Proper site selection for CO₂ geologic storage requires assessing the impact of potential leakage of CO₂ from deep subsurface reservoirs to overlying drinking water aquifers. Although recent studies have largely focused on the mobilization of trace elements in response to the intrusion of CO₂ into such aquifers, in this paper we investigate two other leakage issues and potential effects on groundwater quality: the transport of organic compounds by supercritical CO₂ from deep storage reservoirs and the upward migration of CO₂ with co-injected H₂S. Numerical simulations show that organic compounds that may be present at depth, such as benzene, could be mobilized by supercritical CO₂ and migrate with the leaking CO₂. Modeling results also show that upon the transport of CO₂+H₂S mixtures through a hypothetical leakage pathway, H₂S arrival in the shallower aquifer is delayed in comparison with that of CO₂ due to the preferential dissolution of H₂S into the aqueous phase. The potentially adverse impacts of leakage on shallow groundwater quality may be exacerbated for cases of leaking CO₂+H₂S, compared to intrusion of pure CO₂, possibly leading to the mobilization of thiophilic elements such as arsenic. Geochemical reactions included in the simulations involve adsorption/desorption, reductive dissolution of goethite, precipitation of pyrite, siderite, and arsenic sulfide phases. The models presented are generic in nature, exploring important processes regarding organic compounds and co-injected H₂S, and calling attention to the need for more site-specific studies taking into account the variability and uncertainty of key hydrogeologic and geochemical parameters.