Amanda R. Lawter

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).

Evaluating impacts of CO2 and CH4 gas intrusion into an unconsolidated aquifer: fate of As and Cd

The sequestration of carbon dioxide (CO2) in deep underground reservoirs has been identified as an important strategy to decrease atmospheric CO2 levels and mitigate global warming, but potential risks on overlying aquifers currently lack a complete evaluation. In addition to CO2, other gases such as methane (CH4) may be present in storage reservoirs. This paper explores for the first time the combined effect of leaking CO2 and CH4 gasses on the fate of major, minor and trace elements in an aquifer overlying a potential sequestration site. Emphasis is placed on the fate of arsenic (As) and cadmium (Cd) released from the sediments or present as soluble constituents in the leaking brine. Results from macroscopic batch and column experiments show that the presence of CH4 (at a concentration of 1 % in the mixture CO2/CH4) does not have a significant effect on solution pH or the concentrations of most major elements (such as Ca, Ba, and Mg). However, the concentrations of Mn, Mo, Si and Na are inconsistently affected by the presence of CH4 (i.e., in at least one sediment tested in this study). Cd is not released from the sediments and spiked Cd is mostly removed from the aqueous phase most likely via adsorption. The fate of sediment associated As [mainly sorbed arsenite or As(III) in minerals] and spiked As [i.e., As5+] is complex. Possible mechanisms that control the As behavior in this system are discussed in this paper. Results are significant for CO2 sequestration risk evaluation and site selection and demonstrate the importance of evaluating reservoir brine and gas stream composition during site selection to ensure the safest site is being chosen.

Getters for improved technetium containment in cementitious waste forms

A cementitious waste form, Cast Stone, is a possible candidate technology for the immobilization of low activity nuclear waste (LAW) at the Hanford site. This work focuses on the addition of getter materials to Cast Stone that can sequester Tc from the LAW, and in turn, lower Tc release from the Cast Stone. Two getters which produce different products upon sequestering Tc from LAW were tested: Sn(II) apatite (Sn-A) that removes Tc as a Tc(IV)-oxide and potassium metal sulfide (KMS-2) that removes Tc as a Tc(IV)-sulfide species, allowing for a comparison of stability of the form of Tc upon entering the waste form. The Cast Stone with KMS-2 getter had the best performance with addition equivalent to ∼0.08wt% of the total waste form mass. The observed diffusion (Dobs) of Tc decreased from 4.6±0.2×10-12cm2/s for Cast Stone that did not contain a getter to 5.4±0.4×10-13cm2/s for KMS-2 containing Cast Stone. It was found that Tc-sulfide species are more stable against re-oxidation within getter containing Cast Stone compared with Tc-oxide and is the origin of the decrease in Tc Dobs when using the KMS-2.

Silver-based getters for 129I removal from low-activity waste

Abstract A prominent radionuclide of concern in nuclear wastes, 129I, is present in low-activity wastes (LAW) at the Hanford site. Several Ag-containing materials were tested as immobilization agents, or “getters”, for I (as iodide, I−) removal from deionized (DI) water and a liquid LAW simulant: Ag impregnated activate carbon (Ag–C), Ag exchanged zeolite (Ag–Z), and argentite. In anoxic batch experiments with DI water, the Ag–C and argentite were most effective, with maximum Kd values of 6.2 × 105 mL/g for the Ag–C and 3.7 × 105 mL/g for the argentite after 15 days. Surface area and Ag content were found to influence the performance of the getters in DI water. In the anoxic batch experiments with LAW simulant, Ag–Z vastly outperformed the other getters with Kd values of 2.2 × 104 mL/g at 2 h, which held steady until 15 days, compared with 1.8 × 103 mL/g reached at 15 days by the argentite. All getters were stable over long periods of time (i.e. 40 days) in DI water, while the Ag–Z and argentite were also stable in the LAW simulant. Ag–Z was found to have consistent I removal upon crushing to a smaller particle size and in the presence of O2, making it a strong candidate for the treatment of LAW containing I.

Assessment of Hexavalent Chromium Natural Attenuation for the Hanford Site 100 Area

Hexavalent chromium (Cr(VI)) plumes are present in the 100 Area at the Hanford Site. Remediation efforts are under way with objectives of restoring the groundwater to meet the drinking-water standard (48 µg/L) and protecting the Columbia River by ensuring that discharge of groundwater to the river is below the surface-water quality standard (10 µg/L). Current remedies include application of Pump-and-Treat (P&T) at the 100-D, 100-H, and 100-K Areas and Monitored Natural Attenuation (MNA) at the 100-F/IU Area. Remedy selection is still under way at the other 100 Areas. Additional information about the natural attenuation processes for Cr(VI) is important in all of these cases. In this study, laboratory experiments were conducted to demonstrate and quantify natural attenuation mechanisms using 100 Area sediments and groundwater conditions.

A Critical Review of the Impacts of Leaking CO2 Gas and Brine on Groundwater Quality

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Technetium and iodine aqueous species immobilization and transformations in the presence of strong reductants and calcite-forming solutions: Remedial action implications

At the Hanford Site in southeastern Washington, discharge of radionuclide laden liquid wastes resulted in vadose zone contamination, providing a continuous source of these contaminants to groundwater. The presence of multiple contaminants (i.e., 99Tc and 129I) increases the complexity of finding viable remediation technologies to sequester contaminants in situ and protect groundwater. Although previous studies have shown the efficiency of zero valent iron (ZVI) and sulfur modified iron (SMI) in reducing mobile Tc(VII) to immobile Tc(IV) and iodate incorporation into calcite, the coupled effects from simultaneously using these remedial technologies have not been previously studied. In this first-of-a-kind laboratory study, we used reductants (ZVI or SMI) and calcite-forming solutions to simultaneously remove aqueous Tc(VII) and iodate via reduction and incorporation, respectively. The results confirmed that Tc(VII) was rapidly removed from the aqueous phase via reduction to Tc(IV). Most of the aqueous iodate was transformed to iodide faster than incorporation into calcite occurred, and therefore the I remained in the aqueous phase. These results suggested that this remedial pathway is not efficient in immobilizing iodate when reductants are present. Other experiments suggested that iodate removal via calcite precipitation should occur prior to adding reductants for Tc(VII) removal. When microbes were included in the tests, there was no negative impact on the microbial population but changes in the makeup of the microbial community were observed. These microbial community changes may have an impact on remediation efforts in the long-term that could not be seen in a short-term study. The results underscore the importance of identifying interactions between natural attenuation pathways and remediation technologies that only target individual contaminants.

Incorporation Modes of Iodate in Calcite

Iodate (IO3-) incorporation in calcite (CaCO3) is a potential sequestration pathway for environmental remediation of radioiodine-contaminated sites (e.g., Hanford Site, WA), but the incorporation mechanisms have not been fully elucidated. Ab initio molecular dynamics (AIMD) simulations and extended X-ray absorption fine structure spectroscopy (EXAFS) were combined to determine the local coordination environment of iodate in calcite, the associated charge compensation schemes (CCS), and any tendency for surface segregation. IO3- substituted for CO32- and charge compensation was achieved by substitution of Ca2+ by Na+ or H+. CCS that minimized the I-Na/H distance or placed IO3- at the surface were predicted by density functional theory to be energetically favored, with the exception of HIO3, which was found to be metastable relative to the formation of HCO3-. Iodine K-edge EXAFS spectra were calculated from AIMD trajectories and used to fit the experimental spectrum. The best-fit combination consisted of a significant proportion of surface-segregated IO3- and charge compensation was predominantly by H+. Important implications are therefore that pH should strongly affect the extent of IO3- incorporation and that IO3- accumulated at the surface of CaCO3 particles may undergo mobilization under conditions that promote calcite dissolution. These impacts need to be considered in calcite-based iodate remediation strategies.

Element mobilization and immobilization from carbonate rocks between CO 2 storage reservoirs and the overlying aquifers during a potential CO 2 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.