The pollution of ground water caused by the inflow of drainage water from mudstone excavation sites is a serious issue. It has been recognized that the problem results from the discharge of sulfate generated by the decomposition of pyrite contained in the mudstone. Urban natural ground commonly contains mud sediments, and when the underground environment is altered by underground openings, pollution by acid water originating from these mud sediments may occur. The prediction of pollution is therefore important in constructing underground openings. We used numerical simulation to attempt prediction of the spatial extent of pollution caused by the drainage of acid water with heavy metals from mud sediment.