FRACTURE DETECTION AND GROUNDWATER-FLOW CHARACTERIZATION USING HE ANDRN IN SOIL GASES, MANITOBA, CANADA

Surveys of the distribution and migration of He and Rn were carried ou t in the well-characterized granitic terrane of the AECL Underground R esearch Laboratory (URL), Manitoba as part of a joint AECL Research, U nited Kingdom Department of the Environment, and United States Departm ent of Energy research initiative. The investigations were designed to determine whether concentrations of He and Rn in soil gases could be used to identify locations of groundwater recharge and discharge from bedrock fractures. The results obtained indicate that subsurface trans port of He and possibly Rn in this setting appears to be controlled la rgely by the groundwater flow system in the bedrock. Release of dissol ved gases near the ground surface causes soil gas anomalies, which ref lect discharge from the deeper flow system. In the recharge area of th e deep groundwater flow system at the URL site, He abundances are clos e to the atmospheric level, but the discharge area of the deep flow sy stem is characterized by significant He anomalies (up to 0.5 ppm above atmosphere levels). For Rn, the recharge area has broadly distributed high concentrations, probably caused by local Rn production in U-rich overburden, while the discharge area has only localized concentration s of Rn, which are not at the same location as the He anomalies. The g eneral nature of the groundwater flow regime in both areas is reflecte d in the presence and distribution of the soil gas anomalies. In addit ion, major fractures in bedrock, which act as preferential groundwater flow paths, have been located from soil gas anomalies, even when obsc ured by overburden of variable thickness and character. The distributi on of He in soil gas appears to be most representative of groundwater recharge and discharge conditions in the granitic rock, while Rn may b e useful for locating specific channels where more rapid groundwater d ischarge is occurring from deep fracture zones.