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Uranium Isotopic Method of Tracing Underground Water in the Floridian Aquifer System

AdelThe FGS environmental isotope laboratory is equipped with an alpha spectrometer (eight detectors), a radio-isotope fume hood and a Mega Pure 3A water distillation/deionization unit.

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An unexpected activity deficit of 234U relative to its parent 238U in Wakulla Springs water in the 1960's led to the development of the U isotopic method of water tracing by a team of researchers at Florida State University. They found that ground-waters can have a wide variation in U concentration and 234U/238U ratio depending upon rock/water interactions during infiltration of meteoric waters. However, once circulating in an oxic environment these parameters are quite conservative and can be used to "fingerprint" water masses. Applied to aquifer systems world-wide the method has proven very useful in evaluating water resources. But nowhere has it been used so intensively as in the Floridian Aquifer System. For example, the source waters of several of Florida's first magnitude springs have been identified, the degree of depletion of community and industrial water supplies estimated, and the degree of pollution by mining operations have been ascertained. Even the deepest parts of the Floridian Aquifer System have been studied, where thermal waters are found and where flow paths of sea water intrusion into the deep boulder zone of the aquifer have been mapped.

Recently there has been renewed interest in using U isotopic methods to identify the source waters of Wakulla Springs due to public concern about nitrate pollution. The 234U/238U activity ratio of the waters of Wakulla Springs is very low relative to average groundwater and surface waters and owes its existence to the recharge conditions peculiar to karstic regions. Another application includes assessment of water-rock reactions during aquifer storage and recovery (ASR) practices. The Florida Geological Survey is now in the process of setting up a uranium isotopic analysis laboratory under the direction of Dr. Adel Dabous, who has used the method extensively in studying the ground waters of his native Egypt.



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figure 1 Figure 1.
Feeder Conduits for Wakulla Springs. Samples retrieved by cave divers from individual conduits exhibit a range of 234U/238U activity ratios. A=0.97, B=0.90, C=0.83, Sally Ward=0.96. When activity ratios are combined with U concentration values, the relative feeder volumes can be calculated (After Macesich 1991)
figure 2 Figure 2.
Sources of water feeding Silver Springs. The U activity ratio and U concentration of area well waters (lower diagram, where S=1/C) can be grouped to reveal the relative contribution of regional ground water sectors (upper diagram) to the flow of Silver Springs. (After Osmond et al., 1974)
figure 3 Figure 3.
Inferred ground water flow and mixing beneath the Cody Scarp near Tallahassee based on U activity ratio and U concentration (After Osmond et al., 1968)
figure 4 Figure 4.
How U activity ratio and U concentration of well waters can be used to distinguish low and high transmissivity in Floridian Aquifer System waters in Central Florida (dotted arrows) and polluted and unpolluted waters in the Hillsborough and Polk County phosphate mining area (open arrows) (After Osmond et al., 1985)
figure 5 Figure 5.
Transmissivity zones of the Boulder Zone in the deep Floridian Aquifer System of South Florida. Cross-hatched arrows show where U activity ratios near 1.15 indicate inflow of sea water. (After Cowart et al., 1978)


References    back to top


  • Cowart, J.B., Kaufman, M.I., and Osmond, J.K., 1978, Uranium Isotopic Variations in Ground Waters of the Floridian Aquifer and Boulder Zone of South Florida: Journal of Hydrology, v. 36, p.161-172.


  • Kaufman, M.I., Rydell, H.S., and Osmond, J.K., 1969, U-234/U-238 Disequilibrium as an Aid to Hydrologic Study of the Floridian Aquifer: Journal of Hydrology, v. 9, p.374-386.


  • Macesich, M., 1991, Uranium isotopic disequilibrium study of Wakulla Springs [MS Thesis]: Florida State University, 146 p.


  • Osmond, J.K., Rydell, H.S., and Kaufman, M.I.., 1968, Uranium Disequilibrium in Groundwater: An Isotope Dilution Approach in Hydrologic Investigations: Science, v. 162, p.997-999.


  • Osmond, J.K., Kaufman, M.I., and Cowart, J.B., 1974, Mixing Volume Calculations, Sources, and Aging Trends of Floridian Aquifer Water by Uranium Isotopic Methods: Geochimica et Cosmochimica Acta, v. 38, p.1083-1100.


  • Osmond, J.K., and Cowart, J.B., 1976, Theory and Uses of Natural Uranium Isotopic Variations in Hydrology: Atomic Energy Review, v. 14, p.621-679.


  • Osmond, J.K., and Cowart, J.B., 1977, An investigation of the radioisotopes of the thermal waters of the Peace River area. The Geothermal Nature of Florida: Florida Bureau of Geology, Special Publication, v. 21, p.131-147.


  • Osmond, J.K., Cowart, J.B., and Humphreys, C.L., Wagner, B., 1985, Radioelement Migration in Natural and Mined Phosphate Terrains: Florida Institute of Phosphate Research Publication 05-002-027, Bartow, Florida. 131 pp


  • Osmond, J.K., Cowart, J.B., Dabous, A.A., Cao, H., and Miller, T., 2000, Uranium and Strontium isotope character of waters in the Wakulla Karst Plain: Florida Geology Survey Special Publications No. 46, W. Schmidt, J. Lloyd, and C. Collier, Eds, p. 45-60

Last updated: October 17, 2014

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