Metal release from limestones at high partial-pressures of CO2

- We analyzed metal release from natural limestone rocks at elevated pCO2.
- Calcite dissolution and desorption controlled metal release.
- We simulated kinetic calcite, pyrite dissolution under oxic and suboxic conditions.
- Calcite will release more metals than pyrite in carbonate aquifers across 30 years.

CO2 leakage from underground CO2 sequestration and storage poses potential risks to degradation of water quality in shallow aquifers. Increased CO2 concentrations can result in decreased pH and lead to subsequent metal release from mineral dissolution or desorption from mineral surfaces. Dissolution of carbonate minerals present in aquifer sediments or rocks will buffer pH and is generally thought to reduce the potential risk of metal release in the event of a CO2 leak. As a result, much of the research on geochemical impacts of CO2 leakage has focused on siliciclastic aquifers with little to no carbonate minerals present. However, carbonate minerals contain trace amounts of metals in their crystal structure that will be released into solution with dissolution and may pose a risk to drinking water quality. Here, we perform laboratory water-rock experiments to analyze the potential for metal release due to carbonate mineral dissolution in limestone aquifers. Rock samples from three limestone aquifers were dissolved in batch reactors with varying partial-pressures of CO2 (from 0.01 to 1 bar) in the headspace. As CO2 dissolved into the fluid and decreased the pH, the carbonate minerals dissolved and released metals into solution. The concentrations of calcium, magnesium, strontium, barium, thallium, uranium, and cobalt increased but remained below any regulatory limits. The concentrations of arsenic and nickel increased and exceeded primary drinking water standards set by the USEPA and the State of California, respectively. Potential sources of metals in the rocks were determined through detailed sample characterization using sequential extractions, laser ablation inductively coupled mass spectrometry, and high resolution mineralogical mapping with QEMSCAN. We found that calcite dissolution released more metals to solution than pyrite dissolution or metal desorption from mineral surfaces in these experiments. Geochemical models based on the experimental data were used to evaluate the relative importance of calcite dissolution versus pyrite dissolution over a 30-year time frame. Under both oxic and sub-oxic conditions, calcite dissolution is the dominant source of metals to solution immediately after exposure to CO2. Pyrite dissolution becomes the dominant source at later times as the fluid reaches equilibrium with respect to calcite. For all model scenarios, the cumulative contribution of metals to solution was dominated by calcite dissolution. Results from this study suggest that the pH-buffering benefit of carbonate mineral dissolution in the event of a CO2 leak may be offset by the potentially negative effect of trace metal release from the crystal structure. This study highlights the need for detailed sample characterization at individual sites to identify sources of metals when assessing the potential risk of CO2 leakage into shallow aquifers.