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Assessing Vulnerability of Karstic Floridan Aquifer for Public Supply Well Contamination

This study utilizes multiple tracers and geochemical modeling to assess the vulnerability of a public supply well in the karstic Floridan aquifer system to contamination. The study finds that the well is highly susceptible to contamination due to short-circuiting of water through a highly transmissive zone in the limestone. The mixing of water from the surficial and Floridan aquifers results in elevated concentrations of various contaminants.

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Assessing Vulnerability of Karstic Floridan Aquifer for Public Supply Well Contamination

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  1. Use of multiple tracers and geochemical modeling to assess vulnerability of a public supply well in the karstic Floridan aquifer Brian Katz1, Christy Crandall1, Scott McBride2, andMarian Berndt1 U.S. Geological Survey 1Tallahassee, FL 2Tampa, FL Fifth National Monitoring Conference May 8-11, 2006 San Jose, CA U.S. Department of the Interior U.S. Geological Survey

  2. Take Home Messages • Public supply well in the karstic Floridan aquifer system is highly vulnerable to contamination due to short circuiting of water from surficial material through a highly transmissive zone in the limestone. • Mixing of water in the PSW from the surficial and Floridan aquifers results in elevated concentrations of pesticides, VOCs, nitrate, radon, arsenic and uranium, compared to monitoring wells in the Floridan aquifer.

  3. Floridan Aquifer System- Highly Susceptible to Contamination Mapped karst features Small scale study area Upper Floridan aquifer Tampa, FL

  4. Highly Permeable Surficial material • Marine Terrace deposits • consisting of highly • permeable sands with • some clays (intermediate • confining unit) • Elevations range from 24 • to 80 ft above sea level • Mean rainfall = 55 in/yr • Hillsborough River • Mean annual flow (1939- 2000): 450 cfs • Interactions between river and surficial and Floridan aquifers Deeply incised river Channel

  5. Monitoring Network Area Vulnerability of ground water to contamination

  6. Regional ground- water flow 29 monitoring wells installed in 2003-04, based on contributing areas generated using probabilistic, and fracture models. Monitoring Network

  7. NO3 As, U, 222Rn pesticides Stable isotopes Age tracers Fe VOCs major ions DOC Redox TEAPs Geo-chem. models Flow models solid- phase chem. Geo- physical logs Putting the pieces together • Use of multiple chemical tracers and geochemical modeling to determine: • origin of contaminants in PSW • movement of contaminants to PSW • mixtures of water from different hydrogeologic units in PSW

  8. Assessing Ground-Water Age:SF6 and 3H atmospheric input

  9. SF6 and 3H/3He apparent Ages (yrs) much younger in surficial material than Floridan aquifer

  10. 110 Depth, ft 130 150 Caliper Spec. Cond. 170 Lower specific conductance and much larger open hole in 150-170 ft zone Geophysical logs of public supply well bore (TTP-4)

  11. 138 ft 148 ft 156 ft Optical Televiewer shots show large openings in limestone in Public supply well bore (TTP-4) 160 ft 164 ft

  12. Surface water Pesticides detected in public supply well • Atrazine and Deethylatrazine (CIAT) were detected four times in water from PSW (TTP-4) • Concentrations were highest in 62SRP POND and surficial well at this site.

  13. 6 VOCs detected in samples from public supply well

  14. Nitrate-N concentrations decrease with depth except for public supply well NO3-N concentrations higher in oxic surficial aquifer system than anoxic Floridan aquifer with exception of TTP-4 and other wells in 150-160 ft depth zone.

  15. Naturally occurring contaminants in public supply well reflect mixing of water

  16. Reactions: Mineral dissolution/precipitation Oxidation/reduction of C, S, N, Fe species Surficial aquifer water Public Supply Well water TTP-4 Floridan aquifer water Geochemical Mass-balance mixing models for public supply well (NETPATH) + Model constraints: C, S, Ca, Mg, Fe, redox Model phases: Calcite, dolomite, gypsum, pyrite, FeOOH, CH2O, CO2

  17. Surficial aquifer water 30-60% Public Supply Well water TTP-4 Floridan aquifer water 40-70% Public Supply well water contains30-60% water from surficial aquifer Sensitivity of models evaluated by comparing calculated isotopic compositions to observed compositions +

  18. Conclusions:Public Supply Well Vulnerability Issues Combined chemical, geophysical, and modeling data indicate mixing of water in PSW from surficial aquifer and Floridan aquifer due to short circuiting of water through a highly transmissive zone. • Elevated NO3-N conc. (FAS waters typically have much lower NO3 due to denitrification), • Numerous detections of VOCs, pesticides at low conc. • Elevated As, U, 222Rn conc. compared to water from monitoring wells in the FAS

  19. Acknowledgments: USGS National Water Quality Assessment Program Bruce Bernard Darlene Blum Sandy Eberts Patty Metz Terry Petrosky John Pittman Ann Tihansky City of Temple Terrace Transport of Anthropogenic And Natural Contaminants To Public Supply Wells (TANC) Contact info: bkatz@usgs.gov Brian G. Katz, PhD USGS 2010 Levy Ave. Tallahassee, FL 32310 (850) 942-9500 x3018

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