Project Proposal for 1999
USGS Geologic Division
New Project Proposal- FY 1999
IDENTIFYING INFORMATION
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
E-mail: eshinn@usgs.gov
Phone: 813-893-3100 x3030
Fax: 813-803-2032
Mail Address: US Geological Survey, 600 4th Street South, St.
Petersburg, FL 33701
Program: INATURES
Program Element(s)/Task(s): Element 2; Task 2.1 Freshwater discharge
to Florida Bay
BACKGROUND NARRATIVES
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.
WORK PLAN
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.
References
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.
Deliverables/products
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.
PROJECT SUPPORT REQUIREMENTS
| 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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