USGS Scientific Investigations Report 2008-5023

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Assessing Age Distribution and Contaminant Movement in Ground Water in the Contributing Recharge Area to a Public Supply Well in the Karstic Upper Floridan Aquifer

By Brian G. Katz1, Christy A. Crandall1, W. Scott McBride2, Patty A. Metz2, Marion P. Berndt1, and Sandra M. Eberts3

1U.S. Geological Survey, 2010 Levy Ave., Tallahassee, FL 32310
2 U.S. Geological Survey, 10500 University Center Dr., Ste.215, Tampa, Florida 33612
3 U.S. Geological Survey, 6480 Doubletree Ave., Columbus, Ohio 43229

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Multiple isotopic and other chemical tracers were used to assess age distribution and contaminant movement in ground water within a contributing recharge area to a public supply well (PSW) near Tampa, Florida, as part of the U.S. Geological Survey National Water Quality Assessment Program. During 2003-2006, water samples were collected from the PSW [open interval 39-53 m below land surface (bls)] and 28 monitoring wells in the karstic Upper Floridan aquifer (UFA) and the overlying surficial aquifer system (SAS).  Concentrations of the transient age tracers, SF6 and 3H/3He, in water from the oxic SAS indicated recent recharge (<7 years), but tracer concentrations in water from monitoring wells and the PSW in the anoxic UFA were consistent with binary mixtures that contained varying amounts of recent recharge and tracer-free water (>50 years).  Although ground-water age generally increased with depth in the UFA, monitoring wells that tapped a 43-49 m zone (bls) had higher fractions of young water than water from wells at shallower depths (23-31 m bls).   Mixtures of water in the PSW also were consistent with delta 34S of sulfate, 222Rn, nitrate-N, and VOC concentrations that fell between end-member values for waters from the SAS and various zones in the 87Sr/86Sr in the PSW water were slightly less radiogenic (0.70779) than water from other UFA monitoring wells (0.70790-0.70804) indicating that the PSW also withdraws some water from deep parts of the aquifer.  Geochemical mass-balance mixing models for the PSW indicate a 40-60% contribution of water from the SAS based on various isotopic and chemical indicators. Geophysical data from the PSW borehole indicated a highly transmissive zone at 43-49 m bls, which likely is related to a large solution feature that is hydraulically connected to the surface and SAS, thus yielding the younger age water mentioned above.

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