Groundwater Studies

Noble gas hydrology uses a variety of stable and radioactive isotopes of noble gases as tools to examine groundwater ages, recharge temperatures, water flow patterns, salinization etc. The tritium and helium pair provides an effective tool to estimate groundwater ages of a few decades. It can be used to replace the routinely used tritium. Indeed, the application of tritium and helium measurements in groundwater studies has steadily been gaining momentum. While tritium alone may be used to trace the bomb signal of the early 1960s (and infer a time scale), radioactive decay and dispersion contribute significantly to uncertainties and make it difficult to draw firm conclusions in a given groundwater regime. Combining tritium with its radioactive product ³He has two prominent advantages: 1) The sum of the measured tritium and ³He restores the original tritium signal, and a much better defined bomb signal emerges; 2) The ratio of the 3He to ³H is used to determine a time period for which groundwater has been isolated from the atmosphere (e.g., time since recharge), a straightforward application of radioactive dating. Tritium in rain is shown in Figure 1, which illustrates the inherent difficulty in interpreting tritium concentration in groundwater, especially in younger aquifers.

Helium isotope information is not without complications: ⁴He may be enriched in groundwater as a result of leaching of radiogenic helium produced in the radioactive decays of the Uranium and Thorium series elements. In fact, this particular feature was developed into a tool for uranium prospecting (e.g., Top and Clarke, 1981). ³He may be produced in situ nuclear reactions, interfering with the tritiogenic signal. Both helium isotopes may also be enriched due to a mantle component where regional geology provides connections to a mantle source. In general however, these complicating components may be resolved for many types of aquifers. Tritium- helium-3 age determination has been verified to produce reliable, consistent time scale information, which in turn used in flow-path and -rate estimates (e.g., Schlosser et al., 1998).

References

Clarke, W.B., W.J. Jenkins and Z. Top, Determination of tritium by mass-spectrometric measurement of 3He. International J. Applied Radiation and Isotopes, 27,515-522, 1976.

Schlosser, P. et al., Tritium/ 3He measurements in young groundwater. In Isotope Techniques in the study of Environmental Change, pp. 165-189, 1998, IAEA, Vienna.

Top, Z. and W.B. Clarke, Dissolved helium isotopes and tritium in lakes: further results for uranium prospecting in central Labrador. Economic Geology, 76, 2018-2031, 1981.

Projects

Helium and Radon as Tracers of Groundwater input into Florida Bay (Abstract)
Use of tritium-helium in the Everglades (Abstract)
A study of CFC removal in the Everglades groundwater (Abstract)

Groundwater Studies Noble gas hydrology uses a variety of stable and radioactive isotopes of noble gases as tools to examine groundwater ages, recharge temperatures, water flow patterns, salinization etc. The tritium and helium pair provides an effective tool to estimate groundwater ages of a few decades. It can be used to replace the routinely used tritium. Indeed, the application of tritium and helium measurements in groundwater studies has steadily been gaining momentum. While tritium alone may be used to trace the bomb signal of the early 1960s (and infer a time scale), radioactive decay and dispersion contribute significantly to uncertainties and make it difficult to draw firm conclusions in a given groundwater regime. Combining tritium with its radioactive product ³He has two prominent advantages: 1) The sum of the measured tritium and ³He restores the original tritium signal, and a much better defined bomb signal emerges; 2) The ratio of the 3He to ³H is used to determine a time period for which groundwater has been isolated from the atmosphere (e.g., time since recharge), a straightforward application of radioactive dating. Tritium in rain is shown in Figure 1, which illustrates the inherent difficulty in interpreting tritium concentration in groundwater, especially in younger aquifers. Helium isotope information is not without complications: ⁴He may be enriched in groundwater as a result of leaching of radiogenic helium produced in the radioactive decays of the Uranium and Thorium series elements. In fact, this particular feature was developed into a tool for uranium prospecting (e.g., Top and Clarke, 1981). ³He may be produced in situ nuclear reactions, interfering with the tritiogenic signal. Both helium isotopes may also be enriched due to a mantle component where regional geology provides connections to a mantle source. In general however, these complicating components may be resolved for many types of aquifers. Tritium- helium-3 age determination has been verified to produce reliable, consistent time scale information, which in turn used in flow-path and -rate estimates (e.g., Schlosser et al., 1998). References Clarke, W.B., W.J. Jenkins and Z. Top, Determination of tritium by mass-spectrometric measurement of 3He. International J. Applied Radiation and Isotopes, 27,515-522, 1976. Schlosser, P. et al., Tritium/ 3He measurements in young groundwater. In Isotope Techniques in the study of Environmental Change, pp. 165-189, 1998, IAEA, Vienna. Top, Z. and W.B. Clarke, Dissolved helium isotopes and tritium in lakes: further results for uranium prospecting in central Labrador. Economic Geology, 76, 2018-2031, 1981. Projects Helium and Radon as Tracers of Groundwater input into Florida Bay (Abstract) Use of tritium-helium in the Everglades (Abstract) A study of CFC removal in the Everglades groundwater (Abstract)