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

Abstract

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.

A paleotemperature record derived from dissolved noble gases in groundwater of the Aquia Aquifer (Maryland, USA)

“…He concentrations often increase with groundwater age due to accumulation of radiogenic He produced by ␣-decay of U and Th series nuclides in crustal minerals (e.g., Andrews and Lee, 1979;Torgersen and Clarke, 1985;Stute et al, 1992b;Solomon et al, 1996). There are three potential sources of radiogenic He in groundwater: i) in situ production within the 811 Noble gas paleotemperature record from the Aquia Aquifer aquifer matrix; ii) release of stored He from fresh sediments; and iii) a flux from adjacent layers or even the whole underlying crust.…”

Radiogenic helium in shallow groundwater within a clay till, southwestern Ontario

“…Indeed, the storage of 4 He in crystal grains (e.g., apatite) is the basis for the U-Th/He thermochronometry technique that suggests grains are closed to the diffusive loss of 4 He below He-closure temperatures (see Reiners [2002] for a complete discussion). Solomon et al [1996] suggest that after the erosion and deposition of aquifer grains having a large surface to volume ratio, 4 He rad release rates greater than that resulting from U/Th production can occur for a finite period of time.…”

“…He-4 has thus traditionally been used as a groundwater tracer over a long timescale [Andrews and Lee, 1979;Marine, 1979;Torgersen, 1980;Andrews et al, 1982;Torgersen and Ivey, 1985;Balderer and Lehmann, 1989;Bottomley et al, 1990;Ballentine et al, 1991;Mazor and Bosch, 1991;Stute et al, 1992;Marty et al, 1993]. However, concentrations of 4 He in groundwater that are larger than can be explained by U and Th decay and the water age have been well documented [e.g., Torgersen and Ivey, 1985;Solomon et al, 1996] and previously interpreted as the result of 4 He diffusion from older underlying units (external source).…”

“…[3] Solomon et al [1996] studied 4 He in a shallow aquifer and concluded that elevated 4 He resulted from diffusion out of aquifer solids (internal source) rather than upward transport from the underlying bedrock. Incremental heating studies of 4 He rad release performed on aquifer solids were consistent with the hypothesis of diffusional 4 He rad release from aquifer grains and is interpreted as the release of stored helium.…”

“…Generally, smaller grain sizes (silt) release helium at very high rates that exceed the production from U/Th decay for a shorter period of time compared to larger (sand) grains [Solomon et al, 1996;Hunt, 2000;Sheldon, 2002]. Grains that are or have been exposed to elevated temperatures may release the bulk of their stored helium over a shorter period of time because the helium solid-state diffusion coefficient increases dramatically ($10Â in 20°) with increasing temperature.…”