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Contaminant Release from Solidified Radioactive Wastes Buried in Unsaturated Sediments: Lysimeter Study

Dep. of Agronomy and Horticulture, New Mexico State Univ., Dept 3Q, Las Cruces, NM 88003-0003;

R. J. Serne

Geosciences Dep., Pacific Northwest Lab., P.O. Box 999, Richland, WA 99352.

^* Corresponding author (tljones{at}taipan.nmsu.edu).

ABSTRACT

A lysimeter facility has been used since 1984 to monitor the leaching of buried waste forms under natural, arid conditions. The facility includes 10 bare-soil lysimeters (183 cm diam. by 305 cm deep) containing buried waste forms solidified with Portland III cement, masonry cement, bitumen, and vinyl-ester styrene. From 1984 through 1992, an average of 45 cm of water (27% of area precipitation) have leached through the lysimeters. To date, tritium, ⁶⁰Co, and ¹³⁷Cs have been identified in the lysimeter leachate samples. From 1984 through 1992, >4000 µCi of tritium, representing 76 and 71% of inventory (not decay corrected), have been leached from the two waste forms containing tritium. Cobalt-60 (<0.1% of original Co inventories) has been found in the leachate from each of all six of the waste forms that originally contained >1 mCi of inventory. Mobile Co is believed to be chelated with organic compounds, such as EDTA, present in the waste. Trace amounts of ¹³⁷Cs have occasionally been identified in leachate from two waste forms since 1991. Qualitatively, the field leaching results confirm laboratory studies, suggesting that tritium is readily leached from cement, and that ⁶⁰Co is generally leached more from cement than from vinyl-ester styrene. These data suggest that empirical source-term models should predict leaching based on the amount of water flowing past the waste form, not based on time.

Pacific Northwest Laboratory is operated for the U.S. Dep. of Energy by Battelle Memorial Institute under contract DE-ACO6-76RLO 1830. Work sponsored by NMSU Agric. Exp. Stn., Las Cruces, and the Dep. of Energy's Office of Technology Development.

Received for publication January 22, 1993.