USGS Groundwater Information
Mobilization of arsenic and other trace elements during aquifer storage and recovery, southwest Florida
By Jonathan D. Arthur1, Adel
A. Dabous1, and James B. Cowart 2
Aquifer storage and recovery (ASR) is an effective method of injecting treated or reclaimed water into confined, or semi-confined permeable formations for later withdrawal as needed. This technology is rapidly becoming widely accepted to address water supply shortages. In 1998, only six ASR facilities were in operation in Florida. As of January 2002, 26 ASR facilities exist in Florida and 19 are permitted for construction. More than 100 ASR facilities are in operation worldwide. ASR not only helps meet increasing demands for drinking water, but it has several other applications in industry, agriculture and environmental restoration. A prime example of the latter application is the role of ASR in the Comprehensive Everglades Restoration Project. Approximately 300 ASR wells are proposed in South Florida to capture ~1.7 billion gallons per day and store the water in the Floridan aquifer system (FAS) until it is needed.
Early operational testing of ASR wells in Florida (early 1980's to mid 1990's) focused primarily on engineering aspects to address what was considered the bottom line: what percentage of water can be recovered once it has been injected into aquifer storage zones? Although water quality monitoring accompanied the testing of these wells, little attention was paid to water-rock interactions that may occur during ASR operation, unless those processes affected the ability to store or recover water (e.g., precipitation/plugging).
In 1995, leaching experiments conducted by Florida Geological Survey (FGS) staff demonstrated that not only does uranium occur in appreciable amounts (>25 ppm) within FAS limestones, but also more than 30% of the uranium can be leached from the rocks under oxidizing conditions in the laboratory. This is especially significant relative to ASR in Florida where the storage zone is the reduced FAS. Source (i.e., surface) waters for ASR contain more dissolved oxygen (DO) than native ground water. Once these waters are introduced into a reduced aquifer, selective leaching and/or mineral dissolution may release metals into the injected water. Recognizing the implications of this research, the Florida Department of Environmental Protection (FDEP) funded the Aquifer Storage and Recovery Geochemistry project, which is now in its fifth year at the FGS. Goals of this project include:
Currently, three ASR facilities comprise the focus of our research: NW Hillsborough County Reclaimed water ASR, Rome Avenue ASR (Hillsborough County), and the Punta Gorda ASR facility (Charlotte County). Results of research on the latter two sites, located more than 120 km apart, are summarized herein.
Water quality changes during twelve ASR cycle tests have been evaluated to date. Time-series graphs allow comparison of water chemistry changes during injection, storage and recovery. These graphs also clearly define mixing and mobilization, depending on the initial concentrations of “end member” waters. For example, where concentration of an element is low in injected water, and high in native water (Figure 1), or vice versa, a mixing curve between injected and native ground water is well defined during withdrawal. On the other hand, when the concentration of a metal is low in both injected and native water, but an increase in concentration is observed during recovery, mobilization from the aquifer matrix is indicated. Mobilization of As during recovery at the Punta Gorda ASR facility is evident in Figure 1. This pattern is also observed in the ASR well cluster at Rome Avenue.
Figure 1. Punta Gorda cycle tests: As and Ca distributions. During recovery, mixing of low-Ca injected waters and higher-Ca native ground water is observed. Arsenic concentrations in recharge and native ground waters are less than 10 µg/l. An As peak (up to 50 µg/l) is observed indicating water-rock interaction.
Combined results from cycle tests (Williams and others, 2001; Arthur and others, 2001; unpublished FGS data) suggest that As, Fe, Mn, Ni(?), V(?) and U are mobilized from the aquifer system matrix into the injected waters. Arsenic and U mobilization are the most consistent and well-documented trends observed, with concentrations exceeding 85 µg/l and 6 µg/l, respectively. Most cycle test results are based on water quality during recovery from ASR wells (e.g., Figure 1), however where ASR monitor wells exist, mobilization is observed during injection as injected water moves past storage zone monitor wells. Preliminary results indicate that these mobilization reactions take place on the order of days, perhaps hours. Three pairs of cycle tests indicate that maximum observed As concentrations decrease during successive cycle testing, however, this preliminary observation holds true only where both cycle test injection volumes are similar and exposure of “new” aquifer matrix to the injected water is minimal. This result is not only desired, but expected assuming that the As source is a fixed concentration within the aquifer matrix and it is not replenished through mobilization/precipitation from yet unidentified source waters. In cases involving paired cycle tests where the second injection is a larger volume (thus more exposure to previously unexposed aquifer matrix), the maximum As concentration is greater during the second cycle test of the pair (data not shown).
Figure 2 summarizes As concentrations from nine cycle tests with respect to the volume percent of injected water that has been recovered during each test. Input volumes range from 3.7 to 159 million gallons. Some cycles recovered only 75% while others recovered more than 200% by volume. There is no apparent relation between the maximum As concentrations and the volume of water recharged/injected during the cycle test (data not shown). On the other hand, Figure 2 suggests that maximum As concentrations are not observed until between 50% and more than 100% of the input volume is recovered. This preliminary finding is important to consider in the design and monitoring of cycle tests.
Figure 2. Arsenic concentrations represented by trend lines versus cumulative percent recovery (by volume) of water injected during cycle tests. Vertical arrows indicate estimated maximum As concentration. Horizontal arrows indicate the point beyond which the maximum As concentration would be encountered (i.e., maximum As concentration was not attained before sampling ended).
Different geochemical processes or reactions likely govern U versus As and Ni concentrations as indicated by ASR well data from Rome Avenue (Figure 3). The U concentrations peak earlier than those of As and Ni, an observation corroborated by data reported in earlier cycle tests results (Arthur and others, 2001) from the Punta Gorda ASR well. This not only suggests different geochemical reactions/processes, but also a possible association of As and Ni mobilization.
In addition to U being useful to demonstrate metals mobilization during ASR activities, the activity ratio (AR) of 234U/238U is also useful for identifying mixing and evolution of waters during ASR in a single well (Arthur and others, 2001). Moreover, AR is useful to demonstrate heterogeneity that exists among wells in a single wellfield (Rome Avenue) or between ASR facilities located kilometers apart (Rome Avenue and Punta Gorda). Various models are proposed to account for the observed trends in AR (Cowart and others, 1998; Arthur and others, 2001; Williams and others, 2001), including selective leaching of 234U from thin U-rich grain carbonate coatings, and selective leaching of 234U from homogeneously distributed U within grains, coupled with flowing or non-flowing, aggressive, oxic interstitial water.
The aquifer system matrix
Mineralogical and chemical characterization of the FAS storage zone (Oligocene Suwannee Limestone) at both ASR facilities has been determined through a variety of methods. Mineralogy was determined through binocular description, petrography, x-ray diffraction, scanning electron microscopy and energy dispersive x-ray microprobe analyses. Thirty-six samples have been analyzed for 64 elements using multi-method analytical techniques (XRAL and Actlabs laboratories). Results of chemical analyses for 15 samples are reviewed in Arthur and others, (2001). When compared to global averages for limestones, most FAS average compositions are similar, except for perhaps Cr, which is higher in the FAS rocks. However, the maximum concentrations of selected metals (e.g., As, Hg, Ni, U) far exceed global averages (Table 1).
Mineralogy of the ASR storage zone carbonates is dominantly calcite and dolomite with minor clay minerals and organic material, and trace amounts of quartz, gypsum, and pyrite. Pyrite is euhedral to subhedral, averages less than one micron in size, and is intergranular and intragranular with respect to the calcite grains (Figure 4A). Framboidal pyrite masses are also observed (Figure 4B). Semiquantative microprobe analyses of more than 15 pyrite grains tentatively suggests that As concentrations range from less than five weight percent to below detection (~ two weight percent). Preliminary results of electron microscopy analyses (dot mapping and backscatter imaging) did not reveal other occurrences of detectable As-bearing minerals or mineral coatings.
Figure 4. Backscatter electron images of Suwannee Limestone, Floridan aquifer system. White areas are pyrite occuring as: A - finely disseminated intergranular and intragranular subhedral crystals (Rome Avenue, ASR 3, 255 feet below land surface), and B - framboidal masses along pore spaces within carbonate matrix (Rome Avenue, ASR 5, 251 feet below land surface). Scale bar = 10 µm.
Arsenic mobilization mechanisms
Welch and others (2000) present a comprehensive overview of As hydrogeochemistry. Relative to the FAS, examples of natural As mobilization mechanisms and associated mineral phases include: 1) oxidation of sulfide minerals such as pyrite, which may contain trace elements as lattice substitutes, impurities or in solid solution (Ni, Co, Cu, Pb, As, Zn, and Mn), 2) desorption or dissolution of Fe and Mn hydroxides (including grain or fracture coatings), 3) oxidation-reduction of organic material, which can mobilize organically complexed As, and 4) biological transformations (see Arthur and others, 2001 for more detail and references).
In addition to our recent work, other studies document As mobility during artificial recharge of aquifers (Stuyfzand, 1998; Ruiter and Stuyfzand, 1998, Brun and others, 1998). In a study of 11 deep well recharge experiments, oxidation of pyrite is reported, resulting in mobilization of As, Co, and Ni (Stuyfzand, 1998). Stuyfzand found that As remains relatively mobile while Ni and Co are less mobile and likely co-precipitate or adsorb on to Fe-hydroxides further away from the injection well.
Aquifer storage and recovery is a viable alternative water supply, but consideration of water-rock interaction is an important water quality and thus human health consideration. The Floridan aquifer system matrix is chemically heterogeneous, which is not only exemplified by carbonate geochemical data, but by the variable geochemical response observed in cycle tests from wells only a few hundred meters apart (Rome Avenue ASR wells). In the ground water, mobilization of As, Fe, Mn, U and other metals are observed during ASR activities. Variables affecting this mobility include: 1) native and input water chemistry ( DO, pH), 2) aquifer matrix chemistry/mineralogy, 3) input water - matrix contact time and number of cycle tests, and 4) site-specific hydrogeology/geochemistry. In addition to U being mobile, U activity ratios are useful toward understanding ground water evolution during ASR activities. Due to concerns regarding maximum contaminant levels, the design, construction and operation of ASR facilities, including monitor well placement and monitoring schedules should take into account the possibility of water-rock interaction and mobilization of metals into recovered waters.
Arthur, J.D., Cowart, J.B. and Dabous, A.A., 2001, Florida aquifer storage and recovery geochemical study: Year three progress report: Florida Geological Survey Open File Report 83, 46 p.
Brun, A., Christensen, F.D., Christiansen, J.S., Stuyfzand, P.J., and Timmer, H., 1998, Water quality modeling at the Langerak deep-well recharge site: in Peters, J.H., et al., eds., Artificial Recharge of Groundwater: Rotterdam, Netherlands, A.A. Balkema, 474 p.
Cowart, J.B., Williams, H.K., and Arthur, J.D., 1998, Mobilization of U isotopes by the introduction of surface waters into a carbonate aquifer: Geological Society of America Abstracts with Programs, v. 30, no. 7, p. A-86.
Ruiter, H. and Stuyfzand, P.J., 1998, An experiment on well recharge of oxic water into an anoxic aquifer: in Peters, J.H., et al., eds., Artificial Recharge of Groundwater: Rotterdam, Netherlands, A.A. Balkema, 474 p.
Stuyfzand, P.J., 1998, Quality changes upon injection into anoxic aquifers in the Netherlands: Evaluation of 11 experiments, in Peters, J.H., et al., eds., Artificial Recharge of Groundwater: Rotterdam, Netherlands, A.A. Balkema, 474 p.
Welch, A.H., Westjohn, D.B., Helsel, D.R., and Wanty, R.B., 2000, Arsenic in ground water of the United States: Occurrence and geochemistry: Ground Water, vol. 38, no. 4, p. 589-604.
Williams, H., Cowart, J.B. and Arthur, J.D., 2002, Florida aquifer storage and recovery (ASR) geochemical project: Year One and Year Two Progress Report Submitted to Bureau of Water Facilities Regulation, Florida Department of Environmental Protection, January, 1999, Florida Geological Survey Report of Investigation 100, 131 p.
In George R. Aiken and Eve L. Kuniansky, editors, 2002, U.S. Geological Survey Artificial Recharge Workshop Proceedings, Sacramento, California, April 2-4, 2002: USGS Open-File Report 02-89
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