An assessment of the performance of the Ranger RP1 constructed wetland filter during releases in 1995 and 1996

1997

Internal Report 255
leGras C & Klessa D
Supervising Scientist Division
Department of the Environment

About the report

Energy Resources of Australia Ltd (ERA) has constructed and maintains a wetland filter in the catchment of Retention Pond 1 (RP 1) at the Ranger mine site. The filter comprises eight cells of different sizes with a combined surface area of about 56 000m2 and volume 45 000m3. The effective minimum flow path is approximately 1240 m. The filter was constructed as part of Ranger's water management system, and is designed to remove certain contaminants from the effluent stream of Retention Pond 2 (RP2) before flood irrigation of the amended water elsewhere in the RPl catchment. Various wetland plants are present, including Eleocharis and Nymphya sp.

The constituents of RP2 water that are present at greatly elevated concentrations compared with unaffected waters in neighbouring creeks are: magnesium (Mg); sulfate (SO42-): manganese (Mn) and radionuclides such as uranium (U) and radium (Ra). The wetland filter is moderately to very effective at removing Mn, moderately effective at removing U and almost totally ineffective at removing Mg and SO42-. Manganese, present mainly as Mn2+ in influent water is removed principally by oxidation to manganese oxides during passage through the wetland, which then settle from the water column. Uranium is probably removed mainly by absorption from the water column to iron (Fe) rich particulates. Transit time in the wetland filter is likely to be the main determinant of retention efficiency for U, with influent pH apparently playing a lesser role.

The principal objective of this report is to elucidate the factors responsible for the ameliorative effect of the wetland filter. The behaviour of Ra has not yet been thoroughly assessed and is not discussed further in this report.