Excess Unsupported 210Pb in Lake Sediment from Rocky Mountain Lakes: A Groundwater Effect

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Abstract
Sediment cores from 4 high-altitude (.apprx. 3200 m) lakes in Rocky Mountain National Park, Colorado, were dated by 210Pb chronology. Background (supported) 210Pb activities for the 4 cores range from 0.26-0.93 Beq/g dry weight, high for typical oligotrophic lakes. Integrated unsupported 210Pb ranges from 0.81 (a typical value for most lakes) to 11.0 Beq/cm2. The 210Pb activity in the surface sediments ranges from 1.48-22.2 Beq/g dry weight. Sediment from Lake Louise, the most unusual of the 4, has 22.2 Beq/g dry weight at the sediment surface, an integrated unsupported 210Pb = 11.0 Beq/cm2, and supported 210Pb = 0.74 Beq/g dry weight. 226Ra content of the sediment is insufficient to explain either the high unsupported 210Pb or the 222Rn content of the water column of Lake Louise, which averaged 96.2 Beq/l. 222Rn-rich groundwater entering the lake is the source of the high 222Rn in the water column. This is capable of supporting the unusually high 210Pb flux to the sediment surface. Groudwater with high 222Rn may control the 210Pb budget of lakes where sediment cores have integrated unsupported 210Pb greater than 2 Beq/cm2.