Annual Supply of 238U, 234U, 230Th, 226Ra, 210Pb, 210Po, and 232Th to Lake 239 (Experimental Lakes Area, Ontario) from Terrestrial and Atmospheric Sources

Abstract

The activities of 238U, 234U, 230Th, 226Ra, 210Pb, 210Po, and 232Th were measured in seasonal samples of stream runoff water, suspended and bedload sediments, and bulk precipitation. These nuclides were also determined in stream drainage area soils and bedrock. From these measurements and hydrological data, an estimate of the annual supply of these nuclides to Lake 239 at the Experimental Lakes Area (ELA) was made. Because of low concentrations of major ions, suspended sediments, and low pH, many common stream transport mechanisms for these nuclides are not available, and we propose that dissolved organic carbon is important in this respect. The annual flux (per unit stream drainage area) of these nuclides to the lake is similar to fluxes calculated for larger rivers of the world. Uranium-234 is transported preferentially to 238U at activity ratios of 1–1.3. Radium-226 is carried in solution in proportion to Ca++. The major supply of 210Pb to the lake is from atmospheric fallout on the lake surface area. There are insufficient 222Rn or 210Pb deficits in the soil profiles to balance the measured excess 210Pb in surface soils and lake sediments. Partition coefficients for these nuclides were calculated from data on stream suspended sediments and stream water; U and Ra coefficients were ≈ 104, and Th, 210Pb, and 210Po coefficients were 104 to 105.