Most Hydrogeologic tests to determine suitable geologic sites for disposal of radioactive waste are of short duration when compared to the time span required in the assessment of the safety of the facility. Geochemical studies of fluid and rock offer an opportunity to examine natural processes that have been going on for a period of time comparable to that required for safe retention of radioactive waste. In a study assessing the potential for storing radioactive waste in metamorphic rock at the Savannah River Plant near Aiken, South Carolina, the rate of water movement was determined to be about 0.06 m/yr by analyzing gas dissolved in the water. The gas contained up to 6% helium, which originated from the radioactive decay of natural uranium and thorium in the crystalline rock. The residence time of the water in the rock was calculated to be 840,000 years from the quantity of uranium and thorium in the rock, its rate of radioactive decay, and the quantity of helium dissolved in the water. From this residence time and a flow path of 51 km the average velocity of water through the rock was estimated. This water velocity, which is an average over a broad region (51 km) and a long time period (840,000 years), is more applicable to the assessment of a geologic site for storage of radioactive waste than are velocities estimated from packer tests, pumping tests, or artificial tracer tests, all of which require extensive time and space extrapolations.