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

Abstract

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.