U.S. Department of the Interior
U.S. Geological Survey
Circular 1207

Water Quality in Southern Florida

Summary

Intro to S Florida NAWQA Study Unit

Major Findings

- Nutrients

- DOC and DOM

- Pesticides, VOCs, Trace Elements and Herbicides

- Bottom Sediments and Fish

- Biological Communities

Study Unit Design

Glossary

References

Appendix

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Dissolved organic carbon concentrations are commonly high.

Dissolved organic carbon (DOC) originates from natural sources, such as living and decaying plants, and from human sources. DOC represents about half the dissolved organic matter (DOM) in natural waters (Hem, 1985). In southern Florida, DOC concentrations commonly are high compared with other natural waters in the Nation and may constitute, as DOM, a significant fraction of the dissolved solids (table 2).

Concentrations of DOC ranged from 4.8 to 52.0 mg/L at the SOFL surface-water sites, and from 0.6 to 80 mg/L at the ground-water sites (table 2). The highest DOC concentrations (median of 34.0 mg/L) occurred at Hillsboro Canal at S-6 in the northern Everglades downgradient from the Everglades Agricultural Area, which has highly organic muck soils that are thought to be a source of DOC. In contrast, the Big Cypress Swamp reference site at Tamiami Trail Bridge 105 had the lowest concentrations (median of 9.5 mg/L) of the routinely sampled sites. This site is characterized by natural swamp vegetation and thin carbonate soils and rock. Concentrations of DOC at sites along the Tamiami Trail (1996-97) ranged from 4.8 to 26.9 mg/L (table 2), and tended to be low in the central and eastern Big Cypress Swamp and higher in the western Big Cypress and to the east in the Everglades (Miller and others, 1999).

The amount of DOC in water is significant. It can (1) contribute to water color, which absorbs sunlight and reduces the amount of light available for use by submerged aquatic plants and phytoplankton; (2) serve as a source of carbon for bacterial growth; (3) form complexes with trace elements, such as mercury, and make them more soluble and mobile in water (Reddy and others, 1999); (4) reduce bioavailability of nonionic organic compounds through sorption, entrapment, or sequestering the compounds (Nowell and others, 1999, p. 296); (5) increase the solubilities of relatively insoluble compounds, such as p,p'-DDT, PCBs, and lindane (Chiou and others, 1986); and (6) react with chemicals used to disinfect public water supplies and produce undesirable by-products, such as chloroform and other trihalomethanes, that may have harmful effects on human health.

Naturally occurring dissolved organic matter (DOM) causes the tea-colored water in many of Florida's rivers and swamps. Water color can have significant ecological effects by decreasing available light in the water column and reducing algal and aquatic plant productivity and growth. In the Peace River and upper Charlotte Harbor, for example, color can contribute up to about half of the total light attenuation in the water column, and this high attenuation limits algal phytoplankton and seagrass growth (McPherson and Miller, 1987). The range in color for southern Florida water, including the NAWQA sites, is shown in table 3.