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

Profiles of 4 He in pore water were measured in clay aquitards in SW Ontario. The 4 He distributions are consistent with groundwater velocities that are <6 mm yr −1 , and thus diffusion is the dominant transport mechanism for 4 He. Modeling indicates that the effective diffusion coefficient for 4 He is 6.3 ± 1.6 × 10 −6 cm 2 s −1 . Furthermore, the profiles are consistent with the internal release of 4 He from aquitard sediments at a rate of 0.03–0.13 μcc(STP) kg −1 yr −1 . These rates are also consistent with laboratory release experiments and, on average, are 600 times greater than the production of 4 He from U/Th decay. Modeling and the ratio of 21 Ne to 4 He within the sediments indicate that although the aquitards were deposited about 13 kA BP, the sediments released >70% of initial 4 He for 50 to 60 kA prior to incorporation into the till.