Penetration of Bog Peats and Lake Sediments by Tritium from Atmospheric Fallout

Abstract

Tritium in surface bog peats of the Red Lake wetland, northern Minnesota, exhibits about the same concentration as in atmospheric precipitation, which is still appreciably enriched by tritium from nuclear weapon testing. Past input pulses are not preserved in the peat profile, and the pattern of subsurface decline suggests that most of the modern precipitation rich in tritium is removed rapidly by evapotranspiration and by lateral runoff within the top 1.5 m of the 3.5—m thickness of little—decomposed Sphagnum peat at the crest of the raised bog. However, some bomb tritium has penetrated, presumably by molecular diffusion, down to the discontinuity between the highly permeable Sphagnum peat and the underlying well—decomposed and much less permeable fen peat. In profundal lake sediments of four Minnesota lakes the influence of highly enriched precipitation from the early 1960's has likewise been largely transitory and of major significance only in sediment depths of less than 1 m, although some penetration below the depth has occurred, presumably by molecular diffusion.