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

Profiles of ⁴He in pore water were measured in clay aquitards in SW Ontario. The ⁴He distributions are consistent with groundwater velocities that are <6 mm yr⁻¹, and thus diffusion is the dominant transport mechanism for ⁴He. Modeling indicates that the effective diffusion coefficient for ⁴He is 6.3 ± 1.6 × 10⁻⁶ cm² s⁻¹. Furthermore, the profiles are consistent with the internal release of ⁴He 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 ⁴He from U/Th decay. Modeling and the ratio of ²¹Ne to ⁴He within the sediments indicate that although the aquitards were deposited about 13 kA BP, the sediments released >70% of initial ⁴He for 50 to 60 kA prior to incorporation into the till.