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

PDF version

Pesticides, PCBs, other organics, and trace elements have accumulated in bottom sediment and fish.

Fish sampling in Big Cypress Swamp as part of the southern Florida NAWQA study. [larger image]

Many pesticides, other trace organic compounds, and trace elements are hydrophobic; that is, in aquatic environments they tend to be associated with sediment particles and biological tissues rather than dissolved in water. For this reason, sampling bottom sediment and fish is an effective way to assess the occurrence of these contaminants in the aquatic environment.

The most frequently detected pesticides in bottom sediment at the SOFL sites during 1996-98 were DDT and its breakdown products DDE and DDD, ranging from 2.4 to 308 micrograms per kilogram (µg/kg). DDE exceeded the Canadian sediment quality guidelines probable effects level (PEL) of 6.75 µg/kg (Environment Canada, 1999) at three of the seven fixed sites, including the Hillsboro Canal at S-6 (308 µg/kg). More than 40 organic compounds were detected in bed sediment from the Hillsboro Canal at S-6, including ametryn, chlordane, DDT compounds, dieldrin, endosulfan, and other semivolatile organic compounds, including polycyclic aromatic hydrocarbons (PAHs).

The most frequently detected pesticides in fish at 15 SOFL sites also were DDT and its breakdown products. Largemouth bass and (or) Florida gar were collected at each site, and one or more samples (5-8 whole fish) were analyzed for pesticides. DDT compounds were detected in 25 of the 27 composited fish samples. Concentrations of total DDT ranged from less than 5 to 1,170 µg/kg in Florida gar and from less than 5 to 610 µg/kg in largemouth bass. The most commonly detected and abundant DDT product was p,p'-DDE. Total DDT concentrations exceeded the 200- µg/kg guidelines (Newell and others, 1987) for the protection of fish-eating wildlife in 4 of 27 fish samples. Highest concentrations of total DDT were in canals of the northern Everglades near agricultural lands. For comparison, during 1970-73 concentrations of total DDT in 49 composite fish samples from 12 sites in southern Florida ranged from 6 to 800 µg/kg. In 1978, total DDT concentrations in 23 fish samples ranged from 3 to 1,650 µg/kg (Haag and McPherson, 1997).

Other organochlorine compounds and PCBs detected in the composite whole-fish samples collected during 1995-96 include polychlorinated biphenyls (PCBs) and the pesticides dieldrin, mirex, and various compounds of chlordane (cis-chlordane, oxy-chlordane, trans-chlordane, trans-nonachlor, and cis-nonachlor). Maximum concentrations of pesticides occurred primarily in fish collected in Hillsboro Canal at S-6. PCBs were detected in three separate fish samples; the maximum concentration (140 µg/kg) was in fish collected from Black Creek Canal. Dieldrin and toxaphene were two other pesticides commonly detected in composite fish samples from the Everglades in the early 1970s. Concentrations of dieldrin in largemouth bass were as high as 130 µg/kg, and concentrations of toxaphene were as high as 5,000 µg/kg. In 1995, dieldrin concentrations ranged from less than 5 to 18 µg/kg, and 5 of 27 fish samples had detectable dieldrin. Toxaphene was not detected in fish collected during 1995-96, but the analytical method was not very sensitive for toxaphene (only levels greater than 200 µg/kg could be detected).

Row crops, C-111 basin, southern Florida. [larger image]

The types and amounts of pesticides used in Florida have changed over the years because of new technology, land use, and State and Federal regulations. One of the most frequently detected herbicides in bed sediment in southern Florida, ametryn, is used in relatively small amounts on sugarcane crops (6 tons per year [tons/y]) (Miles and Pfeuffer, 1997). By far, the greatest frequency of insecticide detection was the organochlorine insecticides, such as DDD, DDE, DDT, dieldrin, and heptachlor. DDT was banned for most uses in the Nation in 1973. These insecticides also are the most frequently detected pesticides in bottom sediments (Shahane, 1994). Although most organochlorine pesticides such as DDT and chlordane are no longer sold in the United States, they persist in the environment and continue to pose potential threats to wildlife and humans. Persistent organochlorine pesticides were detected beginning in the late 1960s and early 1970s (Kolipinski and Higer, 1969; McPherson, 1973) in bottom sediment and fish that are a part of the food chain in the Everglades. Reflooding of farm lands for Everglades restoration potentially could lead to mobilization of persistent organochlorine pesticides and foodweb contamination, as occurred in Lake Apoka just north of the study area. Many organochlorine pesticides and PCBs also have been linked to hormone disruption and reproductive problems in aquatic animals (Colborn and others, 1993).

Polynuclear aromatic hydrocarbons (PAHs) are contaminants in soils and sediments and originate from such sources as crude oil and tar and from forest fires and incomplete combustion of fossil fuels. Bottom-sediment samples were collected at 10 sites in a survey of the Barron River Canal in 1998 to evaluate the occurrence of PAHs and other semivolatile organic compounds in the vicinity of the Big Cypress National Preserve (Miller and McPherson, U.S. Geological Survey, in press). PAHs normalized to organic carbon had patterns of distribution that indicated sources to be roads, vehicles, or an old creosote wood-treatment facility. Concentrations of phthalate esters and the trace elements arsenic, cadmium, and zinc in the Barron River Canal appear to have a nonpoint source and to be influenced by local bed-sediment properties, such as sediment particle size and organic content. At some Barron River Canal sites, lead, copper, and zinc, normalized to aluminum, exceeded background levels and may be enriched by human activities. Trace elements in bottomsediment samples from the Barron River Canal sites did not exceed the Canadian PEL for freshwater sediment.

Mercury is a contaminant in the Southern Florida Study Unit.

Figure 11. Mercury concentrations in largemouth bass tissue equaled or exceeded 0.5 µg/g in an area in southern Florida. (Lambou and others, 1991). Median sulfate concentrations at SOFL sites, P-33 and P-34, in mg/L, 1996-98 (P-site data from South Florida Water Management District). [larger image]

Game fish in the Everglades have concentrations of mercury that exceed recommended levels (1.5 micrograms per gram, µg/g) for human consumption (Ware and others, 1990; fig. 11). The maximum concentrations of mercury found in edible portions of largemouth bass (4.4 µg/g) collected from the Everglades exceeded mercury concentrations in game fish from all other parts of the State (Stober and others, 1995). The NAWQA Program specifies that trace elements (including mercury) be determined for fish livers. Thus, the data are not directly comparable to data from studies analyzing fish fillets (the edible portion of the fish). Mercury concentrations in composite samples of largemouth bass livers from the SOFL sites ranged from 0.4 µg/g in Black Creek Canal to 42 µg/g in Miami Canal at S-8. Mercury concentrations in Florida gar livers ranged from 2.1 µg/g in the Caloosahatchee River at Alva to 190 µg/g in the Miami Canal at S-8. Largemouth bass and gar are top predators; on average, mercury concentrations in gar livers were about four times higher than mercury concentrations in largemouth bass livers.

The high concentrations of mercury in fish result from foodweb bioaccumulation of methylmercury, the most biologically available form of mercury in the environment (fig. 12). Buildup in the Everglades food web begins with high rates of bacterial mercury methylation at the sediment-water interface and subsequent transport of methylmercury from this interface into the water column. The Everglades region had one of the highest methylmercury to mercury ratios in sediment of the 21 NAWQA basins sampled in 1998 (Krabbenhoft and others, 1999a).

Mercury methylation rates in the Everglades are affected by complex physical-chemical-biological processes that vary widely from day to night, from season to season, and along spatial gradients. Two of the most important controls on mercury methylation are availability of sulfur and bacterial cycling of sulfur. Sulfur inputs to the Everglades have increased over background levels during the 20th century as a result of runoff containing agricultural fertilizers. Large inputs of sulfur in parts of the northern Everglades have stimulated sulfate reduction that would normally favor methylation, but very high levels of sulfide in the eutrophic areas have an inhibitory effect on methylation rates. Lower levels of sulfur contamination in the central Everglades (fig. 11) have increased sulfate reduction and mercury methylation without the inhibitory effects of excess sulfide on methylation rates (Benoit and others, 1999; Orem and others, 1999, p. 79). Concentrations of mercury are higher in fish and other organisms in the more remote and lownutrient waters of the central and southern Everglades than in the high-nutrient waters of the northern Everglades (Cleckner and others, 1998; Gilmour and others, 1998; Hurley and others, 1998).

Degradation of methylmercury also is a dynamic process that varies from day to night and spatially across the Everglades. Microbial demethylation rates in the sediments do not show strong spatial patterns, whereas photodemethylation rates in the water column do show a strong north to south upward trend, ranging from 2 to 15 percent per day, as a result of decreasing DOM in the water column from north to south and the resulting increase in light penetration in the south (Krabbenhoft and others, 1999b).

Atmospheric sources supply mercury that sustains methylation and food-web biomagnification in the Everglades. The southern Florida area has one of the highest atmospheric mercury deposition rates (25 micrograms per square meter per year, µg/m2/y) in the United States (Krabbenhoft and others, 1999a). Atmospheric inputs have local sources, such as medical, municipal, and industrial incinerators, landfills, power plants, and other urban activities, and global sources. Local sources are considered by most investigators to be primarily responsible for the relatively high atmospheric mercury inputs in southern Florida.