Use of helium as a hydrological tracer
Gupta, Sushil K., Philip S. Moravcik, and L. Stephen Lau
Helium has several characteristics which make it attractive for use as a tracer in hydrological studies. These include its inert nature, relatively high solubility in water (~1%), low molecular diffusion in water, ready availability in commercial quantities, nontoxic nature, and low background atmospheric concentration. The use of helium as a tracer of water movement has become possible through the development 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 (1) a set of experiments undertaken to compare breakthrough of helium with common salt (NaCl) tracer through laboratory sand columns; (2) a set of groundwater tracing experiments conducted in a basaltic aquifer in central O’ahu, Hawai’i; and (3) two laboratory experiments undertaken to evaluate the applicability of helium instrumentation for the tracing of submerged plumes in open water conditions. The test results demonstrate that helium behaves as a conservative tracer during saturated flow through porous media. During unsaturated flow, exchange of helium with air entrained in the porous media reduces its usefulness. During submerged flow of a labeled plume, helium behaves like fluorescein in a relatively tranquil laboratory tank environment for hours but is gradually lost through the air-water interface, thus limiting the usefulness of helium in shallow plume tracing studies to short duration experiments.