EVIDENCE OF TERRESTRIAL DISCHARGE OF DEEP GROUNDWATER ON THE CANADIANSHIELD FROM HELIUM IN SOIL GASES

Assessment of the impact of deep geological disposal of nuclear fuel w astes at a site in the Canadian Shield requires knowledge of the locat ion and size of areas of discharge of deep groundwater from the vicini ty of the underground disposal vault. A strong He anomaly has been det ected in soil gases in a 10 x 10 m area of wetland on the banks of Bog gy Creek, near Lac du Bonnet, Manitoba. The area has He concentrations in near-surface soils as high as 360 nL.L-1 and is assumed to indicat e discharge of He-rich groundwater through a permeable subsurface bedr ock fracture. Elevated Cl- concentrations in groundwater and its use a s a ''deer lick'' support this interpretation. A He flux density of ap proximately 2.1 L.m-2.a-1 is determined from a depth profile of He con centrations at one location in the site. A total He flux of 270 L.a-1 is determined for the entire site, which corresponds to a deep groundw ater discharge of about 26 000 L.a-1. This estimate is comparable with He fluxes and calculated groundwater discharges for two other lake-bo ttom locations on the Canadian Shield.