The objective of this project is to conduct research on the potential use of helium, injected into water, as a tracer to determine pathways used by the water in its movement through aquifers, suboceanic and other hydrologically important environments. Tracers now used for this purpose include various types of salts, dyes and radioactive substances. This research involves development of an instrument, based on use of a detector, the operation of which exploits the know high permeation of helium through quartz glass. Plans are to evaluate the concept on several different types of problems that require the tracing of the movement of water including determination of the hydrological properties of aquifers, locations of interconnections between sources and sinks of pollutants, and tracing of pathways that provide for movement of water through fissured and fractured rocks. The calibration of mathematical models that predict the fate of environmental pollutants that move through subsurface media and that are discharged into surface waters is dependent on use of tracers. Helium is potentially advantageous for use as a tracer in comparison with substances that are not inert and that exist environmentally in relatively large and variable background concentrations. Helium is also relatively abundant in commercial quantities and is reasonably soluble in water. This research may lead to use of helium injection as a tracing method to replace or complement current practices using conventional chemicals, dyes and radioactive substances. **
