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Tracing the Mixing of Groundwater into Coastal Waters Utilizing a New Radiochemical Technique

Project Proposal for 1999

USGS Geologic Division
New Project Proposal- FY 1999

Project Title: Tracing the Mixing of Groundwater into Coastal Waters Utilizing a New Radiochemical Technique: Radium Isotope Systematics to look at the Geologic Control of Aquifers.

Geographical Area: South Florida, Gulf of Mexico
Project Start Date: October 1, 1998
Project End Date: September 31, 2000

Project Chief: Eugene Shinn with Peter Swarzenski
Region/DivisioniTeam/Section: Eastern/Geologic/Coastal & Marine/St. Petersburg, FL
Phone: 813-893-3100 x3030
Fax: 813-803-2032
Mail Address: US Geological Survey, 600 4th Street South, St. Petersburg, FL 33701

Program Element(s)/Task(s): Element 2; Task 2.1 Freshwater discharge to Florida Bay

Project Summary:
We propose to develop 223,224,228,226Ra isotope systematics to address the issue of groundwater flow into Florida Bay. Such methods are critical in accurately evaluating the role of submarine discharge and have direct implications for assessing coastal eutrophication, contamination and overall ecosystem change. Techniques described herein have been successfully utilized to quantify the contribution of groundwater in coastal mixing on time scales of a few days to years. Briefly, radium has a very different geochemical signature in freshwater versus seawater. This attribute, in addition to known source functions, a wide range of half-lives (3.8 days to 1600 years), and elevated groundwater activities make radium ideal to examine subsurface water/sediment transitions. By measuring this suite of radium isotopes in local groundwater wells (which are already in place) as well as in surrounding fresh water, seawater and underlying limestone, we will determine the coastal groundwater discharge rate and magnitude. The application of radium isotopes in this system will provide vital information for establishing a comprehensive water and contaminant budget for Florida Bay.

Project Objectives and Strategy:
There has been a long-term effort by both federal and state agencies to map and assess coastal aquifer systems. Such research is socio-economically highly relevant as heightened agricultural and municipal demand on freshwater can have severe effects on coastal aquifers. For example, saltwater intrusion into the Biscayne aquifer system has been an ongoing, precarious issue for south Florida municipalities, requiring complex research and management. Groundwater can be elevated in nutrients, and the submarine discharge of groundwater into coastal waters can contribute to coastal eutrophication. Regional geologic features of course have a strong control on aquifer characteristics, yet this relationship must be better quantified. This proposal will tie directly into Dr. Shinnís ongoing coastal aquifer work by employing an innovative new radiotracer technique that will uniquely quantify the magnitude and rate of groundwater discharge into coastal Florida Bay.

The distinct chemical characteristics of the radium isotope quartet (e.g., in freshwater Ra is very strongly particle-bound while in seawater it is dissolved) have recently enabled scientists to accurately quantify the coastal discharge of groundwater. Funding agencies such as NSF have realized the need to support research programs that attempt to define the extent and quantity of groundwater discharges into coastal bottom waters. For example, it has been recently reported that near-shore submarine groundwater fluxes along the eastern seaboard (North Atlantic Bight) can contribute significantly to the combined surficial, riverine discharge from that shoreline (Moore, 1996). Such results may have severe implications, not only for establishing accurate estuarine or oceanic mass balance budgets, but these results are also vital for establishing coastal groundwater budgets and associated nutrient flux estimates. This proposed research would thus address the following objectives:

  • Develop the radiochemical capabilities in St Petersburg to utilize the Ra quartet in ground water studies.  Strategy: We will work closely with Dr W.S. Moore USC) to setup, calibrate and utilize coincidence counters. Bringing this capacity to St. Petersburg would significantly our existing radiochemical expertise and expand our versatility to include short duration (days to weeks) coastal processes. The capability would have relevance in all coastal waters where the hydrologic regime is conducive to subterranean marine groundwater discharge. The best method for quantifying short-lived 223,224Ra is a delayed coincidence counting system, as developed specifically by Moore. Swarzenski has worked closely with Moore and will develop this capability at USGS CFCG under his guidance. Such a counting system is relatively inexpensive (under 10K for two), portable (can be brought in the field) and can quantify additional short-lived isotopes such as 227Ac, whose environmental fate is still largely untested but promising for ephemeral sediment/water interface processes (e.g., resuspension).
  • Characterize the temporal and spatial groundwater discharge in the vicinity of the freshwater/saltwater interface of south Florida.  Strategy: We have just now (mid-May, 1998) secured supplemental support (at present still verbal; contact is Dr. Chris Madden, SFWMD) from the South Florida Water Management District to precisely address issues of groundwater flow into Florida Bay. Thus this proposed research plan would foster a collaborative program between the USGS and a water management district. Our sampling efforts would directly overlap with ongoing SFWMD field efforts. We will quantify the Ra quartet in groundwater wells, freshwater, seawater, Holocene and Pleistocene limestone deposits.
  • Assess observed groundwater flow characteristics in south Florida within a regional geologic framework.  Strategy: Take advantage of extensive knowledge on the hydrology of south Florida to develop a correlative relationship between the regional geology and groundwater discharge characteristics. Establishing such a relationship will broaden the predictive applicability of these Ra isotopes to other coastal aquifer systems.
Potential Impacts and Major Products:
It is well known that marine groundwater discharge into certain coastal waters can be very large and is susceptible to heightened freshwater demand. A major potential impact from this study would be to accurately quantify the rate and magnitude of groundwater discharge in south Florida. Such data are conspicuously lacking at present, and would provide a crucial key to developing water budgets and associated nutrient mass balance calculations. This has a direct application to ongoing studies that look, for example, at the flux of sub-surface nutrients that enter Florida Bay. Major products from the proposed study would be:
To develop the capabilities to measure natural activities of short and long lived Ra isotopes in St. Petersburg;
1) To use these isotopes as a means for quantifying groundwater discharge in south Florida. This will provide crucial information for many ongoing projects in Florida Bay (e.g., groundwater discharge - change in nutrient/metal/salinity budgets - change in community structure);
2) To examine groundwater discharge data in context of the regional geology. This will provide a predictive capability that will eventually be applicable to other coastal aquifer systems. This is a theme that is receiving much current attention all along the eastern seaboard; e.g., Chesapeake Bay-DelMarVa Peninsula (F. Manheim).
3) Data reduction and dissemination through presentations at science meetings and peer-reviewed journal articles.

Collaborators, Clients: US Environmental Protection Agency (EPA); South Florida Water Management District (SFWMD); Everglades National Park (ENP); Inter-Agency Florida Bay Science Program.

In fresh water, radium is strongly particle-reactive and tightly attached to the suspended load. In contrast, radium exists primarily in the dissolved phase in seawater. This simple difference in chemical behavior is due to a change in the adsorption coefficient of Ra between fresh and saltwater as well as to a change in the average suspended particle concentration between terrigenous and marine waters (Webster et al., 1993; Bollinger and Mopre, 1994). Desorption of Ra in mid-salinity values has been widely noted in many estuaries and is due to the release surface bound Ra as riverine particles enter high ionic strength estuarine waters. This had been verified in the laboratory with sorption/desorption experiments (Nozaki et al., 1989).

Thorium isotopes are also highly particle reactive and are a continuous source of 223,224Ra that is regenerated on a time scale of days. Frequent mixing and resuspension of the near-shore surficial sediments thus provide an efficient source of the short-lived Ra isotopes without generating the longer-lived 228,226Ra isotopes. 223,224Ra may therefore be used to identify groundwater from coastal water; and, when the initial endmember activity and the 223Ra/224Ra activity ratio have been established (this had been also verified in the field; Swarzenski et al., 1998), then radium activity ratios may be used to provide information on the time elapsed since the surface water was last in contact with bottom sediments.

The following model has been developed to examine the groundwater discharge into upper Florida Bay (Swarzenski et al., 1998):

(image currently unavailable)


Marine geochemists have been able to clearly demonstrate that longer-lived 228/226Ra-activity ratios are an ideal tracer for plume migration studies (Moore et al., 1986; Nozaki et al., 1991). Unlike other tracers of surface water, such an activity ratio can not be modified either by evaporation, precipitation or biological activity, but only by radioactive decay. Because of the relatively long half-life of 228Ra (t = 5.75 yr.) and 226Ra (t = 1600 yr.), these isotopes are not useful for determining mixing rates/processes on time scales of a few days to weeks. The shorter-lived radium isotopes (223,224Ra) may be useful in such studies and applied with similar success as the longer-lived 228/226Ra activity ratios.
To use the activity of excess 224Ra in a water sample as a geochronometer for water movement, one would write a mass balance equation as follows:

(image currently unavailable)

where 224 is the decay constant for 224Ra, 0.191 days-1, 224Rai is the initial amount of 224Ra in the water sample and fEM is the fraction of the endmember (well sample) remaining in the sample. Age determinations calculated in such a manner reflect the time elapsed since the water sample became enriched in Ra by the discharge of groundwater. fEM can be estimated either from salinity or from the distribution of 228,226Ra isotopes. There are four basic assumptions that must be upheld to correctly apply this model:
we can define a single value for the 224Ra activity and salinity over the time of interest;
1) the endmembers can not change over the time period of interest;
2) there can be no inputs/sinks for Ra except for mixing and radioactive decay;
3) the open ocean must contain negligible dissolved excess 224Ra (see Fig. 1 B).

(image currently unavailable)

Using 223Ra and 224Ra in this manner is based on the assumption that the initial 223/224Ra activity ratio must remain constant. This conclusion is reasonable as the long-lived parent isotopes (231Paa and 228Th) have relatively constant activity ratios in estuarine sediments, and the intermediate Th isotopes (227Th and 228Th) are scavenged efficiently in the near-shore water column (this has been already been verified in the field; Swarzenski et al., 1998).
    There are many existing shallow wells situated close to the freshwater - saltwater interface in upper Florida Bay/Everglades (coordinated by E. Shinn, USGS CFCG). Samples from these wells, from surface waters within Florida Bay and the underlying Pleistocene limestone, will be analyzed for short-lived Ra isotopes and occasionally 222Rn In addition to well samples, we will also collect porewater and sediment samples for radiochemical and nutrient analyses. Because seasonal variations can play a major role in regulating coastal groundwater discharge (e.g, changes in evapotranspiration) in Florida Bay, sampling will target seasonal precipitation fluctuations.

Radiochemical Measurements
    Radium (~ 10 L) is quantitatively removed onto Mn-fiber cartridges, which are partially air-dried and placed into an air circulation system containing a scintillation cell/photomultiplier tube (Swarzenski et al., 1998). Such delayed coincidence (scintillation) counters will be used after methods developed by Dr. W.S. Moore (e.g., Moore and Arnold, 1996; Rama and Moore, 1996) to determine the short-lived radium isotopes (via the decay of 224,219Rn and 216,215Po). 222Rn will be measured using a portable radon detector.

Bollinger, M.S. and Moore, W.S. (1993) Evaluation of salt marsh hydrology using radium as a tracer. Geochim. Cosmochim. Acta 57: 2203-2212.

Broecker, W.S. and Peng, T.H. (1981) Tracers in the Sea. Eldigo Press, pp.690.

Fish, J.E. and Stewart, M. (1991) Hydrogeology of the surficial aquifer system, Dade County, Florida. US Geological Survey Water Resources Investigations Report 90-4108, pp. 50.

Moore, W.S. (1996) Large groundwater inputs into coastal waters as revealed by 226Ra enrichment. Nature 380:6 12.

Moore, W.S. and Arnold, R. (1996) Measurement of 223Ra and 224Ra in coastal waters using a delayed coincidence counter. J. Geophys. Res. 101: 1321-1329.

Moore, W.S., Sarmiento, J.L. and Key, R.M. (1986) Tracing the Amazon component of surface Atlantic water using 228Ra, salinity and silica. J. Geophys. Res. 91: 2574-2580.

Nozaki, Y., Kasemsupaya, V. and Tsubota. H. (1989) Mean residence time of the shelf water in the East China and Yellow Seas Determined by 2281226Ra measurements. Geophys. Res. Lett., 16: 1297-1300.

Parker, G.G. et al. (1955) Water resources of southeastern Florida, with special reference to the geology and ground water of the Miami area. US Geological Survey Water Supply Paper 1255, pp. 965.

Rama and Moore, W.S. (1996) Using the radium quartet for evaluating groundwater input and water exchange in salt marshes. Geochim Cosmochim. Acta 60: 46454652.

Swarzenski P.W., Holmes, C., Shinn, G. and Moore, W.S. (1998) Tracing the movement and mixing of groundwater into Florida Bay utilizing a new radiochemical technique: 223Ra and 224Ra isotope systematics. 1998 Florida Bay Science Conference, Univ. of Miami. Proceedings Volume.

Webster, 1.T., Hancock, G.J. and Murray, A.S. (1994) Use of radium isotopes to examine pore-water exchange in an estuary. Limnol. Oceanogr. 39(8) 1917-1927.

Time Line
Our field work will be coordinated closely with ongoing USGS CFCG sampling efforts. We intend to begin sampling in the summer, 1998. Our sampling program will commence through Fall, 1999. Reports, presentations and manuscripts will be generated for peer-reviewed science journals.

We will quantify the rate and magnitude of submarine groundwater into upper Florida Bay. As S. Florida
groundwater has been shown to be nutrient-rich (nutrients are derived in part from extensive agriculture N of the Everglades), this source will be evaluated as a source-term for coastal eutrophication within Florida Bay.

Outreach activities
Knowledge of the role of SGD into Florida Bay is still strikingly lacking, even though this has remained as one of the most important research themes in south Florida reviewed by the Florida Bay Research Council. We will generate Fact-sheets, and present out findings at public symposia.

Proposerís previous experience in the projectís topic or geographical area.
Swarzenski is a marine geochemist whose research during the past ten years has focussed primarily on the biogeochemical behavior of uranium series radionuclides (U, Po, Pb, Ra, Th) in various coastal environments (Framvaren Fjord, Norway, Amazon River, Brazil; Fly River, Papua New Guinea, Mississippi River, USA). He has recently utilized radium isotopes to address groundwater issues in Florida Bay (Swarzenski et al., 1998).

Below are some relevant papers from Swarzenski:
SWARZENSKI P. W., AND MCKEE B. A. (1998) Seasonal uranium distributions in the coastal waters adjacent to the Amazon and Mississippi Rivers. Estuaries, Vol. 21, No. 3.

SWARZENSKI P. W., MCKEE B. A., SORENSEN K. AND TODD J.F. (1998) 210Pb and 210Po, manganese and iron cycling across the O2/H2S interface of a permanently stratified Fjord: Framvaren, Norway. Marine Chemistry, Accepted - In Press.

SWARZENSKI P. W., MCKEE B. A., SKEI J.M., BOOTH J. G. AND TODD J.F. (1998) Uranium across the redox transition zone of a permanently stratified Fjord: Framvaren, Norway. Marine Chemistry, Accepted - In Press.

SWARZENSKI P. W., PORCELLI D. AND MCKEE B.A. Aqueous Uranium Geochemistry in Tropical Environs: An Estuarine Comparison of the Amazon and Fly (Papua New Guinea) Rivers. Geochim. Cosmochim. Ac, (In Prep).

McKee B. M., SWARZENSKI P. W. AND BOOTH, J. G. (1996) The flux of uranium isotopes from river-dominated shelf sediments. In: International. Symposium on the Geochemistry of the Earthís Surface., (IAGG) pp. 85-91.

MOORE W. S., DEMASTER D. D., SMOAK J. M., MCKEE B. A. AND SWARZENSKI P. W. (1996) Radionuclide tracers of sediment-water interactions on the Amazon Shelf. Cont. Shelf Res. 16: 645-665.

SWARZENSKI P. W., MCKEE B. A., AND BOOTH J. G. (1995) Uranium geochemistry on the Amazon Shelf: Chemical phase partitioning and cycling across a salinity gradient. Geochim. et Cosmochim. Acta, 59: 7-18.

McKee B.A., Swarzenski P.W. and Booth J.G. Uranium cycling in river-dominated environments: Revisting the global role of coastal margin sediments. Geochim. et Cosmochim. Acta (in press).

McKee B.A., Booth J.G. and Swarzenski P.W. Sediment deposition, redistribution and accumulation in the Mississippi River Bight. Continental Shelf Res. (in press)

Booth J.G., McKee B.A. and Swarzenski P.W. Factors influencing temporal and spatial variability of uranium concentrations in the Mississippi River. Geochim. et Cosmochim. Acta (submitted).

Shinn and Halley have both worked extensively (> 20 years) in Florida Bay; Holmes has recently examined the geochronological history of sedimentation within the Bay.

Names of Key Project Staff: (From C&MG, St. Pete) FY1999 FY2000
  pp pp
Gene Shinn carbonate hydrology   2 2
Peter Swarzenski marine geochemistry   10 10
Chuck Holmes radiochemistry   2 2
Robert Halley carbonate geochemistry   2 2
Don Hickey Hydrologist   6 6
Chris Reich Hydrologist   6 6
    FTE 1.08 1.08

Major Equipment / Facility Needs: Two Ra counters ($ < 10K)
These two delayed coincidence counters will be constructed/calibrated with the help of Dr. Billy Moore, Univ. of S. Carolina

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