Large-scale storage of carbon dioxide in saline aquifers may cause considerable pressure perturbation and brine migration in the deep formations, which may give rise to a significant influence on the regional groundwater system. With the help of parallel computing techniques, a comprehensive, large-scale numerical simulation of CO 2 geologic storage that predicts not only CO 2 migration but also its impact on regional groundwater flow was performed. As a case study, a hypothetical industrial-scale CO 2 injection in the Tokyo Bay, which is surrounded by the most industrialized area in Japan, was considered, and the impact of down-dip CO 2 injection on up-dip near-surface aquifers was investigated. A regional hydrogeological model with an area of about 60km×70km around the Tokyo Bay was discretized into about 10 million gridblocks. To solve the high-resolution model efficiently, we used a parallelized multiphase flow simulator TOUGH2-MP/ECO2N on a world-class high performance supercomputer in Japan, the Earth Simulator. In the simulation, CO 2 was injected into a storage aquifer at about 1km depths under the Tokyo bay from 10 wells with the total rate of 10 million tons/year for 100 years. Through the model, regional groundwater pressure build-up and seepage changes at land surface are examined. The results suggest that even if containment of CO 2 plume is ensured, pressure buildup in the order of tens of meters can occur in shallow confined layers of extensive regions including urban inlands.