将饮用水供应指定为“受地表水直接影响的地下水”（GWUDI），需要按照地表水监管标准处理该供应。基于氦和氚比率的地下水年龄测定方法可用于补充建立供水GWUDI的现有方法。使用氦氚比的“年龄”定义为水与大气失去接触所经过的时间。使用该年龄测定方法评估GWUDI的前提是，年龄较大的水域距离地表较远（平均而言），因此更具地下水特征。这些信息对于确定供水是否面临来自地表水源的污染风险，因此被称为GWUDI非常有价值。从佛罗里达州迈阿密戴德县的地下水供应中采集的10个样本说明了该方法的价值。 包括17篇参考文献。

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.