SIR 2004-5144
Prepared in cooperation with the
St. Johns River Water Management District,
Southwest Florida Water Management District,
Marion County, and
Florida Department of Environmental Protection
2004
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Abstract Introduction Purpose and Scope Description of Study Area Previous Investigations Site-Numbering System Methods Acknowledgments Hydrogeology Hydrogeologic Units and Hydraulic Properties Ground-Water Flow Silver Springs Land Use and Sources of Nitrogen Land Use, 1977 and 1995 Sources of Nitrogen Land Use in the 10-Year Contributing Area Chemistry of Ground and Spring Water Major Ions and Trace Constituents Dissolved Oxygen and Nutrients Comparison to Data from 1989-90 Relation of Nitrogen Concentrations to Land Use Nitrogen Isotopes Wastewater Indicators Dating Spring Water using Anthropogenic Tracers Summary and Conclusions Selected References Appendix A. Water-Quality Data, 2001-2002 Appendix B. Land-Use and Geologic Codes
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The Silver Springs group, in central Marion County, Florida, has a combined average discharge rate of 796 cubic feet per second and forms the headwaters of the Silver River. The springs support a diverse ecosystem and are an important cultural and economic resource. Concentrations of nitrite-plus-nitrate (nitrate-N) in water from the Main Spring increased from less than 0.5 milligrams per liter (mg/L) in the 1960s to about 1.0 mg/L in 2003. The Upper Floridan aquifer supplies the ground water to support spring discharge. This aquifer is at or near land surface in much of the ground-water basin; nutrients leached at land surface can easily percolate downward into the aquifer. Sources of nitrogen in ground water in the Silver Springs basin include atmospheric deposition, fertilizers used by agricultural and urban activities, and human and animal wastes.
During 2000-2001, 56 wells in the area contributing recharge to Silver Springs were sampled for major ions, nutrients, and some trace constituents. Selected wells also were sampled for a suite of organic constituents commonly found in domestic and industrial wastewater and for the ratio of nitrogen isotopes (15N/14N) to better understand the sources of nitrate. Wells were selected to be representative of both confined and unconfined conditions of the Upper Floridan aquifer, as well as a variety of land-use types. Data from this study were compared to data collected from 25 wells in 1989-90. Concentrations of nitrate-N in ground water during this study ranged from less than the detection limit of 0.02 to 12 mg/L, with a median of 1.2 mg/L. For data from 1989-90, the range was from less than 0.02 to 3.6 mg/L, with a median of 1.04 mg/L.
Water from wells in agricultural land-use areas had the highest median nitrate-N concentration (1.7 mg/L), although it is uncertain if the 12 mg/L maximum concentration was influenced by land-use activities or proximity to a septic tank. The median value for all urban land-use areas was 1.15 mg/L. Because fewer wells were in rangeland or forested areas, those categories were grouped together. The median concentration for that group was 0.09 mg/L.
The ratio of 15N/14N in ground-water samples ranged from -0.5 to 11.5 per mil. The median value for ground-water samples from 35 wells, 4.9 per mil, is near the top of the range that indicates inorganic nitrogen sources. In agricultural areas, the median 15N/14N was 4.8 per mil, indicating mostly inorganic (fertilizer) sources. In urban areas, the median 15N/14N was 5.4 per mil, indicating more influence of organic nitrogen (N) sources. Thus, in both agricultural and urban areas, fertilizer is an important inorganic source of N in ground water (and, therefore, in spring water as well). The influence of organic N is more apparent in urban areas than in agricultural areas. Two distinct 15N/14N values were observed in water from the Main Spring, one indicating an inorganic nitrogen source and the other indicating a mixture of sources with a strong influence of organic nitrogen.
Thirty-five wells and three springs of the Silver Springs group (the Main Spring, the Abyss, and the Blue Grotto) were sampled for a suite of 63 compounds common in wastewater. A total of 38 compounds was detected, nearly all in very low concentrations. The most frequently detected compound was the insecticide N,N-diethyl-meta-toluamide (DEET), which was detected in water from 27 wells and all three springs. The presence or absence of DEET in ground-water samples did not seem to be related to land use; however, hydrogeologic conditions at the well sites (confined or unconfined) generally did affect the presence or absence of DEET in the ground water. DEET also appears to be a useful tracer for the presence of reused water.
Water samples were collected from the Main Spring and two other springs of the Silver Springs group and analyzed for concentrations of dissolved gasses and for chlorofluorocarbons (CFCs), sulfur hexafluoride, and tritium/helium-3 for the purpose of dating the young fraction of ground water. Apparent ground-water ages are based on a piston-flow model, which may not adequately represent a complex flow system. Apparent age for water from the Main Spring is about 27 years for tritium/ helium-3 data, compared to about 15 years based on sulfur hexafluoride. For the Abyss, the tritium/helium age is about 9 years and the sulfur hexafluoride age is about 8 years. For the Blue Grotto, the tritium/helium-3 age is about 19 years, whereas the sulfur hexafluoride age is about 6 years. These results indicate that the flow system to the Abyss may be relatively shallow and rapid and adequately represented by the piston flow model, whereas the flow systems of the other springs are more complex, probably involving mixing with water from deeper zones of the aquifer. Both the nitrogen isotope and tritium/ helium-3 data from the Main Spring apparently indicate that the mixture of spring discharge from predominantly young to mostly older water can change rapidly and may not be easily related to discharge rate. CFC data could not be used for dating because all spring samples contained higher than equilibrium concentrations of CFCs from atmospheric sources.
All three springs are affected by the presence of wastewater in the contributing area, as evidenced by the presence of DEET and high CFC concentrations in all the samples. Although the conduit flow system contributing water to Silver Springs is complex and likely involves mixing of water from both shallow and deep flow zones, the fact that the water is relatively young (less than 30 years) indicates the importance of minimizing nitrogen loading within a contributing area that has ground-water travel times of 30 years or less.
Phelps, G.G., 2004, Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate: U.S. Geological Survey Scientific Investigations Report 2004-5144, 54 p.
U.S. Department of the Interior,
U.S. Geological Survey
Suite 1006
224 West Central Parkway
Altamonte Springs, FL 32714
tgphelps@usgs.gov
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