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The Florida Aquifer Storage and Recovery Geochemical
Study is an ongoing investigation by the FGS to examine water-rock
geochemical interactions that take place during ASR cycle testing.
Water-quality variations and aquifer system characteristics at five
ASR facilities are the focus of the current study. In addition,
geochemical and mineralogical data from cores taken within and
outside the influence of an ASR storage zone is currently being
collected.
FGS research funded by the FDEP UIC Program (Division
of Water Resource Management) has confirmed that understanding
water-rock geochemical interactions is important to the continued
success of ASR in Florida. Results of this investigation indicate
the following: 1) chemical (including isotopic) variability exists
within ground water and carbonates of the Floridan Aquifer System,
2) this variability may result in site-specific geochemical
processes affecting ASR wells and water quality, 3) in some
localities, oxygen-rich surface waters, once injected into the
Floridan Aquifer System causes the release of trace metals such as
arsenic (As), iron (Fe), manganese (Mn), uranium (U) and perhaps
nickel (Ni) into the recharged (and eventually recovered) waters, 4)
the design of recharge-storage-recovery cycle tests and the location
of monitor wells are important aspects of understanding these
geochemical processes. Of the nine cycle tests investigated to date,
most recovered samples exceed the new maximum contaminant level
(MCL) for As (10 ug/l). Research on the source of As in the Floridan
Aquifer System matrix, results of cycle testing in different
hydrogeological settings and the effects of repeated cycles tests
continues. See also
http://water.usgs.gov/ogw/pubs/ofr0289/jda_mobilization.htm
The FGS is also engaged in research as part of the
Comprehensive Everglades Restoration Plan to conduct a geochemical
reconnaissance of the Floridan Aquifer System carbonates in southern
Florida. This investigation will focus on carbonate mineralogy,
mineral chemistry, rock geochemistry (including a series of isotopic
analyses), and bench-scale studies. Two types of bench scale studies
are underway: sequential extraction, which identifies associations
between minerals and metals, and leaching studies that assess
solubility of metals during simulated cycle tests under 
high-dissolved oxygen conditions in the laboratory. Similar
experiments are being conducted for consulting firms, who have
recognized the expertise and the unique analytical capabilities of
the FGS hydrogeochemistry lab (see figure at right).
Results of this research underscore the need for continued research on
the geochemistry of ASR in Florida, especially in consideration of
the proposed 300 ASR wells to be installed as part of the
Comprehensive Everglades Restoration Plan. There exists a need to
improve our understanding of the water-rock dynamics in different
hydrogeological settings in which ASR may be applied.
(Cooperative research with Camp, Dresser and McKee, Inc.)
The purpose of this study is to characterize
bench-scale leachability of Floridian Aquifer System carbonate rocks
in response to high concentrations of dissolve oxygen (DO), and
identify sources of metals in these rocks. This study is divided
into five main parts:1) lithological descriptions of the ASR well
rock samples of Avon Park Formation, Seminole County, 2)
permeability study for both vertical and horizontal core samples of
the ASR well, 3) geochemistry of the water samples (leachate) when
exposed to aquifer rocks under high concentrations of dissolved
oxygen, 4) geochemistry of the aquifer rocks to identify sources of
metals in these rocks, and 5) sequential extraction of the
storage-zone carbonates to identify mineral and non-mineral phases
in the aquifer matrix that may be leachable under ASR conditions.
Four hundred sixty five water samples, seven whole
rock samples, and seven sequential leaching samples have been
analyzed for 64 elements using ICP-MS and ICP-OES analytical
techniques (Activation Laboratories).
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Water quality changes during the four bench-scale
cycles have been evaluated. Time-series graphs allow comparison of
water chemistry changes during periods of aeration and non-aeration
for each cycle. The graphs (see example below) also clearly define
concentrations and mobilization, depending on the initial
concentrations of the source rock. Mobilization of As, U and other
trace metals during four cycles for the Seminole ASR aquifer rocks
is evident. Comparable results for leached As were evident in all
four cycle tests. Leachates collected from cycle test 1 displayed
greater As concentrations compared to the other three cycles. During
this cycle, leachates collected during pump off showed an increase
of As concentrations with time whereas during pump on, As peaked,
then showed decrease in concentrations with time. On the other hand,
other samples showed more than one As peak.
From the whole rock geochemistry data, many trace
metals such as As, Cr, Ni, V, Mo, and U showed high concentrations,
when compared to global averages for limestones. Linear covariations
of As and Mo suggest that Mo is possibly associated in As-bearing
phases such as arsenian pyrite or organic material. Other minerals
or phases containing As in these carbonate rocks include organic
material and Fe- and Mn-oxyhydroxide coatings. Arsenian pyrite is
among the sources of As in the ASR aquifer rocks, however,
sequential extraction studies suggest other phases (e.g., Fe-oxides
and organics) may contain As and associate metals as well. Organic
material may contain U, which is also thought to be associated with
carbonate phases.
Results of sequential extraction bench tests suggest
that dominant As-bearing phases reside within the “organic plus
insoluble residue” fraction of the matrix, which includes sulfide
minerals (see below). Results demonstrate that the “organic plus
insoluble residue” fraction is strongly associated with Al, Cd, Cr,
Co, Fe, La, Ni, Pb, Se, Sr, Th and U, whereas Zn is associated with
carbonate minerals. Organic material is recognized as a source/sink
for uranium. The sources of arsenic may also include organics in
addition to pyrite. These extraction results also provide evidence
for presence of As and other mobilized metals in “non-sulfide”
fractions of the aquifer matrix (e.g., the carbonate and Fe-oxide
fractions ).
The Hydrogeology Section acquired new lab space at the
FGS/DEP annex. The hydrogeochemistry lab (525 sq. ft.) is ready for
operation with an additional 95 sq. ft. for sample storage. A
radio-isotope lab of 209 sq. ft. is equipped with an alpha
spectrometer and a radio-isotope fume hood. This will be used for
uranium isotope studies of groundwater. A Mega Pure 3A Water Still
will accommodate analytical water needs. Seventy-six feet of bench
space is available for laboratory experiments such as bench-scale
leaching and extraction studies of water-rock interaction related to
aquifer storage and recovery (ASR)
The Hydrogeology Section has also acquired a scanning
electron microscope JXA-840A (SEM) and microprobe. The SEM lab is
approximately 277 sq. ft. located adjacent to the hydrogeochemistry
lab. This instrument can be used for many applications in mineral
chemistry, as well as paleontology. Current investigations are
utilizing the state-of-the-art high performance electron probe
microanalyzer that allows high resolution element mapping and
analysis. The microprobe is an energy-dispersive x-ray system with a
light-element window (can detect elements as light as boron) coupled
with a 4-Pi Revolution imaging and analyses computer system.
Discussions are underway to add and wavelength dispersive detector
to the SEM.
Selected Outsourced Research Projects
- Surface geophysical methods demonstration project designed to
define the extent of ASR injection zones (in 3-D, time
transgressive) to improve monitor well placement at ASR facilities
- Bench-scale microcosm studies to simulate and better understand ASR
microbiological- geochemical reactions (planned)
- Characterize baseline indigenous microbial populations (genetics
and physiology) in receiving aquifers at ASR facilities to further
understanding of the affect on water quality and microbial fate and
transport
For more information about
ASR Hydrogeochemical Research, contact Jon Arthur.
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Florida Geological
Survey - Applied Geoscience Services Section
2012 State of Florida