SIR 2004-5177


Prepared in cooperation with the
St. Johns River Water Management District



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

Sharon E. Kroening


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Previous Studies

Purpose and Scope

Description of the Study Area


Hydrogeologic Setting

Hydrologic Setting

Land Use and Land Cover


Approach and Methods

Streamflow Data

Retrospective Water-Quality Data

Data Collected for this Study

Sample Collection and Analysis Methods

Quality Assurance

Continuous Monitoring Data

Data Analysis Methods


Streamflow Characteristics

Temporal Trends

Seasonal Variation

Streamflow Reversals

Flow Duration

Low-Flow Frequency Statistics

Water-Quality Characteristics

Water Temperature

Dissolved Oxygen

Water pH

Specific Conductance

Total Dissolved Solids

Major Ions


Minor Inorganic Constituents




Water Color and Organic Carbon


Total Nitrogen

Ammonia Nitrogen

Organic Plus Ammonia Nitrogen

Nitrate Nitrogen

Total Phosphorus

Orthophosphate Phosphorus



Total Suspended Solids

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

Temporal Trends (1991-99)



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

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

Appendix C. Streamflow duration statistics at selected sites on the St. Johns, Econlockhatchee, and Wekiva Rivers and Blackwater Creek


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 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 point-source 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.

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.

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