The Role of Organic Complexants and Colloids in the Transport of Radionuclides by Groundwater (NUREG/CR-6627, PNNL-12185)

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Publication Information

Manuscript Completed: April 1999
Date Published: January 2000

Prepared by:
D.E. Robertson, C.W. Thomas, S.L. Pratta^a
K.H. Abel, E.A. Lepel, A.J. Schilk^a
E.L. Cooper, F. Caron, K.J. King, R.W.D. Killey, P.G. Hartwig^b
J.F. Mattie, M.K. Haas, E. Romaniszyn, M. Benz, W.G. Evenden^b
S.O. Link^c
P. Vilks^d

^aPacific Norhwest National Laboratory
Richland, WA 99352

Subcontractors:
^bChalk River Laboratories
Chalk River, Ontario, Canada KOJIJO

^cWashington State University at Tri-Cities
Richland, WA 99352

^dWhiteshell Laboratories
Whiteshell, Manitoba, Canada

E. O’Donnell, NRC Project Manager

NRC Job Code L1935

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001

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Abstract

This final report for project JCN L1935, "Role of Organic Complexation and Colloids in the Transport of Radionuclides by Groundwater", describes the results of field and laboratory work conducted during the last two years of the project. It also contains summaries of important observations reported in earlier progress reports NUREG/CR-6429 and NUTREG/CR-6587, and other open literature publications.

The field studies were conducted at two low-level radioactive waste (LLW) management areas within the boundaries of the Chalk River Laboratories, Chalk River, Ontario, Canada, under a cooperative arrangement with the Atomic Energy of Canada, Ltd. At these locations, several well-defined, slightly radioactive groundwater contaminant plumes have developed over the past 25 to 40 years, providing excellent field sites for conducting studies of radionuclide transport by groundwater. This work was conducted to provide NRC, licensees, and other interested parties with information to better understand and predict the groundwater transport of radionuclides from LLW disposal facilities.

These studies have primarily addressed: 1) characterization of the sub-surface geochemical environment near the disposal facilities, 2) identification and quantification of the migrating radionuclides and their chemical speciation in groundwater, 3) the sorption behavior (Kd measurements) of the mobile radionuclide species (cationic and anionic) onto site soils, 4) identification of colloidal radionuclides, and 5) the environmental dynamics of ¹⁴C in the vicinity of a solid LLW disposal facility (published earlier as NUJREG/CR-6587).

The results of these studies indicate that the mobile radionuclides in groundwater at the Chalk River LLW disposal sites are primarily in solution as dissolved anionic organo-radionuclide complexes. These complexes appear to be quite stable over long time periods in the groundwater environment. Ion chromatographic separations of the soluble anionic radionuclide species from both ultra-filtered and 0.45 ffiltered groundwater from the Chemical Pit contaminant plume have demonstrated the presence of multiple chemical species for each radionuclide that are anionic in nature and consist of organo-radionuclide species. There is some colloidal transport, but it is minor compared to the dissolved radionuclide species.

Soil Kd measurements using the natural anionic species as radio-tracers gave much lower Kd values compared to measurements made using commercially available uncomplexed radio-tracers. These experiments confirm that the natural anionic organo-radionuclide species in groundwater are much more mobile than would be predicted from transport modeling using classical literature Kd data bases.

Carbon-14 migrating in groundwater from the Area-C LLW disposal site emerges in a wetland area 300 m downgradient from the waste trenches and enters the ambient vegetation. Several field studies showed that the ¹⁴C almost exclusively enters the vegetation by photosynthetic uptake of ¹⁴CO₂ released from the ground surface, while transpiration uptake directly from the contaminated groundwater is essentially negligible.

These studies will provide performance assessment modelers and regulators with additional information to better assess the rates and mechanisms of radionuclide transport from LLW disposal facilities that become infiltrated with water.