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Laboratory and field experiments were conducted with helium as a ground‐water tracer. Techniques were developed for the addition and extraction of helium from water. A mass spectrometer and a pressure‐volume apparatus were used for helium measurements at concentrations in water ranging from 1.5 to 5.5×10−4 milligrams per liter. In the field investigation, flow was traced through a confined aquifer for a distance of 188 feet. Both laboratory and field experiments showed that helium traveled at a slightly lower velocity than chloride. The advantages of helium as a ground‐water tracer are its safety, low cost, relative ease of analysis, low concentrations required, and chemical inertness. The disadvantages include the relatively large errors in analysis, difficulties of maintaining a constant recharge rate, time required to develop equilibrium conditions in unconfmed aquifers, and possible loss to the atmosphere in unconfined aquifers.

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The ideal ground‐water tracer should correctly depict the movement of water through a porous medium without modifying the transmission characteristics of the system. Cationic materials, including radio‐isotopes and certain organic dyes, are generally unsatisfactory and anionic radio‐isotopes when used without stable carriers may be largely retained by adsorption. Chloride may be satisfactorly used where density effects can be avoided and dispersion of clays is not likely. Organic materials are subject to decomposition by soil microorganisms and hence their value as water tracers are limited. Tritium, when used as tritiated water, may exchange with soil bound water with a resulting reduction in velocity of the tracer front.