The impact of CO2 on shallow groundwater chemistry: observations at a natural analog site and implications for carbon sequestration

In a natural analog study of risks associated with carbon sequestration, impacts of CO2 on shallow groundwater quality have been measured in a sandstone aquifer in New Mexico, USA. Despite relatively high levels of dissolved CO2, originating from depth and producing geysering at one well, pH depression and consequent trace element mobility are relatively minor effects due to the buffering capacity of the aquifer. However, local contamination due to influx of brackish waters in a subset of wells is significant. Geochemical modeling of major ion concentrations suggests that high alkalinity and carbonate mineral dissolution buffers pH changes due to CO2 influx. Analysis of trends in dissolved trace elements, chloride, and CO2 reveal no evidence of in situ trace element mobilization. There is clear evidence, however, that As, U, and Pb are locally co-transported into the aquifer with CO2-rich brackish water. This study illustrates the role that local geochemical conditions will play in determining the effectiveness of monitoring strategies for CO2 leakage. For example, if buffering is significant, pH monitoring may not effectively detect CO2 leakage. This study also highlights potential complications that CO2 carrier fluids, such as brackish waters, pose in monitoring impacts of geologic sequestration.