Risks attributable to water quality changes in shallow potable aquifers from geological carbon sequestration leakage into sediments of variable carbonate content

The consequences of CO2 leakage from geological sequestration into shallow aquifers must be fully understood before such geo-engineering technology can be implemented. A series of CO2 exposure batch reactor experiments were conducted utilizing 8 sediments of varying composition obtained from across Denmark including; siliceous, carbonate and clay materials. Sediments were exposed to CO2 and hydro-geochemical effects were observed in order to improve general understanding of trace metal mobility, quantify carbonate influence, assess risks attributable to fresh water resources from a potential leak and aid monitoring measurement and verification (MMV) program design. Results demonstrate control of water chemistry by sediment mineralogy and most significantly carbonate content, for which a potential semi-logarithmic relationship with pH and alkalinity was observed. In addition, control of water chemistry by calcite equilibrium was inferred for sediments containing >2% total inorganic carbon (TIC), whereby pH minima and alkalinity maxima of approximately 6 and 20 mequiv./l respectively were observed. Carbonate dominated (i.e. >2% TIC) and mixed (i.e. clay containing) sediments showed the most severe changes in water chemistry with large increases in all major and trace elements coupled to minimal reductions in pH due to high buffering capacity. Silicate dominated sediments exhibited small changes in dissolved major ion concentrations and the greatest reductions in pH, therefore displaying the greatest propensity for mobilization of high toxicity pH sensitive trace species.