The effect of pure and impure carbon dioxide (CO2) on geological storage is uncertain. Oxidation of impurities such as sulfur dioxide (SO2) and hydrogen sulfide (H2S), which may be catalyzed by co-injected oxygen or nitrogen oxides, leads to the formation of sulfuric acid (H2SO4) and a decrease in pH of the formation water. We investigated the effect of 0.005 mol L- 1 H2SO4 (corresponding to a worst case of 0.4% SO2 in the flue gas, anticipating total conversion of SO2 into H2SO4) on the reactivity of the reservoir (limestone and sandstone) and cap (marl) rocks of Hontomin (Spain) at P = pCO2 = 10 bar and 60 °C during 24 days using flow-through column experiments. Aqueous element concentrations were measured from fluid extracts obtained periodically throughout the experiments to infer fluid-rock reactions over time. Results were modeled with the CrunchFlow reactive transport code.
The added H2SO4 lowered the pH of the injected brine by ~ 1.5 pH units with respect to the pH of ~ 3.6 of the H2SO4-free brine. In both H2SO4-free and H2SO4-rich brine, calcite dissolution fostered gypsum precipitation. A comparison between the reactivity of the rocks reacted in H2SO4-free and H2SO4-rich brine showed that calcite dissolution and gypsum precipitation rates were increased by 27–48% and 25–75%, respectively, in H2SO4-rich brine. Overall rock porosity increments in H2SO4-rich brine were 2.9–3.6%, 3.7–4.8%, and 2.1–2.7% for sandstone, limestone and marl, respectively. Porosities in H2SO4-rich brine were 6%, 23% and 250%, higher, respectively, than under pure CO2. Modeled porosity increments in the acid inlet zone in H2SO4-rich brine for sandstone, limestone and marl were 28–29%, 44–45% and 24–28%, respectively, corresponding to an increase of 1%, 250% and 25%, respectively, relative to H2SO4-free brine. Gypsum precipitation was consistently higher in marl than in limestone and sandstone, indicating kinetically favorable conditions for gypsum precipitation within the cap rock. Our results provide relevant data for long-term storage simulations of impure CO2 injection.
Enhanced in situ biodenitrification (EIB) is a feasible technology to clean nitrate-polluted groundwater and reach drinking water standards. Aimed at enabling a better monitoring and management of the technology at the field scale, we developed a two-dimensional reactive transport model (RTM) of a cross section (26.5 × 4 m) of a fractured aquifer composed of marls involving both biogeochemical processes and associated isotope fractionation. The RTM was based on the upscaling of a previously developed batch-scale model and on a flow model that was constructed and calibrated on in situ pumping and tracer tests. The RTM was validated using the experimental data provided by Vidal-Gavilan et al. (2013). The model considers several processes including (i) exogenous and endogenous microbial nitrate and sulfate respiration coupled to ethanol oxidation and linked to microbial growth and decay, and (ii) geochemical interactions (dissolution/precipitation of calcite), and (iii) isotopic fractionation of the reaction network (15N–NO3, 18O–NO3, 13C–DIC, 13C–ethanol, 13C–biomass, and 13C–calcite). Most of the calibrated microbiological parameter values at field scale did not change more than one order of magnitude from those obtained at batch scale, which indicates that parameters determined at the batch scale can be used as initial estimates to reproduce field observations provided that groundwater flow is well known. In contrast, the calcite precipitation rate constant increased significantly (fifty times) with respect to batch scale. The incorporation of isotope fractionation into the model allowed to confirm the overall consistency of the model and to test the practical usefulness of assessing the efficiency of EIB through the Rayleigh equation approach. The large underestimation of the Rayleigh equation of the extent of EIB (from 10 to 50%) was caused by the high value of hydrodynamic dispersion observed in this fractured aquifer together with the high reaction rates.
Torres, E.; Couture, R.-M.; Shafei, B.; Ayora, C.; Van Cappellen, P.; Nardi, A. Chemical geology Vol. 419, num. 25, p. 75-91 DOI: 10.1016/j.chemgeo.2015.10.023 Data de publicació: 2015-10-15 Article en revista
The Sancho Reservoir in SW Spain has been impacted by acid mine drainage (AMD) since the Tharsis mine stopped activity in 1998. As a result, the reservoir exhibits lowpH (~3.5) and high aqueous concentrations of sulfate, aluminum, iron and trace metals. Thus far, removal of contaminants by sediment burial has not been as effective as expected in improvingwater qualitywithin the reservoir. To informpotential remediation strategies, a 1-D, non-steady-state reactive transport model with a comprehensive set of equilibrium and kinetic biogeochemical reactions is used to simulate the fate of tracemetals and acidity in sediments affected by AMD. Two realizations of themodel account for the spatial heterogeneity of bottomwater oxygenation. A “permanently oxic” model represents shallow sediments above the thermocline, while a “holomictic” model represents the deeper sediments where bottom water oxygen levels oscillate between completely anoxic and oxic as a result of water-column overturn. The model is calibrated against an extensive dataset on the depth distributions of pore water and solid phase species. Model results imply that, under permanently oxic conditions, the sediments act as a sink for acidity (H+) and aqueous Al, Zn, Cu, Co and Ni, but act as a source of aqueousMn, Fe and As. The latter are released to the overlying water as a result of Mn and Fe (oxy)hydroxide reductive dissolution in the sediments. Below the thermocline, when bottom waters become anoxic, metal sulfides precipitate in the sediment.
When the bottom waters subsequently become oxic, the metal sulfides are oxidized along the downward-penetrating oxygen front and the associated metals are released to the overlying water. On the order of 35% of the sediment pools of sulfide-bound Zn, Cu, Co and Ni, and ~25% of FeS are thus reoxidized. However, overall the sediments act as a net sink for the pollutants considered in the model. On an annual basis, about 10% of the total elemental masses of S, Al, Zn and Cu present in the water column of the reservoir are removed by burial in the sediments, but only ~2% for Co and Ni. For Fe, Mn and As, the corresponding values are 80, 70 and 98% respectively. Themodel predicts that, if AMD input to the reservoirwere to completely cease, the sediments would reach a new steady state with negligible release of aqueous contaminants to the overlying water column within a few years.
Groundwater discharge to the Mundo River (SE, Spain) has been investigated from 2011 to 2013 by means of Rn-222 activities in river water and groundwater. Starting nearby the river source, some 50 km of river channel have been studied. The Mundo River is located in the water stressed region of the Segura River Basin. Identifying and quantifying groundwater discharge to rivers is essential for the Hydrological Plan of the Segura Basin Authority. Four main areas of groundwater discharge to the river have been identified by means of Rn-222. Moreover, groundwater fluxes have been quantified using radon activities and, when possible, have been validated with chloride mass balances. The uncertainty range (+/- 2 sigma) of all water balances has also been assessed. Groundwater discharge (Q(GW)) values estimated by radon mass balances (RMB) and chloride mass balances (CMB) were similar in the river tracts and/or dates in which surface inputs from tributaries were null or negligible. This adds confidence to the Q(GW) values estimated by RMB in the reaches were CMB could not be performed due to the existence of ungauged surface inputs, as is the case of the upper basin of the Mundo River, as well as to the applicability of the method to similar situations. Quantification of groundwater discharge allowed identifying Ayna zone as the main gaining reach of the studied area, with up to 29,553 +/- 8667 m(3) day(-1) in year 2011. Overall, the total Q(GW) estimated by means of RMB for the studied area was 8-16% of the total river flow. The results are coherent with the meteorological conditions of the study period (average rainfall around 450 mm/y) and also with the undisturbed situation of the aquifers discharging to the Mundo River in the considered area. (C) 2014 Elsevier B.V. All rights reserved.
Rodríguez-Escales, P.; van Breukelen, B. M.; Vidal-Gavilan, G.; Soler, A.; Folch, A. Chemical geology Vol. 365, p. 20-29 DOI: 10.1016/j.chemgeo.2013.12.003 Data de publicació: 2014-02 Article en revista
An integrated model of EIB is presented considering biogeochemistry and isotopes.
Adding isotopes allows for the improved quantification of EIB processes. The organic carbon source can induce calcite precipitation or dissolution. Enrichment factors indicate the inverse fractionation of carbon sources. It is the first step in developing an integrated EIB model at the field scale.
Enhanced in-situ biodenitrification (EIB) is a potential technology for remediating nitrate-polluted groundwater. EIB aims to create optimal biodenitrification conditions through the addition of carbon sources, enabling the autochthonous microbial community to degrade nitrate via different redox pathways. Biogeochemical numerical models are useful tools for predicting and designing such biodenitrification applications. Compound-specific stable isotope analysis (CSIA) is another valuable method for determining the degree of nitrate transformation. Therefore, incorporating isotope fractionation in biogeochemical models combines the two tools and is a key step in the development of reactive transport models of EIB under field conditions. In this work, we developed such an integrated model using the Phreeqc code and calibrated the model with batch scale experimental data using either ethanol or glucose as external carbon sources. The model included the following: microbiological processes —exogenous and endogenous nitrate respiration coupled to microbial growth and decay; geochemical processes —precipitation or dissolution of calcite; and isotopic fractionation —δ15N-NO3−, δ18O-NO3−, and δ13C-DIC, incorporating the full δ13C isotope geochemistry involved in EIB. The modeled results fit well with the hydrochemical and isotopic experimental data. The model also incorporated nitrite accumulation observed during the glucose experiment. The biogeochemical model indicates that, depending on the added carbon source, calcite precipitates (using ethanol) or dissolves (using glucose). In both cases, changes in hydraulic conductivity can be induced for actual and long-term EIB applications. The incorporation of isotope fractionation in the model better enables to account for other natural attenuation processes, such as dilution and dispersion, in EIB applications at field scale. Both calibrated enrichment factors (+ 8‰ for ethanol and + 17‰ for glucose) suggest that an inverse fractionation effect occurred (in which the heavy isotope reacts faster than the light isotope) during their oxidation.
Bruno, J.; Duro, L.; De Pablo, J.; Casas, I.; Ayora, C.; Delgado, J.; Gimeno, M.; Peña, J.; Linklater, C.; Villar, L.; Gomez, P. Chemical geology Vol. 151, p. 277-291 Data de publicació: 1998-11 Article en revista