Leakage from deep storage reservoirs is a major risk factor associated with geologic sequestration of carbon dioxide (CO 2 ). Different scientific theories exist concerning the potential implications of such leakage for near-surface environments. The authors of this report reviewed the current literature on how CO 2 leakage (from storage reservoirs) would likely impact the geochemistry of near surface environments such as potable water aquifers and the vadose zone.Experimental and modeling studies highlighted the potential for both beneficial (e.g., CO 2 re-sequestration or contaminant immobilization) and deleterious (e.g., contaminant mobilization) consequences of CO 2 intrusion in these systems. Current knowledge gaps, including the role of CO 2 -induced changes in redox conditions, the influence of CO 2 influx rate, gas composition, organic matter content and microorganisms are discussed in terms of their potential influence on pertinent geochemical processes and the potential for beneficial or deleterious outcomes.Geochemical modeling was used to systematically highlight why closing these knowledge gaps are pivotal. A framework for studying and assessing consequences associated with each factor is also presented in Section 5.6. v