Chemical speciation modelling of groundwater in a shallow glacial sand aquifer. Part II, radionuclide speciation and effect of organics

Abstract

The aims of the work detailed in this report are to gain understanding of the changes that occur in groundwater chemistry following the addition of a) the radionuclides 58C0 and 63Ni, and b) organic complexants. The analytical data used in these modelling studies is as for the first report in this series, and consists of inductively coupled plasma analyses for total element concentrations of three samples. For reasons that are detailed in the text (section 4.3) only two of the three samples were modelled using the PHREEQE geochemical code in conjunction with the UWCC thermodynamic database. Acetate and EDTA were used as analogues for the weakest and strongest binding expected by the natural organic matter present in the aquifer. The results show that at the measured pH, the major form of 58C0 will be the Co2+ ion. In the case of 63Ni, the major aqueous phase species is NiC03O. The speciation of cobalt is unaltered with changes in the pH or Eh, whereas nickel is pH sensitive. The formation of solid phase cobalt or nickel species is not predicted. The addition of 58C0 to the aquifer as a CcbH2EDTA is predicted to result in the dissociation of the complex and consequent formation of the free Co2+ and ZnEDTA2-. Acetate doesn ot perturb either the natural aqueous speciatiorn the speciationo f the added radionuclides. In contrast, using EDTA as an analogue for natural organic matter results in significant perturbations of both the natural speciation and the radionuclide speciation. It was found that 48% of the total iron, 92% of the total cobalt and 99% of the total nickel were complexed by EDTA with an EDTA concentration equivalent to the measured total organic carbon concentration of 3.5 and 2.5 mg dm-3 in sample 2 and 3 respectively