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Chronology and Isotope Geochemistry of Ground Waters in the Florida Keys and Offshore Areas

Project Proposal for 1998

Project Title: Chronology and Isotope Geochemistry of Ground Waters in the Florida Keys and Offshore Areas
Project Start Date: 1996
Project End Date: 1998?
Project Number: Task 6.6
Principle Investigators: J.K. Bohlke, L.N. Plummer, T.B. Coplen, E. Busenberg (USGS, WRD, Reston); E. Shinn (USGS, GD, St. Petersburg)
Project Chief: J.K. Bohlke; E.A. Shinn
Region/Division/Team/Section: ER/WRD/National Research Program; ER/GD/Coastal Geology
E-mail: jkbohlke@usgs.gov, eshinn@usgs.gov
Phone: 703-648-6325; 813-893-3100
Fax: 703-648-5274; 813-893-3366
Mailing Address:
J. K. Bohlke
U.S. Geological Survey
431 National Center, Reston, VA 22092

E. Shinn
U.S. Geological Survey
600 4th Street South, St. Petersburg, FL 33701

Program Element(s)/Task(s) 6 (Florida Bay, Florida Keys, and the Coral Reefs)
Task: 6.6
Collaborators, Clients: The study is designed as a collaboration between researchers in the Water Resources Division and the Geologic Division in USGS. The study complements (and is coordinated with) several other studies in the same area. Those include: Fragile Environment Tasks 6.4, 6.5, and 6.8, all of which deal with various aspects of hydrology, ground- water/surface- water interactions, and sediment diagenesis in Florida Bay and the Keys; EPA-funded radon tracer study; EPA-funded coral and hardbottom monitoring project; EPA-funded seagrass monitoring project; USGS aerial magnetic ground-water salinity mapping project; and other USGS and S. Florida Water Management projects being conducted in Florida Bay. Data from this study should provide useful constraints on the movement and quality of ground water beneath Florida Bay, the Keys, and offshore reefs; that information will be helpful in the design of efficient regulation policies by the Florida Dept. of Environmental Protection, the NOAA Florida Keys National Marine Sanctuary, and Everglades National Park. For example, the Florida Dept. of Environmental Protection is considering alternative sewage-disposal methods for the Florida Keys, and has already modified permitting procedures for shallow injection wells, based 'in part on results of previous studies of nutrients and micro-organisms in marine ground waters. The current project will provide further Information about the extent and rate of movement of potential contamination to aid decision-making about waste-water treatment and disposal in the region. In addition, the study includes testing some new combinations of methods and sample types that could expand the list of approaches available to the scientific community for future investigations of similar problems.


Project Summary: Ground waters are potential sources, sinks, and carriers of nutrients and other contaminants beneath the Florida Keys and offshore regions to the north and south. This project is designed to provide new data on the sources, flow directions, exchange rates, and chemical characteristics of ground waters underlying the region of Florida Bay, the Keys, and offshore reefs. The results, to be derived in part from analyses of environmental tracers and isotopes, will provide general empirical information about subsurface transport processes and their potential impact on surface water chemistry.

Project Justification: A significant issue of concern in South Florida is the potential effect of anthropogenic pollutants from the Florida Keys or elsewhere on the water quality and health of offshore marine ecosystems. It has been suggested that certain contaminants (e.g., bacteria, excess nutrients) found in some offshore ground waters may be transported 'in the subsurface to discharge sites beneath Florida Bay or the reef tract, where they may be contributing to declining ecosystem health. But not much is known about the origins of the ground waters underlying the region, how the subsurface flow systems operate, and what is the fate of contaminants emplaced in ground water in the Keys.

Project Objectives: We propose to test the use of environmental isotopes and tracers, combined with geochemical modeling, for determining the origins and ages of the various ground waters in the region of Florida Bay and the Keys, the extent and velocity of contaminant plumes, the fate of anthropogenic contaminants, and water-aquifer reactions that alter the source characteristics. The empirical approaches being tested include: (1) analyses of CFC's, carbon-14, and tritium and helium isotopes for information about the residence times of water and dissolved species in the subsurface; (2) analyses of stable isotopes of carbon, sulfur, hydrogen, oxygen, and nitrogen for information about sources of, and chemical reactions among, waters, nutrients, and other species; and (3) modeling of chemical reactions and residence time distributions. This work will complement other ongoing or proposed studies in the Keys involving deliberate tracer injection, hydraulic monitoring and modeling, and analyses of additional specific contaminants.

Overall Strategy, Study Design, and Planned Major Products: The major elements of the data gathering part of the study are (1) well installation and selection, to provide regional coverage of various ground-water environments along with depth resolution at selected sites (many of these were done before the study, additional ones have been or are being installed on the basis of the first round of sampling); (2) two intensive field sampling campaigns in which all of the constituents of interest are collected from a subset of the sites (one of these sampling episodes occurred in 1996; the other is planned for summer, 1997 after completion of new wells); (3) laboratory analyses of sample sets (some of these are done in-house; others are contracted out; overall time required can be as much as a year or more). Modeling and interpretation of results take place in preliminary fashion as data become available, but comprehensive and reliable treatments rely upon multicomponent evaluations involving the whole data set. Nevertheless, it is anticipated that several selected components of the study will be written up as separate reports for journal publication, along with a single comprehensive data report and accessory project summaries.


Overall: Samples for analysis are to be collected during 2 intensive field campaigns from observation wells, seepage collectors, and surface waters, to be accessed largely by boat. One of those collections occurred in 1996; another is planned for summer 1997 to complete the regional survey. The following types of analyses are to be done:
1 - Major-element water chemistry and dissolved gases. These data are essential for comparing water types and for calculating potential reaction relations among different waters (e.g., seawater reduction, mixing, etc.).
2 - Nitrogen isotopes in ammonium, nitrate, and N2 gas. Concentrations and isotope mass balances will be examined for evidence relating high ammonium in offshore ground waters to potential sources such as waste water and natural sedimentary organic matter.
3 - Chlorofluorocarbons (CFC's). Contamination levels of CFC's may indicate anthropogenic point sources; ambient levels may be useful in dating the recharge (entry into the subsurface) of the ground waters; no CFC's could indicate ages more than 50 years, or possibly degradation of CFC's.
4 - Tritium and He isotopes. Tritium-helium dating may indicate the residence times of ground waters since recharge from the ocean or the bay or the Keys; these data may be necessary as backup for the CFC's because of the possibility of CFC degradation in some of the reducing waters, or they may be used in conjunction with the CFC's to indicate ground-water mixing or dispersion phenomena.
5 - C-14 and stable carbon isotopes in carbonate, methane, and dissolved organic carbon. Isotopes and chemistry of around waters will be used for calculating reactions that might represent conversion of seawater into reduced marine ground water; information about the sources of organic constituents may be derived; very old waters may be detected.
6- Sulfur isotopes in sulfide and sulfate, plus mass balance of sulfate reduction. Sulfur derived from modern seawater (or Florida Bay) may be distinguishable from sulfur coming out with deep old ground waters; these data are also needed for reaction modeling.
7- Stable oxygen and hydrogen isotopes of water. Water isotopes are potentially useful for distinguishing water units such as evaporated, meteoric, marine, glacial/intergiacial, etc. A map attached to this summary shows the locations of wells installed in several transects across Florida Bay and the Keys to the offshore reef, plus a number of relatively new wells at various locations in Florida Bay. Selected wells in that network will provide a regional survey of shallow ground water in the earlier stages of the study. More detailed sampling will focus on locations in the Keys where contaminants are known to occur in the subsurface, and in other critical or anomalous areas revealed by the regional survey. WRD researchers will coordinate water sampling and analytical work; GD researchers will provide access to wells and background data, handle field logistics, etc. Some data will be collected in the field and some will be produced by a number of different specialty laboratories. All chemical and isotopic data will be recorded with respect to the location, date, and time of collection. The principal investigators will share responsibility for coordinating the analytical work, and for interpreting the results. The senior investigator will assume responsibility for overall coordination and data compilation.

First of 2 major sampling campaigns completed 2/96 (Bohlke, Plummer, Busenberg, Coplen, and Shinn); analyses of those samples began 2/96 and continued to the end of the year (Bohlke, Plummer, Busenberg, Coplen, WRD-NWQL, FIU, LDEO, and others); no deliverables.
Analyses of 1996 samples continued; second of 2 major sampling campaigns planned for 9/97; extended abstract and poster presentation given on preliminary results for Florida Bay Science Conference, Key Largo 12/96 (see ACCOMPLISHMENTS); updated abstract submitted for South Florida Program meeting, Fort Lauderdale 8/97 (same group).
Analyses of 1997 samples will continue; a comprehensive data report will be assembled near the end of FY98 after all data are available; reports will be written during the year on origins and residence times of ground waters, origins and geochemical reactions of nutrients and other mobile species.

Planned Deliverables/Products: One or more data reports will be produced at the conclusion of the study. Interpretive reports on the results of the study will be prepared as appropriate for publication in peer-reviewed journals.

Planned Outreach Activities: Some interpretive coordination and sample sharing, is anticipated with USGS groups studying diagenesis in sediment pore waters from Florida Bay (Task 6.8). Co-sampling and data sharing have been done with researchers at the University of Florida studying ground-water emissions of methane and radon. Clients will be kept informed of study results by participation in Technical Advisory Council meetings, scheduled USGS briefings, and various publications.

Prior Accomplishments in Proposed Area of Work: N/A

New Directions, Expansion of Continuing Project (if applicable): In comparison with the FY96 sample distribution, sampling scheduled for late FY97 is expected to expand the regional coverage of the study significantly within Florida Bay, and to provide additional vertical resolution by way of multi-depth well clusters, seepage meters, and coordination with near-surface sediment pore-water work by other investigators. Effort in FY98 is expected to be mainly aimed at completing chemical and isotopic analyses of those samples, and preparing reports for publication.


Accomplishments and Outcomes, Including Outreach: FY97 has seen continued laboratory analysis of samples collected in FY96, and will include another major sampling trip and associated analyses to complete the proposed regional ground-water survey. Selected new results for 1996 samples include:
(1) Tritium and helium isotope data confirm that the residence times of many of the ground waters in the surface were in the order of decades. These results are qualitatively consistent with the earlier CFC-12 results, but require more quantitative treatment of both sets of data to determine likely flow and mixing scenarios.
(2) Nitrogen isotope ratios (N-15/N-14) in ammonium samples taken near a waste-water injection site were relatively high compared to all other samples. The high values at the injection site are consistent with waste-water sources; whereas the consistently lower values elsewhere could indicate different sources of ammonium such as reactions in relatively uncontaminated sediments (additional isotopic work on sediments and shallow sediment pore waters is planned to constrain this further).
(3) Carbon-14 abundances in dissolved inorganic carbon (mainly bicarbonate) range from less than 10% to about 110% "modern", consistent with a large range of apparent radiocarbon ages. However, much of the variation in apparent ages can be accounted for by chemical reactions between seawater (close to 110 % "modern") and carbonate sediments (close to 0% 'modern").
(4) Reaction modeling with the full set of chemical and isotopic data has just begun. More data from sediment cores will be needed for this-, some of those data will come from other tasks in the program, whereas others will require further isotopic analyses of archived sediment samples.

Deliverables, Products Completed: Shinn, E.A., Reich, C.D., Hickey, D.T., Bohlke, J.K., Plummer, L.N., Coplen, T.B., Busenberg, E., Chariton, J., Burnett, W., Dillon, K., and Corbett, R., 1996, Assessing the origin and fate of ground water in the Florida Keys. 1996 Florida Bay Science Conference, University of Florida IFAS, Program and Abstracts, p. 74-76. [combined summary of 3 related projects]
Bohlke, J.K., Plummer, L.N., Coplen, T.B., and Busenberg, E., 1996, Chronology and isotope geochemistry of ground waters in the Florida Keys and offshore areas: progress report. 1996 Florida Bay Science Conference, University of Florida IFAS, Program and Abstracts, p. 13. [poster presentation]
Bohlke, J.K., Plummer, L.N., Coplen, T.B., and Busenberg, E., Schlosser, P., and Shinn, E.A., 1997, Sources and residence times of marine ground waters beneath the Florida Keys and nearby offshore areas. [abstract submitted for the Program meeting, Fort Lauderdale, FL, August, 1997]


Required Expertise(all for FY96-98): Geologists (well installation, field support, background information); Chemists (chemical and isotopic analyses); Chemical hydrologists (geochemical interpretations, residence time interpretations); Contractors (analytical services for selected constituents); Support staff (analytical services in-house)

Names of Key Project Staff (all for FY96-98; all listed as co-PI's in the initial proposal): J.K. Bohlke, Research Hydrologist-14, Reston; L.N. Plummer, Senior Scientist-16, Reston; T.B. Coplen, Research Chemist-15, Reston; E. Busenberg, Research Hydrologist-14, Reston; E. Shinn, Geologist, St. Petersburg

Major Equipment/Facility Needs: Sampling requires use of a boat large enough to provide a fairly stable platform for 3-4 people plus equipment for approximately 1-2 hours at each site (both in the Bay and in offshore areas); sampling equipment, including pumps and containers will be provided as needed by the sampling team (mostly from WRD, Reston); field laboratory facilities are needed for field measurements, for temporary cold (refrigerated) storage of some samples, for packing and shipping (and for receiving shipments); minor equipment and consumables are needed for analytical work.

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Last updated: 24 April, 2014 @ 01:01 PM (KP)