Element mobilization and immobilization from carbonate rocks between CO2 storage reservoirs and the overlying aquifers during a potential CO2 leakage.

Despite the numerous studies on changes within the reservoir following CO2 injection and the effects of CO2 release into overlying aquifers, little or no literature is available on the effect of CO2 release on rock between the storage reservoirs and subsurface. This is important, because the interactions that occur in this zone between the CO2 storage reservoir and the subsurface may have a significant impact on risk analysis for CO2 storage projects. To address this knowledge gap, relevant rock materials, temperatures and pressures were used to study mineralogical and elemental changes in this intermediate zone. After rocks reacted with CO2-acidified 0.01 M NaCl, liquid analysis showed an increase of major elements (e.g., Ca and Mg) and variable concentrations of potential contaminants (e.g., Sr and Ba); lower aqueous concentrations of these elements were observed in N2 control experiments, likely due to differences in pH between the CO2 and N2 experiments. In experiments with As/Cd and/or organic spikes, representing potential contaminants in the CO2 plume originating in the storage reservoir, most or all of these contaminants were removed from the aqueous phase. SEM and Mössbauer spectroscopy results showed the formation of new minerals and Fe oxides in some CO2-reacted samples, indicating potential for contaminant removal through mineral incorporation or adsorption onto Fe oxides. These experiments show the interactions between the CO2-laden plume and the rock between storage reservoirs and overlying aquifers have the potential to affect the level of risk to overlying groundwater, and should be considered during site selection and risk evaluation.

Coupled Geochemical Impacts of Leaking CO2 and Contaminants from Subsurface Storage Reservoirs on Groundwater Quality.

The leakage of CO2 and the concomitant brine from deep storage reservoirs to overlying groundwater aquifers is considered one of the major potential risks associated with geologic CO2 sequestration (GCS). In this work both batch and column experiments were conducted to determine the fate of trace metals in groundwater in the scenarios of CO2 and metal-contaminated brine leakage. The sediments for this study were from an unconsolidated sand and gravel aquifer in Kansas, containing 0-4 wt % carbonates. Cd (114 µg/L) and As (40 µg/L) were spiked into the reaction system to represent potential contaminants from the reservoir brine. Through this research we demonstrated that Cd and As were adsorbed on the sediments, in spite of the lowered pH due to CO2 dissolution in the groundwater. Cd concentrations in the effluent were below the Cd MCL, even for sediments without detectable carbonate to buffer the pH. Arsenic concentrations in the effluent were also significantly lower than the influent concentration, suggesting that the sediments tested have the capacity to mitigate the coupled adverse effects of CO2 leakage and brine intrusion. The mitigation capacity of sediment is a function of its geochemical properties (e.g., the presence of carbonate minerals, adsorbed As, and phosphate).

Performance of the Fluidized Bed Steam Reforming product under hydraulically unsaturated conditions.

Several candidates for supplemental low-activity waste (LAW) immobilization at the Hanford site in Washington State, USA are being considered. One waste sequestering technology considered is Fluidized Bed Steam Reforming (FBSR). The granular product resulting from the FBSR process is composed primarily of an insoluble sodium aluminosilicate matrix with the dominant phases being feldspathoid minerals with a 1:1:1 molar ratio of Na, Al and Si. To demonstrate the durability of the product, which can be disposed of at the unsaturated Integrated Disposal Facility (IDF) at Hanford, a series of tests has been performed using the Pressurized Unsaturated Flow (PUF) system, which allows for the accelerated weathering of the solid materials. The system maintains hydraulically unsaturated conditions, thus mimicking the open-flow and transport properties that will be present at the IDF. Two materials were tested using the system: 1) the FBSR granular product and 2) the FBSR granular product encapsulated in a geopolymer to form a monolith. Results of the experiments show a trend of relatively constant effluent concentration of Na, Si, Al, and Cs as a function of time from both materials. The elements I and Re show a steady release throughout the yearlong test from the granular material but their concentrations seem to be increasing at one year from the monolith material. This result suggests that these two elements may be present in the sodalite cage structure rather than in the predominant nepheline phase because their release occurs at a different rate compared to nepheline phase. Also, these elements to not seem to reprecipitate when released from the starting material. Calculated one-year release rates for Si are on the order of 10(-6) g/(m(2) d) for the granular material and 10(-5) g/(m(2) d) for the monolith material while Re release is seen to be two orders of magnitude higher than Si release rates. SEM imaging and XRD analysis show how the alteration of the two materials is dependent on their depth in the column. This phenomenom is a result of depth-dependent solution concentrations giving rise chemical environments that may be supersaturated with respect to a number of mineral phases.

Geochemical implications of gas leakage associated with geologic CO2 storage--a qualitative review.

Gas leakage from deep storage reservoirs is a major risk factor associated with geologic carbon sequestration (GCS). A systematic understanding of how such leakage would impact the geochemistry of potable aquifers and the vadose zone is crucial to the maintenance of environmental quality and the widespread acceptance of GCS. This paper reviews the current literature and discusses current knowledge gaps on how elevated CO(2) levels could influence geochemical processes (e.g., adsorption/desorption and dissolution/precipitation) in potable aquifers and the vadose zone. The review revealed that despite an increase in research and evidence for both beneficial and deleterious consequences of CO(2) migration into potable aquifers and the vadose zone, significant knowledge gaps still exist. Primary among these knowledge gaps is the role/influence of pertinent geochemical factors such as redox condition, CO(2) influx rate, gas stream composition, microbial activity, and mineralogy in CO(2)-induced reactions. Although these factors by no means represent an exhaustive list of knowledge gaps we believe that addressing them is pivotal in advancing current scientific knowledge on how leakage from GCS may impact the environment, improving predictions of CO(2)-induced geochemical changes in the subsurface, and facilitating science-based decision- and policy-making on risk associated with geologic carbon sequestration.

Desorption behavior of carbon tetrachloride and chloroform in contaminated low organic carbon aquifer sediments.

Slow release behavior of carbon tetrachloride (CCl(4)) and chloroform (CHCl(3)) in low organic carbon (<0.1%) deep aquifer sediments was quantified by 1-D column desorption studies with intact cores. The compounds had been in contact with the sediments for 30years. Comparison of the CCl(4) distribution coefficient (K(d)) from this study with those from short contact time experiments suggested that CCl(4)K(d)'s calculated from site contaminated sediments of long contact time are likely a factor of 10 or more higher than those calculated from short contact-time lab experiments. A significant portion of the CHCl(3) mass (55% to more than 90%) was resistant to aqueous desorption in sediments with clay contents ranging from 2.0% to 36.7% and organic carbon content ranging from 0.017% to 0.088%. In contrast, CCl(4) showed greatest mass retention (31% or more) only in the highest clay and organic carbon content sediment. Relatively easy solvent extraction of the residual masses of CCl(4) and CHCl(3) from the sediments indicated the compounds were not permanently sequestered. Tracer breakthrough in columns was well behaved, indicating interparticle diffusion was not causing the slow release behavior. Diffusion out of intraparticle pores is suggested to be the main process governing the observed behavior although, diffusion out of natural organic matter cannot be ruled out as a potential contributing factor. The half-life for release of the slow fraction of CHCl(3) mass from sediments was estimated to be in the range of weeks (100h) to months (1100h). Neither CCl(4) or CHCl(3) were detected at measurable levels in the column effluent of one of the sediments even though a significant mass fraction of CHCl(3) was found present on the sediment following desorption suggesting that our estimate of hundreds to thousands of hours for complete release of CHCl(3) masses from such sediment is conservative.

Uranium phases in contaminated sediments below Hanford's U tank farm.

Macroscopic and spectroscopic investigations (XAFS, XRF, and TRLIF) on Hanford contaminated vadose zone sediments from the U-tank farm showed that U(VI) exists as different surface phases as a function of depth below ground surface (bgs). Secondary precipitates of U(VI) silicate precipitates (boltwoodite and uranophane) were present dominantly in shallow-depth sediments (15-16 m bgs), while adsorbed U(VI) phases and polynuclear U(VI) surface precipitates were considered to dominate in intermediate-depth sediments (20-25 m bgs). Only natural uranium was observed in the deeper sediments (> 28 m bgs) with no signs of contact with tank wastes containing Hanford-derived U(VI). Across all depths, most of the U(VI) was preferentially associated with the silt and clay size fractions of sediments. Strong correlation between U(VI) and Ca was found in the shallow-depth sediments, especially for the precipitated U(VI) silicates. Because U(VI) silicate precipitates dominate in the shallow-depth sediments, the released U(VI) concentration by macroscopic (bi)carbonate leaching resulted from both desorption and dissolution processes. Having different U(VI) surface phases in the Hanford contaminated sediments indicates that the U(VI) release mechanism could be complicated and that detailed characterization of the sediments using several different methods would be needed to estimate U(VI) fate and transport correctly in the vadose zone.

U(VI) adsorption on aquifer sediments at the Hanford Site.

Aquifer sediments collected via split-spoon sampling in two new groundwater wells in the 200-UP-1 operable unit at the Hanford Site were characterized and showed typical Ringold Unit E Formation properties dominated by gravel and sand. High iron-oxide content in Fe oxide/clay coatings caused the highest U(VI) adsorption as quantified by batch K(d) values, indicating iron oxides are the key solid adsorbent in the 200-UP-1 sediments that affect U(VI) fate and mobility. Even though U(VI) adsorption on the gravel-sized fraction of the sediments is considered to be negligible, careful characterization should be conducted to determine U(VI) adsorption on gravel, because of presence of Fe oxides coatings and diffusion-controlled adsorption into the gravel particles' interior surfaces. A linear adsorption isotherm was observed up to 10(-6) M (238 microg/L) of total U(VI) concentration in batch U(VI) adsorption tests with varying total U(VI) concentrations in spiked groundwater. U(VI) adsorption decreased with increasing concentrations of dissolved carbonate, because strong anionic aqueous uranium-carbonate complexes formed at high pH and high alkalinity conditions. Noticeable uranium desorption hysteresis was observed in a flow-through column experiment, suggesting that desorption K(d) values for aged uranium-contaminated sediments at the Hanford Site can be larger than adsorption K(d) values determined in short-term laboratory experiments and slow uranium release from contaminated sediments into the groundwater is expected.

Extraction and quantitative analysis of iodine in solid and solution matrixes.

129I is a contaminant of interest in the vadose zone and groundwater at numerous federal and privately owned facilities. Several techniques have been utilized to extract iodine from solid matrixes; however, all of them rely on two fundamental approaches: liquid extraction or chemical/heat-facilitated volatilization. While these methods are typically chosen for their ease of implementation, they do not totally dissolve the solid. We defined a method that produces complete solid dissolution and conducted laboratory tests to assess its efficacy to extract iodine from solid matrixes. Testing consisted of potassium nitrate/potassium hydroxide fusion of the sample, followed by sample dissolution in a mixture of sulfuric acid and sodium bisulfite. The fusion extraction method resulted in complete sample dissolution of all solid matrixes tested. Quantitative analysis of 127I and 129I via inductively coupled plasma mass spectrometry showed better than +/-10% accuracy for certified reference standards, with the linear operating range extending more than 3 orders of magnitude (0.005-5 microg/L). Extraction and analysis of four replicates of standard reference material containing 5 microg/g 127I resulted in an average recovery of 98% with a relative deviation of 6%. This simple and cost-effective technique can be applied to solid samples of varying matrixes with little or no adaptation.

Reduction kinetics of Fe(III), Co(III), U(VI), Cr(VI), and Tc(VII) in cultures of dissimilatory metal-reducing bacteria.

The reduction kinetics of Fe(III)citrate, Fe(III)NTA, Co(III)EDTA-, U(VI)O(2) (2+), Cr(VI)O(4) (2-), and Tc(VII)O(4) (-) were studied in cultures of dissimilatory metal reducing bacteria (DMRB): Shewanella alga strain BrY, Shewanella putrefaciens strain CN32, Shewanella oneidensis strain MR-1, and Geobacter metallireducens strain GS-15. Reduction rates were metal specific with the following rate trend: Fe(III)citrate > or = Fe(III)NTA > Co(III)EDTA- >> UO(2)(2+) > CrO(4)(2-) > TcO(4)(-), except for CrO(4) (2-) when H(2) was used as electron donor. The metal reduction rates were also electron donor dependent with faster rates observed for H(2) than lactate- for all Shewanella species despite higher initial lactate (10 mM) than H2 (0.48 mM). The bioreduction of CrO(4) (2-) was anomalously slower compared to the other metals with H(2) as an electron donor relative to lactate and reduction ceased before all the CrO(4)(2-) had been reduced. Transmission electron microscopic (TEM) and energy-dispersive spectroscopic (EDS) analyses performed on selected solids at experiment termination found precipitates of reduced U and Tc in association with the outer cell membrane and in the periplasm of the bacteria. The kinetic rates of metal reduction were correlated with the precipitation of reduced metal phases and their causal relationship discussed. The experimental rate data were well described by a Monod kinetic expression with respect to the electron acceptor for all metals except CrO(4)(2-), for which the Monod model had to be modified to account for incomplete reduction. However, the Monod models became statistically over-parameterized, resulting in large uncertainties of their parameters. A first-order approximation to the Monod model also effectively described the experimental results, but the rate coefficients exhibited far less uncertainty. The more precise rate coefficients of the first-order model provided a better means than the Monod parameters, to quantitatively compare the reduction rates between metals, electron donors, and DMRB species.