Good science using a range of laboratory, field, and modeling approaches is critical for understanding the benefits, risks, and unforeseen consequences of carbon capture and storage (CCS). As described in Little and Jackson, (1) we presented experimental results to help understand the potential consequences of CO2 leaks into overlying aquifers. This research, and our previous CCS-related work (e.g., refs 2 and 3), was undertaken to improve the efficacy and safety of CCS technologies worldwide. To this end, we are currently extending the work in Little and Jackson (1) to include more sites and to analyze the impact of a broader range of environmental conditions. We are also working to understand more completely the mechanisms for changes in element concentrations observed in our study and in other research (e.g., dissolution of mineral phases or enhanced desorption). Here, we address the specific comments from Gilfillan and Haszeldine (4) in the order in which they were given.