Impact of Elevated CO2 on Trace Element Release from Aquifer Sediments of the San Joaquin Valley, CA

Abstract:

Carbon capture and storage (CCS) is a promising technique for mitigating climate change by storing large volumes of carbon dioxide in deep saline aquifers. In California, the thick marine sediments of the Central and Salinas Valleys have been identified as prime targets for future CO₂ storage. However, the potential impacts on water quality of overlying drinking-water aquifers must be studied before CCS can be implemented. In this study, we compare trace element release from San Joaquin Valley aquifer sediments with a wide range of textural and redox properties. Kinetic batch experiments were performed with artificial groundwater continuously equilibrated under CO₂-saturated (at 1 atm) and background CO₂ (0.002-0.006 atm) conditions, resulting in a shift of nearly 3 pH units. In addition, the reversibility of trace element release was studied by sequentially lowering the CO₂ from 1.0 atm to 0.5 atm to background concentrations (0.002-0.006 atm) for CO₂-saturated systems in order to mimic the dissipation of a CO₂ plume in the aquifer. During exposure to high CO₂, a number of elements displayed enhanced release compared to background CO₂ experiments (Ca, Mg, Li, Si, B, As, Sr, Ni, Fe, Mn, V, Ti, and Co) with concentrations of As, Fe, and Mn exceeding EPA maximum contaminant levels in some cases. On the other hand, Mo and U showed suppressed release. Most intriguing, many of the elements showing enhanced release displayed at least some degree of irreversibility when CO₂ concentrations were decreased to background levels. In fact, in some cases (i.e., for V), an element showed further release when CO₂ concentrations were decreased. These results suggest that there may be longer-term effects on groundwater quality that persist even after the CO₂ plume has dissipated. Several different mechanisms of trace element release including ion exchange, desorption, and carbonate mineral dissolution are explored. Preliminary modeling results suggest that carbonate mineral dissolution can play a key role in driving trace element release even in sediments where carbonates are in low abundance.