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Source of radiogenic helium 4 in shallow aquifers: Implications for dating young groundwater
Author(s) -
Solomon D. K.,
Hunt A.,
Poreda R. J.
Publication year - 1996
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/96wr00600
Subject(s) - aquifer , radiogenic nuclide , groundwater , geology , groundwater flow , shield , hydrogeology , nuclide , hydrology (agriculture) , geochemistry , soil science , petrology , mantle (geology) , geotechnical engineering , physics , quantum mechanics
Radiogenic helium 4 ( 4 He rad ) has been used in numerous studies as a tracer of groundwater age in the range of 10 3 –10 8 years. We have measured 4 He rad along shallow groundwater flow paths at a variety of hydrogeologically distinct sites and postulate its use for dating groundwater as young as 10 1 years. Groundwater travel times and fluid velocities are particularly well documented at one site in northern Ontario because of detailed profiling of tritium, 3 H/ 3 He ratios, and chlorofluorocarbons (CFCs). Metamorphic rocks of the Canadian Shield (>1 Ga) that contain large quantities of 4 He are the protolith of this unconsolidated aquifer and observed 4 He rad values increase linearly with distance along a flow path and with increasing groundwater age. A solute transport model suggests that the aquifer solids are the source of 4 He rad as vertical fluid velocities are too great to allow upward diffusion of 4 He rad from the underlying shield rocks. The apparent rate of 4 He rad release is 130 μcm 3 m −3 yr −1 and is 300 times greater than can be supported by the in situ decay of U and Th series nuclides (i.e., the “steady state” approximation). Laboratory release experiments (conducted by sequentially heating the aquifer solids, measuring the amount of 4 He released, and then extrapolating release rates to the in situ temperature) agree well with the field results and suggest that diffusion from aquifer solids is the source of 4 He rad . The combined laboratory and field release data yield 4 He diffusion coefficients that exhibit an Arrhenius temperature dependence that is similar to 4 He rad diffusion in quartz reported by other researchers. The 4 He rad release rate at the Ontario site is extraordinarily similar to sites in Tennessee, Nebraska, and Germany in spite of major hydrogeologic differences. A model of 4 He diffusion from spherical grains suggests that aquifer solids derived from old protoliths will release 4 He at rates greater than supported by U/Th production for up to 50 million years in fine sands that have typical U/Th concentrations. Both observations and modeling suggest that 4 He may be useful as a groundwater dating tool over a range of tens to hundreds of years. The latter is particularly important because no other groundwater dating techniques are accurate for waters ranging from 40 to about 500 years old.