .006 .020 .028 .039 .019

11 St. Johns River at State Road 415 bridge 107 3.7 .006 .024 .024 .036 .025

12 St. Johns River near Sanford 133 12.8 .02 .011 .028 .046 .035

13 St. Johns River south of the confluence with 

the Wekiva River

105 5.7 .006 .011 .030 .054 .043

16 Wekiva River near the mouth 105 0 .071 .084 .093 .022

17 St. Johns River north of the confluence with 

the Wekiva River

105 1.0 .006 .020 .037 .059 .039

18 St. Johns River downstream of Blue Springs 106 1.9 .006 .026 .042 .058 .033

20 St. Johns River near Hontoon Marina 107 0 .020 .040 .055 .035

21 St. Johns River near DeLand 131 3.8 .01 .020 .040 .060 .040

*Percentile was estimated using a probability plotting procedure.
78 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Total phosphorus concentration data (fig. 36) 

showed increased concentrations at the Cocoa site during 

May-June 2000. An increase in orthophosphate concentrations at this site during the same period was not 

measured. It is likely that any orthophosphate present at 

this site was utilized by phytoplankton and converted into 

organic phosphorus.

Chlorophyll-a

Chlorophyll-a is a photosynthetic pigment whose 

concentration is commonly used as a surrogate for the 

amount of algae in the water. To prevent eutrophication in 

streams, the USEPA recommends that chlorophyll-a

concentrations not exceed 0.40 μg/L in aggregate ecoregion XII, which includes the State of Florida. Retrospective chlorophyll-a concentration data were available from 

the FFWCC, Orange County, SJRWMD, and Volusia 

County. No retrospective data were available from the 

FDEP and the USGS.

Chlorophyll-a concentrations generally were greater 

than 0.4 μg/L in more than 90 percent of the samples from 

the St. Johns, Econlockhatchee, and Wekiva Rivers, 

based on the 1991-2002 data. In the St. Johns River, chlorophyll-a concentrations during 1991-99 (fig. 37) generally were greater at sites downstream of Lake Monroe 

compared to the other sites. Total phosphorus concentrations in the St. Johns River also generally were greater at 

sites downstream of Lake Monroe compared to the other 

sites (table 26), which suggests that increased 

chlorophyll-a concentrations at these same locations may 

have resulted in part from increased phosphorus concentrations. Chlorophyll-a concentrations also were greater at 

the downstream of Lake Harney site compared to the 

upstream of Lake Harney site, although total phosphorus 

concentrations were not substantially different at these 

two sites (table 26). This suggests that increased chlorophyll-a concentrations downstream of the lake may not be 

the result of increased phosphorus inputs to the river. 

Another explanation for increased chlorophyll-a concentrations at the downstream of Lake Harney site is that the 

quiescent conditions in the lake provide more optimal 

conditions for algal growth.

Chlorophyll-a concentrations in the St. Johns River 

generally were greater during the spring and summer lowflow periods. The 1991-99 data generally showed that 

concentrations were greater during the May-June lowflow period compared to other times of the year at sites 

downstream of site 11 (fig. 4). For example, in the 

St. Johns River downstream of Blue Springs (site 18), 

median concentrations in May-June were about 29 μg/L 

compared to about 2 μg/L in November-December 

(fig. 38).

Figure 37. Chlorophyll-a concentrations in the St. Johns River and near the mouths of the Econlockhatchee 

and Wekiva Rivers, 1991-99.
Water-Quality Characteristics 79

During 2000-02, chlorophyll-a concentrations at the 

Cocoa site generally were high during the spring and 

summer low-flow periods, from about April through 

August (fig. 39).Concentrations were high from 

about April through August at the other sites as well. 

Concentrations likely increased during the spring and 

summer low-flow periods in response to increased water 

temperatures. Chlorophyll-a concentrations generally 

remained elevated during the summer until high-flow 

events occurred in response to the wet-season rainfall. 

Decreases in chlorophyll-a concentrations during highflow events likely resulted from light limitation of 

phytoplankton growth due to inputs of highly colored 

organic matter into the St. Johns River (fig. 30). Chlorophyll-a concentrations and water color were inversely 

related (Kendall’s tau = -0.5041, p = 0). This hypothesis is 

consistent with studies (Aldridge and others, 1998; Phlips 

and others, 2000), indicating that light availability 

strongly affects phytoplankton crops in the lower 

St. Johns River.

CHLOROPHYLLCONCENTRATION,

IN MICROGRAMS PER LITER

a

JAN /

FEB.

20

0

40

60

MAR /

APR.

MAY /

JUNE

JULY /

AUG.

SEPT /

OCT.

NOV /

DEC.

13

of

17

10th/90th and

total values

EXPLANATION

SCHEMATIC BOXPLOT

x

x

x

75th percentile

median

25th percentile

upper outside

15

of

18

13

of

17

15

of

17

15

of

18

15

of

17

12

of

16

x

Figure 38. Seasonal variations in chlorophyll-a concentrations in the St. Johns River downstream 

of Blue Springs, Florida, 1991-99.

Figure 39. Chlorophyll-a concentrations and streamflow at the St. Johns River site near Cocoa, 

Florida, 2000-02.
80 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Turbidity

Turbidity is a measure of the scattering of light and is 

interpreted as a measure of the clarity of water. Increased 

water turbidity may result from algal growth or inorganic 

sediment suspended in the water. To protect streams from 

over-enrichment of nitrogen and phosphorus, the USEPA 

has established a recommended ecoregional criterion for 

turbidity of 1.9 nephelometric turbidity units (NTU) for 

aggregate ecoregion XII, which includes the State of 

Florida.

More than 80 percent of the turbidity values in the 

St. Johns River from 2000-02 exceeded 1.9 NTU. In 

contrast, less than 10 percent of the turbidity values from 

the Wekiva River near Sanford and 33 percent of the 

values from Blackwater Creek exceeded the recommended criterion of 1.9 NTU. Median turbidity values in 

the St. Johns River were greater at the Cocoa and Christmas sites (4.8 and 6.1 NTU, respectively) than at the 

Sanford and DeLand sites (3.9 and 2.9 NTU, respectively). Turbidity values had a significant inverse relation 

with streamflow at each of the sites (table 29). High 

turbidity values during low-flow periods were not consistent with inputs of sediment by soil erosion, but suggests 

that algal growth or the resuspension of bottom sediments 

during the spring and summer low-flow period contributes 

turbidity in the St. Johns River.

Total Suspended Solids

Total suspended solids concentration is an indicator 

of the amount of particulate matter in water and is 

measured as the amount of particulates that remains on a 

glass-fiber filter after an aliquot of sample has passed 

through a filtration apparatus. Particulate matter may be 

transported to a stream by erosion or through the resuspension of settled particulate matter in the streambed. 

Photosynthetic activity also can convert carbon and nutrients dissolved in the water into biomass.

Spatial, seasonal, and temporal variations in total 

suspended solids concentrations were similar to those for 

chlorophyll-a. Median total suspended solids concentrations in the St. Johns River during 1991-99 (table 30) 

were highest at sites downstream of State Road 415, 

generally where the highest chlorophyll-a concentrations 

occur in the river (fig. 37). Similar to the results for chlorophyll-a, total suspended solids concentrations generally 

Table 29. Relations between turbidity values and total suspended solids concentrations and streamflow at St. Johns 

River sites, 1991-2002.

[Correlations statistically significant at the 0.05 significance level are shown in bold. Explanation of values: tau, Kendall’s tau; p, p-value; 

n, number of observations. Retrospective data were compiled from the Florida Department of Environmental Protection, Florida Fish and 

Wildlife Conservation Commission, Orange County, St. Johns River Water Management District, Volusia County, and the U.S. Geological 

Survey]

Map ID

(fig. 4) Station name Explanation of values

Turbidity Total suspended solids

U.S. Geological 

Survey data 

(2000-02)

Retrospective 

data 

(1991-99)

U.S. Geological 

Survey data 

(2000-02)

 1 St. Johns River near Cocoa tau

p

n

-0.5172

0

72

Not determined -0.5708

0

72

 3 St. Johns River near Christmas tau

p

n

-.3799

0

72

-.2465

0

126

-.4710

0

72

 8 St. Johns River upstream of Lake 

Harney, near Geneva

tau

p

n

Not sampled -.2393

0

249

Not determined

12 St. Johns River near Sanford tau

p

n

-.3282

0

72

Not determined -.3009

.0002

72

21 St. Johns River near DeLand tau

p

n

-.3282

0

72

-.2132

.0009

108

-.2825

.0005

72
Water-Quality Characteristics 81

were greater during the May-June low-flow period 

compared to the other periods at sites downstream of 

Lake Harney, as indicated by data from the Hontoon 

Marina site (fig. 40). Data from the Christmas site from 

2000-02 (fig. 41) further illustrate that the highest total 

suspended solids concentrations occurred during lowflow conditions.

Higher total suspended solids concentrations during 

low-flow periods combined with similar spatial, seasonal, 

and temporal patterns between chlorophyll-a and total 

suspended solids concentrations suggest that chlorophyll-a probably contributes a substantial portion of the 

suspended solids in the river. Correlations between chlorophyll-a and total suspended solids concentrations at all 

of the St. Johns River sites were significant and positive, 

except for the Cocoa, Christmas, and Orange Mound sites 

from 1991-99 (table 31). The strongest correlations 

between chlorophyll-a and total suspended solids concentrations generally were observed at sites downstream of 

State Road 415. These also were the sites where the highest 

concentrations of chlorophyll-a generally were observed 

(fig. 37).

Human and Veterinary Antibiotics, Other Human 

Prescription and Nonprescription Drugs, Pesticides, 

and Organic Wastewater Constituents

Limited sampling was conducted at the Cocoa (site 1, 

fig. 4) and Sanford (site 12, fig. 4) sites on the St. Johns 

River in 2002 and 2003 to determine the occurrence of 

human and veterinary antibiotics, other human prescription and nonprescription drugs, pesticides, and a suite of 

organic constituents which may indicate domestic or 

industrial waste (app. B). Human prescription and nonprescription drugs analyzed included antacids, antibiotics, a 

stimulant, and anticoagulant, antihypertensive, and antiinflammatory drugs. A limited number (11) of pesticides 

and pesticide degradates were analyzed, including atrazine, bromacil, carbaryl, chlorpyrifos, diazinon, metalaxyl, metolachor, and prometon. Method reporting limits 

for pesticides were relatively high (0.5 μg/L) compared to 

other methods (Furlong and others, 2001). Some of the 

pesticides analyzed, such as bromacil, atrazine, and metolachlor, may be applied to agricultural land in the study 

Table 30. Descriptive statistics of total suspended solids concentrations in the St. Johns, Econlockhatchee, and Wekiva Rivers, 1991-99.

[N, number of samples; IQR, interquartile range; the IQR may not equal the 75th percentile minus the 25th percentile due to rounding. Data not analyzed for 

St. Johns River at Puzzle Lake]

Map ID

(fig. 4) Station name N

Percent 

censored 

data

25th

percentile Median 75thpercentile IQR

 1 St. Johns River near Cocoa 69 91.3 --------------------Not computed------------------

 2 St. Johns River at Beeline Highway 77 55.8 1.62* 3.59* 7.46 5.84

 3 St. Johns River near Christmas 126 67.5 1.77* 3.51* 6.89 5.12

 4 St. Johns River at Orange Mound 29 20.7 3.00 4.00 6.00 3.00

 7 Econlockhatchee River near the mouth 57 57.9 .86* 1.93* 4.00 3.14

 8 St. Johns River upstream of Lake Harney, near Geneva 249 59.8 2.13* 3.47* 6.00 3.87

 9 St. Johns River at north end of Lake Harney 103 61.2 2.54* 4.13* 7.00 4.46

10 St. Johns River at Lemon Bluff 105 56.2 1.93* 3.94* 9.00 7.07

11 St. Johns River at State Road 415 Bridge 106 35.8 3.74* 6.50 11.25 7.51

12 St. Johns River near Sanford 133 27.8 5.00* 8.00 14.50 9.50

13 St. Johns River south of the confluence with the Wekiva River 104 14.4 5.00 11.0 14.75 9.75

16 Wekiva River near the mouth 103 53.4 2.94* 4.44* 6.50 3.56

17 St. Johns River north of the confluence with the Wekiva River 104 23.1 5.00 8.50 12.00 7.00

18 St. Johns River downstream of Blue Springs 105 26.7 7.50* 4.57 12.00 7.43

20 St. Johns River near Hontoon Marina 106 21.7 5.00 8.00 13.25 8.25

21 St. Johns River near DeLand 106 28.3 4.60* 7.00 11.40 6.80

*Percentile estimated using a probability plotting procedure.
82 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

area. Caffeine, cotinine, and sulfamethoxazole were 

analyzed by two methods; two reporting limits are listed 

for these constituents in appendix B.

A total of 20 compounds was detected, including 

pesticides, antibiotics, plasticizers, and detergent metabolites (fig. 42). Atrazine, metolachlor, and cholesterol 

were the most frequently detected compounds. No pesticide concentrations exceeded applicable MCLs established by the USEPA. Atrazine and metolachlor are 

pesticides used to control broadleaf and grassy weeds on 

corn and vegetable crops in Florida (Shahane, 1999). 

Atrazine also is used to control weeds in St. Augustine 

grass (McCarty, 1995). Cholesterol is a steroid and can 

originate from both animal and plant sources (Zaugg and 

others, 2002). The frequent detection of cholesterol is 

consistent with a nationwide study on the occurrence of 

pharmaceuticals, hormones, and organic wastewater 

contaminants in streams in which steroids were detected 

in over 80 percent of the samples analyzed (Kolpin and 

others, 2002). Atrazine was detected in 83 percent of the 

TOTAL SUSPENDED SOLIDS

CONCENTRATION,

IN MILLIGRAMS PER LITER 10

0

20

30

5

15

25

JAN /

FEB.

MAR /

APR.

MAY /

JUNE

JULY /

AUG.

SEPT /

OCT.

NOV /

DEC.

13

of

17

10th/90th and

total values

EXPLANATION

SCHEMATIC BOXPLOT

75th percentile

median

25th percentile

13

of

17

14

of

18

13

of

18

14

of

18

14

of

18

11

of

17

Figure 40. Seasonal variations in total suspended solids concentration in the St. Johns River 

near Hontoon Marina, Florida, 1991-99.

Figure 41. Total suspended solids concentration and streamflow at the St. Johns River site near 

Christmas, Florida, 2000-02.
Water-Quality Characteristics 83

samples analyzed from the Cocoa and Sanford sites; 

metolachlor and cholesterol were detected in 57 percent 

of the samples analyzed. In most cases, concentrations 

were less than the method reporting limits; however, the 

presence of these constituents was confirmed in the laboratory by using mass spectrometry. Reported concentrations less than the method reporting limit are estimated 

concentrations.

More constituents were detected in water from the 

Sanford site compared to the Cocoa site, which likely 

reflects that the Sanford site is downstream from (1) point 

sources delivering municipal wastewater effluent to 

streams in the study area and (2) urban-residential areas 

on septic systems. A total of 20 constituents was detected 

at the Sanford site. The most frequently detected constituents at this site were atrazine, cholesterol, coprostanol, 

and cotinine. Coprostanol is produced almost exclusively 

in the digestive tracts of higher mammals and is often 

correlated with the presence of other sewage-derived 

contaminants (Zaugg and others, 2002). Cotinine is a 

nicotine metabolite. Atrazine was detected in 75 percent 

of the samples analyzed from this site, and cotinine was 

detected in 67 percent samples analyzed. Coprostanol and 

cholesterol were detected in 50 percent of the samples 

Table 31. Kendall’s tau correlation coefficients describing the relation between chlorophyll-a and total suspended solids concentration 

at sites on the St. Johns, Econlockhatchee, and Wekiva Rivers, 1991-99.

[N, number of observations; p-value, probability value. Correlations statistically significant at the 0.05 significance level are shown in bold. Retrospective data 

compiled from the Florida Department of Environmental Protection, Florida Fish and Wildlife Conservation Commission, Orange County, St. Johns River 

Water Management District, Volusia County, and the U.S. Geological Survey]

Map ID

(fig. 4) Station name

Retrospective data 

(1991-99)

U.S. Geological Survey data 

(2000-02)

N Kendall’s

tau p-value N Kendall’stau p-value

 1 St. Johns River near Cocoa 59 0.0158 0.7353 72 0.4887 0.0

 2 St. Johns River at Beeline Highway 29 .4433 .0006 ----------Not sampled----------

 3 St. Johns River near Christmas 76 .01789 .8022 72 .4636 0

 4 St. Johns River at Orange Mound 28 .2154 .1167 ----------Not sampled----------

 5 St. Johns River at Puzzle Lake Total suspended solids data

not compiled

----------Not sampled----------

 7 Econlockhatchee River near the mouth 37 .2628 .0096 ----------Not sampled----------

 8 St. Johns River upstream of Lake Harney, near Geneva 178 .1074 .0278 ----------Not sampled----------

 9 St. Johns River at north end of Lake Harney 105 .3463 0 ----------Not sampled----------

10 St. Johns River at Lemon Bluff 107 .3226 0 ----------Not sampled----------

11 St. Johns River at State Road 415 Bridge 106 .5332 0 ----------Not sampled----------

12 St. Johns River near Sanford 129 .4250 0 71 .6103 0

13 St. Johns River south of the confluence with the Wekiva River 106 .4947 0 ----------Not sampled----------

14 Wekiva River near Sanford ----------Not sampled---------- 12 ---Not determined---

15 Blackwater Creek near Cassia ----------Not sampled---------- 12 ---Not determined---

16 Wekiva River near the mouth 101 .1525 0 ----------Not sampled----------

17 St. Johns River north of the confluence with the Wekiva River 103 .5393 0 ----------Not sampled----------

18 St. Johns River downstream of Blue Springs 103 .5517 0 ----------Not sampled----------

20 St. Johns River near Hontoon Marina 105 .5456 0 ----------Not sampled----------

21 St. Johns River near DeLand 105 .5110 0 72 .5919 0
84 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

analyzed. The other constituents detected at the Sanford 

site, including the antibiotic azithromycin and anhydroerythromycin, were each measured only once.

Atrazine, metolachlor, cholesterol, bromacil, and 

phenol were detected at the Cocoa site. Bromacil is a 

pesticide commonly applied to citrus in Florida. Phenol is 

used as a disinfectant (Zaugg and others, 2002) and was 

reported at oncentrations near reporting limits in field 

blank samples (Kolpin and others, 2002). Atrazine and 

metolachlor were detected in all of the samples analyzed 

from this site; cholesterol was detected in 67 percent of 

the samples analyzed. Bromacil and phenol were each 

detected once. The greatest concentration of atrazine, 

0.77 ug/L, was measured in April 2002.

Although limited, the data at the Cocoa site suggest 

that a greater number of constituents were detected during 

high-flow conditions than during low-flow conditions, 

which suggests these constituents probably are transported by runoff (fig. 43). At this site, five constituents 

were detected during high-flow conditions in August 

2002. In contrast, only one constituent (metolachlor) was 

detected during low-flow conditions in December 2001.

At the Sanford site, the limited data indicate that a 

greater number of constituents generally were detected 

during low-flow conditions, which suggests constituents 

measured at this site resulted from point-source 

discharges or septic tank leachate. Constituent concentrations are expected to be more concentrated during lowflow conditions if point-source discharges and septic tank 

leachate are the sources of constituents because these 

comprise a greater percentage of the streamflow during 

such conditions. At the Sanford site, 12 constituents were 

detected during low-flow conditions in June 2002; 

10 constituents were detected during low-flow conditions 

in December 2002; and 4 constituents (not including 

human prescription and nonprescription drugs) were 

detected in March 2002. In contrast, only one constituent 

(2-butoxy-phosphate ethanol) was detected at this site 

during high-flow conditions in September 2002.

Sampling was conducted once in March 2003 to 

determine the occurrence of the suite of organic constituents that may indicate domestic or industrial waste, pesticides, and human prescription and nonprescription drugs 

at three points in the water treatment process at a pilot 

facility on Lake Monroe. Removal of iron and organic 

matter was necessary at this facility because iron concentrations in the St. Johns River were greater than the 

secondary drinking water regulation established by the 

USEPA during high-flow conditions and because 

dissolved organic carbon concentrations were relatively 

high (the median concentration at the Sanford site was 

19.5 mg/L). Removal of dissolved solids and chloride 

Figure 42. Frequency of detection of human and veterinary antibiotics, other human prescription and nonprescription 

drugs, pesticides, and organic wastewater constituents at the St. Johns River sites near Cocoa and Sanford, Florida, 

water years 2002-03.
Water-Quality Characteristics 85

also was necessary because concentrations of these two 

constituents exceeded secondary drinking water regulations established by the USEPA during low-flow conditions. Conventional water treatment (coagulation/ 

flocculation, high-rate clarification, and filtration) was 

used at the pilot facility to remove iron and organic matter 

from the water, and reverse osmosis was used to remove 

dissolved solids and chloride. Disinfectant (chloramine) 

was applied prior to the reverse-osmosis process to reduce 

biological fouling of the reverse-osmosis membranes.

Water samples were collected to document the 

quality of the untreated influent or raw water to the 

facility, finished water after conventional treatment, and 

the finished water after the reverse-osmosis process. 

Fewer constituents were detected in the reverse-osmosis 

filtered sample than in the raw water or conventionally 

treated water samples. Only one constituent (bromoform) 

was detected in the reverse-osmosis treated water sample, 

which suggests this process removed most of the trace 

organic contaminants from the water. In the raw water 

sample, seven constituents were detected (atrazine; 

cotinine; caffeine; bromacil; phenol; 3,4-dichlorophenol 

isocyanate; and 2-butoxy-phosphate ethanol).

Ten constituents were detected in the conventionally 

treated water sample (anthraquinone, atrazine, cotinine, 

caffeine, bromacil, bromoform, 2-butoxy-phosphate ethanol, carbamazepine, fluoranthene, pentachlorophenol). 

Bromoform, a disinfection byproduct, likely was formed 

in the conventionally treated water after the disinfectant 

was applied. The fact that other constituents were detected 

in finished water that were not detected in raw water may 

have resulted from the removal of matrix components in 

the raw water by the conventional water-treatment 

process. Matrix interferences in raw water samples can 

preclude getting a sufficiently good-quality spectral 

match to be confident of making an identification, 

particularly if the concentration of the interferent is 

much greater than the compound of interest. This can 

be a particular problem with samples that may contain 

substantial amounts of wastewater effluent (E. Furlong, 

U.S. Geological Survey, written commun., 2003). 

Figure 43. Number of human and veterinary antibiotics, other human prescription and nonprescription drugs, 

pesticides, and organic wastewater constituents detected at selected St. Johns River sites near Cocoa and 

Sanford, Florida, water years 2000-02.
86 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Anthraquinone is used as a bird repellent and is used in 

manufacturing textiles (Zaugg and others, 2002). Fluoranthene is a combustion product of coal, oil, gas, or 

wood (Zaugg and others, 2002). Pentachlorophenol is 

used as a wood preservative. Carbamazepine is an anticonvulsant drug, which has been reported as a persistent 

indicator of wastewater contamination in Europe. Ternes 

(1998) reported that carbamazepine was ubiquitously 

present in the aquatic environment in Germany with 

median concentrations of 2.1 μg/L in wastewater effluent 

and 0.25 μg/L in streams. These ubiquitous and high 

concentrations were attributed to the extremely low 

removal rate in wastewater treatment facilities. Only 

about 7 percent of the carbamazepine was removed in a 

wastewater treatment facility near Frankfurt, Germany 

(Ternes, 1998). Investigations in Germany and Switzerland (Ternes, 1998; Tixier and others, 2003) have shown 

that of the pharmaceuticals studied, carbamazepine had 

the highest daily loads in wastewater effluent. Subsequent 

sampling and collection of field-blank and replicate data 

would be useful to better define the occurrence of these 

constituents in the conventionally treated water. Five 

constituents (atrazine, cotinine, caffeine, bromacil, and 

2-butoxy-phosphate ethanol) were detected in both the 

raw and conventionally treated water samples, which 

suggests that the conventional water-treatment process 

used at the pilot facility on Lake Monroe may not remove 

these constituents.

Temporal Trends (1991-99)

Temporal trends in constituent values during 1991-

99 were determined at the Cocoa, Christmas, upstream of 

Lake Harney, and DeLand sites (fig. 4, table 32). Temporal trends were not determined at the Sanford site because 

continuous streamflow data were not available from 

1991-95. Most temporal trends were determined using the 

Seasonal-Kendall test. Temporal trends in nitrate, ammonia nitrogen, total phosphorus, and orthophosphate 

concentrations were determined using Tobit regression 

because at each site there generally were greater than 

5 percent censored data and two reporting limits. All 

Tobit regression models included terms accounting for 

flow- and seasonal-related variability. Temporal trends 

in ammonia nitrogen were not determined at the DeLand 

site because concentrations were not reported from 1996-99. 

Temporal trends in chlorophyll-a concentrations were not 

calculated for the Cocoa site because there were too few 

observations in the beginning of the period of record. 

The criteria for significance of temporal trends were that 

each trend was significant at the 0.05-significance level 

during the 1991-98, 1991-99, and/or 1992-99 periods.

Increased total phosphorus and orthophosphate 

concentrations at the Cocoa, Christmas, and upstream of 

Lake Harney sites were the most notable temporal trends 

in water-quality constituents from 1991-99. The reason 

for the increases in phosphorus concentrations was not 

readily apparent. Higher phosphorus concentrations from 

1991-99 likely were not the result of increased fertilizer 

applications or greater cattle numbers. County-level fertilizer-sales data compiled by the USGS (D. Lorenz, U.S. 

Geological Survey, written commun., 1999) showed no 

increase in the tons of phosphorus fertilizer sold in 

Brevard, Indian River, Lake, Okeechobee, Orange, 

Osceola, Seminole, or Volusia Counties from 1991-98. 

Livestock counts obtained from the Florida Agricultural 

Statistics Service also showed no increase in the number 

of beef cows in these same counties from 1991-99 

(Florida Agricultural Statistics Service, 2004). Flooding 

former agricultural lands to create wetlands in the upper 

St. Johns River Basin may have influenced phosphorus 

concentrations in the river. Phosphorus that accumulated 

on the land surface as a result of fertilizer application or 

livestock manure generation may have remobilized once 

the land was reflooded. In addition, organic phosphorus in 

the soil may have mineralized when the wetland was 

drained. Several studies in Florida (Reddy, 1983; Olila 

and others, 1997) demonstrate that drying and reflooding 

organic soils release phosphorus into the water column. 

However, analyses of wetland data from the upper 

St. Johns River Basin (Marzolf, 2000) generally show no 

significant temporal trends from 1979-98 in total phosphorus or orthophosphate concentrations in the water.

Summary 

Streamflow and water quality of the 90-mile-long 

middle reach of the St. Johns River from downstream of 

Lake Poinsett to near DeLand, Florida, was characterized 

using retrospective (1991-99) and recently collected data 

(2000-02). The Econlockhatchee and Wekiva Rivers are 

the two largest tributaries to the St. Johns River within the 

study area. Upstream and within the study area, much of 

the St. Johns River is characterized by flow through shallow lakes. Wetlands, agriculture, upland forests, and 

urban areas are the predominant land use and land covers. 

Agricultural land is concentrated in the southern part of the 

study area and consists mainly of unimproved pasture. 
Table 32. Temporal trends in selected physical properties, nutrients, and chlorophyll-a at St. Johns River sites, 1991-99.[Correlations statistically significant at the 0.05 significance level are shown in bold. %, percent; p, p-value; N, number of observations. Sites are shown in downstream order. Refrom the Florida Department of Environmental Protection, Florida Fish and Wildlife Conservation Commission, Orange County, St. Johns River Water Management District, VoU.S. Geological Survey]

Map ID

(fig. 4) Site Specific conductance Dissolved oxygen pH Nitrate nitrogen Total organic plus ammonia nitrogen Ammonia nitrogen Total phosphorus O

 1 St. Johns River near Cocoa trend = -2.1%

p = .1415N = 51

trend = -1.7%p = .4156N = 51

trend = 0.8%p = .0148N = 49

trend = -1.9%p = .7207N = 69

trend = 2.4%p = .3139N = 50

trend = -2.4%p = .4853N = 67

trend = 11.7%p = 0N = 67

trp N 

 4 St. Johns River near 

Christmas

trend = -3.7%p = .0543N = 53

trend = 2.3%p = .1695N = 53

trend =1.2%*p = .0446N = 52

trend = 6.4%p = .0501N = 121

trend = 1.3%p = .5840N=54

trend = 9.1%**p = .0002N = 121

trend = 15.0%p = 0N = 120

trp N 

8 St. Johns River upstream 

of Lake Harney, near Geneva

trend = -2.1%p = -.1994N = 54

trend = -.1%p = .8876N = 54

trend = -.3%p = .4.95N = 54

trend = -2.0%p = .2074N = 248

trend = 2.7%p = .1739N = 54

trend = 6.5%***p = .0022N = 143

trend = 10.9%p = 0N = 250

trp N 

21 St. Johns River near 

DeLand

trend = -1.0%p = .3900N = 54

trend = -1.8%p = .2175N = 54

trend = -.04%p = .7323N = 54

trend = -1.9%p = .4503N = 128

trend = -.03%p = .1502N = 54

Not calculated trend = .04%

p = .9719N = 126

trp N 

 *There was no significant temporal trend from 1991-98. Trend = 1.1%, p = 0.1166. **There was no significant temporal trend from 1991-98. Trend = 3.1%, p = 0.3042.***There was no significant temporal trend from 1991-98. Trend = 4.8%, p = 0.0646.
88 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

Urban land is concentrated in the Orlando metropolitan 

area and in the cities of Cocoa and Titusville. Many of the 

original wetlands in the Upper St. Johns River Basin were 

drained to support the production of citrus, row crops, and 

beef cattle. A large construction project began in 1988 to 

restore wetlands in the Upper St. Johns River Basin; 

about 240 square miles of wetlands have been enhanced 

or restored.

Streamflow characteristics of the St. Johns River 

were determined using data collected by the U.S. Geological Survey (USGS) from water years 1933-2000. Waterquality characteristics were described using 1991-99 

retrospective data at 15 sites on the St. Johns River and 

1 site each near the mouths of the Econlockhatchee and 

Wekiva Rivers. Water-quality constituents evaluated 

included physical properties, major ions and other inorganic constituents, nutrients, organic carbon, suspended 

solids, and phytoplankton chlorophyll-a. The 1991-99 

data were augmented with: (1) biweekly water-quality 

data collected at four St. Johns River sites during water 

years 2000-02; (2) continuous field properties data 

collected at these same four sites primarily during water 

years 2001-02; (3) quarterly water-quality data collected 

from sites on the Wekiva River and Blackwater Creek 

from water years 2000-02; and (4) quarterly water-quality 

data collected at Blue Springs from water years 2000-02.

Mean annual streamflow in the St. Johns River 

increased from 1,030 cubic feet per second (ft3

/s) downstream of Lake Poinsett to 2,850 ft3

/s near DeLand, based 

on data from water years 1985 to 2000. Streamflow in the 

St. Johns River generally was lowest in May and highest 

in September and October. Lower streamflows in May 

probably resulted from less rainfall combined with 

greater evapotranspiration rates due to the high air temperatures. High streamflow in September and October 

probably resulted from increased rainfall combined with 

less available storage in lakes and wetlands. Rainfall from 

June through August may fill the lakes and wetlands, 

leaving less available storage for rainfall in September 

and October. The reach of the St. Johns River described 

in this report does not have daily tidal reversals of streamflow; however, periodic streamflow reversals were 

observed in the studied reach of the river. Reverse streamflows typically were measured at least once a year at the 

most downstream sites due to wind and tidal effects. 

Streamflow reversals were measured infrequently during 

periods of extreme low flow at upstream sites as a result 

of wind effects.

The highest dissolved oxygen concentrations were at 

sites downstream from Lake Poinsett, Lake Harney, and 

Lake Monroe. Higher concentrations at these sites may 

have resulted from algal production in the lakes. 

Dissolved oxygen concentrations and water pH values 

were substantially lower during high-flow conditions than 

during low-flow conditions. Low dissolved oxygen 

concentrations may have resulted from the input of water 

from marsh areas or from the subsequent decomposition 

of organic matter transported to the river during high-flow 

events. Low water pH values during high-flow conditions 

likely resulted from the increased dissolved organic 

carbon concentrations in the river. Dissolved organic 

carbon primarily is composed of fulvic and hydrophilic 

acids that lower the pH of the water.

More than 50 percent of the samples collected from 

the St. Johns River from 2000-02 had total dissolved 

solids concentrations greater than the secondary drinking 

water regulation of 500 milligrams per liter (mg/L) established by the U.S. Environmental Protection Agency 

(USEPA) and the State of Florida. Chloride and sulfate 

concentrations also exceeded the secondary drinking 

water regulation of 250 mg/L established by the USEPA 

and the State of Florida, primarily during summer lowflow periods. Chloride concentrations exceeded 250 mg/L 

at all sites; sulfate concentrations exceeded 250 mg/L 

only at the two most upstream sites.

Concentrations of total dissolved solids and other 

inorganic constituents in the St. Johns River were 

inversely related with streamflow. Most major ion 

concentrations, total dissolved solids concentrations, and 

specific conductance values varied substantially at the 

Christmas, Sanford, and DeLand sites during low-flow 

periods in 2000-01, probably reflecting wind and tidal 

effects. Bromide concentrations as high as 17 mg/L were 

measured at the most upstream site from 2000-02. 

Temporal variations in bromide were characterized by 

sharp peaks in concentration during low-flow periods. 

Peaks in bromide concentrations tended to coincide with 

peaks in chloride concentrations (except during 2001) 

because ground water affected by relict seawater likely 

was the source of both constituents.

Other constituents varied directly with streamflow. 

Sulfide concentrations as high as 6 mg/L were measured 

during high-flow periods. Increased sulfide concentrations 

likely resulted from the decomposition of organic matter or 

the reduction of sulfate. Total iron concentrations greater 

than the secondary drinking water regulation of 300 micrograms per liter (μg/L) established by the USEPA and the 

State of Florida were measured at all St. Johns River sites 

from 2000-02. Total iron concentrations exceeded 

300 μg/L most frequently at the Christmas site, located in 
References 89

the upstream part of the study area. Total and dissolved 

iron concentrations also increased substantially during 

high-flow conditions in the St. Johns River. 

During 2000-02, more than 95 percent of the samples 

from the St. Johns River sites had water color values that 

exceeded the secondary drinking water regulation of 15 

platinum-cobalt units established by the USEPA and the 

State of Florida. Median dissolved organic carbon 

concentrations ranged from 15 to 26 mg/L during 

2000-02; concentrations as high as 42 mg/L were 

measured. Water color values and dissolved organic 

carbon concentrations were significantly greater during 

high-flow conditions than during low-flow conditions. 

Specific ultraviolet light absorbance data indicated the 

organic carbon in the river during high-flow conditions 

was aromatic in composition and likely originated from 

terrestrially derived sources compared to other times of 

the year when organic carbon was likely derived from 

algae.

Ammonia nitrogen and orthophosphate phosphorus 

were present in the St. Johns River, indicating that 

bioavailable forms of nutrients generally were present. 

Ammonia nitrogen concentrations generally were greater 

during high-flow conditions than during low-flow conditions. Nitrate nitrogen concentrations during 1991-99 

were lower in the upstream part of the river compared to 

other sites, which may be related to nitrate being used as 

a nutrient by terrestrial or aquatic plants or denitrification 

in wetland areas. High nitrate concentrations at sites 

downstream of the confluence with the Wekiva River 

may be due to nitrogen-enriched ground-water inflow. 

Seasonal variations in nitrate concentrations during 2000-02 

were characterized by sharp peaks in concentration from 

about October through February, likely resulting from 

nitrification.

Chlorophyll-a and total suspended solids concentrations in the St. Johns River generally were highest in May 

and June compared to other times of the year. Low chlorophyll-a concentrations in July and August may have 

resulted from light limitation because of increased 

concentrations of highly colored organic matter in the 

river. High total suspended solids concentrations during 

the May and June low-flow period suggest that runoff of 

soil eroded from the land surface is not the primary mechanism in which suspended solids are contributed to the 

river, but rather that suspended solids primarily are 

contributed by algal production or the resuspension of 

bottom sediments.

The occurrence of human and veterinary antibiotics, 

other human prescription and nonprescription drugs, 

pesticides, and a suite of organic constituents that may 

indicate domestic or industrial waste were described at 

two sites using limited data collected in water years 2002-03 

and at a pilot water-treatment facility on Lake Monroe 

using data from one sampling event conducted in March 

2003. A total of 20 constituents was detected in water 

from the St. Johns River, which included pesticides, 

antibiotics, plasticizers, and detergent metabolites. No 

constituent concentrations exceeded applicable Maximum Contaminant Levels established by the USEPA. 

Atrazine, metolachlor, and cholesterol were the most 

frequently detected compounds. More constituents were 

detected in water from the Sanford site (20) compared to 

the Cocoa site (5), which likely reflects the location of the 

Sanford site downstream from point sources delivering 

municipal wastewater effluent to streams in the study area 

and downstream from urban-residential areas on septic 

systems. Limited data suggest these constituents are 

derived from runoff to the Cocoa site and from pointsource discharges or septic tank leachate at the Sanford 

site.

The most notable temporal trend in water-quality 

constituents during 1991-99 was increased concentrations 

of total phosphorus and dissolved orthophosphate at three 

upstream sites on the St. Johns River. The reason for the 

increases in phosphorus concentrations at these sites was 

not apparent. Increased fertilizer sales, increased livestock populations, or increased phosphorus concentrations in recently restored wetlands do not account for this 

trend.

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94 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002
 95

Appendixes
Appendix A. Sites with retrospective water-quality data compiled for this report 

[FDEP, Florida Department of Environmental Protection; FFWCC, Florida Fish and Wildlife Conservation Commission; Orange County,

Orange County Department of Environmental Protection; SJRWMD, St. Johns River Water Management District; Volusia County, Volusia 

County Department of Environmental Health; USGS, U.S. Geological Survey]

Agency 

station 

identifier

Station name Sampling 

agency

Period 

of 

record

LPO Lake Poinsett Outlet at State Road 520 SJRWMD 1979-1999

SJ19A St. Johns River at Beeline Highway Orange County 1988-1999

SJR528 St. Johns River at bridge on 528 SJRWMD 1996-1999

SJ21 St. Johns River at State Road 50, west Orange County 1980-1999

SRS Seminole Ranch south boundary at State Road 50 along 

St. Johns River

SJRWMD 1991-1999

SJ24 St. Johns River at Orange Mound Orange County 1981-1999

SJ25 St. Johns River at south Puzzle Lake Orange County 1980-1981, 1983-1999

GFCCR0173 Econlockhatchee River at Tree Line FFWCC 1973-1996

SJ26 Econlockhatchee River at mouth Orange County 1980-1999

20010012 St. Johns River at Florida Highway No. 46 FDEP 1968-1973, 1975-1995

SJ27 St. Johns River at State Road 46 Orange County 1980-1999

SRN Seminole Ranch north boundary at State Road 46 along 

St. Johns River

SJRWMD 1982-1989, 1991-1999

SJ02 St. Johns River at south end Lake Harney Volusia County 1991-1999

SJ04 St. Johns River at north end Lake Harney Volusia County 1991-1999

GFCCR0188 St. Johns River exit to Lake Harney FFWCC 1981-1996

SJ05 St. Johns River at Lemon Bluff Volusia County 1991-1999

SJ08 St. Johns River south of State Road 415 bridge Volusia County 1991-1999

20010003 St. Johns River at U.S. Highway 17-92 FDEP 1968-1995

20010003 St. Johns River at U.S. Highway 17-92 SJRWMD 1995-1999

SJ12 St. Johns River at U.S. Highway 17-92 Volusia County 1991-1999

GFCCR0197 St. Johns River 1⁄2 mile south of Wekiva River FFWCC 1987-1996

SJ13 St. Johns River at channel marker 101 Volusia County 1991-1999

SJ14 Wekiva River 1 mi. upstream from St. Johns Volusia County 1991-1999

GFCCR0196 Wekiva River part first sharp turn to right FFWCC 1987-1996

GFCCR0199 St. Johns River north of Wekiva River FFWCC 1987-1996

SJ15 St. Johns River at channel marker 93 Volusia County 1991-1999

SJ16 St. Johns River at channel marker 69 Volusia County 1991-1999

SJ18 St. Johns River at channel marker 53 Volusia County 1991-1999

SJ19 St. Johns River 50 yds. south of State Road 44 bridge Volusia County 1991-1999

02236000 St. Johns River near DeLand USGS 1948-1995

96 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002
Appendix B 97

Appendix B. Water-quality constituents analyzed in stream water collected during this study

[mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemen per centimeter; °C, degrees Celcius; NTU, nephelometric turbidity units; UV, 

ultraviolet; nm, nanometers; N, nitrogen; P, phosphorus; USEPA, U.S. Environmental Protection Agency. Concentrations denoted with an asterisk are estimates]

Constituent Use of 

organic constituent

Method 

reporting limit Analytical method

Total sulfide, mg/L 1 Iodometric titration (Fishman and Friedman, 1989)

Dissolved calcium, mg/L .02 USEPA method 200.7 (USEPA, 1994)

Total calcium, mg/L .02 Do.

Dissolved magnesium, mg/L .03 Do.

Total magnesium, mg/L .03 Do.

Dissolved sodium, mg/L .1 Atomic absorption spectrometry (Fishman and 

Friedman, 1989)

Dissolved potassium, mg/L .1 Do.

Dissolved chloride, mg/L .05 USEPA method 300.0 (USEPA, 1993)

Dissolved sulfate, mg/L .07 Do.

Dissolved silica, mg/L .01 USEPA method 200.7 (USEPA, 1994)

Dissolved barium, μg/L .5 Do.

Total barium, μg/L .5 Do.

Dissolved iron, μg/L 2 Do.

Total iron, μg/L 2 Do.

Dissolved strontium, μg/L .5 Do.

Total strontium, μg/L .5 Do.

Dissolved bromide, mg/L .05 USEPA method 300.0 (USEPA, 1993)

Dissolved ammonia nitrogen, mg/L as N .01 Colorimetry (Fishman and Friedman, 1989)

Total ammonia nitrogen, mg/L as N .01 Do.

Dissolved nitrite nitrogen, mg/L as N .01 Do.

Total nitrite nitrogen, mg/L as N .01 Do.

Total organic plus ammonia nitrogen, 

mg/L as N

.2 Do.

Total nitrite plus nitrate nitrogen, 

mg/L as N

.02 Do.

Dissolved nitrite plus nitrate nitrogen, 

mg/L as N

.02 Do.

Total phosphorus, mg/L .02 Do.

Dissolved orthophosphorus, mg/L as P .01 Do.

Total orthophosphorus, mg/L as P .01 Do.

Total organic carbon, mg/L .1 USEPA method 415.1 (USEPA, 1979)

Dissolved organic carbon, mg/L .1 Do.

Turbidity, NTU .05 Turbidimeter (Fishman and Friedman, 1989)

Color, platinum-cobalt units 5 Visual comparison (Fishman and Friedman, 1989)

Specific conductance, μS/cm 1 Wheatstone Bridge (Fishman and Friedman, 1989)

Water pH, pH units .1 Glass electrode (Fishman and Friedman, 1989)

Total non-filterable residue at 105 °C 

(total suspended solids), mg/L

1 Gravimetric (Fishman and Friedman, 1989)

Dissolved residue at 180 °C (total 

dissolved solids), mg/L

1 Do.

Phytoplankton chlorophyll-a, μg/L .1 Standard Methods SM10200H (American Public 

Health Association, 1986)

Acid-neutralizing capacity, mg/L 1 Titration (Fishman and Friedman, 1989)
98 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

UV absorbance at 254 nm .001 Standard Methods SM5910B (American Public 

Health Association, 1986)

Sulfamethizole, μg/L antibiotic .05 Liquid chromatography/mass spectrometry 

(Kolpin and others, 2002)

Sulfathiazole, μg/L antibiotic .05 Do.

Sulfamerazine, μg/L antibiotic .05 Do.

Sulfamethazine, μg/L antibiotic .05 Do.

Sulfachlorpyridazine, μg/L antibiotic .05 Do.

Sulfamethoxazole, μg/L antibiotic .05 Do.

Sulfamethoxazole, μg/L antibiotic .064* Liquid chromatography/mass spectrometry 

(Cahill and others, 2004)

Sulfadimethoxine, μg/L antibiotic .05 Liquid chromatography/mass spectrometry 

(Kolpin and others, 2002)

Tylosin, μg/L antibiotic .02 Do.

Roxithromycin, μg/L antibiotic .02 Do.

Erthromycin-H2O, μg/L erthromycin metabolite .02 Do.

Tetracycline, μg/L antibiotic .05 Do.

Oxytetracycline, μg/L antibiotic .05 Do.

Chlortetracycline, μg/L antibiotic .05 Do.

Doxycycline, μg/L antibiotic .05 Do.

Norfloxaxin, μg/L antibiotic .02 Do.

Enrofloxaxin, μg/L antibiotic .02 Do.

Ciprofloxaxin, μg/L antibiotic .02 Do.

Sarafloxacin, μg/L antibiotic .02 Do.

Lincomycin, μg/L antibiotic .02 Do.

Carbadox, μg/L antibiotic .05 Do.

Trimethoprim, μg/L antibiotic .02 Do.

Virginiamycin, μg/L antibiotic .2 Do.

Methotrexate, μg/L antibiotic .05 Do.

Demeclocycline, μg/L antibiotic .05 Do.

Minocycline, μg/L antibiotic .05 Do.

Metformin antidiabetic not 

determined

Liquid chromatography/mass spectrometry 

(Cahill and others, 2004)

Cotinine, μg/L nicotine metabolite .014 Do.

Cotinine, μg/L nicotine metabolite 1.0 Gas chromatography/mass spectrometry 

(Kolpin and others, 2002)

Albuterol (Salbutamol), μg/L antiasthmatic .023 Liquid chromatography/mass spectrometry 

(Cahill and others, 2004)

Cimetidine, μg/L antacid .012* Do.

Acetaminophen, μg/L antipyretic .036 Do.

Ranitidine, μg/L antacid .013* Do.

1,7-dimethylxanthine, μg/L caffeine metabolite .144 Do.

Trimethoprim, μg/L antibiotic .013 Do.

Diltiazem, μg/L antihypertensive .016* Do.

Fluoxetine, μg/L antidepressant .014* Do.

Appendix B. Water-quality constituents analyzed in stream water collected during this study (Continued)

[mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemen per centimeter; °C, degrees Celcius; NTU, nephelometric turbidity units; UV, 

ultraviolet; nm, nanometers; N, nitrogen; P, phosphorus; USEPA, U.S. Environmental Protection Agency. Concentrations denoted with an asterisk are estimates]

Constituent Use of 

organic constituent

Method 

reporting limit Analytical method
Appendix B 99

Warfarin, μg/L anticoagulant .011 Liquid chromatography/mass spectrometry 

(Cahill and others, 2004)

Ibuprofen, μg/L anti-inflammatory .042* Do.

Gemfibrozil, μg/L antihyperlipidemic .013* Do.

Caffeine, μg/L stimulant .016 Do.

Caffeine, μg/L stimulant .5 Gas chromatography/mass spectrometry 

(Kolpin and others, 2002)

Dehydronifedipine, μg/L antianginal .015 Liquid chromatography/mass spectrometry 

(Cahill and others, 2004)

Codeine, μg/L analgesic .015 Do.

Thiabendazole, μg/L pesticide .011 Do.

Diphenhydramine, μg/L antihistamine .015* Do.

Azithromycin, μg/L antibiotic .005 Do.

Erythromycin, μg/L antibiotic .009 Do.

Carbamazapine, μg/L anticonvulsant .011 Do.

Miconazole, μg/L antifungal .018 Do.

Tetrachloroethylene, μg/L solvent, degreaser .5 Gas chromatography/mass spectrometry 

(Kolpin and others, 2002)

Bromoform, μg/L disinfection byproduct .5 Do.

Isopropyl benzene (cumene), μg/L manufacturing phenol/acetone, 

fuels, and paint thinner

.5 Do.

Phenol, μg/L disinfectant .5 Do.

1,4-dichlorobenzene, μg/L deodorizer .5 Do.

d-limonene, μg/L fungicide, antimicrobial, antiviral, 

fragrance in aerosols

.5 Do.

acetophenone, μg/L fragrance .5 Do.

para-cresol, μg/L wood preservative .5 Do.

Isophorone, μg/L solvent for lacquer, plastic, oil, 

silicon, resin

.5 Do.

Camphor, μg/L flavor, odorant, ointments .5 Do.

Isoborneol, μg/L fragrance in perfumery, 

disinfectants

.5 Do.

Menthol, μg/L cigarettes, cough drops, liniments, 

mouthwash

.5 Do.

Napthalene, μg/L polycyclic aromatic hydrocarbon .5 Do.

Methyl salicylate, μg/L liniment, food, beverage, 

UV-absorbing liquids

.5 Do.

Dichlorvos, μg/L insecticide 1.0 Do.

Isoquinoline, μg/L flavors and fragrances .5 Do.

2-methylnapthalene, μg/L 2-5 percent of gasoline, diesel fuel, 

or crude oil

.5 Do.

Indole, μg/L pesticide inert ingredient, fragrance 

in coffee

.5 Do.

3,4-dichlorophenyl isocyanate, μg/L diuron degradate .5 Do.

1-methylnapthalene, μg/L 2-5percent of gasoline, diesel fuel, 

or crude oil

.5 Do.

Appendix B. Water-quality constituents analyzed in stream water collected during this study (Continued)

[mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemen per centimeter; °C, degrees Celcius; NTU, nephelometric turbidity units; UV, 

ultraviolet; nm, nanometers; N, nitrogen; P, phosphorus; USEPA, U.S. Environmental Protection Agency. Concentrations denoted with an asterisk are estimates]

Constituent Use of 

organic constituent

Method 

reporting limit Analytical method
100 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002

3-methyl-1H-indole (skatol), μg/L fragrance, stench in feces and 

coal tar

1.0 Gas chromatography/mass spectrometry 

(Kolpin and others, 2002)

2,6-dimethylnapthalene, μg/L diesel, kerosene .5 Do.

3-tert-butyl-4-hydroxyanisole 

(BHA), μg/L

antioxidant, general preservative 5.0 Do.

N,N-diethyltoluamide (DEET), μg/L insect repellent .5 Do.

5-methyl-1H-benzotriazle, μg/L anticorrosive 2.0 Do.

Diethyl phthalate, μg/L plasticizer .5 Do.

4-tert-octylphenol, μg/L nonionic detergent metabolite 1.0 Do.

Benzophenone, μg/L fixative for perfumes and soaps .5 Do.

Tributyl phosphate, μg/L antifoaming agent, flame retardant .5 Do.

Triethyl citrate (ethyl citrate), μg/L cosmetics, pharmaceuticals .5 Do.

Total para-nonylphenol, μg/L nonionic detergent 5.0 Do.

Prometon, μg/L herbicide (noncrop only) .5 Do.

Pentachlorophenol, μg/L herbicide, fungicide, wood 

preservative, termite control

2.0 Do.

Atrazine, μg/L herbicide .5 Do.

tri(2-chloroethyl) phosphate, μg/L fire retardant .5 Do.

4-n-octylphenol, μg/L nonionic detergent metabolite 1.0 Do.

Diazinon, μg/L insecticide .5 Do.

Phenanthrene, μg/L polycyclic aromatic hydrocarbon .5 Do.

Anthracene, μg/L polycyclic aromatic hydrocarbon .5 Do.

Tonalide (AHTN), μg/L musk fragrance .5 Do.

Carbazole, μg/L insecticide, manufacture of dyes, 

explosives, and lubricant

.5 Do.

Galaxolide (HHCB), μg/L musk fragrance .5 Do.

Monoethoxyl octylphenol (OPEO1), 

μg/L

nonionic detergent metabolite 1.0 Do.

4-cumylphenol, μg/L nonionic detergent metabolite 1.0 Do.

Carbaryl, μg/L insecticide 1.0 Do.

Metalaxyl, μg/L herbicide, fungicide .5 Do.

Bromacil, μg/L herbicide .5 Do.

Metolachlor, μg/L herbicide .5 Do.

Chlorpyrifos, μg/L insecticide .5 Do.

Anthraquinone, μg/L manufacture dyes/textiles .5 Do.

Total nonylphenol ethoxylates 

(NPEO1), μg/L

nonionic detergent metabolite 5.0 Do.

Fluoranthene, μg/L component of coal tar and asphalt, 

combustion product

.5 Do.

Triclosan, μg/L antimicrobial disinfectant 1.0 Do.

Pyrene, μg/L polycyclic aromatic hydrocarbon .5 Do.

Diethoxyl octylphenol OPEO2, μg/L nonionic detergent metabolite 1.0 Do.

Bisphenol A, μg/L plasticizer 1.0 Do.

Total nonylphenol ethoxylates 

(NPEO2), μg/L

nonionic detergent metabolite 5.0 Do.

Appendix B. Water-quality constituents analyzed in stream water collected during this study (Continued)

[mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemen per centimeter; °C, degrees Celcius; NTU, nephelometric turbidity units; UV, 

ultraviolet; nm, nanometers; N, nitrogen; P, phosphorus; USEPA, U.S. Environmental Protection Agency. Concentrations denoted with an asterisk are estimates]

Constituent Use of 

organic constituent

Method 

reporting limit Analytical method
Appendix B 101

Tri(dichloroisopropyl) phosphate, μg/L fire retardant .5 Gas chromatography/mass spectrometry 

(Kolpin and others, 2002)

Triphenyl phosphate, μg/L plasticizer .5 Do.

2-butoxy-phosphate ethano,l μg/L plasticizer .5 Do.

Polybrominated diphenyl ether 

(PBDE4-1), μg/L

plastics 10 Do.

Polybrominated diphenyl ether 

(PBDE4-2), μg/L

plastics 10 Do.

Diethylhexyl phthalate, μg/L plasticizer .5 Do.

Polybrominated diphenyl ether 

(PBDE4-3), μg/L

plastics 10 Do.

Polybrominated diphenyl ether 

(PBDE5-1), μg/L

plastics 10 Do.

Polybrominated diphenyl ether 

(PBDE5-2), μg/L

plastics 10 Do.

Benzo(a) pyrene, μg/L polycyclic aromatic hydrocarbon .5 Do.

Polybrominated diphenyl ether 

(PBDE5-3), μg/L

plastics 10 Do.

Polybrominated diphenyl ether 

(PBDE6-1), μg/L

plastics 10 Do.

Polybrominated diphenyl ether 

(PBDE6-2), μg/L

plastics 10 Do.

3-beta-coprostanol, μg/L fecal steroid 2 Do.

Cholesterol, μg/L plant/animal steroid 2 Do.

beta-sitosterol, μg/L plant steroid 2 Do.

Stigmastanol, μg/L plant steroid 2 Do.

Appendix B. Water-quality constituents analyzed in stream water collected during this study (Continued)

[mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemen per centimeter; °C, degrees Celcius; NTU, nephelometric turbidity units; UV, 

ultraviolet; nm, nanometers; N, nitrogen; P, phosphorus; USEPA, U.S. Environmental Protection Agency. Concentrations denoted with an asterisk are estimates]

Constituent Use of 

organic constituent

Method 

reporting limit Analytical method
Appendix C. Streamflow duration statistics at selected sites on the St. Johns, Econlockhatchee, and Wekiva Rivers and 

Blackwater Creek

[NA, not available]

Map ID

(fig. 4)

Station 

number Station name

Drainage 

area 

(square 

miles)

Period of 

record 

(water years)

Flow, in cubic feet per second, which was equaled or exceeded for 

percentage of time indicated

99 95 90 50 10 5 1

1 02232400 St. Johns River near Cocoa 1,331 1985-2000

1957-2000

4.8

7.4

51

45

105

92

595

591

2,560

2,400

3,640

3,390

5,310

5,300

3 02232500 St. Johns River near Christmas 1,539 1985-2000

1957-2000

-8.3

7.2

76

56

148

115

833

802

3,270

3,200

4,090

4,130

5,140

6,240

6 02233500 Econlockhatchee River near 

Chuluota

241 1985-2000 35 44 52 152 782 1,170 2,010

8 02234000 St. Johns River upstream of 

Lake Harney, near Geneva

2,043 1985-2000

1957-2000

39

56

176

136

282

233

1,220

1,210

4,770

4,750

6,070

6,120

7,830

NA

12 02234500 St. Johns River near Sanford 1957-2000 -979 -156 236 1,530 5,720 7,900 NA

14 02235000 Wekiva River near Sanford 189 1985-2000

1968-2000

156

156

179

181

202

203

255

263

411

433

528

541

821

804

15 02235200 Black Water Creek near Cassia 126 1985-2000

1968-1969 and 

1985-2000

2.4

2.49

5.1

5.55

8.2

8.64

30

31.2

135

144

196

215

332

366

21 02236000 St. Johns River near Deland 3,066 1985-2000

1957-2000

-808

-1,035

294

162

760

688

2,250

2,460

6,140

6,370

7,450

7,560

9,590

10,750

102 Streamflow and Water-Quality Characteristics at Selected Sites of the St. Johns River in Central Florida, 1933 to 2002