Utilities have tried to ensure that radiological hazards to the environment and residents are kept as low as reasonably achievable by monitoring and controlling planned releases. However, since groundwater contamination was reported to occur due to unplanned releases mostly in the United States nuclear power plants, the interest of the stakeholders has increased to a point where it is now one of the most important issues in the United States nuclear power industry. This paper aims to help to implement an effective on-site groundwater monitoring program at domestic nuclear power plants by briefing the experiences of the United States nuclear power plants on groundwater contaminations and groundwater monitoring, and responses of the United States nuclear industry and regulator body for them.