Designation of a potable water supply as a "groundwater under the direct influence" of surface water (GWUDI) will require treatment of that supply to surface water regulatory standards. A groundwater age-dating method based upon the use of helium and tritium ratios can be used to supplement current methods for establishing a water supply GWUDI. "Age" using helium-tritium ratios is defined as the time elapsed since water has lost contact with the atmosphere. The premise for using this age dating method for evaluating GWUDI is that waters characterized by a larger age are farther removed from the surface (on average) and therefore are more characteristic of groundwater. Such information is valuable in determining whether a water supply is at risk from contamination from surface sources and therefore characterized as GWUDI. Ten samples collected from a groundwater supply located in Miami-Dade County, Florida, illustrate the value of this method. Includes 17 references.