PNNL

Abstract

Vertical distributions of Co-60 are determined in soil cores obtained from a 10-hectare grassland, where anaerobically-digested sludge was applied by surface spraying from 1986 to 1995 on the U.S. Department of Energy’s Oak Ridge Reservation. These results, along with historical application records, are used to estimate vertical-migration rates and perform a mass balance. The presence of Co-60 results solely from the sludge-application process. Soil, vegetation, and surface-water samples were collected. Eleven soil cores were sectioned into 3-cm increments and analyzed by gamma-ray spectrometry. No Co-60 was detected in the vegetation or water samples. The downward migration rate of Co-60 in the upper 15 cm of soil ranged from 0.50 to 0.73 cm/yr. About 98 percent, 0.020 ± 0.011 Bq/cm2, of Co-60 remained in the upper 15 cm of soil, which compared favorably with the expected Co-60 activity based on historical records of 0.019 ± 0.010 Bq/cm2.