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 are present in most surface-water samples.

Pesticide application in vegetable farming area. [larger image]

Pesticides are widely used in southern Florida to control insects, fungi, weeds, and other undesirable organisms. These compounds vary in their toxicity, persistence, and transport. Some of the more persistent pesticides, such as DDT, chlordane, dieldrin, and aldrin, have been discontinued for use in Florida, but their residues persist in the environment. Although pesticides usually are applied to specific areas and directed at specific organisms, these compounds often become widely distributed and pose potential hazards to nontarget organisms.

Intensive pesticide sampling (up to weekly sampling) was conducted at three SOFL sites (IFS sites, see Glossary) that represented three different agricultural land uses — mixed vegetable crops (C-111 basin), sugarcane (S-6), and citrus (U.S. Sugar). Pesticides were detected in all but one sample from the three intensive-sampling sites during 1996-98. The most frequently detected pesticides included those with highest annual application rates, such as the herbicides atrazine, bromacil, simazine, 2-4-D, and diuron. Atrazine, the most frequently detected pesticide overall, was detected in about 90 percent of all samples. The other most frequently detected pesticides overall were metachlor, simazine, tebuthiuron, norflurazon, bromacil, and diuron.

Concentrations of pesticides in water were seasonal and related to land use. Concentrations of atrazine peaked at all three sites in late winter and spring (fig. 7). Concentrations were highest at the S-6 site, where some samples had atrazine concentrations that exceeded the Canadian aquatic-life criterion of 2 µg/L and the USEPA MCL of 3 µg/L. Concentrations at the other two sites were significantly lower, with maximum values less than 1 µg/L.

Figure 7. Concentrations of atrazine at the Intensive Fixed Sites, August 1996 - December 1998, showing similar seasonal occurrence patterns but different concentrations. [larger image]

A pesticide of particular concern, endosulfan, was detected mainly at the Canal C-111 site (fig. 8). Endosulfan was detected by the South Florida Water Management District (SFWMD) over a number of years in the C-111 basin at levels considered to be a threat to aquatic life in the basin and in nearby Florida Bay (Miles and Pfeuffer, 1997). During the intensive sampling period of the SOFL study (1996-98), endosulfan concentrations were 0.05 µg/L or less (fig. 8), which is just below the Florida Department of Environmental Protection criterion (0.056 mg/L) for Class III (recreation, propagation and maintenance of a healthy, well-balanced population of fish and wildlife) freshwater. Detections of endosulfan were frequent in 1996 but became less frequent in the following 2 years, as the use of this pesticide was discouraged and an alternate, imidacloprid, was introduced.

Mixtures of pesticides were common in samples from SOFL and other national NAWQA sites (see figure on page 13). The effects of pesticide mixtures on biota or humans are not included in criteria, which are based on the results of single-species, single-chemical toxicity tests conducted in the laboratory. As a result, analyses of individual pesticides may underestimate potential adverse effects of contaminants on biota (Nowell and others, 1999).

Pesticides were detected in ground water from more than 85 percent of the 108 SOFL wells and beneath every type of land use studied. No pesticide concentration exceeded USEPA or State of Florida drinking-water standards or health advisories.

Regional patterns of pesticides, VOCs, and trace elements are evident in ground water.

Residential land near Fort Lauderdale. [larger image]

VOCs commonly were detected in water from shallow, residential land-use wells and deeper publicsupply (study-unit survey) wells in the Biscayne aquifer (fig. 9). Vinyl chloride, trichloroethylene, tetrachloroethylene, cis-1,2-dichloroethene, and methyl tert-butyl ether (MTBE) were detected more commonly in the older residential and industrial areas and in public-supply wells near mixed agricultural lands. Toluene, p-isopropyltoluene, and 1,2,4-trimethyl-benzene commonly were detected in the newer residential areas and in public-supply wells near mixed agricultural lands. Two samples from publicsupply wells in more industrialized areas had vinyl chloride concentrations (4.68 and 3.18 µg/L) slightly above the USEPA MCL of 2 µg/L.

The NAWQA Program uses Federal drinking-water standards and guidelines to assess the quality of drinking water in potential surface- or ground-water sources prior to treatment and distribution. This complements many ongoing Federal, State, and local monitoring programs that assess drinking water after treatment and distribution.

Urban and agricultural activities are sources of trace-element contamination in ground water. Arsenic and copper are used as fungicides in citrus groves. Arsenic (in the herbicide monosodium methanearsonate) is used in turf-grass maintenance on golf courses, and concentrations of arsenic in shallow ground water are sometimes elevated (Swancar, 1996). Arsenic has been implicated as causing several cancers. Because of this health concern, the USEPA is considering lowering the MCL for arsenic from 50 to about 5 µg/L. Concentrations of arsenic in shallow ground water exceeded 5 µg/L in some of the urban and citrus land-use SOFL wells. Concentrations of copper in the SOFL ground-water samples reached 19 µg/L, which is well below the drinking-water MCL of 1,300 µg/L.

Uranium and radon-222, two naturally occurring radioactive elements that are potential carcinogens, exceeded drinking-water standards in some shallow ground water in the SOFL Study Unit. Uranium exceeded the MCL in 5 of 116 samples. Radon-222, a gaseous radionuclide that, when released to the air and inhaled is a significant cause of lung cancer, exceeded the proposed MCL of 300 picocuries per liter (piC/L) in more than 75 percent of the samples from the Biscayne aquifer.

Transport of Herbicides and their Breakdown Products

To evaluate the geochemical transport of herbicides, water and bed-sediment samples were collected in May 1997 and February 1998 from six SOFL sites (fig. 10) representing different land uses. The samples were analyzed after the methods of Thurman and others (1990) and Meyer and others (1993) for a suite of herbicides and breakdown products. Low levels (0.05 to 2.5 µg/L) of one or more herbicides were detected in water at all sites, including atrazine at every site. Other herbicides detected include ametryn, prometryn, and metolachlor at the sugarcane site, simazine and metolachlor at a mixed-agricultural (vegetable) site, and ametryn, simazine, and terbutryn at a citrus site. Atrazine (at trace levels) and ametryn (exceeding 40 micrograms per kilogram (µg/kg)) were detected in the sediment samples from the sugarcane site (S-6) in both years. The only other herbicides detected in sediments were trace levels of ametryn and alachlor at the mixed-agricultural (vegetable) site (Canal C-111). A breakdown product of alachlor, 2,6-diethylaniline, was detected in water at the same site. At the sugarcane site, S-6, the ratio of sediment-to-water concentration for ametryn was 240 (1997) and 580 (1998) and was zero for atrazine both years, which indicates that ametryn is transported primarily in sediment and atrazine is transported primarily in water.

The ratio of the concentration of deethylatrazine to the parent herbicide atrazine (DAR) has been used as an indicator of herbicide transport and surface- and ground-water interaction in the Midwestern States. Generally, a ratio greater than 1.0 indicates slow unsaturated zone transport and ground-water contributions to surface water (Adams and Thurman, 1991; Thurman and others, 1991; 1992). An elevated ratio also can be caused by photodecomposition of atrazine to deethylatrazine during atmospheric transport of herbicides (Goolsby and others, 1997). A ratio less than 0.1 indicates rapid overland-flow transport to surface water shortly after herbicide application. In southern Florida, DAR values were less than 0.1 at the sugarcane site (S-6), suggesting rapid transport of the herbicide into canal water shortly after herbicide application. The DAR at three other sites was between 0.1 and 1.0, which suggests post-application runoff. The DAR value of 1.0 at the background site (Br-105) presumably is from low-level atmospheric transport and photodecomposition because this site is remote from any farm runoff or ground-water sources of atrazine. The higher values above 1.0 at the citrus site (U.S. Sugar) and Canal C-111 at S-178 could indicate groundwater contributions (as in the Midwest) or more rapid photodegradation of atrazine in southern Florida because of higher temperatures, stronger sunlight, or greater soil organic carbon content and soil moisture than in the Midwest.