Sedimentation Rates and Sediment Core Profiles of 238U and 232Th Decay Chain Radionuclides in a Lake Affected by Uranium Mining and Milling

Abstract

Radionuclide concentration profiles in sediment cores from three deep basins of Quirke Lake, Ontario, showed enrichment in the surficial layers related to the period of uranium mining in the watershed, based on sedimentation rates determined from a milling-related pH decline indicated by fossil diatoms. Recent sedimentation rates are 96 ± 7 to 185 ± 14 g∙m⁻²∙yr⁻¹. ²¹⁰Pb profiles could not be used to determine sedimentation rates due to loadings of ²²⁶Ra and ²¹⁰Ph from mining and milling activities. Profiles of ²³⁸U and ²³²Th decay chain radionuclides showed an enrichment of 1–3 orders of magnitude in surficial sediments relative to background in deeper sediments. Radionudide levels in surficial sediments exceeded those reported in lake sediments in uncontaminated systems and, for some radionuclides, approached or equalled levels in sediments contaminated with uranium and radium refining residues. Radionudide activity ratios in the surficial layers of the Quirke Lake cores and in downstream sediments demonstrated the relative mobilities of the elements in the watershed. Net sediment loading rates for radionuclides in the three basins were 130–230 mg∙m⁻²∙yr⁻¹ for U, 340–4000 Bq∙m⁻²∙yr⁻¹ for other members of the ²³⁸U decay chain, and 210–530 Bq∙m⁻²∙yr⁻¹ for ²³²Th and ²²⁸Th.