Regional survey for He anomalies in Canadian shield lakes: sources of variation and implications for nuclear fuel waste management

Bottom-water He concentration (=[He]) was measured in the late summers of 1989, 1990 and 1991 in the waters of lake basins at the Experimental Lakes Area in northwestern Ontario, Canada. A total of 32 basins were sampled every year. Helium concentrations were relatively stable from year to year, exhibiting a mean coefficient of variation of 34%. Total [He] ranged from values below the atmospheric equilibrium concentration of 47 nl He/l H₂O to a maximum of 5364 nl/l measured in Lake 625 in 1991. Total [He] exhibited a two-phase distribution, with a large subpopulation of basins having only modest He enrichment (geometric mean G.M.= 77nl/l, geometric standard deviation G.S.D.= 1.65, n = 39), and a small subpopulation of basins, including Lakes 625, 634 and 615, with large He anomalies (G.M.= 692nl/l,G.S.D.= 3.08, n = 6). Using hydrological, morphometric and physical data for each lake, bottom-water [He] was predicted. A model including lake order and lake width accounted for 22% of the total variance in [He]. These results support the hypothesis that excess He in lake-bottom-water originates with deep groundwater discharge via fractures in the underlying granite.