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

Sediment cores from four high-altitude (approximately 3200 m) lakes in Rocky Mountain National Park, Colorado, were dated by 210Pb chronology. Background (supported) 210Pb activities for the four cores range from 0.26 to 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 to 22.2 Beq/g dry weight. Sediment from Lake Louise, the most unusual of the four, 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 Rn content of the water column of Lake Louise, which averaged 96.2 Beq/L. We concluded that 222Rn-rich groundwater entering the lake is the source of the high 222Rn in the water column. This, in turn, is capable of supporting the unusually high 210Pb flux to the sediment surface. Groundwater with high 222Rn may control the 210Pb budget of lakes where sediment cores have integrated unsupported 210Pb greater than 2 Beq/cm2.