SUMMARY OF MAJOR FINDINGS
The environment in southern Florida is being degraded
by human activities. Native biota have been reduced greatly in abundance
and diversity by drainage, development, alteration of water flows, degradation
of water quality, and by continuing invasions of exotic species.The Everglades
ecosystem, which is adapted to water that has an extremely low phosphorus
concentration, is being altered by agricultural activities that produce
high levels of phosphorus in water. Nutrient loading in the major rivers
is contributing to overenrichment of Lake Okeechobee and estuaries such
as Charlotte Harbor. Mercury has accumulated in Everglades game fish,
and consumption of the fish poses a potential human health risk. Mercury
has accumulated in the Everglades food web because natural conditions
and human influences enhance methylation of mercury to its organic form
and because high atmospheric mercury deposition rates (among the highest
in the Nation) sustain mercury methylation.
Federal and State agencies and environmental groups
agree that parts of southern Florida should be restored to predevelopment
conditions. Restoration will require massive changes in the water-management
system to restore pre-development drainage patterns, improve water quality,
and protect native biota.
|Urban, agricultural, Native American,
public lands and other important features in the Southern Florida
NAWQA Study Unit (McPherson and Halley, 1996).
Major findings on water quality and biology from this
- Concentrations of total phosphorus (TP) at the Southern
Florida (SOFL) National Water- Quality Assessment (NAWQA) Program sites
were above Everglades background levels and exceeded the U.S. Environmental
Protection Agency’s (USEPA) Everglades water-quality standard of 0.01
milligram per liter (mg/L). A major source of the high TP is fertilizer
- Concentrations of dissolved organic carbon (DOC)
in southern Florida water were relatively high compared with those in
other waters of the Nation. High DOC concentrations provide food for
bacteria to grow, reduce light penetration in the water, and enhance
transport and cycling of pesticides and trace elements such as mercury.
- Pesticides were detected in almost all SOFL samples.
Most concentrations were below aquatic-life criteria; however, the criteria
do not address potential effects of mixtures of pesticides and their
degradation products, which were common in the samples.
- Organochlorine pesticides, such as DDT and its degradation
products, are still prevalent in bottom sediment and fish tissue at
the SOFL sites, even though most uses of these compounds have been discontinued
in recent decades. The mobilization of these pesticides by the reflooding
of Everglades farm lands could lead to food-web contamination.
- Of 21 NAWQA basins nationwide, the Everglades has
the second highest ratio of methylmercury to mercury in sediment.This
enrichment in methyl-mercury enhances mercury uptake by the biota.
- The frequency of external anomalies (lesions, ulcers,
and tumors) on fish collected at two SOFL agricultural canal sites was
in the top 25 percent of 144 NAWQA sites sampled nationwide. Anomalies
can be indications that fish are stressed by contamination.
- Exotic animals and plants are a threat to native
biota. Ten of the 54 exotic fish species established in the region were
collected at the SOFL sites. Several herbicides used to control exotic
plants were detected in surface water.
Major Influences on Surface Water and Ecology
- Drainage modifications and wetland destruction.
- Runoff from agricultural and urban areas.
- High concentrations of DOC and its effects
on the transport of mercury and the attenuation of light.
- Deliberate or accidental release of exotic
In much of the SOFL region, ground water in the surficial
aquifers, such as the Biscayne aquifer, is of good quality and usually
meets Federal and State drinking-water quality standards. Contaminants
are usually in low concentrations, presumably because of rapid flushing
and recharge as a result of high annual rainfall (about 55 inches) and
shallow aquifers and porous limestone that allow the easy interchange
of surface and ground water. However, because of the shallow aquifers
and porous limestone, ground water is vulnerable to surface contamination
and to saltwater intrusion.
Major findings on ground-water quality from this study
include the following:
- Nitrate concentrations were below the drinking-water
standard (10 mg/L) in 108 SOFL wells (Biscayne and other surfical aquifers),
except for two shallow wells in the unnamed surficial aquifer of the
- Pesticides were detected in more than 85 percent
of the SOFL wells and beneath every type of land use studied, but no
concentrations exceeded any USEPA or State of Florida drinking-water
- Pesticides detected in shallow ground water were
associated with specific land uses. For example, the herbicides bromacil
and norflurazon were detected almost exclusively in citrus areas. Metolachlor
and simazine were common in mixed agricultural areas near the southern
- Volatile organic carbon compounds (VOCs) commonly
were detected in water from shallow and deep wells in the Biscayne aquifer.
Concentrations of one industrial VOC, vinyl chloride, exceeded the USEPA
maximum contaminant level (MCL) of 2 micrograms per liter (µg/L)
for drinking water in two samples.
- Radon-222 radioactivity exceeded the proposed MCL
(300 picocuries per liter ([pCi/L]) in the majority of samples from
the Biscayne aquifer, including untreated water from the public-supply
Major Influences on Ground Water
- Porous, shallow limestone aquifers overlain
by thin layers of sandy, permeable soils.
- Water-management practices involving canals,
pumps, gates, locks, and saltwater-control structures.
- Agricultural and urban land-use practices
and aquatic-weed control.
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U.S. Geological Survey Circular
McPherson, B.F., Miller, R.L., Haag, K.H., and Bradner, Anne, 2000, Water Quality in Southern Florida Florida,199698: U.S. Geological Survey Circular 1207, 32 p., on-line at https://pubs.water.usgs.gov/circ1207/