Ground water colloid properties from the Bangombé system

• The Bangombé ground water colloid concentration for 50–400 nm was low (80–300 ppb).
• These colloids are made of silica, ferrihydride and organics.
• The 235U/238U ratio in water samples from reactor zone shows depletion (7.09 × 10−3).
• The Kp values of trivalent elements (Sc, Y, Ln’s) range from 10+5 to 10+6 ml g−1.
• The Kp of U is ∼10+5 ml g−1 and 2–4% of U is associated with the colloid phase.

The concentration and the role of colloids in the transport of elements in the vicinity of a fossil reactor at Bangombé, Gabon, were assessed. Colloid sampling was conducted in seven boreholes around and in the extinguished natural reactor. The ground waters are of Na–Mg–Ca–HCO3 type, with variable salinities, pH 4.6–6.8 and anaerobic Eh values. Filtered ground water and colloid samples were taken from the reactor and the surroundings. Filtered fluids and colloid samples collected on membranes and resuspended in solution were analysed by ICP-MS and ICP-AES in order to examine the element association in the colloid phase within the size range 3000 to 400 to 50 nm. The colloid concentrations for the size 400 to 50 nm range from 80 to 300 ng ml−1. They consist of silica particles associated with ferrihydrite coated with organics. Trace element results show that metals including Pb, Sc, Y, La, Ce, Pr, Nd, Bi, Th and U are associated to various degrees with the colloid phase. The distribution ratios of these trace elements between the water and the colloid phase (Kp) were experimentally determined. The high Pb distribution ratios of 10+7 ml g−1 are specifically discussed. Values range from 10+6 to 10+5 ml g−1for the trivalent elements (Sc, Y, La, Ce, … , Bi). For uranium, a Kp of the order of 10+5 ml g−1 may be calculated and compared with data gained using the surface complexation model. These Kp values suggest that the uranium is partially sorbed or associated with ground water colloids. Measurements from the reactor zone show that about 2–4% of the uranium is associated with the colloid phase, which contributes partially to the uranium transport. The rather low colloid concentrations are due to the relatively high concentrations of Ca, Mg and Na in these quasi-neutral waters. These soluble elements contribute to the attachment of the colloids, restricting their transport. This indicates that the colloid phase may not be an important transport medium for the radionuclides in the Bangombé system if their association is reversible. The Bangombé colloid results are compared with those studied for other systems.

Comparison of lanthanide concentrations associated with the colloid phase (400–50 nm): BAX03(Col) (this work), with the so called “dissolved” (<450 nm): BAX03(GW) (Stille et al., 2003), leachates from particles (>450 nm): BAX03(Leach) (Stille et al., 2003) from the rock: BAX03R(Leach) (Stille et al., 2003), and from the reactor zone: BAX03(Rock1180) (Hidaka et al., 2005).Figure optionsDownload as PowerPoint slide