Groundwater residence times and sources of solutes in - G-WADI

Groundwater Residence Times and Sources of
Solutes in the Upper Floridan Aquifer, Southeast, USA
Stephen Osborn ([email protected])
December 2006
Location:
Figures and Captions
Florida and Georgia, southeast U.S.A.
(Figure 1).
Main problem illustrated:
•What is the residence time of
groundwater in the Upper Floridan
Aquifer (UFA)?
•What is the source of solutes in the
UFA?
Summary:
The Floridan aquifer underlies an area
of more than 250,000 square kilometers
in the southeastern USA and is a major
source of fresh water for public, industrial,
and agricultural use. Estimates of
groundwater age and source of solutes
can be used to determine recharge rates,
identify recharge areas, and improve
groundwater flow models. Improved
hydrologic data may have direct bearing on
best use management practices aimed at
preventing the adverse affects of salt water
intrusion, deterioration of water quality,
and depletion of water resources, which on
a human time scale, are non-renewable.
Figure 1. Extent of the Floridan aquifer: Florida, Georgia, Alabama, and South
Figure
1. Extent
of1990).
the Floridan aquifer: Florida, Georgia, Alabama, and South Carolina
Carolina
(Miller,
(Miller, 1990).
Tracers used:
δ13CDIC, 14CDIC, 3He, 87Sr/86Sr, Noble Gases
Hydrogeological setting:
The Floridan aquifer is defined to be at least 10 times more permeable than its bounding confining units. It consists
of an upper (UFA) and a lower (LFA) section of limestone separated by a semi-confining unit of varying mineralogy
and lithology. Both sections generally thicken as they get deeper to the south from southern Georgia where the aquifer
outcrops. Central Florida has been slightly uplifted into a mid-peninsula arch where karst features such as sinkholes
(Figure 2), caverns, and depression springs are exposed. Aquifer transmissivity is highly coincident with the axis of the
arch and consistent with the high porosity of karst features. The Floridan aquifer discharges at coastal boundaries, the
continental shelf, streams, springs, sinkholes, and due to pumping activities. The LFA is generally not pumped for
drinking water as it is more saline due to the presence of saltwater, it is deeper, and the UFA has traditionally yielded
ample fresh water supplies for most uses.
People affected, environmental, ecological impacts:
Saltwater intrusion into the UFA at coastal boundaries
has become a major issue as growing coastal communities
like Tampa Bay, Savannah, and Jacksonville pump fresh
water from the aquifer. Where the aquifer is too saline
to be potable, mainly in southern Florida, treated sewage
and industrial wastes have been pumped by deep-well
injection into the subterranean karst features of the LFA.
This is aimed at creating floating reservoirs of freshwater
for future use. In other areas, drainage wells have diverted
surface water runoff directly into the UFA.
Figure 2.Figure
Sinkhole
in central
(Miller,
1990).
2. Sinkhole
in Florida
central Florida
(Miller,
1990).
Water sampling and analysis summary:
Groundwater samples from the UFA were collected along flow paths from a recharge area in southeastern Georgia for
analysis of radiogenic 3He and noble gases temperatures. In central Florida, groundwater samples were collected from
the UFA along flow paths from recharge to discharge at coastal boundaries (Figure 3) for analysis of the 14C (percent
modern carbon, pmc) of dissolved inorganic carbon (DIC). In a study conducted in northern Florida, surface lake water
and subsurface groundwater samples were collected. Groundwater samples were collected sequentially from a surficial
aquifer to the UFA at depth. These samples were analyzed for δ13CDIC values and 87Sr/86Sr ratios.
Figure 3. Potentiometric map depicting sample locations and flow paths, and graphs of modeled 14C ages along flow paths (Plummer and
Sprinkle, 2001).
Figure
3. Potentiometric map depicting sample locations and flow paths, and graphs of
modeled 14C ages along flow paths (Plummer and Sprinkle, 2001).
Results of tracer studies:
Helium excess measured in UFA samples generally
increases with distance from the recharge area
from 0.5x10-8 to 42x10-8 cc STP g-1 (Figure 4).
The noble gas temperatures are separated into two
groups of approximately 17.7 °C (up-gradient) and
approximately 13.7 °C (down-gradient). The transition
of approximately 4±0.6 °C between the two groups of
noble gas temperatures occurs at a He excess of 10x10-8
cc STP g-1, which based on published He accumulation
rates dates the upper limit of the transition at
approximately 20 thousand years before present (ka) .
In central Florida, the modeled 14C ages in the UFA
Figure 4. Helium excess and noble gas temperatures along a
transect in southeast Georgia (Clark et al., 1997). The helium
generally increase along the flow path from
recharge
Figure 4. Helium excess
and
gasnoble
temperatures
a transect in southeas
excess is the
topnoble
number and
gas temperaturealong
is the bottom
areas in the center of the state (Figure 3). Deviations
of each dataThe
set. helium excess is the top number and noble g
Georgia (Clark etnumber
al., 1997).
from the decreasing trend in specific wells are explained
temperature is the bottom number of each data set.
by mixing of waters with different 14C values
that have been locally recharged. The modeled
14
C (NETPATH) adjusted age of groundwater
at discharge areas per flow path are between
15 ka and 30 ka. The increased reaction of
infiltrating water with carbonate rocks with
depth is depicted in Figure 5. The mineralogy
of the host rock changes from the silisiclastics
of the surficial aquifer (high 87Sr/86Sr ratio and
low δ13C) to the carbonates of the UFA (low
87
Sr/86Sr ratio and high δ13C) as the sample
depth increases (along the arrow in Figure 5).
Findings and conclusions:
Figure 5. 87Sr/86Sr vs. δ13C for groundwater in northern Florida (Katz and
The increasing He excess trend suggests
Bullen, 1996).
that groundwater age increases as well.
The transition of noble gas temperatures
Figure 5. 87Sr/86Sr vs. G13C for groundwater in northern Florida (Katz and Bullen, 1996).
corresponds with the timing of the last glacial period (LGP). Furthermore, the decreased temperature of
approximately 4±0.6 °C is consistent with published values from other areas in the southern USA for the LGP.
The adjusted 14C ages of DIC show that most water from the unconfined parts of the UFA were recharged during
the last 15 to 30 ka and that 14C values can potentially be used to identify where the UFA is locally recharged.
The decreasing 87Sr/86Sr ratio and increasing δ13CDIC value with depth indicates that the dissolution of calcite and
dolomite is the main source of solutes in the UFA.
Take home message:
Much of the groundwater in the UFA at coastal boundaries is non-renewable on a human time scale. Thus,
groundwater pumping by coastal communities should be managed cautiously in order to prevent depletion of fresh
water resources, salt water intrusion, and deterioration of water quality, while providing drinking water to the people
in the region.
Credits:
The research presented herein was supported by the National Science Foundation, the U.S. Geological Survey, and
the Florida Department of Environmental Protection.
Further readings:
Clark, J.F., M. Stute, P. Schlosser, and S. Drenkard, A tracer study of the Floridan aquifer in southeastern Georgia: Implications for groundwater
flow and paleoclimate, Water Resources Res., 33, no. 2, 281-289, 1997.
Katz, B.G., and T.D. Bullen, The combined use of 87Sr/86Sr and carbon and water isotopes to study the hydrochemical interaction between
groundwater and lakewater in mantled karst, Geochimica et Cosmochimica Acta, 60, no. 24, 5075-5087, 1996.
Miller, J.A., Groundwater Atlas of the United States, Alabama Florida, Georgia, and South Carolina, U.S. Geological Survey, HA 730-G,
published in 1990, http://capp.water.usgs.gov/gwa/index.html, 2006.
Plummer, L.N., and C.L. Sprinkle, Radiocarbon dating of dissolved inorganic carbon in groundwater from confined parts of the Upper Floridan
aquifer, Florida, USA, Hydrogeology Journal, 9, 127-150, 2001.