Uranium-Thorium Series Geochemical Tracers

Radon

Two isotopes of radon are potentially of importance in coastal groundwater studies. ²²²Rn (t_1/2 3.8 days) is produced by the α decay of ²²⁶Ra in the ²³⁸U decay series, whereas ²²⁰Rn (t_1/2 = 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.

Figure A. Uranium-Thorium Decay Series for Radon. To view a larger image, click on the image above.

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.

Click on the images below to open a printable version of the USGS Report in Adobe Reader.

OFR 2004-1226 (808 KB PDF)

Submarine ground water discharge and its role in coastal processes and ecosystems http://sofia.usgs.gov/publications/ofr/2004-1226/

FS 2004-3117 (1.16 MB PDF)

Novel geophysical and geochemical techniques used to study submarine groundwater discharge in Biscayne Bay, Florida http://sofia.usgs.gov/publications/fs/2004-3117/

OFR 2004-1369 (1.03 MB PDF)

An autonomous, electromagnetic seepage meter to study coastal groundwater/ surface-water exchange http://sofia.usgs.gov/publications/ofr/2004-1369/

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	Uranium-Thorium Series Geochemical Tracers Radon Two isotopes of radon are potentially of importance in coastal groundwater studies. ²²²Rn (t_1/2 3.8 days) is produced by the α decay of ²²⁶Ra in the ²³⁸U decay series, whereas ²²⁰Rn (t_1/2 = 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.

			Figure A. Uranium-Thorium Decay Series for Radon. To view a larger image, click on the image above.
	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.
To view additional information about Uranium/Thorium series tracers, please click on the links below: U/Th Series Radionuclides as Coastal Groundwater Tracers (909 KB PDF) Remaining uncertainties in the use of Rn-222 as a quantitative tracer of submarine groundwater discharge (157 KB PDF)

Print Publications: Reports Click on the images below to open a printable version of the USGS Report in Adobe Reader. NOTE: PDF files may be viewed using Adobe Reader public domain software. If unable to access the PDF files, please contact aharrison@usgs.gov.


OFR 2004-1226 (808 KB PDF) Submarine ground water discharge and its role in coastal processes and ecosystems http://sofia.usgs.gov/publications/ofr/2004-1226/		FS 2004-3117 (1.16 MB PDF) Novel geophysical and geochemical techniques used to study submarine groundwater discharge in Biscayne Bay, Florida http://sofia.usgs.gov/publications/fs/2004-3117/
	OFR 2004-1369 (1.03 MB PDF) An autonomous, electromagnetic seepage meter to study coastal groundwater/ surface-water exchange http://sofia.usgs.gov/publications/ofr/2004-1369/

Uranium-Thorium Series Geochemical Tracers

Radon

Print Publications: Reports