PARENT SESSION

IP11 Investigations of the Columbia River and Estuary (USA)
B113 & B114
8:00 AM - 12:00 PM, Thursday

(IP088) Uptake and depuration of ⁹⁰Sr/⁹⁰Y by Columbia River periphyton communities.

Fellows, R¹, Bunn, A¹, McKinstry, C¹, Degerman, T¹, Miller, B¹, ¹ Pacific Northwest National Laboratory, Richland, WA, USA

ABSTRACT- Low amounts of strontium-90 (⁹⁰Sr) are currently entering the Columbia River at the U.S. Department of Energys Hanford Site in eastern Washington State. As part of a comprehensive study on the biological fate and transport of ⁹⁰Sr at the Columbia groundwater/river interface, periphyton communities were collected from the Columbia River and cultured on glass slides. These were exposed to river water containing differing concentrations (1.0x10⁵, 5.0x10⁴, and 1.0x10⁴ pCi L^-1) of ⁹⁰Sr (as SrCl₂). Exposures lasted for 120 h followed by 72-h depuration periods in non-labeled river water. At selected time intervals, periphyton was scraped from slides into centrifuge tubes, washed with sterile river water, dried, weighed, ashed and counted using liquid scintillation. In the environment, ⁹⁰Sr (t=28.2 yr) exists in secular equilibrium (equal counts from both) with its daughter product yttrium-90 (⁹⁰Y; t=64.1 h). Most studies of Sr uptake employ ⁸⁵Sr, ⁸⁹Sr, or chemically separate ⁹⁰Sr from ⁹⁰Y to avoid interference of ⁹⁰Y on the counting process. We counted all samples within 60 h of sampling and again after 27 d (> 10 half-lives of ⁹⁰Y). The 60-h counts were consistently 2 to 3 times greater (pCi g^-1 dry wt.) than those of the same samples after 27 d. Removal of the ⁹⁰Sr/⁹⁰Y from the exposure system produced depuration curves closely resembling ⁹⁰Y decay kinetics. Calculations using counts and ⁹⁰Y decay correction indicated that assimilation and retention of ⁹⁰Y by the periphyton was almost an order of magnitude greater than ⁹⁰Sr. The ^- decay product of ⁹⁰Y (935 keV average, 2.26 MeV max) is much more energetic than the ^- of ⁹⁰Sr (196 keV average, 546 keV max) and accounts for most of the radioisotope dosage imparted to a biological organism following ⁹⁰Sr incorporation. Effects of such a higher dose to the primary producers of a freshwater ecosystem are unknown.

Key words: Strontium-90, periphyton, Yttrium-90, Columbia River