Geochemical assessment of CO2 perturbation in a shallow aquifer evaluated by a push–pull field experiment

• Published in: International journal of greenhouse gas control Vol. 21; pp. 23 - 32
• Main Authors: Rillard, Jean, Gombert, Philippe, Toulhoat, Pierre, Zuddas, Pierpaolo
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.02.2014; Elsevier
• Subjects: CO2 leakage; Earth Sciences; Environmental Sciences; Field kinetics of reaction; Geological CO2 sequestration; Sciences of the Universe; Shallow groundwater chemistry; Single-well push–pull test; Trace metals elements; CO2 leakage; Geological CO2 sequestration; Shallow groundwater chemistry; Single-well push–pull test; Field kinetics of reaction; Trace metals elements
• ISSN: 1750-5836; 1878-0148
• DOI: 10.1016/j.ijggc.2013.11.019

•A push–pull test was performed to study the impact of CO2 perturbation in aquifers.•Results show an increase in Mn, Zn, As, Ca, Mg and alkalinity and decreases in pH.•Main path reactions were dissolution of dolomite, ferrihydrite and siderite.•An original method was used to estimate a field-scale kinetic rate of metal release.•Kinetic rate increased by 2 orders of magnitude over the CO2 perturbation. A field experiment was conducted using a push–pull test method in a shallow aquifer to investigate the potential impact of CO2 leakage on groundwater chemistry. The push–pull test was performed using a volume of groundwater previously pumped from the aquifer that was saturated in CO2 and introduced into a fractured sandstone aquifer before re-pumping it. Groundwater pH, alkalinity, electric conductivity, redox potential and FeII were measured on-site. A specific protocol was established to avoid oxidation during sampling. Field measurements and laboratory analyses showed rapid and systematic changes in pH and alkalinity as well as an increase in the aqueous concentrations of major cations (Ca, Mg) and trace element species (Fe, Mn, Zn, As). Thermodynamic calculations taking into account both redox and pH sensitive reactions indicated that trace elements may be mobilized as the result of the dissolution of metal oxide minerals. A simplified kinetic model, based on quantitative analyses provided by a mixing model, showed that the trace element release rate is ruled by a reaction of complex order with respect to pH, suggesting the influence of metal complexation reactions, involving bicarbonate and sulfate anions. Results suggest that, in the case of potential CO2 leakage in subsurface aquifers, the remobilization of bivalent metal cations (Fe, Mn, Zn) is relatively high, while it is limited for other elements such as As, Ca or Mg. This study provides a new data set for evaluating the impact of CO2 leakage in shallow aquifers and proposes specific methods for analyzing reaction pathways and kinetic reaction rates at the field scale.