USGS Coastal and Marine Geology Program > Pacific Coastal & Marine Science Center > Research Projects > Coastal Aquifer Project: Submarine Groundwater Discharge > Methods & Tools > Uranium-Thorium Series Geochemical Tracers
Two isotopes of radon are potentially of importance in coastal groundwater studies. 222Rn (t1/2 3.8 days) is produced by the α decay of 226Ra in the 238U decay series, whereas 220Rn (t1/2 = 55.6 s) is formed by the R decay of 224Ra (Figure A). Radon, with an atomic number of 86, is the heaviest of the noble gases and, therefore, in groundwater is not easily ionized and so does not react with aquifer surfaces. As a consequence, 222Rn is highly mobile with respect to transfer from the aquifer matrix to pore water and frequently has the highest observed groundwater activities (Figure B). The production of Rn from the decay of Ra is accompanied by a recoil in the direction opposite to the emitted α particle. The recoil range of an Rn atom is on the order of 40 nm in solids, 95 nm in water, and 64000 nm in air. Krishnaswami et al. suggested that 222Rn and all other U/Th series isotopes produced by α decay are supplied at similar rates by such recoil. Therefore, the activities of 222Rn in groundwater may be used to calculate the recoil rate for all U/Th series nuclides produced by α recoil. The only loss term for 222Rn is radioactive decay, and with a 3.8 day half-life, it will likely reach steady-state activities in most groundwater systems.
Observed groundwater 222Rn activities typically correspond to 222Rn release rates of up to ~10% of the amount being produced in the aquifer rock. This implies that ~20% of the 226Ra in the host rock should exist within recoil distance of the surface. Such high recoil rates cannot easily be supported by recoil from typical aquifer grain sizes with uniform parent Ra activities. Instead, various other causes for such high release rates have been invoked. For example, it is possible that 226Ra must be absorbed on very small grains or present on secondary phases, or 226Ra adsorbed on surfaces could preferentially produce 222Rn by weathering processes. Another process that has been suggested for the elevated supply of 222Rn into the unsaturated zone is the leaching of radionuclides from adjacent minerals. Where fluctuations in the water table yield ephemerally saturated conditions, the decreased stopping power of air allows atoms ejected from minerals to be implanted across pore spaces. These atoms will then be available for subsequent leaching, which would affect the supply of 222Rn from ephemerally flooded sediments.
It is likely that U and Th may be heterogeneously enriched within aquifers in fine-grained clays or other aquicludes with low hydraulic conductivities that are not part of the main water-bearing deposits.
Figure B. Five-day time series of (A) bottom water 222Rn activities (dpm/L), (B) bidirectional electromagnetic (EM) seepage meter results (cm/d); the zero discharge line is shown for reference, and (C) bottom water conductivities (mS/cm) collected from the top of a EM seepage meter dome. The reference line in (C) denotes the mean conductivity (40.7 mS/cm) of the bottom waters.