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

Nutrient enrichment is prevalent in surface water.

Water quality has been degraded in large parts of southern Florida by human activities that result in high nutrient concentrations and over-enrichment. Nutrient concentrations at the SOFL NAWQA sites are elevated when compared with Everglades background concentrations (see box below). The high nutrient concentrations, primarily from agricultural runoff, have contributed to overenrichment of surface water, including Lake Okeechobee, estuaries such as Charlotte Harbor, and the northern Everglades. The high phosphorus concentrations in agricultural runoff entering the northern Everglades are a significant cause of ecosystem degradation.

The USEPA recently (May 26, 1999) approved a new water-quality standard of 0.01 mg/L or less for phosphorus in the Miccosukee Federal Indian reservation lands of the Everglades. The State of Florida is reviewing additional scientific information and plans to adopt a numerical phosphorus standard for other parts of the Everglades.

	Nutrient Concentrations Vary Widely in Southern Florida and the Nation
Concentrations of nutrients in surface water at the SOFL sites vary widely compared with national NAWQA median concentrations. Generally, the high nutrient concentrations in southern Florida are at canal and river sites north of the Tamiami Trail and are associated with agricultural activities. Concentrations of phosphorus in surface water of the more pristine Everglades (south of the Tamiami Trail) are more than an order of magnitude lower than national background concentrations. The Everglades ecosystem is adapted to water that has extremely low phosphorus concentrations and is being adversely affected by agricultural runoff with high concentrations of phosphorus. Concentrations of phosphorus in southern Florida are highest in the Peace River (fig. 4) and are well above national median concentrations. The high concentrations in the Peace River are associated with natural sources (phosphatic deposits) as well as mining and agricultural activities in the basin. Concentrations of nutrients in shallow ground water at SOFL wells generally are lower than or comparable with the national background concentrations. Median concentrations of nitrate in the southern Florida wells are onefourth the national background concentration, and median concentrations of orthophosphate are comparable to the national background concentration. Concentrations of nutrients at the SOFL sites are plotted against corresponding national nutrient ranges for different land uses in Appendix A. Nutrient concentrations of the national NAWQA and the SOFL NAWQA study areas, national background, and Everglades background sites in mg/L [SW, surface water; GW, shallow ground water; <, less than; --, no data] Nutrient NAWQA Background National, 1993-98, median2 SOFL, range of medians3, 05/96-09/98 National1 (undeveloped areas) Everglades4 05/96-09/98 Total phosphorus (SW) 0.12 0.006-0.79 0.1 <0.004 Total nitrogen (SW) 1.3 0.7-2.6 1.0 1.1 Nitrite plus nitrate (SW) 0.71 0.006-0.59 0.6 0.005 Ammonia (SW) 0.046 0.017-0.25 0.1 <0.015 Ammonia (GW) 0.02 0.24-0.438 -- -- Nitrite plus nitrate (GW) 1.85 <0.05-0.44 2.0 -- Orthophosphate (GW) 0.11 0.013-0.028 0.02 -- 1U.S. Geological Survey, 1999. 2Flow-weighted concentrations. 3Medians for routinely sampled (BFS) sites (SW) and for land-use categories (GW). 4Everglades National Park, site P-34, medians; data from South Florida Water Management District (1992).

All routinely sampled SOFL sites (BFS sites, see Glossary) had phosphorus concentrations that exceeded the USEPA Everglades standard of 0.01 mg/L, but the two southern sites, Canal C-111 and Tamiami Canal at bridge 105, had median concentrations near the 0.01-mg/L Everglades standard (fig. 4). The ENP reference sites had median phosphorus concentrations below 0.01 mg/L and are characteristic of pristine Everglades water.

Figure 5. Total phosphorus concentrations increased as discharge and water levels declined at Tamiami Canal at Bridge 105, Big Cypress National Preserve. [larger image]

Seasonal changes in total phosphorus concentrations often are related to changes in water levels and flows. Generally, concentrations were below 0.02 milligram per liter (mg/L) at the Big Cypress Swamp reference site (Br 105) during 1996-98, but increased to more than 0.10 mg/L as water flows and levels declined during the dry season (fig. 5). The increase in phosphorus concentrations occurs when fish, wading birds, and other aquatic organisms congregate in ponded waters and their wastes contribute nutrients to the remaining inundated areas. The relatively greater ground-water contributions to surface water during the dry season also may increase nutrient concentrations. The background marsh sites in the ENP had very low concentrations of phosphorus (less than 0.01 mg/L) throughout the 1996-98 sampling period, and effects of seasonal low water levels on nutrients were not evident.

The dominant source of phosphorus loading in southern Florida is fertilizer (fig. 6). Manure and atmospheric sources are also important in some subbasins. Annual phosphorus loads estimated for selected canals and rivers were highest in the Peace River and lowest in the eastern Big Cypress Swamp. Much more phosphorus has been transported seaward from the northern Everglades and Okeechobee basins by the Caloosahatchee River, St. Lucie Canal, and major canals of Palm Beach County than is transported seaward in the southern Everglades (Haag and others, 1996).

Nutrient concentrations in ground water are highly variable.

Figure 6. Estimated phosphorus loading from point and nonpoint sources in surface-water basins in southern Florida (Haag and others, 1996). [larger image]

High nitrate concentrations (greater than 10 mg/L) in drinking water can cause a life-threatening illness in infants known as "bluebaby syndrome." Nitrate concentrations in ground water were low (commonly not detected) in most of the 108 wells sampled in the SOFL Study Unit, but a few occasionally were elevated in areas with agricultural land use. Wells located in or near the mixed-agricultural land-use area (including public-supply wells) tended to yield water with detectable concentrations of nitrate, but no nitrate concentrations exceeded the drinking-water standard of 10 mg/L. More than half of the wells located in citrus groves yielded water with no detectable nitrate concentrations, but two wells had nitrate concentrations above the drinking-water standard. With a few exceptions, shallow Biscayne aquifer wells in the urban land-use area contained water with relatively low concentrations of nitrate.

Ammonia concentrations in ground water in the SOFL Study Unit were relatively high compared with concentrations in other NAWQA Study Units across the Nation. The median ammonia concentration (0.396 mg/L) in the SOFL citrus land-use survey was the highest of 47 NAWQA agricultural surveys nationwide. Median concentrations of ammonia in the SOFL urban and study-unit survey (public water supply) ranked second and fifth highest, respectively, in these categories nationally.

Dissolved phosphorus concentrations in SOFL wells ranged from 0.001 to 0.79 mg/L; most land-use areas had median concentrations less than 0.01 mg/L (table 1). Concentrations above 0.05 mg/L occurred in some wells in all landuse and public-supply surveys, but highest concentrations generally were in the deeper public-supply wells of the Biscayne aquifer and in the citrus land-use area. The source of the relatively high concentrations of phosphorus in ground water may be fertilizers or naturally occurring phosphatic materials associated with silt and clays.