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Hydrogeology Section

Aquifer Storage and Recovery Geochemical Studies

Aquifer storage and recovery is a cost-effective, viable solution to address drinking-water shortages in Florida. ASR wells are Class 5 injection wells regulated by the Underground Injection Control Program of the Florida Department of Environmental Protection. Twenty-six ASR facilities are in operation in Florida and more than 15 sites are under development. Some of the sites include reclaimed water ASR facilities, which are also cost-effective solutions to local water shortages. The FGS is working with the FDEP Underground Injection Control (UIC) Program, the University of South Florida, SWFWMD, SFWMD and consulting firms to characterize water-rock geochemistry during ASR at the bench scale and in the field.

Focus Areas

Schematic of ASR site showing ASR well (gray) and monitor well (dashed). As water is recharged (blue) it follows paths of highest permeability. The shape of the zone of recharged water is also dictated by density differences between recharged and native water. During recharge and storage, a potential exists for water-rock geochemical reactions to occur (denoted by orange). As water is recovered, soluble metals may reflect the trend shown in the graph of concentration (C) versus time since start of recovery (T).

Schematic of  ASR site showing ASR and monitor well

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

Comprehensive Everglades Restoration Program Geochemistry Project

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 Well proposed to be sampled for Floridan Aquifer System carbonate geochemistry study 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.

Bench Scale Geochemical Assessment of Water-rock Interaction: Seminole County ASR
(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).

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.Schematic of  ASR site showing ASR and monitor wellDuring 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 ).

Hydrogeochemistry Lab

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.Hydrogeochemistry Lab and

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.

Last updated: November 10, 2014

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