CHARACTERIZATION OF ORGANICS IN WHITESHELL RESEARCH AREA GROUNDWATER AND THE IMPLICATIONS FOR RADIONUCLIDE TRANSPORT

Naturally occurring organics in groundwaters of the Whiteshell Researc h Area (WRA) of southern Manitoba were investigated to assess their po tential role in radionuclide transport within granite fractures of the Canadian Shield. A survey of dissolved organic carbon concentrations, carried out to determine the variability in the organic content of th ese groundwaters, showed average concentrations of 0.8 +/- 0.1 mg/L fo r Fracture Zone 2, 0.8 +/- 0.4 mg/L for near-vertical fractures and 2. 3 +/- 0.8 mg/L for deeper saline groundwater. The dissolved organic ca rbon consisted mainly of hydrophilic neutral compounds (60 to 75%), an d hydrophobic and hydrophilic acids (23 to 39%), along with very small amounts of hydrophobic bases and neutrals, and hydrophilic bases. Sig nificant contamination from drilling operations and plastics, mostly i n the form of hydrophobic neutrals, was observed in some samples. Thes e compounds would not contribute to radionuclide migration in a natura l system and are not included in the assessment of the organic complex ing capacity of these groundwaters. Based on groundwater concentration s of hydrophobic and hydrophilic acids and on the measured carboxylic functional group content of these acids, the average complexing capaci ty of natural organics in Whiteshell Research Area groundwaters was ca lculated to be 2.9 x 10(-6) eq/L. The ability of these organics to com plex radionuclides was tested using conditional stability constants fr om the literature for humic complex formation with Am(III), Th(IV), an d U(VI). The chemistry of U(VI) was predicted to be dominated by inorg anic complexes and not significantly affected by organics. The literat ure contains a wide range in values of stability constants for Am(III) and Th(TV) complexes with humics. The aqueous chemistry of Am(III) an d Th(VI) could be affected by complexation with natural organics if th e higher stability constant values are valid for WRA groundwaters. Cro wn Copyright (C) 1996 Published by Elsevier Science Ltd.