USE OF INJECTED HELIUM AS A HYDROLOGICAL TRACER

Helium has several characteristics which make it attractive for use as a tracer in hydrological studies. These include its inert nature, rel atively high solubility in water (approximately 1%), low molecular dif fusion in water, ready availability in commercial quantities, nontoxic nature, and low background atmospheric concentration. The use of heli um as a tracer of water movement has become possible through the devel opment of an instrument which takes advantage of the fact that at room temperature helium diffuses through a quartz glass membrane at a rate of three to four orders of magnitude greater than any other gas. This paper describes: (i) a set of experiments undertaken to compare break through of helium with common salt (NaCl) tracer through laboratory sa nd columns; (ii) a set of groundwater tracing experiments conducted in a basaltic aquifer in Central O'ahur, Hawaii and (iii) two laboratory experiments undertaken to evaluate the applicability of helium instru mentation for the tracing of submerged plumes in open water conditions . The test results demonstrate that helium behaves as a conservative t racer during saturated flow through porous media. During unsaturated f low, exchange of helium with air entrained in the porous media reduces its usefulness. During submerged flow of a labelled plume, helium beh aves like fluorescein in a relatively tranquil laboratory tank environ ment for hours but is grudually lost through the air-water interface, thus limiting the usefulness of helium in shallow plume tracing studie s to short duration experiments.