Diffusion of radiogenic helium in shallow groundwater: implications for crustal degassing

The clay till of Sarnia and fractured bedrock of Smithvilie, Ontario provide wellcharacterized groundwater systems in which to examine the diffusive transport and crustal degassing of helium. Although poorly defined in the literature, the application of effective diffusion coefficients accurately describes the transport of 4He in geologic media and is examined at both sites. In addition, the sites provide an opportunity to determine the crustal degassing flux in shallow groundwater systems for comparison to large sedimentary basins. The effective diffusion coefficients of 4He in clay till and fractured shale are examined by numerical simulation of measured groundwater 4He concentrations at both sites. Effective diffusion coefficients of 6.3 x 10"6 cm2/s and 1.48 x 10'7 cm2/s were determined for the clay till and the Rochester Shale, respectively. A mass balance of methane substantiates that diffusion is the primary means of mass transport through the shale and define a CH4 effective diffusion coefficient of 3.7 x 1CT8 cm2/s. The model results emphasize the importance of applying an effective diffusion coefficient to describe the transport of helium through geologic media. The internal release rates and degassing fluxes are determined for both sites. The He degassing fluxes out of the clay till (2.7 x 108 atoms4He/m2/s) and Rochester Shale (1.22 -1.70 x 108 atoms4He/m2/s) are similar to the crustal degassing fluxes reported in the literature. The results of this study suggest the importance of the release of stored helium to the crustal degassing flux. Furthermore, the results indicate that a significant percentage of ancient stored helium is released to the atmosphere during erosional processes that cause grain size reduction and are not measurable in groundwater. A new dissolved gas sampling method was developed to permit sample collection from small diameter peizometers and/or low permeability units. The results of field and laboratory analysis indicate that the samplers equilibrate in -8 hours in advection-dominated systems, and within two weeks in a controlled diffusion-dominated system. The samplers allow for high quality dissolved gas samples with minimal effort, time, and expense. The samplers eliminate sample loss and contamination common to other methods of obtaining dissolved gas samples.