SOURCE OF RADIOGENIC HELIUM 4 IN SHALLOW AQUIFERS - IMPLICATIONS FOR DATING YOUNG GROUNDWATER

Radiogenic helium 4 (He-4(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 He-4(rad) along shallow groundwater flow paths at a variety o f hydrogeologically distinct sites and postulate its use for dating gr oundwater as young as 10(1) years. Groundwater travel times and fluid velocities are particularly well documented at one site in northern On tario because of detailed profiling of tritium, H-3/He-3 ratios, and c hlorofluorocarbons (CFCs). Metamorphic rocks of the Canadian Shield (> 1 Ga) that contain large quantities of He-4 are the protolith of this unconsolidated aquifer and observed He-4(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 sour ce of He-4(rad) as vertical fluid velocities are too great to allow up ward diffusion of He-4(rad) from the underlying shield rocks. The appa rent rate of He-4(rad) release is 130 mu cm(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 so lids, measuring the amount of He-4 released, and then extrapolating re lease rates to the in situ temperature) agree well with the field resu lts and suggest that diffusion from aquifer solids is the source of He -4(rad). The combined laboratory and held release data yield He-4 diff usion coefficients that exhibit an Arrhenius temperature dependence th at is similar to He-4 diffusion in quartz reported by other researcher s. The He-4(rad) release rate at the Ontario site is extraordinarily s imilar to sites in Tennessee, Nebraska, and Germany in spite of major hydrogeologic differences. A model of He-4 diffusion from spherical gr ains suggests that aquifer solids' derived from old protoliths will re lease He-4 at rates greater than supported by U/Th production for up t o 50 million years in fine sands that have typical U/Th concentrations . Both observations and modeling suggest that He-4 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.