NOBLE GAS AND TRITIUM LABORATORY
Noble gases are the elements that make up Group 18 of the periodic table. Under standard conditions, they are odorless, colorless, monatomic gases with very low chemical reactivity. The six naturally occurring noble gases are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and the radioactive radon (Rn). Their inert nature allows them to serve as excellent tracers in complex hydrological systems. Each of the non-radioactive noble gases has a series of stable isotopes, which can be measured for their information on hydrogeological processes and conditions .
DATING GROUNDWATER WITH T -3He
Groundwater age can be estimated for any tritium-bearing groundwater from its decay although dispersion of the initial tritium concentration at time of recharge and subsequent mixing are complicating factors. Further, initial tritium is often poorly known in most groundwater systems due to seasonal interannual, and spatial variability. We can remove the need for initial tritium concentration if the accumulated daughter, tritigenic 3He, is also measured in the groundwater at the time of sampling, according to the formula:
The measured 3Het expressed in TU (1 3He per 1018 H) must be corrected for atmospheric 3He that accompanies the groundwater during discharge. The atmospheric 3He includes both a dissolved component and an excess air component that can be entrained as microbubbles during recharge. Contributions of 3He from excess air are determined by the excess Ne in the sample, for which its solubility is less sensitive to temperature than for He. Application of this method is described with practical examples in Clark (2015).
The accumulation of geogenic 4He (from alpha decay in the U-Th decay series) is a geochronometer that can be applied over a wide range of time scales. Uncertainties arise from its high diffusivity, which can add helium from deep crustal sources into shallow confined aquifers. However, if the diffusive flux can be constrained, then in situ production from U and Th concentrations can provide estimates of groundwater age. This method is limited by the uncertainty in resolving geogenic production in excess of atmospheric 4He that is incorporated during recharge. Sufficiently high U and Th concentrations within the aquifer allow for the dating of submodern groundwaters that are a few hundred to many thousands of years old. Geogenic 4He concentrations can also be used for uranium exploration, and for tracing the origin of crustal fluids.