Fracture detection and groundwater flow characterization using He and Rn in soil gases, Manitoba, Canada

Surveys of the distribution and migration of He and Rn carried out in well-characterized granitic terrain indicate that subsurface transport of He and possibly Rn appears to be controlled largely by the groundwater flow system in the bedrock. Release of dissolved gases near the ground surface caused soil gas anomalies, which reflect discharge from the deeper flow system. In the recharge area of the deep groundwater flow system, He abundances are close to the atmospheric level, but the discharge area is characterized by significant He anomalies. For Rn, the recharge area has broadly distributed high concentrations, probably caused by local Rn production in U-rich overburden, while the discharge era has only localized concentrations of Rn, not at the same location as the He anomalies. The general nature of the groundwater flow regime in both areas is reflected in the presence and distribution of the soil gas anomalies. Major fractures in bedrock, which act as preferential groundwater flow paths, have been located from soil gas anomalies, even when obscured by overburden of variable thickness and character. The distribution of He in soil gas appears to be most representative of groundwater recharge and discharge conditions in the granitic rock, while Rn may be useful for locating specific channels where more rapid groundwater discharge is occurring from deep fracture zones. (author)