IDENTIFICATION OF DEEP GROUNDWATER DISCHARGE AREAS IN THE BOGGY CREEKCATCHMENT, SOUTHEASTERN MANITOBA, USING EXCESS AQUEOUS HELIUM

The hydrogeology of the Lac du Bonnet batholith at the Whiteshell Rese arch Area (WRA) in southeastern Manitoba has been studied extensively. Although the general structure of the batholith is well known, its ne ar-surface structure is less well known because of the overburden. Con sequently, the precise locations of groundwater discharge from deep fr actures into local surface water remain uncertain. Deep groundwater in the WRA is highly enriched with radiogenic He from the decay of natur al U and Th ([He] is up to 50 mL.L-1). Surface waters normally equilib rate with atmospheric He and contain a standard [He] of 47 nL.L-1. We hypothesized that areas of deep groundwater discharge from the Lac du Bonnet batholith should be characterized by elevated aqueous He concen trations. We surveyed Boggy Creek and Boggy Lake at the WRA in the lat e winter of 1989 for such He anomalies. In Boggy Creek, three signific ant anomalies were identified, whereas in Boggy Lake several were foun d, the largest occupying the southeast quarter of the 80 ha lake. All the He anomalies were closely associated with the mapped or projected locations of faults in the underlying granite. A calculation of the to tal mass of excess He trapped under winter ice in Boggy Lake suggests that the total volume of deep groundwater seepage is small.