2.4 Radiogenic Tracers

Radiogenic tracers are atoms of a chemical element that are produced by radioactive decay and may themselves be radioactive or stable. Information about groundwater residence times sometimes can be obtained from the accumulation rates of radiogenic isotopes with low background concentrations. This approach may be applied in aquifers with relatively long groundwater residence times containing components of 4He from radioactive decay of U and Th (e.g., Mazor and Bosch, 1992; Solomon et al., 1996). The technique requires independent estimates of the local production and release rates of the radiogenic isotopes. However, obtaining enough measurements to understand potential spatial variations in production rates is difficult. 222Rn is also produced through U and Th decay, but its short half-life (3.8 days) means that secular equilibrium concentrations are achieved after a relatively short time (~20 days). (Secular equilibrium means that the rate of production is exactly balanced by the rate of decay, and so the concentration does not change.) Thus, over longer timescales, 222Rn concentrations in groundwater are essentially unrelated to residence time and primarily determined by the production rate (Figure 5).

222Rn is only produced by radioactive decay of 226Ra (which is part of the U and Th decay chain). Therefore, in principle, measurements of 222Rn can be used to date groundwater over periods up to approximately 20 days. Helium can be present from other sources. It has a significant concentration in the atmosphere, so is present in water that recharges the groundwater system. Consequently, helium is most useful for dating groundwater over longer timescales, when He produced from radioactive decay of U and Th isotopes within an aquifer (referred to as radiogenic 4He) is likely to dominate helium from other sources, including diffusion from underlying strata.

When groundwater containing accumulated radiogenic tracers is removed from the aquifer that contains the parent materials, for example by discharging to a stream, then concentrations of these tracers no longer increase. 222Rn concentrations will decrease due to radioactive decay, and concentrations of 4He and 222Rn will decrease due to exchange with the atmosphere. If these exchange rates are known, or can be measured, then concentrations of these dissolved gases within surface water bodies can be used to determine the time since groundwater discharged to the surface (Section 3.6).

Schematic illustration of changes in concentration of helium and radon with time after water enters an aquifer system
Figure 5 – Schematic illustration of changes in concentration of helium and radon with time after water enters an aquifer system. Both are produced by radioactive decay of other elements. However, as radon is itself radioactive, the rate of increase in concentration decreases over time, until eventually a constant concentration is reached (after approximately 20 days). Helium does not decay or degrade, so its concentration continues to increase over time (Cook, 2020).