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Soda Ash Treatment of a Strontium-90-Contaminated Groundwater Seep¹

ABSTRACT

A ⁹⁰Sr-contaminated groundwater seep on the perimeter of a low-level radioactive solid waste disposal area at the Oak Ridge National Laboratory (ORNL) was treated by burying 315 kg of soda ash in the groundwater flow path leading to the seep, and placing 45 kg of soda ash on the surface of the seep. The concentration of ⁹⁰Sr in the seep water fell from an average of 7000 Bq L^–1 to 900 Bq L^–1 for the 90 d after burial, followed by a period of gradual rise back to pretreatment levels over the next 100 d. The electrical conductivity and pH of the seep water increased following soda ash burial, while water hardness fell. Hardness was highly correlated (r = 0.84) with ⁹⁰Sr concentrations over the entire 2-year observation period, indicating the similar behavior of ⁹⁰Sr and soluble Ca and Mg. This in situ softening of, and ⁹⁰Sr precipitation from, the seep water was achieved by coprecipitation of ⁹⁰Sr with Ca(Mg)CO₃ until the buried soda ash was depleted by dissolution in the groundwater. The soda ash treatment of groundwater seeps appears to be most practical as an interim technique for those situations requiring an immediate, but temporary, corrective action. During this limited but effective period, more permanent corrective actions could be planned at the source of contamination. The electrical conductivity, pH, and hardness of the larger surface stream, into which this seep discharges, were not affected by the soda ash burial, most likely due to the approximately 2000-fold dilution effected by this stream.

Key Words: radioactive waste • groundwater contamination • carbonate precipitation

¹ Research sponsored by the Office of Defense Waste and By-Product Management, U.S. Department of Energy, under contract W-7405-ENG-26 with Union Carbide Corp. Publication no. 2054, Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830.

² Research Staff Member and Senior Laboratory Technician, Environ. Sci. Div., Oak Ridge National Laboratory.

Received for publication November 3, 1982.