A method was developed to exclude modern carbon from ground water samples by purging the ground water from the well in a traditional manner to remove all vestiges of water stored in the well. This purging technique allows fresh ground water to enter the well bore from the formation. Prior to the traditional purging step, a head of nitrogen gas is introduced above the water column to completely displace all atmospheric gas from the well bore. Reintroduction of atmospheric gas into the headspace of the well is prevented during the traditional ground water purging step by continuously passing nitrogen gas into the well bore. The nitrogen gas is slowly injected in a “bottom-up” direction through a hose that extends almost to the static water table. Charles’s law was used to calculate the volume of nitrogen gas that is required to displace all the atmospheric gas from the headspace in the well before any ground water is purged from the well. Measurements taken at the pressure regulator are used to ensure that a sufficient volume of nitrogen gas is injected into the headspace of the well. In addition, nitrogen gas is also used to purge atmospheric gas from the sample containers. This method was used successfully to demonstrate that the age of the ground water below a proposed radioactive waste repository is at least 40,000 years BP.