Enhanced transport of heavy metals due to a CO2-acidified brine

The leakage of carbon dioxide (CO2) from deep geological storage into shallow aquifers has the potential to reduce the pH of the groundwater, which can facilitate dissolution and/or desorption of major cations and naturally occurring hazardous trace elements. Earlier studies show that desorption is the controlling mechanism for long term mobilization of solutes. In this paper, we study the mechanism of pH-dependent desorption of a cation (barium, Ba2+) and its transport behavior through a porous medium with a reactive surface dominated by iron oxide. We performed column flood experiments and numerical simulations applying conditions that resemble CO2-acidified brine leaking in a carbonate free potable shallow aquifer. The results show the formation of a retarded concentration front of desorbing Ba2+, with peak having a maximum concentration much larger than the initial value. The amplitude of the peak is sensitive to the pH of the acidified brine and the overall desorbed Ba2+ is controlled by the initial aquifer pH. This peak can locally exceed the Maximum Contaminant Level (MCL) regulated by the U.S. EPA presenting a health hazard.