Sediment characteristics and concentration gradients in the RP1 constructed wetland filter

Department of the Environment, 1998

Internal Report 293
Klessa D, Hunt C & leGras CI
Supervising Scientist Division

About the report

The compartmentalisation of contaminants, derived from restricted release zone water, into sediment of the Retention Pond #1 Constructed Wetland Filter (RPI CWF) at Ranger was studied. Following two seasons during which RP2 water was processed by the RPI CWF, sediment was sampled from the 0–1, 1–3, 3–5 and 5–10 cm depths of the cells. Sediment samples were assayed for pH, electrical conductivity, cation exchange capacity (CEC), exchangeable bases, available P and total N, P, C, U, Cu, Cd, Pb, Co, Zn, Mn and Fe. Sodium bicarbonate (0.5 M) buffered at pH 8.5 was also used to extract a labile uranium fraction from sediment.

Sediment was characterised by a moderate to slightly acid pH (4.6–6.5), very low to moderate CEC (0.4–11.3 cmol/kg) and low organic C content (0.6–1.6%). Total U concentration decreased in an approximately exponential fashion with path length and depth from a maximum of 570 mg/kg in Cell I surface sediment. On average, 51 % of total U was extracted by bicarbonate. It was estimated that around 64–77% of the retained U load from the polishing of RP2 water could be accounted for in the sediment of the wetland filter.

The speciation of U in influent water to the RPI CWF is discussed. The more acid pH and CO2 enriched environment of sediment pore water compared to the overlying water column will favour the speciation and sorption of the mononuclear cation and hemicarbonate forms of U which are preferentially adsorbed by oxyhydroxide surfaces. Total U was highly correlated with total Mn which might imply the active sequestering of U by Mn oxide surfaces. A similarly high correlation was found between total Mn and Co which was attributed largely to native soil minerals. However, the relative enrichment of surface sediment by Mn and Co, especialIy in the front cells, suggests that the formation of Mn oxides and the sorption of Co is an active process.