Source of radiogenic helium 4 in shallow aquifers: Implications for dating young groundwater

Radiogenic helium 4 (4Herad) has been used in numerous studies as a tracer of groundwater age in the range of 103–108 years. We have measured 4Herad along shallow groundwater flow paths at a variety of hydrogeologically distinct sites and postulate its use for dating groundwater as young as 101 years. Groundwater travel times and fluid velocities are particularly well documented at one site in northern Ontario because of detailed profiling of tritium, 3H/3He ratios, and chlorofluorocarbons (CFCs). Metamorphic rocks of the Canadian Shield (>1 Ga) that contain large quantities of 4He are the protolith of this unconsolidated aquifer and observed 4Herad 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 4Herad as vertical fluid velocities are too great to allow upward diffusion of 4Herad from the underlying shield rocks. The apparent rate of 4Herad release is 130 μcm3 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 4He 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 4Herad. The combined laboratory and field release data yield 4He diffusion coefficients that exhibit an Arrhenius temperature dependence that is similar to4Herad diffusion in quartz reported by other researchers. The 4Herad 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 4He diffusion from spherical grains suggests that aquifer solids derived from old protoliths will release 4He 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 4He 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.