Migration of Technetium, Iodine, Neptunium, and Uranium in the Peat of Two Minerotrophic Mires

Abstract

AbstractMires may become receptors of hazardous wastes transported by the groundwater from surface or underground disposal facilities. Should the contributing water sources contain dissolved solutes, they may sorb onto the organic matter in the mire, precipitate out of solution, or form more or less mobile complexes or colloids. Our study investigated the vertical migration of Tc, I, Np, and U in two types of mires typical of the Precambrian Shield. We also report seasonal groundwater level fluctuations and chemical composition changes in the mires. Our results indicate dramatic seasonal changes in the pH and chemical composition of the groundwater, particularly the concentrations of Na, Mg, Ca, and SO4. Iodine, in spite of being sorbed to the peat with a sorption coefficient, Kd, of 0.2 to 64 L kg−1, migrated quickly through the peat in the porewater. The aeration status and the botanical origin of the peat seem to be factors in the retardation of I. Iodine accumulated on the mire surface in late summer when the water table was lower. However, Tc, U, and Np were quickly immobilized in the reducing environment of the mire and did not migrate very far from their entry point. Geometric mean Kd values for U in the three peats varied from 2600 to 19 000 L kg−1. The Kd values for U did not depend on porewater concentration, humification, or aeration. A comparison between Kd values for U obtained in situ and in laboratory studies showed that our incubation‐sorption system may be a satisfactory technique for U. Neptunium Kd values varied from 31 to 2600 L kg−1 and Tc Kd values were > 2 L kg−1 in these three peats. The Kd values are general indicators of the mobility of these elements in mires, the most mobile being I followed by Tc, Np, and U.